Shear Strength Reduction in Progress of Shear Displacement on the Landslide Near Dam Reservoir

Abstract

Landslides may cause enormous human casualties, economic losses, infrastructure damages and the change of physical environment and geomorphology. Landslides on the upstream of dam reservoir obviously occur when reservoir impoundment has affected groundwater level of the upstream slope that penetrates into the susceptible soil layers which can fall down to the reservoir. The falling landslide mass into reservoir would significantly reduce the reservoir capacity as sustainable water resource for human and agriculture needs. The study of shear strength reduction in landslide mass is important aspart of the landslide dynamics science which covers mobility after failure associated with landslide hazard level, risk assessment, landslide velocity and affected area. In this paper, the shear strength reduction in progress of shear displacement on the landslide near dam reservoir is obtained by means of ring shear tests. Examined samples were taken from the 2008 deep-seated large-scale landslide near Aratozawa reservoir that is located in Miyagi Prefecture, Japan. The observed data indicated that during eight months before landslide event on 14 June 2008, the Aratozawa reservoir was impounded significantly and caused high pore pressure in the suspected slip surface layer. The undrained cyclic loading tests and earthquake loading tests are conducted to explain the shear strength reduction in progress of shear displacement and high initial pore pressure from pre-failure state to the steady-state motion. The results of the tests implied that the sample from secondary collapse zone was the weakest specimen which is vulnerable to be mobilized in a long distant shear displacement. In addition, the ring shear test results indicated that the rapid shear strength reduction of landslide mass was mainly caused by initial high pore pressure and exacerbated by the 2008 Iwate-Miyagi inland earthquake as a main shock in the case of deep and large-scale landslide near Aratozawa dam reservoir.