FEATURES OF SLIPPERY STRATA FOR PIANSHAN LANDSLIDES IN QINGJIANG RIVER CATCHMENT OF WESTERN HUBEI PROVINCE

The Dalong formation of Permain and the Daye formation of Triassic are the main slippery strata in Qingjiang River catchment of western Hubei province. Pianshan landslide is a typical example. Its lower part is composed of thin-bed limestone of Daye formation. Its slide belt is formed with mudstone or shale of the top of Dalong formation. Depending on field investigation, sampling and mechanical tests in laboratory, following results on the mineral and chemical compositions and mechanical properties are obtained for the two formation materials. The main clay mineral in the mudstone is montmorillonite with good swelling and shrinkage character. The content of clay minerals is 40%～75%.The strength of the mudstone decreases obviously after argillization, companying with typical creep deformation. For long term strength properties, the cohesion is 23kPa and the internal friction angle is 18.1,which are achieved with rheological tests. The amount of clay minerals in the thin bed limestone is high. The main components are illite and chlorite and the bedding surfaces are very smooth. The surfaces are abraded and the clay is oriented between the layers due to the bedding slipping deformation. The strength of interlayers decreases obviously and forms the slide surface inside the landslide body when the layers are saturated. There are good mutual effect and acceleration between the landslide evolution and Qingjiang River development. The similar geological structure and water reciprocating strip should be taken seriously.