Seismic Site Classification and Empirical Correlation Between Standard Penetration Test N Value and Shear Wave Velocity for Guwahati Based on Thorough Subsoil Investigation Data

Abstract

Guwahati, the Gateway of India in the northeast, is a large business and development center. Past seismic scenarios suggest moderate to significant effects of regional earthquakes (EQs) in Guwahati in terms of liquefaction as well as building damages. Considering the role of local soil in amplifying EQ-generated ground motions and controlling surface damages, present study attempts seismic site classification of subsoil of Guwahati. Subsoil is explored based on 43 geophysical tests and 244 borelogs gathered from different resources. Based on the borehole data, 4 numbers of 2D cross-sections are developed from different parts of Guwahati, clearly indicating that a majority of the locations are composed of clay of intermediate to high plasticity while at specific locations only, layers of sand are found at selective depths. Further, seismic site classification based on 30 m average SPT-N suggests that a major part of Guwahati falls under seismic site class (SSC) D such as Balaji Temple and Airport. However, Assam Zoo, Pan Bazaar, IIT campus, Dhol Gobinda and Maligaon show SSC E clearly indicating the presence of soft soil deposits at these locations. Similar site classification is also attempted from MASW test-based 30 m average shear wave velocity (VS30). VS30-based site classification also categorizes most of Guwahati under SSC D. However, there are locations in the southern part of Guwahati which belong to SSC C as well. Mismatch in SSC based on two different test findings for Indian soil found here are consistent with previous studies. Further, three empirical correlations based on both SPT-N and VS profiles at 22 test locations are developed for: (1) clayey; (2) sandy and (3) all soil types. Proposed correlation for all soil types is validated graphically and is found closely matching with similar correlations for Turkey and Lucknow.