Proceedings of Indian Geotechnical Conference

December 15-17, 2011, Kochi (Paper No.F-256)

PROBABILISTIC ASSESSMENT OF LIQUEFACTION POTENTIAL FOR THE STATE OF

GUJARAT

K.S. Vipin, Endeavour Research Fellow, University of Wollongong, NSW, Australia. ks.vipin@gmail.com

T.G. Sitharam, Professor, Dept. of Civil Engg. IISc, Bangalore, India. sitharam@civil.iisc.ernet.in

K. Sreevalsa, Research Scholar, Dept. of Civil Engg, IISc, Bangalore, India. sreevals@civil.iisc.ernet.in

ABSTRACT: The Bhuj earthquake of 2001 is considered as the deadliest stable continental shield region earthquake. One of

the main reasons for heavy damage during the Bhuj earthquake was the effects of site amplification and liquefaction. This paper

tries to evaluate one of these geotechnical aspects – liquefaction, based on a performance-based approach. The seismic hazard

assessment of Gujarat (at bed rock level) was evaluated using Probabilistic Seismic hazard Assessment (PSHA) method. Based

on the rock level peak horizontal acceleration (PHA) values obtained, the surface level peak ground acceleration (PGA) values

were evaluated by amplification coefficients suggested by Raghu Kanth and Iyengar for Peninsular India. Using the PGA

values, the liquefaction potential was evaluated based on a performance-based approach. The liquefaction potential was

evaluated based on the corrected SPT value required to prevent liquefaction. The spatial variation of SPT values required to

prevent liquefaction for return periods of 475 and 2500 years are presented in this paper. The results obtained in this study can be used for macro level planning and liquefaction hazard mitigation of Gujarat.

Keywords: Liquefaction, Seismic Hazard, PSHA, SPT, performance-based approach

INTRODUCTION

Gujarat, which is located on the western part of India, is one

of the Indian states with very high industrial activity. The state accounts for about 22 % for India’s export. Ahmadabad,

The capital city of Gujarat, is rated as the third fastest

growing city in the world. The location map of Gujarat along will all the district head quarters is given in Fig. 1. The Kutch

region of Gujarat happens to be one of the most seismically

active regions in the stable continental shield region in the

world. Two devastating earthquakes have struck this region in

the past (1819 MW – 8.3 and 2001 MW – 7.7). The Bhuj

earthquake of 2001 is termed as the deadliest continental

shield earthquake ever occurred in the world. This

earthquake has caused widespread damage at Bhuj and

Ahmedabad region. Seismic soil liquefaction effects were

observed in more than 10,000 km2 area in Rann of Kutch

region during this earthquake.

The seismic zonation map of India [1] divides the country

into four different seismic zones. The Rann of Kutch region

of Gujarat falls in zone V, the zone with highest seismic

hazard. This is the only regions in the stable continental

shield region which comes under seismic zone V. Moreover,

it can be seen that there is considerable spatial variation of

seismic hazard in Gujarat [1] (Fig. 2).

The most devastating geotechnical effect of earthquakes is the

seismic soil liquefaction. Liquefaction occurs due to the

decrease in effective stress because of the sudden dynamic

earthquake load. The devastating effects of liquefaction were

first observed during the Niigata and Alaska earthquakes in 1964. In India, widespread liquefaction effects were observed

during the Bhuj earthquake in 2001. The remote sensing

satellite image showing the extent of liquefaction during Bhuj Earthquake is given in Fig. 3. This image shows clearly the

liquefied area in the Bhuj region of Gujarat during the deadly

earthquake in 2001.

Fig. 1 Location map of Gujarat

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K.S. Vipin, T.G. Sitharam & K. Sreevalsa

Fig. 2 Seismic zonation of Gujarat [1]

Fig. 3 Satellite image of Liquefaction at Gujarat [13]

On a broad scale, the steps involved in liquefaction potential

evaluation can be divided into two steps. The first step is the

evaluation of earthquake loading and the second step is the

evaluation of soil resistance to liquefaction. The evaluation of

earthquake loading requires analysis of seismotectonic

properties of the region, collection of earthquake details and

the evaluation of peak ground acceleration. The soil

resistance depends on the properties of soil, age and type of

soil deposit and depth of ground water table. The important

steps involved in the liquefaction potential evaluation are:

assessment of peak horizontal acceleration (PHA) at bed rock

level; evaluation of surface level peak ground acceleration

(PGA), considering local site effects and evaluation of liquefaction potential based on the PGA values and the soil

properties.

While trying to evaluate the liquefaction potential for a vast

region like Gujarat, it is practically impossible to get the

geotechnical properties for evaluating the liquefaction

potential. Hence, in the present paper the liquefaction

potential of Gujarat is evaluated using a performance-based

approach suggested by [2].

LIQUEFACTION HAZARD ASSESSMENT OF

GUJARAT

The important steps involved in liquefaction potential

evaluation using performance based approach are discussed

below.

Preparation of Earthquake Catalogue The seismic hazard assessment starts with preparing the

earthquake catalogue and identifying the seismic sources. An

earlier attempt to compile the earthquake catalogue for

Gujarat was done earlier by [14]. The earthquakes which are

occurring outside the study area will also contribute to the seismic hazard in the study area. Hence, the earthquake

catalogue for Gujarat was prepared by collecting data from an

area, which is within a radius of 300 km from the boundary of

the state [3]. The collected earthquake data were in different

magnitude scales. They were homogenized into a single

earthquake magnitude (moment magnitude, MW) using the

method suggested by [15] and then the catalogue was

declustered. This processed earthquake catalogue was used

for the seismic hazard analysis.

Selection of Seismic Sources Two types of seismic sources were considered in the analysis

(i) linear sources and (ii) zoneless approach. The linear

seismic sources were identified with the help of

Seismotectonic Atlas [4], which is one of the best documents listing the linear seismic sources in India and adjoining areas.

During the analysis it was notices that there were some

earthquake events which were not associated with any of the identified linear sources. In order to consider the effects of

these earthquake events, another type of source, based on

zoneless approach [5] was also considered.

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Probabilistic assessment of liquefaction potential for the state of gujarat

Ground Motion Prediction Equations The accuracy of prediction of ground motion depends on the

selection of Ground motion prediction equations (GMPE).

The only GMPE available for Peninsular India was developed

by [6]. Hence the GMPE developed for regions of similar

tectonic setup were also used in this study. [7] Suggest that

the ground motion attenuation in Eastern North America

(ENA) and Peninsular India are comparable. Hence two of

the GMPEs developed for ENA were also used in the analysis

– [8] and [9].

Site Response For evaluation of liquefaction potential, the surface level peak

ground acceleration is required. The peak horizontal

acceleration (PHA) values given by the GMPE will be at the

seismic bed rock level. Hence, to obtain the surface level PGA values, the effect of site response need to considered. In

the present study, since the study area is very vast, the site

characterization was done using NEHRP [10] site

classification scheme. The entire state of Gujarat was

assumed to be in site class D. The surface level acceleration

values were evaluated for site class D using appropriate

amplification factors [6] and these values were used for the

liquefaction potential evaluation. Since the soil in other site

classes is stiff, it may not liquefy and the PGA values for

these site classes were not evaluated.

PERFORMANCE BASED LIQUEFACTION

POTENTIAL EVALUATION

The deterministic liquefaction potential evaluation methods

require complete geotechnical details for evaluating the

liquefaction potential of a region [11]. Since the study area is

very vast, it will not be possible to get the geotechnical data

for the entire study area. Using the performance based

approach [2; 11; 12], liquefaction potential of a region can be

evaluated in terms of corrected SPT values required to

prevent liquefaction for a given return period. Readers can

refer to [2; 11; 12] for more details of performance-based approach in liquefaction potential evaluation.

RESULTS AND DISCUSSIONS

The seismic hazard analysis for the state of Gujarat was

evaluated using probabilistic method. A performance-based

approach was adopted to evaluate the corrected SPT values

required to prevent liquefaction. One of the main factors,

which will affect the liquefaction potential, is the depth of

water table. Since detailed data on this was not available, the

ground water table was assumed at ground surface. Due to

this, the results obtained in this study will indicate the worst

scenario of liquefaction for Gujarat.

The liquefaction hazard curve, showing the variation of

corrected SPT values (N1,60,cs) with return period for some of the major cities in Gujarat is shown in Fig. 4. The liquefaction

return period for these locations can be obtained directly from these curves. The corrected SPT value can be obtained from

the geotechnical test and based on this value, the liquefaction

return period can be obtained. Just to cite as an example, if

the corrected SPT value for a site at Bhuj is 20, then it can be

concluded that the location is safe against liquefaction for a

return period of 100 years (approximately). Whereas, if the

same SPT value is considered for Surat, the liquefaction

return period is approximately 8x103 years. From this, it can

be seen that the Bhuj is more vulnerable to liquefaction

hazard than Surat.

1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

1.E+07

0 5 10 15 20 25 30 35 40

An

nu

al ra

e o

f E

xceed

an

ce

Corrected N Value

Vadodara

Surat

Ahmadabad

Rajkot

Jamnagar

Bhuj

Fig. 4 Liquefaction Return Period for Major cities in Gujarat

The spatial variation of corrected SPT values (N1,60,cs)

required to prevent liquefaction for a return period of 475 years is shown in Fig. 5. The regions where the required

N1,60,cs values is less than 10 can be taken as safe against

liquefaction for a return period of 475 years. It is clear from

the results that the liquefaction susceptibility is very high for

the Rann of Kuchh region of Gujarat. This exactly matches

with the liquefaction pattern observed during 2001 Bhuj

earthquake.

The Spatial variation of N1,60,cs values required to prevent

liquefaction for a return period of 2500 years is shown in Fig.

6. Evan though the pattern of variation matches with that of

Fig. 5, the required SPT values are higher for a return period

of 2500 years. This is because of increase in PGA values with

increase in return period. The spatial variation of liquefaction

potential values obtained for both 475 and 2500 year return

period matches well with the liquefaction observed during the

Bhuj earthquake during 2001 (Fig. 3).

CONCLUDING REMARKS

The liquefaction potential evaluation clearly points out that

the liquefaction hazard is high at the Bhuj region of Gujarat.

The liquefaction hazard is relatively low for southern and

eastern regions. The liquefaction hazard pattern obtained in

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K.S. Vipin, T.G. Sitharam & K. Sreevalsa

the present study matches well with the actual liquefaction

observed during the Bhuj earthquake in 2001.

Fig. 5 Spatial variation of N1,60,cs values required to prevent

liquefaction for a return period of 475 years

Fig. 6 Spatial variation of N1,60,cs values required to prevent

liquefaction for a return period of 2500 years

ACKNOWLEDGEMENTS

The first author acknowledges the support received from

Indian Institute of Science through Senior Research

Fellowship (SRA). Authors would like to thank Ministry of

Earth Science, India for funding the project titled “Site

characterization of Indio-Gangetic plain with studies of site response and Liquefaction” (Ref. No. ME

SCO/MVC/TGS/003).

REFERENCES

1. BIS-1893 (2002), Indian Standard Criteria for

Earthquake Resistant Design of Structures. Part 1 -

General Provisions and Buildings, Bureau of Indian

Standards, New Delhi.

2. Kramer, S.L. and Mayfield, R.T. (2007), Return period of

soil liquefaction. Journal of Geotechnical and

Geoenvironmental Engineering, 133(7), 802 - 813.

3. Regulatory Guide 1.165 (1997), Identification and

characterization of seismic sources and determination of

safe shutdown earthquake ground motion, U.S. Nuclear

Regulatory Commission.

4. SEISAT (2000), Seismotectonic Atlas of India,

Geological Survey of India, New Delhi.

5. Frankel, A. (1995), Mapping seismic hazard in the

Central Eastern United States, Seismological Research Letters, 66(4), 8 - 21.

6. Raghu Kanth, S.T.G. and Iyengar, R.N. (2007),

Estimation of seismic spectral acceleration in Peninsular

India, Journal of Earth System Sciences, 116(3), 199 -

214.

7. Cramer, C.H. and Kumar, A. (2003), 2001 Bhuj, India,

earthquake engineering seismoscope recordings and

Eastern North America ground motion attenuation

relations, Bulletin of the Seismological Society of

America, 93, 1390 – 1394.

8. Campbell, K.W. and Bozorgnia, Y. (2003), Updated near-source ground motion (attenuation) relations for the

horizontal and vertical components of peak ground

acceleration and acceleration response spectra, Bulletin

of the Seismological Society of America, 93, 314 - 331.

9. Atkinson, G.M. and Boore, D.M. (2006), Earthquake

Ground-Motion prediction equations for Eastern North

America, Bulletin of the Seismological Society of

America, 96(6): 2181 - 2205.

10. BSSC (2003), NEHRP recommended provisions for

seismic regulations for new buildings and other structures

(FEMA 450), Part 1: Provisions, Building Seismic Safety Council for the Federal Emergency Management

Agency, Washington, D.C., USA.

11. Vipin, K.S., Anbazhagan, P. and Sitharam, T.G. (2010),

Probabilistic evaluation of seismic soil liquefaction

potential based on SPT data, Natural Hazards, 53, 547 –

560.

12. Vipin, K.S. and Sitharam, T.G. (2011), Evaluation of

Liquefaction Return Period Based on Local Site Classes:

Probabilistic Performance Based Logic Tree Approach,

International Journal of Geotechnical Engineering, 5, 245

– 254.

13. http://photojournal.jpl.nasa.gov/catalog/PIA03895.

14. Yadav, R.B.S., Tripathi, J.N., Rastogi, B.K. and Chopra,

S. (2010), Probabilistic Assessment of Earthquake

Hazard in Gujarat and Adjoining Region of India, Pure and Applied Geophysics, 165, 1813 – 1833.

15. Scordilis, E.M. (2006), Empirical global relations

converting ms and mb to moment magnitude, J Seismology, 10, 225 – 236.

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