Shear behavior of bedding fault material on the basal layer of DGB landslide

Daguangbao (DGB) Landslide (12 × 10⁸ m³) is the largest landslide triggered by the 2008 Ms8.0 Wenchuan Earthquake, in which basal shear failure develops on an interlayer fault at 400 m deep under the ground. After the landslide, a 1.8 km long (in the sliding direction) oblique shear face is exposed. Different kinds of materials in the interlayer fault of DGB landslide are taken for direct shear test, medium scale shear test, in-situ shear test and ring shear test. The test results show that fault material cohesion ranges from 20 to 320 kPa and internal friction angle from 15° to 41°. Shearing strength of interlayer fault materials is related to fragmentation degree of structure. The lower fragmentation degree the more obvious strain softening characteristics of materials, the higher fragmentation degree the poorer shearing resistance of materials. Compared with argillaceous materials in the same fault, the mylonitic materials are of higher shear strength and internal friction angle. Both mylonitic materials and breccia materials are strong in liquefying. In saturated undrained cases, shear strength of fault materials could drop to 9.7°, with S3 down to 0. In saturated undrained dynamic shear conditions, fault internal friction angle could be reduced to 23.1° and 4.2°. It is concluded that low friction feature of fault materials caused by the influence of groundwater is the main reason for destabilization of DGB landslide.