Nonlinear Ground Response Analysis of Kolkata Soil

Abstract

Seismic bed-rock motions get amplified or de-amplified depending on the overlying soil deposit and very often found to cause disaster to the overlying structures. Kolkata city is situated on a deep alluvial deposit of Gangetic basin and consists of distinct soil formations, namely, normal Kolkata deposit, consisting of a thick soft silty clay/clayey silt deposit underlain by stiff clayey soil/silty sand down to considerable depth, and river channel deposit which consists of a deep deposit of medium/dense/very dense sandy soil present mainly along the Adi Ganga channel. So, in such a scenario when sediment thickness is quite high the study of local site effects should be of primary importance. Few studies have been carried out on local site effects of Kolkata soil using equivalent linear ground response analysis. Although for a weaker motion, equivalent linear (EL) criteria may closely predict the nonlinear response of soil, but when a soil is subjected to a strong motion, the EL assumption of soil behavior may not hold true. In this paper, nonlinear ground response analysis has been carried out using DEEPSOIL program to predict ground response of both normal Kolkata deposit and river channel deposit. Synthetic acceleration time history compatible with IS-Code rock level spectra for Zone-III has been used in the study. Variation of peak ground acceleration (PGA), spectral acceleration (SA), and pore water pressure (PWP) has been studied for both the type of deposits.  A number of Vs-N correlations have been used to calculate shear wave velocity in the analysis to study their effects on the site response.