20 - OEC, NB, RC - Articol Bridges

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“Highway and Bridge Engineering 2014”, International SymposiumIaşi, România, December 6th, 2014

Geotechnical investigations and solutions for the stability of aroad built on a sloping ground

Oana Elena Colț1, Nicolae Boțu1, Răzvan Chirilă1 1 Department of Transportation Infrastructure and Foundations, Gheorghe Asachi Technical University,

Ia și, 700050, Romania

Summary

This article presents the investigation works made in order to find efficient

solutions for the consolidation of an instable slope located near Cluj Napoca,

where a secondary road will be built. For the evaluation of the local and general

stability of the emplacement, after a local evaluation and observation of the site, it

was decided to perform field and laboratory investigations. In order to determine

all the characteristics needed, a piezometer and inclinometer monitoring

programme of the slope was introduced. After processing all the results, the

parameters that characterize the evolution of active landslides that affected some

sections of the road, were established. The necessary consolidation solutions

needed to control the areas affected by instability, for the maximum safety of the

road, are presented in the end of this article.

KEYWORDS: geotechnical investigations, instability, consolidation solutions, piezometer, inclinometer

1. INTRODUCTION

Geotechnical investigation programme of the land usually includes field

investigations, laboratory work and monitoring activities. According to EN ISO22475-1 July 2008 there are three types of sampling methods by which certainclasses of samples of soil quality can be obtained, as follows:

- Category A by which samples of quality classes 1-5 can be obtained;

- Category B by which samples of quality classes 3-5 can be obtained;- Category C by which samples of quality class 5 can be obtained.

Table 1. Quality classes of ground samples and sampling categories

Quality classes of ground samples 1 2 3 4 5

Sampling categories

A

B

C



2 O.Colț, N. Boțu, R. Chirilă

Laboratory test programme has to be be performed in accordance to the proposedobjective namely stability analyzed on potential sliding areas and dimensioning

consolidation works, where necessary.

In this case, achieving shear strength parameters will be performed on all the

ground layers that are encountered, and punctually in the possible sliding surfaces,zones with reduced consistency of the ground samples taken from the stable layer.

The article analyzes a site located in Cluj - Romania where is to be achieved a bypass road. In this area the route crosses a slope, which in some areas shows

potential slide phenomenon.

Figure 1. Site localization

The analyzed site was affected by landslides in the past and currently has a wavy

appearance. In 2007, in the southern part of the Polus complex, a landslideoccurred, caused by the excavation work done at the base of the slope for building

a retaining wall and the absence of an effective groundwater drainage works. Inorder to solve the situation were executed earthworks and two terraces wereimplemented, whose configurations were adopted to ensure the stability of theslope. Currently, geometrically, the slope consists of 4 terraces bounded by slopeswith gradients of approx. 1: 1.



“Highway and Bridge Engineering 2014” International Symposium 3

2. GEOTECHNICAL GROUND INVESTIGATION

From a geomorphological point of view, the site belongs to Transylvanian

Depression unit. The given Neozoic age of the Transylvanian Depression made thestructure of the area to correspond to two different structural levels: one forming

the foundation, and another representing the depression filling.Many field visits were initially made to establish the site situation in the initial

phase. Distinctive elements were identified on the ground, both natural andanthropogenic, by which three areas with potential future road damage weredefined.

The first area between the kilometer positions 0+450 and 1+150 extends on a slope

with a maximum gradient of approx. 60º, transversal direction on road route. Thisarea presents fractures, ruptures, ground waving and slightly opposed surfaces.

Figure 2. Slope appearance

Slope area between the kilometer positions 1+700 and 2+000 was affected in 2010

by an important phenomenon of instability, due to the construction of a subsidizedhousing. To achieve these properties, excavation works are required and site accessroads, too, resulting in the excavation slopes with heights up to 2.0 m.

In the area of the complex of buildings, landslide phenomena were identified on theentire slope surface in the form of detachment fronts of unstable land mass.Currently, the slope seems to be in a state of equilibrium, but it is still an obvious

presence of a high risk of reactivation of landslide phenomena, due to the terrainconfiguration that allows deep infiltration of the surface water.




Figure 3. Slope appearance

The third area where we identified distinct elements of landslide phenomena is between the kilometer positions 2+000 and 2+200. Direct observations were performed directly on site, making some research on the situation of land during

2007 - 2010, when were reported large landslide phenomenon.

Based on the norms governing geotechnical documentation, a plan was establishedto investigate the site, through geotechnical drilling, laboratory tests andgeotechnical monitoring programme.

Therefore, a total of 18 geotechnical drillings were made with disturbed andundisturbed sampling, with maximum quality class 2 (according to the Table 1). Atotal of six boreholes were equipped with tubing in order to undertake specific

inclinometer measurements. Drillings were marked by F01 ÷ F18 (Figure 3) andwere between 10.0m and 36.0m depths.

Table 2. Location of prospecting and monitoring works

Drilling

code

Position Drilling

depth

Inclinometer

monitoring

Absolute

rate

STERERO`70

Absolute coordinates

[m] [m] X Y

F01 km1+736

21.50 no +386.31 387628.641 584046.012

F02 km1+736

23.00 no +378.83 387614.689 584083.500

F03 km2+000

21.00 no +370.00 387878.439 584124.458

F04 km2+090

17.50 no +372.00 387962.619 584153.170

F05 km2+000

15.00 yes +376.00 387883.787 584108.016




F06 km2+200

15.00 yes +375.00 388064.932 584183.824

F07 km1+125

15.00 yes +408.87 387081.404 583772.258

F08 km1+125

10.00 no +404.18 387074.144 583796.131

F09 km2+090 36.00 yes +375.96 387969.283 584137.499

F10 km2+150

14.00 no +382.00 388022.061 584145.986

F11 km2+045

10.00 no +382.00 387932.385 584096.007

F12 km1+450

10.00 no +398.50 387373.352 583917.830

F13 km1+300

10.00 no +408.00 387252.765 583825.916

F14 km0+700

15.00 yes +419.15 386817.127 583510.731

F15 km

0+300

10.50 no +393.00 386445.928 583364.694

F16 km0+745

10.50 no +409.50 386799.258 583578.100

F17 km0+745

10.50 no +421.41 386860.656 583552.474

F18 km0+845

15.00 yes +406.62 386824.079 583662.761

Figure 3. Location of prospecting and monitoring works




Laboratory tests on soil samples taken from the field were made in closeaccordance with the proposed objective, stability analyzes and dimensioning of

consolidation works.

The following tables reveals characteristic values of geotechnical parameters

obtained from laboratory tests.

Table 3. Results of physical tests - drilling F09

Layer

name

Laye

r

thick

ness

(m)

w

(%)

IP

(%)

IC

( - )

γ

kN/m3

γd

kN/m3

n

(%)

e

( - )

Sr

( - )

Vegetab

le soil

0.30 – – – – – – – –

Dust

and

sand

dust

5.20 13.90÷17.92

8.73 ÷23.39

1.00÷1.21

18.11 15.73 40.64 0.68 0.60

Brown

dusty

sand

with

grey

areas

3.50 13.42÷16.35

– – 16.52 14.20 45.39 0.83 0.52

Reddish

dust

1.50 16.25 16.37 1.03 17.39 14.96 43.34 0.76 0.57

Grey

dusty

sand

2.00 13.98 – – – – – – –

Reddish

dust

1.00 16.17 14.95 1.03 17.72 15.25 42.22 0.73 0.60

Grey

dustysand

0.50 12.74 – – – – – – –

Reddish

dust

with

intercal

ations of

grey

sand

3.00 15.54÷17.44

15.18÷18.46

1.02÷1.03

17.55 15.19 42.46 0.74 0.57

Grey

clayey

sand

3.00 19.84÷21.01

21.46÷22.08

0.76÷0.86

17.08 14.11 46.13 0.86 0.66

Grey

dusty

1.00 16.57 – – 18.00 15.44 40.61 0.68 0.64




clayey dust anddust

36.00 100.33 2.70 19 56

Values of the geotechnical parameters (Xd) were determined from the characteristic

values with the relationship: k d

X X

(1)

3. STABILITY ANALYSIS AND GEOTECHNICAL MONITORING

Transversal lithological profiles were drawn on the greatest slope lines in order toanalyze stability on sloping areas and possible sliding.//the possibility of slidingThe problem of slopes stability and of the various ground construction mainlydepends on the strength characteristics of the layers constituting the massive. The

stability of these masses is usually estimated by different methods and theoreticalcalculation, depending on its type (cohesive or non-cohesive), the slope of themassif, calculation assumptions, or, practically, by measuring deformations directly

on the ground (slope monitoring systems).

The physical and mechanical characteristics determined correctly allowinterpretation of deformation processes that may be encountered in the field, andcalculations of massive strength and stability of the ground under the slope,allowing the adoption of design solutions that ensure their stability, both in the process of consolidation and for new projections works (roads, embankments, earth

dams, artificial slopes).

The stability calculations were performed using the values of shear strength parameters determined in the laboratory within the additional field and laboratory

investigations programme.

3.1. Modelling site through computer programme

Stability calculation of the site was performed using automatic calculation

specialized software GEO5, using limit equilibrium method Bishop in severalassumptions.

Limit equilibrium methods allow the assumption that the same safety factor has a

constant value for any point of failure surface and stability conditions and ischaracterized by its average value.




Figure 4. Civil 3D surface generation and drawing profiles

Figure 5. Transversal litologic profile 6-6’




The parameter values used in geotechnical stability analysis (Table 5) resultedfrom consideration of the values given after laboratory tests, which were applied on

partial factors of SR EN 1997-1-2004.

Table 5 Input data used in stability analysis

No.

layer

Layer name γ

kN/m3

Φef (°) cef (kPa) γsat

kN/m3

1. Vegetable soil 16.80 14.40 5.70 18.00

2. Clay and silty clay 19.20 12.70 27.80 19.90

3. Brown reddish sandy clay 18.50 15.20 17.10 19.20

4. Brown-grey dusty sand 17.45 13.80 5.20 18.32

5. Reddish clay 19.95 7.20 19.30 20.80

6. Reddish clayey dust 17.90 12.80 20.70 18.50

7. Grey dusty sand, consolidated 17.96 25.60 9.80 18.10

After running stability analysis to optimize sliding surface in a static regime, aminimum safety factor Fs = 1.01 was obtained, (Figure 6, Table 6), corresponding

to potential sliding surfaces located at a depth of 11.20 m (depth reported to F06drilling position).

Figure 6. Location of sliding surface in GEO 5 programme

Table 6. Stability analysis results for 6-6 ' profile

Analysis profile Obtained results

Profile 6-6’ The amount of active forces Fa = 3784.33 kN/m

The amount of passive forces Fp = 3821.67 kN/m

The moment of force whichcauses the sliding

Ma = 328971.98 kNm/m

The moment of forces that oppose Mp = 332218.00 kNm/m




sliding

Stability factor Fs = 1.01

3.2. Geotechnical monitoring of potential slide areas

Geotechnical boreholes were equipped for monitoring the activity of the slope,with ABS plastic inclinometer tubes - 70.0 mm diameter, fitted with 4 grooves forguiding the inclinometer probe.

The space between the borehole walls and inclinometer tubing was filled with

granular material sort 4-8 mm, to ensure the taking over deformations in thesurrounding ground mass. After drilling equipping, the upper inclinometer tubingwas protected with metal tube and reinforced concrete slab.

In order to achieve measurements, RST Digital Mems Inclinometer Systemequipment (Figure 5.21) has been used, with a biaxial probe. For recordingdeformations of the four directions, two probe passes along the inclinometer tubingwere required (Figure 5.22, Figure 5.24). According to the certificates provided by

the manufacturer, the maximum error of the device is ± 2 mm / 25m.

Figure 7. RST Digital Mems Inclinometer System

The monitoring activity was performed between November 2013 and April 2014

through seven stages of records, according to the schedule specified in Table 7.

Table 7. Monitoring schedule

Inclinometer borehole F05 F06 F07 F09 F14 F18

Date of making measurement Stage

13 November 2013 C0 C0 C0 C0 C0 C0

02 December 2013 C1 C1 C1 C1 C1 C1

16 December 2013 C2 C2 C2 C2 C2 C2




10 January 2014 C3 C3 C3 C3 C3 C3

29 January 2014 C4 C4 C4 C4 C4 C4

19 February 2014 C5 C5 C5 C5 C5 C5

13 March 2014 C6 C6 C6 C6 C6 C6

03 April 2014 C7 C7 C7 C7 C7 C7

Inclinometer monitoring records and results certify that from the whole area

analyzed the phenomenon of instability, it is active only in the characteristic profiles of 6-6 'and 7-7' zones (section between km 1+920 – 2+160 - upstreamPOLUS shopping complex).

Based on visual observations on the site, where specific elements have been

identified for the existence of the instability phenomena and monitoring the results,it appears that the landslides in the analyzed area, with the exception of 1+920 –2+160, is in a temporary equilibrium, classified in the stabilized landslidescategory.

Figure 8. Relative displacement and displacement vector of inclinometer F05




Figure 9. Maximum displacement vector evolution of inclinometer F05

Taking into account the values of inclinometer movements recorded during themonitoring programme and classification of land movement in the very slowlandslides category (as GT 006-97), the actual situation on site can be assimilated

with a subunit safety factor, near the limit of equilibrium state.

4. CONSOLIDATION SOLUTIONS

Through geotechnical investigations, stability analyzes and geotechnicalmonitoring programme, the necessity for strengthen solutions implementation washighlighted in order that the road will be exploited in safe conditions.

Thus, the following measures and methods to improve the slopes for respective

location were analyzed and disposed:

- Geometric methods to achieve softer slopes, the creation of berms and

banquets - to ensure minimum width of the road;

- Drainage methods – a depth drainage for lowering groundwater andsurface drains to remove water from seepage;

- Mechanical methods - consolidating elements able to take up the forces

given by the ground action.

Engineering consolidation works of unstable slopes are subjected to lateral forceexerted by massive ground with which they come into contact. The intensity oflateral forces depend on the possibility of displacement and deformation of the

support element and on shear strength mobilization on potential sliding plane. Thesupport structures will be dimensioned for this purpose based on existing failure

theories and through computer programmes.

5. CONCLUSIONS

The case study presented in this article confirms the importance of geotechnical

investigations for different locations as being the ways of communication, in orderto know the mechanism of deformable ground.

Determining the number and type of investigation both by drillings and in situmonitoring works for designing a road is often a difficult problem due to the cost

related to investment value. However, experience has shown that a complete set of

investigations leads to an economic and safe design.




The implementation of such a project by creating a new road system on a slope,which includes cut and fill, produces significant changes in the state of the massive

efforts.

The presence inside the massive of groundwater in permeable layers constitutes a

danger to the stability of the slope, due to local and general hydrodynamicentrainment of fine particles and decreasing shear strength parameters.

The analysis of case study shows that the additional information obtained from themonitoring of landslides allow correct identification of parameters that characterize

landslide and its classification into different categories; highlighting the activelandslide areas and areas that are in a state of temporary equilibrium; developmentof calculations of inverse analysis for assessing the stability reserve given theknown position of the sliding surface.

Based on information from the literature regarding monitoring planning, it was

considered useful to highlight the issues that involves the knowledge of the phenomenon of slip, and puts under control of the affected areas to reduce the riskof instability.

As a final conclusion we can say that through some clear and actual stages ongeotechnical investigation of a slope leads to optimal design and eliminates the riskof roads exploitation.

References

1. Boțu N., Lungu I., Boți I. (2007). Evaluation of the geotechnical risk in the hilly zones within the

city of Ia și, Computation Civil Engineering, page. 299-307;2. Boțu N., Chirilă R., Grigore D. (2014) Stability analysis of a national road using shear strength

reduction technique, 14th International Multidisciplinary Scientific GeoConference SGEM 2014,Section Hydrogeology, Engineering Geology and Geotechnics, Slope stability – models andmanagement – ISBN 978-619-7105-08-7, ISSN 1314-2704, page. 847-852;

3. Chirilă R. (2013) Over the shear strength, mobilization, models and theories for ground failure ,Doctoral School Report, Gheorghe Asachi Technical University of Iasi;

4. Chirilă R., Grigore D., Mușat V. (2014) Modeling and stability calculations for transportinfrastructure by numerical methods, XV Danube-European Conference on GeotechnicalEngineering, Vienna, Austria, ISBN 978-3-902583-01-6, page.529-534;

5. Duncan J.M., Wright S.G. (2005) Soil Strength and Slope Stability;6. Grigore D. (2014). Study on research and monitoring landslides, PhD Thesis, Gheorghe Asachi

Technical University of Iasi;7. Grigore D., Chirilă R., Mușat V., Boțu N. (2014) Using the results from inclinometer

measurement for choosing road consolidation solution, XV Danube-European Conference onGeotechnical Engineering, Vienna, Austria, ISBN 978-3-902583-01-6, page. 523-528;

8. Manea S. (1998). Risk assessment of slipping slope, Conspress Publisher, Bucharest;9. Mușat V. (1988) Contributions in slope stability problem, PhD Thesis, Polytechnic Institute Iași

Stanciu A., Lungu I. (2006). Foundations, Technical Publishing, Bucharest.

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