Proceedings of Indian Geotechnical Conference

December 15-17, 2011, Kochi (Paper No. F –174)

INFLUENCE OF FREQUENCY OF SEISMIC SHAKING ON THE PERFORMANCE OF

REINFORCED SOIL SLOPES

N. Srilatha, Ph.D. Student, e-mail: srilatha@civil.iisc.ernet.in

G. Madhavi Latha, Associate Professor, e-mail: madhavi@civil.iisc.ernet.in

C.G. Puttappa, Professor, e-mail: puttappacg@gmail.com

ABSTRACT: This paper studies the effect of frequency of seismic base shaking on the performance of model reinforced

soil slopes through series of laboratory model tests. Construction of model soil slopes in the laminar box mounted on

shaking table, instrumentation and results from the shaking table tests are discussed in detail. The soil used in these tests is

sandy clay. The slope of the soil slope and the quantity and location of reinforcement are varied in different tests. These

slopes are of height 600 mm. Acceleration of shaking is kept constant as 0.3 g in all the tests to maximize the response.

Biaxial geogrids are used as reinforcement and the slope is constructed in lifts with geogrids placed at different heights.

Frequency of base shaking is varied from 2 Hz to 16 Hz in different tests. It is observed from these tests that the frequency

of shaking has significant influence on the performance of the reinforced slopes. The performance is compared in terms of

the deformation of the slope and the acceleration amplifications measured at different elevations. Root Mean Square (RMS)

accelerations computed at different elevations showed consistent trends with reference to the input frequency. Higher

frequencies not resulted in higher deformations or acceleration amplifications always. The performance of the slope is

getting locally minimized at certain levels of frequency; the reasons for the same are explored and discussed in the paper.

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. However, the knowledge on the performance of

these reinforced soil slopes under seismic conditions is not

studied by many. Good performance in terms of ductility of

geosynthetic reinforced slopes and walls against seismic

loading has been identified in physical model tests [1,2,

3,4,5].This paper aims at understanding the effect of

frequency of seismic base shaking on the performance of

model reinforced soil slopes through series of laboratory

model tests. The performance is compared in terms of the

deformation of the slope and the acceleration amplifications

measured at different elevations. It is observed from these

tests that the frequency of shaking has significant influence

on the performance of the reinforced slopes.

SHAKING TABLE

A computer controlled servo hydraulic single axis shaking

table is used to simulate the horizontal shaking action. The

pay 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

and 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

aluminum 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.

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 34%, 23% and 20% respectively. The maximum

dry unit weight obtained from the standard proctor

compaction test was 17.67 kN/m3 with an optimum

moisture content of 16.31%.

Reinforcement A Biaxial geogrid made of polypropylene is used in this

study. The properties of geogrid are determined from

standard multi-rib tension tests as per ASTM D-6637-01.

The properties of this geogrids are listed in table 1.

Table 1 Properties of the geogrid

Parameter Value

Ultimate tensile strength 26 kN/m

Initial modulus 183 kN/m

Secant modulus at 5% strain 125 kN/m

Mass per unit area 0.22 kg/m2

Aperture size 35 mm × 35 mm

Aperture shape Square

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N. Srilatha, G. Madhavi Latha, & C.G.Puttappa

MODEL CONSTRUCTION A polyethylene sheet was used to cover the inside of the

laminar box to cover the gap between the rectangular panels

and also to minimize the friction between the model and the

laminar box. 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 unit weight and water

content were kept as 15 kN/m3 and 10% 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 the interface of the compacted

soil layers. During the process of compaction the

accelerometers, A1, A2 and A3 were embedded in soil at

elevations 170 mm, 370 mm and 570 mm from the base of

the slope, whereas one accelerometer, A0, was fixed to the

bottom of the shaking table to measure base acceleration.

Three displacement transducers, U1, U2 and U3, were

positioned along the face of the slope at elevations 200 mm,

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

horizontal displacements. Fig. 1 shows the schematic

diagram of a typical reinforced soil slope model constructed

in the laminar box. Fig. 2 shows the completed 45º slope

with instrumentation.

Fig. 1 Schematic of typical reinforced 2 –layered Slope.

Fig. 2 Photograph of completed 45º slope

MODEL TESTS AND RESULTS

Series of model tests were conducted by varying the slope

angle, reinforcement spacing and the frequency. The base

acceleration was kept as 0.3 g for all the tests. The

frequency was varied from 2 Hz to 16 Hz in different tests.

In all the tests, the model slope was subjected to 40 cycles

of sinusoidal motion of shaking table at the intended base

acceleration. Table 2 gives the details of model tests and the

test parameters.

Table 2 Details of model tests and test parameters

Test

code

Frequency

(Hz)

No. of

reinforcing

layers

slope

URT1 2 0 45º

RT 1 2 2 45º

URT2 5 0 45º

RT 2 5 2 45º

URT3 7 0 45º

RT 3 7 2 45º

URT4 10 0 45º

URT5 12 0 45º

URT6 16 0 45º

URT7 2 0 60º

URT8 5 0 60º

URT9 7 0 60º

RT4 2 2 60º

Displacements measured by U1, corresponding to the

sensor placed at 200 mm from the base of the slope are

shown in Fig. 3. Typical variation of displacement with

number of cycles of dynamic loading at different elevations

for tests URT1-URT6 at the end of 40 cycles of sinusoidal

motion is shown in Fig. 4(a). The elevations are normalized

with respect to the height of the slope. Frequency of motion

in these tests ranges from 2 to 16 Hz.

Fig. 3 Variation of displacement recorded by U1 with

number of cycles in test URT2.

It can be clearly seen that slope displacement was

maximum under a frequency of 7 Hz. Horizontal

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Influence of frequency of seismic shaking on the performance of reinforced soil slopes

displacements are presented after normalizing the elevation

(z) by the total height of the slope (H).To simplify the

presentation of acceleration response of the slope, RMS

acceleration amplification factors (RMSA amplification

factors) are used to represent the acceleration. These factors

are calculated using the root mean square (RMS) method

applied to the acceleration-time history for each

accelerometer device [6]. RMSA amplification factors for

different model slopes at different elevations are presented

in Fig. 4(b). Maximum amplification occurred at the top of

the slope and the acceleration response of the slope

increased with the increase in frequency, until 7 Hz. The

response at 7 Hz is highly amplified and later on decreased

for 10 Hz and again showed increasing trend, exhibiting

maximum response at 16 Hz.

(a) Displacement profiles

(b) RMSA amplification factors

Fig. 4 Response of unreinforced 45° model slopes at

various frequencies after 40 cycles of shaking at 0.3 g.

The effect of frequency on the response of reinforced slopes

is shown in Fig. 5 for tests RT1, RT2 and RT3 carried out

at frequencies of 2, 5 and 7 Hz respectively. All these

slopes are reinforced with two layers of geogrid as

explained earlier. The displacement response increased

from 2Hz – 7 Hz, maximum being at 7 Hz whereas there is

no clear trend in case of acceleration response. These

observations highlight the role of fundamental (resonance)

frequency of the system and the proximity of the base

excitation frequency to this resonance frequency [6].

(a) Displacement profiles

(b) RMSA amplification factors

Fig. 5 Response of 45° reinforced model slopes at various

frequencies after 40 cycles of shaking at 0.3 g.

Fig. 6 compares the normalized face displacements and

RMSA profiles of unreinforced soil slope of 60° at various

frequencies at the end of 40 cycles. It could be observed

that the displacement response is the maximum at 7Hz and

acceleration response is same for 5 Hz and 7 Hz.

Reinforced slope displaced less than that of unreinforced

slope for slope angle of 45° as shown in Fig. 7. In fact the

factor of safety against static failure is as high as 15-20 and

hence the slope does not need any reinforcement for static

stability. The reinforcement provided is helpful for

improving the seismic stability of the slope.

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N. Srilatha, G. Madhavi Latha, & C.G.Puttappa

(a) Displacement profiles

(b) RMSA amplification factors

Fig. 6 Response of unreinforced 60° model slope at various

frequencies after 40 cycles of shaking at 0.3 g.

Fig. 7(a) Comparison of displacement profiles for

unreinforced and reinforced soil slopes of 45°at 7 Hz.

Fig. 7(b) Comparison of RMSA profiles for unreinforced

and reinforced soil slopes of 45° at 7 Hz.

CONCLUSIONS

The tested model slopes are generally showing maximum

response in terms of displacements and acceleration

amplifications at 7 Hz. These results demonstrate the

importance of fundamental frequency in the designs. This is

true for the reinforced slopes also.

REFERENCES

1. Bathurst, R. J., Hatami, K. & Alfaro, M. C. (2002a)

“Geosynthetic reinforced soil walls and slopes: seismic

Aspects”, Chapter 14, Geosynthetics and their applications (ed. S. K. Shukla), pp. 327–392. London:

Thomas Telford.

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

Seismic behaviour of geosynthetic-reinforced slope

with

over load by shaking table tests. Slopes and Retaining structures under r static and seismic Conditions, ASCE

GSP 140, CDROM.

3. Perez, A. (1999), Seismic response of geosynthetic reinforced steep slopes, M.S. Thesis, University of

Washington, USA.

4. 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 126, 1664-1672.

5. Ching - Chuan Huang, Jeng - Chong Horng , Wen-

Jong Chang , Jiunn - Shyang Chiou ,Chia - Han

Chen(2011), Dynamic behavior of reinforced walls –

Horizontal displacement response, Geotextiles and

Geomembranes

29, 257-267.

6. Kramer, S. L. (1996). Geotechnical Earthquake

Engineering, Prentice Hall, Upper Saddle River, NJ.

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