Investigation of the lumpiness of a blasted mass for various diameters of a cylindrical explosive charge

Abstract

The question concerning the relationship between the lumpiness of a blasted mass and the diameter of bore charges is one of the basic questions raised during the development of a method for computing lumpiness.There is no single opinion concerning the character of the effect of the charge diameter on lumpiness in the technical literature.Some specialists consider that the lumpiness of a blasted rock mass remains constant when the same specific explosive consumption and geometric similitude between the diameter of the charges and the dimensions of the grid in which they are arranged is observed in a specific rock mass.This position is reflected, for example, in "Technical rules for conducting blasting operations in power construction" [I], where the lumpiness computation is based on studies by V. K. Rubtsov.In other publications, which are comparatively small in number, it is stated that an increase in the diameter of the charges is accompanied by an improvement in the size reduction of the blasted mass due to an increase in the reaction time of the stress waves with increasing scale of the blast.Finally, it is indicated in the majority of studies conducted in recent years that an increase in the diameter of the charge will lead to degradation and size reduction; in this case, it is usually associated with an increase in the number of natural cracks between charges as their diameter increases, and, consequently, the distance between them, i.e., with the factor of the independence between the natural fissured state of the rock masses and the scale of blasting.Thus, Kutuzov and Rubtsov [2] and Kutuzov [3] describe the relation between the required specific consumption of explosives and the diameter of the charge by the equation q ~(0,6 + 3.3d.x),where d is the diameter of the bore charge in m, and x is the average dimension of the cleavage in the mass in m.The possibility of reducing the scale effect of the variation in the coarseness of material size reduction to the effect of the natural jointing of the rock mass alone raises doubts, since then it should not be observed in a solid or slightly cracked medium; this contradicts most practical observations. The purpose of the present study was to evaluate experimentally the effect of variation in charge diameter on the lumpiness of a blasted mass under conditions of geometric similitude between the dimensions (diameter and length) of the charges and concrete blocks (solid and cracked) in which the blasting was carried out.The blocks were cylinders of a ceanent--sand mix with a through axial channel for placement of the charge.The compressive strength of 4 x 4 x 4 cm control cubes formed from this material was 42 MPa.Some of the blocks were prepared as solid blocks, and others as sectional blocks formed from concentrically placed tubular elements between which inserts of a thin perforated paper were placed to simulate cracking.The number of elements in the composite blocks numbered two or three, depending on the block diameter.The distance between "cracks" was 5-6 cm.S...