Hydrodynamic monitoring of a preexisting landslide during reservoir filling - a case history of Xietan Landslide, the Three Gorges Reservoir

Abstract

水庫岸坡失穩是伴隨水電工程建設而產生的一種地質災害,一般認為主要是由于水庫蓄水或運營導致岸坡水動力條件的不利演化造成的。國內外水庫岸坡失穩的實例很多,但是岸坡的水動力條件演化過程主要以滲流計算所獲得的滲流場作為依據,缺乏系統完整的現場監測資料。以三峽水庫蓄水為契機,在湖北秭歸縣泄灘古滑坡體上建立了一套自動水文監測系統,監測內容包括水庫水位、滑坡體水位、滲壓與降雨量等。從監測成果來看,岸坡的水位(或滲壓)上升速度明顯滯后于庫水位上升速度,滯后時間與岸坡的滲透性有關,滑帶、滑坡影響帶和基巖的滲透性相對較弱,滯后時間較長,但是滑坡體的滲透性良好,滯后時間較短;水位觀測井的觀測成果存在誤導,需要結合觀測井結構進行分析,建議盡可能使用標準管結構;降雨引起的滑坡體水位上升量和滯后時間與降雨強度關系密切,降雨強度越大水位上升量越大,水位開始上漲的時間越短。因此,對降雨的影響分析有必要考慮雨型,確定合適的基準時間單位。就泄灘滑坡而言,以小時為基本時間單位較為合適。

Reservoir slope instability is a geological hazard induced by construction of hydroelectric project and is generally believed to be caused mainly by the unfavorable change of hydrodynamic condition in the slope due to reservoir filling and/or operation. Many cases of reservoir slope instability can be found both in China and abroad, the evolution of the hydrodynamic conditions, however, is mainly obtained by seepage analysis, while the systematic and complete field instrumentation data can rarely be found. Taking the opportunity of filling the Three Gorges Reservoir, an automatic hydrologic monitoring system is implemented to monitor the hydrodynamic evolution of a preexisting landslide, Xietan landslide in Zigui County, Hubei Province, China. The system includes the monitoring of reservoir water level, water level in boreholes, seepage pressure and precipitation, etc. The comprehensive analysis of the monitored data shows that the rising speed of water level (or seepage pressure) on the reservoir bank lags behind that of the reservoir filling and the lagging time is closely related to the permeability of materials of the bank. The weaker the permeability of the slip band, sliding-disturbed zone and bed rock are, the longer their lagging time is. If the permeability of the main body is quite good, then only a short lagging time can be observed. Water level data from observation wells are misleading and should be analyzed with the well structure, and standpipe should be used whenever possible. Rainfall-induced water level rising in the main body and its lagging time are greatly influenced by rainfall intensity. Heavy rainfall takes less time to infiltrate into the slope and induces higher water level rising. Therefore, for the analysis of rainfall influence, a basic time unit should be determined according to the rain type. Basic time unit of hour is feasible for Xietan landslide.