Numerical Modeling to Investigate the Influence of Jointing on Production Blast Fragmentation

Blasthole pattern selection is often decided without any evaluation of the structure of the rock mass to be blasted. This is in part due to production constraints that can determine the position of a given blast in the pit, thereby making it difficult to correctly accommodate the rock structure in the pattern orientation. An improved understanding of the influence of blasthole pattern in relation to the jointing in the bench could aid blast design selection to achieve the required fragmentation. Numerical modelling using a hybrid finite/discrete element program coupled with a non-ideal detonation model was conducted to evaluate different orientations of a single joint set, and the influence of the blasthole spacing-to-burden (SB) ratio within different configurations of two joint sets. A comparison of the particle size distribution curves for the range of blast deigns and rock masses modelled highlighted the general trends that: wide jointing produces finer fragmentation than tighter jointing; wider SB ratios produce finer fragmentation in square jointing; and an equilateral SB ratio produces finer fragmentation in angled jointing. 1. INTRODUCTION The aim of this study was to use numerical modelling to undertake an analysis of the influence of rock structure on fragmentation. While rock structure includes, folding, faulting, bedding planes and joint sets, for this study only jointing is investigated. However, even within jointing there can be a significant variation in how they present in the rock mass, for example in the top image of Figure 1, there are consistently spaced joint sets, while in the bottom image there are abrupt changes in the joint spacing and resulting in-situ block sizes. Previous studies into the influence of rock structure and discontinuities have tended to be conducted in the laboratory. The detailed work by Fornery et al., (1983) investigated the influence of discontinuities on the transmission of blast induce stress waves, highlighting the resulting complex wave patterns. A considerable amount of work has been undertaken by researchers in India, constructing laboratory scaled benches from slabs of limestone bonded together with plaster of paris. Bhandari (1983) conducted a series of single blasthole tests with vertical and sub-vertical joint sets orientated parallel and perpendicular to the free face (Figure 2) and found that jointing perpendicular to the face produced excessive backbreak, while parallel jointing caused poor toe breakage.