Mizuki NEGISHI
Effects of groundwater level on slope failure induced by rainfall
Hirofumi TOYOTA
Slope failure during rainfall is closely related to the amount of moisture in the slope. When moisture content of soils increases, an increase of shear stress due to the increase of soils' weight and shear strength reduction caused by the loss of suction occur. Therefore, in order to quantitatively evaluate the stability of the slope, it is necessary to grasp moisture distribution of inside of the slope and their strength properties.
Model slope tests using rainfall simulator considering rainfall history were carried out using three types of drainage to specify the moisture distribution of slops related to the drainage. In the experiments, groundwater level, suction, moisture content and displacement of the surface were measured to examine the relation between those parameters and slop failures.
Moreover, box shear tests were conducted to measure strength of soil and friction between soil and iron plate. However, shear testing for unsaturated soil is not convenient because of long duration of testing time, necessity of special equipment and difficulty of test method. Simple testing method for unsaturated soil was proposed to estimate the strength constants of unsaturated soil. And slope stability analysis was performed using a commercial FEM analysis to simulate the model slope failure using soil parameters obtained from the proposed simple method.
The following conclusions were obtained in this study.
1. The results of slope stability analysis, where soil parameters obtained from the proposed simple testing method for unsaturated soil were used, have a good agreement with those of model slope tests. Therefore, the validity of the proposed method was shown from the performance of the analysis.
2. The failure of the model slope is highly dependent on the groundwater level of the slope.
3. In small model slope such as the depth of about 50 cm, it is necessary for simulation of slope failure to accurately determine the cohesion of saturated soil.