Shear wave velocity-based liquefaction geohazard study of alluvial deposit in the south of the foothills of tectonically active Himalayan region

Moderate-size earthquakes, and the presence of water saturated soil in the near surface can trigger the liquefaction geohazard causing buildings to settle / tilt or collapse, damaging bridges, dams, and roads. A number of paleo-seismic research have focused on the Himalayan area as a potential site for liquefaction. The present study site is in the south of the tectonically active Himalayan foothills and lies in earthquake Seismic Zone III. Therefore, the region can experience earthquakes from nearby regions and can potentially damage civil infrastructures due to liquefaction. The objective of this paper is to determine the susceptibility of alluvial soil deposits to liquefaction for seismic hazard and risk mitigation. Liquefaction geohazard study of alluvial deposits was carried out using shear wave velocity (Vs) profiling. Preliminary assessment of the soil is made by building the average shear wave velocity map up to 30 m depth (Vs30) and by constructing the corrected shear wave velocity (V-s1) maps. It was observed from the Vs30 map that a major portion of the studied area lies in Site Class CD and only a small portion lies in Site Class D. Moreover, it is also noticed from the V(s1 )map that a smaller section of the area has V(s1 )lower than the upper limit of V-s1(& lowast; )(215 m/s) below which liquefaction may occur. The region showing lower values of V(s1 )is further examined for liquefaction hazard as per the guidelines given by Andrus et al. (2004). Resistance of the soil to liquefaction, stated as cyclic resistance ratio (CRR), and the magnitude of cyclic loading on the soil induced by the earthquake shaking, stated as cyclic stress ratio (CSR) are computed for the area. Several maps of factor of safety (FS) for different depths are prepared by taking the ratio of CSR and CRR. When FS < 1, the soil is considered prone to liquefaction. Furthermore, susceptibility of soil to liquefaction against different peak horizontal ground surface acceleration (PHGSA) and varying depth of water table is also evaluated in terms of factor of safety. It is observed from this study that for lower levels of PHGSA (up to 0.175 g) the soil can be considered safe. However, the soil becomes more vulnerable to liquefaction when PHGSA is above 0.175 g and with rising water table. The comparison of the factor of safety (FS) obtained using the SPT-N method and the Vs-derived approach shows consistent results, with both methods confirming the absence of liquefaction in the studied soil layers.