Models for prediction of rock responses to blasting have been developed to evaluate alternative blast designs for specific applications and rock types. Design optimisation, however, further requires monitoring and measurement to ensure control over detonation sequencing, and to identify the more important influences controlling rock response.
A model of rock fragmentation by blasting has been developed by the Julius Kruttschnitt Mineral Research Centre in Australia, based on the results of measurements and monitoring at more than 25 mine sites, totalling more than 200 weeks of site work over the past three years. The model has permitted the definition of rock damage from blasting, and is being used to optimise blast design for the achievement of specific blasting objectives.
The model has the primary feature of directly utilising fracture data from fracture mapping and stereoplots, and identifies the existing fracturing as the primary influence on both fragmentation and damage. Perhaps of equal importance is the development of monitoring and measurement techniques used for the collection of data required by the model. The use of these monitoring techniques has highlighted some of the basic faults in blasting, including lack of control over charge initiation, variability in explosive and initiator performance, lack of detailed design assessment procedures, and an inadequate understanding of the mechanisms of damage and the factors controlling damage. In most applications to current operations, modelling is considered to be of secondary importance to basic control - over charging procedures, design, and product specifications.
The results of some basic blast monitoring are presented to emphasise the application of monitoring to the optimisation of blast performance. The improvements to productivity resulting from the monitoring have been Substantial, and beyond the current scope of modelling. Mines have reported savings in excess of $2 million per year in explosive and drilling costs alone, with powder factors being reduced by up to 50%, while improving fragmentation, reducing damage, and controlling ground vibration.
The JKMRC fragmentation and damage model is based on the following observations: 1. fracturing has the largest single influence on fragmentation and damage; 2. the degree of breakage of a rock is dependent on the size of the particle and the energy imparted to it; 3. during blasting, breakage or fracturing is primarily produced by shock energy or brisance, and rock displacement is produced by gas energy.
The basic assumption of this mechanistic model of breakage can therefore be stated: if the size of particle and the input energy to it are known, then the degree of breakage of the particle can be predicted. This follows from observations of the breakage response of thousands of rock particles in laboratory breakage tests.
(Figure in full paper)
The intensity of fracturing is measured by line scanning techniques, whereby the points of intersections of all natural fractures with a scan line are measured, together with the dip and dip directions of the fractures.
