INTRODUCTION
Pre-split blasting is a technique used to reduce damage to excavation profiles during blasting by pre-forming a continuous fracture between parallel boreholes lightly charged with decoupled explosives along the line of the required surface. Various theories and hypotheses have been presented to explain the phenomenom of pre-split blasting (see for instance Aso, 1966; Carrasco and Saperstein, 1977; Griffin, 1973 and Kutter, 1967) but no totally satisfactory explanation of the mechanics of fracture formation and extension has been provided.
Most previous approaches (Aso, 1966; Griffin, 1973 and Paine, 1961) have tended to concentrate on the mathematics of interaction between stress waves from adjacent sources. Interaction of stresses induced by expanding gases following detonation has been considered of minor importance. However, decoupling introduced during pre-splitting is specifically designed to reduce dynamic effects and to emphasize rock stresses resulting from expansion of detonation products. It could indeed be argued that the phenomenom has more in common with hydrofracture than with the conventional use of high explosives.
MECHANICS OF PRE-SPLITTING
Energy release and transfer to the rock body from an explosive detonating in a borehole in rock is a complex process, being affected partly by the relative impedances of the explosive and rock and the efficiency of the coupling, and partly by the pressures exerted by expanding gases in the borehole. It is useful to differentiate between these two aspects of the process by describing them as the dynamic and quasi-static components of energy release.
The dynamic component comprises initially a plastic headwave, decaying rapidly to form a radially expanding compression wave. The energy in the wave, its shape and velocity are related to the explosive energy and the degree of coupling of the explosive and rock and their relative impedances. The initial high energy in the wave is dissipated by local crushing at the borehole periphery and/or limited radial cracking parallel to the direction of maximum compression. According to Carrasco and Saperstein (1977) these cracks are initiated near to but not at the hole surface. Since the wave velocity is approximately three times the maximum crack propagation velocity (Edgeston and Barstow, 1941), extension of cracks by wave action is minimal and intact rocks generally have a high resistance to transient compression. The main effect of the wave is in loosening discontinuities in the rock through tensile reflection at interfaces which cross the wave path.
As the headwave leaves the zone of the borehole, the borehole itself is pressurised by the build up of the gases which are a byproduct of the rapid combustion characterised by detonation. These exert a high quasi-static pressure on the borehole sidewall. The effect of this pressure is to induce compressive radial and, more important, tensile tangential stresses around the borehole which effectively open the existing limited length cracks produced by the dynamic wave. This results in two effects:
The opening of the crack surfaces on/or near the borehole circumference will induce tensile stresses at the tips of the cracks, creating conditions for crack extension.
