Fractured rock seepage characteristics and its impact on slope stability

Catchments characterized by shallow soil and deep stony layers in weathered gneiss(otherwise known as ‘rock-soil dual entity’ catchments) have very low stability in comparison with common slopes with similar soil and weather rock layers. To clarify the mechanism leading to low stability of ‘rock-soil dual entity’ catchments as well as slopes and to identify the countermeasures retaining slope stability,catchment slope hydrological processes such as rainfall characteristics,soil water movement and seepage were monitored and analyzed during rainfall events,using auto-recording ombrometer,time domain reflector(TDR),and tipping bucket flow meter.The results showed that:1) rainfall characteristics such as intensity and duration,soil moisture content of slope before rain events,and slope geology were the major elements driving water seepage and slope stability.2) during heavy rainfall events,saturated zone instantaneously existed at the soil-gneiss interface,about 50 cm beneath the slope surface.Water in the instantaneous saturated zone softened the slope surface and caused sliding.3) because of spatial variations in the slope layer and the formation of instantaneous saturated zone at the soil-gneiss interface,preferential flow occurred in the slope.This phenomenon,on the one hand is good for the formation of slope runoff and further,for water supply in the downstream;but highly increases the possibility of landslide on the other hand.To retain slope stability,instantaneous saturated water is suggested to be artificially drained to reduce the incidence of hydraulic pressure build-up by collecting seepage water on slopes.