Seismic Response of Geosynthetic Reinforced Soil Slopes
Latha, G. Madhavi Varman, N.Associate Professor Graduate Student
email: [email protected] email: [email protected]
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
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