Effect of Pressure Gradient on Critical Shear Stress of Cohesive Soils

Abstract

Part of the difficulty in simulating or understanding the erosion of cohesive soils is the near impossibility of replicating field conditions, including the constantly varying pore water pressure and resulting seepage pressures in response to changing overlying flow depth and groundwater conditions. Unlike granular soils, for which pore pressures respond to imposed conditions in a relatively short time, pore pressures in cohesive soils tend to take considerable time to reach steady state under new imposed conditions. Consequently, pore pressure gradients can exist within the soil matrix. Herein, we evaluate the influence of pore pressure gradients on critical shear stress for entrainment of cohesive soils. The Erosionometer, a test device previously introduced for fast and accurate determination of the critical shear stress of cohesive soils based on physical shearing and uplifting of the soil surface, is modified to test for the critical shear stress while a hydraulic pressure gradient is applied to the sample surface. Tests on multiple cohesive soils demonstrate that the critical shear stress increases linearly with increasing drainage gradient (downward seepage pressure).