The Effects of Positive Pore Pressure on Sliding and Toppling of Rock Blocks with Some Considerations of Intact Rock Effects

Abstract

The primary types of rock slope failures are block slides, rock topples and wedge slides. Unfavorable orientations for rock discontinuities provide the impetus for rock slope movement. Discontinuities that intersect the rock slope at angles greater than ϕd, the angle of sliding resistance, are considered to be potentially unstable under dry conditions. Types of discontinuities (weakness planes) are bedding, joints, faults, shear zones and foliation. Topples occur when the dimensions of the block and the slope angle of the base plane result in an overturning moment that exceeds the resisting moment. When positive pore pressures are present in the boundary fractures of rock blocks the driving force for sliding is increased and the resisting force is reduced. For topple-prone blocks, pore pressures provide additional overturning moments which reduce the factor of safety. Using limit equilibrium analysis and simple static relationships the details of block sliding and toppling are presented in a step by step fashion in this paper. Solved example problems are included. The purpose is to provide the reader with a clear understanding of the procedure for setting up and solving these problems using a basic statics approach. Intact rock or rock bridges also provide added shearing resistance along discontinuities. A brief evaluation of this phenomenon under positive pore pressure conditions is also considered. Results show that the factor of safety for sliding and toppling is greatly reduced when pore pressures are present.