Abstract: The large scale clayey soil landslides are frequently occurred in the middle and lower reaches of Wei River. The historical Bojishan Landslide is located at the north slope of urban Baoji City.  As one of the high risk landslides monitored by Bureaus of Land Resources of both Baoji City and Shanxi Province, this ancient landslide has been observed having a long term rheological deformation and is justified to have high sliding probability. In order to accurately interpret the monitored data and predict the trend of the slope movement, the sliding zone, its dimension as well as the deformation pattern need to be evaluated. The comprehensive sliding zone investigation consisted of characterizing of the soil surface soil, rock and groundwater conditions through the test boring program. The mineral component, physical and mechanical properties of the sliding zone are also evaluated. The following results are concluded: 1) The sliding zone of Bojishan Landslide is within the high clay content stratum of Claystone of Sanmen Formation. The observed progressive rheological movements are also in this soil stratum. 2) The clay mineral component in the sliding zone consists predominantly of montmorillonite with strong fissuring and expansion characteristics. Under the action of the groundwater, the soil stratum is fissured and a continuous shearing zone could be formed and therefore triggering the sliding.

Key words: Clayey Soil Landslides, Triggering Mechanism, Clay Minerals, Rheological Deformation