Shear strength behaviour of silty soil in Kolkata region

Abstract

Silty soil / clayey silt / silty clay is a commonly available soil found in the upper reaches, down to a depth of 3-4m to even 15-16m in some locations, of soil deposit in and around Kolkata, India. This silty soil as available in the Kolkata region is generally light brown in color, with low plasticity, and it has low to moderate strength on saturation. Such deposits are also found in the alluvial deposits of north India, particularly in Ganga basin areas. Hence it is very much important to study the behavior of such soil in order to gain some knowledge of their properties, particularly their shear strength under various conditions to meet the requirements from engineering practices. Though a number of studies have been conducted in this direction, in general, the properties of reported silty soil may not be applicable for to all types of soil. Therefore, to better analyze the characteristics of locally available silty soil, this thesis highlights the shear strength behaviour of the silty soil as collected from a depth of 2-3m of the Rajarhat zone of Kolkata and analyzes their shear strength parameters at different dry densities and moisture contents based on the results of the conventional direct shear test. The dry sample exhibited maximum friction angle and after adding water to the sample, there is a sharp decline in the value of friction angle which goes on decreasing with further addition of water, due to the fact that water acting like a lubricant and is decreasing the friction. It is further noticed that there is a general trend of rising in the value of friction angle with the rise in the dry density. These findings ascertains that the shear strength of the silty soil of the Kolkata region also increase with the increase in dry densities and decrease with the increase of moisture content for same dry density. Also it can be ascertained from the test results that like granular soils, silt also shows greater shear stress with the increase of normal stress. Finally, this study reveals that, for different moisture content (w) and dry density (γd) internal friction angle (φ) varies from 24.22 degree to 33.14 degree and cohesion (c) varies from 0.024 to 0.232 kg/cm2 respectively. Maximum friction angle has been found for dry soil while maximum cohesion for soil at 4% water content.