Predicting blast-induced damage for open pit mines using numerical modelling software and field observations

Abstract

Blasting is the most common method used to fragment rock in the mining industry. However, given the violent nature of explosives and the high variability of results that can occur from blast to blast, there is potential to cause significant damage to the final walls of an open pit, which can lead to slope stability problems, catch bench filling, long-term rock fall hazards and ramp closure. Blasts need to be designed to suit the characteristics of the rock to be broken. Characteristics of the existing rock mass such as natural jointing, joint orientation, joint condition, and the strength of the rock, all need to be accounted for prior to designing a blast. In general, blasting engineers rely on a combination of empirical analysis and rules of thumb for blast designs. The uncertainty involved with these techniques can lead to significant problems in open pit mining. At the bench scale of an open pit mine, the loss of the bench crest is a concern, however at the full pit scale, bench deterioration can jeopardize worker safety and lead to potential closure of the mine. The results of a blast can be highly variable – a blast design that yields favorable results on one side of a pit can have detrimental effects on another wall of the pit or at different elevations in the pit, based on the characteristics of the rock. It often takes multiple iterations of blast designs to achieve an optimal result, which is costly and time consuming for the company that operates the mine. The purpose of this thesis is to evaluate the effectiveness of a relatively new software package, Blo-Up, that combines both a finite difference continuum code and a distinct element code in order to model the entire blasting process from start to finish. The main focus of the research will be to examine blast induced damage sustained to final pit walls and provide techniques for minimizing damage. The specific areas of the study are: 1) To confirm the software is able to give results similar to those observed in the field; 2) To model pre-split designs in homogeneous rock; 3) To model pre-split designs in jointed rock masses; 4) To model the effect of a production hole detonation on inclined pre-split holes, as opposed to vertical pre-split holes, and 5) To model effects of large scale production blasts on final wall stability. For the purposes of this review, kimberlite rock was chosen to be the focus of the study due to its ductile characteristics, which makes controlled blasting difficult. The main findings of the research are as follows: 1) The software is able to replicate blast outcomes observed in the field; 2) The importance of tailoring the pre-split design to the rock mass is critical, and 3) The main production blast must be well balanced if the explosive energy is to be evenly distributed through the system.