Seismic site classification and correlation between VS and SPT-N for deep soil sites in Indo-Gangetic Basin

Abstract

The Indo-Gangetic Basin (IGB) is occupied by the thick deposits of different types of loose to dense soil due to the active sedimentation. Till date, no detailed study has been carried out on seismic site characterization, and classification considering shear wave velocity >50 m depth. In this study, surface wave survey was used for determining the shear wave velocity (VS) variation with depth at 276 locations in the IGB using low-frequency geophones by performing combined active (Multichannel analysis of surface waves, MASW) and passive (Ambient Noise) surveys. To study the spatial variability of VS, based on the sediment deposition and geological variability, the whole IGB has been divided into Punjab-Haryana region (PHR), Uttar Pradesh region (UPR), and Bihar region (BR). Firstly, using the least square, orthogonal, and mixed effect approaches, a new correlation between the VS and SPT-N has been developed for all the three regions separately. The VS and SPT-N correlation has been developed considering both corrected and uncorrected SPT-N value. The residuals have been determined using the observed and the predicted VS values from newly developed VS and SPT-N correlation from each approach. For the same SPT-N value, correlation for UPR region is predicting high VS value as compared to other two regions. Studying the variation of residuals with VS, it is determined that the mixed effect is performing better for VS ≤ 250 m/s and orthogonal for VS > 250 m/s. At higher N-value, difference between VS values for all the three regions are noticeable. Further, the newly developed relationships are compared with the existing VS and SPT-N relationship. Additionally, the IGB is classified based on the average shear wave velocity in the top 30 m (VS30) as per the National Earthquake Hazards Reduction Program (NEHRP) and compared with Eurocode 8 (EC8). The VS30 is high in the southern part of the BR and UPR and low near to the active channel deposition and major part of the IGB is classified as seismic site class D. Most of the IGB sites have experienced severe damage either due to local site effects or liquefaction during past earthquakes. Therefore, the spatial variability of the average shear wave velocity at different depths, i.e., at top 5,10,15,20, 30 50, 100, 150, 200, 250 and 300 m are also estimated and compared with the available basement depth map.