Application of synthetic nets as an enabler of optimised pit slopes at skorpion zinc mine

Abstract

Pit slope optimization is a fundamental design requirement for open pit mines. Several input parameters are incorporated into the slope design process including the slope angle, commodity price, recoveries and equipment size among others. The quality of the rock mass determines how steep the slopes should be designed for a safe and economic open pit mine. Despite rigorous efforts to optimize pit slopes, rock falls from extremely poor zones still occur. A complex and highly heterogeneous rock mass at Skorpion Zinc mine presents a challenge in steepening the hanging wall slopes for maximum resource recovery; competent limestone rock hosts intrusions and shear zones with extremely weathered sheared sericite schists. These ground conditions have resulted in significant rock falls from shear zones onto existing ramps and often-disrupting production. This paper presents how synthetic high wall safety nets have been applied at Skorpion Zinc mine in optimizing the pit slope angles, reducing clean-up cost and enhancing safety. Slope Stability analysis, using Rocscience software, was conducted on interim pit designs, incorporating angles of slopes with and without safety nets. Results show that the slopes were more stable when covered with synthetic nets than the ones left without support. Hence, the hanging wall inter-ramp slope angles were increased from the actual slope angle of 54° to 61° with an optimal angle being 61°. At this optimal slope angle, Probability of Failure (PoF) of <25% and Factor of Safety (FoS) of 1.2, there is a cost-saving of 8.42% of the total waste stripping i.e., a saving of US $9.6 m. The use of synthetic nets has therefore, enhanced the confidence in optimizing pit slope angles and justified the application in rock fall management at Skorpion Zinc Mine.