Monitoring a preexisting landslide during reservoir filling

Abstract

Reservoir slope instability is a geological hazard induced by filling a reservoir and is generally believed to be caused mainly by unfavorable changes in the hydrodynamic condition in the slope during reservoir filling and/or operation. Although many cases of reservoir slope instability have been reported, very few field instrumentation data exist describing how the hydrodynamic condition changes and relates to landslide reactivation. Taking the opportunity of filling the Three Gorges Reservoir, a monitoring system was implemented to monitor the hydrodynamic and deformation response of a preexisting landslide, the Xietan landslide in Zigui County, Hubei Province, China. The monitoring items included reservoir level, water level in boreholes, pore water pressure, rainfall precipitation and deformation of borehole using inclinometers. The results indicate that there exist two phreatic surfaces in the slope before reservoir filling, the upper one is at around the top surface of the slip band and the lower one is in the bedrock. The less permeable slip band and sliding-disturbed zone constitute the relatively impermeable layer between the two surfaces. The stabilization effect of the reverse seepage during reservoir filling is temporary and limited. Reservoir filling is the main cause of reactivation.