Timing Effects on Fragmentation by Blasting

Abstract

Rock fragmentation has always been an indicator for the efficiency of blasts in mines. Several suggestions have been made by blast operators and specialists to improve the rock fracturing mechanisms in order to obtain smaller fragments. The order of blast hole initiations together with the timing interval between holes has been observed to affect the blast results. In this study, a series of small-scale tests, simulating bench blasting have been made to establish the effect of delays in the sequence of blast initiation. The blasts were performed in high strength grout blocks, which were cast to provide a similar to rock condition excluding the possible structural weaknesses of the rock material. Homogeneity of the grout also helped to create a proper testing environment, ensuring the comparability of the blast results from different specimens. The grout used in the experiments had a strength of 50 MPa, density of 2.2 g/m3, and P-wave velocity of 4000 m/s. Unwanted reflections of blast generated stress waves were eliminated by confining the blocks using a yoke. The tests were made with a range of inter hole delays from 0 to 2000 μs. The fragments achieved after each blast were collected and screened to analyze fragmentation from different delays. In general, coarse fragmentation was obtained from short delays while back break was minimal. Relatively longer delays resulted in better fragmentation with more damage to the back of the blocks. The work continued to investigate the effect of blast gas on fragmentation by conducting similar tests in blocks but by placing copper pipes in the blast holes to control the gas propagation through the blast zone. Although gas penetration did not seem to be fully inhibited by the copper pipes, the new design yielded different results in the delay range which had produced optimum fragmentation. The current study is considered to be a start point for more investigations in the field of efficient blasting with regard to the requirements for the mine to mill process.