Analysis of the extent of fracturing by destress blasting is presented in this paper. The fracture pattern is obtained through dynamic modelling of discrete fractures by using a new numerical modelling procedure. The analysis indicates that the application of destress blasting in hard rock mines most likely renders psychological advantage rather than factual benefits. This analysis is in agreement with an analysis of field application of destress blasting, which is presented elsewhere. It is observed from the results of numerical modelling that blasting in confinement will result in fractures aligned along the principal stress axis. This alignment is not propitious to the desired destressing or stress relaxation.
The concept of destress blasting evolved from field observations that limited induced fracturing around an excavation front, which can be observed around the excavation front, reduces the load carrying capacity of the rock and hence relieves it from build-up of high stresses up to perilous levels (Roux et al., 1957). Destress blasting is thought of as a means of maintaining and extending this fracturing. Authors of this paper and field evidence, however, suggest that the success of a destress blasting program is not alone dependant on the amount of fracturing induced. It is more important to determine how these fractures will develop in the rock mass under the influence of confinement. A critical analysis of field evidence is presented elsewhere (Saharan and Mitri, 2005a) and it is shown in that particular analysis that the following state will emerge from blasting in confined rock mass. Furthermore, field observations indicate that destress blasting is even unable to generate new fracture sets due to high confinement (Adams and Jager, 1980; Brauner, 1994; Karwoski et al., 1979 and Scoble et al., 1987). This is one of the reasons for change in beliefs of destress blasting practitioners in South Africa. Now, it is believed by them that the main objective of destress blasting is to activate and propagate the existing fracture networks in rock mass rather than to create new ones (Toper et al., 1997). The following state, however, will emerge with respect to the extension of the existing fractures as indicated by Saharan and Mitri (2005a). Unfortunately, if the quantifiable data pertaining to the nature of fractures due to rock blasting is scarce then such data for the blasting of rock subjected to high stress confinement is rare (Saharan et al., 2005). It is no wonder because a majority of the research is targeted to explore rock fragmentation phenomenon in open-pit mining due to the bulk economic interests involved. The objective of this study, which is presented in two companion publications, is to provide a systematic and quantitative basis for dynamic rock fracturing by destress blasting through analyses of a series of dynamic numerical modelling exercises. This series is the first initiative to build the necessary knowledge base in order to close the gap between the concepts and practices of destress blasting in hard rock mines.
