Though ground subsidence is generally associated with softer rock formations, several occurrences of cap rock failure and sink holes have been reported over open stopes of hard rock mines. Lead-zinc mining is being practised using underground stoping methods in the state of Rajasthan in northern part of India. In most of the cases the orebody occurs as steeply pitching deposit, but often some limbs or lenses of the main orebody occur flatly dipping. This makes them prone to sudden collapse, when extracted using open stoping methods at shallow and moderate depth of cover. It is therefore important to design the stoping method to avoid collapse of the surface crown.
One such case example of a lead-zinc mine, where the orebody occurs almost flat below private agricultural land and in the vicinity of some important surface structures, is on the spot light. A stoping method and an extraction sequence are designed, using elasto-plastic numerical modelling to control surface subsidence, ensure operational safety and maximise ore recovery. The subsidence control measures adopted include leaving optimally designed post-pillars and back filling of the stoped out areas with cemented rock fill (CRF). About 5% of ore is designed to be left in post-pillars. With side confinement offered by the CRF on to these post-pillars, they shall ensure safety and stability of the surface crown during stoping operations. In addition to these measures cable bolts of 10m length are also used for reinforcement of the stope back and to ensure roof stability during extraction. Several hit and miss attempts using elasto-plastic numerical modelling have been done to arrive at the optimum extraction sequence and positioning of the post-pillars. The height of yield zones predicted from the numerical model is used as the main criterion to optimise the design. A scheme for cap rock monitoring is also devised to ensure ground stability. The designed stoping scheme has been successfully implemented in the mine.
Lead-Zinc deposits of high grade are being mined in the Northern part of India for several centuries using underground stoping methods of mining. Some methods involve backfilling for better ore recovery and ground stability. One of the cheapest form of back filling technique effectively used in some underground mines is the Cemented Rock Fill (CRF) (Stone, 1993). Ground subsidence is mostly associated with soft rock formations and flatly dipping deposits such as coal (Whittaker and Reddish, 1989). However several occurances of subsidence and sink holes have been reported over open stopes of hard rock mines as well (Szwedzicki, 1999, Hutchinson et al., 2002). It is hence important to design a stoping method with backfilling most of the voids prior to mine closure to ensure long-term stability of the surface. In this paper a case example of an underground hard rock mine with a flatly dipping orebody is presented. A stoping method in tandem with CRF backfilling is designed using three-dimensional elasto-plastic numerical modeling so as to ensure ground stability during and after extraction.
