The Shangwan giant old landslide in Qinghai province

belonging to Upper Pleistocene series loess of Quaternary System and Guide Group mudstone of Neogene System

is a typical multi-stage rotating landslide and the sliding surfaces are mainly in the mudstone of Guide Group. Based on the full use of the materials obtained by the field landslide survey and drilling test

through analyzing the spatial position relation between key layers (loess soil and conglomerate deposits) in the landslide deposits and mudstone deposits

as well as the sliding surface morphology of the old landslide body and the sludge (dammed lake sediment) on the top of the loess

it is confirmed that the Shangwan giant old landslide body had mainly experienced two large-scale slides. And then

with the theory of groundwater dynamics

the groundwater level can be calculated before the two slides

and a numerical model of landslide deformation and destruction mechanism can be established. Combining the regional neotectonic movement characteristics

the dynamic change of underground water level

the feature of significant decrease in strength of mudstone in the saturated state and the FLAC3D numerical simulation results

it is concluded that the first sliding of the old landslide is mainly caused by river incision

and three stages of slope deformation in the process of the river incision are provided. The first sliding results in the steepening of slope’s free face

the concentration of shear stress at the slope toe

and the formation of a dammed lake in the front of the landslide

which leads to an accumulated traction sliding caused by the rising of ground water level and the softening of the mudstone.