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Mine waste dump instability Moffitt, Karen

Abstract

Advancements in coal mining methods and rates have led to the production of larger volumes of waste rock. Consequently, large waste rock dumps in B.C. are currently being constructed by end-dumping to heights of up to 400 m with face angles of 37-38°. Segments of the foundation slopes underlying these mine dumps are frequently as steep as 30°. These developments have resulted in an increase in the frequency and size of waste dump failures that have not been adequately predicted or explained in terms of conventional slope stability analysis. Failure is often rapid with runout distances of up to 2 km causing increased concern within the industry over potential impacts to the environment and risks to the safety of personnel, equipment and infrastructure. A comprehensive review of documented field behaviour carried out for the purpose of this study has indicated common patterns of deformation that suggest the concept of a unique 'double wedge' mode of failure responsible for these large runout events. This study focuses on the development of a numerical model capable of capturing the commonly observed patterns of deformation, and investigating the development of the ensuing failure mechanism. Stressdeformation numerical analyses carried out using the computer code FLAC have yielded a good correlation with observed field behaviour and provided significant insight into the coincident stresses and deformations within the dump. Analyses indicate that while creep effects in the waste dump could cause significant effects on the magnitudes of displacement, the overall dump stability is governed by the strength of the foundation soils underlying the toe region. Consideration of both the stress and velocity fields within the framework of nonassociated plasticity leads to factors of safety for various dump heights and foundation slopes that are approximately 66% less than predicted from limit equilibrium analysis. The results suggest potential inadequacies of conventional limit equilibrium analysis techniques when applied to the stability and design of waste dumps on steeply sloping terrain.

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