Assessment of Effective Shear Strength Parameters in Triaxial CU Shear Test

Abstract

Test condition and shear strength parameters should be chosen to represent the field conditions as closely as possible. To stimulate the field condition, triaxial shear test was conducted with consolidated-undrained (CU) method on MI (inorganic silt of medium plasticity) type of soil. Laboratory specimens were remoulded at 98% of maximum dry density (MDD) at varying water content. Remoulded specimens were further saturated without any volume change with the help of back pressure system in the triaxial machine. The deviator stress vs. strain graphs and pore water pressure vs. strain graphs are plotted at incremental confining pressures (σ₃) and analysed at different remoulded water content (w). The difference in deviator stress and pore water pressure obtained between the specimens at respective confining pressures seems to be very less, which suggests that difference in the water deficiency between the specimens and the consequent pore water tension is greatly reduced after saturation. The maximum stresses obtained are also employed to understand modified failure envelopes and shear strength parameters with respect to total and effective stress measurement. The total angle of internal friction (ϕ) obtained is lower than the effective angle (ϕ′), whereas cohesion (c) obtained is higher than effective cohesion (c′). The difference in the results suggests that effective stress analysis should be preferred, as it is more rational. This study helps to compare the results of total and effective stress analysis and how pore water pressure (u) plays an important role in effective stress analysis.