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 pressure 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 (fp =31.1°?cp=158kPa), and a formula for the critical failure line (tc =s'n* tan34.6°) were also obtained.
Taiwan is located in the subtropical climate area of West Pacific Ocean. Typhoons bring heavy rainfalls in rainy season and significantly influence the terrain on this island. Moreover, slopes in the southwestern Taiwan are mostly composed of soft sandstone and mudstone. Because the soft rocks are typically poorly cemented and are easily decomposed by water, sandstone slopes usually collapse after heavy rainfalls. This slope failure can be classified to be shallow, burst type and high speed failures, and is greatly governed by the rainfall intensity. To understand the failure mechanics of the soft rock slope, the shear mechanics of soft sandstone is conducted under the fast sheared with large displacement condition. Conventionally, the quick direct shear test is applied to evaluate the shear strengths of rocks and soils. However, the limited shear displacement of the direct shear apparatus causes that the sample cannot get into its real residual state. Additionally, the reversal direct shear test can not test under the constant sheared section. Because of these two reasons, higher shear strength parameters might be obtained. Consequently, the ring shear test equipment, which enables the large displacement of single sheared direction under constant sheared section, is carried out in this study to get shear strengths of soft sandstone.
2. SLIDING-SURFACE LIQUEFACTION AND SHEAR ZONE FAILURE THEORY
In this study, the undrained ring shear test was conducted to investigate the slide phenomenon occurring in a soft sandstone slope after a rainfall.