ICOVP, 13th International Conference on Vibration Problems

29th November 2nd December, 2017, Indian Institute of Technology Guwahati, INDIA

EFFECT OF STRONG MOTION PARAMETERS ON THE RESPONSE OF SOIL

USING CYCLIC TRIAXIAL TESTS

SHIV SHANKAR KUMARCR, PR, A. MURALI KRISHNA, ARINDAM DEY

Indian Institute of Technology Guwahati, Assam 781039, India

k.shiv@iitg.ernet.in, amurali@iitg.ernet.in, arindam.dey@iitg.ernet.in

Abstract. Dynamic response of soil is significantly affected by stress history and the frequency contents of

irregular vibration corresponding to PGA levels. This paper addresses the effect of strong motion parameters

on the response of saturated sandy soil. Stress-controlled cyclic triaxial tests have been conducted at different

relative densities (30%-90%) and confining pressures (50-150kPa). Cyclic loading in terms of irregular

stress, evaluated using simplified procedure proposed by Seed and Idriss (1971), was applied on the test

specimens. The responses have been presented in terms of excess pore-water pressure ratio and shear strain

accumulation in the soil specimens. The results indicated that the accumulated shear strains and excess pore-

water pressures in the soil specimens is significantly affected by the increase in confining depth and

simultaneous changes in the relative density. Further, the study emphasized that the strong motions scaled to

the same PGA levels produce substantially differing soil response due to the variation in the associated strong

motion parameters.

Keywords: Irregular seismic excitation, Strong motion parameters, Cyclic triaxial test, Cohesionless soil,

Shear strain, Excess pore-water pressure

1. Introduction

Dynamic behaviour of soils and associated liquefaction aspects are very important

considerations in earthquake geotechnical engineering. To study the dynamic behaviour

and subsequent evaluation of dynamic soil properties, different laboratory tests using

variety of test specimen and loading conditions are conducted (Seed and Idriss, 1970;

Hardin and Drnevich, 1972; Iwasaki et al., 1978; Kokusho et al., 1982; Seed et al., 1986;

Chung et al., 1989; Vucetic and Dobry, 1991; Ishibashi and Zhang, 1993; Stokoe et al.,

1995; Sitharam and Govindaraju, 2003). Most of the tests, in general, use regular harmonic

excitations as cyclic loading. However, it is very well established that the behaviour of soil

under real earthquake excitation (irregular) is largely different in comparison to the regular

harmonic excitations. This is mainly due to wide range of frequency content and associated

ground motion parameters in the irregular motions. Shear stresses induced by real-time

strong motions are extremely irregular and possesses erratic temporal variation of

magnitude and frequency. Hence, it is very essential to study the behaviour of soil under

real seismic excitations. Instances of investigation of soil response in the laboratory using

real earthquake motion are very limited (Ishihara and Yasuda, 1972, 1973, 1975;

Tsukamoto et al., 2004; Sawada et al., 2006). The dynamic behaviour of soils is influenced

by various parameters, namely stress or strain levels (magnitude of earthquake), soil type,

saturation state of soil and in-situ stress conditions. Several researchers have performed

cyclic triaxial tests under variety of test conditions to anticipate the effect of the above-

mentioned parameters (Seed and Lee, 1966; Dobry et al., 1982; Vucetic and Dobry, 1988;

mailto:k.shiv@iitg.ernet.in,%20amurali@iitg.ernet.in,

Kumar, Krishna, Dey

2

Ladd et al., 1989; Ishihara, 1996). Due to the versatility in simulating medium to large

strains in the soil sample, cyclic triaxial tests have been extensively used for investigating

the dynamic behaviour of soil. In most of the literatures, tests were conducted with

different irregular excitations at either a particular relative density or a particular confining

pressure. The present study deals with the effect of relative density, confining pressure and

the strong motion parameters on the response of soil in terms of accumulated shear strains

and excess pore-water pressure.

In the present study, response of Brahmaputra sand subjected to irregular seismic

excitations has been investigated through cyclic triaxial tests. The tests have been

conducted at different confining pressures (c = 50, 100, and 150 kPa) on the specimens

prepared at different relative densities (Dr = 30, 60 and 90%). Test specimens subjected to

different confining pressures represent the soils in the field located at different confining

depths i.e. 5 m, 10 m and 15 m. The specimens were subjected to three different real

earthquake excitations, namely Kobe (1995; PGA = 0.834g), Bhuj (2001; PGA = 0.103g),

and Tezpur (2012; Scaled PGA = 0.36g) strong motions. The results were reported in terms

of the excess pore-water pressure and shear strain accumulations.

2. Experimental Investigation 2.1. Soil Characteristics

Brahmaputra river sand (BS), collected from Guwahati region, Assam (India) has been

used for the present study. The particle size distribution of the sand, obtained from sieve

analysis (IS: 2720-IV), is shown in Fig. 1 which reflects that the soil falls within the zone

of severely liquefaction susceptible soils (Tsuchida, 1970; Ishihara et al. 1980; Xenaki and

Athanasopoulos, 2003). The specific gravity of the sand was found to be 2.7 (IS: 2720-III).

The minimum and maximum dry unit weight (IS: 2720-XIV) were found to be 13.85

kN/m3 and 16.84 kN/m3, respectively. The physical properties of the sand are summarized

in Table 1. Based on the obtained results, BS classified as poorly graded sand (SP) (ASTM

D2487).

1E-3 0.01 0.1 1 100

20

40

60

80

100

Per

cen

tag

e fi

ner

Praticle size (mm)

Boundry for partially

liquefiable zone

Boundry for severely

liquefiable zone

Brahmaputra sand

Figure.1. Particle size distribution

2.2. Testing Apparatus

Cyclic triaxial apparatus, facilitating both monotonic as well as cyclic loading, was used

for the experimental investigations. The apparatus consists a loading frame of 100 kN,

fitted with a pneumatic dynamic actuator, having a displacement range and operational

Effect of strong motion parameters on the response of soil using cyclic triaxial tests

3

frequency range of 0-30 mm and 0.01-10 Hz, respectively. The details of instrumentations

available with the apparatus has been described in Kumar et al. (2017a).

Figure.2. Cyclic triaxial setup and components

2.3. Sample Preparation

Dry pluviation technique was adopted to prepare the cylindrical specimens of BS having

70 mm diameter and 140 mm height (ASTM D5311). A nominal vacuum pressure of 15-

20 kPa has been used to maintain verticality of the specimen (Ishihara et al. 1978). In order

to achieve a quick saturation and a substantial replacement of the pore-air, carbon dioxide

(CO2), in gaseous form, was flushed through the specimen, for 10-15 minutes, with a

pressure lesser than the applied cell pressure, followed by flushing with de-aired water. To

attain the saturation, the cell pressure (CP) and back pressure (BP) were then gradually

increased in stages while maintaining an almost constant differential pressure of 10 kPa

and checking the pore pressure parameter (B) after each increment of CP. The saturation

process was terminated as the back pressure (BP) reached 200 kPa and the corresponding

B-value was obtained to be greater than 0.96. The test specimens were consolidated to the

targeted effective stress levels, and were subsequently subjected to cyclic loading.

2.4. Irregular Seismic Excitations

Three different irregular stress time histories, computed from the acceleration histories of

Bhuj (2001; PGA = 0.103g), Tezpur (2012; Scaled PGA = 0.36g) and Kobe (1995; PGA =

0.834g) strong motions, have been used to evaluate the dynamic response of soil. The

associated ground motion parameters of these earthquakes obtained from seismosignal are

presented in Table 1. Figure 3 represents the acceleration histories and their corresponding

frequency content obtained by the Fast Fourier Transformations (FFT) of the strong

motions. Frequency-domain representation indicates the variation of energy content over a

frequency band. It is observed that the significant energy content of Bhuj and Kobe strong

motions are congregated over a frequency band of 0.5-4 Hz, while the same for Tezpur

motion is found to be at 2-15 Hz. Apart from these acceleration histories of different PGA,

all the ground motions were scaled to similar PGA levels (0.103 g and 0.36 g) for few of

Kumar, Krishna, Dey

4

the tests. Table 2 presents the ground motion parameters of scaled PGA to 0.103g of

different earthquake motions. It can be observed that the three scaled ground motions

(PGA = 0.103g) are also different because of the associated ground motion parameters

such as predominant period (fundamental frequency), duration and energy levels.

0 10 20 30 40 50 60-0.15-0.10-0.050.000.050.100.15

Bhuj (0.103g)

0 10 20 30 40 50 60-0.4

-0.2

0.0

0.2

0.4

Tezpur (0.36g)

Acc

ele

rati

on

(g)

0 10 20 30 40 50 60

-1.0

-0.5

0.0

0.5

1.0

Kobe (0.834g)

Time (s)

0 5 10 15 20 25 300.00

0.04

0.08

0.12

0 5 10 15 20 25 300.00

0.04

0.08

0.12

0.16

0 5 10 15 20 25 300.0

0.2

0.4

0.6

0.81.5 Hz

2.4 Hz

1.2 Hz

Bhuj motion

Tezpur motion

FF

T (

g-s

)

Frequency (Hz)

Kobe motion

Figure.3. Acceleration time histories and frequency domain representation of input motion

Table.1. Strong motion parameters for different earthquakes used for the analysis Strong

motion parameters 2001 Bhuj

2012

Tezpur

2012 Tezpur

(Scaled motion)

1995

Kobe

Magnitude 7.7 5.0 5.0 6.9

Station Ahmadabad TZP TZP KJMA

Site Class B B B B

Distance from source 238 km - - 0.6 km

Max. PGA (g) 0.103 0.027 0.36 0.834

Predominant period (sec) 0.26 0.08 0.08 0.36

Mean period (sec) 0.59 0.167 0.167 0.64

Bracketed duration (sec) 55.8 34.05 28.30 21.90

Significant duration (sec) 14.84 14.38 14.38 8.38

Arias intensity (m/sec) 0.268 0.003 0.556 8.38

Specific energy density (cm2/sec) 438.561 0.173 33.82 7573.270

Cumulative absolute velocity (cm/sec) 470.413 46.67 653.42 2118.588

vmax/amax (sec) 0.136 0.021 0.021 0.103

Table.2. Ground motion parameters of scaled PGA = 0.103g of different earthquake

motions Strong

motion parameters 2001 Bhuj 2012 Tezpur 1995 Kobe

Predominant period (s) 0.26 0.08 0.36

Mean period (s) 0.60 0.167 0.64

Bracketed duration (s) 55.18 28.3 21.92

Significant duration (s) 14.3 15.35 8.38

Arias intensity (m/s) 0.26 0.05 0.14

Specific energy density (cm2/s) 438.56 2.9 128.57

vmax/amax (s) 0.13 0.02 0.10

Cyclic loading has been applied on the test specimen in terms of cyclic stress history,

which was evaluated based on the accelerations and the effective stress on the sample. It

was considered that the specimens tested at different effective confining stress are assumed

to be located at different confining depths below the ground level. To evaluate the irregular

shear stress () history induced by a real-time earthquake at any depth z within a soil

Effect of strong motion parameters on the response of soil using cyclic triaxial tests

5

deposit, the approach proposed by Seed and Idriss (1971), as exhibited by Eqn. 1, has been

adopted.

acc. ( )

= ' c dg

rg

(1)

1.0 0.00765 ; 9.15dr z for z m (2a)

1.174 0.0267 ; 9.15 23dr z for z m (2b)

acc. ( )2 = 2 ' d c d

gr

g (3)

where, acc. (g) is the acceleration time history, c is the effective confining stress and rd is

the stress reduction factor (Eqn. 2; Youd et al. 2001) accounting for the deformable

characteristics of the soil specimen. The deviatoric stress history, to be applied during the

experiment, was evaluated from the strong motion stress histories as per Eqn. 3. Cyclic

stress ratio (CSR) of the cyclic loading can be evaluated as /c. Figure 4 shows typical applied deviatoric stress time histories of different ground motions with different PGA for

a test specimen at 100 kPa confining stress i.e.at an approximated confining depth of 10 m.

0 10 20 30 40 50 60-20

-10

0

10

20

0 10 20 30 40 50 60-80

-40

0

40

80

0 10 20 30 40 50 60-150-100

-500

50100150

Bhuj motion (0.103g)

'c= 100kPa

Tezpur motion (0.36g)

Dev

iato

ric

stre

ss (

kP

a)

Kobe motion (0.834g)

Time (s) Figure.4. Typical variation of deviator stress at c = 100 kPa for input excitations

Table.3. Investigation parameters of irregular excitations Soil Irregular excitation PGA (g) Dr (%) Confining depth (m)

Sand

Bhuj 0.103

30 5, 10, 15 Tezpur 0.360

Kobe 0.834

Bhuj 0.103

30 10, 15 Tezpur 0.360

Kobe 0.834

Bhuj

0.103 30, 60, 90 10 Tezpur

Kobe

Bhuj

0.360 60 10 Tezpur

Kobe

Kumar, Krishna, Dey

6

3. Results and Discussions

Three strong motion excitations (Bhuj, Tezpur and Kobe motions as described earlier)

have been chosen to study the behaviour of BS specimens under irregular seismic

excitations at different relative densities and confining depths. Stress-controlled cyclic

triaxial tests at undrained condition were conducted on BS specimens, summarized in

Table 3. Test specimens were prepared at different relative densities (30, 60 and 90%) and

were subjected to different confining pressures i.e. 50, 100 and 150 kPa representing the

approximate soil confining depths to be 5, 10 and 15 m, respectively. Cyclic loading was

applied on the soil specimens in the form of irregular excitations as explained earlier. Test

results were presented in terms of developed shear strains and excess pore water pressures.

Excess pore-water pressures (ue) are represented as excess pore-water pressure ratio (ru =

ue /c).

3.1. Effect of relative density

Relative density, indicates the compactness of soil specimen and also an indicative of the

degree of inter-particle interaction, plays a major role in defining the dynamic behaviour of

cohesionless soils. The effect of relative density on the onset of liquefaction of the BS

specimens at a confining stress of 100 kPa and subjected to Bhuj motion is presented in

Fig. 5. It can be observed that ru decreases with the increase in Dr; maximum ru values of

0.13, 0.09 and 0.08 are obtained for test specimens at Dr of 30%, 60% and 90%,

respectively. Owing to the higher ratio of solid particles in a fixed volume representing a

denser state, the quantity of induced pore-water pressure decreases and hence, a reduced ru

was observed during the dynamic shaking. As a consequence, tests conducted at higher

relative densities revealed lesser shear strain accumulation (< 0.04%). Specimens when

subjected to Bhuj motion, no liquefaction was observed suggested by ru

Effect of strong motion parameters on the response of soil using cyclic triaxial tests

7

shear modulus (G) of different specimens, evaluated as a ratio of the maximum shear stress

(applied) to the maximum shear strains (observed) for a given test, and as obvious, the

shear modulus is found to increase with the increase in the relative density. Higher PGA

also reflects lesser value of G because of the significant increase in ru.

0 10 20 30 40 50 60-0.06

-0.04

-0.02

0.00

0.02

0.04

0.00

0.09

0.18

0.27

0.36 Shear strain ru

Dr = 90%

Dr = 60%

Dr = 30%

Bhuj motion (0.103g)

0 10 20 30 40 50 60-0.06

-0.04

-0.02

0.00

0.02

0.04

Time (s)

Sh

ear s

train

(%

)

0.00

0.09

0.18

0.27

0.36

Exces

s p

ore

pre

ssu

re r

ati

o (r u

)

0 10 20 30 40 50 60-0.06

-0.04

-0.02

0.00

0.02

0.04

'c= 100 kPa

0.00

0.09

0.18

0.27

0.36

Figure.5. Strain accumulation and excess PWP ratio in BS specimens confined at 100 kPa

for different Dr subjected Bhuj motion

Table.4. Summary of investigations on BS specimens prepared at different relative density Input

motion

Dr

(%)

c

(kPa) CSRmax

max

(%)

max

(kPa)

G

(MPa) ru,max Liquefaction

Bhuj

30

100 0.092

0.03 30.67 0.13

No 60 0.03 9.2 30.67 0.09

90 0.02 46.00 0.08

Tezpur

30

100 0.327

0.70 4.66 1.0

Yes 60 0.52 32.7 6.27 1.0

90 0.50 6.52 0.9

Kobe

30

100 0.756

15.0 0.50 1.0

Yes 60 12.0 75.6 0.63 1.0

90 5.00 1.51 1.0

3.2. Effect of confining depth

Effect of confining depth has been represented in terms of different confining pressures

and the resulting shear stresses due to a given earthquake motion. In this attempt, the test

specimens prepared at Dr = 30% are considered for different confining depths. Figure 6

shows the results of such specimens subjected to different irregular stress loadings. Figure

6a illustrates the accumulation of shear strain () and development of ru in the BS specimen

subjected to Bhuj motion (PGA = 0.103g). It is observed that the max is nearly 0.01%,

0.03% and 0.03% at confining depths of 5 m, 10 m and 15 m, respectively. An increase in

the confining depth implies that the sample has been subjected to higher shear stress,

which resulted in increased shear strain. Maximum excess pore pressures observed are

very low (ru,max = 0.1

Kumar, Krishna, Dey

8

different depths, 10 and 15 m (with c = 100 and 150 kPa) showed identical response in

terms of shear strain and pore pressure, which is again attributed to the low PGA level.

BS specimens at different depths subjected to scaled Tezpur motion (PGA = 0.36g) with

CSR ranging between 0.28-0.35 exhibited higher peak shear strains in the range of 0.06-

1.8% (Fig. 6). Specimens subjected to confining stresses 100 and 150 kPa exhibited a clear

onset of liquefaction as ru reaches nearly 1, while it was significantly lesser (ru,max = 0.25

6%) has also been

reported from ground response analysis studies using SHAKE and DEEPSOIL (Suetomi

and Yoshida, 1998; Kumar et al. 2014a,b; Singhai et al. 2016). Specimens subjected to

Kobe motion (PGA = 0.834g with CSR range of 0.65-0.80) exhibited ru = 1 at any of the

confining pressures and a substantial residual shear strain (> 5%), thus clearly exhibiting

the occurrence of liquefaction in the specimen. From the above illustration, it can be stated

that the behaviour of BS specimens at different confining pressure is indicative of their

supposed behaviour at different depths in the field subjected to strong motions.

0 10 20 30 40 50 60-0.10

-0.05

0.00

0.05

0.10

0.0

0.2

0.4

0.6

ru

Shear strain

'c = 50 kPa

Tezpur motion (0.36g); Dr = 30%

Shear strain

ru

Shear strain

ru

0 10 20 30 40 50 60-3

-2

-1

0

1

2

Time (s)

Sh

ear s

tra

in (

%)

0

1

2

3

Ex

ces

s p

ore

pre

ssu

re r

ati

o (r

u)

0 10 20 30 40 50 60-1.5

-1.0

-0.5

0.0

0.5

1.0'c = 100 kPa

'c = 150 kPa

0

1

2

Figure.6. Strain accumulation and excess PWP ratio histories of BS specimens at Dr =

30% and different c for Tezpur motion

Table. 5. Summary of investigations on BS specimen subjected to different c Input

motion

c

(kPa)

Dr

(%) CSRmax

max

(%)

max

(kPa)

G

(MPa) ru,max Liquefaction

Bhuj

(0.103g)

50

30

0.097 0.01 4.85 48.50 0.05

No 100 0.092 0.03 9.2 30.67 0.13

150 0.078 0.03 11.7 39.00 0.13

Tezpur

(0.36g)

50

30

0.346 0.06 17.3 28.84 0.25 No

100 0.327 0.70 32.7 4.67 1.00 Yes

150 0.278 1.80 41.7 2.32 1.00

Kobe

(0.834g)

50

30

0.802 8.00 40.1 0.50 1.00

Yes 100 0.756 15.0 75.6 0.50 1.00

150 0.645 18.0 96.7 0.54 1.00

Table 5 summarizes the results demonstrating the effect of confining pressures (depth) and

PGA levels of chosen strong motions. It can be stated that the BS specimens will manifest

Effect of strong motion parameters on the response of soil using cyclic triaxial tests

9

the onset of liquefaction behaviour beyond a PGA value of 0.36g. It has been noticed that

the developed maximum shear strain (max) exceeds 0.5% for soils exhibiting liquefaction.

Furthermore, at particular Dr and confining depth, the input motion of higher PGA reflect

higher strain accumulation. Table 5 also illustrates that for a particular Dr, generation of

excess PWP or liquefaction susceptibility of soil increases with the increase of confining

depth for all input motions.

0 10 20 30 40 50 60-0.06

-0.04

-0.02

0.00

0.02

0.04

-0.050.000.050.100.150.200.25

Shear strain

ru

Dr = 60%, 'c= 100 kPa

Bhuj motion (0.103g)

0 10 20 30 40 50 60-0.06

-0.04

-0.02

0.00

0.02

0.04Kobe motion (0.103g)

Time (s)

Sh

ear s

tra

in (

%)

-0.050.000.050.100.150.200.25

Exces

s p

ore

pre

ssu

re r

ati

o (r u

)

0 10 20 30 40 50 60-0.06

-0.04

-0.02

0.00

0.02

0.04Tezpur motion (0.103g)

Shear strain ru

Shear strain ru

-0.050.000.050.100.150.200.25

0 10 20 30 40 50 60-8-6-4-2024

0

1

2

3

ru

Shear strain

Dr = 60%, 'c = 100 kPa

Bhuj motion (0.36g)

0 10 20 30 40 50 60-8-6-4-2024

Kobe motion (0.36g)

Time (s)

Sh

ear s

train

(%

)

Shear strain ru

0

1

2

3

Exces

s p

ore

pre

ssu

re r

ati

o (r

u)

0 10 20 30 40 50 60-1.0

-0.5

0.0

0.5 Tezpur motion (0.36g)

ruShear strain

0

1

2

3

Figure.7. Strain accumulation and excess PWP ratio from scaled earthquake motion of

PGA 0.103g and 0.36g at Dr = 60% and c = 100 kPa

3.3. Effect of similarly scaled strong motions

The results obtained from the experimental investigations conducted on the BS specimens

based on irregular stress loading evaluated from scaled PGA to 0.103g and 0.36g, of

different strong motions. Figure 7 depicts the effect of the three earthquake motions (with

same PGA) on the BS specimens prepared at Dr = 60% and subjected to c = 100 kPa (10

m depth). It was observed that none of the ground motions, when scaled to PGA = 0.103g,

could initiate liquefaction in the BS specimen; while, liquefaction was observed in the

a

b

Kumar, Krishna, Dey

10

specimens for any of ground motions scaled to 0.36g. Though the PGA is same, the

specimens exhibited different shear strain levels under different excitations. The overall

behaviour of the specimens can clearly observed from the summary of the results presented

in Table 6. It can be noted that BS specimens, prepared at particular Dr and c, subjected

to the similarly scaled strong motions results in different magnitudes of maximum shear

strain due to the variation in the associated strong motion parameters. From the table, it can

be stated that Tezpur motion shows lowest values of max and ru,max in comparison to the

Bhuj and Kobe motion, the highest magnitudes being manifested by the Bhuj motion. The

reason for such behaviour is attributed to the varying strong motion parameters (Table 2)

such as arias intensity, specific energy density, which exhibited similar trend of variation

as that observed from the response of test results. Although other strong motions

parameters such as predominant period, mean period, bracketed duration and significant

duration (Table 2) might have a direct or indirect effect on the observed responses,

however, the study of their individual effects is outside the scope of the present attempt.

Table .6. Summary of results subjected to ground motions with same PGA Input

motion

Scaled

PGA(g)

Dr

(%)

c

(kPa) CSRmax

max

(kPa)

max

(%)

G

(MPa) ru,max Liquefaction

Bhuj

0.103 60 100 0.092 9.2

0.026 35.38 0.09

No Tezpur 0.015 62.00 0.036

Kobe 0.019 48.42 0.06

Bhuj

0.36 60 100 0.327 32.7

4.00 0.817 1.00

Yes Tezpur 0.52 6.29 1.00

Kobe 2.50 1.30 1.00

4. Conclusions

The present study illustrates the effect of irregular seismic excitations (Bhuj, Tezpur and

Kobe motion) on the dynamic response of Brahmaputra sand. The generation of excess

pore water pressure, accumulation of shear strain due to cyclic loading and the onset of

liquefaction was observed to be significantly affected by the state of the specimen

manifested in terms of relative density and confining depth. Based on the present study, the

following conclusions can be drawn:

1. Due to higher inter-particle interaction and degree of compactness, an increase in the relative density leads to the decrement of the accumulated shear strain and the

excess pore-water pressure ratio. The effect on accumulated shear strain is more

prominent for higher PGA of the input motions, while lower PGA induces

prominent effect on the developed excess pore-water pressure.

2. BS specimens subjected to any of the strong motions exhibited onset of liquefaction when the maximum shear strain exceeded 0.5%, and hence, this magnitude of shear

strain can be stated as the threshold shear strain for liquefaction.

3. The time taken for the onset of liquefaction is governed by the nature of the applied strong motion. An impulsive strong motion, e.g. Tezpur/Kobe motion will exhibit a

quick liquefaction phenomenon, while Bhuj motion will exhibit a gradual

attainment of the onset due to its gradual rise towards PGA.

4. An increase in the confining depth leads to the enhancement in the accumulated shear strain and excess pore-water pressure ratio, and hence, increases the

liquefaction susceptibility of the sample. Such effect gets more pronounced with

higher PGA of the input motion.

Effect of strong motion parameters on the response of soil using cyclic triaxial tests

11

5. Based on the results of BS specimens subjected to similarly scaled strong motions, it can be stated that specimens at any relative density will liquefy under the

following optimum conditions: PGA > 0.36g, CSR > 0.3 and max > 0.5%.

6. Cohesionless soil specimens subjected to similarly scaled strong motions exhibits varying accumulated shear strains and excess pore-water pressure ratios due to the

variation of the other associated strong motion parameters such as arias intensity,

specific energy density, predominant period, mean period, bracketed duration and

significant duration. Individual and simultaneous effect of the strong motion

parameters is bound to affect the dynamic response of cohesionless soil and needs

to be studied in rigorous and minute detail.

Reference

[1] ASTM D2487, Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System), ASTM International, West Conshohocken, PA, 2006.

[2] ASTM D5311, Test Method for Load Controlled Cyclic Triaxial Strength of Soil, Annual book of ASTM standards. ASTM International, West Conshohocken, PA, 2011.

[3] Chang C.Y., Power M.S., Tang Y.K., Mok C.M., Evidence of Nonlinear Soil Response during a Moderate Earthquake, Proceeding of the 12th International Conference on Soil Mechanics and

Foundation Engineering. Rio de Janeiro, 3, 14, 1989.

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