Investigation and monitoring on a rainfall-induced deep-seated landslide

The ground movement of Xiashan village landside of Zhejiang province, China, commenced in 1958 due to typhoon rainstorms followed by two subsequent slips in 1989 and 2001. The reactivation of the large-scale landslide has become a serious concern to the safety of a nearby reservoir and the local villagers. A pilot testing programwas established in 2007 using a variety of landslide monitoring facilities. The slope is of a chair-shaped form with Quaternary residual soil overlying the Pliocene sedimentary and basalt layers. The pilot test program included GPS, inclinometers of conventional and fixed types, piezometer, and rain gauge. Two potential sliding planes were identified in the inclinometers showing the possible slip mode along more than one bedding plane. A shallow slip surface dipping a low angle (10 degrees) lies in the residual soil, while a deep slip surface is located along the weak plane within the Pliocene strata. The piezometer reading indicated high groundwater level and a certain level of correlation with the displacement velocity. A critical groundwater depth was determined, and its usage as a warning threshold was discussed. In view of the very important location about an existing dam, it is strongly recommended that surface and internal drainage facilities be provided to ensure the longterm stability of the landslide.