Depth of Input for Ground Response Analysis of Deep Soil Sites

Geotechnical earthquake engineers have been studying the ground response of deep soil deposits under earthquake loadings. The site amplification is correlated not only to soil thickness, but, also to soil properties such as shear modulus, damping and depth of half-space. There are no clear guidelines on the depth at which input motion is to be given. This paper presents the effect of depth of input for site response analysis of deep soil deposits. The influence of depth to the half-space is studied by conducting the 1D response analysis for soil column data for West Tennessee and Charleston site soil profiles. Two bedrock motions are given as input at various depths starting from 40, 60, 80,100, 120, 140, 160, 180, 200, 300, 400, and 500 m. From the limited analysis, it is observed that the variation of PSA and PGA values beyond 300 m are constant. Hence, the effective depth of soil column for the response analysis of deep soils shall be taken as 300-400 m.