Abstract: The stability analysis of slope has been a classic research field in geomechanics and geotechnical engineering. Numerous research results and analytical methods have been proposed in this regard. To overcome the defects of imbalance thrust force method adopted in the practical specification, both two-dimensional and three-dimensional formulations of strength margin and overload margin downslide thrust are developed, based on the principle of the vector sum analytical method of slope stability. Two 2D numerical examples and a 3D numerical example are investigated. The results of 2D numerical examples indicate that the variation of obliquity of sliding surface has less significant influence on the direction of vector downslide thrust than on the direction of residual downslide thrust. The vector downslide thrust curve is smoother than the residual downslide thrust curve, which helps avoid the problem that the precision of the residual downslide thrust is significantly influenced by the variation of obliquity of the polyline-shaped sliding surface. The results of vector downslide thrust and residual downslide thrust are the same when the sliding surface is a single polyline, and the result of 3D numerical example by the proposed method is consistent with the common understanding of the slope stability. The proposed procedures are used to calculate the downslide thrusts of a landslide in the western China under three working conditions, including the gravity condition, rainfall condition and seismic load condition. The results show that the vector downslide thrust result has been better improved compared to the residual downslide thrust. Finally, the potential improvements in the vector downslide thrust method are discussed.

Key words: slope, vector sum method, imbalance thrust force method, residual downslide thrust