Subsurface Profiling of Shallow Bedrock Sites by Integrated Approach and Site-specific 2d Site Response Analysis

Abstract

Estimation of reliable site effects and amplifications are important for seismic design. There are several 1D and 2D large scale site response analyses were reported in the literature and very limited shallow bedrock small scale 2D site responses were carried out. Hence in this study, an attempt has been made to generate the small-scale subsurface profiles in 2D using an integrated geophysical approach and a site-specific 2D nonlinear site response analysis at selected shallow bedrock sites in Peninsular India. Tarapur, Kalpakkam, Bangalore, Hampi, and Vizag are the shallow bedrock sites selected for the study. The subsurface profiling of these locations was done by an integrated geophysical approach by carrying out Multichannel Analysis of Surface Waves (MASW) survey, Ground Penetrating Radar (GPR) survey, Electrical Resistivity Tomography (ERT) and conventional drilling boreholes and measurement of Standard Penetration Test (SPT) N values. The study shows none of the important sites are having a uniform and homogenous subsurface layer even though likewise is assumed in several numerical models. The subsurface profiles generated by the integrated geophysical approaches are modeled in the FLAC - 2D program and site response parameters are estimated. Thirteen representative intraplate ground motions from all around the world are selected for the site response analysis based on the seismicity of the region. Results of the site response analysis were expressed in terms of short-period amplification factor (Fc) and long period amplification factor (Fs) for the intraplate shallow bedrock region. The results show that the Fc gives higher values than Fs in the case of stiff soil and it is showing a reverse trend in the case of soft soil. The trend of Fc is becoming more complex than that of the Fs especially in the case of stiff soil sites. Also, the 2D site response results were compared with conventional 1D nonlinear site response results using the program DEEPSOIL. This study shows that the actual 2D response parameters are highly varying with spatial variability of the subsurface and cannot be captured through the conventional 1D site response analysis. Further, a parametric study has been carried out to quantitatively evaluate the effect of different parameters on 2D site response analysis. Finally, from all these studies, we can conclude that stiffness of the subsurface profile, amplitude of ground motion, predominant period of ground motion, depth of subsurface profile, shape (spatial variability) of the subsurface layers, and inclination of the subsurface layer are the major factors that highly influencing the 2D response parameters of shallow bedrock sites.