Determination of fault-related sloughage in open stopes

Open stope mining is a common bulk mining method that is often affected by fault-related dilution where dominant geological weaknesses or faults exist. Invariably, most metalliferous orebodies extracted by open stope mining are associated with faults by their mode of genesis. Hence, a better understanding of the manner by which faults increase the risk for sloughage and a method to estimate the extent of sloughage in stopes near a fault are required. Elastic-plastic finite element stress models were used to show that faults increase the size of the relaxed zone in stope walls and that this relaxation increases the risk of sloughage. A criterion for fault-related sloughage, defined as the stress level below which rock blocks unravel, is established and a procedure is presented for estimating the size and shape of the fault-enhanced relaxed zone for any given set of ground conditions and stope geometry. The equivalent linear overbreak/slough, ELOS, concept is used to quantify the dilution. The procedure is applicable to blocky and moderately jointed rock masses, and 2D or 3D numerical codes can be used. Knowledge of the extent of potential sloughage helps in support design as it provides an estimate of support demand and cablebolt length. Two case histories are presented to illustrate the procedure.