Stability assessment for the landslide undergoing the effects of water level fluctuation in the Three Gorges Reservoir area, China

摘要

The Majiagou landslide, as a cause for study, is located on the left side of the Zhaxihe River, a tributary of the Yangtze River. After the water level of the Three Gorges Reservoir rose from 95m to 135m in June 2003, the Majiagou landslide occurred including a 20m- long, 3～5cm wide, (locally 10cmwide) fissure at its back that occurred in a time-frame of 3 months. The implication is, that the reservoir impounding reactivated the landslide. The slide mass of the Majiagou landslide is composed of a thin layer of silty clay on the top with low permeability and a thick layer of angular pebbles as the main portion beneath the silt clay which has high permeability. The rate of water level fluctuation is between 0.6 m/d and 4.0m/d with regard to the altitude between 145 m and 175 m in the reservoir during its normal operational state. Under these conditions, the FEM method was applied to simulate the groundwater changes in the slide mass, coinciding with the reservoir water level fluctuation. The results suggest that the groundwater level almost changes with the reservoir water level simultaneously within the slide mass, which means that the groundwater gradient is very gentle. Therefore, the effect of the reservoir water level fluctuation on the landslide stability is mainly by the action of buoyant force. Without considering dam failure, the seepage force is so little as to be negligible. The stability of the landslide in cases of different water levels is then calculated using the Morgenstern-Price method, and the results show that the factors of safety decrease with the water level rise. As the water level increases to 165m, the factors of safety are at the minimum value, increasing with the water level rise. The minimum value reflects that the landslide is in a critical state, so stabilizing design was applied using stabilizing piles as well as a water drainage system. The project was completed in early 2006, and the water level has risen to 156m in Oct. 2006. Monitoring data illustrated that there is no further deformation of the landslide and the stabilizing piles, so the stabilizing work is effective.