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ORI GIN AL PA PER
Shaking table tests to investigate the influence of variousfactors on the liquefaction resistance of sands
Renjitha Mary Varghese • G. Madhavi Latha
Received: 22 February 2013 / Accepted: 9 March 2014 / Published online: 21 March 2014� Springer Science+Business Media Dordrecht 2014
Abstract This paper presents the shaking table studies to investigate the factors that
influence the liquefaction resistance of sand. A uniaxial shaking table with a perspex model
container was used for the model tests, and saturated sand beds were prepared using wet
pluviation method. The models were subjected to horizontal base shaking, and the varia-
tion of pore water pressure was measured. Three series of tests varying the acceleration and
frequency of base shaking and density of the soil were carried out on sand beds simulating
free field condition. Liquefaction was visualized in some model tests, which was also
established through pore water pressure ratios. Effective stress was calculated at the point
of pore water pressure measurement, and the number of cycles required to liquefy the sand
bed were estimated and matched with visual observations. It was observed that there was a
gradual variation in pore water pressure with change in base acceleration at a given
frequency of shaking. The variation in pore water pressure is not significant for the range of
frequency used in the tests. The frequency of base shaking at which the sand starts to
liquefy when the sand bed is subjected to any specific base acceleration depends on the
density of sand, and it was observed that the sand does not liquefy at any other frequency
less than this. A substantial improvement in liquefaction resistance of the sand was
observed with the increase in soil density, inferring that soil densification is a simple
technique that can be applied to increase the liquefaction resistance.
Keywords Shaking table � Liquefaction � Acceleration � Frequency �Density of sand � Model tests
R. M. Varghese � G. Madhavi Latha (&)Department of Civil Engineering, Indian Institute of Science, Bangalore 560012, Indiae-mail: madhavi@civil.iisc.ernet.in
R. M. Varghesee-mail: renjitha@civil.iisc.ernet.in
123
Nat Hazards (2014) 73:1337–1351DOI 10.1007/s11069-014-1142-3
1 Introduction
Liquefaction of cohesionless soils is one of the potential research areas of recent times
because the awareness toward the liquefaction-related damages during seismic events and
the need for mitigation of liquefaction hazards has been increased worldwide. The reasons
for liquefaction are explained by many researchers (Castro 1975; Martin et al. 1975;
Ishihara et al. 1975; Seed 1979; Vaid and Chern 1983; Seed et al. 1985; Poulos et al. 1985).
Liquefaction is associated with the pore water pressure development in the soil due to the
reduction in the volume during undrained loading condition. Development of pore water
pressure starts with the rearrangement of soil grains in loose saturated cohesionless soils
when earthquake waves pass through them. The soil grains try to rearrange themselves to
take a denser form, and pore water gets pressurized during this process because of loss in
the grain contacts. The increment in the pore water pressure is directly proportional to the
decrement in the effective stress of the soil. Apparently the decrease in grain-to-grain
contact or effective stress causes decrease in shear strength of the cohesionless soil and
finally leads to liquefaction or complete loss of shear strength. Liquefaction response of
soil mainly depends on the initial stress and other state parameters of the soil apart from the
ground motion parameters of the seismic shaking (Lee and Seed 1967; Castro and Poulos
1977; Vaid and Finn 1978; Vaid et al. 1985).
Liquefaction of soils has been successfully investigated by several researchers using
reduced scale models in shaking table and centrifuge. Hushmand et al. (1988) and Dobry
et al. (1995) carried out centrifuge tests on saturated sand deposits to understand the time
histories of accelerations and pore pressures during cyclic shaking in centrifuge models.
Use of shaking table tests to understand liquefaction of soils has been demonstrated by
several researchers (Ye et al. 2013; Ha et al. 2011). Sasaki et al. (1992) carried out shaking
table tests to understand the ground displacements during liquefaction and concluded that
the displacements are due to large shear strains in sand which is softened by the generation
of excess pore pressures. Mohajeri and Towhata (2003) and Towhata et al. (2006) carried
out 1-g shaking table tests on soil models prepared in laminar box to study the rate-
dependent behavior of liquefied soils. Kokusho (1999) carried out shaking table tests to
understand the effect of drainage conditions on the liquefaction-induced deformations.
Ueng et al. (2010) used a biaxial laminar shear box mounted on shaking table to study the
settlements in saturated clean deposits of sand and related the volumetric strain in liquefied
sand to the relative density for various shaking durations and earthquake magnitudes.
Maheshwari et al. (2012) reported increase in liquefaction resistance of reinforced sand
through shaking table tests and concluded that the type and amount of reinforcement in the
soil layer has influence on its liquefaction potential.
The low stress levels in 1-g model tests are always argued about and the applicability of
results from these tests for real conditions is questioned. To overcome this difficulty, few
researchers proposed procedures to maintain similitude between shaking table tests and
field conditions. Toyota et al. (2004) carried out 1-g shaking table tests on Toyoura sand to
study the effect of shaking acceleration and frequency on the liquefaction response. These
tests were carried out with reduced density to account for the reduced stress levels in 1-g
model tests. Iai (1989) derived a similitude for the shaking table tests on saturated soil-
structure-fluid model in 1 g gravitational field using e basic equations of equilibrium and
constitutive laws of soil.
There are still several gaps in understanding the influence of various factors that
influence the liquefaction behavior of sand. This paper presents the results of 1-g free field
liquefaction studies on sand through uniaxial shaking table tests. The effect of acceleration
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and frequency of base shaking and relative density of sand on the liquefaction potential and
pore water pressure development are studied through systematic series of shaking table
tests.
2 Testing material
The shaking table tests are performed on poorly graded sand. Figure 1 shows the grain size
distribution curve of the test material. The grain size distribution curve is falling within the
range of gradation for liquefiable sand given by Xenaki and Athanasopoulos (2003). Other
properties of the sand are given in Table 1.
The scanning electron microscopy image in Fig. 2 shows that the test sand particles are
sub-angular in shape. Angularity of particles reduces the liquefaction susceptibility of the
sand because of stable interlocking of grains.
3 Experimental setup and model preparation
Experiments are carried out on a uniaxial shaking table having a pay load of capacity one
tonne. A transparent perspex model container of size 1,200 9 500 9 800 mm was
mounted over the shaking table for preparing the sand bed. It was verified that the natural
frequency of the whole setup does not coincide with the natural frequency of the soil
sample to avoid resonance. Natural frequency of the shaking table used in this study is
100 Hz. Natural frequency of the sand bed in the study is estimated as 30 Hz from the
shear wave velocity of the sand bed (Kramer 2009), which is in turn estimated as 72 m/s
from the void ratio and initial mean effective confining pressure (Hardin and Richart 1963).
In any liquefaction analysis, sample preparation is the major issue because the density
and water content of the sand should be maintained uniform throughout the sample, which
is not an easy task. Different sample preparation methods are available in literature to
evaluate liquefaction resistance of cohesionless soil. They include the following: (1) dry
pluviation method, (2) wet pluviation method, (3) moist tamping method, (4) sedimenta-
tion method. Mulilis et al. (1977) described the effect of sample preparation on sand
liquefaction and concluded that the liquefaction response of a soil specimen significantly
depends on the method of sample preparation. For the present study, wet pluviation method
was adopted for sample preparation. As per literature, wet pluviation method simulates the
natural sand deposition compared to other methods. A 600-mm-thick sand bed was pre-
pared inside the perspex box mounted on the shaking table. Precalculated amount of water
exactly equal to the saturated water content (27 %) of the soil was poured in the perspex
box before pouring the sand. Sand was poured loosely into the water using a conical
hopper. An inverted solid cone with a 60� angle was attached at the end of the funnel to get
the uniform distribution of loose soil. Vaid and Negussey (1988) proved that the sample
prepared by pluviation under water will be homogeneous, and it is almost independent of
height of fall. Figure 3 shows the sequence of stages in sample preparation.
Miniature pore water pressure transducer of range 0–20 kPa with minimal self-weight
of 1.2 gms was used to monitor the pore water pressure variation during the test. Pore
water pressure transducer was kept exactly at mid-height along the central line of the sand
bed so that it is free from any of the boundary effects, and the thread and cable arrangement
ensures the relative movement of the sensor along with the table while shaking. Figure 4
shows the experimental setup before shaking.
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4 Testing and methodology
Saturated sand bed with uniform relative density was prepared in the model container to
measure the response of soil to different seismic shaking conditions. Shaking table along
with the model container was subjected to displacement-controlled sinusoidal base shak-
ing. Acceleration amplitude and frequency of the shaking were varied in different tests.
Three series of shaking table tests were carried to understand the effects of base accel-
eration, frequency and density of sand bed on the pore water pressure development and
liquefaction response of sand bed. Table 2 gives the test matrix used for this study. The
letter symbols A, F and D in the test notation represent the test series to study the effect of
acceleration, frequency and density, respectively. In test series A, frequency of shaking and
relative density of sand bed were kept uniform as 1 Hz and 43 %, respectively, while
varying the acceleration of shaking from 0.1 g to 0.15 g. In series F, acceleration of
shaking and relative density of sand bed were kept uniform as 0.1 g and 43 %, respec-
tively, while varying the frequency of shaking from 1 to 4 Hz. Test series D was intended
Fig. 1 Grain size distribution curve of sand used for tests
Table 1 Properties of sand usedfor shaking table tests
Property Value
Maximum dry unit weight (kN/m3) 17.67
Minimum dry unit weight (kN/m3) 14.23
Specific gravity 2.65
Maximum void ratio 0.862
Minimum void ratio 0.5
Coefficient of uniformity (Cu) 2.857
Coefficient of curvature (Cc) 1.35
D10 0.23
Particle shape Sub-angular
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to study the effect of density of the sand bed in which the acceleration and frequency were
kept as 0.15 g and 1 Hz uniformly, and relative density was varied between 43 and 67 %.
Light stirring and vibration was used for achieving higher initial densities. Water content in
Fig. 2 SEM image of the testing material
Fig. 3 Stages of sample preparation. a Place the perspex box on shaking table. b Fill the box withcalculated water for saturation. c Fix the sensor at position. d Hang the conical hopper. e Adjust the height offall for sand pluviation. f Prepared sand bed of required density
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this bed corresponds to the saturated water content of the sand. Test series with multiple
notations denote overlapping tests across the series.
Variation of pore water pressure at the midpoint of the sand bed was monitored with the
number of shaking cycles for each test. Initiation of liquefaction is identified with respect
to pore water pressure ratio, which is defined as the ratio between excess pore water
pressure developed during dynamic loading (Du) to the initial effective vertical stress (rv0).
The soil bed is said to be liquefied when the pore water pressure ratio reaches one, where
the effective vertical stress becomes zero. Number of cycles for liquefaction was deter-
mined from the data, by calculating the pore water pressure ratio at every cycle and the
number of cycles required for the pore water pressure ratio to reach a value of unity.
5 Results and discussion
5.1 Effect of acceleration on sand liquefaction
It is observed that for the given soil, the liquefaction was initiated at 0.15 g acceleration
amplitude at a frequency of 1 Hz, and the number of cycles required for liquefaction was
six. It was clearly observed that the response of sand is substantially affected by the small
variations in acceleration amplitude. Flow liquefaction with complete loss of shear strength
was visually observed in the sand bed at 0.15 g acceleration and 1 Hz frequency. For all
the acceleration amplitudes less than this, there was limited liquefaction, exhibited by
increase in pore pressure ratio and decrease in effective stress.
Figures 5 and 6 show the variations of pore pressure ratio and effective stress,
respectively, with varying acceleration amplitude. It can be seen that for the test sample
subjected to 0.15 g acceleration at 1 Hz frequency, pore pressure ratio reached 1.0 in six
cycles and the effective stress dropped to zero at this point, concurring with the visual
observation of flow liquefaction of sand at the end of six cycles of shaking. For all other
accelerations, pore pressure ratio increased with number of cycles, but it was much less
than 1.0. It is also observed that the order of increase in pore pressure ratio or decrease in
effective stress is in the order of increase in acceleration amplitude. For example, maxi-
mum pore pressure ratio was 0.13 for acceleration amplitude of 0.12 g, whereas it was 0.2
for 0.13 g, 0.27 for 0.14 g and 1.0 for 0.15 g. Figure 7 shows the sand bed liquefied in the
test with base acceleration of 0.15 g.
Fig. 4 Prepared sand bed before liquefaction
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Reduction in time to reach the peak pore water pressure was observed as the acceler-
ation value increases. Figure 8 indicates the increase in maximum pore water pressure with
respect to acceleration. It can be seen that the influence of acceleration amplitude on the
pore pressure development is highly nonlinear. The influence is less significant at lower
acceleration amplitudes, and at higher accelerations, even a small change in the amplitude
can cause drastic build-up of pore water pressure, demonstrating the quick loss of strength
during liquefaction.
Seed et al. (1976) presented the variation of pore pressure ratio with the number of
cycles during the process of liquefaction and showed a typical band for this variation. The
pore pressures measured in the present study for the case of liquefied sand bed (0.15 g and
1 Hz input base motion) is compared against the plot presented by Seed et al. (1976), and it
is found that the pore water pressure generation is following the presented trend as the
variation of pore pressure ratio with the number of cycles is falling within the boundary.
Here, N represents the number of cycles and Nl represents the number of cycles for
liquefaction. De Alba et al. (1975) also observed that the rate of pore pressure development
Table 2 Test matrix
Variable Test Relative density (%) Acceleration(g) Frequency (Hz)
Acceleration AFT1 43 0.1 1
AT2 43 0.12 1
AT3 43 0.13 1
AT4 43 0.14 1
ADT5 43 0.15 1
Frequency FT1 43 0.1 2
FT2 43 0.1 3
FT3 43 0.1 4
Density DT1 58 0.15 1
DT2 67 0.15 1
Fig. 5 Variation in porepressure ratio for sand bedsubjected to differentaccelerations at 1 Hz frequency
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depends on Nl. and presented an equation to calculate the pore pressure ratio at any cycle of
loading during liquefaction as:
Pore water pressure ratio; ru ¼1
2
� �þ 1
p
� �sin�1 2
N
Nl
� �ð1=aÞ�1
" #ð1Þ
where N is the number of cycles, Nl is the number of cycles for liquefaction and a is a
coefficient that depends on the soil properties and test conditions and usually taken as 0.7
for cohesionless soils. Figure 9 shows the rate of pore water pressure development during
the experiments in present study compared with the range presented by Seed et al. (1976)
and the calculated values using the equation given by De Alba et al. (1975).
Fig. 6 Variation in effective stress for sand bed subjected to different accelerations at 1 Hz frequency
Fig. 7 Sand bed subjected to flow liquefaction (Base acceleration 0.15 g, frequency 1 Hz)
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In this series of tests, cyclic resistance ratio (CRR) of sand beds was same for all the
tests, because the density and water content were same. However, these sand beds were
subjected to different levels of cyclic stresses, because of change in ground motion
accelerations, which varies the cyclic stress ratio (CSR) of the sand beds. Hence, the pore
water pressures developed in the sand beds were different in different tests. Liquefaction
was initiated in the sample, when the CSR was equal to CRR (Kramer 2009), which
happened when the acceleration of shaking was 0.15 g.
Fig. 8 Variation in maximum pore water pressure with different accelerations at 1 Hz
Fig. 9 Rate of pore water pressure generation during the test with 0.15 g and 1 Hz
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5.2 Effect of frequency on sand liquefaction
To study the effect of dynamic loading frequency on sand liquefaction, sand beds are
subjected to base shaking of different frequencies, keeping the acceleration and relative
density of sand same. These tests are carried out at an acceleration amplitude of 0.1 g with
relative density of sand as 43 %, changing the frequency of shaking from 1 Hz to 4 Hz.
Variation of pore pressure ratio in these tests is shown in Fig. 10. From Fig. 10, it can be
observed that the pore pressure ratio is not altered significantly with frequency for fre-
quency range 1–3 Hz. Sand bed is not liquefied completely in these tests. However, for the
test with 4 Hz frequency, in which the sand bed was subjected to flow liquefaction in 19
cycles of shaking, pore pressure ratio increased drastically and reached a value of unity
when sand is liquefied.
Figure 11 gives reduction in effective stress due to pore water pressure development for
different frequencies. It is clear that the drop in effective stress is not significant at lower
frequency levels. Only for a frequency of 4 Hz, the effective stress dropped down to zero,
and hence, this frequency can be termed as threshold frequency for liquefaction for the
given sand at acceleration amplitude of 0.1 g. The maximum pore water pressure devel-
opment can be seen only in threshold frequency for a given acceleration amplitude. This
can be further confirmed from Fig. 12 in which the variation of pore water pressure with
the frequency of base shaking is presented. Up to the threshold frequency, the maximum
pore water pressure developed remained almost the same and it increased drastically for
the threshold frequency.
Figure 13 presents the rate of pore water pressure development for the test with 0.1 g
and 4 Hz where the soil was liquefied and the comparison of the same with the band
presented by Seed et al. (1976) and the calculated variation as per De Alba et al. (1975). It
is observed that initially the measured response is little different from the predictions given
in the literature as the pore water pressure ratio with the number of cycles is falling outside
the given band. Also significant variation in the measured rate of pore water pressure
generation and calculated response as per De Alba et al. (1975) was observed up to a pore
water pressure ratio of 0.7. But at later stages, the experimental trend coincided with the
expected path.
5.3 Effect of relative density of sand on liquefaction
To understand the effect of density of sand on its liquefaction potential, tests were carried
out on sand beds with relative density of the sand varying from 43 to 67 %, keeping the
acceleration amplitude and frequency of base shaking as 0.15 g and 1 Hz, respectively.
Results from these tests are shown in Figs. 14 and 15. It was observed that the sand bed got
completely liquefied for the relative densities of 43 and 58 %, where as it did not liquefy
when the relative density was 67 %.
When pore pressure ratios are compared (Fig. 14), in case of sand bed with relative
density of 67 %, the maximum pore pressure ratio achieved was only 0.3, much less than
the unity, showing that the soil needs extremely higher cyclic loading to liquefy. Figure 15
shows that even though there was not much difference in initial vertical effective stress, the
pore water pressure development is considerably affected by the density of the sand bed.
This can be explained clearly using state parameter of soil at different densities. State
parameter represents the difference in void ratios of sand in present state and the sand in
critical state. Loose sands have void ratio close to the critical void ratio compared to the
dense sands, and hence, less amount of cyclic shaking is enough to bring them to a
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liquefied state, whereas denser sands have lesser void ratio and more shaking is required to
initiate liquefaction in these samples. Liquefaction resistance of the soil increases with
density. Hence, soil densification is the simplest method to improve the liquefaction
resistance of the soils. The cyclic loading parameters used for this series of tests represent a
seismic event of moderate magnitude. Using densification techniques, such as vibroflota-
tion, heavy tamping and blasting, the relative density can be improved further to withstand
seismic events of much higher magnitudes.
In this series of tests, sand beds were subjected to same level of cyclic stresses, because
the ground motion accelerations are same. Hence, the cyclic stress ratio (CSR) of the sand
beds was same. However, cyclic resistance ratio (CRR) of sand beds was different for
Fig. 10 Variation in pore water pressure ratio for sand bed subjected to different frequencies of shaking at0.1 g
Fig. 11 Variation in effectivestress for sand bed subjected todifferent frequencies of shakingat 0.1 g
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different models, because the density is different. Hence, the pore water pressures
developed in the sand beds were different in different tests. Sand beds of lower relative
densities (43 and 58 %) got liquefied, because the CSR value exceeded the CRR values,
and the sand bed of 67 % relative density could stay without liquefying because of higher
CRR value.
Change in the geometry of test bed will have some influence on the values of ground
shaking parameters that initiate liquefaction in the sand bed in scaled tests. Also, 1-g
shaking table studies suffer from scale effects on stress intensities. Hence, the results from
these studies cannot be directly extrapolated for estimating the ground motion parameters
Fig. 12 Variation in maximum pore water pressure for sand bed subjected to different frequencies ofshaking at 0.1 g
Fig. 13 Rate of pore waterpressure generation during thetest 0.1 g and 4 Hz
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that could initiate liquefaction in field. However, qualitative conclusions drawn from this
study are valid for understanding the initiation of liquefaction in saturated sand beds.
6 Conclusions
The following conclusions are drawn from the present study.
• Saturated sands subjected to base shaking get liquefied. The number of cycles needed to
liquefy the soils depends on the acceleration amplitude, frequency of base shaking and
relative density of sand.
Fig. 14 Variation in pore waterpressure ratio of sand bed withdifferent relative densities
Fig. 15 Variation in effective stress of sand with different relative densities
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• Acceleration amplitude is the most important cyclic loading parameter that controls
liquefaction. Higher the acceleration, less number of cycles needed for liquefaction and
higher the pore water pressure developed at any stage.
• There is a threshold frequency for the given sand to achieve initial flow liquefaction at
any given acceleration amplitude. As the frequency increases, the liquefaction
resistance of the sand decreases.
• Pore pressures developed in the model tests depended both on the cyclic stress induced
by the shaking and the cyclic resistance of the sand bed. Liquefaction is initiated when
the cyclic stress ratio exceeded the cyclic resistance ratio.
• Density of sand plays a major role in its liquefaction response. Loose soils, whose state
parameter is less, liquefy faster. Denser sands need higher cyclic loads for liquefaction.
Limited liquefaction of denser sand is achieved at lower cyclic loads, where the pore
pressures increase to certain extent but the pore pressure ratio can never reach unity and
the effective stress does not drop to zero.
• Soil densification is the simplest and effective technique for improving the liquefaction
resistance of sands. Increase in relative density of sand from 58 to 67 % showed drastic
improvement in the liquefaction resistance of the sand used in present study.
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