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The improvement of blasting methods for mineral extraction requires a more accurate pictuce of the mechanical laws of the accompanying processes.Rock masses are usually fractured by means of group firing of explosive charges.The explosion may be simultaneous or delayed, depending on the blasting scheme employed.Experimental data [i] show that in the case of widely spaced explosions the stress field is mainly formed by the two nearest charges, the other members of the series exerting very little effect.When investigating blasting in rows we therefore, of course, examine the interaction of two charges. This communication gives a method for calculating the stress field induced by firing two cylindrical explosive charges.The behavior of strong rock under dynamic loads will be described by the model of a solid body with internal friction.Shemyakin [2] gave a detailed analysis of the propagation of one-dimensional stress waves in a medium with friction; in particular, he derived the law of attenuation of the maximum amplitudes in a cylindrical wave:where o r is the radial component of the stress tensor in cylindrical coordinates, r is the distance from the charge center, a-v/(l --~) (~is Poisson's ratio), and c is a function determined from the boundary condition.The greater attenuation i~ comparison with am elastic wave (a r = c/~r) is due to friction, which is denoted by =.Let us examine the explosion of two cylindrical charges.In the cross section z = const (the z axis runs along the borehole axis), sufficiently distant from the end faces of the charges, we can consider the plane problem, which we solve for an infinite medium (i.e., we disregard the effect of free surfaces).In calculating the overall stress field induced by two cylindrical charges (i) and (2) located at a distance a from one another (Fig. i), we use the superposition of one-dimensional stress fields induced by each charge.A similar procedure was used by Kucheryavyi [3] to calculate the static stress field.For simplicity, in the stress field we disregard the effect of relief ("long" charges are examined), i.e., the stress curve due to each charge has the form shown in Fig. 2. We can state a priori that disregard of relief will have no effect on the qualitative results and that quantitative estimates of the stress field will be of the nature of upper limits.The stresses at each point of the plane (x, y) are determined from the laws of tensor projection. h=l Institute of Mining, Siberian Branch, Academy of Sciences of the USSR, Novosibirsk.

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