Predicting the stability of rockpasses from the geological structure

Although rockpasses are a key element of any mine’s operations, often very little attention is given to their design compared with the design of other elements of the mine. Design, in this context, includes location, orientation, size, shape, length, method of excavation, support, system geometry, and operating principles. Literature on rockpasses contains numerous descriptions of failures of passes and rockpass systems. In many cases these failures have significant cost implications, both directly in terms of rehabilitation or replacement, and indirectly as loss in production. Problems in passes are defined as any occurrences in the pass or pass system that cause the pass or system to operate at less than its desired performance, which performance includes the production rate and designed life or planned total tonnage capacity of the pass. Three publications are referred to specifically in the context of problems with passes—those by Hambley (1987), Emmerich (1992) and Hagan and Archeampong (1999). Of the numerous problems that occur in passes and factors of influence, the following are associated with geological material, geological structure and stress level: Collapses—these may occur within the pass owing to adverse geological structure. Such collapse material may itself block the pass and cause a hangup, or, with continuing collapse, a large opening may result, which can lead to major instability and blockage of the pass Scaling—scaling from the surfaces of the pass under high stress conditions leads to an increase in the size of the pass. It is aggravated by the abrading effect of the rock falling down the pass. Stress scaling will also interact with the geological material and structure—the scaling will be worse in weaker rocks and, as a result of scaling enlargement, a geological structural collapse may occur (for example, Minney, 1990) Wear—owing to the passage of the rock down the pass, wear (abrading and plucking out of rock blocks) may lead to enlargement of the pass and ultimately geological structural collapse. Wear will be enhanced in weak rock material and in the presence of stress scaling. Wear is usually greatest on the footwall surface of the pass, but may not be when interaction takes place with weak rock materials and stress scaling Location of passes—the quality of the rock mass in which the pass is located may have the most significant effect on the performance of the pass. Often the location of passes is dictated by other factors such as proximity to the shaft system, or proximity to the orebody, and there is little scope for choice of location. However, owing to the major influence of rock mass quality, the location of the passes should be optimized as far as possible Predicting the stability of rockpasses from the geological structure