DEFORMATION AND FAILURE MECHANISM OF OUTANG LANDSLIDE IN THREE GORGES RESERVOIR AREA

The Outang landslide is a giant ancient consequent bedding rockslide of about 9.0107m3 in volume and 1.78km2 in area. It is a huge threat to the safety of lives and property of more than 3900 people. It involves the entire town relocation, which can be one of the serious landslide disasters in this area. On the basis of field investigation and engineering-geological exploration, the special geologic environment and characters of Outang landslide are examined in detail. The deformation characteristics of the landslide are studied in-depth with field monitoring data. On this basis the deformation and failure mechanism of Outang landslide are analyzed from the two following aspects. They are the geologic origin and the environment origin. The deformation trend of the landslide is also predicted using the stability calculation result. The result shows that:(1)The Outang landslide is a multi-phase consequent bedding rockslide with three landslide phases. It has the characteristics of apparent dip slide in 3D spatial form. (2)The particular geological factors include the landform, stratum lithology, geological structure and are the internal factors of landslide formation. (3)The cyclical fluctuation of the reservoir water and the concentrated rainfall are the external environment factors inducing the landslide deformation. Analysis shows that the landslide deformation is significantly related to the water level reduction and the concentrated rainfall. Because the reducing speed of reservoir water level is much faster than that of the underground water, it forms seepage force in the landslide body apparently, and promotes the landslide deformation. The concentrated rainfall increases the self-weight and reduces the strength of slope mass, which makes a lot of water enrichment in easy sliding soft layer, softens sliding zone and hastens the landslide creep deformation. (4)The third phase landslide mass and the western side strong deformation zone have the possibility of unstable statement under extreme conditions. So it is inferred that the major failure mode of the landslide is a local buckling failure. In view of this, it is suggested to strengthen monitoring and to take corresponding engineering measures.