Conventional slope stability methods have been found to have several limitations when applied to the design and analysis of pit slopes in strong, fractured rock. Recent analysis of pit wall stability for the operating open pit at Diavik Diamond Mines has led to the exploration of new techniques to estimate the impact of intact rock bridges within pervasive fracture sets on the factor of safety for the pit slopes. The kimberlite pipes being mined from the A154 pit are hosted in a strong, moderately fractured granitic rock mass. Although located within the zone of continuous permafrost, the pipes are being mined in unfrozen rock below the shallow waters of Lac De Gras. To facilitate mining from surface, a water containing dike was constructed and the lake above the pipes was dewatered prior to mining. Rock structure is comprised primarily of shallow and sub-vertical joints with an average continuity of less than 10 m and a relatively small proportion of structure that extends across benches. An assessment of the rock structure using the Discrete Fracture Network approach has suggested that the critical failure surface is a step-path type mechanism with shear along a combination of shallow joints, steep joints, and intact rock bridges. This paper presents a methodology for evaluating the proportion of a failure surface that will require shearing through the rock mass, and simplified approaches to consider the impact of rock bridges on overall pit wall stability.
Diavik Diamond Mines Inc. (DDMI) operates an open pit diamond mine in the Northwest Territories, Canada. In 2002 open pit operations began to exploit portions of two kimberlite pipes, the A154N and A154S; a second open pit is currently being developed to mine the A418 kimberlite pipe and a third pipe is the subject of a feasibility study.