The importance of the reflected stress wave in rock blasting

Abstract The fragmentation process in rock blasting is usually considered from one of two extreme viewpoints. In the first, stress waves are largely ignored and the pattern of failure is usually given in terms of the large gas pressures built up during the explosion process and the static stress field would exist for the particular geometry of loading. In the second, stress-wave reflection at free surfaces and ‘spall’ processes are considered as dominant. Though ‘spall’ fractures become increasingly important for large charges producing intense shocks the situation considered in this paper is for charges which cause fracture growth from the region of the borehole alone. In earlier work little importance has been given to the effect which the reflected waves might have on the developing fractures. It is suggested here that the reflected waves play a major role in determining which fractures develop from the region of the borehole and in which directions they propagate. The new interpretation does not in any way minimize the influence of the gas pressures as the main driving forces for opening the fractures and throwing the rock forward. Experiments are described in which Perspex models were used to simulate rock blasting situations: the results give strong evidence for the importance of reflected waves. It is shown that the stress-wave interaction can also satisfactorily explain earlier results on the differences in break-out angle between concentrated and extended charges in rock blasting. Finally it is suggested that control of the stress wave interactions, either by the choice of geometry of the free surface or in the positioning and time of firing of neighbouring charges, could have practical significance.