EVALUATION OF DYNAMIC RESPONSE OF DEEP SOILS

Abstract

Many seismically vulnerable regions in the India and worldwide are located on deep soil deposits which extend to depths of several hundred meters. It has been recognized that the earthquake response is altered by geological condition at the location of building, which is known as "site effects". As seismic waves propagates through uppermost layer of soil and rock, these layers serve as filter that can increase duration and amplitude of earthquake within narrow frequency band. The amplification of these waves is largely controlled by mechanical properties of these layers or more precisely, the variation of their impedance (impedance is the product of material density and seismic velocity). Typically, densities and shear wave velocities of near surface soil layers is smaller than bedrock below. The energy content is proportional to PVS2 ,thus the amplitude of wave tends to increases as it passes through overlying soil layers. An attempt has made in this dissertation to study seismic site response of deep soils, for this one three different hypothetical homogeneous sites such as soft soil, medium soil and hard soil on bedrock, with depth of half space is varying from 30m to 2000m from ground surface. One dimensional equivalent linear ground response analyses are carried out using computer package DEEPSOIL.A special emphasis is given to investigate effects of level of shaking on deep soil response under dynamic loading condition. Nonlinear behavior of soil is predominant at higher level of shaking, due to this there is shifting of site characteristic frequency towards lower value has been observed. So for reliable seismic response analyses, it is important to consider actual site characteristic frequency. An attempt has also made to generate actual site characteristic frequency in terms of intensity of shaking. Some case studies of Shanghai city deep soil and Kanpur soil have made to better understanding of actual deep soil effects.