Fracturing of rocks by complex changes

Abstract

During the fracture of rocks by shothole and borehole charges, the efficiency of the shock waves increases with the hardness of the rock [ 1].Modem procedures involving the use of shotholes and boreholes do not permit maximum possible utilization of the shock-wave energy, because during the initiation of the charges by electric detonators the detonation front is located at a large angle to the charge axis, and most of the energy of the detonation wave passes into the rock mass; when the shothole (borehole) is packed full of explosives, a considerable part of the explosion energy is expended on overerushing the adjoining rock beds by shear deformations. This shortcoming may be eliminated by using more complex charges.An additional charge ( Fig. 1: 2) with a much higher detonation velocity than the main charge 1 is located along the axis of the latter. Like the main charge, the additional charge is oriented with respect to the hole axis by centering rings 3. The gap between the charge and the walls of the holeis filled with water 4. Initiation of such a charge is effected by a plain detonator 5. When the latter is fired, the detonation of the additional charge will outstrip that of the main charge owing to the discrepancy between their velocities. Figure 2 shows the formation of the detonation front of the main charge without allowing for the effect of the water sheath on the detonation velocity of the boundary layers of explosive. Let us assume that initiation took place at a point 0, downwards along the charge axis. We can then establish the dependence of the propagation of the detonation front of the main charge axis. We can then establish the dependence of the propagation of the detonation front of the main charge with respect to the exposed surface as the function /3 = f(vt, vz), where ~ is the slope of the detonation front towards the charge axis (we will assume that the charge axis is parallel to the exposed surface) and vz and vz are the detonation velocities of the main and supplementary charges, respectively.A time t has elapsed from the beginning of initiation. Disregarding the diameter of the additional charge because it is so small in comparison with that of the main charge, we can evidently represent the detonation front 6 E/:::r!Lii as the surface of a cone with a base diameter AB. From the triangle AOC we determine tan /5 = AO/OC or, denoting AO and OC by vlt and vzt, respectively, we get tan B = vz/vz. Hence L, From an analysis of the operat ion of such a charge and of Eq.(1), we can infer that the slope of the detonation front towards the exposed surface decreases with increasing difference between the detonation velocities of the additional and main charges; if the detonation velocities are equal (which is the case for a uniform charge), t5 is 45*. With a decrease in 8, the normaI component of the detonation pressure on the charge cavity increases. It will be seen from Fig. 2 thatwhere P2 is the nomaal component of the detonation pressure and Pz is the pressure in the detonation wave front of the...