Breaking action of contour charges

Abstract

Contour charges are now widely used in blasting in the USSR and elsewhere.A number of authors have dealt with the action of contour charges on rock. Unfortunately, most of their work is based on a priori laws representing the action of the charge on the rock, and they do not completely elucidate the essence of the phenomena occurring in the blasting of contour charges.Most of the experimental data in the literature obtained from models of the action of Contour charges in laboratory conditions are qualitative; they serve as a good guide in the empirical choice of charging parameters, but cannot act as a methodological guide for engineering calculations.Borovikov [1] gives a theory of the action of a contour charge on the rock, and from this there follows a method of calculation; he also gives the results of an experimental model of the action of a charge, consistent with the theory, but somewhat divergent from the results of full-scale measurements made by foreign investigators [2].In this article we qualitatively refine the essence of the physical phenomena occurring during explosion of a contour charge, and give data on the quantitative verification of the results in [1, 2] on the full scale.Borovtkov [1] assumes that the basic factor determining the intensity of the shock wave excited by a contour charge in rock, and hence the action of the charge on the latter, is the pressure at the shock wave front propagated in the air gap from the explosive charge to the borehole walls. In our opinion this assumption does not agree with present ideas on the fracture of rock by blasting and is inconsistent with many practical results in the use of contour charges.In fact, in practical conditions it is obvious that a considerable part of the explosive cartridges constituting the contour charges is in direct contact with the walls of the borehole. The pressure acting at the point of contact with the borehole wall is equal to the pressure at the front of the detonation wave. If, however, we adopt the hypothesis that the explosive cartridge is centered in the borehole, then according to Borovikov [1], ~he intensity of the wave propagatedin the rockfrom the point of contact is determined by the pressure at the detonatiou wave front, and as a result the rock is intensively brokeu up in the vicinity of the contact so that its structure is completely destroyed, i.e., the rock is crushed by the pressure of the detonation products. In reality, many observations, ~uclud[ng those of the present authors, reveal that with the actual practical charge parameters the structure of the rock in the borehole walls is always preserved, and that the breakage is in the form of cracks localized in the vicinity of a plane system of contour charges. Figure 1 shows a line drawing copied from a photograph of a marble slab cut from a block separated from the massif by contour charges of Ammonite 6zhv with a linear charging density of 0.5 kg/m, places in boreholes 105 mm in diameter.This contradiction results from the fact that Borovikov [1] did not ta...

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