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Improved performance of soft clay foundations using stone columns
and geocell-sand mattress
Sujit Kumar Dash a Mukul Chandra Bora b
a Department of Civil Engineering Indian Institute of Technology Kharagpur Kharagpur 721 302 Indiab Department of Civil Engineering Indian Institute of Technology Guwahati Guwahati 781 039 India
a r t i c l e i n f o
Article history
Received 23 November 2012
Received in revised form
23 August 2013
Accepted 31 August 2013
Available online 21 September 2013
Keywords
Foundation
Geosynthetics
Stone columns
Ground improvement
a b s t r a c t
A series of experiments have been carried out to develop an understanding of the performance
improvement of soft clay foundation beds using stone column-geocell sand mattress as reinforcement It
is found that with the provision of stone columns of adequate length and spacing about three fold
increases in bearing capacity can be achieved While with geocell-sand mattress it is about seven times
that of the unreinforced clay But if combined together the stone column-geocell mattress composite
reinforcement can improve the bearing capacity of soft clay bed as high as by ten fold The optimum
length and spacing of stone columns giving maximum performance improvement are respectively 5
times and 25 times of their diameter The critical height of geocell mattress can be taken equal to the
diameter of the footing beyond which further increase in bearing capacity of the composite foundation
bed is marginal
2013 Elsevier Ltd All rights reserved
1 Introduction
Rapid urbanisation and growth of infrastructure in the present
days has resulted in dramatically increased demand for land space
This has compelled the building industry to improve the soft soil
grounds which otherwise are unsuitable for construction activities
Amongst the various ground improvement techniques used stone
columns and geosynthetic reinforcement are probably the most
popular ones This is primarily due to their simplicity ease of
construction and overall economy that 1047297nds favour with the
practicing engineers
A stone column is a column of stones made through opening up
a vertical cylindrical hole in the soft clay bed and subsequently
1047297lling it up with compacted stone aggregates Due to higher
strength and stiffness the stone columns sustain larger proportion
of the applied load than their soft soil counterpart leading tosigni1047297cant performance improvement of foundation beds (Hughes
and Withers 1974 Juran and Guermazi 1988 Christoulas et al
2000 Wood et al 2000 McKelvey et al 2004 Ambily and
Gandhi 2007 Black et al 2007 Cimentada et al 2011 Dash and
Bora 2013) Moreover being highly permeable the stone columns
act as vertical drains facilitating consolidation of the soft clay
around and thereby improving the long term performance of the
foundation system
Geocell reinforcement is a latest development in the avenues of
geosynthetics It is a three dimensional polymeric honeycomb like
structure of cells interconnected at joints that the reinforcing
mechanism is primarily through all-round con1047297nement of soils
Besides geocells intercept the potential failure planes and their
rigidity forces them deeper into the foundation soil This induces a
higher surcharge loading on the failure plane giving rise to
increased load carrying capacity (Webster and Watkins 1977 Bush
et al 1990 Cowland and Wong 1993 Dash et al 2001 2003a
2003b 2004 Zhou and Wen 2008 Sireesh et al 2009
Leshchinsky and Ling 2013 Tanyu et al 2013)
Review of literature shows that geocell-sand mattress and stone
columns are effective means of performance improvement of soft
clay foundations Their individual applications though have been
intensely studied but combined application of both has remained
unexplored It is expected that the geocell-sand mattress with stonecolumns underneath shall further enhance the load carrying ca-
pacity of the foundation system Moreover a cushion of sand is
generally provided over the stone columns for the purpose of
drainage Limited research reported in the literature indicates that
this sand layer when reinforced with planar geosynthetics can
noticeably improve the bearing capacity of the foundation system
(Deb et al 2007 Abdullah and Edil 2007 Deb et al 2011) Arulrajah
et al (2009) have reported the use of a geogrid-soil platform over
stone columns in the construction of high speed railway embank-
ments in Malaysia In this arrangement the reinforced soil cushion
serves as a 1047298exible raft over the stone columns similar to that of a Corresponding author Tel thorn91 3222 282418 fax thorn91 3222 282254
E-mail address sujitciviliitkgpernetin (SK Dash)
Contents lists available at ScienceDirect
Geotextiles and Geomembranes
j o u r n a l h o m e p a g e w w w e l s e v i e r co m l o c a t e g e o t e x m em
0266-1144$ e see front matter 2013 Elsevier Ltd All rights reserved
httpdxdoiorg101016jgeotexmem201309001
Geotextiles and Geomembranes 41 (2013) 26e35
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piled raft system leading to improved load capacity However geo-
cell is a superior form of reinforcement over the planar one ( Dash
et al 2004 Madhavi and Vidya 2007 Latha and Somwanshi
2009) This is mostly due to its three dimensional con1047297ning struc-
ture that prevents lateral spreading of soil Therefore the sand
cushion over the stone columns if reinforced with geocells is ex-
pected to produce enhanced performance improvement These as-
pects are studied herein through a series of laboratory-scale model
tests The results have been analysed in developing an under-
standing of the behaviour of clay foundations reinforced with the
stone column-geocell mattress composite system
2 Details of model tests
21 Planning of experiments
Schematic sketch of a typical test con1047297guration is shown in
Fig 1 The stone columns were left 1047298oating in the clay bed This was
to simulate the situation commonly encountered in coastal areas
wherein soft clay deposits extend over very large depths that the
stone columns are generally terminated in the clay itself The col-
umns were placed in triangular pattern at a regular spacing S
(Fig 2) In all the tests diameter of stone columns (dsc) was kept
constant as 100 mm
Geocells were formed in chevron pattern (Fig 3) as it gives
better performance improvement over the diamond pattern (Dash
et al 2001) Diameter of geocells (dgc) taken as equivalent diam-
eter of geocell pocket opening was kept constant as 08D
throughout (D diameter of footing) The geocell mattresses were
placed at a constant depth (u) of 01D from the base of the footing
which was found to be the optimum location giving maximum
performance improvement (Dash et al 2008)
In total twelve series of model load tests were conducted the
details of which are presented in Table 1 Within each series onlyone parameter was varied This was to understand the in1047298uence of
this particular parameter on the overall behaviour of the founda-
tion system while the others were kept constant Tests in series 1
were performed on unreinforced clay beds Series 2 and 3 consisted
of testing the stone column reinforced clay beds wherein the in-
1047298uence of length (L) and spacing (S ) of the columns werestudied In
all these tests there was no sand cushion over the clay beds The
effect of height of geocell-sand mattress (h) was studied under
series 4 Subsequently tests in series 5e12 were designed to
LStone
column
Clay
Sand h
u
Geocell layer
DT DTDT DTDT DTDTDT
Footing
D D D
(00) xDxD
Fig 1 Schematic diagram of test con1047297guration
S
dsc
S
Stone column
Fig 2 Layout of stone columns
transverse member diagonal member
bodkin joint
b
b
Fig 3 Geocell system in chevron pattern plan view
Table 1
Details of model tests
Testseries
Typeof reinforcement
Details of parameters investigated
1 Un rei nforc ed cl ay bed wi th c u of 5 kPa
2 SC Variable parameter Ldsc frac14 1 3 5 7
Constant parameter S dsc frac14 25
3 SC Variable parameter S dsc frac14 15 25 35
Constant parameter Ldsc frac14 5
4 GC Variable parameter hD frac14 053 09 11 16
Constant parameter dgcD frac14 08 bD frac14 6
5 GC thorn SC Variable parameter Ldsc frac14 1 3 5 7
Constant parameter
hD frac14 053 dgcD frac14 08 bD frac14 6 S dsc frac14 25
6 GC thorn SC Variable parameter Ldsc frac14 1 3 5 7
Constant parameter
hD frac14 09 dgcD frac14 08 bD frac14 6 S dsc frac14 25
7 GC thorn SC Variable parameter Ldsc frac14 1 3 5 7
Constant parameter
hD frac14 11 dgcD frac14 08 bD frac14 6 S dsc frac14 25
8 GC thorn SC Variable parameter Ldsc frac14 1 3 5 7
Constant parameter
hD frac14 16 dgcD frac14 08 bD frac14 6 S dsc frac14 25
9 GC thorn SC Variable parameter S dsc frac14 15 25 35
Constant parameter
hD frac14 053 dgcD frac14 08 bD frac14 6 Ldsc frac14 5
10 GC thorn SC Variable parameter S dsc frac14 15 25 35
Constant parameter
hD frac14 09 dgcD frac14 08 bD frac14 6 Ldsc frac14 5
11 GC thorn SC Variable parameter S dsc frac14 15 25 35
Constant parameter
hD frac14 11 dgcD frac14 08 bD frac14 6 Ldsc frac14 5
12 GC thorn SC Variable parameter S dsc frac14 15 25 35
Constant parameter
hD frac14 16 dgcD frac14 08 bD frac14 6 Ldsc frac14 5
Note SC Stone columns GC Geocell-sand mattress
SK Dash MC Bora Geotextiles and Geomembranes 41 (2013) 26 e 35 27
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investigate the combined application of both the reinforcements
ie stone columns and geocell-sand mattress
22 Materials used
The model clay beds were prepared using a locally available soil
that had 70 fractions 1047297ner than 75 mm (Fig 4) Its liquid limit
plastic limit and plasticity index were found to be 40 21 and 19
respectively (ASTM D4318 2005) As per the Uni1047297ed Soil Classi1047297-
cation System (USCS ASTM D2487 2006) the soil can be classi1047297ed
as clay with low plasticity (CL)
The stone columns were formed out of a poorly graded crushed
granite aggregate with particle sizes in the range of 2e10 mm (d10
d30 d50 of 220 315 490 mm respectively Fig 4) and uniformity
coef 1047297cient of 232 Its direct shear friction angle (peak) at place-
ment density of 153 kNm3 was found to be 48 The diameter of
model stone columns (100 mm) and size of aggregates used
(d50 frac14 49 mm) were approximately in 17 scale representation of
prototype stone columns of 700 mm diameter with averageaggregate size of 35 mm
The geocell reinforcement used was fabricated from a biaxial
geogrid having aperture size of 36 mm 36 mm ultimate tensile
strength of 193 kNm and 5 strain secant modulus of 135 kNm
(ASTM D6637 2001) The joints of the geocells formed out of 6 mm
wide and 3 mm thick polypropylene strips had a tensile strength of
475 kNm (ASTM D4884 2009) Such low strength of joints was
adopted to scale down the overall strength of the geocells that it is
suitable for the model tests The geocells were 1047297lled with a poorly
graded sand that had average particle size of 044 mm and uni-
formity coef 1047297cient of 252 (Fig 4) In all the tests the sand was
placed at 80 of relative density Its peak friction angle obtained
through triaxial compression tests in the pressure range of 100e
200 kPa was 44
23 Test setup
The model tests were carried out in a laboratory scale test bed-
cum-loading frame assembly (Fig 5) The test beds were prepared
in a steel tank of 1000 mm long1000 mm wide and 1300 mm high
To avoid yielding during tests the four sides of the tank were
braced laterally on their outer surfaces with steel channels The
footing used was made of a rigid steel plate and measured 150 mm
in diameter (D) In order to create a rough base condition a thin
layer of sand was glued onto its bottom In all the tests the footing
was placed at the centre of the test tank Loading was applied
through an automated hydraulic jack system supported against a
reaction frame 1047297
xed onto the ground The load transmitted to the
footing was recorded through an electronic load cell of 20 kN ca-
pacity with an accuracy of 001 N The settlements of the footing
were measured by two linear variable differential transducers
(LVDTs) placed at diametrically opposite ends (DT1 DT2 Fig 1)Deformations (heavesettlement) on foundation bed too were
measured by LVDTs placed through small plastic strips on the soil
surface (DT3eDT8 Fig 1) The LVDTs were of 50 mm travel with an
accuracy of 3 microns The load cell and the LVDTs were connected
to a computer controlled data acquisition system
Selig and McKee (1961) and Chummer (1972) have observed
that the failure wedge in the foundation bed extends over a dis-
tance of about 2e25 times the footing width away from its centre
In the present tests the distance of tank walls from centre of footing
being more than 33D the slip planes are not likely to be interfered
with Besides the geocell mattress being 1047298exible deforms down-
ward under the footing loading and thereby gets pulled away from
the tank side walls reducing the boundary effect to a practically
negligible value Indeed Dash et al (2003a) through instrumentedmodel tests have observed that the footing loading in nearly
similar test conditions did not induce any additional pressure on
the tank walls
The stone columns used had a maximum length of 700 mm (ie
L frac14 7dsc) and the geocell mattresses used had maximum height of
255 mm (ie h frac14 17D) Therefore the minimum clear spacing be-
tween the stone column base and the tank bottom maintained in
the tests was 345 mm [ie 1300 (700 thorn 255)] This is about 345
times the diameter of the stone column (dsc) Mayerhof and Sastry
(1978) have observed that the failure zone below a rigid pile ex-
tends over a depth of about 2 times its diameter The stone columns
being 1047298exible this depth would further be less A stress analysis
considering the dispersion in geocell mattress (Dash et al 2007)
and group action of stone columns (similar to that of rigid piles in
clay Bowel1988) was carried out It shows that for height of geocell
mattress of 17D and stone column length of 7dsc the stressinduced
at the bottom of the test-tank was less than 2 of the applied
pressure In view of this it can be said that the test-tank used in the
present investigation was considerably large enough and not likely
to interfere with the failure zones and hence the experimental re-
sults Besides the con1047297nement due to the tank walls simulated the
actual 1047297eld conditions for the interior columns in a large group
(Ambily and Gandhi 2007)
24 Test bed preparation
The clay was pulverised mixed with predetermined amount of
water and for moisture equilibrium was kept in airtight containers
0001 0010 0100 1000 10000 100000
Particle size (mm)
0
20
40
60
80
100
P e r c e n
t f i n e r b y w e i g h t
stone aggregate
sand
clay
Fig 4 Particle size distribution of stone aggregate sand and clay soil
Fig 5 Test set-up
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for about a week The test bed was prepared in lifts of 005 mthickness For each lift the amount of soil required to produce the
desired bulk density was weighted out placed in the test-tank
levelled and compacted Compaction was done through a wooden
board and a drop hammer using depth marking on the sides of the
tank as guide The compaction energy applied was 299 kJm3 Un-
disturbed soil samples were collected from different locations and
their properties were evaluated Sampling was done through a thin
cylindrical sampler that was pressed into the clay bed andextracted
out with the soil withinApartfrom the in situ density and moisture
content the specimens were tested for the vane shear strength as
well The average moisture content degree of saturation bulk unit
weight and shear strength of the clay in the test beds were found
to be 36 100 1805 kNm3 and 5 kPa respectively Their coef 1047297-
cient of variability was in the range of 15The stone columns were constructed by a replacement method
An open-ended stainless steel pipe of 100 mm outer diameter and
15 mm of wall thickness smeared with petroleum jelly (to reduce
friction) was pushed into the clay bed until it reached the depth of
the column to be formed Subsequently the clay within the pipe
was scooped out through a helical auger of 90 mm diameter To
minimise suction effect a maximum of 100 mm was removed at a
time On completion the internal wall of the pipe was cleaned and
stone aggregates were charged in The stones were added in
batches of 06 kg and compacted to height of 50 mm through a
circular steel tamper of 09 kg with 30 blows of 200 mm drop
leading to a density of 153 kNm3 that corresponds to 65 of
relative density The pipe was then slowly raised ensuring a mini-
mum of 25 mm overlap within the aggregate that the clay outsidedoesnrsquot intrude in This procedure was continued until the stone
column was completely formed (ie till top of clay bed) The stone
column reinforced clay bed thus formed was loaded with a seating
pressure of 25 kNm2 over the entire area for 4 h This was to
achieve uniformity in the test bed (Malarvizhi and Ilamparuthi
2007)
The geocell reinforcement was prepared from geogrid strips
placed in transverse and diagonal directions and connected
together with bodkin joints (Bush et al 1990) The jointwasformed
by pulling the ribs of the diagonal geogrid up through the trans-
verse geogrid and slipping a dowel (plastic strip) through the loop
created (Carroll and Curtis 1990) Three-dimensional view of a
typical geocell structure placedover the clay bed is shown in Fig 6
The geocells were 1047297
lled with sand through raining Compared to
unreinforced case with geocell reinforcement the height of raining
required to achieve the target density was relatively more how-
ever the difference was not much This was because the geocells
being made of geogrids had more than 80 of open area thereforedid not affect much the free 1047298ow of sand during raining The dif-
ference in the placement densities of the sand at various locations
in the test bed measured through in situ sampling was found to be
less than 15
25 Test procedure
In all the tests loading was applied in strain controlled manner
at the rate of 2 mmmin This relatively faster rate of loading was
intended to produce undrained response in the saturated clay bed
It is one of the worst 1047297eld conditions expected as in this case the
angle of friction of the soil tends to be zero leading to large
reduction in the bearing capacity Such phenomenon is common
during rainy seasons particularly in case of railways and highwayswhere the loading is transient in nature In all the tests load was
applied until the footing settlement reached 40 mm Through a
computer controlled system the load-deformation data were
continuously recorded
On completion of tests the deformed shape of the stone columns
were mapped This was done through careful removal of stone
aggregates and 1047297lling the shaft with Plaster of Paris (CaSO4$05H2O)
paste After being hardened the Plaster of Paris column was taken
out and measured for its shape and size The stone column de-
formations thus obtained are presented in terms of radial strain
(r d r o)r o wherein r d is the deformed radius and r o is the original
radius
3 Results and discussion
Typical pressure-settlement responses of clay bed alone that
with geocell and geocell-stone column composite reinforcement
are presented in Fig 7 The settlement s reported is the average of
the readings taken at both ends of the footing (DT 1 and DT2 Fig 1)
It could be observed that in case of unreinforced clay the slope of
the pressure-settlement response continues to increase until set-
tlement (sD) of about 15 and thereafter tends to become nearly
vertical This means that the soil has undergone clear failure and
therefore couldnrsquot support additional pressure anymore However
with geocell reinforcement (Clay thorn GC) the bearing pressure con-
tinues to increase even at settlement (sD) as high as 25 although
the overall improvement is relatively less But with stone columns
in the clay subgrade underneath geocell mattress (Clay thorn GC thorn SC)
Fig 6 Typical geocell layer in the test bed
0
4
8
12
16
20
24
28
F o o t i n g s e t t l e m e n
t s D
( )
0 20 40 60 80 100 120 140 160
Bearing pressure (kPa)
Clay
Clay+GC
Clay+GC+SC (Ldsc= 1)
Clay+GC+SC (Ldsc= 3)
Clay+GC+SC (Ldsc= 5)
Clay+GC+SC (Ldsc= 7)
Fig 7 Footing pressure-settlement responses in1047298uence of length of stone columns in
composite foundation bed (hD frac14 053 S dsc frac14 25) e Test series 5
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the bearing capacity has improved signi1047297cantly much higher than
that with geocell mattress alone Besides the stiffness of the
foundation bed too has increased substantially indicated through
reduced slope of the pressure settlement response Similar
behaviour is noticed in all other cases as well This is attributed to
the increased resistance against deformation provided by the stone
columns through mobilisation of friction and stiffness of the stone
aggregates that provides additional support to the geocell mattress
As a result of which the geocell-sand mattress that behaves as a
subgrade supported beam (Dash et al 2007) stands effectively
against the footing loading leading to improved performance of thefoundation system
The increase in the bearing capacity due to stone columns
geocell mattress and stone column-geocell mattress composite
reinforcement is quanti1047297ed through nondimensional improvement
factors IFsc IFgc IFgcsc respectively de1047297ned as the ratio of bearing
pressure with reinforcement (qsc qgc qgcsc) to that of unreinforced
clay bed (qu) both taken at equal settlement of footing ( sD) The
values of these improvement factors for different test cases and
footing settlements are presented in Table 2 It is evident that with
stone columns the bearing capacity of soft clay can be enhanced by
37 fold (IFsc frac14 37 Test series 3) and with geocell reinforcement it
can be up to 787 fold (IFgc frac14 787 Test series 4) But if coupled
together (ie stone columns and geocell mattress) the composite
reinforcement can enhance the bearing capacity of the soft clay as
high as by 10 fold (IFgcsc frac14 102 Test series 12) Hence it can be said
that the stone column-geocell composite is a superior form of
reinforcement that can give better performance improvement over
the conventional ones ie stone column geocell mattress In1047298u-
ence of different parameters on the overall performance of such
composite foundation systems are discussed in the following
sections
31 In 1047298uence of length of stone columns
In1047298uence of length of stone columns (L) in the composite
foundation bed has been studied for four different heights of
geocells h frac14 053D 09D 11D and 16D (Test series 5e8) It is of
interest to note that increasing the column length from 3 to 5 dsc
leads to sharp increase in performance both in terms of increased
load carrying capacity (IFgcsc Table 2) and reduced settlement
(Fig 7) However with further increase in length (L) to 7dsc the
additional improvement is much less Hence it can be said that in
the composite foundation system the optimum length of stone
columns giving maximum performance improvement is about 5
times their diameter (ie 5dsc) This observation however is from
small-scale models and needs to be veri1047297ed through prototype
tests The results are further analysed in terms of the improvement
factor ratios (ie IFgcscIFgc and IFgcscIFsc) that brings out the
Table 2
Bearing capacity improvement factors (IF IFsc IFgc IF gcsc)
Test series Reinforcement Variable parameter Bearing capacity improvement factor (IF)
sD 1 sD 5 sD 9 sD 13 sD 17 sD 21 sD 25 sD 27
2 SC Ldsc frac14 1 169 125 114 119 123 126 128 130
SC Ldsc frac14 3 181 154 153 160 162 166 173 173
SC Ldsc frac14 5 359 287 293 312 319 331 340 344
SC Ldsc frac14 7 369 320 317 337 340 348 356 360
3 SC S dsc frac14 15 366 290 320 345 349 361 367 370
SC S dsc frac14 25 359 287 293 312 319 331 340 344
SC S dsc frac14 35 197 147 167 184 181 186 190 193
4 GC hD frac14 053 134 150 164 186 195 211 226 232
GC hD frac14 09 210 218 283 303 335 359 411 433
GC hD frac14 11 290 410 477 557 600 660 712 739
GC hD frac14 16 420 481 543 614 654 708 764 787
5 GC thorn SC Ldsc frac14 1 172 198 191 201 215 230 246 253
GC thorn SC Ldsc frac14 3 187 222 223 236 248 266 288 295
GC thorn SC Ldsc frac14 5 412 426 429 432 441 473 502 513
GC thorn SC Ldsc frac14 7 439 479 487 498 502 530 557 569
6 GC thorn SC Ldsc frac14 1 224 280 288 312 345 379 417 435
GC thorn SC Ldsc frac14 3 300 333 328 348 370 408 436 450
GC thorn SC Ldsc frac14 5 429 463 470 509 545 592 643 673
GC thorn SC Ldsc frac14 7 536 555 565 589 612 668 720 746
7 GC thorn SC Ldsc frac14 1 366 506 518 574 615 686 746 778
GC thorn SC Ldsc frac14 3 502 598 594 631 669 722 777 801
GC thorn SC Ldsc frac14 5 566 665 663 700 740 801 850 874GC thorn SC Ldsc frac14 7 700 738 724 755 798 850 913 942
8 GC thorn SC Ldsc frac14 1 341 480 555 640 720 801 872 901
GC thorn SC Ldsc frac14 3 388 529 600 700 755 825 902 924
GC thorn SC Ldsc frac14 5 548 646 684 741 796 859 928 959
GC thorn SC Ldsc frac14 7 610 700 738 782 832 893 956 988
9 GC thorn SC S dsc frac14 15 454 482 484 495 516 553 579 588
GC thorn SC S dsc frac14 25 412 426 429 432 441 473 502 513
GC thorn SC S dsc frac14 35 328 344 350 352 354 365 379 389
10 GC thorn SC S dsc frac14 15 501 546 557 585 624 675 724 747
GC thorn SC S dsc frac14 25 429 463 470 509 545 592 643 673
GC thorn SC S dsc frac14 35 340 354 364 408 416 455 490 506
11 GC thorn SC S dsc frac14 15 610 723 734 760 797 853 907 937
GC thorn SC S dsc frac14 25 566 665 663 700 740 801 850 874
GC thorn SC S dsc frac14 35 530 584 597 617 665 730 789 822
12 GC thorn SC S dsc frac14 15 635 713 729 781 840 920 987 102
GC thorn SC S dsc frac14 25 548 646 684 741 796 859 928 959
GC thorn SC S dsc frac14 35 510 629 643 700 750 819 882 906Note Boldface values indicate the maximum improvement factors for different reinforcements (ie SC GC and SC thorn GC)
SK Dash MC Bora Geotextiles and Geomembranes 41 (2013) 26 e 3530
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contribution of individual reinforcement components (ie geocell
sand mattress stone columns) as has been explained below
IFgcsc
IFscfrac14
qgcsc
qu
qsc
qu
frac14 qgcsc
qsc(1)
Thus the factor (IFgcscIFsc) can be taken as the contribution of
geocell mattress sharing the surcharge loading in the composite
foundation system Similarly the factor IFgcscIFgc (ie qgcscqgc)
represents the contribution of stone columns Typical improvement
factor ratios IFgcscIFgc and IFgcscIFsc for the case with hD frac14 053
are depicted in Fig 8 and Fig 9 respectively It is evident that the
contribution of stone columns IFgcscIFgc increases with increase in
length and the improvement is the maximum when Ldsc ratio
changes from 3 to 5 This may be because the shorter columns (L
dsc lt 3) due to inadequate skin resistance have suffered punching
failure and therefore didnrsquot contribute signi1047297cantly towards the
load carrying capacity of the system In contrast the long columns
(Ldsc 5) owing to large skin resistance mobilised through
increased surface area have effectively stood against the footing
loading giving rise to visible increase in performance improvement
Further con1047297rmation of the column failure modes was obtained
from the post test deformation pro1047297les a typical of which for the
central stone columns are shown in Fig 10 It could be observed
that when short in length (Ldsc frac14 1 and 3) the bulging in the stone
columns is marginal indicating that the column has mostly been
punched down But with increase in length (Ldsc 5) it has
effectively stood against the footing and therefore has bulged
signi1047297cantly Furthermore with column Ldsc ratio of 1 the
contribution of geocell mattress IFgcscIFsc was the maximum
which however reduced as the column length increased and
remained almost constant for Ldsc 5 (Fig 9) This can be analysed
through the responses of the 1047297ll surface depicted in Fig 11 The
surface deformations (d) reported herein are the average of the
readings taken at distance D on both sides of the footing (DT5 andDT6 Fig 1) It can be seen that with increase in the length of stone
columns settlement (thornd) on the 1047297ll surface has reduced Corre-
spondingly heave in the adjacent region ( x frac14 2D and 3D) too was
found to have reduced This is attributed to the increased resistance
of stone columns that inhibits settlement and thereby heave in the
foundation bed With reduced deformations in the soil around the
strength mobilised in the geocells reduces and so is its contribution
to the performance improvement Stone column length beyond the
0 4 8 12 16 20 24 28
Footing settlement sD()
0
1
2
3
4
5
I m p r o v e m e n t f a c t o r r a t i o I F g c s c I F g c
Clay+GC+SC(Ldsc= 1)
Clay+GC+SC(Ldsc= 3)
Clay+GC+SC(Ldsc= 5)
Clay+GC+SC(Ldsc= 7)
Fig 8 Improvement factor ratio-footing settlement responses contribution of stone
columns in composite foundation bed (hD frac14 053 S dsc frac14 25) e Test series 5
0 4 8 12 16 20 24 28
Footing settlement sD()
0
1
2
3
4
5
I m p r o v e m e n t f a c t o r r a t i o I F g c s c I F s c
Clay+GC+SC(Ldsc= 1)
Clay+GC+SC(Ldsc= 3)
Clay+GC+SC(Ldsc= 5)
Clay+GC+SC(Ldsc= 7)
Fig 9 Improvement factor ratio-footing settlement responses contribution of geocell
mattress in composite foundation bed (hD frac14
053 S dsc frac14
25) e
Test series 5
0
1
2
3
4
5
6
7
L e n g t h o f s t o n e c o l u m n L d s c
0 5 10 15 20 25 30
Radial strain (rd-ro)ro ()
Ldsc= 1
Ldsc= 3
Ldsc= 5
Ldsc= 7
Fig 10 Post-test deformed pro1047297le of central stone column in composite foundation
beds (hD frac14 053 S dsc frac14 25) e Test series 5
0 4 8 12 16 20 24 28
Footing settlement sD()
0
1
2
3
4
5
A v e r a g e s u r f a c e d e f o r a m a t i o n
δ D ( )
Clay
Clay+GC
Clay+GC+SC(Ldsc= 1)
Clay+GC+SC(Ldsc= 3)
Clay+GC+SC(Ldsc= 5)
Clay+GC+SC(Ldsc= 7)
Fig 11 Surface deformation-footing settlement responses in1047298uence of length of stone
column in composite foundation bed ( xDfrac14
1 hD frac14
053 S dsc frac14
25) e
Test series 5
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optimum (ie 5dsc) though enhances the skin resistance but it
mostly remains unutilised due to excessive bulging at the top As a
result the responses of the stone columns and that of the geocell
mattress beyond Ldsc ratio of 5 have almost been stabilised Thepresent 1047297ndings are in agreement with the observations of Hughes
and Withers (1974) McKelvey et al (2004) that increasing the
length of stone columns beyond a certain point adds little to the
increase in bearing capacity however can help reducing the set-
tlement in the foundation bed
32 In 1047298uence of spacing of stone columns
Effect of column spacing (S) in the composite foundation beds
was studied under Test series 9e12 Typical responses are shown in
Fig12 Itcan beseen thatat low settlements (sD lt 5) the slope of
the pressure-settlement plots with stone columns are much less
than the case without This indicates that when intact the stone
columns irrespective of their spacing have provided additionalstiffness to the foundation system This however was not the case
when the settlement was large primarily because the columns had
bulged
With relatively widely spaced stone columns (S frac14 35dsc) stiff-
ness of the composite foundation system is almost comparable to
that with geocell reinforcement alone (both the responses are
nearly parallel) It could be because at large spacing the group
action of the peripheral stone columns diminishes that they behave
as individual entities leading to reduced lateral resistance onto the
central con1047297ned region In the absence of adequate con1047297nement
from the surrounding the central stone column underneath the
footing bulged prematurely and therefore couldnrsquot enhance the
stiffness of the foundation system Indeed the post test observa-
tions have shown that with large spacing the central stone columnhad bulged more As the spacing reduces the group action in the
rings of stone columns builds up inducing con1047297nement in the
central region that provides increased support against column
bulging leading to enhanced performance improvement
In general the bearing capacity of the composite foundation bed
was more when the spacing of stone columns was less ( Fig 12)
However the improvement (IFgcsc) with the column spacing (S )
reducing from 35dsc to 25dsc was relatively more than that from
25dsc to 15dsc (Table 2 test series 9e12) Further analysis shows
that the contribution of stone columns in the composite system
IFgcscIFgc was the maximum when they were placed close
(S frac14 15dsc) and reduced as the spacing was increased (Fig 13) In
contrast when the stone columns underneath were placed wide
apart (S frac14 35dsc) the geocell mattress has carried maximum load
IFgcscIFsc which however reduced signi1047297cantly as the spacing of
columns was reduced to 25dsc (Fig 14) Under footing loading the
stone columns with wider spacing have deformed more As the
underlying support yields the geocell mattress through mobi-lisation of anchorage from the adjacent stable region bridges over
and thereby shares higher proportion of the surcharge pressure
The marginal difference in the load factor ratio IFgcscIFsc with the
spacing (S ) changing from 25dsc to 15dsc indicates that further
change in the contribution of geocell mattress is practically negli-
gible With reduced spacing increasedpercentage of weak clay gets
replaced by the stiffer stone columns This gives rise to more uni-
formity of stress in the foundation bed that it deforms less Indeed
reduced settlement and heave on the 1047297ll surface observed with
reduced spacing of stone columns testi1047297es that the deformations in
the foundation bed have reduced down As a result the strain in the
overlying geocell reinforcement reduces leading to reduced mobi-
lisation of its strength and stiffness In such case the geocell
mattress behaves more of like a pedestal a load transmittingmember Whereas with wider spacing (S frac14 35dsc) of stone col-
umns underneath it behaves as a load sustaining member like a
centrally loaded slab resting over columns It can therefore be said
that when the spacing reduces from 35dsc to 25dsc there is sig-
ni1047297cant change in the behaviourof stone columns that it shifts from
near isolated to an interacting response giving rise to large
improvement in the performance of the system Hence the opti-
mum spacing of stone columns in the composite foundation beds
can be taken as 25dsc
0
4
8
12
16
20
24
28
F o o t i n g s e t t l e m e
n t s D ( )
0 20 40 60 80 100 120 140 160
Bearing pressure (kPa)
Clay
Clay+GC
Clay+GC+SC(Sdsc= 15)
Clay+GC+SC(Sdsc= 25)
Clay+GC+SC(Sdsc= 35)
Fig12 Footing pressure-settlement responses in1047298uence of spacing of stone columns
in composite foundation bed (hD frac14 053 Ldsc frac14 5) e Test series 9
0 4 8 12 16 20 24 28
Footing settlement sD()
0
1
2
3
4
I m p r o v e m e n t
f a c t o r r a t i o I F g c s c I F g c
Clay+GC+SC(Sdsc= 15)
Clay+GC+SC(Sdsc= 25)
Clay+GC+SC(Sdsc= 35)
Fig 13 Improvement factor ratio-footing settlement responses contribution of stone
columns in composite foundation bed (hD frac14 09 Ldsc frac14 5) e Test series 10
0 4 8 12 16 20 24 28
Footing settlement sD()
0
1
2
3
4
I m p r o v e m e n t f a c t o r r a t i o I F g c s c I F s c
Clay+GC+SC(Sdsc= 15)
Clay+GC+SC(Sdsc= 25)
Clay+GC+SC(Sdsc= 35)
Fig 14 Improvement factor ratio-footing settlement responses contribution of geo-
cell mattress in composite foundation bed (hD frac14
09 Ldsc frac14
5) e
Test series 10
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33 In 1047298uence of height of geocell mattress
Typical responses of the composite system for three differentheights of geocell mattress (h frac14 053D 09D 16D) are shown in
Figs 8 9 13 14 15 and 16 respectively It could be observed that
when shallow in height (h frac14 053D) the geocell mattress has under
performed that the stone columns have shared nearly three times
more load (IFgcscIFgc frac14 3 Fig 8) However with increase in height
(h) the contribution of stone columns has reduced and that of
geocell mattress (IFgcscIFsc) has gone up When geocells are rela-
tively deep (h frac14 16D) the improvement factor ratio IFgcscIFgc is
just in the range of 1e12 (Fig 15) that the load carried by the stone
columns is at the most 20 that of the geocell mattress The data
presented in Fig 16 indeed shows such a response wherein the
value of improvement factor ratio IFgcscIFsc is as high as 65
indicating that most of the footing pressure has been sustained by
the geocell mattress A deep geocell mattress possesses highmoment of inertia that enables it to stand against the footing
penetration Besides with increase in height (h) the geocell area
deriving anchorage from the in1047297ll soil increases and so is the
anchorage resistance Therefore the geocell mattress takes a large
proportion of the footing loading on its own that the stone columns
underneath mostly remain dormant and thereby contribute less to
the performance improvement Visibly less bulging observed in the
post-test exhumed stone columns (Fig 17) establishes that they
indeed had under performed in sharing the surcharge loading
The improvement due to the geocell-stone column composite
reinforcements are summarised in Table 2 (Test series 5e12) It can
be seen that for height of geocell mattress h frac14 053D 09D 11D and
16D the maximum bearing capacity improvement IFgcsc frac14 569
747 942 and 102 respectively This highlights that the increase in
performance improvement with height of geocell mattress
increasing beyond 11D is relatively less A possible reason for this
could be the stress concentration induced local buckling and
yielding of geocells right under the footing that the global increasein strength and stiffness of the system due to increase in height of
the mattress remains immobilised Indeed the maximum differ-
ence in the values of the factor IFgcscIFsc (ie the load shared by the
geocell mattress) for the case with h frac14 16D and 11D was less than
5 Therefore height of geocell mattress equal to about the diam-
eter of the footing (h frac14 D) can be taken as the optimum one giving
maximum possible performance improvement in the composite
foundation beds However full-scale tests are required to verify this
observation
It is of interest to note that even geocell mattress of medium
height h frac14 09D when combined with stone columns can provide
bearing capacity improvement (IFgcsc frac14 746 Test series 6) as high
as that of a deep geocell mattress h frac14 16D (IFgcsc frac14 787 Test series
4) Due to practical constraints at times it might be dif 1047297cult to
accommodate a relatively large height geocell mattress In such
situation provision of stone columns in the underlying subgrade
would be a viable alternative to manage with geocell mattress of
relatively smaller height
4 Scale effect
Owing to reduced size model tests the results presented in this
paper are prone to scale effects Therefore further studies using
full-scale tests are required to verify these observations However
using a suitable scaling law the results from the present study can
be extrapolated to the prototype case (Fakher and Jones 1996)
The major physical parameters in1047298uencing the response of
geocell-stone column reinforced foundation systems can be
0 4 8 12 16 20 24 28
Footing settlement sD()
00
05
10
15
20
25
30
I m p r o v e m e n t
f c c t o r r a t i o I F g c s c I F g c
Clay+GC+SC(Ldsc= 1)
Clay+GC+SC(Ldsc= 3)
Clay+GC+SC(Ldsc= 5)
Clay+GC+SC(Ldsc= 7)
Fig 15 Improvement factor ratio-footing settlement responses contribution of stone
columns in composite foundation bed (hD frac14 16 S dsc frac14 25) e Test series 8
0 4 8 12 16 20 24 28
Footing settlement sD()
0
2
4
6
8
10
I m p r o v e m e n t f a c t o r r a t i o I F g c s c I F s c
Clay+GC+SC(Ldsc= 1)
Clay+GC+SC(Ldsc= 3)
Clay+GC+SC(Ldsc= 5)
Clay+GC+SC(Ldsc= 7)
Fig 16 Improvement factor ratio-footing settlement responses contribution of geo-
cell mattress in composite foundation bed (hD frac14
16 S dsc frac14
25) e
Test series 8
0
1
2
3
4
5
6
7
L e n g t h o f s t o n e c o l u m n L d s c
0 5 10 15 20 25 30
Radial strain (rd - ro)ro ()
Ldsc= 1
Ldsc= 3
Ldsc= 5
Ldsc= 7
Fig 17 Post-test deformed pro1047297le of central stone column in composite foundation
beds (hD frac14 16 S dsc frac14 25) e Test series 8
SK Dash MC Bora Geotextiles and Geomembranes 41 (2013) 26 e 35 33
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summarised as D h dgc K dsc L d50 S ɸ c u g G where ɸ is the
angle of internal friction of soil and stone aggregate K is the stiff-
ness of geocell reinforcement g is the unit weight of soil and
aggregate G is the shear modulus of the soil and aggregate Other
parameters have been de1047297ned previously The function ( f ) that
governs the composite foundation system can be written as
f
D h dgc K dsc L d50 S c ug Gf s qgcsc qu
frac14 0 (2)
It comprises of 15 parameters and has two fundamental di-
mensions (ie length and force) and therefore can be studied by 13
independent parameters (p1 p2 p3 p4p13 Buckingham
1914) Hence equation (2) can be written as
For a prototype footing ( p) with diameter n times that of the
model (m)
Dp
Dmfrac14 n (4)
For similarity to be maintained the p terms both for model and
prototype need to be equal and therefore considering the p9 term
Gp
Dpgpfrac14
Gm
Dmgm(5)
Assuming that the soils used in the model and prototype do
have same unit weight (Pinto and Cousens 1999) equation (5)
reduces to
Gp
Gmfrac14 Dp
Dmfrac14 n (6)
Considering similarity of the p10 terms
K pgp
G2p
frac14 K mgm
G2m
K p
G2p
frac14 K mG2
m
K pK m
frac14G2
p
G2m
frac14 n2 (7)
As can be seen the strength of prototype geocells should be of n2
times that of the model geocell where n is the scale factor The
geocells used in the present tests have tensile strength of 475 kN
m Therefore the results from the present study to be applicable in
practice the prototype geocells should have tensile strength of
475n2 kNm However the geometric parameters such as pocket
size and height of geocells length diameter and spacing of stone
columns etc have shown a linear variation with the footing size D
5 Conclusions
Review of literature shows that both geocell-sand mattress and
stone columns are effective means of reinforcing the weak soils
Their individual applications though have been intensely studied
by many researchers but combined application of both has
remained unexplored The experimental results obtained in the
present study con1047297rm that such composite reinforcement is an
added advantage over the conventional ones ie stone column or
geocell mattress With provision of stone columns the bearing
capacity of soft clay beds can be increased by 37 fold and with
geocell reinforcement it is of the order of 78 fold When coupled
together ie stone column-geocell mattress combined the bearing
capacity was increased by 102 fold Additionally visible reduction
in slope of pressure settlement responses indicates that the stone
column-geocell composite reinforcement can increase the stiffness
of the foundation bed signi1047297cantly leading to large scale reduction
in footing settlement
The load carrying capacity of the geocell-stone column rein-
forced foundation bed increases with increase in length of stone
columns until 5dsc beyond which further rate of improvement has
reduced down Similarly reducing the spacing of stone columns
below 25dsc does notattract much of additional performance in the
composite system Besides with height of geocells increasing
beyond 11D the performance improvement is found to have
reduced This is possibly due to the stress concentration induced
buckling and yielding of geocells right under the footing that the
increase in strength and stiffness of the system due to increase in
height of the mattress remains immobilised Hence it can be said
that the critical height of geocell mattress giving optimum per-
formance improvement in the composite foundation bed is equal
to about the diameter of the footing (D)
At times practical constraints may prevent in going for large
height geocell mattress or long stone columns severely compro-
mising the performance of the system In such situations the
geocell-stone column composite reinforcement provides an effec-
tive solution for adequate performance improvement and optimum
design of foundations on soft clay This is inferred from the present
study that a shallow height geocell mattress along with medium
length stone columns can provide comparable performance im-
provements as that with deep geocells or long stone columnsHowever full-scale testing and 1047297eld trails are required to verify
these observations
The 1047297ndings of the present study can be of use in the design and
construction of structures over soft clay deposits such as railways
highways foundations for liquid storage tanks large stabilised
areas for parking platforms for oil exploration etc The authors
have also conducted tests with basal geogrid underneath the geo-
cell mattress and the results shall be reported in a subsequent
paper
Acknowledgement
The authors are thankful to the anonymous reviewers for their
valuable comments and suggestions for improvements of the pre-
sentations in the paper
Notation
C c coef 1047297cient of curvature
C u coef 1047297cient of uniformity
D diameter of footing
dgc diameter of geocells
dsc diameter of stone column
emax maximum void ratio
emin minimum void ratio
h height of geocell mattress
g ethp1p2p3p4p13THORN frac14 g
s
D
h
D
dgc
D
h
dgc
dsc
D
L
D
S
D
d50
dsc
G
Dg
K g
G2
c uDg
qgcsc
qu
f
frac14 0 (3)
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IFsc bearing capacity improvement factor due to stone column
reinforcement
IFgc bearing capacity improvement factor due to geocell
reinforcement
IFgcsc bearing capacity improvement factor due to stone
column-geocell composite reinforcement
L length of stone column
qsc bearing pressure with stone column reinforcement
qgc bearing pressure with geocell reinforcement
qgcsc bearing pressure with stone column-geocell composite
reinforcement
r d deformed radius of stone column
r o original radius of stone column
S spacing of stone columns
s settlement of footing
u depth of placement of geocell mattress
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piled raft system leading to improved load capacity However geo-
cell is a superior form of reinforcement over the planar one ( Dash
et al 2004 Madhavi and Vidya 2007 Latha and Somwanshi
2009) This is mostly due to its three dimensional con1047297ning struc-
ture that prevents lateral spreading of soil Therefore the sand
cushion over the stone columns if reinforced with geocells is ex-
pected to produce enhanced performance improvement These as-
pects are studied herein through a series of laboratory-scale model
tests The results have been analysed in developing an under-
standing of the behaviour of clay foundations reinforced with the
stone column-geocell mattress composite system
2 Details of model tests
21 Planning of experiments
Schematic sketch of a typical test con1047297guration is shown in
Fig 1 The stone columns were left 1047298oating in the clay bed This was
to simulate the situation commonly encountered in coastal areas
wherein soft clay deposits extend over very large depths that the
stone columns are generally terminated in the clay itself The col-
umns were placed in triangular pattern at a regular spacing S
(Fig 2) In all the tests diameter of stone columns (dsc) was kept
constant as 100 mm
Geocells were formed in chevron pattern (Fig 3) as it gives
better performance improvement over the diamond pattern (Dash
et al 2001) Diameter of geocells (dgc) taken as equivalent diam-
eter of geocell pocket opening was kept constant as 08D
throughout (D diameter of footing) The geocell mattresses were
placed at a constant depth (u) of 01D from the base of the footing
which was found to be the optimum location giving maximum
performance improvement (Dash et al 2008)
In total twelve series of model load tests were conducted the
details of which are presented in Table 1 Within each series onlyone parameter was varied This was to understand the in1047298uence of
this particular parameter on the overall behaviour of the founda-
tion system while the others were kept constant Tests in series 1
were performed on unreinforced clay beds Series 2 and 3 consisted
of testing the stone column reinforced clay beds wherein the in-
1047298uence of length (L) and spacing (S ) of the columns werestudied In
all these tests there was no sand cushion over the clay beds The
effect of height of geocell-sand mattress (h) was studied under
series 4 Subsequently tests in series 5e12 were designed to
LStone
column
Clay
Sand h
u
Geocell layer
DT DTDT DTDT DTDTDT
Footing
D D D
(00) xDxD
Fig 1 Schematic diagram of test con1047297guration
S
dsc
S
Stone column
Fig 2 Layout of stone columns
transverse member diagonal member
bodkin joint
b
b
Fig 3 Geocell system in chevron pattern plan view
Table 1
Details of model tests
Testseries
Typeof reinforcement
Details of parameters investigated
1 Un rei nforc ed cl ay bed wi th c u of 5 kPa
2 SC Variable parameter Ldsc frac14 1 3 5 7
Constant parameter S dsc frac14 25
3 SC Variable parameter S dsc frac14 15 25 35
Constant parameter Ldsc frac14 5
4 GC Variable parameter hD frac14 053 09 11 16
Constant parameter dgcD frac14 08 bD frac14 6
5 GC thorn SC Variable parameter Ldsc frac14 1 3 5 7
Constant parameter
hD frac14 053 dgcD frac14 08 bD frac14 6 S dsc frac14 25
6 GC thorn SC Variable parameter Ldsc frac14 1 3 5 7
Constant parameter
hD frac14 09 dgcD frac14 08 bD frac14 6 S dsc frac14 25
7 GC thorn SC Variable parameter Ldsc frac14 1 3 5 7
Constant parameter
hD frac14 11 dgcD frac14 08 bD frac14 6 S dsc frac14 25
8 GC thorn SC Variable parameter Ldsc frac14 1 3 5 7
Constant parameter
hD frac14 16 dgcD frac14 08 bD frac14 6 S dsc frac14 25
9 GC thorn SC Variable parameter S dsc frac14 15 25 35
Constant parameter
hD frac14 053 dgcD frac14 08 bD frac14 6 Ldsc frac14 5
10 GC thorn SC Variable parameter S dsc frac14 15 25 35
Constant parameter
hD frac14 09 dgcD frac14 08 bD frac14 6 Ldsc frac14 5
11 GC thorn SC Variable parameter S dsc frac14 15 25 35
Constant parameter
hD frac14 11 dgcD frac14 08 bD frac14 6 Ldsc frac14 5
12 GC thorn SC Variable parameter S dsc frac14 15 25 35
Constant parameter
hD frac14 16 dgcD frac14 08 bD frac14 6 Ldsc frac14 5
Note SC Stone columns GC Geocell-sand mattress
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investigate the combined application of both the reinforcements
ie stone columns and geocell-sand mattress
22 Materials used
The model clay beds were prepared using a locally available soil
that had 70 fractions 1047297ner than 75 mm (Fig 4) Its liquid limit
plastic limit and plasticity index were found to be 40 21 and 19
respectively (ASTM D4318 2005) As per the Uni1047297ed Soil Classi1047297-
cation System (USCS ASTM D2487 2006) the soil can be classi1047297ed
as clay with low plasticity (CL)
The stone columns were formed out of a poorly graded crushed
granite aggregate with particle sizes in the range of 2e10 mm (d10
d30 d50 of 220 315 490 mm respectively Fig 4) and uniformity
coef 1047297cient of 232 Its direct shear friction angle (peak) at place-
ment density of 153 kNm3 was found to be 48 The diameter of
model stone columns (100 mm) and size of aggregates used
(d50 frac14 49 mm) were approximately in 17 scale representation of
prototype stone columns of 700 mm diameter with averageaggregate size of 35 mm
The geocell reinforcement used was fabricated from a biaxial
geogrid having aperture size of 36 mm 36 mm ultimate tensile
strength of 193 kNm and 5 strain secant modulus of 135 kNm
(ASTM D6637 2001) The joints of the geocells formed out of 6 mm
wide and 3 mm thick polypropylene strips had a tensile strength of
475 kNm (ASTM D4884 2009) Such low strength of joints was
adopted to scale down the overall strength of the geocells that it is
suitable for the model tests The geocells were 1047297lled with a poorly
graded sand that had average particle size of 044 mm and uni-
formity coef 1047297cient of 252 (Fig 4) In all the tests the sand was
placed at 80 of relative density Its peak friction angle obtained
through triaxial compression tests in the pressure range of 100e
200 kPa was 44
23 Test setup
The model tests were carried out in a laboratory scale test bed-
cum-loading frame assembly (Fig 5) The test beds were prepared
in a steel tank of 1000 mm long1000 mm wide and 1300 mm high
To avoid yielding during tests the four sides of the tank were
braced laterally on their outer surfaces with steel channels The
footing used was made of a rigid steel plate and measured 150 mm
in diameter (D) In order to create a rough base condition a thin
layer of sand was glued onto its bottom In all the tests the footing
was placed at the centre of the test tank Loading was applied
through an automated hydraulic jack system supported against a
reaction frame 1047297
xed onto the ground The load transmitted to the
footing was recorded through an electronic load cell of 20 kN ca-
pacity with an accuracy of 001 N The settlements of the footing
were measured by two linear variable differential transducers
(LVDTs) placed at diametrically opposite ends (DT1 DT2 Fig 1)Deformations (heavesettlement) on foundation bed too were
measured by LVDTs placed through small plastic strips on the soil
surface (DT3eDT8 Fig 1) The LVDTs were of 50 mm travel with an
accuracy of 3 microns The load cell and the LVDTs were connected
to a computer controlled data acquisition system
Selig and McKee (1961) and Chummer (1972) have observed
that the failure wedge in the foundation bed extends over a dis-
tance of about 2e25 times the footing width away from its centre
In the present tests the distance of tank walls from centre of footing
being more than 33D the slip planes are not likely to be interfered
with Besides the geocell mattress being 1047298exible deforms down-
ward under the footing loading and thereby gets pulled away from
the tank side walls reducing the boundary effect to a practically
negligible value Indeed Dash et al (2003a) through instrumentedmodel tests have observed that the footing loading in nearly
similar test conditions did not induce any additional pressure on
the tank walls
The stone columns used had a maximum length of 700 mm (ie
L frac14 7dsc) and the geocell mattresses used had maximum height of
255 mm (ie h frac14 17D) Therefore the minimum clear spacing be-
tween the stone column base and the tank bottom maintained in
the tests was 345 mm [ie 1300 (700 thorn 255)] This is about 345
times the diameter of the stone column (dsc) Mayerhof and Sastry
(1978) have observed that the failure zone below a rigid pile ex-
tends over a depth of about 2 times its diameter The stone columns
being 1047298exible this depth would further be less A stress analysis
considering the dispersion in geocell mattress (Dash et al 2007)
and group action of stone columns (similar to that of rigid piles in
clay Bowel1988) was carried out It shows that for height of geocell
mattress of 17D and stone column length of 7dsc the stressinduced
at the bottom of the test-tank was less than 2 of the applied
pressure In view of this it can be said that the test-tank used in the
present investigation was considerably large enough and not likely
to interfere with the failure zones and hence the experimental re-
sults Besides the con1047297nement due to the tank walls simulated the
actual 1047297eld conditions for the interior columns in a large group
(Ambily and Gandhi 2007)
24 Test bed preparation
The clay was pulverised mixed with predetermined amount of
water and for moisture equilibrium was kept in airtight containers
0001 0010 0100 1000 10000 100000
Particle size (mm)
0
20
40
60
80
100
P e r c e n
t f i n e r b y w e i g h t
stone aggregate
sand
clay
Fig 4 Particle size distribution of stone aggregate sand and clay soil
Fig 5 Test set-up
SK Dash MC Bora Geotextiles and Geomembranes 41 (2013) 26 e 3528
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for about a week The test bed was prepared in lifts of 005 mthickness For each lift the amount of soil required to produce the
desired bulk density was weighted out placed in the test-tank
levelled and compacted Compaction was done through a wooden
board and a drop hammer using depth marking on the sides of the
tank as guide The compaction energy applied was 299 kJm3 Un-
disturbed soil samples were collected from different locations and
their properties were evaluated Sampling was done through a thin
cylindrical sampler that was pressed into the clay bed andextracted
out with the soil withinApartfrom the in situ density and moisture
content the specimens were tested for the vane shear strength as
well The average moisture content degree of saturation bulk unit
weight and shear strength of the clay in the test beds were found
to be 36 100 1805 kNm3 and 5 kPa respectively Their coef 1047297-
cient of variability was in the range of 15The stone columns were constructed by a replacement method
An open-ended stainless steel pipe of 100 mm outer diameter and
15 mm of wall thickness smeared with petroleum jelly (to reduce
friction) was pushed into the clay bed until it reached the depth of
the column to be formed Subsequently the clay within the pipe
was scooped out through a helical auger of 90 mm diameter To
minimise suction effect a maximum of 100 mm was removed at a
time On completion the internal wall of the pipe was cleaned and
stone aggregates were charged in The stones were added in
batches of 06 kg and compacted to height of 50 mm through a
circular steel tamper of 09 kg with 30 blows of 200 mm drop
leading to a density of 153 kNm3 that corresponds to 65 of
relative density The pipe was then slowly raised ensuring a mini-
mum of 25 mm overlap within the aggregate that the clay outsidedoesnrsquot intrude in This procedure was continued until the stone
column was completely formed (ie till top of clay bed) The stone
column reinforced clay bed thus formed was loaded with a seating
pressure of 25 kNm2 over the entire area for 4 h This was to
achieve uniformity in the test bed (Malarvizhi and Ilamparuthi
2007)
The geocell reinforcement was prepared from geogrid strips
placed in transverse and diagonal directions and connected
together with bodkin joints (Bush et al 1990) The jointwasformed
by pulling the ribs of the diagonal geogrid up through the trans-
verse geogrid and slipping a dowel (plastic strip) through the loop
created (Carroll and Curtis 1990) Three-dimensional view of a
typical geocell structure placedover the clay bed is shown in Fig 6
The geocells were 1047297
lled with sand through raining Compared to
unreinforced case with geocell reinforcement the height of raining
required to achieve the target density was relatively more how-
ever the difference was not much This was because the geocells
being made of geogrids had more than 80 of open area thereforedid not affect much the free 1047298ow of sand during raining The dif-
ference in the placement densities of the sand at various locations
in the test bed measured through in situ sampling was found to be
less than 15
25 Test procedure
In all the tests loading was applied in strain controlled manner
at the rate of 2 mmmin This relatively faster rate of loading was
intended to produce undrained response in the saturated clay bed
It is one of the worst 1047297eld conditions expected as in this case the
angle of friction of the soil tends to be zero leading to large
reduction in the bearing capacity Such phenomenon is common
during rainy seasons particularly in case of railways and highwayswhere the loading is transient in nature In all the tests load was
applied until the footing settlement reached 40 mm Through a
computer controlled system the load-deformation data were
continuously recorded
On completion of tests the deformed shape of the stone columns
were mapped This was done through careful removal of stone
aggregates and 1047297lling the shaft with Plaster of Paris (CaSO4$05H2O)
paste After being hardened the Plaster of Paris column was taken
out and measured for its shape and size The stone column de-
formations thus obtained are presented in terms of radial strain
(r d r o)r o wherein r d is the deformed radius and r o is the original
radius
3 Results and discussion
Typical pressure-settlement responses of clay bed alone that
with geocell and geocell-stone column composite reinforcement
are presented in Fig 7 The settlement s reported is the average of
the readings taken at both ends of the footing (DT 1 and DT2 Fig 1)
It could be observed that in case of unreinforced clay the slope of
the pressure-settlement response continues to increase until set-
tlement (sD) of about 15 and thereafter tends to become nearly
vertical This means that the soil has undergone clear failure and
therefore couldnrsquot support additional pressure anymore However
with geocell reinforcement (Clay thorn GC) the bearing pressure con-
tinues to increase even at settlement (sD) as high as 25 although
the overall improvement is relatively less But with stone columns
in the clay subgrade underneath geocell mattress (Clay thorn GC thorn SC)
Fig 6 Typical geocell layer in the test bed
0
4
8
12
16
20
24
28
F o o t i n g s e t t l e m e n
t s D
( )
0 20 40 60 80 100 120 140 160
Bearing pressure (kPa)
Clay
Clay+GC
Clay+GC+SC (Ldsc= 1)
Clay+GC+SC (Ldsc= 3)
Clay+GC+SC (Ldsc= 5)
Clay+GC+SC (Ldsc= 7)
Fig 7 Footing pressure-settlement responses in1047298uence of length of stone columns in
composite foundation bed (hD frac14 053 S dsc frac14 25) e Test series 5
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the bearing capacity has improved signi1047297cantly much higher than
that with geocell mattress alone Besides the stiffness of the
foundation bed too has increased substantially indicated through
reduced slope of the pressure settlement response Similar
behaviour is noticed in all other cases as well This is attributed to
the increased resistance against deformation provided by the stone
columns through mobilisation of friction and stiffness of the stone
aggregates that provides additional support to the geocell mattress
As a result of which the geocell-sand mattress that behaves as a
subgrade supported beam (Dash et al 2007) stands effectively
against the footing loading leading to improved performance of thefoundation system
The increase in the bearing capacity due to stone columns
geocell mattress and stone column-geocell mattress composite
reinforcement is quanti1047297ed through nondimensional improvement
factors IFsc IFgc IFgcsc respectively de1047297ned as the ratio of bearing
pressure with reinforcement (qsc qgc qgcsc) to that of unreinforced
clay bed (qu) both taken at equal settlement of footing ( sD) The
values of these improvement factors for different test cases and
footing settlements are presented in Table 2 It is evident that with
stone columns the bearing capacity of soft clay can be enhanced by
37 fold (IFsc frac14 37 Test series 3) and with geocell reinforcement it
can be up to 787 fold (IFgc frac14 787 Test series 4) But if coupled
together (ie stone columns and geocell mattress) the composite
reinforcement can enhance the bearing capacity of the soft clay as
high as by 10 fold (IFgcsc frac14 102 Test series 12) Hence it can be said
that the stone column-geocell composite is a superior form of
reinforcement that can give better performance improvement over
the conventional ones ie stone column geocell mattress In1047298u-
ence of different parameters on the overall performance of such
composite foundation systems are discussed in the following
sections
31 In 1047298uence of length of stone columns
In1047298uence of length of stone columns (L) in the composite
foundation bed has been studied for four different heights of
geocells h frac14 053D 09D 11D and 16D (Test series 5e8) It is of
interest to note that increasing the column length from 3 to 5 dsc
leads to sharp increase in performance both in terms of increased
load carrying capacity (IFgcsc Table 2) and reduced settlement
(Fig 7) However with further increase in length (L) to 7dsc the
additional improvement is much less Hence it can be said that in
the composite foundation system the optimum length of stone
columns giving maximum performance improvement is about 5
times their diameter (ie 5dsc) This observation however is from
small-scale models and needs to be veri1047297ed through prototype
tests The results are further analysed in terms of the improvement
factor ratios (ie IFgcscIFgc and IFgcscIFsc) that brings out the
Table 2
Bearing capacity improvement factors (IF IFsc IFgc IF gcsc)
Test series Reinforcement Variable parameter Bearing capacity improvement factor (IF)
sD 1 sD 5 sD 9 sD 13 sD 17 sD 21 sD 25 sD 27
2 SC Ldsc frac14 1 169 125 114 119 123 126 128 130
SC Ldsc frac14 3 181 154 153 160 162 166 173 173
SC Ldsc frac14 5 359 287 293 312 319 331 340 344
SC Ldsc frac14 7 369 320 317 337 340 348 356 360
3 SC S dsc frac14 15 366 290 320 345 349 361 367 370
SC S dsc frac14 25 359 287 293 312 319 331 340 344
SC S dsc frac14 35 197 147 167 184 181 186 190 193
4 GC hD frac14 053 134 150 164 186 195 211 226 232
GC hD frac14 09 210 218 283 303 335 359 411 433
GC hD frac14 11 290 410 477 557 600 660 712 739
GC hD frac14 16 420 481 543 614 654 708 764 787
5 GC thorn SC Ldsc frac14 1 172 198 191 201 215 230 246 253
GC thorn SC Ldsc frac14 3 187 222 223 236 248 266 288 295
GC thorn SC Ldsc frac14 5 412 426 429 432 441 473 502 513
GC thorn SC Ldsc frac14 7 439 479 487 498 502 530 557 569
6 GC thorn SC Ldsc frac14 1 224 280 288 312 345 379 417 435
GC thorn SC Ldsc frac14 3 300 333 328 348 370 408 436 450
GC thorn SC Ldsc frac14 5 429 463 470 509 545 592 643 673
GC thorn SC Ldsc frac14 7 536 555 565 589 612 668 720 746
7 GC thorn SC Ldsc frac14 1 366 506 518 574 615 686 746 778
GC thorn SC Ldsc frac14 3 502 598 594 631 669 722 777 801
GC thorn SC Ldsc frac14 5 566 665 663 700 740 801 850 874GC thorn SC Ldsc frac14 7 700 738 724 755 798 850 913 942
8 GC thorn SC Ldsc frac14 1 341 480 555 640 720 801 872 901
GC thorn SC Ldsc frac14 3 388 529 600 700 755 825 902 924
GC thorn SC Ldsc frac14 5 548 646 684 741 796 859 928 959
GC thorn SC Ldsc frac14 7 610 700 738 782 832 893 956 988
9 GC thorn SC S dsc frac14 15 454 482 484 495 516 553 579 588
GC thorn SC S dsc frac14 25 412 426 429 432 441 473 502 513
GC thorn SC S dsc frac14 35 328 344 350 352 354 365 379 389
10 GC thorn SC S dsc frac14 15 501 546 557 585 624 675 724 747
GC thorn SC S dsc frac14 25 429 463 470 509 545 592 643 673
GC thorn SC S dsc frac14 35 340 354 364 408 416 455 490 506
11 GC thorn SC S dsc frac14 15 610 723 734 760 797 853 907 937
GC thorn SC S dsc frac14 25 566 665 663 700 740 801 850 874
GC thorn SC S dsc frac14 35 530 584 597 617 665 730 789 822
12 GC thorn SC S dsc frac14 15 635 713 729 781 840 920 987 102
GC thorn SC S dsc frac14 25 548 646 684 741 796 859 928 959
GC thorn SC S dsc frac14 35 510 629 643 700 750 819 882 906Note Boldface values indicate the maximum improvement factors for different reinforcements (ie SC GC and SC thorn GC)
SK Dash MC Bora Geotextiles and Geomembranes 41 (2013) 26 e 3530
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contribution of individual reinforcement components (ie geocell
sand mattress stone columns) as has been explained below
IFgcsc
IFscfrac14
qgcsc
qu
qsc
qu
frac14 qgcsc
qsc(1)
Thus the factor (IFgcscIFsc) can be taken as the contribution of
geocell mattress sharing the surcharge loading in the composite
foundation system Similarly the factor IFgcscIFgc (ie qgcscqgc)
represents the contribution of stone columns Typical improvement
factor ratios IFgcscIFgc and IFgcscIFsc for the case with hD frac14 053
are depicted in Fig 8 and Fig 9 respectively It is evident that the
contribution of stone columns IFgcscIFgc increases with increase in
length and the improvement is the maximum when Ldsc ratio
changes from 3 to 5 This may be because the shorter columns (L
dsc lt 3) due to inadequate skin resistance have suffered punching
failure and therefore didnrsquot contribute signi1047297cantly towards the
load carrying capacity of the system In contrast the long columns
(Ldsc 5) owing to large skin resistance mobilised through
increased surface area have effectively stood against the footing
loading giving rise to visible increase in performance improvement
Further con1047297rmation of the column failure modes was obtained
from the post test deformation pro1047297les a typical of which for the
central stone columns are shown in Fig 10 It could be observed
that when short in length (Ldsc frac14 1 and 3) the bulging in the stone
columns is marginal indicating that the column has mostly been
punched down But with increase in length (Ldsc 5) it has
effectively stood against the footing and therefore has bulged
signi1047297cantly Furthermore with column Ldsc ratio of 1 the
contribution of geocell mattress IFgcscIFsc was the maximum
which however reduced as the column length increased and
remained almost constant for Ldsc 5 (Fig 9) This can be analysed
through the responses of the 1047297ll surface depicted in Fig 11 The
surface deformations (d) reported herein are the average of the
readings taken at distance D on both sides of the footing (DT5 andDT6 Fig 1) It can be seen that with increase in the length of stone
columns settlement (thornd) on the 1047297ll surface has reduced Corre-
spondingly heave in the adjacent region ( x frac14 2D and 3D) too was
found to have reduced This is attributed to the increased resistance
of stone columns that inhibits settlement and thereby heave in the
foundation bed With reduced deformations in the soil around the
strength mobilised in the geocells reduces and so is its contribution
to the performance improvement Stone column length beyond the
0 4 8 12 16 20 24 28
Footing settlement sD()
0
1
2
3
4
5
I m p r o v e m e n t f a c t o r r a t i o I F g c s c I F g c
Clay+GC+SC(Ldsc= 1)
Clay+GC+SC(Ldsc= 3)
Clay+GC+SC(Ldsc= 5)
Clay+GC+SC(Ldsc= 7)
Fig 8 Improvement factor ratio-footing settlement responses contribution of stone
columns in composite foundation bed (hD frac14 053 S dsc frac14 25) e Test series 5
0 4 8 12 16 20 24 28
Footing settlement sD()
0
1
2
3
4
5
I m p r o v e m e n t f a c t o r r a t i o I F g c s c I F s c
Clay+GC+SC(Ldsc= 1)
Clay+GC+SC(Ldsc= 3)
Clay+GC+SC(Ldsc= 5)
Clay+GC+SC(Ldsc= 7)
Fig 9 Improvement factor ratio-footing settlement responses contribution of geocell
mattress in composite foundation bed (hD frac14
053 S dsc frac14
25) e
Test series 5
0
1
2
3
4
5
6
7
L e n g t h o f s t o n e c o l u m n L d s c
0 5 10 15 20 25 30
Radial strain (rd-ro)ro ()
Ldsc= 1
Ldsc= 3
Ldsc= 5
Ldsc= 7
Fig 10 Post-test deformed pro1047297le of central stone column in composite foundation
beds (hD frac14 053 S dsc frac14 25) e Test series 5
0 4 8 12 16 20 24 28
Footing settlement sD()
0
1
2
3
4
5
A v e r a g e s u r f a c e d e f o r a m a t i o n
δ D ( )
Clay
Clay+GC
Clay+GC+SC(Ldsc= 1)
Clay+GC+SC(Ldsc= 3)
Clay+GC+SC(Ldsc= 5)
Clay+GC+SC(Ldsc= 7)
Fig 11 Surface deformation-footing settlement responses in1047298uence of length of stone
column in composite foundation bed ( xDfrac14
1 hD frac14
053 S dsc frac14
25) e
Test series 5
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optimum (ie 5dsc) though enhances the skin resistance but it
mostly remains unutilised due to excessive bulging at the top As a
result the responses of the stone columns and that of the geocell
mattress beyond Ldsc ratio of 5 have almost been stabilised Thepresent 1047297ndings are in agreement with the observations of Hughes
and Withers (1974) McKelvey et al (2004) that increasing the
length of stone columns beyond a certain point adds little to the
increase in bearing capacity however can help reducing the set-
tlement in the foundation bed
32 In 1047298uence of spacing of stone columns
Effect of column spacing (S) in the composite foundation beds
was studied under Test series 9e12 Typical responses are shown in
Fig12 Itcan beseen thatat low settlements (sD lt 5) the slope of
the pressure-settlement plots with stone columns are much less
than the case without This indicates that when intact the stone
columns irrespective of their spacing have provided additionalstiffness to the foundation system This however was not the case
when the settlement was large primarily because the columns had
bulged
With relatively widely spaced stone columns (S frac14 35dsc) stiff-
ness of the composite foundation system is almost comparable to
that with geocell reinforcement alone (both the responses are
nearly parallel) It could be because at large spacing the group
action of the peripheral stone columns diminishes that they behave
as individual entities leading to reduced lateral resistance onto the
central con1047297ned region In the absence of adequate con1047297nement
from the surrounding the central stone column underneath the
footing bulged prematurely and therefore couldnrsquot enhance the
stiffness of the foundation system Indeed the post test observa-
tions have shown that with large spacing the central stone columnhad bulged more As the spacing reduces the group action in the
rings of stone columns builds up inducing con1047297nement in the
central region that provides increased support against column
bulging leading to enhanced performance improvement
In general the bearing capacity of the composite foundation bed
was more when the spacing of stone columns was less ( Fig 12)
However the improvement (IFgcsc) with the column spacing (S )
reducing from 35dsc to 25dsc was relatively more than that from
25dsc to 15dsc (Table 2 test series 9e12) Further analysis shows
that the contribution of stone columns in the composite system
IFgcscIFgc was the maximum when they were placed close
(S frac14 15dsc) and reduced as the spacing was increased (Fig 13) In
contrast when the stone columns underneath were placed wide
apart (S frac14 35dsc) the geocell mattress has carried maximum load
IFgcscIFsc which however reduced signi1047297cantly as the spacing of
columns was reduced to 25dsc (Fig 14) Under footing loading the
stone columns with wider spacing have deformed more As the
underlying support yields the geocell mattress through mobi-lisation of anchorage from the adjacent stable region bridges over
and thereby shares higher proportion of the surcharge pressure
The marginal difference in the load factor ratio IFgcscIFsc with the
spacing (S ) changing from 25dsc to 15dsc indicates that further
change in the contribution of geocell mattress is practically negli-
gible With reduced spacing increasedpercentage of weak clay gets
replaced by the stiffer stone columns This gives rise to more uni-
formity of stress in the foundation bed that it deforms less Indeed
reduced settlement and heave on the 1047297ll surface observed with
reduced spacing of stone columns testi1047297es that the deformations in
the foundation bed have reduced down As a result the strain in the
overlying geocell reinforcement reduces leading to reduced mobi-
lisation of its strength and stiffness In such case the geocell
mattress behaves more of like a pedestal a load transmittingmember Whereas with wider spacing (S frac14 35dsc) of stone col-
umns underneath it behaves as a load sustaining member like a
centrally loaded slab resting over columns It can therefore be said
that when the spacing reduces from 35dsc to 25dsc there is sig-
ni1047297cant change in the behaviourof stone columns that it shifts from
near isolated to an interacting response giving rise to large
improvement in the performance of the system Hence the opti-
mum spacing of stone columns in the composite foundation beds
can be taken as 25dsc
0
4
8
12
16
20
24
28
F o o t i n g s e t t l e m e
n t s D ( )
0 20 40 60 80 100 120 140 160
Bearing pressure (kPa)
Clay
Clay+GC
Clay+GC+SC(Sdsc= 15)
Clay+GC+SC(Sdsc= 25)
Clay+GC+SC(Sdsc= 35)
Fig12 Footing pressure-settlement responses in1047298uence of spacing of stone columns
in composite foundation bed (hD frac14 053 Ldsc frac14 5) e Test series 9
0 4 8 12 16 20 24 28
Footing settlement sD()
0
1
2
3
4
I m p r o v e m e n t
f a c t o r r a t i o I F g c s c I F g c
Clay+GC+SC(Sdsc= 15)
Clay+GC+SC(Sdsc= 25)
Clay+GC+SC(Sdsc= 35)
Fig 13 Improvement factor ratio-footing settlement responses contribution of stone
columns in composite foundation bed (hD frac14 09 Ldsc frac14 5) e Test series 10
0 4 8 12 16 20 24 28
Footing settlement sD()
0
1
2
3
4
I m p r o v e m e n t f a c t o r r a t i o I F g c s c I F s c
Clay+GC+SC(Sdsc= 15)
Clay+GC+SC(Sdsc= 25)
Clay+GC+SC(Sdsc= 35)
Fig 14 Improvement factor ratio-footing settlement responses contribution of geo-
cell mattress in composite foundation bed (hD frac14
09 Ldsc frac14
5) e
Test series 10
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33 In 1047298uence of height of geocell mattress
Typical responses of the composite system for three differentheights of geocell mattress (h frac14 053D 09D 16D) are shown in
Figs 8 9 13 14 15 and 16 respectively It could be observed that
when shallow in height (h frac14 053D) the geocell mattress has under
performed that the stone columns have shared nearly three times
more load (IFgcscIFgc frac14 3 Fig 8) However with increase in height
(h) the contribution of stone columns has reduced and that of
geocell mattress (IFgcscIFsc) has gone up When geocells are rela-
tively deep (h frac14 16D) the improvement factor ratio IFgcscIFgc is
just in the range of 1e12 (Fig 15) that the load carried by the stone
columns is at the most 20 that of the geocell mattress The data
presented in Fig 16 indeed shows such a response wherein the
value of improvement factor ratio IFgcscIFsc is as high as 65
indicating that most of the footing pressure has been sustained by
the geocell mattress A deep geocell mattress possesses highmoment of inertia that enables it to stand against the footing
penetration Besides with increase in height (h) the geocell area
deriving anchorage from the in1047297ll soil increases and so is the
anchorage resistance Therefore the geocell mattress takes a large
proportion of the footing loading on its own that the stone columns
underneath mostly remain dormant and thereby contribute less to
the performance improvement Visibly less bulging observed in the
post-test exhumed stone columns (Fig 17) establishes that they
indeed had under performed in sharing the surcharge loading
The improvement due to the geocell-stone column composite
reinforcements are summarised in Table 2 (Test series 5e12) It can
be seen that for height of geocell mattress h frac14 053D 09D 11D and
16D the maximum bearing capacity improvement IFgcsc frac14 569
747 942 and 102 respectively This highlights that the increase in
performance improvement with height of geocell mattress
increasing beyond 11D is relatively less A possible reason for this
could be the stress concentration induced local buckling and
yielding of geocells right under the footing that the global increasein strength and stiffness of the system due to increase in height of
the mattress remains immobilised Indeed the maximum differ-
ence in the values of the factor IFgcscIFsc (ie the load shared by the
geocell mattress) for the case with h frac14 16D and 11D was less than
5 Therefore height of geocell mattress equal to about the diam-
eter of the footing (h frac14 D) can be taken as the optimum one giving
maximum possible performance improvement in the composite
foundation beds However full-scale tests are required to verify this
observation
It is of interest to note that even geocell mattress of medium
height h frac14 09D when combined with stone columns can provide
bearing capacity improvement (IFgcsc frac14 746 Test series 6) as high
as that of a deep geocell mattress h frac14 16D (IFgcsc frac14 787 Test series
4) Due to practical constraints at times it might be dif 1047297cult to
accommodate a relatively large height geocell mattress In such
situation provision of stone columns in the underlying subgrade
would be a viable alternative to manage with geocell mattress of
relatively smaller height
4 Scale effect
Owing to reduced size model tests the results presented in this
paper are prone to scale effects Therefore further studies using
full-scale tests are required to verify these observations However
using a suitable scaling law the results from the present study can
be extrapolated to the prototype case (Fakher and Jones 1996)
The major physical parameters in1047298uencing the response of
geocell-stone column reinforced foundation systems can be
0 4 8 12 16 20 24 28
Footing settlement sD()
00
05
10
15
20
25
30
I m p r o v e m e n t
f c c t o r r a t i o I F g c s c I F g c
Clay+GC+SC(Ldsc= 1)
Clay+GC+SC(Ldsc= 3)
Clay+GC+SC(Ldsc= 5)
Clay+GC+SC(Ldsc= 7)
Fig 15 Improvement factor ratio-footing settlement responses contribution of stone
columns in composite foundation bed (hD frac14 16 S dsc frac14 25) e Test series 8
0 4 8 12 16 20 24 28
Footing settlement sD()
0
2
4
6
8
10
I m p r o v e m e n t f a c t o r r a t i o I F g c s c I F s c
Clay+GC+SC(Ldsc= 1)
Clay+GC+SC(Ldsc= 3)
Clay+GC+SC(Ldsc= 5)
Clay+GC+SC(Ldsc= 7)
Fig 16 Improvement factor ratio-footing settlement responses contribution of geo-
cell mattress in composite foundation bed (hD frac14
16 S dsc frac14
25) e
Test series 8
0
1
2
3
4
5
6
7
L e n g t h o f s t o n e c o l u m n L d s c
0 5 10 15 20 25 30
Radial strain (rd - ro)ro ()
Ldsc= 1
Ldsc= 3
Ldsc= 5
Ldsc= 7
Fig 17 Post-test deformed pro1047297le of central stone column in composite foundation
beds (hD frac14 16 S dsc frac14 25) e Test series 8
SK Dash MC Bora Geotextiles and Geomembranes 41 (2013) 26 e 35 33
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summarised as D h dgc K dsc L d50 S ɸ c u g G where ɸ is the
angle of internal friction of soil and stone aggregate K is the stiff-
ness of geocell reinforcement g is the unit weight of soil and
aggregate G is the shear modulus of the soil and aggregate Other
parameters have been de1047297ned previously The function ( f ) that
governs the composite foundation system can be written as
f
D h dgc K dsc L d50 S c ug Gf s qgcsc qu
frac14 0 (2)
It comprises of 15 parameters and has two fundamental di-
mensions (ie length and force) and therefore can be studied by 13
independent parameters (p1 p2 p3 p4p13 Buckingham
1914) Hence equation (2) can be written as
For a prototype footing ( p) with diameter n times that of the
model (m)
Dp
Dmfrac14 n (4)
For similarity to be maintained the p terms both for model and
prototype need to be equal and therefore considering the p9 term
Gp
Dpgpfrac14
Gm
Dmgm(5)
Assuming that the soils used in the model and prototype do
have same unit weight (Pinto and Cousens 1999) equation (5)
reduces to
Gp
Gmfrac14 Dp
Dmfrac14 n (6)
Considering similarity of the p10 terms
K pgp
G2p
frac14 K mgm
G2m
K p
G2p
frac14 K mG2
m
K pK m
frac14G2
p
G2m
frac14 n2 (7)
As can be seen the strength of prototype geocells should be of n2
times that of the model geocell where n is the scale factor The
geocells used in the present tests have tensile strength of 475 kN
m Therefore the results from the present study to be applicable in
practice the prototype geocells should have tensile strength of
475n2 kNm However the geometric parameters such as pocket
size and height of geocells length diameter and spacing of stone
columns etc have shown a linear variation with the footing size D
5 Conclusions
Review of literature shows that both geocell-sand mattress and
stone columns are effective means of reinforcing the weak soils
Their individual applications though have been intensely studied
by many researchers but combined application of both has
remained unexplored The experimental results obtained in the
present study con1047297rm that such composite reinforcement is an
added advantage over the conventional ones ie stone column or
geocell mattress With provision of stone columns the bearing
capacity of soft clay beds can be increased by 37 fold and with
geocell reinforcement it is of the order of 78 fold When coupled
together ie stone column-geocell mattress combined the bearing
capacity was increased by 102 fold Additionally visible reduction
in slope of pressure settlement responses indicates that the stone
column-geocell composite reinforcement can increase the stiffness
of the foundation bed signi1047297cantly leading to large scale reduction
in footing settlement
The load carrying capacity of the geocell-stone column rein-
forced foundation bed increases with increase in length of stone
columns until 5dsc beyond which further rate of improvement has
reduced down Similarly reducing the spacing of stone columns
below 25dsc does notattract much of additional performance in the
composite system Besides with height of geocells increasing
beyond 11D the performance improvement is found to have
reduced This is possibly due to the stress concentration induced
buckling and yielding of geocells right under the footing that the
increase in strength and stiffness of the system due to increase in
height of the mattress remains immobilised Hence it can be said
that the critical height of geocell mattress giving optimum per-
formance improvement in the composite foundation bed is equal
to about the diameter of the footing (D)
At times practical constraints may prevent in going for large
height geocell mattress or long stone columns severely compro-
mising the performance of the system In such situations the
geocell-stone column composite reinforcement provides an effec-
tive solution for adequate performance improvement and optimum
design of foundations on soft clay This is inferred from the present
study that a shallow height geocell mattress along with medium
length stone columns can provide comparable performance im-
provements as that with deep geocells or long stone columnsHowever full-scale testing and 1047297eld trails are required to verify
these observations
The 1047297ndings of the present study can be of use in the design and
construction of structures over soft clay deposits such as railways
highways foundations for liquid storage tanks large stabilised
areas for parking platforms for oil exploration etc The authors
have also conducted tests with basal geogrid underneath the geo-
cell mattress and the results shall be reported in a subsequent
paper
Acknowledgement
The authors are thankful to the anonymous reviewers for their
valuable comments and suggestions for improvements of the pre-
sentations in the paper
Notation
C c coef 1047297cient of curvature
C u coef 1047297cient of uniformity
D diameter of footing
dgc diameter of geocells
dsc diameter of stone column
emax maximum void ratio
emin minimum void ratio
h height of geocell mattress
g ethp1p2p3p4p13THORN frac14 g
s
D
h
D
dgc
D
h
dgc
dsc
D
L
D
S
D
d50
dsc
G
Dg
K g
G2
c uDg
qgcsc
qu
f
frac14 0 (3)
SK Dash MC Bora Geotextiles and Geomembranes 41 (2013) 26 e 3534
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IFsc bearing capacity improvement factor due to stone column
reinforcement
IFgc bearing capacity improvement factor due to geocell
reinforcement
IFgcsc bearing capacity improvement factor due to stone
column-geocell composite reinforcement
L length of stone column
qsc bearing pressure with stone column reinforcement
qgc bearing pressure with geocell reinforcement
qgcsc bearing pressure with stone column-geocell composite
reinforcement
r d deformed radius of stone column
r o original radius of stone column
S spacing of stone columns
s settlement of footing
u depth of placement of geocell mattress
References
Abdullah CH Edil TB 2007 Behaviour of geogrid-reinforced load transfer plat-forms for embankment on rammed aggregate piers Geosynth Int 14 (3) 141e153
Ambily AP Gandhi SR 2007 Behaviour of stone columns based on experimental
and FEM analysis J Geotech Geoenviron Eng ASCE 133 (4) 405e
515Arulrajah A Abdullah A Bouzza A 2009 Ground improvement technique for
railway embankments Ground Improv 162 (1) 3e14ASTM D2487 2006 Standard Practice for Classi1047297cation of Soils for Engineering
Purposes (Uni1047297ed Soil Classi1047297cation System) ASTM International West Con-shohocken PA httpdxdoiorg101520D2487-06E01
ASTM D4318 2005 Standard Test Methods for Liquid Limit Plastic Limit andPlasticity Index of Soils ASTM International West Conshohocken PA httpdxdoiorg101520D4318-05
ASTM D4884 2009 Standard Test Method for Strength of Strewn Thermally BondedSeams of Geotextiles ASTM International West Conshohocken PA httpdxdoiorg101520D4884e09
ASTM D6637 2001 Standard Test Methods for Determining Tensile Properties of Geogrid by Single or Multi-rib Tensile Methods ASTM International WestConshohocken PA httpdxdoiorg101520D6637e01R09
Black J Sivakumar V MeKinley JD 2007 Performance of clay samples reinforcedwith vertical granular columns Can Geotech J 44 (1) 89e95
Bowel JE 1988 Foundation Analysis and Design fourth ed McGraw-HillSingapore
Buckingham E 1914 On physically similar systems Phys Rev APS 4 345 e376Bush DI Jenner CG Bassett RH 1990 The design and construction of geocell
foundation mattresses supporting embankments over soft ground GeotextGeomembr 9 (1) 83e98
Carroll Jr RG Curtis VC 1990 Geogrid connections Geotext Geomembr 9 (4e6)515e530
Cimentada A Costa AD Izal JC Sagaseta C 2011 Laboratory study on radialconsolidation and deformation in clay reinforced with stone columns CanGeotech J 48 (1) 36e52
Christoulas S Bouckvalaas G Giannaros C 2000 An experimental study on themodel stone columns Soils Found 40 (6) 11e22
Chummer AV 1972 Bearing capacity theory from experimental results J SoilMech Found Div ASCE 98 (12) 1311e1324
Cowland JW Wong SCK 1993 Performance of a road embankment on soft claysupported on a geocell mattress foundation GeotextGeomembr12 (8) 687e705
Dash SK Bora MC 2013 2013 In1047298uence of geosynthetic encasement on theperformance of stone columns 1047298oating in soft clay Can Geotech J 50 (7) 754e
765Dash SK Krishnaswamy NR Rajagopal K 2001 Bearing capacity of strip footing
supported on geocell-reinforced sand Geotext Geomembr 19 (4) 235e256Dash SK Sireesh S Sitharam TG 2003a Model studies on circular footing
supported on geocell reinforced sand underlain by soft clay Geotext Geo-membr 21 (4) 197e219
Dash SK Sireesh S Sitharam TG 2003b Behaviour of geocell reinforced sandbeds under circular footing Ground Improv 7 (3) 111e115
Dash SK Rajagopal K Krishnaswamy NR 2004 Performance of different geo-synthetic reinforcement materials in sand foundations Geosynth Int 11 (1)35e42
Dash SK Rajagopal K Krishnaswamy NR 2007 Behaviour of geocell reinforcedsand beds under strip loading Can Geotech J 44 (7) 905e916
Dash SK Reddy PDT Raghukanth STG 2008 Subgrade modulus of geocell-reinforced sand foundations Ground Improv 12 (2) 79e87
Deb K Chandra S Basudhar PK 2007 Generalised model for geosynthetic-reinforced granular 1047297ll-soft soil with stone columns Int J Geomech ASCE 7(4) 266e276
Deb K Samadhiya NK Namdeo JB 2011 Laboratory model studies on unrein-forced and geogrid-reinforced sand bed over stone column-improved soft clayGeotext Geomembr 29 (2) 190e196
Fakher A Jones CJFP 1996 Discussion of bearing capacity of rectangular footingson geogrid reinforced sand by Yetimoglu T Wu JTH Saglamer AJ GeotechEng ASCE 122 (4) 326e327
Hughes JMO Withers NJ 1974 Reinforcing of soft cohesive soils with stonecolumns Ground Eng 7 (3) 42e49
Juran I Guermazi A 1988 Settlement response of soft soils reinforced by com-pacted sand columns J Geotech Eng ASCE 114 (8) 930
e942
Latha GM Somwanshi A 2009 Effect of reinforcement form on the bearing ca-pacity of square footings on sand Geotext Geomembr 27 (6) 409e422
Leshchinsky B Ling H 2013 Effects of geocell con1047297nement on strength anddeformation behaviour of gravel J Geotech Geoenviron Eng ASCE 139 (2)340e352
Madhavi GL Vidya SM 2007 Effects of reinforcement form on the behaviour of geosynthetic reinforced sand Geotext Geomembr 25 (1) 23e32
Malarvizhi Ilamparuthi 2007 Comparative study on the behaviour of encasedstone column and conventional stone column Soils Found 47 (5) 873e885
McKelvey D Sivakumar V Bell A Graham J 2004 Modeling vibrated Stonecolumns on soft clay Geotech Eng Proc Inst Civil Eng 157 (3) 137e149
Mayerhof GG Sastry VVRN 1978 Bearing capacity of piles in layered soils part2 Can Geotech J 15 (2) 183e189
Pinto MIM Cousens TW1999 Modelling a geotextile-reinforced brick-faced soilretaining wall Geosynth Int 6 (5) 417e447
Selig ET McKee KE 1961 Static and dynamic behaviour of small footings J Soil
Mech Found Div ASCE 87 (6) 29e
47Sireesh S Sitharam TG Dash SK 2009 Bearing capacity of circular footing ongeocell-sand mattress overlying clay bed with void Geotext Geomembr 27 (2)89e98
Tanyu BF Aydilek AH Lau AW Edil TB Benson CH 2013 Laboratory eval-uation of geocell-reinforced gravel subbase over poor subgrades Geosynth Int20 (2) 47e61
Webster SL Watkins JE 1977 Investigation of Construction Techniques forTactical Bridge Approach Roads across Soft Ground Technical Report S-77e1United States Army Corps of Engineers Waterways Experiment Station Mis-sissippi USA
Wood DM Hu W Nash DFT 2000 Group effects in stone column foundationsmodel tests Geotechnique 50 (6) 689e698
Zhou H Wen X 2008 Model studies on geogrid or geocell-reinforced sandcushion on soft soil Geotext Geomembr 26 (3) 231e238
SK Dash MC Bora Geotextiles and Geomembranes 41 (2013) 26 e 35 35
8212019 Articol d Citit
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investigate the combined application of both the reinforcements
ie stone columns and geocell-sand mattress
22 Materials used
The model clay beds were prepared using a locally available soil
that had 70 fractions 1047297ner than 75 mm (Fig 4) Its liquid limit
plastic limit and plasticity index were found to be 40 21 and 19
respectively (ASTM D4318 2005) As per the Uni1047297ed Soil Classi1047297-
cation System (USCS ASTM D2487 2006) the soil can be classi1047297ed
as clay with low plasticity (CL)
The stone columns were formed out of a poorly graded crushed
granite aggregate with particle sizes in the range of 2e10 mm (d10
d30 d50 of 220 315 490 mm respectively Fig 4) and uniformity
coef 1047297cient of 232 Its direct shear friction angle (peak) at place-
ment density of 153 kNm3 was found to be 48 The diameter of
model stone columns (100 mm) and size of aggregates used
(d50 frac14 49 mm) were approximately in 17 scale representation of
prototype stone columns of 700 mm diameter with averageaggregate size of 35 mm
The geocell reinforcement used was fabricated from a biaxial
geogrid having aperture size of 36 mm 36 mm ultimate tensile
strength of 193 kNm and 5 strain secant modulus of 135 kNm
(ASTM D6637 2001) The joints of the geocells formed out of 6 mm
wide and 3 mm thick polypropylene strips had a tensile strength of
475 kNm (ASTM D4884 2009) Such low strength of joints was
adopted to scale down the overall strength of the geocells that it is
suitable for the model tests The geocells were 1047297lled with a poorly
graded sand that had average particle size of 044 mm and uni-
formity coef 1047297cient of 252 (Fig 4) In all the tests the sand was
placed at 80 of relative density Its peak friction angle obtained
through triaxial compression tests in the pressure range of 100e
200 kPa was 44
23 Test setup
The model tests were carried out in a laboratory scale test bed-
cum-loading frame assembly (Fig 5) The test beds were prepared
in a steel tank of 1000 mm long1000 mm wide and 1300 mm high
To avoid yielding during tests the four sides of the tank were
braced laterally on their outer surfaces with steel channels The
footing used was made of a rigid steel plate and measured 150 mm
in diameter (D) In order to create a rough base condition a thin
layer of sand was glued onto its bottom In all the tests the footing
was placed at the centre of the test tank Loading was applied
through an automated hydraulic jack system supported against a
reaction frame 1047297
xed onto the ground The load transmitted to the
footing was recorded through an electronic load cell of 20 kN ca-
pacity with an accuracy of 001 N The settlements of the footing
were measured by two linear variable differential transducers
(LVDTs) placed at diametrically opposite ends (DT1 DT2 Fig 1)Deformations (heavesettlement) on foundation bed too were
measured by LVDTs placed through small plastic strips on the soil
surface (DT3eDT8 Fig 1) The LVDTs were of 50 mm travel with an
accuracy of 3 microns The load cell and the LVDTs were connected
to a computer controlled data acquisition system
Selig and McKee (1961) and Chummer (1972) have observed
that the failure wedge in the foundation bed extends over a dis-
tance of about 2e25 times the footing width away from its centre
In the present tests the distance of tank walls from centre of footing
being more than 33D the slip planes are not likely to be interfered
with Besides the geocell mattress being 1047298exible deforms down-
ward under the footing loading and thereby gets pulled away from
the tank side walls reducing the boundary effect to a practically
negligible value Indeed Dash et al (2003a) through instrumentedmodel tests have observed that the footing loading in nearly
similar test conditions did not induce any additional pressure on
the tank walls
The stone columns used had a maximum length of 700 mm (ie
L frac14 7dsc) and the geocell mattresses used had maximum height of
255 mm (ie h frac14 17D) Therefore the minimum clear spacing be-
tween the stone column base and the tank bottom maintained in
the tests was 345 mm [ie 1300 (700 thorn 255)] This is about 345
times the diameter of the stone column (dsc) Mayerhof and Sastry
(1978) have observed that the failure zone below a rigid pile ex-
tends over a depth of about 2 times its diameter The stone columns
being 1047298exible this depth would further be less A stress analysis
considering the dispersion in geocell mattress (Dash et al 2007)
and group action of stone columns (similar to that of rigid piles in
clay Bowel1988) was carried out It shows that for height of geocell
mattress of 17D and stone column length of 7dsc the stressinduced
at the bottom of the test-tank was less than 2 of the applied
pressure In view of this it can be said that the test-tank used in the
present investigation was considerably large enough and not likely
to interfere with the failure zones and hence the experimental re-
sults Besides the con1047297nement due to the tank walls simulated the
actual 1047297eld conditions for the interior columns in a large group
(Ambily and Gandhi 2007)
24 Test bed preparation
The clay was pulverised mixed with predetermined amount of
water and for moisture equilibrium was kept in airtight containers
0001 0010 0100 1000 10000 100000
Particle size (mm)
0
20
40
60
80
100
P e r c e n
t f i n e r b y w e i g h t
stone aggregate
sand
clay
Fig 4 Particle size distribution of stone aggregate sand and clay soil
Fig 5 Test set-up
SK Dash MC Bora Geotextiles and Geomembranes 41 (2013) 26 e 3528
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for about a week The test bed was prepared in lifts of 005 mthickness For each lift the amount of soil required to produce the
desired bulk density was weighted out placed in the test-tank
levelled and compacted Compaction was done through a wooden
board and a drop hammer using depth marking on the sides of the
tank as guide The compaction energy applied was 299 kJm3 Un-
disturbed soil samples were collected from different locations and
their properties were evaluated Sampling was done through a thin
cylindrical sampler that was pressed into the clay bed andextracted
out with the soil withinApartfrom the in situ density and moisture
content the specimens were tested for the vane shear strength as
well The average moisture content degree of saturation bulk unit
weight and shear strength of the clay in the test beds were found
to be 36 100 1805 kNm3 and 5 kPa respectively Their coef 1047297-
cient of variability was in the range of 15The stone columns were constructed by a replacement method
An open-ended stainless steel pipe of 100 mm outer diameter and
15 mm of wall thickness smeared with petroleum jelly (to reduce
friction) was pushed into the clay bed until it reached the depth of
the column to be formed Subsequently the clay within the pipe
was scooped out through a helical auger of 90 mm diameter To
minimise suction effect a maximum of 100 mm was removed at a
time On completion the internal wall of the pipe was cleaned and
stone aggregates were charged in The stones were added in
batches of 06 kg and compacted to height of 50 mm through a
circular steel tamper of 09 kg with 30 blows of 200 mm drop
leading to a density of 153 kNm3 that corresponds to 65 of
relative density The pipe was then slowly raised ensuring a mini-
mum of 25 mm overlap within the aggregate that the clay outsidedoesnrsquot intrude in This procedure was continued until the stone
column was completely formed (ie till top of clay bed) The stone
column reinforced clay bed thus formed was loaded with a seating
pressure of 25 kNm2 over the entire area for 4 h This was to
achieve uniformity in the test bed (Malarvizhi and Ilamparuthi
2007)
The geocell reinforcement was prepared from geogrid strips
placed in transverse and diagonal directions and connected
together with bodkin joints (Bush et al 1990) The jointwasformed
by pulling the ribs of the diagonal geogrid up through the trans-
verse geogrid and slipping a dowel (plastic strip) through the loop
created (Carroll and Curtis 1990) Three-dimensional view of a
typical geocell structure placedover the clay bed is shown in Fig 6
The geocells were 1047297
lled with sand through raining Compared to
unreinforced case with geocell reinforcement the height of raining
required to achieve the target density was relatively more how-
ever the difference was not much This was because the geocells
being made of geogrids had more than 80 of open area thereforedid not affect much the free 1047298ow of sand during raining The dif-
ference in the placement densities of the sand at various locations
in the test bed measured through in situ sampling was found to be
less than 15
25 Test procedure
In all the tests loading was applied in strain controlled manner
at the rate of 2 mmmin This relatively faster rate of loading was
intended to produce undrained response in the saturated clay bed
It is one of the worst 1047297eld conditions expected as in this case the
angle of friction of the soil tends to be zero leading to large
reduction in the bearing capacity Such phenomenon is common
during rainy seasons particularly in case of railways and highwayswhere the loading is transient in nature In all the tests load was
applied until the footing settlement reached 40 mm Through a
computer controlled system the load-deformation data were
continuously recorded
On completion of tests the deformed shape of the stone columns
were mapped This was done through careful removal of stone
aggregates and 1047297lling the shaft with Plaster of Paris (CaSO4$05H2O)
paste After being hardened the Plaster of Paris column was taken
out and measured for its shape and size The stone column de-
formations thus obtained are presented in terms of radial strain
(r d r o)r o wherein r d is the deformed radius and r o is the original
radius
3 Results and discussion
Typical pressure-settlement responses of clay bed alone that
with geocell and geocell-stone column composite reinforcement
are presented in Fig 7 The settlement s reported is the average of
the readings taken at both ends of the footing (DT 1 and DT2 Fig 1)
It could be observed that in case of unreinforced clay the slope of
the pressure-settlement response continues to increase until set-
tlement (sD) of about 15 and thereafter tends to become nearly
vertical This means that the soil has undergone clear failure and
therefore couldnrsquot support additional pressure anymore However
with geocell reinforcement (Clay thorn GC) the bearing pressure con-
tinues to increase even at settlement (sD) as high as 25 although
the overall improvement is relatively less But with stone columns
in the clay subgrade underneath geocell mattress (Clay thorn GC thorn SC)
Fig 6 Typical geocell layer in the test bed
0
4
8
12
16
20
24
28
F o o t i n g s e t t l e m e n
t s D
( )
0 20 40 60 80 100 120 140 160
Bearing pressure (kPa)
Clay
Clay+GC
Clay+GC+SC (Ldsc= 1)
Clay+GC+SC (Ldsc= 3)
Clay+GC+SC (Ldsc= 5)
Clay+GC+SC (Ldsc= 7)
Fig 7 Footing pressure-settlement responses in1047298uence of length of stone columns in
composite foundation bed (hD frac14 053 S dsc frac14 25) e Test series 5
SK Dash MC Bora Geotextiles and Geomembranes 41 (2013) 26 e 35 29
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the bearing capacity has improved signi1047297cantly much higher than
that with geocell mattress alone Besides the stiffness of the
foundation bed too has increased substantially indicated through
reduced slope of the pressure settlement response Similar
behaviour is noticed in all other cases as well This is attributed to
the increased resistance against deformation provided by the stone
columns through mobilisation of friction and stiffness of the stone
aggregates that provides additional support to the geocell mattress
As a result of which the geocell-sand mattress that behaves as a
subgrade supported beam (Dash et al 2007) stands effectively
against the footing loading leading to improved performance of thefoundation system
The increase in the bearing capacity due to stone columns
geocell mattress and stone column-geocell mattress composite
reinforcement is quanti1047297ed through nondimensional improvement
factors IFsc IFgc IFgcsc respectively de1047297ned as the ratio of bearing
pressure with reinforcement (qsc qgc qgcsc) to that of unreinforced
clay bed (qu) both taken at equal settlement of footing ( sD) The
values of these improvement factors for different test cases and
footing settlements are presented in Table 2 It is evident that with
stone columns the bearing capacity of soft clay can be enhanced by
37 fold (IFsc frac14 37 Test series 3) and with geocell reinforcement it
can be up to 787 fold (IFgc frac14 787 Test series 4) But if coupled
together (ie stone columns and geocell mattress) the composite
reinforcement can enhance the bearing capacity of the soft clay as
high as by 10 fold (IFgcsc frac14 102 Test series 12) Hence it can be said
that the stone column-geocell composite is a superior form of
reinforcement that can give better performance improvement over
the conventional ones ie stone column geocell mattress In1047298u-
ence of different parameters on the overall performance of such
composite foundation systems are discussed in the following
sections
31 In 1047298uence of length of stone columns
In1047298uence of length of stone columns (L) in the composite
foundation bed has been studied for four different heights of
geocells h frac14 053D 09D 11D and 16D (Test series 5e8) It is of
interest to note that increasing the column length from 3 to 5 dsc
leads to sharp increase in performance both in terms of increased
load carrying capacity (IFgcsc Table 2) and reduced settlement
(Fig 7) However with further increase in length (L) to 7dsc the
additional improvement is much less Hence it can be said that in
the composite foundation system the optimum length of stone
columns giving maximum performance improvement is about 5
times their diameter (ie 5dsc) This observation however is from
small-scale models and needs to be veri1047297ed through prototype
tests The results are further analysed in terms of the improvement
factor ratios (ie IFgcscIFgc and IFgcscIFsc) that brings out the
Table 2
Bearing capacity improvement factors (IF IFsc IFgc IF gcsc)
Test series Reinforcement Variable parameter Bearing capacity improvement factor (IF)
sD 1 sD 5 sD 9 sD 13 sD 17 sD 21 sD 25 sD 27
2 SC Ldsc frac14 1 169 125 114 119 123 126 128 130
SC Ldsc frac14 3 181 154 153 160 162 166 173 173
SC Ldsc frac14 5 359 287 293 312 319 331 340 344
SC Ldsc frac14 7 369 320 317 337 340 348 356 360
3 SC S dsc frac14 15 366 290 320 345 349 361 367 370
SC S dsc frac14 25 359 287 293 312 319 331 340 344
SC S dsc frac14 35 197 147 167 184 181 186 190 193
4 GC hD frac14 053 134 150 164 186 195 211 226 232
GC hD frac14 09 210 218 283 303 335 359 411 433
GC hD frac14 11 290 410 477 557 600 660 712 739
GC hD frac14 16 420 481 543 614 654 708 764 787
5 GC thorn SC Ldsc frac14 1 172 198 191 201 215 230 246 253
GC thorn SC Ldsc frac14 3 187 222 223 236 248 266 288 295
GC thorn SC Ldsc frac14 5 412 426 429 432 441 473 502 513
GC thorn SC Ldsc frac14 7 439 479 487 498 502 530 557 569
6 GC thorn SC Ldsc frac14 1 224 280 288 312 345 379 417 435
GC thorn SC Ldsc frac14 3 300 333 328 348 370 408 436 450
GC thorn SC Ldsc frac14 5 429 463 470 509 545 592 643 673
GC thorn SC Ldsc frac14 7 536 555 565 589 612 668 720 746
7 GC thorn SC Ldsc frac14 1 366 506 518 574 615 686 746 778
GC thorn SC Ldsc frac14 3 502 598 594 631 669 722 777 801
GC thorn SC Ldsc frac14 5 566 665 663 700 740 801 850 874GC thorn SC Ldsc frac14 7 700 738 724 755 798 850 913 942
8 GC thorn SC Ldsc frac14 1 341 480 555 640 720 801 872 901
GC thorn SC Ldsc frac14 3 388 529 600 700 755 825 902 924
GC thorn SC Ldsc frac14 5 548 646 684 741 796 859 928 959
GC thorn SC Ldsc frac14 7 610 700 738 782 832 893 956 988
9 GC thorn SC S dsc frac14 15 454 482 484 495 516 553 579 588
GC thorn SC S dsc frac14 25 412 426 429 432 441 473 502 513
GC thorn SC S dsc frac14 35 328 344 350 352 354 365 379 389
10 GC thorn SC S dsc frac14 15 501 546 557 585 624 675 724 747
GC thorn SC S dsc frac14 25 429 463 470 509 545 592 643 673
GC thorn SC S dsc frac14 35 340 354 364 408 416 455 490 506
11 GC thorn SC S dsc frac14 15 610 723 734 760 797 853 907 937
GC thorn SC S dsc frac14 25 566 665 663 700 740 801 850 874
GC thorn SC S dsc frac14 35 530 584 597 617 665 730 789 822
12 GC thorn SC S dsc frac14 15 635 713 729 781 840 920 987 102
GC thorn SC S dsc frac14 25 548 646 684 741 796 859 928 959
GC thorn SC S dsc frac14 35 510 629 643 700 750 819 882 906Note Boldface values indicate the maximum improvement factors for different reinforcements (ie SC GC and SC thorn GC)
SK Dash MC Bora Geotextiles and Geomembranes 41 (2013) 26 e 3530
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contribution of individual reinforcement components (ie geocell
sand mattress stone columns) as has been explained below
IFgcsc
IFscfrac14
qgcsc
qu
qsc
qu
frac14 qgcsc
qsc(1)
Thus the factor (IFgcscIFsc) can be taken as the contribution of
geocell mattress sharing the surcharge loading in the composite
foundation system Similarly the factor IFgcscIFgc (ie qgcscqgc)
represents the contribution of stone columns Typical improvement
factor ratios IFgcscIFgc and IFgcscIFsc for the case with hD frac14 053
are depicted in Fig 8 and Fig 9 respectively It is evident that the
contribution of stone columns IFgcscIFgc increases with increase in
length and the improvement is the maximum when Ldsc ratio
changes from 3 to 5 This may be because the shorter columns (L
dsc lt 3) due to inadequate skin resistance have suffered punching
failure and therefore didnrsquot contribute signi1047297cantly towards the
load carrying capacity of the system In contrast the long columns
(Ldsc 5) owing to large skin resistance mobilised through
increased surface area have effectively stood against the footing
loading giving rise to visible increase in performance improvement
Further con1047297rmation of the column failure modes was obtained
from the post test deformation pro1047297les a typical of which for the
central stone columns are shown in Fig 10 It could be observed
that when short in length (Ldsc frac14 1 and 3) the bulging in the stone
columns is marginal indicating that the column has mostly been
punched down But with increase in length (Ldsc 5) it has
effectively stood against the footing and therefore has bulged
signi1047297cantly Furthermore with column Ldsc ratio of 1 the
contribution of geocell mattress IFgcscIFsc was the maximum
which however reduced as the column length increased and
remained almost constant for Ldsc 5 (Fig 9) This can be analysed
through the responses of the 1047297ll surface depicted in Fig 11 The
surface deformations (d) reported herein are the average of the
readings taken at distance D on both sides of the footing (DT5 andDT6 Fig 1) It can be seen that with increase in the length of stone
columns settlement (thornd) on the 1047297ll surface has reduced Corre-
spondingly heave in the adjacent region ( x frac14 2D and 3D) too was
found to have reduced This is attributed to the increased resistance
of stone columns that inhibits settlement and thereby heave in the
foundation bed With reduced deformations in the soil around the
strength mobilised in the geocells reduces and so is its contribution
to the performance improvement Stone column length beyond the
0 4 8 12 16 20 24 28
Footing settlement sD()
0
1
2
3
4
5
I m p r o v e m e n t f a c t o r r a t i o I F g c s c I F g c
Clay+GC+SC(Ldsc= 1)
Clay+GC+SC(Ldsc= 3)
Clay+GC+SC(Ldsc= 5)
Clay+GC+SC(Ldsc= 7)
Fig 8 Improvement factor ratio-footing settlement responses contribution of stone
columns in composite foundation bed (hD frac14 053 S dsc frac14 25) e Test series 5
0 4 8 12 16 20 24 28
Footing settlement sD()
0
1
2
3
4
5
I m p r o v e m e n t f a c t o r r a t i o I F g c s c I F s c
Clay+GC+SC(Ldsc= 1)
Clay+GC+SC(Ldsc= 3)
Clay+GC+SC(Ldsc= 5)
Clay+GC+SC(Ldsc= 7)
Fig 9 Improvement factor ratio-footing settlement responses contribution of geocell
mattress in composite foundation bed (hD frac14
053 S dsc frac14
25) e
Test series 5
0
1
2
3
4
5
6
7
L e n g t h o f s t o n e c o l u m n L d s c
0 5 10 15 20 25 30
Radial strain (rd-ro)ro ()
Ldsc= 1
Ldsc= 3
Ldsc= 5
Ldsc= 7
Fig 10 Post-test deformed pro1047297le of central stone column in composite foundation
beds (hD frac14 053 S dsc frac14 25) e Test series 5
0 4 8 12 16 20 24 28
Footing settlement sD()
0
1
2
3
4
5
A v e r a g e s u r f a c e d e f o r a m a t i o n
δ D ( )
Clay
Clay+GC
Clay+GC+SC(Ldsc= 1)
Clay+GC+SC(Ldsc= 3)
Clay+GC+SC(Ldsc= 5)
Clay+GC+SC(Ldsc= 7)
Fig 11 Surface deformation-footing settlement responses in1047298uence of length of stone
column in composite foundation bed ( xDfrac14
1 hD frac14
053 S dsc frac14
25) e
Test series 5
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optimum (ie 5dsc) though enhances the skin resistance but it
mostly remains unutilised due to excessive bulging at the top As a
result the responses of the stone columns and that of the geocell
mattress beyond Ldsc ratio of 5 have almost been stabilised Thepresent 1047297ndings are in agreement with the observations of Hughes
and Withers (1974) McKelvey et al (2004) that increasing the
length of stone columns beyond a certain point adds little to the
increase in bearing capacity however can help reducing the set-
tlement in the foundation bed
32 In 1047298uence of spacing of stone columns
Effect of column spacing (S) in the composite foundation beds
was studied under Test series 9e12 Typical responses are shown in
Fig12 Itcan beseen thatat low settlements (sD lt 5) the slope of
the pressure-settlement plots with stone columns are much less
than the case without This indicates that when intact the stone
columns irrespective of their spacing have provided additionalstiffness to the foundation system This however was not the case
when the settlement was large primarily because the columns had
bulged
With relatively widely spaced stone columns (S frac14 35dsc) stiff-
ness of the composite foundation system is almost comparable to
that with geocell reinforcement alone (both the responses are
nearly parallel) It could be because at large spacing the group
action of the peripheral stone columns diminishes that they behave
as individual entities leading to reduced lateral resistance onto the
central con1047297ned region In the absence of adequate con1047297nement
from the surrounding the central stone column underneath the
footing bulged prematurely and therefore couldnrsquot enhance the
stiffness of the foundation system Indeed the post test observa-
tions have shown that with large spacing the central stone columnhad bulged more As the spacing reduces the group action in the
rings of stone columns builds up inducing con1047297nement in the
central region that provides increased support against column
bulging leading to enhanced performance improvement
In general the bearing capacity of the composite foundation bed
was more when the spacing of stone columns was less ( Fig 12)
However the improvement (IFgcsc) with the column spacing (S )
reducing from 35dsc to 25dsc was relatively more than that from
25dsc to 15dsc (Table 2 test series 9e12) Further analysis shows
that the contribution of stone columns in the composite system
IFgcscIFgc was the maximum when they were placed close
(S frac14 15dsc) and reduced as the spacing was increased (Fig 13) In
contrast when the stone columns underneath were placed wide
apart (S frac14 35dsc) the geocell mattress has carried maximum load
IFgcscIFsc which however reduced signi1047297cantly as the spacing of
columns was reduced to 25dsc (Fig 14) Under footing loading the
stone columns with wider spacing have deformed more As the
underlying support yields the geocell mattress through mobi-lisation of anchorage from the adjacent stable region bridges over
and thereby shares higher proportion of the surcharge pressure
The marginal difference in the load factor ratio IFgcscIFsc with the
spacing (S ) changing from 25dsc to 15dsc indicates that further
change in the contribution of geocell mattress is practically negli-
gible With reduced spacing increasedpercentage of weak clay gets
replaced by the stiffer stone columns This gives rise to more uni-
formity of stress in the foundation bed that it deforms less Indeed
reduced settlement and heave on the 1047297ll surface observed with
reduced spacing of stone columns testi1047297es that the deformations in
the foundation bed have reduced down As a result the strain in the
overlying geocell reinforcement reduces leading to reduced mobi-
lisation of its strength and stiffness In such case the geocell
mattress behaves more of like a pedestal a load transmittingmember Whereas with wider spacing (S frac14 35dsc) of stone col-
umns underneath it behaves as a load sustaining member like a
centrally loaded slab resting over columns It can therefore be said
that when the spacing reduces from 35dsc to 25dsc there is sig-
ni1047297cant change in the behaviourof stone columns that it shifts from
near isolated to an interacting response giving rise to large
improvement in the performance of the system Hence the opti-
mum spacing of stone columns in the composite foundation beds
can be taken as 25dsc
0
4
8
12
16
20
24
28
F o o t i n g s e t t l e m e
n t s D ( )
0 20 40 60 80 100 120 140 160
Bearing pressure (kPa)
Clay
Clay+GC
Clay+GC+SC(Sdsc= 15)
Clay+GC+SC(Sdsc= 25)
Clay+GC+SC(Sdsc= 35)
Fig12 Footing pressure-settlement responses in1047298uence of spacing of stone columns
in composite foundation bed (hD frac14 053 Ldsc frac14 5) e Test series 9
0 4 8 12 16 20 24 28
Footing settlement sD()
0
1
2
3
4
I m p r o v e m e n t
f a c t o r r a t i o I F g c s c I F g c
Clay+GC+SC(Sdsc= 15)
Clay+GC+SC(Sdsc= 25)
Clay+GC+SC(Sdsc= 35)
Fig 13 Improvement factor ratio-footing settlement responses contribution of stone
columns in composite foundation bed (hD frac14 09 Ldsc frac14 5) e Test series 10
0 4 8 12 16 20 24 28
Footing settlement sD()
0
1
2
3
4
I m p r o v e m e n t f a c t o r r a t i o I F g c s c I F s c
Clay+GC+SC(Sdsc= 15)
Clay+GC+SC(Sdsc= 25)
Clay+GC+SC(Sdsc= 35)
Fig 14 Improvement factor ratio-footing settlement responses contribution of geo-
cell mattress in composite foundation bed (hD frac14
09 Ldsc frac14
5) e
Test series 10
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33 In 1047298uence of height of geocell mattress
Typical responses of the composite system for three differentheights of geocell mattress (h frac14 053D 09D 16D) are shown in
Figs 8 9 13 14 15 and 16 respectively It could be observed that
when shallow in height (h frac14 053D) the geocell mattress has under
performed that the stone columns have shared nearly three times
more load (IFgcscIFgc frac14 3 Fig 8) However with increase in height
(h) the contribution of stone columns has reduced and that of
geocell mattress (IFgcscIFsc) has gone up When geocells are rela-
tively deep (h frac14 16D) the improvement factor ratio IFgcscIFgc is
just in the range of 1e12 (Fig 15) that the load carried by the stone
columns is at the most 20 that of the geocell mattress The data
presented in Fig 16 indeed shows such a response wherein the
value of improvement factor ratio IFgcscIFsc is as high as 65
indicating that most of the footing pressure has been sustained by
the geocell mattress A deep geocell mattress possesses highmoment of inertia that enables it to stand against the footing
penetration Besides with increase in height (h) the geocell area
deriving anchorage from the in1047297ll soil increases and so is the
anchorage resistance Therefore the geocell mattress takes a large
proportion of the footing loading on its own that the stone columns
underneath mostly remain dormant and thereby contribute less to
the performance improvement Visibly less bulging observed in the
post-test exhumed stone columns (Fig 17) establishes that they
indeed had under performed in sharing the surcharge loading
The improvement due to the geocell-stone column composite
reinforcements are summarised in Table 2 (Test series 5e12) It can
be seen that for height of geocell mattress h frac14 053D 09D 11D and
16D the maximum bearing capacity improvement IFgcsc frac14 569
747 942 and 102 respectively This highlights that the increase in
performance improvement with height of geocell mattress
increasing beyond 11D is relatively less A possible reason for this
could be the stress concentration induced local buckling and
yielding of geocells right under the footing that the global increasein strength and stiffness of the system due to increase in height of
the mattress remains immobilised Indeed the maximum differ-
ence in the values of the factor IFgcscIFsc (ie the load shared by the
geocell mattress) for the case with h frac14 16D and 11D was less than
5 Therefore height of geocell mattress equal to about the diam-
eter of the footing (h frac14 D) can be taken as the optimum one giving
maximum possible performance improvement in the composite
foundation beds However full-scale tests are required to verify this
observation
It is of interest to note that even geocell mattress of medium
height h frac14 09D when combined with stone columns can provide
bearing capacity improvement (IFgcsc frac14 746 Test series 6) as high
as that of a deep geocell mattress h frac14 16D (IFgcsc frac14 787 Test series
4) Due to practical constraints at times it might be dif 1047297cult to
accommodate a relatively large height geocell mattress In such
situation provision of stone columns in the underlying subgrade
would be a viable alternative to manage with geocell mattress of
relatively smaller height
4 Scale effect
Owing to reduced size model tests the results presented in this
paper are prone to scale effects Therefore further studies using
full-scale tests are required to verify these observations However
using a suitable scaling law the results from the present study can
be extrapolated to the prototype case (Fakher and Jones 1996)
The major physical parameters in1047298uencing the response of
geocell-stone column reinforced foundation systems can be
0 4 8 12 16 20 24 28
Footing settlement sD()
00
05
10
15
20
25
30
I m p r o v e m e n t
f c c t o r r a t i o I F g c s c I F g c
Clay+GC+SC(Ldsc= 1)
Clay+GC+SC(Ldsc= 3)
Clay+GC+SC(Ldsc= 5)
Clay+GC+SC(Ldsc= 7)
Fig 15 Improvement factor ratio-footing settlement responses contribution of stone
columns in composite foundation bed (hD frac14 16 S dsc frac14 25) e Test series 8
0 4 8 12 16 20 24 28
Footing settlement sD()
0
2
4
6
8
10
I m p r o v e m e n t f a c t o r r a t i o I F g c s c I F s c
Clay+GC+SC(Ldsc= 1)
Clay+GC+SC(Ldsc= 3)
Clay+GC+SC(Ldsc= 5)
Clay+GC+SC(Ldsc= 7)
Fig 16 Improvement factor ratio-footing settlement responses contribution of geo-
cell mattress in composite foundation bed (hD frac14
16 S dsc frac14
25) e
Test series 8
0
1
2
3
4
5
6
7
L e n g t h o f s t o n e c o l u m n L d s c
0 5 10 15 20 25 30
Radial strain (rd - ro)ro ()
Ldsc= 1
Ldsc= 3
Ldsc= 5
Ldsc= 7
Fig 17 Post-test deformed pro1047297le of central stone column in composite foundation
beds (hD frac14 16 S dsc frac14 25) e Test series 8
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summarised as D h dgc K dsc L d50 S ɸ c u g G where ɸ is the
angle of internal friction of soil and stone aggregate K is the stiff-
ness of geocell reinforcement g is the unit weight of soil and
aggregate G is the shear modulus of the soil and aggregate Other
parameters have been de1047297ned previously The function ( f ) that
governs the composite foundation system can be written as
f
D h dgc K dsc L d50 S c ug Gf s qgcsc qu
frac14 0 (2)
It comprises of 15 parameters and has two fundamental di-
mensions (ie length and force) and therefore can be studied by 13
independent parameters (p1 p2 p3 p4p13 Buckingham
1914) Hence equation (2) can be written as
For a prototype footing ( p) with diameter n times that of the
model (m)
Dp
Dmfrac14 n (4)
For similarity to be maintained the p terms both for model and
prototype need to be equal and therefore considering the p9 term
Gp
Dpgpfrac14
Gm
Dmgm(5)
Assuming that the soils used in the model and prototype do
have same unit weight (Pinto and Cousens 1999) equation (5)
reduces to
Gp
Gmfrac14 Dp
Dmfrac14 n (6)
Considering similarity of the p10 terms
K pgp
G2p
frac14 K mgm
G2m
K p
G2p
frac14 K mG2
m
K pK m
frac14G2
p
G2m
frac14 n2 (7)
As can be seen the strength of prototype geocells should be of n2
times that of the model geocell where n is the scale factor The
geocells used in the present tests have tensile strength of 475 kN
m Therefore the results from the present study to be applicable in
practice the prototype geocells should have tensile strength of
475n2 kNm However the geometric parameters such as pocket
size and height of geocells length diameter and spacing of stone
columns etc have shown a linear variation with the footing size D
5 Conclusions
Review of literature shows that both geocell-sand mattress and
stone columns are effective means of reinforcing the weak soils
Their individual applications though have been intensely studied
by many researchers but combined application of both has
remained unexplored The experimental results obtained in the
present study con1047297rm that such composite reinforcement is an
added advantage over the conventional ones ie stone column or
geocell mattress With provision of stone columns the bearing
capacity of soft clay beds can be increased by 37 fold and with
geocell reinforcement it is of the order of 78 fold When coupled
together ie stone column-geocell mattress combined the bearing
capacity was increased by 102 fold Additionally visible reduction
in slope of pressure settlement responses indicates that the stone
column-geocell composite reinforcement can increase the stiffness
of the foundation bed signi1047297cantly leading to large scale reduction
in footing settlement
The load carrying capacity of the geocell-stone column rein-
forced foundation bed increases with increase in length of stone
columns until 5dsc beyond which further rate of improvement has
reduced down Similarly reducing the spacing of stone columns
below 25dsc does notattract much of additional performance in the
composite system Besides with height of geocells increasing
beyond 11D the performance improvement is found to have
reduced This is possibly due to the stress concentration induced
buckling and yielding of geocells right under the footing that the
increase in strength and stiffness of the system due to increase in
height of the mattress remains immobilised Hence it can be said
that the critical height of geocell mattress giving optimum per-
formance improvement in the composite foundation bed is equal
to about the diameter of the footing (D)
At times practical constraints may prevent in going for large
height geocell mattress or long stone columns severely compro-
mising the performance of the system In such situations the
geocell-stone column composite reinforcement provides an effec-
tive solution for adequate performance improvement and optimum
design of foundations on soft clay This is inferred from the present
study that a shallow height geocell mattress along with medium
length stone columns can provide comparable performance im-
provements as that with deep geocells or long stone columnsHowever full-scale testing and 1047297eld trails are required to verify
these observations
The 1047297ndings of the present study can be of use in the design and
construction of structures over soft clay deposits such as railways
highways foundations for liquid storage tanks large stabilised
areas for parking platforms for oil exploration etc The authors
have also conducted tests with basal geogrid underneath the geo-
cell mattress and the results shall be reported in a subsequent
paper
Acknowledgement
The authors are thankful to the anonymous reviewers for their
valuable comments and suggestions for improvements of the pre-
sentations in the paper
Notation
C c coef 1047297cient of curvature
C u coef 1047297cient of uniformity
D diameter of footing
dgc diameter of geocells
dsc diameter of stone column
emax maximum void ratio
emin minimum void ratio
h height of geocell mattress
g ethp1p2p3p4p13THORN frac14 g
s
D
h
D
dgc
D
h
dgc
dsc
D
L
D
S
D
d50
dsc
G
Dg
K g
G2
c uDg
qgcsc
qu
f
frac14 0 (3)
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IFsc bearing capacity improvement factor due to stone column
reinforcement
IFgc bearing capacity improvement factor due to geocell
reinforcement
IFgcsc bearing capacity improvement factor due to stone
column-geocell composite reinforcement
L length of stone column
qsc bearing pressure with stone column reinforcement
qgc bearing pressure with geocell reinforcement
qgcsc bearing pressure with stone column-geocell composite
reinforcement
r d deformed radius of stone column
r o original radius of stone column
S spacing of stone columns
s settlement of footing
u depth of placement of geocell mattress
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Deb K Samadhiya NK Namdeo JB 2011 Laboratory model studies on unrein-forced and geogrid-reinforced sand bed over stone column-improved soft clayGeotext Geomembr 29 (2) 190e196
Fakher A Jones CJFP 1996 Discussion of bearing capacity of rectangular footingson geogrid reinforced sand by Yetimoglu T Wu JTH Saglamer AJ GeotechEng ASCE 122 (4) 326e327
Hughes JMO Withers NJ 1974 Reinforcing of soft cohesive soils with stonecolumns Ground Eng 7 (3) 42e49
Juran I Guermazi A 1988 Settlement response of soft soils reinforced by com-pacted sand columns J Geotech Eng ASCE 114 (8) 930
e942
Latha GM Somwanshi A 2009 Effect of reinforcement form on the bearing ca-pacity of square footings on sand Geotext Geomembr 27 (6) 409e422
Leshchinsky B Ling H 2013 Effects of geocell con1047297nement on strength anddeformation behaviour of gravel J Geotech Geoenviron Eng ASCE 139 (2)340e352
Madhavi GL Vidya SM 2007 Effects of reinforcement form on the behaviour of geosynthetic reinforced sand Geotext Geomembr 25 (1) 23e32
Malarvizhi Ilamparuthi 2007 Comparative study on the behaviour of encasedstone column and conventional stone column Soils Found 47 (5) 873e885
McKelvey D Sivakumar V Bell A Graham J 2004 Modeling vibrated Stonecolumns on soft clay Geotech Eng Proc Inst Civil Eng 157 (3) 137e149
Mayerhof GG Sastry VVRN 1978 Bearing capacity of piles in layered soils part2 Can Geotech J 15 (2) 183e189
Pinto MIM Cousens TW1999 Modelling a geotextile-reinforced brick-faced soilretaining wall Geosynth Int 6 (5) 417e447
Selig ET McKee KE 1961 Static and dynamic behaviour of small footings J Soil
Mech Found Div ASCE 87 (6) 29e
47Sireesh S Sitharam TG Dash SK 2009 Bearing capacity of circular footing ongeocell-sand mattress overlying clay bed with void Geotext Geomembr 27 (2)89e98
Tanyu BF Aydilek AH Lau AW Edil TB Benson CH 2013 Laboratory eval-uation of geocell-reinforced gravel subbase over poor subgrades Geosynth Int20 (2) 47e61
Webster SL Watkins JE 1977 Investigation of Construction Techniques forTactical Bridge Approach Roads across Soft Ground Technical Report S-77e1United States Army Corps of Engineers Waterways Experiment Station Mis-sissippi USA
Wood DM Hu W Nash DFT 2000 Group effects in stone column foundationsmodel tests Geotechnique 50 (6) 689e698
Zhou H Wen X 2008 Model studies on geogrid or geocell-reinforced sandcushion on soft soil Geotext Geomembr 26 (3) 231e238
SK Dash MC Bora Geotextiles and Geomembranes 41 (2013) 26 e 35 35
8212019 Articol d Citit
httpslidepdfcomreaderfullarticol-d-citit 410
for about a week The test bed was prepared in lifts of 005 mthickness For each lift the amount of soil required to produce the
desired bulk density was weighted out placed in the test-tank
levelled and compacted Compaction was done through a wooden
board and a drop hammer using depth marking on the sides of the
tank as guide The compaction energy applied was 299 kJm3 Un-
disturbed soil samples were collected from different locations and
their properties were evaluated Sampling was done through a thin
cylindrical sampler that was pressed into the clay bed andextracted
out with the soil withinApartfrom the in situ density and moisture
content the specimens were tested for the vane shear strength as
well The average moisture content degree of saturation bulk unit
weight and shear strength of the clay in the test beds were found
to be 36 100 1805 kNm3 and 5 kPa respectively Their coef 1047297-
cient of variability was in the range of 15The stone columns were constructed by a replacement method
An open-ended stainless steel pipe of 100 mm outer diameter and
15 mm of wall thickness smeared with petroleum jelly (to reduce
friction) was pushed into the clay bed until it reached the depth of
the column to be formed Subsequently the clay within the pipe
was scooped out through a helical auger of 90 mm diameter To
minimise suction effect a maximum of 100 mm was removed at a
time On completion the internal wall of the pipe was cleaned and
stone aggregates were charged in The stones were added in
batches of 06 kg and compacted to height of 50 mm through a
circular steel tamper of 09 kg with 30 blows of 200 mm drop
leading to a density of 153 kNm3 that corresponds to 65 of
relative density The pipe was then slowly raised ensuring a mini-
mum of 25 mm overlap within the aggregate that the clay outsidedoesnrsquot intrude in This procedure was continued until the stone
column was completely formed (ie till top of clay bed) The stone
column reinforced clay bed thus formed was loaded with a seating
pressure of 25 kNm2 over the entire area for 4 h This was to
achieve uniformity in the test bed (Malarvizhi and Ilamparuthi
2007)
The geocell reinforcement was prepared from geogrid strips
placed in transverse and diagonal directions and connected
together with bodkin joints (Bush et al 1990) The jointwasformed
by pulling the ribs of the diagonal geogrid up through the trans-
verse geogrid and slipping a dowel (plastic strip) through the loop
created (Carroll and Curtis 1990) Three-dimensional view of a
typical geocell structure placedover the clay bed is shown in Fig 6
The geocells were 1047297
lled with sand through raining Compared to
unreinforced case with geocell reinforcement the height of raining
required to achieve the target density was relatively more how-
ever the difference was not much This was because the geocells
being made of geogrids had more than 80 of open area thereforedid not affect much the free 1047298ow of sand during raining The dif-
ference in the placement densities of the sand at various locations
in the test bed measured through in situ sampling was found to be
less than 15
25 Test procedure
In all the tests loading was applied in strain controlled manner
at the rate of 2 mmmin This relatively faster rate of loading was
intended to produce undrained response in the saturated clay bed
It is one of the worst 1047297eld conditions expected as in this case the
angle of friction of the soil tends to be zero leading to large
reduction in the bearing capacity Such phenomenon is common
during rainy seasons particularly in case of railways and highwayswhere the loading is transient in nature In all the tests load was
applied until the footing settlement reached 40 mm Through a
computer controlled system the load-deformation data were
continuously recorded
On completion of tests the deformed shape of the stone columns
were mapped This was done through careful removal of stone
aggregates and 1047297lling the shaft with Plaster of Paris (CaSO4$05H2O)
paste After being hardened the Plaster of Paris column was taken
out and measured for its shape and size The stone column de-
formations thus obtained are presented in terms of radial strain
(r d r o)r o wherein r d is the deformed radius and r o is the original
radius
3 Results and discussion
Typical pressure-settlement responses of clay bed alone that
with geocell and geocell-stone column composite reinforcement
are presented in Fig 7 The settlement s reported is the average of
the readings taken at both ends of the footing (DT 1 and DT2 Fig 1)
It could be observed that in case of unreinforced clay the slope of
the pressure-settlement response continues to increase until set-
tlement (sD) of about 15 and thereafter tends to become nearly
vertical This means that the soil has undergone clear failure and
therefore couldnrsquot support additional pressure anymore However
with geocell reinforcement (Clay thorn GC) the bearing pressure con-
tinues to increase even at settlement (sD) as high as 25 although
the overall improvement is relatively less But with stone columns
in the clay subgrade underneath geocell mattress (Clay thorn GC thorn SC)
Fig 6 Typical geocell layer in the test bed
0
4
8
12
16
20
24
28
F o o t i n g s e t t l e m e n
t s D
( )
0 20 40 60 80 100 120 140 160
Bearing pressure (kPa)
Clay
Clay+GC
Clay+GC+SC (Ldsc= 1)
Clay+GC+SC (Ldsc= 3)
Clay+GC+SC (Ldsc= 5)
Clay+GC+SC (Ldsc= 7)
Fig 7 Footing pressure-settlement responses in1047298uence of length of stone columns in
composite foundation bed (hD frac14 053 S dsc frac14 25) e Test series 5
SK Dash MC Bora Geotextiles and Geomembranes 41 (2013) 26 e 35 29
8212019 Articol d Citit
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the bearing capacity has improved signi1047297cantly much higher than
that with geocell mattress alone Besides the stiffness of the
foundation bed too has increased substantially indicated through
reduced slope of the pressure settlement response Similar
behaviour is noticed in all other cases as well This is attributed to
the increased resistance against deformation provided by the stone
columns through mobilisation of friction and stiffness of the stone
aggregates that provides additional support to the geocell mattress
As a result of which the geocell-sand mattress that behaves as a
subgrade supported beam (Dash et al 2007) stands effectively
against the footing loading leading to improved performance of thefoundation system
The increase in the bearing capacity due to stone columns
geocell mattress and stone column-geocell mattress composite
reinforcement is quanti1047297ed through nondimensional improvement
factors IFsc IFgc IFgcsc respectively de1047297ned as the ratio of bearing
pressure with reinforcement (qsc qgc qgcsc) to that of unreinforced
clay bed (qu) both taken at equal settlement of footing ( sD) The
values of these improvement factors for different test cases and
footing settlements are presented in Table 2 It is evident that with
stone columns the bearing capacity of soft clay can be enhanced by
37 fold (IFsc frac14 37 Test series 3) and with geocell reinforcement it
can be up to 787 fold (IFgc frac14 787 Test series 4) But if coupled
together (ie stone columns and geocell mattress) the composite
reinforcement can enhance the bearing capacity of the soft clay as
high as by 10 fold (IFgcsc frac14 102 Test series 12) Hence it can be said
that the stone column-geocell composite is a superior form of
reinforcement that can give better performance improvement over
the conventional ones ie stone column geocell mattress In1047298u-
ence of different parameters on the overall performance of such
composite foundation systems are discussed in the following
sections
31 In 1047298uence of length of stone columns
In1047298uence of length of stone columns (L) in the composite
foundation bed has been studied for four different heights of
geocells h frac14 053D 09D 11D and 16D (Test series 5e8) It is of
interest to note that increasing the column length from 3 to 5 dsc
leads to sharp increase in performance both in terms of increased
load carrying capacity (IFgcsc Table 2) and reduced settlement
(Fig 7) However with further increase in length (L) to 7dsc the
additional improvement is much less Hence it can be said that in
the composite foundation system the optimum length of stone
columns giving maximum performance improvement is about 5
times their diameter (ie 5dsc) This observation however is from
small-scale models and needs to be veri1047297ed through prototype
tests The results are further analysed in terms of the improvement
factor ratios (ie IFgcscIFgc and IFgcscIFsc) that brings out the
Table 2
Bearing capacity improvement factors (IF IFsc IFgc IF gcsc)
Test series Reinforcement Variable parameter Bearing capacity improvement factor (IF)
sD 1 sD 5 sD 9 sD 13 sD 17 sD 21 sD 25 sD 27
2 SC Ldsc frac14 1 169 125 114 119 123 126 128 130
SC Ldsc frac14 3 181 154 153 160 162 166 173 173
SC Ldsc frac14 5 359 287 293 312 319 331 340 344
SC Ldsc frac14 7 369 320 317 337 340 348 356 360
3 SC S dsc frac14 15 366 290 320 345 349 361 367 370
SC S dsc frac14 25 359 287 293 312 319 331 340 344
SC S dsc frac14 35 197 147 167 184 181 186 190 193
4 GC hD frac14 053 134 150 164 186 195 211 226 232
GC hD frac14 09 210 218 283 303 335 359 411 433
GC hD frac14 11 290 410 477 557 600 660 712 739
GC hD frac14 16 420 481 543 614 654 708 764 787
5 GC thorn SC Ldsc frac14 1 172 198 191 201 215 230 246 253
GC thorn SC Ldsc frac14 3 187 222 223 236 248 266 288 295
GC thorn SC Ldsc frac14 5 412 426 429 432 441 473 502 513
GC thorn SC Ldsc frac14 7 439 479 487 498 502 530 557 569
6 GC thorn SC Ldsc frac14 1 224 280 288 312 345 379 417 435
GC thorn SC Ldsc frac14 3 300 333 328 348 370 408 436 450
GC thorn SC Ldsc frac14 5 429 463 470 509 545 592 643 673
GC thorn SC Ldsc frac14 7 536 555 565 589 612 668 720 746
7 GC thorn SC Ldsc frac14 1 366 506 518 574 615 686 746 778
GC thorn SC Ldsc frac14 3 502 598 594 631 669 722 777 801
GC thorn SC Ldsc frac14 5 566 665 663 700 740 801 850 874GC thorn SC Ldsc frac14 7 700 738 724 755 798 850 913 942
8 GC thorn SC Ldsc frac14 1 341 480 555 640 720 801 872 901
GC thorn SC Ldsc frac14 3 388 529 600 700 755 825 902 924
GC thorn SC Ldsc frac14 5 548 646 684 741 796 859 928 959
GC thorn SC Ldsc frac14 7 610 700 738 782 832 893 956 988
9 GC thorn SC S dsc frac14 15 454 482 484 495 516 553 579 588
GC thorn SC S dsc frac14 25 412 426 429 432 441 473 502 513
GC thorn SC S dsc frac14 35 328 344 350 352 354 365 379 389
10 GC thorn SC S dsc frac14 15 501 546 557 585 624 675 724 747
GC thorn SC S dsc frac14 25 429 463 470 509 545 592 643 673
GC thorn SC S dsc frac14 35 340 354 364 408 416 455 490 506
11 GC thorn SC S dsc frac14 15 610 723 734 760 797 853 907 937
GC thorn SC S dsc frac14 25 566 665 663 700 740 801 850 874
GC thorn SC S dsc frac14 35 530 584 597 617 665 730 789 822
12 GC thorn SC S dsc frac14 15 635 713 729 781 840 920 987 102
GC thorn SC S dsc frac14 25 548 646 684 741 796 859 928 959
GC thorn SC S dsc frac14 35 510 629 643 700 750 819 882 906Note Boldface values indicate the maximum improvement factors for different reinforcements (ie SC GC and SC thorn GC)
SK Dash MC Bora Geotextiles and Geomembranes 41 (2013) 26 e 3530
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contribution of individual reinforcement components (ie geocell
sand mattress stone columns) as has been explained below
IFgcsc
IFscfrac14
qgcsc
qu
qsc
qu
frac14 qgcsc
qsc(1)
Thus the factor (IFgcscIFsc) can be taken as the contribution of
geocell mattress sharing the surcharge loading in the composite
foundation system Similarly the factor IFgcscIFgc (ie qgcscqgc)
represents the contribution of stone columns Typical improvement
factor ratios IFgcscIFgc and IFgcscIFsc for the case with hD frac14 053
are depicted in Fig 8 and Fig 9 respectively It is evident that the
contribution of stone columns IFgcscIFgc increases with increase in
length and the improvement is the maximum when Ldsc ratio
changes from 3 to 5 This may be because the shorter columns (L
dsc lt 3) due to inadequate skin resistance have suffered punching
failure and therefore didnrsquot contribute signi1047297cantly towards the
load carrying capacity of the system In contrast the long columns
(Ldsc 5) owing to large skin resistance mobilised through
increased surface area have effectively stood against the footing
loading giving rise to visible increase in performance improvement
Further con1047297rmation of the column failure modes was obtained
from the post test deformation pro1047297les a typical of which for the
central stone columns are shown in Fig 10 It could be observed
that when short in length (Ldsc frac14 1 and 3) the bulging in the stone
columns is marginal indicating that the column has mostly been
punched down But with increase in length (Ldsc 5) it has
effectively stood against the footing and therefore has bulged
signi1047297cantly Furthermore with column Ldsc ratio of 1 the
contribution of geocell mattress IFgcscIFsc was the maximum
which however reduced as the column length increased and
remained almost constant for Ldsc 5 (Fig 9) This can be analysed
through the responses of the 1047297ll surface depicted in Fig 11 The
surface deformations (d) reported herein are the average of the
readings taken at distance D on both sides of the footing (DT5 andDT6 Fig 1) It can be seen that with increase in the length of stone
columns settlement (thornd) on the 1047297ll surface has reduced Corre-
spondingly heave in the adjacent region ( x frac14 2D and 3D) too was
found to have reduced This is attributed to the increased resistance
of stone columns that inhibits settlement and thereby heave in the
foundation bed With reduced deformations in the soil around the
strength mobilised in the geocells reduces and so is its contribution
to the performance improvement Stone column length beyond the
0 4 8 12 16 20 24 28
Footing settlement sD()
0
1
2
3
4
5
I m p r o v e m e n t f a c t o r r a t i o I F g c s c I F g c
Clay+GC+SC(Ldsc= 1)
Clay+GC+SC(Ldsc= 3)
Clay+GC+SC(Ldsc= 5)
Clay+GC+SC(Ldsc= 7)
Fig 8 Improvement factor ratio-footing settlement responses contribution of stone
columns in composite foundation bed (hD frac14 053 S dsc frac14 25) e Test series 5
0 4 8 12 16 20 24 28
Footing settlement sD()
0
1
2
3
4
5
I m p r o v e m e n t f a c t o r r a t i o I F g c s c I F s c
Clay+GC+SC(Ldsc= 1)
Clay+GC+SC(Ldsc= 3)
Clay+GC+SC(Ldsc= 5)
Clay+GC+SC(Ldsc= 7)
Fig 9 Improvement factor ratio-footing settlement responses contribution of geocell
mattress in composite foundation bed (hD frac14
053 S dsc frac14
25) e
Test series 5
0
1
2
3
4
5
6
7
L e n g t h o f s t o n e c o l u m n L d s c
0 5 10 15 20 25 30
Radial strain (rd-ro)ro ()
Ldsc= 1
Ldsc= 3
Ldsc= 5
Ldsc= 7
Fig 10 Post-test deformed pro1047297le of central stone column in composite foundation
beds (hD frac14 053 S dsc frac14 25) e Test series 5
0 4 8 12 16 20 24 28
Footing settlement sD()
0
1
2
3
4
5
A v e r a g e s u r f a c e d e f o r a m a t i o n
δ D ( )
Clay
Clay+GC
Clay+GC+SC(Ldsc= 1)
Clay+GC+SC(Ldsc= 3)
Clay+GC+SC(Ldsc= 5)
Clay+GC+SC(Ldsc= 7)
Fig 11 Surface deformation-footing settlement responses in1047298uence of length of stone
column in composite foundation bed ( xDfrac14
1 hD frac14
053 S dsc frac14
25) e
Test series 5
SK Dash MC Bora Geotextiles and Geomembranes 41 (2013) 26 e 35 31
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optimum (ie 5dsc) though enhances the skin resistance but it
mostly remains unutilised due to excessive bulging at the top As a
result the responses of the stone columns and that of the geocell
mattress beyond Ldsc ratio of 5 have almost been stabilised Thepresent 1047297ndings are in agreement with the observations of Hughes
and Withers (1974) McKelvey et al (2004) that increasing the
length of stone columns beyond a certain point adds little to the
increase in bearing capacity however can help reducing the set-
tlement in the foundation bed
32 In 1047298uence of spacing of stone columns
Effect of column spacing (S) in the composite foundation beds
was studied under Test series 9e12 Typical responses are shown in
Fig12 Itcan beseen thatat low settlements (sD lt 5) the slope of
the pressure-settlement plots with stone columns are much less
than the case without This indicates that when intact the stone
columns irrespective of their spacing have provided additionalstiffness to the foundation system This however was not the case
when the settlement was large primarily because the columns had
bulged
With relatively widely spaced stone columns (S frac14 35dsc) stiff-
ness of the composite foundation system is almost comparable to
that with geocell reinforcement alone (both the responses are
nearly parallel) It could be because at large spacing the group
action of the peripheral stone columns diminishes that they behave
as individual entities leading to reduced lateral resistance onto the
central con1047297ned region In the absence of adequate con1047297nement
from the surrounding the central stone column underneath the
footing bulged prematurely and therefore couldnrsquot enhance the
stiffness of the foundation system Indeed the post test observa-
tions have shown that with large spacing the central stone columnhad bulged more As the spacing reduces the group action in the
rings of stone columns builds up inducing con1047297nement in the
central region that provides increased support against column
bulging leading to enhanced performance improvement
In general the bearing capacity of the composite foundation bed
was more when the spacing of stone columns was less ( Fig 12)
However the improvement (IFgcsc) with the column spacing (S )
reducing from 35dsc to 25dsc was relatively more than that from
25dsc to 15dsc (Table 2 test series 9e12) Further analysis shows
that the contribution of stone columns in the composite system
IFgcscIFgc was the maximum when they were placed close
(S frac14 15dsc) and reduced as the spacing was increased (Fig 13) In
contrast when the stone columns underneath were placed wide
apart (S frac14 35dsc) the geocell mattress has carried maximum load
IFgcscIFsc which however reduced signi1047297cantly as the spacing of
columns was reduced to 25dsc (Fig 14) Under footing loading the
stone columns with wider spacing have deformed more As the
underlying support yields the geocell mattress through mobi-lisation of anchorage from the adjacent stable region bridges over
and thereby shares higher proportion of the surcharge pressure
The marginal difference in the load factor ratio IFgcscIFsc with the
spacing (S ) changing from 25dsc to 15dsc indicates that further
change in the contribution of geocell mattress is practically negli-
gible With reduced spacing increasedpercentage of weak clay gets
replaced by the stiffer stone columns This gives rise to more uni-
formity of stress in the foundation bed that it deforms less Indeed
reduced settlement and heave on the 1047297ll surface observed with
reduced spacing of stone columns testi1047297es that the deformations in
the foundation bed have reduced down As a result the strain in the
overlying geocell reinforcement reduces leading to reduced mobi-
lisation of its strength and stiffness In such case the geocell
mattress behaves more of like a pedestal a load transmittingmember Whereas with wider spacing (S frac14 35dsc) of stone col-
umns underneath it behaves as a load sustaining member like a
centrally loaded slab resting over columns It can therefore be said
that when the spacing reduces from 35dsc to 25dsc there is sig-
ni1047297cant change in the behaviourof stone columns that it shifts from
near isolated to an interacting response giving rise to large
improvement in the performance of the system Hence the opti-
mum spacing of stone columns in the composite foundation beds
can be taken as 25dsc
0
4
8
12
16
20
24
28
F o o t i n g s e t t l e m e
n t s D ( )
0 20 40 60 80 100 120 140 160
Bearing pressure (kPa)
Clay
Clay+GC
Clay+GC+SC(Sdsc= 15)
Clay+GC+SC(Sdsc= 25)
Clay+GC+SC(Sdsc= 35)
Fig12 Footing pressure-settlement responses in1047298uence of spacing of stone columns
in composite foundation bed (hD frac14 053 Ldsc frac14 5) e Test series 9
0 4 8 12 16 20 24 28
Footing settlement sD()
0
1
2
3
4
I m p r o v e m e n t
f a c t o r r a t i o I F g c s c I F g c
Clay+GC+SC(Sdsc= 15)
Clay+GC+SC(Sdsc= 25)
Clay+GC+SC(Sdsc= 35)
Fig 13 Improvement factor ratio-footing settlement responses contribution of stone
columns in composite foundation bed (hD frac14 09 Ldsc frac14 5) e Test series 10
0 4 8 12 16 20 24 28
Footing settlement sD()
0
1
2
3
4
I m p r o v e m e n t f a c t o r r a t i o I F g c s c I F s c
Clay+GC+SC(Sdsc= 15)
Clay+GC+SC(Sdsc= 25)
Clay+GC+SC(Sdsc= 35)
Fig 14 Improvement factor ratio-footing settlement responses contribution of geo-
cell mattress in composite foundation bed (hD frac14
09 Ldsc frac14
5) e
Test series 10
SK Dash MC Bora Geotextiles and Geomembranes 41 (2013) 26 e 3532
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33 In 1047298uence of height of geocell mattress
Typical responses of the composite system for three differentheights of geocell mattress (h frac14 053D 09D 16D) are shown in
Figs 8 9 13 14 15 and 16 respectively It could be observed that
when shallow in height (h frac14 053D) the geocell mattress has under
performed that the stone columns have shared nearly three times
more load (IFgcscIFgc frac14 3 Fig 8) However with increase in height
(h) the contribution of stone columns has reduced and that of
geocell mattress (IFgcscIFsc) has gone up When geocells are rela-
tively deep (h frac14 16D) the improvement factor ratio IFgcscIFgc is
just in the range of 1e12 (Fig 15) that the load carried by the stone
columns is at the most 20 that of the geocell mattress The data
presented in Fig 16 indeed shows such a response wherein the
value of improvement factor ratio IFgcscIFsc is as high as 65
indicating that most of the footing pressure has been sustained by
the geocell mattress A deep geocell mattress possesses highmoment of inertia that enables it to stand against the footing
penetration Besides with increase in height (h) the geocell area
deriving anchorage from the in1047297ll soil increases and so is the
anchorage resistance Therefore the geocell mattress takes a large
proportion of the footing loading on its own that the stone columns
underneath mostly remain dormant and thereby contribute less to
the performance improvement Visibly less bulging observed in the
post-test exhumed stone columns (Fig 17) establishes that they
indeed had under performed in sharing the surcharge loading
The improvement due to the geocell-stone column composite
reinforcements are summarised in Table 2 (Test series 5e12) It can
be seen that for height of geocell mattress h frac14 053D 09D 11D and
16D the maximum bearing capacity improvement IFgcsc frac14 569
747 942 and 102 respectively This highlights that the increase in
performance improvement with height of geocell mattress
increasing beyond 11D is relatively less A possible reason for this
could be the stress concentration induced local buckling and
yielding of geocells right under the footing that the global increasein strength and stiffness of the system due to increase in height of
the mattress remains immobilised Indeed the maximum differ-
ence in the values of the factor IFgcscIFsc (ie the load shared by the
geocell mattress) for the case with h frac14 16D and 11D was less than
5 Therefore height of geocell mattress equal to about the diam-
eter of the footing (h frac14 D) can be taken as the optimum one giving
maximum possible performance improvement in the composite
foundation beds However full-scale tests are required to verify this
observation
It is of interest to note that even geocell mattress of medium
height h frac14 09D when combined with stone columns can provide
bearing capacity improvement (IFgcsc frac14 746 Test series 6) as high
as that of a deep geocell mattress h frac14 16D (IFgcsc frac14 787 Test series
4) Due to practical constraints at times it might be dif 1047297cult to
accommodate a relatively large height geocell mattress In such
situation provision of stone columns in the underlying subgrade
would be a viable alternative to manage with geocell mattress of
relatively smaller height
4 Scale effect
Owing to reduced size model tests the results presented in this
paper are prone to scale effects Therefore further studies using
full-scale tests are required to verify these observations However
using a suitable scaling law the results from the present study can
be extrapolated to the prototype case (Fakher and Jones 1996)
The major physical parameters in1047298uencing the response of
geocell-stone column reinforced foundation systems can be
0 4 8 12 16 20 24 28
Footing settlement sD()
00
05
10
15
20
25
30
I m p r o v e m e n t
f c c t o r r a t i o I F g c s c I F g c
Clay+GC+SC(Ldsc= 1)
Clay+GC+SC(Ldsc= 3)
Clay+GC+SC(Ldsc= 5)
Clay+GC+SC(Ldsc= 7)
Fig 15 Improvement factor ratio-footing settlement responses contribution of stone
columns in composite foundation bed (hD frac14 16 S dsc frac14 25) e Test series 8
0 4 8 12 16 20 24 28
Footing settlement sD()
0
2
4
6
8
10
I m p r o v e m e n t f a c t o r r a t i o I F g c s c I F s c
Clay+GC+SC(Ldsc= 1)
Clay+GC+SC(Ldsc= 3)
Clay+GC+SC(Ldsc= 5)
Clay+GC+SC(Ldsc= 7)
Fig 16 Improvement factor ratio-footing settlement responses contribution of geo-
cell mattress in composite foundation bed (hD frac14
16 S dsc frac14
25) e
Test series 8
0
1
2
3
4
5
6
7
L e n g t h o f s t o n e c o l u m n L d s c
0 5 10 15 20 25 30
Radial strain (rd - ro)ro ()
Ldsc= 1
Ldsc= 3
Ldsc= 5
Ldsc= 7
Fig 17 Post-test deformed pro1047297le of central stone column in composite foundation
beds (hD frac14 16 S dsc frac14 25) e Test series 8
SK Dash MC Bora Geotextiles and Geomembranes 41 (2013) 26 e 35 33
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summarised as D h dgc K dsc L d50 S ɸ c u g G where ɸ is the
angle of internal friction of soil and stone aggregate K is the stiff-
ness of geocell reinforcement g is the unit weight of soil and
aggregate G is the shear modulus of the soil and aggregate Other
parameters have been de1047297ned previously The function ( f ) that
governs the composite foundation system can be written as
f
D h dgc K dsc L d50 S c ug Gf s qgcsc qu
frac14 0 (2)
It comprises of 15 parameters and has two fundamental di-
mensions (ie length and force) and therefore can be studied by 13
independent parameters (p1 p2 p3 p4p13 Buckingham
1914) Hence equation (2) can be written as
For a prototype footing ( p) with diameter n times that of the
model (m)
Dp
Dmfrac14 n (4)
For similarity to be maintained the p terms both for model and
prototype need to be equal and therefore considering the p9 term
Gp
Dpgpfrac14
Gm
Dmgm(5)
Assuming that the soils used in the model and prototype do
have same unit weight (Pinto and Cousens 1999) equation (5)
reduces to
Gp
Gmfrac14 Dp
Dmfrac14 n (6)
Considering similarity of the p10 terms
K pgp
G2p
frac14 K mgm
G2m
K p
G2p
frac14 K mG2
m
K pK m
frac14G2
p
G2m
frac14 n2 (7)
As can be seen the strength of prototype geocells should be of n2
times that of the model geocell where n is the scale factor The
geocells used in the present tests have tensile strength of 475 kN
m Therefore the results from the present study to be applicable in
practice the prototype geocells should have tensile strength of
475n2 kNm However the geometric parameters such as pocket
size and height of geocells length diameter and spacing of stone
columns etc have shown a linear variation with the footing size D
5 Conclusions
Review of literature shows that both geocell-sand mattress and
stone columns are effective means of reinforcing the weak soils
Their individual applications though have been intensely studied
by many researchers but combined application of both has
remained unexplored The experimental results obtained in the
present study con1047297rm that such composite reinforcement is an
added advantage over the conventional ones ie stone column or
geocell mattress With provision of stone columns the bearing
capacity of soft clay beds can be increased by 37 fold and with
geocell reinforcement it is of the order of 78 fold When coupled
together ie stone column-geocell mattress combined the bearing
capacity was increased by 102 fold Additionally visible reduction
in slope of pressure settlement responses indicates that the stone
column-geocell composite reinforcement can increase the stiffness
of the foundation bed signi1047297cantly leading to large scale reduction
in footing settlement
The load carrying capacity of the geocell-stone column rein-
forced foundation bed increases with increase in length of stone
columns until 5dsc beyond which further rate of improvement has
reduced down Similarly reducing the spacing of stone columns
below 25dsc does notattract much of additional performance in the
composite system Besides with height of geocells increasing
beyond 11D the performance improvement is found to have
reduced This is possibly due to the stress concentration induced
buckling and yielding of geocells right under the footing that the
increase in strength and stiffness of the system due to increase in
height of the mattress remains immobilised Hence it can be said
that the critical height of geocell mattress giving optimum per-
formance improvement in the composite foundation bed is equal
to about the diameter of the footing (D)
At times practical constraints may prevent in going for large
height geocell mattress or long stone columns severely compro-
mising the performance of the system In such situations the
geocell-stone column composite reinforcement provides an effec-
tive solution for adequate performance improvement and optimum
design of foundations on soft clay This is inferred from the present
study that a shallow height geocell mattress along with medium
length stone columns can provide comparable performance im-
provements as that with deep geocells or long stone columnsHowever full-scale testing and 1047297eld trails are required to verify
these observations
The 1047297ndings of the present study can be of use in the design and
construction of structures over soft clay deposits such as railways
highways foundations for liquid storage tanks large stabilised
areas for parking platforms for oil exploration etc The authors
have also conducted tests with basal geogrid underneath the geo-
cell mattress and the results shall be reported in a subsequent
paper
Acknowledgement
The authors are thankful to the anonymous reviewers for their
valuable comments and suggestions for improvements of the pre-
sentations in the paper
Notation
C c coef 1047297cient of curvature
C u coef 1047297cient of uniformity
D diameter of footing
dgc diameter of geocells
dsc diameter of stone column
emax maximum void ratio
emin minimum void ratio
h height of geocell mattress
g ethp1p2p3p4p13THORN frac14 g
s
D
h
D
dgc
D
h
dgc
dsc
D
L
D
S
D
d50
dsc
G
Dg
K g
G2
c uDg
qgcsc
qu
f
frac14 0 (3)
SK Dash MC Bora Geotextiles and Geomembranes 41 (2013) 26 e 3534
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IFsc bearing capacity improvement factor due to stone column
reinforcement
IFgc bearing capacity improvement factor due to geocell
reinforcement
IFgcsc bearing capacity improvement factor due to stone
column-geocell composite reinforcement
L length of stone column
qsc bearing pressure with stone column reinforcement
qgc bearing pressure with geocell reinforcement
qgcsc bearing pressure with stone column-geocell composite
reinforcement
r d deformed radius of stone column
r o original radius of stone column
S spacing of stone columns
s settlement of footing
u depth of placement of geocell mattress
References
Abdullah CH Edil TB 2007 Behaviour of geogrid-reinforced load transfer plat-forms for embankment on rammed aggregate piers Geosynth Int 14 (3) 141e153
Ambily AP Gandhi SR 2007 Behaviour of stone columns based on experimental
and FEM analysis J Geotech Geoenviron Eng ASCE 133 (4) 405e
515Arulrajah A Abdullah A Bouzza A 2009 Ground improvement technique for
railway embankments Ground Improv 162 (1) 3e14ASTM D2487 2006 Standard Practice for Classi1047297cation of Soils for Engineering
Purposes (Uni1047297ed Soil Classi1047297cation System) ASTM International West Con-shohocken PA httpdxdoiorg101520D2487-06E01
ASTM D4318 2005 Standard Test Methods for Liquid Limit Plastic Limit andPlasticity Index of Soils ASTM International West Conshohocken PA httpdxdoiorg101520D4318-05
ASTM D4884 2009 Standard Test Method for Strength of Strewn Thermally BondedSeams of Geotextiles ASTM International West Conshohocken PA httpdxdoiorg101520D4884e09
ASTM D6637 2001 Standard Test Methods for Determining Tensile Properties of Geogrid by Single or Multi-rib Tensile Methods ASTM International WestConshohocken PA httpdxdoiorg101520D6637e01R09
Black J Sivakumar V MeKinley JD 2007 Performance of clay samples reinforcedwith vertical granular columns Can Geotech J 44 (1) 89e95
Bowel JE 1988 Foundation Analysis and Design fourth ed McGraw-HillSingapore
Buckingham E 1914 On physically similar systems Phys Rev APS 4 345 e376Bush DI Jenner CG Bassett RH 1990 The design and construction of geocell
foundation mattresses supporting embankments over soft ground GeotextGeomembr 9 (1) 83e98
Carroll Jr RG Curtis VC 1990 Geogrid connections Geotext Geomembr 9 (4e6)515e530
Cimentada A Costa AD Izal JC Sagaseta C 2011 Laboratory study on radialconsolidation and deformation in clay reinforced with stone columns CanGeotech J 48 (1) 36e52
Christoulas S Bouckvalaas G Giannaros C 2000 An experimental study on themodel stone columns Soils Found 40 (6) 11e22
Chummer AV 1972 Bearing capacity theory from experimental results J SoilMech Found Div ASCE 98 (12) 1311e1324
Cowland JW Wong SCK 1993 Performance of a road embankment on soft claysupported on a geocell mattress foundation GeotextGeomembr12 (8) 687e705
Dash SK Bora MC 2013 2013 In1047298uence of geosynthetic encasement on theperformance of stone columns 1047298oating in soft clay Can Geotech J 50 (7) 754e
765Dash SK Krishnaswamy NR Rajagopal K 2001 Bearing capacity of strip footing
supported on geocell-reinforced sand Geotext Geomembr 19 (4) 235e256Dash SK Sireesh S Sitharam TG 2003a Model studies on circular footing
supported on geocell reinforced sand underlain by soft clay Geotext Geo-membr 21 (4) 197e219
Dash SK Sireesh S Sitharam TG 2003b Behaviour of geocell reinforced sandbeds under circular footing Ground Improv 7 (3) 111e115
Dash SK Rajagopal K Krishnaswamy NR 2004 Performance of different geo-synthetic reinforcement materials in sand foundations Geosynth Int 11 (1)35e42
Dash SK Rajagopal K Krishnaswamy NR 2007 Behaviour of geocell reinforcedsand beds under strip loading Can Geotech J 44 (7) 905e916
Dash SK Reddy PDT Raghukanth STG 2008 Subgrade modulus of geocell-reinforced sand foundations Ground Improv 12 (2) 79e87
Deb K Chandra S Basudhar PK 2007 Generalised model for geosynthetic-reinforced granular 1047297ll-soft soil with stone columns Int J Geomech ASCE 7(4) 266e276
Deb K Samadhiya NK Namdeo JB 2011 Laboratory model studies on unrein-forced and geogrid-reinforced sand bed over stone column-improved soft clayGeotext Geomembr 29 (2) 190e196
Fakher A Jones CJFP 1996 Discussion of bearing capacity of rectangular footingson geogrid reinforced sand by Yetimoglu T Wu JTH Saglamer AJ GeotechEng ASCE 122 (4) 326e327
Hughes JMO Withers NJ 1974 Reinforcing of soft cohesive soils with stonecolumns Ground Eng 7 (3) 42e49
Juran I Guermazi A 1988 Settlement response of soft soils reinforced by com-pacted sand columns J Geotech Eng ASCE 114 (8) 930
e942
Latha GM Somwanshi A 2009 Effect of reinforcement form on the bearing ca-pacity of square footings on sand Geotext Geomembr 27 (6) 409e422
Leshchinsky B Ling H 2013 Effects of geocell con1047297nement on strength anddeformation behaviour of gravel J Geotech Geoenviron Eng ASCE 139 (2)340e352
Madhavi GL Vidya SM 2007 Effects of reinforcement form on the behaviour of geosynthetic reinforced sand Geotext Geomembr 25 (1) 23e32
Malarvizhi Ilamparuthi 2007 Comparative study on the behaviour of encasedstone column and conventional stone column Soils Found 47 (5) 873e885
McKelvey D Sivakumar V Bell A Graham J 2004 Modeling vibrated Stonecolumns on soft clay Geotech Eng Proc Inst Civil Eng 157 (3) 137e149
Mayerhof GG Sastry VVRN 1978 Bearing capacity of piles in layered soils part2 Can Geotech J 15 (2) 183e189
Pinto MIM Cousens TW1999 Modelling a geotextile-reinforced brick-faced soilretaining wall Geosynth Int 6 (5) 417e447
Selig ET McKee KE 1961 Static and dynamic behaviour of small footings J Soil
Mech Found Div ASCE 87 (6) 29e
47Sireesh S Sitharam TG Dash SK 2009 Bearing capacity of circular footing ongeocell-sand mattress overlying clay bed with void Geotext Geomembr 27 (2)89e98
Tanyu BF Aydilek AH Lau AW Edil TB Benson CH 2013 Laboratory eval-uation of geocell-reinforced gravel subbase over poor subgrades Geosynth Int20 (2) 47e61
Webster SL Watkins JE 1977 Investigation of Construction Techniques forTactical Bridge Approach Roads across Soft Ground Technical Report S-77e1United States Army Corps of Engineers Waterways Experiment Station Mis-sissippi USA
Wood DM Hu W Nash DFT 2000 Group effects in stone column foundationsmodel tests Geotechnique 50 (6) 689e698
Zhou H Wen X 2008 Model studies on geogrid or geocell-reinforced sandcushion on soft soil Geotext Geomembr 26 (3) 231e238
SK Dash MC Bora Geotextiles and Geomembranes 41 (2013) 26 e 35 35
8212019 Articol d Citit
httpslidepdfcomreaderfullarticol-d-citit 510
the bearing capacity has improved signi1047297cantly much higher than
that with geocell mattress alone Besides the stiffness of the
foundation bed too has increased substantially indicated through
reduced slope of the pressure settlement response Similar
behaviour is noticed in all other cases as well This is attributed to
the increased resistance against deformation provided by the stone
columns through mobilisation of friction and stiffness of the stone
aggregates that provides additional support to the geocell mattress
As a result of which the geocell-sand mattress that behaves as a
subgrade supported beam (Dash et al 2007) stands effectively
against the footing loading leading to improved performance of thefoundation system
The increase in the bearing capacity due to stone columns
geocell mattress and stone column-geocell mattress composite
reinforcement is quanti1047297ed through nondimensional improvement
factors IFsc IFgc IFgcsc respectively de1047297ned as the ratio of bearing
pressure with reinforcement (qsc qgc qgcsc) to that of unreinforced
clay bed (qu) both taken at equal settlement of footing ( sD) The
values of these improvement factors for different test cases and
footing settlements are presented in Table 2 It is evident that with
stone columns the bearing capacity of soft clay can be enhanced by
37 fold (IFsc frac14 37 Test series 3) and with geocell reinforcement it
can be up to 787 fold (IFgc frac14 787 Test series 4) But if coupled
together (ie stone columns and geocell mattress) the composite
reinforcement can enhance the bearing capacity of the soft clay as
high as by 10 fold (IFgcsc frac14 102 Test series 12) Hence it can be said
that the stone column-geocell composite is a superior form of
reinforcement that can give better performance improvement over
the conventional ones ie stone column geocell mattress In1047298u-
ence of different parameters on the overall performance of such
composite foundation systems are discussed in the following
sections
31 In 1047298uence of length of stone columns
In1047298uence of length of stone columns (L) in the composite
foundation bed has been studied for four different heights of
geocells h frac14 053D 09D 11D and 16D (Test series 5e8) It is of
interest to note that increasing the column length from 3 to 5 dsc
leads to sharp increase in performance both in terms of increased
load carrying capacity (IFgcsc Table 2) and reduced settlement
(Fig 7) However with further increase in length (L) to 7dsc the
additional improvement is much less Hence it can be said that in
the composite foundation system the optimum length of stone
columns giving maximum performance improvement is about 5
times their diameter (ie 5dsc) This observation however is from
small-scale models and needs to be veri1047297ed through prototype
tests The results are further analysed in terms of the improvement
factor ratios (ie IFgcscIFgc and IFgcscIFsc) that brings out the
Table 2
Bearing capacity improvement factors (IF IFsc IFgc IF gcsc)
Test series Reinforcement Variable parameter Bearing capacity improvement factor (IF)
sD 1 sD 5 sD 9 sD 13 sD 17 sD 21 sD 25 sD 27
2 SC Ldsc frac14 1 169 125 114 119 123 126 128 130
SC Ldsc frac14 3 181 154 153 160 162 166 173 173
SC Ldsc frac14 5 359 287 293 312 319 331 340 344
SC Ldsc frac14 7 369 320 317 337 340 348 356 360
3 SC S dsc frac14 15 366 290 320 345 349 361 367 370
SC S dsc frac14 25 359 287 293 312 319 331 340 344
SC S dsc frac14 35 197 147 167 184 181 186 190 193
4 GC hD frac14 053 134 150 164 186 195 211 226 232
GC hD frac14 09 210 218 283 303 335 359 411 433
GC hD frac14 11 290 410 477 557 600 660 712 739
GC hD frac14 16 420 481 543 614 654 708 764 787
5 GC thorn SC Ldsc frac14 1 172 198 191 201 215 230 246 253
GC thorn SC Ldsc frac14 3 187 222 223 236 248 266 288 295
GC thorn SC Ldsc frac14 5 412 426 429 432 441 473 502 513
GC thorn SC Ldsc frac14 7 439 479 487 498 502 530 557 569
6 GC thorn SC Ldsc frac14 1 224 280 288 312 345 379 417 435
GC thorn SC Ldsc frac14 3 300 333 328 348 370 408 436 450
GC thorn SC Ldsc frac14 5 429 463 470 509 545 592 643 673
GC thorn SC Ldsc frac14 7 536 555 565 589 612 668 720 746
7 GC thorn SC Ldsc frac14 1 366 506 518 574 615 686 746 778
GC thorn SC Ldsc frac14 3 502 598 594 631 669 722 777 801
GC thorn SC Ldsc frac14 5 566 665 663 700 740 801 850 874GC thorn SC Ldsc frac14 7 700 738 724 755 798 850 913 942
8 GC thorn SC Ldsc frac14 1 341 480 555 640 720 801 872 901
GC thorn SC Ldsc frac14 3 388 529 600 700 755 825 902 924
GC thorn SC Ldsc frac14 5 548 646 684 741 796 859 928 959
GC thorn SC Ldsc frac14 7 610 700 738 782 832 893 956 988
9 GC thorn SC S dsc frac14 15 454 482 484 495 516 553 579 588
GC thorn SC S dsc frac14 25 412 426 429 432 441 473 502 513
GC thorn SC S dsc frac14 35 328 344 350 352 354 365 379 389
10 GC thorn SC S dsc frac14 15 501 546 557 585 624 675 724 747
GC thorn SC S dsc frac14 25 429 463 470 509 545 592 643 673
GC thorn SC S dsc frac14 35 340 354 364 408 416 455 490 506
11 GC thorn SC S dsc frac14 15 610 723 734 760 797 853 907 937
GC thorn SC S dsc frac14 25 566 665 663 700 740 801 850 874
GC thorn SC S dsc frac14 35 530 584 597 617 665 730 789 822
12 GC thorn SC S dsc frac14 15 635 713 729 781 840 920 987 102
GC thorn SC S dsc frac14 25 548 646 684 741 796 859 928 959
GC thorn SC S dsc frac14 35 510 629 643 700 750 819 882 906Note Boldface values indicate the maximum improvement factors for different reinforcements (ie SC GC and SC thorn GC)
SK Dash MC Bora Geotextiles and Geomembranes 41 (2013) 26 e 3530
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contribution of individual reinforcement components (ie geocell
sand mattress stone columns) as has been explained below
IFgcsc
IFscfrac14
qgcsc
qu
qsc
qu
frac14 qgcsc
qsc(1)
Thus the factor (IFgcscIFsc) can be taken as the contribution of
geocell mattress sharing the surcharge loading in the composite
foundation system Similarly the factor IFgcscIFgc (ie qgcscqgc)
represents the contribution of stone columns Typical improvement
factor ratios IFgcscIFgc and IFgcscIFsc for the case with hD frac14 053
are depicted in Fig 8 and Fig 9 respectively It is evident that the
contribution of stone columns IFgcscIFgc increases with increase in
length and the improvement is the maximum when Ldsc ratio
changes from 3 to 5 This may be because the shorter columns (L
dsc lt 3) due to inadequate skin resistance have suffered punching
failure and therefore didnrsquot contribute signi1047297cantly towards the
load carrying capacity of the system In contrast the long columns
(Ldsc 5) owing to large skin resistance mobilised through
increased surface area have effectively stood against the footing
loading giving rise to visible increase in performance improvement
Further con1047297rmation of the column failure modes was obtained
from the post test deformation pro1047297les a typical of which for the
central stone columns are shown in Fig 10 It could be observed
that when short in length (Ldsc frac14 1 and 3) the bulging in the stone
columns is marginal indicating that the column has mostly been
punched down But with increase in length (Ldsc 5) it has
effectively stood against the footing and therefore has bulged
signi1047297cantly Furthermore with column Ldsc ratio of 1 the
contribution of geocell mattress IFgcscIFsc was the maximum
which however reduced as the column length increased and
remained almost constant for Ldsc 5 (Fig 9) This can be analysed
through the responses of the 1047297ll surface depicted in Fig 11 The
surface deformations (d) reported herein are the average of the
readings taken at distance D on both sides of the footing (DT5 andDT6 Fig 1) It can be seen that with increase in the length of stone
columns settlement (thornd) on the 1047297ll surface has reduced Corre-
spondingly heave in the adjacent region ( x frac14 2D and 3D) too was
found to have reduced This is attributed to the increased resistance
of stone columns that inhibits settlement and thereby heave in the
foundation bed With reduced deformations in the soil around the
strength mobilised in the geocells reduces and so is its contribution
to the performance improvement Stone column length beyond the
0 4 8 12 16 20 24 28
Footing settlement sD()
0
1
2
3
4
5
I m p r o v e m e n t f a c t o r r a t i o I F g c s c I F g c
Clay+GC+SC(Ldsc= 1)
Clay+GC+SC(Ldsc= 3)
Clay+GC+SC(Ldsc= 5)
Clay+GC+SC(Ldsc= 7)
Fig 8 Improvement factor ratio-footing settlement responses contribution of stone
columns in composite foundation bed (hD frac14 053 S dsc frac14 25) e Test series 5
0 4 8 12 16 20 24 28
Footing settlement sD()
0
1
2
3
4
5
I m p r o v e m e n t f a c t o r r a t i o I F g c s c I F s c
Clay+GC+SC(Ldsc= 1)
Clay+GC+SC(Ldsc= 3)
Clay+GC+SC(Ldsc= 5)
Clay+GC+SC(Ldsc= 7)
Fig 9 Improvement factor ratio-footing settlement responses contribution of geocell
mattress in composite foundation bed (hD frac14
053 S dsc frac14
25) e
Test series 5
0
1
2
3
4
5
6
7
L e n g t h o f s t o n e c o l u m n L d s c
0 5 10 15 20 25 30
Radial strain (rd-ro)ro ()
Ldsc= 1
Ldsc= 3
Ldsc= 5
Ldsc= 7
Fig 10 Post-test deformed pro1047297le of central stone column in composite foundation
beds (hD frac14 053 S dsc frac14 25) e Test series 5
0 4 8 12 16 20 24 28
Footing settlement sD()
0
1
2
3
4
5
A v e r a g e s u r f a c e d e f o r a m a t i o n
δ D ( )
Clay
Clay+GC
Clay+GC+SC(Ldsc= 1)
Clay+GC+SC(Ldsc= 3)
Clay+GC+SC(Ldsc= 5)
Clay+GC+SC(Ldsc= 7)
Fig 11 Surface deformation-footing settlement responses in1047298uence of length of stone
column in composite foundation bed ( xDfrac14
1 hD frac14
053 S dsc frac14
25) e
Test series 5
SK Dash MC Bora Geotextiles and Geomembranes 41 (2013) 26 e 35 31
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optimum (ie 5dsc) though enhances the skin resistance but it
mostly remains unutilised due to excessive bulging at the top As a
result the responses of the stone columns and that of the geocell
mattress beyond Ldsc ratio of 5 have almost been stabilised Thepresent 1047297ndings are in agreement with the observations of Hughes
and Withers (1974) McKelvey et al (2004) that increasing the
length of stone columns beyond a certain point adds little to the
increase in bearing capacity however can help reducing the set-
tlement in the foundation bed
32 In 1047298uence of spacing of stone columns
Effect of column spacing (S) in the composite foundation beds
was studied under Test series 9e12 Typical responses are shown in
Fig12 Itcan beseen thatat low settlements (sD lt 5) the slope of
the pressure-settlement plots with stone columns are much less
than the case without This indicates that when intact the stone
columns irrespective of their spacing have provided additionalstiffness to the foundation system This however was not the case
when the settlement was large primarily because the columns had
bulged
With relatively widely spaced stone columns (S frac14 35dsc) stiff-
ness of the composite foundation system is almost comparable to
that with geocell reinforcement alone (both the responses are
nearly parallel) It could be because at large spacing the group
action of the peripheral stone columns diminishes that they behave
as individual entities leading to reduced lateral resistance onto the
central con1047297ned region In the absence of adequate con1047297nement
from the surrounding the central stone column underneath the
footing bulged prematurely and therefore couldnrsquot enhance the
stiffness of the foundation system Indeed the post test observa-
tions have shown that with large spacing the central stone columnhad bulged more As the spacing reduces the group action in the
rings of stone columns builds up inducing con1047297nement in the
central region that provides increased support against column
bulging leading to enhanced performance improvement
In general the bearing capacity of the composite foundation bed
was more when the spacing of stone columns was less ( Fig 12)
However the improvement (IFgcsc) with the column spacing (S )
reducing from 35dsc to 25dsc was relatively more than that from
25dsc to 15dsc (Table 2 test series 9e12) Further analysis shows
that the contribution of stone columns in the composite system
IFgcscIFgc was the maximum when they were placed close
(S frac14 15dsc) and reduced as the spacing was increased (Fig 13) In
contrast when the stone columns underneath were placed wide
apart (S frac14 35dsc) the geocell mattress has carried maximum load
IFgcscIFsc which however reduced signi1047297cantly as the spacing of
columns was reduced to 25dsc (Fig 14) Under footing loading the
stone columns with wider spacing have deformed more As the
underlying support yields the geocell mattress through mobi-lisation of anchorage from the adjacent stable region bridges over
and thereby shares higher proportion of the surcharge pressure
The marginal difference in the load factor ratio IFgcscIFsc with the
spacing (S ) changing from 25dsc to 15dsc indicates that further
change in the contribution of geocell mattress is practically negli-
gible With reduced spacing increasedpercentage of weak clay gets
replaced by the stiffer stone columns This gives rise to more uni-
formity of stress in the foundation bed that it deforms less Indeed
reduced settlement and heave on the 1047297ll surface observed with
reduced spacing of stone columns testi1047297es that the deformations in
the foundation bed have reduced down As a result the strain in the
overlying geocell reinforcement reduces leading to reduced mobi-
lisation of its strength and stiffness In such case the geocell
mattress behaves more of like a pedestal a load transmittingmember Whereas with wider spacing (S frac14 35dsc) of stone col-
umns underneath it behaves as a load sustaining member like a
centrally loaded slab resting over columns It can therefore be said
that when the spacing reduces from 35dsc to 25dsc there is sig-
ni1047297cant change in the behaviourof stone columns that it shifts from
near isolated to an interacting response giving rise to large
improvement in the performance of the system Hence the opti-
mum spacing of stone columns in the composite foundation beds
can be taken as 25dsc
0
4
8
12
16
20
24
28
F o o t i n g s e t t l e m e
n t s D ( )
0 20 40 60 80 100 120 140 160
Bearing pressure (kPa)
Clay
Clay+GC
Clay+GC+SC(Sdsc= 15)
Clay+GC+SC(Sdsc= 25)
Clay+GC+SC(Sdsc= 35)
Fig12 Footing pressure-settlement responses in1047298uence of spacing of stone columns
in composite foundation bed (hD frac14 053 Ldsc frac14 5) e Test series 9
0 4 8 12 16 20 24 28
Footing settlement sD()
0
1
2
3
4
I m p r o v e m e n t
f a c t o r r a t i o I F g c s c I F g c
Clay+GC+SC(Sdsc= 15)
Clay+GC+SC(Sdsc= 25)
Clay+GC+SC(Sdsc= 35)
Fig 13 Improvement factor ratio-footing settlement responses contribution of stone
columns in composite foundation bed (hD frac14 09 Ldsc frac14 5) e Test series 10
0 4 8 12 16 20 24 28
Footing settlement sD()
0
1
2
3
4
I m p r o v e m e n t f a c t o r r a t i o I F g c s c I F s c
Clay+GC+SC(Sdsc= 15)
Clay+GC+SC(Sdsc= 25)
Clay+GC+SC(Sdsc= 35)
Fig 14 Improvement factor ratio-footing settlement responses contribution of geo-
cell mattress in composite foundation bed (hD frac14
09 Ldsc frac14
5) e
Test series 10
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33 In 1047298uence of height of geocell mattress
Typical responses of the composite system for three differentheights of geocell mattress (h frac14 053D 09D 16D) are shown in
Figs 8 9 13 14 15 and 16 respectively It could be observed that
when shallow in height (h frac14 053D) the geocell mattress has under
performed that the stone columns have shared nearly three times
more load (IFgcscIFgc frac14 3 Fig 8) However with increase in height
(h) the contribution of stone columns has reduced and that of
geocell mattress (IFgcscIFsc) has gone up When geocells are rela-
tively deep (h frac14 16D) the improvement factor ratio IFgcscIFgc is
just in the range of 1e12 (Fig 15) that the load carried by the stone
columns is at the most 20 that of the geocell mattress The data
presented in Fig 16 indeed shows such a response wherein the
value of improvement factor ratio IFgcscIFsc is as high as 65
indicating that most of the footing pressure has been sustained by
the geocell mattress A deep geocell mattress possesses highmoment of inertia that enables it to stand against the footing
penetration Besides with increase in height (h) the geocell area
deriving anchorage from the in1047297ll soil increases and so is the
anchorage resistance Therefore the geocell mattress takes a large
proportion of the footing loading on its own that the stone columns
underneath mostly remain dormant and thereby contribute less to
the performance improvement Visibly less bulging observed in the
post-test exhumed stone columns (Fig 17) establishes that they
indeed had under performed in sharing the surcharge loading
The improvement due to the geocell-stone column composite
reinforcements are summarised in Table 2 (Test series 5e12) It can
be seen that for height of geocell mattress h frac14 053D 09D 11D and
16D the maximum bearing capacity improvement IFgcsc frac14 569
747 942 and 102 respectively This highlights that the increase in
performance improvement with height of geocell mattress
increasing beyond 11D is relatively less A possible reason for this
could be the stress concentration induced local buckling and
yielding of geocells right under the footing that the global increasein strength and stiffness of the system due to increase in height of
the mattress remains immobilised Indeed the maximum differ-
ence in the values of the factor IFgcscIFsc (ie the load shared by the
geocell mattress) for the case with h frac14 16D and 11D was less than
5 Therefore height of geocell mattress equal to about the diam-
eter of the footing (h frac14 D) can be taken as the optimum one giving
maximum possible performance improvement in the composite
foundation beds However full-scale tests are required to verify this
observation
It is of interest to note that even geocell mattress of medium
height h frac14 09D when combined with stone columns can provide
bearing capacity improvement (IFgcsc frac14 746 Test series 6) as high
as that of a deep geocell mattress h frac14 16D (IFgcsc frac14 787 Test series
4) Due to practical constraints at times it might be dif 1047297cult to
accommodate a relatively large height geocell mattress In such
situation provision of stone columns in the underlying subgrade
would be a viable alternative to manage with geocell mattress of
relatively smaller height
4 Scale effect
Owing to reduced size model tests the results presented in this
paper are prone to scale effects Therefore further studies using
full-scale tests are required to verify these observations However
using a suitable scaling law the results from the present study can
be extrapolated to the prototype case (Fakher and Jones 1996)
The major physical parameters in1047298uencing the response of
geocell-stone column reinforced foundation systems can be
0 4 8 12 16 20 24 28
Footing settlement sD()
00
05
10
15
20
25
30
I m p r o v e m e n t
f c c t o r r a t i o I F g c s c I F g c
Clay+GC+SC(Ldsc= 1)
Clay+GC+SC(Ldsc= 3)
Clay+GC+SC(Ldsc= 5)
Clay+GC+SC(Ldsc= 7)
Fig 15 Improvement factor ratio-footing settlement responses contribution of stone
columns in composite foundation bed (hD frac14 16 S dsc frac14 25) e Test series 8
0 4 8 12 16 20 24 28
Footing settlement sD()
0
2
4
6
8
10
I m p r o v e m e n t f a c t o r r a t i o I F g c s c I F s c
Clay+GC+SC(Ldsc= 1)
Clay+GC+SC(Ldsc= 3)
Clay+GC+SC(Ldsc= 5)
Clay+GC+SC(Ldsc= 7)
Fig 16 Improvement factor ratio-footing settlement responses contribution of geo-
cell mattress in composite foundation bed (hD frac14
16 S dsc frac14
25) e
Test series 8
0
1
2
3
4
5
6
7
L e n g t h o f s t o n e c o l u m n L d s c
0 5 10 15 20 25 30
Radial strain (rd - ro)ro ()
Ldsc= 1
Ldsc= 3
Ldsc= 5
Ldsc= 7
Fig 17 Post-test deformed pro1047297le of central stone column in composite foundation
beds (hD frac14 16 S dsc frac14 25) e Test series 8
SK Dash MC Bora Geotextiles and Geomembranes 41 (2013) 26 e 35 33
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summarised as D h dgc K dsc L d50 S ɸ c u g G where ɸ is the
angle of internal friction of soil and stone aggregate K is the stiff-
ness of geocell reinforcement g is the unit weight of soil and
aggregate G is the shear modulus of the soil and aggregate Other
parameters have been de1047297ned previously The function ( f ) that
governs the composite foundation system can be written as
f
D h dgc K dsc L d50 S c ug Gf s qgcsc qu
frac14 0 (2)
It comprises of 15 parameters and has two fundamental di-
mensions (ie length and force) and therefore can be studied by 13
independent parameters (p1 p2 p3 p4p13 Buckingham
1914) Hence equation (2) can be written as
For a prototype footing ( p) with diameter n times that of the
model (m)
Dp
Dmfrac14 n (4)
For similarity to be maintained the p terms both for model and
prototype need to be equal and therefore considering the p9 term
Gp
Dpgpfrac14
Gm
Dmgm(5)
Assuming that the soils used in the model and prototype do
have same unit weight (Pinto and Cousens 1999) equation (5)
reduces to
Gp
Gmfrac14 Dp
Dmfrac14 n (6)
Considering similarity of the p10 terms
K pgp
G2p
frac14 K mgm
G2m
K p
G2p
frac14 K mG2
m
K pK m
frac14G2
p
G2m
frac14 n2 (7)
As can be seen the strength of prototype geocells should be of n2
times that of the model geocell where n is the scale factor The
geocells used in the present tests have tensile strength of 475 kN
m Therefore the results from the present study to be applicable in
practice the prototype geocells should have tensile strength of
475n2 kNm However the geometric parameters such as pocket
size and height of geocells length diameter and spacing of stone
columns etc have shown a linear variation with the footing size D
5 Conclusions
Review of literature shows that both geocell-sand mattress and
stone columns are effective means of reinforcing the weak soils
Their individual applications though have been intensely studied
by many researchers but combined application of both has
remained unexplored The experimental results obtained in the
present study con1047297rm that such composite reinforcement is an
added advantage over the conventional ones ie stone column or
geocell mattress With provision of stone columns the bearing
capacity of soft clay beds can be increased by 37 fold and with
geocell reinforcement it is of the order of 78 fold When coupled
together ie stone column-geocell mattress combined the bearing
capacity was increased by 102 fold Additionally visible reduction
in slope of pressure settlement responses indicates that the stone
column-geocell composite reinforcement can increase the stiffness
of the foundation bed signi1047297cantly leading to large scale reduction
in footing settlement
The load carrying capacity of the geocell-stone column rein-
forced foundation bed increases with increase in length of stone
columns until 5dsc beyond which further rate of improvement has
reduced down Similarly reducing the spacing of stone columns
below 25dsc does notattract much of additional performance in the
composite system Besides with height of geocells increasing
beyond 11D the performance improvement is found to have
reduced This is possibly due to the stress concentration induced
buckling and yielding of geocells right under the footing that the
increase in strength and stiffness of the system due to increase in
height of the mattress remains immobilised Hence it can be said
that the critical height of geocell mattress giving optimum per-
formance improvement in the composite foundation bed is equal
to about the diameter of the footing (D)
At times practical constraints may prevent in going for large
height geocell mattress or long stone columns severely compro-
mising the performance of the system In such situations the
geocell-stone column composite reinforcement provides an effec-
tive solution for adequate performance improvement and optimum
design of foundations on soft clay This is inferred from the present
study that a shallow height geocell mattress along with medium
length stone columns can provide comparable performance im-
provements as that with deep geocells or long stone columnsHowever full-scale testing and 1047297eld trails are required to verify
these observations
The 1047297ndings of the present study can be of use in the design and
construction of structures over soft clay deposits such as railways
highways foundations for liquid storage tanks large stabilised
areas for parking platforms for oil exploration etc The authors
have also conducted tests with basal geogrid underneath the geo-
cell mattress and the results shall be reported in a subsequent
paper
Acknowledgement
The authors are thankful to the anonymous reviewers for their
valuable comments and suggestions for improvements of the pre-
sentations in the paper
Notation
C c coef 1047297cient of curvature
C u coef 1047297cient of uniformity
D diameter of footing
dgc diameter of geocells
dsc diameter of stone column
emax maximum void ratio
emin minimum void ratio
h height of geocell mattress
g ethp1p2p3p4p13THORN frac14 g
s
D
h
D
dgc
D
h
dgc
dsc
D
L
D
S
D
d50
dsc
G
Dg
K g
G2
c uDg
qgcsc
qu
f
frac14 0 (3)
SK Dash MC Bora Geotextiles and Geomembranes 41 (2013) 26 e 3534
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IFsc bearing capacity improvement factor due to stone column
reinforcement
IFgc bearing capacity improvement factor due to geocell
reinforcement
IFgcsc bearing capacity improvement factor due to stone
column-geocell composite reinforcement
L length of stone column
qsc bearing pressure with stone column reinforcement
qgc bearing pressure with geocell reinforcement
qgcsc bearing pressure with stone column-geocell composite
reinforcement
r d deformed radius of stone column
r o original radius of stone column
S spacing of stone columns
s settlement of footing
u depth of placement of geocell mattress
References
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Ambily AP Gandhi SR 2007 Behaviour of stone columns based on experimental
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515Arulrajah A Abdullah A Bouzza A 2009 Ground improvement technique for
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ASTM D4318 2005 Standard Test Methods for Liquid Limit Plastic Limit andPlasticity Index of Soils ASTM International West Conshohocken PA httpdxdoiorg101520D4318-05
ASTM D4884 2009 Standard Test Method for Strength of Strewn Thermally BondedSeams of Geotextiles ASTM International West Conshohocken PA httpdxdoiorg101520D4884e09
ASTM D6637 2001 Standard Test Methods for Determining Tensile Properties of Geogrid by Single or Multi-rib Tensile Methods ASTM International WestConshohocken PA httpdxdoiorg101520D6637e01R09
Black J Sivakumar V MeKinley JD 2007 Performance of clay samples reinforcedwith vertical granular columns Can Geotech J 44 (1) 89e95
Bowel JE 1988 Foundation Analysis and Design fourth ed McGraw-HillSingapore
Buckingham E 1914 On physically similar systems Phys Rev APS 4 345 e376Bush DI Jenner CG Bassett RH 1990 The design and construction of geocell
foundation mattresses supporting embankments over soft ground GeotextGeomembr 9 (1) 83e98
Carroll Jr RG Curtis VC 1990 Geogrid connections Geotext Geomembr 9 (4e6)515e530
Cimentada A Costa AD Izal JC Sagaseta C 2011 Laboratory study on radialconsolidation and deformation in clay reinforced with stone columns CanGeotech J 48 (1) 36e52
Christoulas S Bouckvalaas G Giannaros C 2000 An experimental study on themodel stone columns Soils Found 40 (6) 11e22
Chummer AV 1972 Bearing capacity theory from experimental results J SoilMech Found Div ASCE 98 (12) 1311e1324
Cowland JW Wong SCK 1993 Performance of a road embankment on soft claysupported on a geocell mattress foundation GeotextGeomembr12 (8) 687e705
Dash SK Bora MC 2013 2013 In1047298uence of geosynthetic encasement on theperformance of stone columns 1047298oating in soft clay Can Geotech J 50 (7) 754e
765Dash SK Krishnaswamy NR Rajagopal K 2001 Bearing capacity of strip footing
supported on geocell-reinforced sand Geotext Geomembr 19 (4) 235e256Dash SK Sireesh S Sitharam TG 2003a Model studies on circular footing
supported on geocell reinforced sand underlain by soft clay Geotext Geo-membr 21 (4) 197e219
Dash SK Sireesh S Sitharam TG 2003b Behaviour of geocell reinforced sandbeds under circular footing Ground Improv 7 (3) 111e115
Dash SK Rajagopal K Krishnaswamy NR 2004 Performance of different geo-synthetic reinforcement materials in sand foundations Geosynth Int 11 (1)35e42
Dash SK Rajagopal K Krishnaswamy NR 2007 Behaviour of geocell reinforcedsand beds under strip loading Can Geotech J 44 (7) 905e916
Dash SK Reddy PDT Raghukanth STG 2008 Subgrade modulus of geocell-reinforced sand foundations Ground Improv 12 (2) 79e87
Deb K Chandra S Basudhar PK 2007 Generalised model for geosynthetic-reinforced granular 1047297ll-soft soil with stone columns Int J Geomech ASCE 7(4) 266e276
Deb K Samadhiya NK Namdeo JB 2011 Laboratory model studies on unrein-forced and geogrid-reinforced sand bed over stone column-improved soft clayGeotext Geomembr 29 (2) 190e196
Fakher A Jones CJFP 1996 Discussion of bearing capacity of rectangular footingson geogrid reinforced sand by Yetimoglu T Wu JTH Saglamer AJ GeotechEng ASCE 122 (4) 326e327
Hughes JMO Withers NJ 1974 Reinforcing of soft cohesive soils with stonecolumns Ground Eng 7 (3) 42e49
Juran I Guermazi A 1988 Settlement response of soft soils reinforced by com-pacted sand columns J Geotech Eng ASCE 114 (8) 930
e942
Latha GM Somwanshi A 2009 Effect of reinforcement form on the bearing ca-pacity of square footings on sand Geotext Geomembr 27 (6) 409e422
Leshchinsky B Ling H 2013 Effects of geocell con1047297nement on strength anddeformation behaviour of gravel J Geotech Geoenviron Eng ASCE 139 (2)340e352
Madhavi GL Vidya SM 2007 Effects of reinforcement form on the behaviour of geosynthetic reinforced sand Geotext Geomembr 25 (1) 23e32
Malarvizhi Ilamparuthi 2007 Comparative study on the behaviour of encasedstone column and conventional stone column Soils Found 47 (5) 873e885
McKelvey D Sivakumar V Bell A Graham J 2004 Modeling vibrated Stonecolumns on soft clay Geotech Eng Proc Inst Civil Eng 157 (3) 137e149
Mayerhof GG Sastry VVRN 1978 Bearing capacity of piles in layered soils part2 Can Geotech J 15 (2) 183e189
Pinto MIM Cousens TW1999 Modelling a geotextile-reinforced brick-faced soilretaining wall Geosynth Int 6 (5) 417e447
Selig ET McKee KE 1961 Static and dynamic behaviour of small footings J Soil
Mech Found Div ASCE 87 (6) 29e
47Sireesh S Sitharam TG Dash SK 2009 Bearing capacity of circular footing ongeocell-sand mattress overlying clay bed with void Geotext Geomembr 27 (2)89e98
Tanyu BF Aydilek AH Lau AW Edil TB Benson CH 2013 Laboratory eval-uation of geocell-reinforced gravel subbase over poor subgrades Geosynth Int20 (2) 47e61
Webster SL Watkins JE 1977 Investigation of Construction Techniques forTactical Bridge Approach Roads across Soft Ground Technical Report S-77e1United States Army Corps of Engineers Waterways Experiment Station Mis-sissippi USA
Wood DM Hu W Nash DFT 2000 Group effects in stone column foundationsmodel tests Geotechnique 50 (6) 689e698
Zhou H Wen X 2008 Model studies on geogrid or geocell-reinforced sandcushion on soft soil Geotext Geomembr 26 (3) 231e238
SK Dash MC Bora Geotextiles and Geomembranes 41 (2013) 26 e 35 35
8212019 Articol d Citit
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contribution of individual reinforcement components (ie geocell
sand mattress stone columns) as has been explained below
IFgcsc
IFscfrac14
qgcsc
qu
qsc
qu
frac14 qgcsc
qsc(1)
Thus the factor (IFgcscIFsc) can be taken as the contribution of
geocell mattress sharing the surcharge loading in the composite
foundation system Similarly the factor IFgcscIFgc (ie qgcscqgc)
represents the contribution of stone columns Typical improvement
factor ratios IFgcscIFgc and IFgcscIFsc for the case with hD frac14 053
are depicted in Fig 8 and Fig 9 respectively It is evident that the
contribution of stone columns IFgcscIFgc increases with increase in
length and the improvement is the maximum when Ldsc ratio
changes from 3 to 5 This may be because the shorter columns (L
dsc lt 3) due to inadequate skin resistance have suffered punching
failure and therefore didnrsquot contribute signi1047297cantly towards the
load carrying capacity of the system In contrast the long columns
(Ldsc 5) owing to large skin resistance mobilised through
increased surface area have effectively stood against the footing
loading giving rise to visible increase in performance improvement
Further con1047297rmation of the column failure modes was obtained
from the post test deformation pro1047297les a typical of which for the
central stone columns are shown in Fig 10 It could be observed
that when short in length (Ldsc frac14 1 and 3) the bulging in the stone
columns is marginal indicating that the column has mostly been
punched down But with increase in length (Ldsc 5) it has
effectively stood against the footing and therefore has bulged
signi1047297cantly Furthermore with column Ldsc ratio of 1 the
contribution of geocell mattress IFgcscIFsc was the maximum
which however reduced as the column length increased and
remained almost constant for Ldsc 5 (Fig 9) This can be analysed
through the responses of the 1047297ll surface depicted in Fig 11 The
surface deformations (d) reported herein are the average of the
readings taken at distance D on both sides of the footing (DT5 andDT6 Fig 1) It can be seen that with increase in the length of stone
columns settlement (thornd) on the 1047297ll surface has reduced Corre-
spondingly heave in the adjacent region ( x frac14 2D and 3D) too was
found to have reduced This is attributed to the increased resistance
of stone columns that inhibits settlement and thereby heave in the
foundation bed With reduced deformations in the soil around the
strength mobilised in the geocells reduces and so is its contribution
to the performance improvement Stone column length beyond the
0 4 8 12 16 20 24 28
Footing settlement sD()
0
1
2
3
4
5
I m p r o v e m e n t f a c t o r r a t i o I F g c s c I F g c
Clay+GC+SC(Ldsc= 1)
Clay+GC+SC(Ldsc= 3)
Clay+GC+SC(Ldsc= 5)
Clay+GC+SC(Ldsc= 7)
Fig 8 Improvement factor ratio-footing settlement responses contribution of stone
columns in composite foundation bed (hD frac14 053 S dsc frac14 25) e Test series 5
0 4 8 12 16 20 24 28
Footing settlement sD()
0
1
2
3
4
5
I m p r o v e m e n t f a c t o r r a t i o I F g c s c I F s c
Clay+GC+SC(Ldsc= 1)
Clay+GC+SC(Ldsc= 3)
Clay+GC+SC(Ldsc= 5)
Clay+GC+SC(Ldsc= 7)
Fig 9 Improvement factor ratio-footing settlement responses contribution of geocell
mattress in composite foundation bed (hD frac14
053 S dsc frac14
25) e
Test series 5
0
1
2
3
4
5
6
7
L e n g t h o f s t o n e c o l u m n L d s c
0 5 10 15 20 25 30
Radial strain (rd-ro)ro ()
Ldsc= 1
Ldsc= 3
Ldsc= 5
Ldsc= 7
Fig 10 Post-test deformed pro1047297le of central stone column in composite foundation
beds (hD frac14 053 S dsc frac14 25) e Test series 5
0 4 8 12 16 20 24 28
Footing settlement sD()
0
1
2
3
4
5
A v e r a g e s u r f a c e d e f o r a m a t i o n
δ D ( )
Clay
Clay+GC
Clay+GC+SC(Ldsc= 1)
Clay+GC+SC(Ldsc= 3)
Clay+GC+SC(Ldsc= 5)
Clay+GC+SC(Ldsc= 7)
Fig 11 Surface deformation-footing settlement responses in1047298uence of length of stone
column in composite foundation bed ( xDfrac14
1 hD frac14
053 S dsc frac14
25) e
Test series 5
SK Dash MC Bora Geotextiles and Geomembranes 41 (2013) 26 e 35 31
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optimum (ie 5dsc) though enhances the skin resistance but it
mostly remains unutilised due to excessive bulging at the top As a
result the responses of the stone columns and that of the geocell
mattress beyond Ldsc ratio of 5 have almost been stabilised Thepresent 1047297ndings are in agreement with the observations of Hughes
and Withers (1974) McKelvey et al (2004) that increasing the
length of stone columns beyond a certain point adds little to the
increase in bearing capacity however can help reducing the set-
tlement in the foundation bed
32 In 1047298uence of spacing of stone columns
Effect of column spacing (S) in the composite foundation beds
was studied under Test series 9e12 Typical responses are shown in
Fig12 Itcan beseen thatat low settlements (sD lt 5) the slope of
the pressure-settlement plots with stone columns are much less
than the case without This indicates that when intact the stone
columns irrespective of their spacing have provided additionalstiffness to the foundation system This however was not the case
when the settlement was large primarily because the columns had
bulged
With relatively widely spaced stone columns (S frac14 35dsc) stiff-
ness of the composite foundation system is almost comparable to
that with geocell reinforcement alone (both the responses are
nearly parallel) It could be because at large spacing the group
action of the peripheral stone columns diminishes that they behave
as individual entities leading to reduced lateral resistance onto the
central con1047297ned region In the absence of adequate con1047297nement
from the surrounding the central stone column underneath the
footing bulged prematurely and therefore couldnrsquot enhance the
stiffness of the foundation system Indeed the post test observa-
tions have shown that with large spacing the central stone columnhad bulged more As the spacing reduces the group action in the
rings of stone columns builds up inducing con1047297nement in the
central region that provides increased support against column
bulging leading to enhanced performance improvement
In general the bearing capacity of the composite foundation bed
was more when the spacing of stone columns was less ( Fig 12)
However the improvement (IFgcsc) with the column spacing (S )
reducing from 35dsc to 25dsc was relatively more than that from
25dsc to 15dsc (Table 2 test series 9e12) Further analysis shows
that the contribution of stone columns in the composite system
IFgcscIFgc was the maximum when they were placed close
(S frac14 15dsc) and reduced as the spacing was increased (Fig 13) In
contrast when the stone columns underneath were placed wide
apart (S frac14 35dsc) the geocell mattress has carried maximum load
IFgcscIFsc which however reduced signi1047297cantly as the spacing of
columns was reduced to 25dsc (Fig 14) Under footing loading the
stone columns with wider spacing have deformed more As the
underlying support yields the geocell mattress through mobi-lisation of anchorage from the adjacent stable region bridges over
and thereby shares higher proportion of the surcharge pressure
The marginal difference in the load factor ratio IFgcscIFsc with the
spacing (S ) changing from 25dsc to 15dsc indicates that further
change in the contribution of geocell mattress is practically negli-
gible With reduced spacing increasedpercentage of weak clay gets
replaced by the stiffer stone columns This gives rise to more uni-
formity of stress in the foundation bed that it deforms less Indeed
reduced settlement and heave on the 1047297ll surface observed with
reduced spacing of stone columns testi1047297es that the deformations in
the foundation bed have reduced down As a result the strain in the
overlying geocell reinforcement reduces leading to reduced mobi-
lisation of its strength and stiffness In such case the geocell
mattress behaves more of like a pedestal a load transmittingmember Whereas with wider spacing (S frac14 35dsc) of stone col-
umns underneath it behaves as a load sustaining member like a
centrally loaded slab resting over columns It can therefore be said
that when the spacing reduces from 35dsc to 25dsc there is sig-
ni1047297cant change in the behaviourof stone columns that it shifts from
near isolated to an interacting response giving rise to large
improvement in the performance of the system Hence the opti-
mum spacing of stone columns in the composite foundation beds
can be taken as 25dsc
0
4
8
12
16
20
24
28
F o o t i n g s e t t l e m e
n t s D ( )
0 20 40 60 80 100 120 140 160
Bearing pressure (kPa)
Clay
Clay+GC
Clay+GC+SC(Sdsc= 15)
Clay+GC+SC(Sdsc= 25)
Clay+GC+SC(Sdsc= 35)
Fig12 Footing pressure-settlement responses in1047298uence of spacing of stone columns
in composite foundation bed (hD frac14 053 Ldsc frac14 5) e Test series 9
0 4 8 12 16 20 24 28
Footing settlement sD()
0
1
2
3
4
I m p r o v e m e n t
f a c t o r r a t i o I F g c s c I F g c
Clay+GC+SC(Sdsc= 15)
Clay+GC+SC(Sdsc= 25)
Clay+GC+SC(Sdsc= 35)
Fig 13 Improvement factor ratio-footing settlement responses contribution of stone
columns in composite foundation bed (hD frac14 09 Ldsc frac14 5) e Test series 10
0 4 8 12 16 20 24 28
Footing settlement sD()
0
1
2
3
4
I m p r o v e m e n t f a c t o r r a t i o I F g c s c I F s c
Clay+GC+SC(Sdsc= 15)
Clay+GC+SC(Sdsc= 25)
Clay+GC+SC(Sdsc= 35)
Fig 14 Improvement factor ratio-footing settlement responses contribution of geo-
cell mattress in composite foundation bed (hD frac14
09 Ldsc frac14
5) e
Test series 10
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33 In 1047298uence of height of geocell mattress
Typical responses of the composite system for three differentheights of geocell mattress (h frac14 053D 09D 16D) are shown in
Figs 8 9 13 14 15 and 16 respectively It could be observed that
when shallow in height (h frac14 053D) the geocell mattress has under
performed that the stone columns have shared nearly three times
more load (IFgcscIFgc frac14 3 Fig 8) However with increase in height
(h) the contribution of stone columns has reduced and that of
geocell mattress (IFgcscIFsc) has gone up When geocells are rela-
tively deep (h frac14 16D) the improvement factor ratio IFgcscIFgc is
just in the range of 1e12 (Fig 15) that the load carried by the stone
columns is at the most 20 that of the geocell mattress The data
presented in Fig 16 indeed shows such a response wherein the
value of improvement factor ratio IFgcscIFsc is as high as 65
indicating that most of the footing pressure has been sustained by
the geocell mattress A deep geocell mattress possesses highmoment of inertia that enables it to stand against the footing
penetration Besides with increase in height (h) the geocell area
deriving anchorage from the in1047297ll soil increases and so is the
anchorage resistance Therefore the geocell mattress takes a large
proportion of the footing loading on its own that the stone columns
underneath mostly remain dormant and thereby contribute less to
the performance improvement Visibly less bulging observed in the
post-test exhumed stone columns (Fig 17) establishes that they
indeed had under performed in sharing the surcharge loading
The improvement due to the geocell-stone column composite
reinforcements are summarised in Table 2 (Test series 5e12) It can
be seen that for height of geocell mattress h frac14 053D 09D 11D and
16D the maximum bearing capacity improvement IFgcsc frac14 569
747 942 and 102 respectively This highlights that the increase in
performance improvement with height of geocell mattress
increasing beyond 11D is relatively less A possible reason for this
could be the stress concentration induced local buckling and
yielding of geocells right under the footing that the global increasein strength and stiffness of the system due to increase in height of
the mattress remains immobilised Indeed the maximum differ-
ence in the values of the factor IFgcscIFsc (ie the load shared by the
geocell mattress) for the case with h frac14 16D and 11D was less than
5 Therefore height of geocell mattress equal to about the diam-
eter of the footing (h frac14 D) can be taken as the optimum one giving
maximum possible performance improvement in the composite
foundation beds However full-scale tests are required to verify this
observation
It is of interest to note that even geocell mattress of medium
height h frac14 09D when combined with stone columns can provide
bearing capacity improvement (IFgcsc frac14 746 Test series 6) as high
as that of a deep geocell mattress h frac14 16D (IFgcsc frac14 787 Test series
4) Due to practical constraints at times it might be dif 1047297cult to
accommodate a relatively large height geocell mattress In such
situation provision of stone columns in the underlying subgrade
would be a viable alternative to manage with geocell mattress of
relatively smaller height
4 Scale effect
Owing to reduced size model tests the results presented in this
paper are prone to scale effects Therefore further studies using
full-scale tests are required to verify these observations However
using a suitable scaling law the results from the present study can
be extrapolated to the prototype case (Fakher and Jones 1996)
The major physical parameters in1047298uencing the response of
geocell-stone column reinforced foundation systems can be
0 4 8 12 16 20 24 28
Footing settlement sD()
00
05
10
15
20
25
30
I m p r o v e m e n t
f c c t o r r a t i o I F g c s c I F g c
Clay+GC+SC(Ldsc= 1)
Clay+GC+SC(Ldsc= 3)
Clay+GC+SC(Ldsc= 5)
Clay+GC+SC(Ldsc= 7)
Fig 15 Improvement factor ratio-footing settlement responses contribution of stone
columns in composite foundation bed (hD frac14 16 S dsc frac14 25) e Test series 8
0 4 8 12 16 20 24 28
Footing settlement sD()
0
2
4
6
8
10
I m p r o v e m e n t f a c t o r r a t i o I F g c s c I F s c
Clay+GC+SC(Ldsc= 1)
Clay+GC+SC(Ldsc= 3)
Clay+GC+SC(Ldsc= 5)
Clay+GC+SC(Ldsc= 7)
Fig 16 Improvement factor ratio-footing settlement responses contribution of geo-
cell mattress in composite foundation bed (hD frac14
16 S dsc frac14
25) e
Test series 8
0
1
2
3
4
5
6
7
L e n g t h o f s t o n e c o l u m n L d s c
0 5 10 15 20 25 30
Radial strain (rd - ro)ro ()
Ldsc= 1
Ldsc= 3
Ldsc= 5
Ldsc= 7
Fig 17 Post-test deformed pro1047297le of central stone column in composite foundation
beds (hD frac14 16 S dsc frac14 25) e Test series 8
SK Dash MC Bora Geotextiles and Geomembranes 41 (2013) 26 e 35 33
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summarised as D h dgc K dsc L d50 S ɸ c u g G where ɸ is the
angle of internal friction of soil and stone aggregate K is the stiff-
ness of geocell reinforcement g is the unit weight of soil and
aggregate G is the shear modulus of the soil and aggregate Other
parameters have been de1047297ned previously The function ( f ) that
governs the composite foundation system can be written as
f
D h dgc K dsc L d50 S c ug Gf s qgcsc qu
frac14 0 (2)
It comprises of 15 parameters and has two fundamental di-
mensions (ie length and force) and therefore can be studied by 13
independent parameters (p1 p2 p3 p4p13 Buckingham
1914) Hence equation (2) can be written as
For a prototype footing ( p) with diameter n times that of the
model (m)
Dp
Dmfrac14 n (4)
For similarity to be maintained the p terms both for model and
prototype need to be equal and therefore considering the p9 term
Gp
Dpgpfrac14
Gm
Dmgm(5)
Assuming that the soils used in the model and prototype do
have same unit weight (Pinto and Cousens 1999) equation (5)
reduces to
Gp
Gmfrac14 Dp
Dmfrac14 n (6)
Considering similarity of the p10 terms
K pgp
G2p
frac14 K mgm
G2m
K p
G2p
frac14 K mG2
m
K pK m
frac14G2
p
G2m
frac14 n2 (7)
As can be seen the strength of prototype geocells should be of n2
times that of the model geocell where n is the scale factor The
geocells used in the present tests have tensile strength of 475 kN
m Therefore the results from the present study to be applicable in
practice the prototype geocells should have tensile strength of
475n2 kNm However the geometric parameters such as pocket
size and height of geocells length diameter and spacing of stone
columns etc have shown a linear variation with the footing size D
5 Conclusions
Review of literature shows that both geocell-sand mattress and
stone columns are effective means of reinforcing the weak soils
Their individual applications though have been intensely studied
by many researchers but combined application of both has
remained unexplored The experimental results obtained in the
present study con1047297rm that such composite reinforcement is an
added advantage over the conventional ones ie stone column or
geocell mattress With provision of stone columns the bearing
capacity of soft clay beds can be increased by 37 fold and with
geocell reinforcement it is of the order of 78 fold When coupled
together ie stone column-geocell mattress combined the bearing
capacity was increased by 102 fold Additionally visible reduction
in slope of pressure settlement responses indicates that the stone
column-geocell composite reinforcement can increase the stiffness
of the foundation bed signi1047297cantly leading to large scale reduction
in footing settlement
The load carrying capacity of the geocell-stone column rein-
forced foundation bed increases with increase in length of stone
columns until 5dsc beyond which further rate of improvement has
reduced down Similarly reducing the spacing of stone columns
below 25dsc does notattract much of additional performance in the
composite system Besides with height of geocells increasing
beyond 11D the performance improvement is found to have
reduced This is possibly due to the stress concentration induced
buckling and yielding of geocells right under the footing that the
increase in strength and stiffness of the system due to increase in
height of the mattress remains immobilised Hence it can be said
that the critical height of geocell mattress giving optimum per-
formance improvement in the composite foundation bed is equal
to about the diameter of the footing (D)
At times practical constraints may prevent in going for large
height geocell mattress or long stone columns severely compro-
mising the performance of the system In such situations the
geocell-stone column composite reinforcement provides an effec-
tive solution for adequate performance improvement and optimum
design of foundations on soft clay This is inferred from the present
study that a shallow height geocell mattress along with medium
length stone columns can provide comparable performance im-
provements as that with deep geocells or long stone columnsHowever full-scale testing and 1047297eld trails are required to verify
these observations
The 1047297ndings of the present study can be of use in the design and
construction of structures over soft clay deposits such as railways
highways foundations for liquid storage tanks large stabilised
areas for parking platforms for oil exploration etc The authors
have also conducted tests with basal geogrid underneath the geo-
cell mattress and the results shall be reported in a subsequent
paper
Acknowledgement
The authors are thankful to the anonymous reviewers for their
valuable comments and suggestions for improvements of the pre-
sentations in the paper
Notation
C c coef 1047297cient of curvature
C u coef 1047297cient of uniformity
D diameter of footing
dgc diameter of geocells
dsc diameter of stone column
emax maximum void ratio
emin minimum void ratio
h height of geocell mattress
g ethp1p2p3p4p13THORN frac14 g
s
D
h
D
dgc
D
h
dgc
dsc
D
L
D
S
D
d50
dsc
G
Dg
K g
G2
c uDg
qgcsc
qu
f
frac14 0 (3)
SK Dash MC Bora Geotextiles and Geomembranes 41 (2013) 26 e 3534
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IFsc bearing capacity improvement factor due to stone column
reinforcement
IFgc bearing capacity improvement factor due to geocell
reinforcement
IFgcsc bearing capacity improvement factor due to stone
column-geocell composite reinforcement
L length of stone column
qsc bearing pressure with stone column reinforcement
qgc bearing pressure with geocell reinforcement
qgcsc bearing pressure with stone column-geocell composite
reinforcement
r d deformed radius of stone column
r o original radius of stone column
S spacing of stone columns
s settlement of footing
u depth of placement of geocell mattress
References
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Ambily AP Gandhi SR 2007 Behaviour of stone columns based on experimental
and FEM analysis J Geotech Geoenviron Eng ASCE 133 (4) 405e
515Arulrajah A Abdullah A Bouzza A 2009 Ground improvement technique for
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ASTM D4318 2005 Standard Test Methods for Liquid Limit Plastic Limit andPlasticity Index of Soils ASTM International West Conshohocken PA httpdxdoiorg101520D4318-05
ASTM D4884 2009 Standard Test Method for Strength of Strewn Thermally BondedSeams of Geotextiles ASTM International West Conshohocken PA httpdxdoiorg101520D4884e09
ASTM D6637 2001 Standard Test Methods for Determining Tensile Properties of Geogrid by Single or Multi-rib Tensile Methods ASTM International WestConshohocken PA httpdxdoiorg101520D6637e01R09
Black J Sivakumar V MeKinley JD 2007 Performance of clay samples reinforcedwith vertical granular columns Can Geotech J 44 (1) 89e95
Bowel JE 1988 Foundation Analysis and Design fourth ed McGraw-HillSingapore
Buckingham E 1914 On physically similar systems Phys Rev APS 4 345 e376Bush DI Jenner CG Bassett RH 1990 The design and construction of geocell
foundation mattresses supporting embankments over soft ground GeotextGeomembr 9 (1) 83e98
Carroll Jr RG Curtis VC 1990 Geogrid connections Geotext Geomembr 9 (4e6)515e530
Cimentada A Costa AD Izal JC Sagaseta C 2011 Laboratory study on radialconsolidation and deformation in clay reinforced with stone columns CanGeotech J 48 (1) 36e52
Christoulas S Bouckvalaas G Giannaros C 2000 An experimental study on themodel stone columns Soils Found 40 (6) 11e22
Chummer AV 1972 Bearing capacity theory from experimental results J SoilMech Found Div ASCE 98 (12) 1311e1324
Cowland JW Wong SCK 1993 Performance of a road embankment on soft claysupported on a geocell mattress foundation GeotextGeomembr12 (8) 687e705
Dash SK Bora MC 2013 2013 In1047298uence of geosynthetic encasement on theperformance of stone columns 1047298oating in soft clay Can Geotech J 50 (7) 754e
765Dash SK Krishnaswamy NR Rajagopal K 2001 Bearing capacity of strip footing
supported on geocell-reinforced sand Geotext Geomembr 19 (4) 235e256Dash SK Sireesh S Sitharam TG 2003a Model studies on circular footing
supported on geocell reinforced sand underlain by soft clay Geotext Geo-membr 21 (4) 197e219
Dash SK Sireesh S Sitharam TG 2003b Behaviour of geocell reinforced sandbeds under circular footing Ground Improv 7 (3) 111e115
Dash SK Rajagopal K Krishnaswamy NR 2004 Performance of different geo-synthetic reinforcement materials in sand foundations Geosynth Int 11 (1)35e42
Dash SK Rajagopal K Krishnaswamy NR 2007 Behaviour of geocell reinforcedsand beds under strip loading Can Geotech J 44 (7) 905e916
Dash SK Reddy PDT Raghukanth STG 2008 Subgrade modulus of geocell-reinforced sand foundations Ground Improv 12 (2) 79e87
Deb K Chandra S Basudhar PK 2007 Generalised model for geosynthetic-reinforced granular 1047297ll-soft soil with stone columns Int J Geomech ASCE 7(4) 266e276
Deb K Samadhiya NK Namdeo JB 2011 Laboratory model studies on unrein-forced and geogrid-reinforced sand bed over stone column-improved soft clayGeotext Geomembr 29 (2) 190e196
Fakher A Jones CJFP 1996 Discussion of bearing capacity of rectangular footingson geogrid reinforced sand by Yetimoglu T Wu JTH Saglamer AJ GeotechEng ASCE 122 (4) 326e327
Hughes JMO Withers NJ 1974 Reinforcing of soft cohesive soils with stonecolumns Ground Eng 7 (3) 42e49
Juran I Guermazi A 1988 Settlement response of soft soils reinforced by com-pacted sand columns J Geotech Eng ASCE 114 (8) 930
e942
Latha GM Somwanshi A 2009 Effect of reinforcement form on the bearing ca-pacity of square footings on sand Geotext Geomembr 27 (6) 409e422
Leshchinsky B Ling H 2013 Effects of geocell con1047297nement on strength anddeformation behaviour of gravel J Geotech Geoenviron Eng ASCE 139 (2)340e352
Madhavi GL Vidya SM 2007 Effects of reinforcement form on the behaviour of geosynthetic reinforced sand Geotext Geomembr 25 (1) 23e32
Malarvizhi Ilamparuthi 2007 Comparative study on the behaviour of encasedstone column and conventional stone column Soils Found 47 (5) 873e885
McKelvey D Sivakumar V Bell A Graham J 2004 Modeling vibrated Stonecolumns on soft clay Geotech Eng Proc Inst Civil Eng 157 (3) 137e149
Mayerhof GG Sastry VVRN 1978 Bearing capacity of piles in layered soils part2 Can Geotech J 15 (2) 183e189
Pinto MIM Cousens TW1999 Modelling a geotextile-reinforced brick-faced soilretaining wall Geosynth Int 6 (5) 417e447
Selig ET McKee KE 1961 Static and dynamic behaviour of small footings J Soil
Mech Found Div ASCE 87 (6) 29e
47Sireesh S Sitharam TG Dash SK 2009 Bearing capacity of circular footing ongeocell-sand mattress overlying clay bed with void Geotext Geomembr 27 (2)89e98
Tanyu BF Aydilek AH Lau AW Edil TB Benson CH 2013 Laboratory eval-uation of geocell-reinforced gravel subbase over poor subgrades Geosynth Int20 (2) 47e61
Webster SL Watkins JE 1977 Investigation of Construction Techniques forTactical Bridge Approach Roads across Soft Ground Technical Report S-77e1United States Army Corps of Engineers Waterways Experiment Station Mis-sissippi USA
Wood DM Hu W Nash DFT 2000 Group effects in stone column foundationsmodel tests Geotechnique 50 (6) 689e698
Zhou H Wen X 2008 Model studies on geogrid or geocell-reinforced sandcushion on soft soil Geotext Geomembr 26 (3) 231e238
SK Dash MC Bora Geotextiles and Geomembranes 41 (2013) 26 e 35 35
8212019 Articol d Citit
httpslidepdfcomreaderfullarticol-d-citit 710
optimum (ie 5dsc) though enhances the skin resistance but it
mostly remains unutilised due to excessive bulging at the top As a
result the responses of the stone columns and that of the geocell
mattress beyond Ldsc ratio of 5 have almost been stabilised Thepresent 1047297ndings are in agreement with the observations of Hughes
and Withers (1974) McKelvey et al (2004) that increasing the
length of stone columns beyond a certain point adds little to the
increase in bearing capacity however can help reducing the set-
tlement in the foundation bed
32 In 1047298uence of spacing of stone columns
Effect of column spacing (S) in the composite foundation beds
was studied under Test series 9e12 Typical responses are shown in
Fig12 Itcan beseen thatat low settlements (sD lt 5) the slope of
the pressure-settlement plots with stone columns are much less
than the case without This indicates that when intact the stone
columns irrespective of their spacing have provided additionalstiffness to the foundation system This however was not the case
when the settlement was large primarily because the columns had
bulged
With relatively widely spaced stone columns (S frac14 35dsc) stiff-
ness of the composite foundation system is almost comparable to
that with geocell reinforcement alone (both the responses are
nearly parallel) It could be because at large spacing the group
action of the peripheral stone columns diminishes that they behave
as individual entities leading to reduced lateral resistance onto the
central con1047297ned region In the absence of adequate con1047297nement
from the surrounding the central stone column underneath the
footing bulged prematurely and therefore couldnrsquot enhance the
stiffness of the foundation system Indeed the post test observa-
tions have shown that with large spacing the central stone columnhad bulged more As the spacing reduces the group action in the
rings of stone columns builds up inducing con1047297nement in the
central region that provides increased support against column
bulging leading to enhanced performance improvement
In general the bearing capacity of the composite foundation bed
was more when the spacing of stone columns was less ( Fig 12)
However the improvement (IFgcsc) with the column spacing (S )
reducing from 35dsc to 25dsc was relatively more than that from
25dsc to 15dsc (Table 2 test series 9e12) Further analysis shows
that the contribution of stone columns in the composite system
IFgcscIFgc was the maximum when they were placed close
(S frac14 15dsc) and reduced as the spacing was increased (Fig 13) In
contrast when the stone columns underneath were placed wide
apart (S frac14 35dsc) the geocell mattress has carried maximum load
IFgcscIFsc which however reduced signi1047297cantly as the spacing of
columns was reduced to 25dsc (Fig 14) Under footing loading the
stone columns with wider spacing have deformed more As the
underlying support yields the geocell mattress through mobi-lisation of anchorage from the adjacent stable region bridges over
and thereby shares higher proportion of the surcharge pressure
The marginal difference in the load factor ratio IFgcscIFsc with the
spacing (S ) changing from 25dsc to 15dsc indicates that further
change in the contribution of geocell mattress is practically negli-
gible With reduced spacing increasedpercentage of weak clay gets
replaced by the stiffer stone columns This gives rise to more uni-
formity of stress in the foundation bed that it deforms less Indeed
reduced settlement and heave on the 1047297ll surface observed with
reduced spacing of stone columns testi1047297es that the deformations in
the foundation bed have reduced down As a result the strain in the
overlying geocell reinforcement reduces leading to reduced mobi-
lisation of its strength and stiffness In such case the geocell
mattress behaves more of like a pedestal a load transmittingmember Whereas with wider spacing (S frac14 35dsc) of stone col-
umns underneath it behaves as a load sustaining member like a
centrally loaded slab resting over columns It can therefore be said
that when the spacing reduces from 35dsc to 25dsc there is sig-
ni1047297cant change in the behaviourof stone columns that it shifts from
near isolated to an interacting response giving rise to large
improvement in the performance of the system Hence the opti-
mum spacing of stone columns in the composite foundation beds
can be taken as 25dsc
0
4
8
12
16
20
24
28
F o o t i n g s e t t l e m e
n t s D ( )
0 20 40 60 80 100 120 140 160
Bearing pressure (kPa)
Clay
Clay+GC
Clay+GC+SC(Sdsc= 15)
Clay+GC+SC(Sdsc= 25)
Clay+GC+SC(Sdsc= 35)
Fig12 Footing pressure-settlement responses in1047298uence of spacing of stone columns
in composite foundation bed (hD frac14 053 Ldsc frac14 5) e Test series 9
0 4 8 12 16 20 24 28
Footing settlement sD()
0
1
2
3
4
I m p r o v e m e n t
f a c t o r r a t i o I F g c s c I F g c
Clay+GC+SC(Sdsc= 15)
Clay+GC+SC(Sdsc= 25)
Clay+GC+SC(Sdsc= 35)
Fig 13 Improvement factor ratio-footing settlement responses contribution of stone
columns in composite foundation bed (hD frac14 09 Ldsc frac14 5) e Test series 10
0 4 8 12 16 20 24 28
Footing settlement sD()
0
1
2
3
4
I m p r o v e m e n t f a c t o r r a t i o I F g c s c I F s c
Clay+GC+SC(Sdsc= 15)
Clay+GC+SC(Sdsc= 25)
Clay+GC+SC(Sdsc= 35)
Fig 14 Improvement factor ratio-footing settlement responses contribution of geo-
cell mattress in composite foundation bed (hD frac14
09 Ldsc frac14
5) e
Test series 10
SK Dash MC Bora Geotextiles and Geomembranes 41 (2013) 26 e 3532
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33 In 1047298uence of height of geocell mattress
Typical responses of the composite system for three differentheights of geocell mattress (h frac14 053D 09D 16D) are shown in
Figs 8 9 13 14 15 and 16 respectively It could be observed that
when shallow in height (h frac14 053D) the geocell mattress has under
performed that the stone columns have shared nearly three times
more load (IFgcscIFgc frac14 3 Fig 8) However with increase in height
(h) the contribution of stone columns has reduced and that of
geocell mattress (IFgcscIFsc) has gone up When geocells are rela-
tively deep (h frac14 16D) the improvement factor ratio IFgcscIFgc is
just in the range of 1e12 (Fig 15) that the load carried by the stone
columns is at the most 20 that of the geocell mattress The data
presented in Fig 16 indeed shows such a response wherein the
value of improvement factor ratio IFgcscIFsc is as high as 65
indicating that most of the footing pressure has been sustained by
the geocell mattress A deep geocell mattress possesses highmoment of inertia that enables it to stand against the footing
penetration Besides with increase in height (h) the geocell area
deriving anchorage from the in1047297ll soil increases and so is the
anchorage resistance Therefore the geocell mattress takes a large
proportion of the footing loading on its own that the stone columns
underneath mostly remain dormant and thereby contribute less to
the performance improvement Visibly less bulging observed in the
post-test exhumed stone columns (Fig 17) establishes that they
indeed had under performed in sharing the surcharge loading
The improvement due to the geocell-stone column composite
reinforcements are summarised in Table 2 (Test series 5e12) It can
be seen that for height of geocell mattress h frac14 053D 09D 11D and
16D the maximum bearing capacity improvement IFgcsc frac14 569
747 942 and 102 respectively This highlights that the increase in
performance improvement with height of geocell mattress
increasing beyond 11D is relatively less A possible reason for this
could be the stress concentration induced local buckling and
yielding of geocells right under the footing that the global increasein strength and stiffness of the system due to increase in height of
the mattress remains immobilised Indeed the maximum differ-
ence in the values of the factor IFgcscIFsc (ie the load shared by the
geocell mattress) for the case with h frac14 16D and 11D was less than
5 Therefore height of geocell mattress equal to about the diam-
eter of the footing (h frac14 D) can be taken as the optimum one giving
maximum possible performance improvement in the composite
foundation beds However full-scale tests are required to verify this
observation
It is of interest to note that even geocell mattress of medium
height h frac14 09D when combined with stone columns can provide
bearing capacity improvement (IFgcsc frac14 746 Test series 6) as high
as that of a deep geocell mattress h frac14 16D (IFgcsc frac14 787 Test series
4) Due to practical constraints at times it might be dif 1047297cult to
accommodate a relatively large height geocell mattress In such
situation provision of stone columns in the underlying subgrade
would be a viable alternative to manage with geocell mattress of
relatively smaller height
4 Scale effect
Owing to reduced size model tests the results presented in this
paper are prone to scale effects Therefore further studies using
full-scale tests are required to verify these observations However
using a suitable scaling law the results from the present study can
be extrapolated to the prototype case (Fakher and Jones 1996)
The major physical parameters in1047298uencing the response of
geocell-stone column reinforced foundation systems can be
0 4 8 12 16 20 24 28
Footing settlement sD()
00
05
10
15
20
25
30
I m p r o v e m e n t
f c c t o r r a t i o I F g c s c I F g c
Clay+GC+SC(Ldsc= 1)
Clay+GC+SC(Ldsc= 3)
Clay+GC+SC(Ldsc= 5)
Clay+GC+SC(Ldsc= 7)
Fig 15 Improvement factor ratio-footing settlement responses contribution of stone
columns in composite foundation bed (hD frac14 16 S dsc frac14 25) e Test series 8
0 4 8 12 16 20 24 28
Footing settlement sD()
0
2
4
6
8
10
I m p r o v e m e n t f a c t o r r a t i o I F g c s c I F s c
Clay+GC+SC(Ldsc= 1)
Clay+GC+SC(Ldsc= 3)
Clay+GC+SC(Ldsc= 5)
Clay+GC+SC(Ldsc= 7)
Fig 16 Improvement factor ratio-footing settlement responses contribution of geo-
cell mattress in composite foundation bed (hD frac14
16 S dsc frac14
25) e
Test series 8
0
1
2
3
4
5
6
7
L e n g t h o f s t o n e c o l u m n L d s c
0 5 10 15 20 25 30
Radial strain (rd - ro)ro ()
Ldsc= 1
Ldsc= 3
Ldsc= 5
Ldsc= 7
Fig 17 Post-test deformed pro1047297le of central stone column in composite foundation
beds (hD frac14 16 S dsc frac14 25) e Test series 8
SK Dash MC Bora Geotextiles and Geomembranes 41 (2013) 26 e 35 33
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summarised as D h dgc K dsc L d50 S ɸ c u g G where ɸ is the
angle of internal friction of soil and stone aggregate K is the stiff-
ness of geocell reinforcement g is the unit weight of soil and
aggregate G is the shear modulus of the soil and aggregate Other
parameters have been de1047297ned previously The function ( f ) that
governs the composite foundation system can be written as
f
D h dgc K dsc L d50 S c ug Gf s qgcsc qu
frac14 0 (2)
It comprises of 15 parameters and has two fundamental di-
mensions (ie length and force) and therefore can be studied by 13
independent parameters (p1 p2 p3 p4p13 Buckingham
1914) Hence equation (2) can be written as
For a prototype footing ( p) with diameter n times that of the
model (m)
Dp
Dmfrac14 n (4)
For similarity to be maintained the p terms both for model and
prototype need to be equal and therefore considering the p9 term
Gp
Dpgpfrac14
Gm
Dmgm(5)
Assuming that the soils used in the model and prototype do
have same unit weight (Pinto and Cousens 1999) equation (5)
reduces to
Gp
Gmfrac14 Dp
Dmfrac14 n (6)
Considering similarity of the p10 terms
K pgp
G2p
frac14 K mgm
G2m
K p
G2p
frac14 K mG2
m
K pK m
frac14G2
p
G2m
frac14 n2 (7)
As can be seen the strength of prototype geocells should be of n2
times that of the model geocell where n is the scale factor The
geocells used in the present tests have tensile strength of 475 kN
m Therefore the results from the present study to be applicable in
practice the prototype geocells should have tensile strength of
475n2 kNm However the geometric parameters such as pocket
size and height of geocells length diameter and spacing of stone
columns etc have shown a linear variation with the footing size D
5 Conclusions
Review of literature shows that both geocell-sand mattress and
stone columns are effective means of reinforcing the weak soils
Their individual applications though have been intensely studied
by many researchers but combined application of both has
remained unexplored The experimental results obtained in the
present study con1047297rm that such composite reinforcement is an
added advantage over the conventional ones ie stone column or
geocell mattress With provision of stone columns the bearing
capacity of soft clay beds can be increased by 37 fold and with
geocell reinforcement it is of the order of 78 fold When coupled
together ie stone column-geocell mattress combined the bearing
capacity was increased by 102 fold Additionally visible reduction
in slope of pressure settlement responses indicates that the stone
column-geocell composite reinforcement can increase the stiffness
of the foundation bed signi1047297cantly leading to large scale reduction
in footing settlement
The load carrying capacity of the geocell-stone column rein-
forced foundation bed increases with increase in length of stone
columns until 5dsc beyond which further rate of improvement has
reduced down Similarly reducing the spacing of stone columns
below 25dsc does notattract much of additional performance in the
composite system Besides with height of geocells increasing
beyond 11D the performance improvement is found to have
reduced This is possibly due to the stress concentration induced
buckling and yielding of geocells right under the footing that the
increase in strength and stiffness of the system due to increase in
height of the mattress remains immobilised Hence it can be said
that the critical height of geocell mattress giving optimum per-
formance improvement in the composite foundation bed is equal
to about the diameter of the footing (D)
At times practical constraints may prevent in going for large
height geocell mattress or long stone columns severely compro-
mising the performance of the system In such situations the
geocell-stone column composite reinforcement provides an effec-
tive solution for adequate performance improvement and optimum
design of foundations on soft clay This is inferred from the present
study that a shallow height geocell mattress along with medium
length stone columns can provide comparable performance im-
provements as that with deep geocells or long stone columnsHowever full-scale testing and 1047297eld trails are required to verify
these observations
The 1047297ndings of the present study can be of use in the design and
construction of structures over soft clay deposits such as railways
highways foundations for liquid storage tanks large stabilised
areas for parking platforms for oil exploration etc The authors
have also conducted tests with basal geogrid underneath the geo-
cell mattress and the results shall be reported in a subsequent
paper
Acknowledgement
The authors are thankful to the anonymous reviewers for their
valuable comments and suggestions for improvements of the pre-
sentations in the paper
Notation
C c coef 1047297cient of curvature
C u coef 1047297cient of uniformity
D diameter of footing
dgc diameter of geocells
dsc diameter of stone column
emax maximum void ratio
emin minimum void ratio
h height of geocell mattress
g ethp1p2p3p4p13THORN frac14 g
s
D
h
D
dgc
D
h
dgc
dsc
D
L
D
S
D
d50
dsc
G
Dg
K g
G2
c uDg
qgcsc
qu
f
frac14 0 (3)
SK Dash MC Bora Geotextiles and Geomembranes 41 (2013) 26 e 3534
8212019 Articol d Citit
httpslidepdfcomreaderfullarticol-d-citit 1010
IFsc bearing capacity improvement factor due to stone column
reinforcement
IFgc bearing capacity improvement factor due to geocell
reinforcement
IFgcsc bearing capacity improvement factor due to stone
column-geocell composite reinforcement
L length of stone column
qsc bearing pressure with stone column reinforcement
qgc bearing pressure with geocell reinforcement
qgcsc bearing pressure with stone column-geocell composite
reinforcement
r d deformed radius of stone column
r o original radius of stone column
S spacing of stone columns
s settlement of footing
u depth of placement of geocell mattress
References
Abdullah CH Edil TB 2007 Behaviour of geogrid-reinforced load transfer plat-forms for embankment on rammed aggregate piers Geosynth Int 14 (3) 141e153
Ambily AP Gandhi SR 2007 Behaviour of stone columns based on experimental
and FEM analysis J Geotech Geoenviron Eng ASCE 133 (4) 405e
515Arulrajah A Abdullah A Bouzza A 2009 Ground improvement technique for
railway embankments Ground Improv 162 (1) 3e14ASTM D2487 2006 Standard Practice for Classi1047297cation of Soils for Engineering
Purposes (Uni1047297ed Soil Classi1047297cation System) ASTM International West Con-shohocken PA httpdxdoiorg101520D2487-06E01
ASTM D4318 2005 Standard Test Methods for Liquid Limit Plastic Limit andPlasticity Index of Soils ASTM International West Conshohocken PA httpdxdoiorg101520D4318-05
ASTM D4884 2009 Standard Test Method for Strength of Strewn Thermally BondedSeams of Geotextiles ASTM International West Conshohocken PA httpdxdoiorg101520D4884e09
ASTM D6637 2001 Standard Test Methods for Determining Tensile Properties of Geogrid by Single or Multi-rib Tensile Methods ASTM International WestConshohocken PA httpdxdoiorg101520D6637e01R09
Black J Sivakumar V MeKinley JD 2007 Performance of clay samples reinforcedwith vertical granular columns Can Geotech J 44 (1) 89e95
Bowel JE 1988 Foundation Analysis and Design fourth ed McGraw-HillSingapore
Buckingham E 1914 On physically similar systems Phys Rev APS 4 345 e376Bush DI Jenner CG Bassett RH 1990 The design and construction of geocell
foundation mattresses supporting embankments over soft ground GeotextGeomembr 9 (1) 83e98
Carroll Jr RG Curtis VC 1990 Geogrid connections Geotext Geomembr 9 (4e6)515e530
Cimentada A Costa AD Izal JC Sagaseta C 2011 Laboratory study on radialconsolidation and deformation in clay reinforced with stone columns CanGeotech J 48 (1) 36e52
Christoulas S Bouckvalaas G Giannaros C 2000 An experimental study on themodel stone columns Soils Found 40 (6) 11e22
Chummer AV 1972 Bearing capacity theory from experimental results J SoilMech Found Div ASCE 98 (12) 1311e1324
Cowland JW Wong SCK 1993 Performance of a road embankment on soft claysupported on a geocell mattress foundation GeotextGeomembr12 (8) 687e705
Dash SK Bora MC 2013 2013 In1047298uence of geosynthetic encasement on theperformance of stone columns 1047298oating in soft clay Can Geotech J 50 (7) 754e
765Dash SK Krishnaswamy NR Rajagopal K 2001 Bearing capacity of strip footing
supported on geocell-reinforced sand Geotext Geomembr 19 (4) 235e256Dash SK Sireesh S Sitharam TG 2003a Model studies on circular footing
supported on geocell reinforced sand underlain by soft clay Geotext Geo-membr 21 (4) 197e219
Dash SK Sireesh S Sitharam TG 2003b Behaviour of geocell reinforced sandbeds under circular footing Ground Improv 7 (3) 111e115
Dash SK Rajagopal K Krishnaswamy NR 2004 Performance of different geo-synthetic reinforcement materials in sand foundations Geosynth Int 11 (1)35e42
Dash SK Rajagopal K Krishnaswamy NR 2007 Behaviour of geocell reinforcedsand beds under strip loading Can Geotech J 44 (7) 905e916
Dash SK Reddy PDT Raghukanth STG 2008 Subgrade modulus of geocell-reinforced sand foundations Ground Improv 12 (2) 79e87
Deb K Chandra S Basudhar PK 2007 Generalised model for geosynthetic-reinforced granular 1047297ll-soft soil with stone columns Int J Geomech ASCE 7(4) 266e276
Deb K Samadhiya NK Namdeo JB 2011 Laboratory model studies on unrein-forced and geogrid-reinforced sand bed over stone column-improved soft clayGeotext Geomembr 29 (2) 190e196
Fakher A Jones CJFP 1996 Discussion of bearing capacity of rectangular footingson geogrid reinforced sand by Yetimoglu T Wu JTH Saglamer AJ GeotechEng ASCE 122 (4) 326e327
Hughes JMO Withers NJ 1974 Reinforcing of soft cohesive soils with stonecolumns Ground Eng 7 (3) 42e49
Juran I Guermazi A 1988 Settlement response of soft soils reinforced by com-pacted sand columns J Geotech Eng ASCE 114 (8) 930
e942
Latha GM Somwanshi A 2009 Effect of reinforcement form on the bearing ca-pacity of square footings on sand Geotext Geomembr 27 (6) 409e422
Leshchinsky B Ling H 2013 Effects of geocell con1047297nement on strength anddeformation behaviour of gravel J Geotech Geoenviron Eng ASCE 139 (2)340e352
Madhavi GL Vidya SM 2007 Effects of reinforcement form on the behaviour of geosynthetic reinforced sand Geotext Geomembr 25 (1) 23e32
Malarvizhi Ilamparuthi 2007 Comparative study on the behaviour of encasedstone column and conventional stone column Soils Found 47 (5) 873e885
McKelvey D Sivakumar V Bell A Graham J 2004 Modeling vibrated Stonecolumns on soft clay Geotech Eng Proc Inst Civil Eng 157 (3) 137e149
Mayerhof GG Sastry VVRN 1978 Bearing capacity of piles in layered soils part2 Can Geotech J 15 (2) 183e189
Pinto MIM Cousens TW1999 Modelling a geotextile-reinforced brick-faced soilretaining wall Geosynth Int 6 (5) 417e447
Selig ET McKee KE 1961 Static and dynamic behaviour of small footings J Soil
Mech Found Div ASCE 87 (6) 29e
47Sireesh S Sitharam TG Dash SK 2009 Bearing capacity of circular footing ongeocell-sand mattress overlying clay bed with void Geotext Geomembr 27 (2)89e98
Tanyu BF Aydilek AH Lau AW Edil TB Benson CH 2013 Laboratory eval-uation of geocell-reinforced gravel subbase over poor subgrades Geosynth Int20 (2) 47e61
Webster SL Watkins JE 1977 Investigation of Construction Techniques forTactical Bridge Approach Roads across Soft Ground Technical Report S-77e1United States Army Corps of Engineers Waterways Experiment Station Mis-sissippi USA
Wood DM Hu W Nash DFT 2000 Group effects in stone column foundationsmodel tests Geotechnique 50 (6) 689e698
Zhou H Wen X 2008 Model studies on geogrid or geocell-reinforced sandcushion on soft soil Geotext Geomembr 26 (3) 231e238
SK Dash MC Bora Geotextiles and Geomembranes 41 (2013) 26 e 35 35
8212019 Articol d Citit
httpslidepdfcomreaderfullarticol-d-citit 810
33 In 1047298uence of height of geocell mattress
Typical responses of the composite system for three differentheights of geocell mattress (h frac14 053D 09D 16D) are shown in
Figs 8 9 13 14 15 and 16 respectively It could be observed that
when shallow in height (h frac14 053D) the geocell mattress has under
performed that the stone columns have shared nearly three times
more load (IFgcscIFgc frac14 3 Fig 8) However with increase in height
(h) the contribution of stone columns has reduced and that of
geocell mattress (IFgcscIFsc) has gone up When geocells are rela-
tively deep (h frac14 16D) the improvement factor ratio IFgcscIFgc is
just in the range of 1e12 (Fig 15) that the load carried by the stone
columns is at the most 20 that of the geocell mattress The data
presented in Fig 16 indeed shows such a response wherein the
value of improvement factor ratio IFgcscIFsc is as high as 65
indicating that most of the footing pressure has been sustained by
the geocell mattress A deep geocell mattress possesses highmoment of inertia that enables it to stand against the footing
penetration Besides with increase in height (h) the geocell area
deriving anchorage from the in1047297ll soil increases and so is the
anchorage resistance Therefore the geocell mattress takes a large
proportion of the footing loading on its own that the stone columns
underneath mostly remain dormant and thereby contribute less to
the performance improvement Visibly less bulging observed in the
post-test exhumed stone columns (Fig 17) establishes that they
indeed had under performed in sharing the surcharge loading
The improvement due to the geocell-stone column composite
reinforcements are summarised in Table 2 (Test series 5e12) It can
be seen that for height of geocell mattress h frac14 053D 09D 11D and
16D the maximum bearing capacity improvement IFgcsc frac14 569
747 942 and 102 respectively This highlights that the increase in
performance improvement with height of geocell mattress
increasing beyond 11D is relatively less A possible reason for this
could be the stress concentration induced local buckling and
yielding of geocells right under the footing that the global increasein strength and stiffness of the system due to increase in height of
the mattress remains immobilised Indeed the maximum differ-
ence in the values of the factor IFgcscIFsc (ie the load shared by the
geocell mattress) for the case with h frac14 16D and 11D was less than
5 Therefore height of geocell mattress equal to about the diam-
eter of the footing (h frac14 D) can be taken as the optimum one giving
maximum possible performance improvement in the composite
foundation beds However full-scale tests are required to verify this
observation
It is of interest to note that even geocell mattress of medium
height h frac14 09D when combined with stone columns can provide
bearing capacity improvement (IFgcsc frac14 746 Test series 6) as high
as that of a deep geocell mattress h frac14 16D (IFgcsc frac14 787 Test series
4) Due to practical constraints at times it might be dif 1047297cult to
accommodate a relatively large height geocell mattress In such
situation provision of stone columns in the underlying subgrade
would be a viable alternative to manage with geocell mattress of
relatively smaller height
4 Scale effect
Owing to reduced size model tests the results presented in this
paper are prone to scale effects Therefore further studies using
full-scale tests are required to verify these observations However
using a suitable scaling law the results from the present study can
be extrapolated to the prototype case (Fakher and Jones 1996)
The major physical parameters in1047298uencing the response of
geocell-stone column reinforced foundation systems can be
0 4 8 12 16 20 24 28
Footing settlement sD()
00
05
10
15
20
25
30
I m p r o v e m e n t
f c c t o r r a t i o I F g c s c I F g c
Clay+GC+SC(Ldsc= 1)
Clay+GC+SC(Ldsc= 3)
Clay+GC+SC(Ldsc= 5)
Clay+GC+SC(Ldsc= 7)
Fig 15 Improvement factor ratio-footing settlement responses contribution of stone
columns in composite foundation bed (hD frac14 16 S dsc frac14 25) e Test series 8
0 4 8 12 16 20 24 28
Footing settlement sD()
0
2
4
6
8
10
I m p r o v e m e n t f a c t o r r a t i o I F g c s c I F s c
Clay+GC+SC(Ldsc= 1)
Clay+GC+SC(Ldsc= 3)
Clay+GC+SC(Ldsc= 5)
Clay+GC+SC(Ldsc= 7)
Fig 16 Improvement factor ratio-footing settlement responses contribution of geo-
cell mattress in composite foundation bed (hD frac14
16 S dsc frac14
25) e
Test series 8
0
1
2
3
4
5
6
7
L e n g t h o f s t o n e c o l u m n L d s c
0 5 10 15 20 25 30
Radial strain (rd - ro)ro ()
Ldsc= 1
Ldsc= 3
Ldsc= 5
Ldsc= 7
Fig 17 Post-test deformed pro1047297le of central stone column in composite foundation
beds (hD frac14 16 S dsc frac14 25) e Test series 8
SK Dash MC Bora Geotextiles and Geomembranes 41 (2013) 26 e 35 33
8212019 Articol d Citit
httpslidepdfcomreaderfullarticol-d-citit 910
summarised as D h dgc K dsc L d50 S ɸ c u g G where ɸ is the
angle of internal friction of soil and stone aggregate K is the stiff-
ness of geocell reinforcement g is the unit weight of soil and
aggregate G is the shear modulus of the soil and aggregate Other
parameters have been de1047297ned previously The function ( f ) that
governs the composite foundation system can be written as
f
D h dgc K dsc L d50 S c ug Gf s qgcsc qu
frac14 0 (2)
It comprises of 15 parameters and has two fundamental di-
mensions (ie length and force) and therefore can be studied by 13
independent parameters (p1 p2 p3 p4p13 Buckingham
1914) Hence equation (2) can be written as
For a prototype footing ( p) with diameter n times that of the
model (m)
Dp
Dmfrac14 n (4)
For similarity to be maintained the p terms both for model and
prototype need to be equal and therefore considering the p9 term
Gp
Dpgpfrac14
Gm
Dmgm(5)
Assuming that the soils used in the model and prototype do
have same unit weight (Pinto and Cousens 1999) equation (5)
reduces to
Gp
Gmfrac14 Dp
Dmfrac14 n (6)
Considering similarity of the p10 terms
K pgp
G2p
frac14 K mgm
G2m
K p
G2p
frac14 K mG2
m
K pK m
frac14G2
p
G2m
frac14 n2 (7)
As can be seen the strength of prototype geocells should be of n2
times that of the model geocell where n is the scale factor The
geocells used in the present tests have tensile strength of 475 kN
m Therefore the results from the present study to be applicable in
practice the prototype geocells should have tensile strength of
475n2 kNm However the geometric parameters such as pocket
size and height of geocells length diameter and spacing of stone
columns etc have shown a linear variation with the footing size D
5 Conclusions
Review of literature shows that both geocell-sand mattress and
stone columns are effective means of reinforcing the weak soils
Their individual applications though have been intensely studied
by many researchers but combined application of both has
remained unexplored The experimental results obtained in the
present study con1047297rm that such composite reinforcement is an
added advantage over the conventional ones ie stone column or
geocell mattress With provision of stone columns the bearing
capacity of soft clay beds can be increased by 37 fold and with
geocell reinforcement it is of the order of 78 fold When coupled
together ie stone column-geocell mattress combined the bearing
capacity was increased by 102 fold Additionally visible reduction
in slope of pressure settlement responses indicates that the stone
column-geocell composite reinforcement can increase the stiffness
of the foundation bed signi1047297cantly leading to large scale reduction
in footing settlement
The load carrying capacity of the geocell-stone column rein-
forced foundation bed increases with increase in length of stone
columns until 5dsc beyond which further rate of improvement has
reduced down Similarly reducing the spacing of stone columns
below 25dsc does notattract much of additional performance in the
composite system Besides with height of geocells increasing
beyond 11D the performance improvement is found to have
reduced This is possibly due to the stress concentration induced
buckling and yielding of geocells right under the footing that the
increase in strength and stiffness of the system due to increase in
height of the mattress remains immobilised Hence it can be said
that the critical height of geocell mattress giving optimum per-
formance improvement in the composite foundation bed is equal
to about the diameter of the footing (D)
At times practical constraints may prevent in going for large
height geocell mattress or long stone columns severely compro-
mising the performance of the system In such situations the
geocell-stone column composite reinforcement provides an effec-
tive solution for adequate performance improvement and optimum
design of foundations on soft clay This is inferred from the present
study that a shallow height geocell mattress along with medium
length stone columns can provide comparable performance im-
provements as that with deep geocells or long stone columnsHowever full-scale testing and 1047297eld trails are required to verify
these observations
The 1047297ndings of the present study can be of use in the design and
construction of structures over soft clay deposits such as railways
highways foundations for liquid storage tanks large stabilised
areas for parking platforms for oil exploration etc The authors
have also conducted tests with basal geogrid underneath the geo-
cell mattress and the results shall be reported in a subsequent
paper
Acknowledgement
The authors are thankful to the anonymous reviewers for their
valuable comments and suggestions for improvements of the pre-
sentations in the paper
Notation
C c coef 1047297cient of curvature
C u coef 1047297cient of uniformity
D diameter of footing
dgc diameter of geocells
dsc diameter of stone column
emax maximum void ratio
emin minimum void ratio
h height of geocell mattress
g ethp1p2p3p4p13THORN frac14 g
s
D
h
D
dgc
D
h
dgc
dsc
D
L
D
S
D
d50
dsc
G
Dg
K g
G2
c uDg
qgcsc
qu
f
frac14 0 (3)
SK Dash MC Bora Geotextiles and Geomembranes 41 (2013) 26 e 3534
8212019 Articol d Citit
httpslidepdfcomreaderfullarticol-d-citit 1010
IFsc bearing capacity improvement factor due to stone column
reinforcement
IFgc bearing capacity improvement factor due to geocell
reinforcement
IFgcsc bearing capacity improvement factor due to stone
column-geocell composite reinforcement
L length of stone column
qsc bearing pressure with stone column reinforcement
qgc bearing pressure with geocell reinforcement
qgcsc bearing pressure with stone column-geocell composite
reinforcement
r d deformed radius of stone column
r o original radius of stone column
S spacing of stone columns
s settlement of footing
u depth of placement of geocell mattress
References
Abdullah CH Edil TB 2007 Behaviour of geogrid-reinforced load transfer plat-forms for embankment on rammed aggregate piers Geosynth Int 14 (3) 141e153
Ambily AP Gandhi SR 2007 Behaviour of stone columns based on experimental
and FEM analysis J Geotech Geoenviron Eng ASCE 133 (4) 405e
515Arulrajah A Abdullah A Bouzza A 2009 Ground improvement technique for
railway embankments Ground Improv 162 (1) 3e14ASTM D2487 2006 Standard Practice for Classi1047297cation of Soils for Engineering
Purposes (Uni1047297ed Soil Classi1047297cation System) ASTM International West Con-shohocken PA httpdxdoiorg101520D2487-06E01
ASTM D4318 2005 Standard Test Methods for Liquid Limit Plastic Limit andPlasticity Index of Soils ASTM International West Conshohocken PA httpdxdoiorg101520D4318-05
ASTM D4884 2009 Standard Test Method for Strength of Strewn Thermally BondedSeams of Geotextiles ASTM International West Conshohocken PA httpdxdoiorg101520D4884e09
ASTM D6637 2001 Standard Test Methods for Determining Tensile Properties of Geogrid by Single or Multi-rib Tensile Methods ASTM International WestConshohocken PA httpdxdoiorg101520D6637e01R09
Black J Sivakumar V MeKinley JD 2007 Performance of clay samples reinforcedwith vertical granular columns Can Geotech J 44 (1) 89e95
Bowel JE 1988 Foundation Analysis and Design fourth ed McGraw-HillSingapore
Buckingham E 1914 On physically similar systems Phys Rev APS 4 345 e376Bush DI Jenner CG Bassett RH 1990 The design and construction of geocell
foundation mattresses supporting embankments over soft ground GeotextGeomembr 9 (1) 83e98
Carroll Jr RG Curtis VC 1990 Geogrid connections Geotext Geomembr 9 (4e6)515e530
Cimentada A Costa AD Izal JC Sagaseta C 2011 Laboratory study on radialconsolidation and deformation in clay reinforced with stone columns CanGeotech J 48 (1) 36e52
Christoulas S Bouckvalaas G Giannaros C 2000 An experimental study on themodel stone columns Soils Found 40 (6) 11e22
Chummer AV 1972 Bearing capacity theory from experimental results J SoilMech Found Div ASCE 98 (12) 1311e1324
Cowland JW Wong SCK 1993 Performance of a road embankment on soft claysupported on a geocell mattress foundation GeotextGeomembr12 (8) 687e705
Dash SK Bora MC 2013 2013 In1047298uence of geosynthetic encasement on theperformance of stone columns 1047298oating in soft clay Can Geotech J 50 (7) 754e
765Dash SK Krishnaswamy NR Rajagopal K 2001 Bearing capacity of strip footing
supported on geocell-reinforced sand Geotext Geomembr 19 (4) 235e256Dash SK Sireesh S Sitharam TG 2003a Model studies on circular footing
supported on geocell reinforced sand underlain by soft clay Geotext Geo-membr 21 (4) 197e219
Dash SK Sireesh S Sitharam TG 2003b Behaviour of geocell reinforced sandbeds under circular footing Ground Improv 7 (3) 111e115
Dash SK Rajagopal K Krishnaswamy NR 2004 Performance of different geo-synthetic reinforcement materials in sand foundations Geosynth Int 11 (1)35e42
Dash SK Rajagopal K Krishnaswamy NR 2007 Behaviour of geocell reinforcedsand beds under strip loading Can Geotech J 44 (7) 905e916
Dash SK Reddy PDT Raghukanth STG 2008 Subgrade modulus of geocell-reinforced sand foundations Ground Improv 12 (2) 79e87
Deb K Chandra S Basudhar PK 2007 Generalised model for geosynthetic-reinforced granular 1047297ll-soft soil with stone columns Int J Geomech ASCE 7(4) 266e276
Deb K Samadhiya NK Namdeo JB 2011 Laboratory model studies on unrein-forced and geogrid-reinforced sand bed over stone column-improved soft clayGeotext Geomembr 29 (2) 190e196
Fakher A Jones CJFP 1996 Discussion of bearing capacity of rectangular footingson geogrid reinforced sand by Yetimoglu T Wu JTH Saglamer AJ GeotechEng ASCE 122 (4) 326e327
Hughes JMO Withers NJ 1974 Reinforcing of soft cohesive soils with stonecolumns Ground Eng 7 (3) 42e49
Juran I Guermazi A 1988 Settlement response of soft soils reinforced by com-pacted sand columns J Geotech Eng ASCE 114 (8) 930
e942
Latha GM Somwanshi A 2009 Effect of reinforcement form on the bearing ca-pacity of square footings on sand Geotext Geomembr 27 (6) 409e422
Leshchinsky B Ling H 2013 Effects of geocell con1047297nement on strength anddeformation behaviour of gravel J Geotech Geoenviron Eng ASCE 139 (2)340e352
Madhavi GL Vidya SM 2007 Effects of reinforcement form on the behaviour of geosynthetic reinforced sand Geotext Geomembr 25 (1) 23e32
Malarvizhi Ilamparuthi 2007 Comparative study on the behaviour of encasedstone column and conventional stone column Soils Found 47 (5) 873e885
McKelvey D Sivakumar V Bell A Graham J 2004 Modeling vibrated Stonecolumns on soft clay Geotech Eng Proc Inst Civil Eng 157 (3) 137e149
Mayerhof GG Sastry VVRN 1978 Bearing capacity of piles in layered soils part2 Can Geotech J 15 (2) 183e189
Pinto MIM Cousens TW1999 Modelling a geotextile-reinforced brick-faced soilretaining wall Geosynth Int 6 (5) 417e447
Selig ET McKee KE 1961 Static and dynamic behaviour of small footings J Soil
Mech Found Div ASCE 87 (6) 29e
47Sireesh S Sitharam TG Dash SK 2009 Bearing capacity of circular footing ongeocell-sand mattress overlying clay bed with void Geotext Geomembr 27 (2)89e98
Tanyu BF Aydilek AH Lau AW Edil TB Benson CH 2013 Laboratory eval-uation of geocell-reinforced gravel subbase over poor subgrades Geosynth Int20 (2) 47e61
Webster SL Watkins JE 1977 Investigation of Construction Techniques forTactical Bridge Approach Roads across Soft Ground Technical Report S-77e1United States Army Corps of Engineers Waterways Experiment Station Mis-sissippi USA
Wood DM Hu W Nash DFT 2000 Group effects in stone column foundationsmodel tests Geotechnique 50 (6) 689e698
Zhou H Wen X 2008 Model studies on geogrid or geocell-reinforced sandcushion on soft soil Geotext Geomembr 26 (3) 231e238
SK Dash MC Bora Geotextiles and Geomembranes 41 (2013) 26 e 35 35
8212019 Articol d Citit
httpslidepdfcomreaderfullarticol-d-citit 910
summarised as D h dgc K dsc L d50 S ɸ c u g G where ɸ is the
angle of internal friction of soil and stone aggregate K is the stiff-
ness of geocell reinforcement g is the unit weight of soil and
aggregate G is the shear modulus of the soil and aggregate Other
parameters have been de1047297ned previously The function ( f ) that
governs the composite foundation system can be written as
f
D h dgc K dsc L d50 S c ug Gf s qgcsc qu
frac14 0 (2)
It comprises of 15 parameters and has two fundamental di-
mensions (ie length and force) and therefore can be studied by 13
independent parameters (p1 p2 p3 p4p13 Buckingham
1914) Hence equation (2) can be written as
For a prototype footing ( p) with diameter n times that of the
model (m)
Dp
Dmfrac14 n (4)
For similarity to be maintained the p terms both for model and
prototype need to be equal and therefore considering the p9 term
Gp
Dpgpfrac14
Gm
Dmgm(5)
Assuming that the soils used in the model and prototype do
have same unit weight (Pinto and Cousens 1999) equation (5)
reduces to
Gp
Gmfrac14 Dp
Dmfrac14 n (6)
Considering similarity of the p10 terms
K pgp
G2p
frac14 K mgm
G2m
K p
G2p
frac14 K mG2
m
K pK m
frac14G2
p
G2m
frac14 n2 (7)
As can be seen the strength of prototype geocells should be of n2
times that of the model geocell where n is the scale factor The
geocells used in the present tests have tensile strength of 475 kN
m Therefore the results from the present study to be applicable in
practice the prototype geocells should have tensile strength of
475n2 kNm However the geometric parameters such as pocket
size and height of geocells length diameter and spacing of stone
columns etc have shown a linear variation with the footing size D
5 Conclusions
Review of literature shows that both geocell-sand mattress and
stone columns are effective means of reinforcing the weak soils
Their individual applications though have been intensely studied
by many researchers but combined application of both has
remained unexplored The experimental results obtained in the
present study con1047297rm that such composite reinforcement is an
added advantage over the conventional ones ie stone column or
geocell mattress With provision of stone columns the bearing
capacity of soft clay beds can be increased by 37 fold and with
geocell reinforcement it is of the order of 78 fold When coupled
together ie stone column-geocell mattress combined the bearing
capacity was increased by 102 fold Additionally visible reduction
in slope of pressure settlement responses indicates that the stone
column-geocell composite reinforcement can increase the stiffness
of the foundation bed signi1047297cantly leading to large scale reduction
in footing settlement
The load carrying capacity of the geocell-stone column rein-
forced foundation bed increases with increase in length of stone
columns until 5dsc beyond which further rate of improvement has
reduced down Similarly reducing the spacing of stone columns
below 25dsc does notattract much of additional performance in the
composite system Besides with height of geocells increasing
beyond 11D the performance improvement is found to have
reduced This is possibly due to the stress concentration induced
buckling and yielding of geocells right under the footing that the
increase in strength and stiffness of the system due to increase in
height of the mattress remains immobilised Hence it can be said
that the critical height of geocell mattress giving optimum per-
formance improvement in the composite foundation bed is equal
to about the diameter of the footing (D)
At times practical constraints may prevent in going for large
height geocell mattress or long stone columns severely compro-
mising the performance of the system In such situations the
geocell-stone column composite reinforcement provides an effec-
tive solution for adequate performance improvement and optimum
design of foundations on soft clay This is inferred from the present
study that a shallow height geocell mattress along with medium
length stone columns can provide comparable performance im-
provements as that with deep geocells or long stone columnsHowever full-scale testing and 1047297eld trails are required to verify
these observations
The 1047297ndings of the present study can be of use in the design and
construction of structures over soft clay deposits such as railways
highways foundations for liquid storage tanks large stabilised
areas for parking platforms for oil exploration etc The authors
have also conducted tests with basal geogrid underneath the geo-
cell mattress and the results shall be reported in a subsequent
paper
Acknowledgement
The authors are thankful to the anonymous reviewers for their
valuable comments and suggestions for improvements of the pre-
sentations in the paper
Notation
C c coef 1047297cient of curvature
C u coef 1047297cient of uniformity
D diameter of footing
dgc diameter of geocells
dsc diameter of stone column
emax maximum void ratio
emin minimum void ratio
h height of geocell mattress
g ethp1p2p3p4p13THORN frac14 g
s
D
h
D
dgc
D
h
dgc
dsc
D
L
D
S
D
d50
dsc
G
Dg
K g
G2
c uDg
qgcsc
qu
f
frac14 0 (3)
SK Dash MC Bora Geotextiles and Geomembranes 41 (2013) 26 e 3534
8212019 Articol d Citit
httpslidepdfcomreaderfullarticol-d-citit 1010
IFsc bearing capacity improvement factor due to stone column
reinforcement
IFgc bearing capacity improvement factor due to geocell
reinforcement
IFgcsc bearing capacity improvement factor due to stone
column-geocell composite reinforcement
L length of stone column
qsc bearing pressure with stone column reinforcement
qgc bearing pressure with geocell reinforcement
qgcsc bearing pressure with stone column-geocell composite
reinforcement
r d deformed radius of stone column
r o original radius of stone column
S spacing of stone columns
s settlement of footing
u depth of placement of geocell mattress
References
Abdullah CH Edil TB 2007 Behaviour of geogrid-reinforced load transfer plat-forms for embankment on rammed aggregate piers Geosynth Int 14 (3) 141e153
Ambily AP Gandhi SR 2007 Behaviour of stone columns based on experimental
and FEM analysis J Geotech Geoenviron Eng ASCE 133 (4) 405e
515Arulrajah A Abdullah A Bouzza A 2009 Ground improvement technique for
railway embankments Ground Improv 162 (1) 3e14ASTM D2487 2006 Standard Practice for Classi1047297cation of Soils for Engineering
Purposes (Uni1047297ed Soil Classi1047297cation System) ASTM International West Con-shohocken PA httpdxdoiorg101520D2487-06E01
ASTM D4318 2005 Standard Test Methods for Liquid Limit Plastic Limit andPlasticity Index of Soils ASTM International West Conshohocken PA httpdxdoiorg101520D4318-05
ASTM D4884 2009 Standard Test Method for Strength of Strewn Thermally BondedSeams of Geotextiles ASTM International West Conshohocken PA httpdxdoiorg101520D4884e09
ASTM D6637 2001 Standard Test Methods for Determining Tensile Properties of Geogrid by Single or Multi-rib Tensile Methods ASTM International WestConshohocken PA httpdxdoiorg101520D6637e01R09
Black J Sivakumar V MeKinley JD 2007 Performance of clay samples reinforcedwith vertical granular columns Can Geotech J 44 (1) 89e95
Bowel JE 1988 Foundation Analysis and Design fourth ed McGraw-HillSingapore
Buckingham E 1914 On physically similar systems Phys Rev APS 4 345 e376Bush DI Jenner CG Bassett RH 1990 The design and construction of geocell
foundation mattresses supporting embankments over soft ground GeotextGeomembr 9 (1) 83e98
Carroll Jr RG Curtis VC 1990 Geogrid connections Geotext Geomembr 9 (4e6)515e530
Cimentada A Costa AD Izal JC Sagaseta C 2011 Laboratory study on radialconsolidation and deformation in clay reinforced with stone columns CanGeotech J 48 (1) 36e52
Christoulas S Bouckvalaas G Giannaros C 2000 An experimental study on themodel stone columns Soils Found 40 (6) 11e22
Chummer AV 1972 Bearing capacity theory from experimental results J SoilMech Found Div ASCE 98 (12) 1311e1324
Cowland JW Wong SCK 1993 Performance of a road embankment on soft claysupported on a geocell mattress foundation GeotextGeomembr12 (8) 687e705
Dash SK Bora MC 2013 2013 In1047298uence of geosynthetic encasement on theperformance of stone columns 1047298oating in soft clay Can Geotech J 50 (7) 754e
765Dash SK Krishnaswamy NR Rajagopal K 2001 Bearing capacity of strip footing
supported on geocell-reinforced sand Geotext Geomembr 19 (4) 235e256Dash SK Sireesh S Sitharam TG 2003a Model studies on circular footing
supported on geocell reinforced sand underlain by soft clay Geotext Geo-membr 21 (4) 197e219
Dash SK Sireesh S Sitharam TG 2003b Behaviour of geocell reinforced sandbeds under circular footing Ground Improv 7 (3) 111e115
Dash SK Rajagopal K Krishnaswamy NR 2004 Performance of different geo-synthetic reinforcement materials in sand foundations Geosynth Int 11 (1)35e42
Dash SK Rajagopal K Krishnaswamy NR 2007 Behaviour of geocell reinforcedsand beds under strip loading Can Geotech J 44 (7) 905e916
Dash SK Reddy PDT Raghukanth STG 2008 Subgrade modulus of geocell-reinforced sand foundations Ground Improv 12 (2) 79e87
Deb K Chandra S Basudhar PK 2007 Generalised model for geosynthetic-reinforced granular 1047297ll-soft soil with stone columns Int J Geomech ASCE 7(4) 266e276
Deb K Samadhiya NK Namdeo JB 2011 Laboratory model studies on unrein-forced and geogrid-reinforced sand bed over stone column-improved soft clayGeotext Geomembr 29 (2) 190e196
Fakher A Jones CJFP 1996 Discussion of bearing capacity of rectangular footingson geogrid reinforced sand by Yetimoglu T Wu JTH Saglamer AJ GeotechEng ASCE 122 (4) 326e327
Hughes JMO Withers NJ 1974 Reinforcing of soft cohesive soils with stonecolumns Ground Eng 7 (3) 42e49
Juran I Guermazi A 1988 Settlement response of soft soils reinforced by com-pacted sand columns J Geotech Eng ASCE 114 (8) 930
e942
Latha GM Somwanshi A 2009 Effect of reinforcement form on the bearing ca-pacity of square footings on sand Geotext Geomembr 27 (6) 409e422
Leshchinsky B Ling H 2013 Effects of geocell con1047297nement on strength anddeformation behaviour of gravel J Geotech Geoenviron Eng ASCE 139 (2)340e352
Madhavi GL Vidya SM 2007 Effects of reinforcement form on the behaviour of geosynthetic reinforced sand Geotext Geomembr 25 (1) 23e32
Malarvizhi Ilamparuthi 2007 Comparative study on the behaviour of encasedstone column and conventional stone column Soils Found 47 (5) 873e885
McKelvey D Sivakumar V Bell A Graham J 2004 Modeling vibrated Stonecolumns on soft clay Geotech Eng Proc Inst Civil Eng 157 (3) 137e149
Mayerhof GG Sastry VVRN 1978 Bearing capacity of piles in layered soils part2 Can Geotech J 15 (2) 183e189
Pinto MIM Cousens TW1999 Modelling a geotextile-reinforced brick-faced soilretaining wall Geosynth Int 6 (5) 417e447
Selig ET McKee KE 1961 Static and dynamic behaviour of small footings J Soil
Mech Found Div ASCE 87 (6) 29e
47Sireesh S Sitharam TG Dash SK 2009 Bearing capacity of circular footing ongeocell-sand mattress overlying clay bed with void Geotext Geomembr 27 (2)89e98
Tanyu BF Aydilek AH Lau AW Edil TB Benson CH 2013 Laboratory eval-uation of geocell-reinforced gravel subbase over poor subgrades Geosynth Int20 (2) 47e61
Webster SL Watkins JE 1977 Investigation of Construction Techniques forTactical Bridge Approach Roads across Soft Ground Technical Report S-77e1United States Army Corps of Engineers Waterways Experiment Station Mis-sissippi USA
Wood DM Hu W Nash DFT 2000 Group effects in stone column foundationsmodel tests Geotechnique 50 (6) 689e698
Zhou H Wen X 2008 Model studies on geogrid or geocell-reinforced sandcushion on soft soil Geotext Geomembr 26 (3) 231e238
SK Dash MC Bora Geotextiles and Geomembranes 41 (2013) 26 e 35 35
8212019 Articol d Citit
httpslidepdfcomreaderfullarticol-d-citit 1010
IFsc bearing capacity improvement factor due to stone column
reinforcement
IFgc bearing capacity improvement factor due to geocell
reinforcement
IFgcsc bearing capacity improvement factor due to stone
column-geocell composite reinforcement
L length of stone column
qsc bearing pressure with stone column reinforcement
qgc bearing pressure with geocell reinforcement
qgcsc bearing pressure with stone column-geocell composite
reinforcement
r d deformed radius of stone column
r o original radius of stone column
S spacing of stone columns
s settlement of footing
u depth of placement of geocell mattress
References
Abdullah CH Edil TB 2007 Behaviour of geogrid-reinforced load transfer plat-forms for embankment on rammed aggregate piers Geosynth Int 14 (3) 141e153
Ambily AP Gandhi SR 2007 Behaviour of stone columns based on experimental
and FEM analysis J Geotech Geoenviron Eng ASCE 133 (4) 405e
515Arulrajah A Abdullah A Bouzza A 2009 Ground improvement technique for
railway embankments Ground Improv 162 (1) 3e14ASTM D2487 2006 Standard Practice for Classi1047297cation of Soils for Engineering
Purposes (Uni1047297ed Soil Classi1047297cation System) ASTM International West Con-shohocken PA httpdxdoiorg101520D2487-06E01
ASTM D4318 2005 Standard Test Methods for Liquid Limit Plastic Limit andPlasticity Index of Soils ASTM International West Conshohocken PA httpdxdoiorg101520D4318-05
ASTM D4884 2009 Standard Test Method for Strength of Strewn Thermally BondedSeams of Geotextiles ASTM International West Conshohocken PA httpdxdoiorg101520D4884e09
ASTM D6637 2001 Standard Test Methods for Determining Tensile Properties of Geogrid by Single or Multi-rib Tensile Methods ASTM International WestConshohocken PA httpdxdoiorg101520D6637e01R09
Black J Sivakumar V MeKinley JD 2007 Performance of clay samples reinforcedwith vertical granular columns Can Geotech J 44 (1) 89e95
Bowel JE 1988 Foundation Analysis and Design fourth ed McGraw-HillSingapore
Buckingham E 1914 On physically similar systems Phys Rev APS 4 345 e376Bush DI Jenner CG Bassett RH 1990 The design and construction of geocell
foundation mattresses supporting embankments over soft ground GeotextGeomembr 9 (1) 83e98
Carroll Jr RG Curtis VC 1990 Geogrid connections Geotext Geomembr 9 (4e6)515e530
Cimentada A Costa AD Izal JC Sagaseta C 2011 Laboratory study on radialconsolidation and deformation in clay reinforced with stone columns CanGeotech J 48 (1) 36e52
Christoulas S Bouckvalaas G Giannaros C 2000 An experimental study on themodel stone columns Soils Found 40 (6) 11e22
Chummer AV 1972 Bearing capacity theory from experimental results J SoilMech Found Div ASCE 98 (12) 1311e1324
Cowland JW Wong SCK 1993 Performance of a road embankment on soft claysupported on a geocell mattress foundation GeotextGeomembr12 (8) 687e705
Dash SK Bora MC 2013 2013 In1047298uence of geosynthetic encasement on theperformance of stone columns 1047298oating in soft clay Can Geotech J 50 (7) 754e
765Dash SK Krishnaswamy NR Rajagopal K 2001 Bearing capacity of strip footing
supported on geocell-reinforced sand Geotext Geomembr 19 (4) 235e256Dash SK Sireesh S Sitharam TG 2003a Model studies on circular footing
supported on geocell reinforced sand underlain by soft clay Geotext Geo-membr 21 (4) 197e219
Dash SK Sireesh S Sitharam TG 2003b Behaviour of geocell reinforced sandbeds under circular footing Ground Improv 7 (3) 111e115
Dash SK Rajagopal K Krishnaswamy NR 2004 Performance of different geo-synthetic reinforcement materials in sand foundations Geosynth Int 11 (1)35e42
Dash SK Rajagopal K Krishnaswamy NR 2007 Behaviour of geocell reinforcedsand beds under strip loading Can Geotech J 44 (7) 905e916
Dash SK Reddy PDT Raghukanth STG 2008 Subgrade modulus of geocell-reinforced sand foundations Ground Improv 12 (2) 79e87
Deb K Chandra S Basudhar PK 2007 Generalised model for geosynthetic-reinforced granular 1047297ll-soft soil with stone columns Int J Geomech ASCE 7(4) 266e276
Deb K Samadhiya NK Namdeo JB 2011 Laboratory model studies on unrein-forced and geogrid-reinforced sand bed over stone column-improved soft clayGeotext Geomembr 29 (2) 190e196
Fakher A Jones CJFP 1996 Discussion of bearing capacity of rectangular footingson geogrid reinforced sand by Yetimoglu T Wu JTH Saglamer AJ GeotechEng ASCE 122 (4) 326e327
Hughes JMO Withers NJ 1974 Reinforcing of soft cohesive soils with stonecolumns Ground Eng 7 (3) 42e49
Juran I Guermazi A 1988 Settlement response of soft soils reinforced by com-pacted sand columns J Geotech Eng ASCE 114 (8) 930
e942
Latha GM Somwanshi A 2009 Effect of reinforcement form on the bearing ca-pacity of square footings on sand Geotext Geomembr 27 (6) 409e422
Leshchinsky B Ling H 2013 Effects of geocell con1047297nement on strength anddeformation behaviour of gravel J Geotech Geoenviron Eng ASCE 139 (2)340e352
Madhavi GL Vidya SM 2007 Effects of reinforcement form on the behaviour of geosynthetic reinforced sand Geotext Geomembr 25 (1) 23e32
Malarvizhi Ilamparuthi 2007 Comparative study on the behaviour of encasedstone column and conventional stone column Soils Found 47 (5) 873e885
McKelvey D Sivakumar V Bell A Graham J 2004 Modeling vibrated Stonecolumns on soft clay Geotech Eng Proc Inst Civil Eng 157 (3) 137e149
Mayerhof GG Sastry VVRN 1978 Bearing capacity of piles in layered soils part2 Can Geotech J 15 (2) 183e189
Pinto MIM Cousens TW1999 Modelling a geotextile-reinforced brick-faced soilretaining wall Geosynth Int 6 (5) 417e447
Selig ET McKee KE 1961 Static and dynamic behaviour of small footings J Soil
Mech Found Div ASCE 87 (6) 29e
47Sireesh S Sitharam TG Dash SK 2009 Bearing capacity of circular footing ongeocell-sand mattress overlying clay bed with void Geotext Geomembr 27 (2)89e98
Tanyu BF Aydilek AH Lau AW Edil TB Benson CH 2013 Laboratory eval-uation of geocell-reinforced gravel subbase over poor subgrades Geosynth Int20 (2) 47e61
Webster SL Watkins JE 1977 Investigation of Construction Techniques forTactical Bridge Approach Roads across Soft Ground Technical Report S-77e1United States Army Corps of Engineers Waterways Experiment Station Mis-sissippi USA
Wood DM Hu W Nash DFT 2000 Group effects in stone column foundationsmodel tests Geotechnique 50 (6) 689e698
Zhou H Wen X 2008 Model studies on geogrid or geocell-reinforced sandcushion on soft soil Geotext Geomembr 26 (3) 231e238
SK Dash MC Bora Geotextiles and Geomembranes 41 (2013) 26 e 35 35