Geotechnical Characterization of Hillslope Soils of Guwahati Region

Abstract

Hillslopes within the city of Guwahati consist of geological stratification that characterize progressive stages of residual weathering, which can be categorized as basal rock, decomposed granitic rock and corestones, saprolitic and lateritic residual overburden. Saprolite formation is the layer of residual soil derived from isovolumetric weathering of the bedrock. Undisturbed saprolite formation looks very compact and retains much of the parent rock structure and fabric, but actually is extremely porous due to washing out of the finer particles. Such soils are very friable and once disturbed are exceedingly susceptible to landslides. Several rainfall-induced landslide occurrences have been reported in the hillslopes of Guwahati region. Characterization of these soils is important for proper analysis of hillslope stability of this region. Laboratory test has been performed on disturbed and undisturbed soil samples to assess the geotechnical characteristics. Test to determine grain size distribution, specific gravity, Atterberg’s limits, in situ dry density, shear strength parameters, and permeability were conducted. Hillslopes within Guwahati region consist of residual soils in unsaturated condition. The soil–water characteristic curve, which is an important characteristic of unsaturated soil, is determined and compared with that obtained from empirical methods using grain size distribution and Atterberg’s limits provided in the literature. Unconsolidated undrained triaxial test was conducted on statically compacted samples remoulded to in situ density at different water content to understand the effect of degree of saturation and moisture content on the shear strength of the soil. Such detailed characterization would provide the requisite understanding for an efficient analysis of the rainfall-induced natural hillslope failure in this region.