In hard rock stability of exposed pit slopes is primarily defined by weak zones in the rock mass, such as joints, contacts of layers, weak interlayers and tectonic faults. The weakening impact of all types of the weak zones is defined by their orientation in relation to one another and to the exposed surfaces; their size; frequency and shear strength along the weak zones. Traditionally, laboratory shear strength tests define cohesion and friction angle for joints over a small area (approximately 20-30 cm2). It is impossible to model all features of the large joints, such as roughness, undulation and stepping properties on such a small scale. However, it is the large joints of hundreds and thousands of square meters in size, along which large failures occurrences, that threaten production safety of open pit operations.
Methodology of back analysis of cohesion & friction of joints based on pit slope failure events of mining operations in Russia and Chile. Design of bench parameters can be based on the friction angles along joints measured in laboratory conditions. Selection of the cohesion value along joints depends on the project strategy (conservative/pessimistic or aggressive/optimistic) and the acceptable level of risk.
