SITE CHARACTERIZATION AND LIQUEFACTION POTENTIAL ASSESSMENT OF AN URBAN HABITAT IN INDO-GANGETIC PLAINS

Abstract

In India and worldwide, several seismically prone regions are situated in deep soil deposits having deep basement depths. Local site conditions play an important role in modifying the ground motion at the surface when the seismic waves propagate in the upward direction from rock to the soil layers. The risk becomes manifold when the soil strata which is liquefaction susceptible is in the vicinity of seismogenic sources capable of generating moderate to great earthquakes. Such conditions exist at locations which are very near to the mountain building where the fan areas of rivers constitute deep basins in juxtaposition which are filled with alluvium with time. Implicitly, it requires proper ground response analysis to estimate the seismic damage in terms of liquefaction potential which becomes of paramount interest due to presence of deep soils in this region. One such example from Himalayas is being explored in the present study. Roorkee, an important town in the IGB at the foothills of the Himalaya is vulnerable to strong ground motion due to presence of seismogenic sources already with high hazard values. The city has experienced many damaging earthquakes in the past. Seismotectonically, it lies on seismogenic feature namely, Delhi-Hardwar ridge which is a linear NE-trending ridge with western boundary defined by Dehradun-Mahendragarh fault. The work has been carried out in various steps, defining the objectives of the study, such as, site characterization using the geophysical, geological and the geotechnical data; compilation of seismicity and geology of the study area and to develop a seismo-tectonic model for the region, predict strong ground motion for various exceedance probabilities, assessment of liquefaction potential of soil using numerical modeling approach and proposing mitigation measure for the region for increasing the liquefaction resistance of soil. Site characterization is carried out using the geophysical techniques, geotechnical and geological methods. The upper layers are characterized using Multichannel analysis of surface waves (MASW) and Micro-tremor analysis using ambient noise for the estimation of shear wave velocity profiles. The general soil depths below Roorkee has been explored the help of geological information and the bore logs available combining with the geophysical methods. Based on the information from experimental data, data available from earlier records and the geological data from deep seismic sounding (GSI, 2000), the soil profiles in terms of layers, thickness of soil, Gmax, VS are reported. Site characterization of thick alluvium below Roorkee shows that basement depth of 3000 m above which engineering bed rock is overlaid by soft soil cover.