Seismic Response of Geosynthetic Reinforced Soil Slopes

Latha, G. Madhavi Varman, N.Associate Professor Graduate Student

email: madhavi@civil.iisc.ernet.in email: nandivarman.am@gmail.com

Department of Civil Engineering, Indian Institute of Science, Bangalore

ABSTRACT

This paper describes shaking table model studies conducted on unreinforced and reinforced soil slopes. A computer

controlled hydraulically driven single degree of freedom shake table was used in these tests. Model soil slopes

were constructed using clayey sand in a laminar box. Two different slope angles were used in tests. Two different

types of reinforcement, namely, a woven geotextile and a biaxial geogrid were used in the tests. The models were

instrumented with ultrasonic displacement transducers and accelerometers at different locations. The slopes were

subjected to horizontal base shaking of known acceleration and frequency and the displacements and accelerations

were monitored for time increments. Numerical modelling of the geosynthetic reinforced soil slopes subjected to

base shaking is also presented. Comparison of displacements and accelerations of tests with different slope angles

and with reinforcing layers of different stiffness values resulted in some important observations regarding the

response of these soil slopes under seismic shaking conditions.

Indian Geotechnical Conference – 2010, GEOtrendz

December 16–18, 2010

IGS Mumbai Chapter & IIT Bombay

1. INTRODUCTION

Soil reinforcement to increase the performance of slopes

by reducing deformations and to build steep slopes in less

space has been a potential topic of interest to geotechnical

engineers from the past two decades. However, very few

studies are available on the seismic response of reinforced

soil slopes, e g. Perez (1999), Perez and Holtz (2004), Lo

Grasso et al. (2005), Nova- Roessig and Sitar (2006) and

Huang et al. (2008). This paper aims at understanding the

performance of geosynthetic reinforced soil slopes under

seismic conditions through laboratory shaking table tests.

The relative performance of slopes with and without

reinforcement, the effect of slope angle and reinforcement

type is studied by carrying out shaking table tests on models

of soil slopes in the shaking table and also by simulating

them in numerical model.

2. SHAKING TABLE

A computer controlled servo hydraulic single axis shaking

table is used to simulate the horizontal shaking action. The

play load capacity of this shaking table is 1000 Kg and the

loading platform is of size 1 m ´ 1m. The operating

frequency range is 0.05 Hz to 50 Hz. Accelerometers and

ultrasonic non-contact displacement transducers are used

to measure the response of the model slope during shaking.

Accelerometers are of analog voltage output type with a

full-scale acceleration range of ±2g along both the x and y

axes, with sensitivity of 0.001g. The sensing range of the

ultrasonic displacement transducers is 30 mm to 300 mm

output response time of 30 ms.

The laminar box used in this study is rectangular in cross

section with inside dimensions of 500 mm ´ 1000 mm and

800 mm deep made up of fifteen rectangular hollow layers

machined from solid aluminium separated by linear roller

bearings arranged to permit relative movement between the

layers with minimum friction and the bottom most layer is

rigidly connected to the solid base of 15 mm thickness.

3. MATERIALS USED

Soil

The soil used to construct the model slopes is classified as

clayey sand (SC) as per the unified soil classification system.

The liquid limit, plastic limit and shrinkage limit of the

soil are 37%, 18.5% and 16% respectively. The maximum

dry unit weight (MDD) and optimum moisture content

(OMC) obtained from the standard proctor compaction test

were 18.4 kN/m3 and 15% respectively. The cohesion and

friction able obtained from consolidated undrained (CU)

triaxial test were 15 kPa and 11° at OMC and MDD.

Geosynthetics

The geotextile used is a polypropylene woven fabric. The

properties of geotextile are given in Table1.

172 G. Madhavi Latha and N. Varman

Table 1: Properties of the Geotextiles

Warp 55.5 KN/m Ultimate tensile strength as per ASTM D-4595 Weft 46.0 KN/m

Warf 38% Elongation at break

Weft 21.3%

Thickness 1 mm

Mass per unit area 230 gm/m2

Secant modulus at 2% axial strain 152 KN/m

The geogrid used in the model tests is made of

polypropylene. The properties of geogrid are listed in

Table 2.

Table 2: Properties of the Geogrid

Parameter Value

Ultimate tensile strength ASTM

D-6637-01

26 KN/m

Initial modulus 183 KN/m

Secant modulus at 5% strain 125 KN/m

Unit weight 0.22 Kg/m2

Aperture size 35 mm × 35 mm

Aperture shape Square

4. MODEL CONSTRUCTION

The soil was compacted in layers of equal height of size

850 mm × 500 mm in plan and 600 mm in height in the

laminar box. The remaining space in the laminar box was

kept empty for mounting the displacement transducers and

that space was packed with concrete cubes during

compaction. The unit weight and water content were kept

as 15.5 KN/m3 and 17% respectively in all the model tests.

A mass of 5 kg was dropped from a height of 450 mm on

150 mm × 150 mm square steel base plate with fixed guide

rod at the centre of the base plate to achieve the desired

unit weight for each layer. Reinforcement was placed at

Fig. 1: Schematic of Geosynthetic Reinforced Model Slope

the interface of the compacted soil layers. During the

process of compaction the accelerometers were embedded

in soil at elevations 170 mm, 370 mm and 570 mm from

the base of the slope, whereas one accelerometer was fixed

to the bottom of the shaking table to measure the base

acceleration. Three displacement transducers were

positioned along the face of the slope at elevations 200

mm, 350 mm and 500 mm from the base of the slope to

measure the horizontal displacements. The compacted soil

was trimmed to the required slope geometry. Figure 1 shows

the schematic of two layered geotextile reinforced model

slope of angle 45°. Figure 2 shows the completed slope of

angle 45p with instrumentation.

Fig. 2: Completed Model Slope with Instrumentation

5. MODEL TESTS AND RESULTS

Series of model tests were conducted by varying the slope

angle, reinforcement spacing and changing the

reinforcement material. Some tests were also conducted

with varying base acceleration. In all the tests, the model

slope was subjected to 30 cycles of sinusoidal motion of

shaking table at the intended base acceleration. Frequency

of shaking was kept as 2 Hz in all the tests. Table 3 gives

the details of model tests and the test parameters.

Table 3: Details of Model Tests and Test Parameters

Test Reinforcing

Material

Number of Reinforcing

Layers

Slope Angle

Accele- ration

T1 unreinforced - 45 0.1g

T2 unreinforced - 45 0.3g

T3 geotextile 2 45 0.3g

T4 geotextile 3 45 0.3g

T5 geogrid 3 45 0.3g

T6 unreinforced - 60 0.3g

T7 geotextile 2 60 0.3g

T8 geotextile 3 60 0.3g

Typical variation of displacements with number of

cycles of dynamic loading at different elevations for the

test on unreinforced soil slope of angle 45°, which was

tested at 0.3 g acceleration and 2 Hz frequency, is shown

in Figure 3. Displacements measured by U1 correspond to

elevation of 200 mm from the base of the slope.

Figure 4 presents the final face displacement profile

of different model slopes at the end of 20 cycles of sinusoidal

motion. It should be noted that the figure provides

comparison of displacement profiles of unreinforced slope

of angle 45° tested at different accelerations (T1 & T2). It

Seismic Response of Geosynthetic Reinforced Soil Slopes 173

could be observed that the unreinforced slope model

displaced more when subjected to 0.3g acceleration

compared to the same slope subjected to 0.1g acceleration.

As the deformations are very low for 0.1g acceleration,

base acceleration is kept as 0.3g for all further model tests,

to keep the response high.

Fig. 3: Variation of Displacement with Number Cycles

for the Test T2

Fig. 4: Displacement Profiles for Unreinforced Slope

Subjected to Different Acceleration

Figure 5 compares the face displacement profile of

unreinforced and two layer geotextile reinforced soil slope

of angle 45° at the end of 20 cycles (T2 & T3). Similarly

Figure 6 compares the face displacement profiles of

unreinforced and two layer geotextile reinforced soil slope

of angle 60° at the end of 20 cycles (T6 & T7). It could be

observed that the two layer geotextile reinforced slope

displaced less than that of unreinforced slope for slope angle

45°. But in case of 60° slope angle, both unreinforced slope

and reinforced performed similarly.

Fig. 5: Comparison of Displacement Profiles for Unreinforced

and Reinforced Soil Slopes of 45° Angle

The effect of type of reinforcement on the displacement

profile of the slope is studied by comparing the results from

model tests with 3 layer geotextile and geogrid

reinforcements. Figure 7 shows the comparison for 45°

slope angle at the end of 20 cycles (T4 & T5). It can be

observed from the figure that the geotextile reinforcement

is proving to be more efficient in controlling the

deformations. The geotextile used in this study is of high

tensile strength and modulus compared to the geogrid used

and hence proved to be more beneficial.

At the end of 20 cycles

0

100

200

300

400

500

600

0 0.2 0.4 0.6 0.8 1

Displacement(mm)

Ele

va

tio

n f

rom

bo

tto

m(m

m)

Unreinforced

Reinforced 2 layer

Fig. 6: Comparison of Displacement Profiles for Unreinforced

and Reinforced Soil Slopes of 60° Angle

Fig. 7: Comparison of Displacement Profiles for Geotextiles

and Geogrid Reinforced Soil Slopes of 45° Angle

The effect of number of reinforcing layers is also

studied for both the slope angles with geotextile

reinforcement. Results from model tests with 2-layer and

3-layer geotextile reinforcement with 60° slope angle (T7

& T8) are compared in Figure 8 in terms of slope

displacement. It can be observed that the extra reinforcing

layer is not beneficial for reducing the slope movements.

Fig. 8: Comparison of Displacement Profiles for 60° Model

Slopes Reinforced with Two and Three Layers of Geotextiles

174 G. Madhavi Latha and N. Varman

6. NUMERICAL MODELING

Shaking table tests on geotextile reinforced soil slopes are

simulated in the numerical model using the computer

program GEO-STUDIO 2004. In this study the SLOPE/W

and QUAKE/W modules of this program are used to

construct the slopes and to apply dynamic base shaking.

Basic factor of safety calculations of the model slopes using

the Slope/W module showed very high values (13-20),

indicating that the slopes are generally stable in static

conditions. Dynamic analysis is carried out using Quake/

W module. Soil is modeled as a Mohr-Coulomb material

because of simplicity with reasonable accuracy. Young’s

modulus of the soil is computed as 25 MPa from the results

of triaxial tests and Poisson’s ratio is taken as 0.2 as

suggested by Bowles (1997) for this kind of soils. The

geotextile is modeled as a structural beam element of

flexible material with the properties obtained from

laboratory experiments. Figure 9 shows the discretization

of elements and boundary condition of the model.

Base width (mm)

Hei

gh

t of

slo

pe

mo

del

(m

m)

Fig. 9: Model Slope of Angle 45° Simulated in GEOSTUDIO

The 30 cycles of sinusoidal motion of shaking at the

intended acceleration of 0.3g was given as an input

acceleration to all models in numerical analysis. This was

done by creating an example file in QUAKE/W using the

measured base acceleration-time history data from the

Fig. 10: Comparison of Experimental and Numerical Results

experiment. The example file is then imported to the

analysis setting to simulate the numerical analysis. Figure

10 shows the comparison of displacement profile at the

end of 20 cycles between the numerical simulation and the

physical model test of two layer geotextile reinforced slope

of angle 45°. It can be observed that there is a close match

between the experimental and numerical results.

7. CONCLUSIONS

• The reinforced soil slopes performed better than the

unreinforced slopes for a slope angle of 45°. For a steeper

slope (60°), the effect of reinforcement is not seen in

the tests. Further tests and analysis are required to

understand this behaviour.

• The geotextile reinforcement gave better performance

than the geogrid reinforcement used because of its

higher tensile modulus

• Results from numerical analysis are reasonably

matching with the results from physical model tests

REFERENCES

Bowles, J.E. (1997). Foundation Analysis and Design, 5th

Ed., McGraw-Hill.

Huang, C.C., Horng, J.C. and Charng, J.J. (2008). Seismic

stability of reinforced slopes: Failure mechanism and

displacements. Geosynthetics International, 15(5), 333-

349.

Lo Grasso, A.S., Maugeri M and Recalcati, P. (2005).

Seismic behaviour of geosynthetic-reinforced slopes

with overload by shaking table tests. Slopes and

Retaining structures under static and seismic

conditions, ASCE GSP 140, CDROM.

Nova-Roessig, L. and Sitar, N. (2006). Centrifuge model

studies of the seismic response of reinforced soil slopes.

Journal of Geotechnical and Geoenvironmental

Engineering, ASCE, 132(3), 380-400.

Perez, A. (1999). Seismic response of Geosynthetic

Reinforced Steep Slopes. M.S. Thesis, University of

Washington, USA.

Perez, A. and Holtz, R.D. (2004). Seismic Response of

Reinforced Steep Soil Slopes: Results of Shaking Table

Study. Geotechnical Engineering for Transportation

Projects, ASCE GSP No. 126, 1664-1672.