Probabilistic Assessment of Liquefaction Potential of Guwahati City

Abstract

This paper presents a probabilistic assessment of the liquefaction potential of Guwahati City, in the North Eastern region of India. North Eastern region of India is one of the six most seismically active regions of the world. The assessment was done for saturated cohesionless deposits. Probabilistic liquefaction assessment is done in terms of probability of liquefaction. According to many researchers, probability of liquefaction is a better index for assessment of liquefaction than factor of safety obtained by the deterministic approach. In this papera comparative study of liquefaction potential of soil sites susceptible to liquefaction have been done using eight SPT based probabilistic approaches. Most of the approaches are based on logistic regression analysis of field performance data to determine empirical equations for evaluating the probability of liquefaction. Departing from logistic regression approach othershave used a Bayesian mapping function that relates reliability index and factor of safety to the probability of liquefaction. Standard penetration test (SPT) N values, engineering properties of the soils and depth of water table were taken from a data base of 200 boreholes up to 30 m depth covering an area of 262 km2 in Guwahati city. Standard penetration test was done at every 1.5 m interval to determine the N value of the soil with depth. Undisturbed and disturbed soil samples were collected to determine the engineering properties of the soils in the laboratory. A design peak ground acceleration of 0.36 g was used since Guwahati falls in zone V, a high earthquake risk zone, according to the seismic zoning map of India. Liquefaction susceptibility from the methods is presented as maps showing zones of levels of risk of liquefaction. The SPT-based liquefaction evaluation probabilistic procedures have been found to yield significantly different predictions. Comparisons of the methods in the probabilistic approach have shown a difference in the values of probabilities in the same depth. However, on common comparison, the soil layers susceptible to liquefaction in different zones of the city have been identified.