This paper describes a numerical modelling technique that can be used to model destress blasting in sequential mining operations at depth. Stresses and mining induced strain energy densities are computed at the end of each mining step. Destress blasting, a technique used to relieve an area of underground mine of high stress concentrations which might otherwise result in rockburst, is simulated by modifying the deformation properties of the rockmass in the destressed zone. The results indicate that large reductions in strain energy densities and stresses are attainable in the fractured zone of a destress blast, thereby supporting the hypothesis that distress blasting can be used to alleviate the hazards of rockbursts of the strain type.
There has been a growing concern in recent years world-wide over the continued incidence of rockbursts in underground hard rock mines which are getting deeper as the rich, more economically accessible deposits of the shallower depths get depleted. To cite statistics from some Canadian sources as examples, detailed records of rockburst incidents including related damage, injuries and fatalities are best illustrated by the records of the Ontario Ministry of Labour [I]. From 1928 to 1990, almost 4000 rockbursts and 57 associated fatalities were reported to the Ministry or its predecessor, the Department of Mines. In Sigma mine in Northern Quebec, between February 1961 and May 1993, about 95 rockburst occurrences were recorded. As a result, greater research efforts have been placed on the understanding of the nature of rockbursts, their control, and the accurate monitoring of stress build-up and seismic activities in mine structures which are usually the precursors of rockbursts. Apace with this research effort, emphasis has been placed over the past years on a particular form of a technique which, when properly applied, alleviates stress build-up in mine structures and therefore controls the occurrence of rockbursts. This technique is termed "Destress Blasting" and is explained later in this paper. The technique has been shown to be effective in some of the most important mining districts of the world such as South Africa, Idaho Mining district in the US, and in the deep mines of the Canadian shield around Sudbury. This paper concerns itself with the application of numerical modelling to the study of destress blasting technique in hard rock mines at depth. With the advent of high speed, large capacity computers, various numerical modelling tools are now available for modelling mine structures found in rock masses having different geomechanical properties and different configurations. The numerical modelling reported in this paper is based on the concept of calculation of mining induced strain energy densities around mine cavities [2] which was later applied to Campbell Red Lake mine [3]. The idea is based on the assumption that rockburst phenomena can be attributed to the sudden release of energy in a volume of highly stressed rock. The paper clearly shows that large reductions in the stresses and mining-induced strain energy in highly stressed zones of hard rock mines can be achieved by destressing.
