Experimental Study on Residual Shear Strength of Soil Using Undrained Ring Shear Apparatus

In general, studies of residual shear strength are minimal for soils on landslide-prone slopes in Sri Lanka. This study focuses on determining the residual shear strength of the soil samples taken from the Athwelthota landslide area, by conducting a series of undrained monotonic shear-stress control ring shear tests. The shear behavior of soil under different effective normal stress conditions was observed and further, the shearing behavior of soil samples with different contents of silt and clay was analyzed. The results showed that a small cohesion was exhibited at low effective normal stress conditions and the mobilized friction angle became constant at higher normal stresses. It was also found that peak and steady-state shear strengths decreased when the fine content increased, resulting a higher brittleness index. And the brittleness index decreases with the increase of effective normal stress. Monitoring the pore water pressure during the shearing shows that it increased with shear displacement. A significant reduction in the shear strength could result from the shear failure due to the buildup of excess pore-water pressure within the shear zone. The specimen confined under identical stress conditions generated higher excess pore water pressure within the shear zone when the fine content was higher.