This paper researches on the characteristics the formation conditions and mechanism of the toppling deformation occurring on the medium-steep bedding rock slopes.The longitudinal valley is well developed along the Bailong River trunk streams influenced by regional geotectonic conditions. The Silurian sandy slate and phyllite are well developed in this area,especially the soft thin-layered sericite quartz and carbonaceous phyllite exposed in Zhouqu and Wudu regions.They leading to the lower stability of bank slopes.Particularly,the landslides are more likely to occur in the bedding slopes as the failure modes of sliding-toppling and sliding-tension.Further,a special toppling failure mode is founded on the steep bedding rock slopes,namely,the termed sliding-toppling mode,based on the field investigations associated with Ⅶ# landslide situated in the Shuiboxia hydropower station, the Qingyaling landslide and the Mengj iagangou landslide situated in the Bikou hydroelectric power station on the Bailong River.It can be concluded that the sliding-toppling mode is primarily well developed on the steep bedding rock slopes with alternating soft and hard layers.These rock slopes generally have a favorable free surface condition with the angle of slope greater than 40°and the strata dip angle greater than 65°.The formation mechanism is induced by the slope stress field,water or earthquake.The initial deformation occurs at the lower part of the slope,along with the sliding-toppling at the upper part.Finally,the continuous toppling failure surface appears in the basal plane and results in the landslide.

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-com-pression occurs in the front of landslide;and groundwater gradually dissipates.