Underground mining in gold, base metals and coal deposits has occurred for centuries, and even millennia in some parts of the world. The mechanization of mining and economic appeal of open pit mining in the last 30 years has facilitated the conversion of many historic underground mines into large open pits. Historic underground workings or voids below the working floor of an open pit and voids behind or intersected by pit slopes have the potential to collapse and induce uncontrolled ground movement which may cause harm to personnel or equipment as well as impact upon the economic profitability of the operation. Identification and demarcation of controlled exclusion zones from potential voids beneath the working floor is common practice to prevent personnel and equipment entering at-risk areas. However, the way these potential void risk areas are determined and managed vary significantly from operation to operation. Risk management strategies for working in close proximity to historic underground workings are dependent on several factors including ground conditions, historic underground mining method and size of voids (open or filled stopes, room and pillar, caving, horizontal and vertical developments) and void status (open or filled, surveyed or estimated size). Blasting to collapse void areas and controlled excavation practice are paramount for safe mining. This paper discusses various approaches of managing risk in open pits with historic underground workings below from gold and base metals deposits in Australia and Africa.
Underground mining for the last hundred to thousand plus years around the world has left behind voids of different size and geometry. The voids are also located in ground conditions of varying strength, geological structure, weathering and alteration, which may change over time. Near-surface void collapses have caused problems in open pit mining operations for many years, although this problem may also present a risk to the public in urban and rural settings.
Reliably identifying the presence of near-surface voids is paramount when commencing open pit mining in a region with history of underground mining. Although the underground mining method may have been various forms of stoping, room and pillar, or caving, the first aspect of almost all void management processes is identification as shown in Fig. 1.