Failure characteristics of gently inclined shallow landslides in Nanjiang, southwest of China

Abstract

Large number of rain-induced landslides occurred on 16 September 2011 in Nanjiang, southwest of China. The typical depth of failure surface was from 1 to 5m at the gently inclined interface between the superficial red silty clay and bedrock. In order to explain the failure characteristics of these shallow landslides, we conducted a rainfall infiltration simulation test, calculated rainfall infiltration depth using modified Green-Ampt (G-A) infiltration model as well as considered the atmospheric effects on the slope failures. Results show that the permeability of red clay is extremely low and the rainfall infiltration depth is limited to around 0.5m during a rainstorm. The rainfall infiltration capacity can be enhanced within the depth of atmospheric influence, which is determined as 5.4m in the study area. With the increase of soil depth and the lower degree of weathering, the void ratio and permeability are gradually reduced; thus, the rainfall infiltration continues following a decreasing trend until the bedrock is reached. Within the shallow landslides in the study area, under the condition of rainfall, the slope can keep in a stable state when the depth of wetting front is less than the thickness of soil layer. When the wetting front proceeds to the interface between the silty clay and bedrock, the potential slip surface can develop on such soil-bedrock interface, in relation to the increased pore water pressure and ground water level. Consequently, shallow failures will occur along the soil-bedrock interface in a depth of 1–5m.