Investigating the shear mechanics of kuan-miao sandstone using undrained ring shear test

In Southwestern Taiwan, heavy rainfall usually triggers the collapse of soft rock slopes, which subsequently undergoes large shear displacements. Kuan-Miao sandstone is a typical soft rock distributed in Southwestern Taiwan and has an average void ratio of 0.39, average coefficient of permeability of 5.9*10^-5 (cm/sec), and slake durability index (Id₁) of 0%. A series of undrained ring shear tests on saturated Kuan-Miao sandstone were conducted to determine the shear strength and shear behavior under large shear displacement. From the ring shear test results, similar failure mechanism of sandstone slope destroyed by a rainfall can also be observed. Intact Kuan-Miao sandstone samples were used to perform a series of ring shear tests under the shear rate 1.5mm/min. All samples were saturated before shearing, and the shear resistance, pore water pressure as well as the height variation of each sample was monitored during the test. The ring shear test results show that excess pore water ressure is generated in the saturated sandstone specimen during shearing. As the effective normal stress decreases, the effective stress path moves to the left, approaching to the critical failure line. Additionally, when the shear resistance reaches the peak strength, grains are crushed in the shear zone, and the effective stress path touches the critical failure line. At this point, the stress state of the specimen moves down along the critical failure line until reaching steady state with a constant pore water pressure, i.e. the true residual state. Finally, the shear strength parameters of Kuan-Miao sandstone (φ_p =31.1°'c_p=158kPa), and a formula for the critical failure line (τ_c =σ_n * tan34.6°) were also obtained.