Shear Behavior Of Loess And Loess Landslides

Loess landslides are among the most hazardous of geological processes in the Chinese Loess Plateau Area, causing thousands of casualties and damage every year. It is an important topic to understand the mechanisms of loess landslides and to analyze their stability, because this is fundamental importance for the hazard prevention and risk assessment. Nevertheless, the shear behavior of the loess and the mechanism of loess landsides have been poorly understood. To prevent and mitigate the further occurrence of loess landlsides in Chinese Loess Plateau Area, better understand the shear strength of unsaturated and saturated loess, analyze the evolution and triggered mechanism of unsaturated loess slopes, as well as the mechanism of saturated loess landslides moving with high velocities and long run-out distances and seasonal reactivation induced by irrigation, and to afford the reliabile parameters for slope stability and liquefaction hazard assessment, therefore, in this research, fundamental tests were performed on loess to study its drained and undrained shear behaviors under large shear displacement, different shera methods and mult-test programs using a ring-shear apparatus.For understanding shear behavior of unsaturated loess, as well as the evolution and triggered mechanism of unsaturated loess landslides, the effects of water content, shear displacement and normal stress on drained shear behavior of unsaturated loess were examined by means of the single ring shear test program and multistage ring shear test program. Additionally, the effects of void ratio and salt concentration in pore water on undrained shear behavior of loess were conducted to help better understanding the liquefaction of saturated loess and seasonal reactivation of irrigation-induced loess landslides. These test results were applied to explain the mechanisms of the unsaturated loess landslide in the Jouzhoutai terrace and the irrigation-induced loess landslides in the Heifangtai terrace within Lanzhou area.From the results of drained ring-shear tests in unsaturated loess, it showed that water content, shear displacement, normal stress and shear test methods had obvious influence on their shear behaviors and shear strength parameters (i.e., cohesion and residual friction angle). When water content of the samples increases, their shear resistances decrease, and their cohesion firstly increase to a certain value and then rapidly decrease zero, and residual frication angle is first to decrease until the certain value after which lightly increase, thickness and water migration degree of shear zone increase. Yet, shear resistance, cohesion, residual friction angle, as well as thickness and water migration degree of shear zone increase with increasing of shear displacement. Furthermore, an increase in normal stress causes increase in shear resistance, thickness and water migration degree of shear zone. Additionally, cohesion is greater in multistage test method than single test method, whereas the residual friction angle is almost same in the two methods.According to the results of undrained ring-shear tests in saturated loess, it showed that its shear behavior was extremely sensitive to changes in void raito of the samples and salt concentration in pore water. It is found from the test results that peak shear strength and steady-state strength of the saturated samples increase with the decrease of void ratio. Furthermore, the peak shear strength and steady-state strength increase with increase of NaCl concentration until a certain value, after which they decrease with further increasing NaCl concentration. Meanwhile, the peak shear strength and steady-state strength of the retrieved samples decrease that of the original sample when salt within these retrieved samples were removed or diluted by deaired distilled water. The assessment results of liquefaction behavior of saturated loess showed that static liquefaction resistance increase with the decrease of void ratio and salt concentration in pore water. Moreover, brittle index, which is related to void ratio, depends on the type of pore water pressure increase, i.e., the brittle index increase with the decrease of void ratio when the increase of pore water pressure occur simultaneously with shear failure, whereas the brittle index increase with the decrease of void ratio when the increase of pore water pressure occur during collapse period. Meanwhile, the steady-state line of the saturated loess is built based on relationship between void ratio and steady-state strength.Based on the above test results, the mechanisms of unconscious failure on unsaturated loess slopes without triggers and the seasonal reactivation of loess mudflow triggered by irrigation are explained. For the unsaturated loess landslides, these failures are due to the decrease in shear strength of shear zone, which is attributed to the increase of water content of the shear zone in shear. For the seasonal reactivation of the irrigation induced loess landslides, this is related to seasonal change in shear strength of loess due to the variation of salt concentration in pore water.This study examines that the shear behavior of loess on drained and undrained ring-shear tests, and explains that mechanism of the change in the shear resistance and shear strength parameters, and applies these results to better understand the unconscious failure on unsaturated loess slopes and the seasonal reactivation of irrigation-induced loess landslides.