Abstract: Landslides of quaternary deposits along the gentle bedrock-cover interface frequently occur during the construction of Bazhong-Dazhou-Wanzhou expressway. Strengths of unsaturated soils affected by groundwater and excavation are the two main factors of the landslides. On the basis of unsaturated direct-shear tests, the unsaturated-analysis soft wares of SEEP/W and SIGMA/W are used in the analysis of sliding mechanism. The results shows that distribution of groundwater seepage produces the soft soil along the bedrock-cover interface. The distribution of groundwater seepage leads to the distribution of matric suction which causes the deterioration of shear strength from top to bottom. Eventually, the soft soil with the weakest strength distributed along the bedrock-cover interface. With the cutting of the deposits with horizontal beddings, the soft soil apparently surrenders and deforms. Tension failures appear at the back of landslide; the middle part of landslide creeps along the bedrock-cover interface; shear-compression occurs in the front of landslide; and groundwater gradually dissipates.