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Examination of toppling behaviour in large rock slopes using the UDEC computer code Nichol, Susan L.


ABSTRACT The question of how to predict the catastrophic failure of a slowly deforming rock mass, be it a natural or engineered slope, remains unresolved despite a large body of research on the topic. The transition of the deformation mechanism from slow, self-stabilizing toppling to rapid, catastrophic detachment continues to hold the interest of researchers due to the proximity of many such deforming slopes to vital infrastructure such as major transportation routes and hydroelectric power facilities. The idea that certain key parameters may influence toppling behaviour in a quantifiable way was examined through a qualitative study of a large rock slope carried out using the Universal Distinct Element Code (UDEC). The slope was modelled using variations of intact rock strength, discontinuity orientation and persistence, and toe conditions. While it was not feasible to consider every possible parameter, this study allowed the influence of small changes to be tracked, and boundaries of behaviour to be mapped. The study showed that stable flexural toppling develops in rock masses characterized by weak to medium-strength rock with relatively few cross-joints. Stresses in these cases remain sub-parallel to the ground surface in the upper portions of the slope. The study demonstrated that deformation of slopes undergoing flexural toppling is relatively slow and generally does not accelerate. With slow rotation, discontinuity dips become sufficiently shallow that flexural toppling is no longer kinematically feasible and these slopes ultimately stabilize.. With more persistent cross-joints and stronger rock, the study showed that deformation generally accelerates and catastrophic brittle toppling failure is more likely to occur. This type of failure is strongly promoted by toe undercutting. Stress distributions show sub-vertical stresses dominating the upper portions of the slope. Simplified models of Mystery Creek (catastrophic failure) and Mount Breakenridge (stabilized) were able to demonstrate these contrasting types of behaviour.

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