Geological control on slope failure mechanisms in the open pit at the Venetia Mine

The Limpopo Belt is a complex accretionary terrane that has undergone numerous deformational events. Tectonically juxtaposed lithology, open to isoclinal folding, cross-cutting and re-activated shear zones, and closely interlayered metamorphic gneisses and schists make pit slope design and maintenance risky. Pit slope design effects the stripping ratio and the bottom-line profitability of a mine. The geological model is the basis on which a pit design starts. At Venetia Mine the model is a tight, northward verging syncline that plunges shallowly towards the east-northeast. The geology has been modeled three-dimensionally using GEMCOM software. The jointing patterns have been studied and hypothetically related to the geology. The synform fold model implies three major pit slope design sectors, the (a) southern limb, (b) fold hinge zone and (c) northern limb of the fold. The southern limb experiences predominantly planar failure, a problem that has resulted in a reduction in the pit slope angle from 51degrees to 37degrees and 440 in two of the southern domains respectively. The northern limb undergoes bench-scale toppling and wedge failure. The hinge zone suffers only from local wedge failure. Bench-scale folding and brittle faulting have created more local problems. Some faults create large slope-scale wedge failures. These geological variations and the relative orientation/position of the excavation necessitated the definition of a total of 11 geotechnical domains, each with an individual pit slope design. The improved understanding of the geology and its impact on the rockmass behaviour will lead to improved blasting practices and steeper slope angles.