The relation between the mode of fragmentation and the stress field parameters in a fractured medium

Abstract

The mechanism of fragmentation of fissured rocks acted on by blasting has certain features which distinguish it from the mechanism of fragmentation of monolithic media. In rocks with a developed system of fine fissures, the pressure of the explosion products in the cavity falls more rapidly than in monolithic rocks because the compressed gases penetrate tile fissures. The stress field is less intense and the stresses are rapidly attenuated as we go further away from the charge. This is due to absorption of the energy of the stress field as it passes through the fissures and is reflected from their wails. A large part of the stress field energy is converted to kinetic energy of the moving fragments. In this case the percussive effect of the explosion is much reduced and the piston action of the gases becomes more important. In rocks broken up into individual blocks by large sparse fissures, only the block containing the charge chamber is intensely fragmented; the remainder of the rock mass fails apart along the fissures. In rocks with this type of structure, the piston action of the explosion is unimportant, owing to the monolithic structure of the individual blocks and the low density of the fissures (i.e., the small number of cracks per unit volume).The fragment-size distribution of the blasted rubble will be largely determined by the structural elements of the spatial fissure network. A more efficient utilization of the energy stored in the stress field formed by the explosion is possible, in the first place, in conditions which hinder the opening-up of the fissures. The stress field intensity falls off less rapidly if there are only tightly closed fissures or fissures narrower than the absolute deformation of the rock in the region of localization of the fissures. In the second place, the rock must not move as an integral whole, i.e., the velocity field must have a nonzero gradient. In this case there is an additional mechanism for transfer of the stress field energy to the more distant zones in the rock: this is essentially due to collisions between moving rock fragments. A series of such collisions causes a state of stress in the more distant parts of the rock, and thus part of the kinetic energy is re-transformed to energy of the stress field.We used model materials to study the influence of fissuration on the fragmentation of a medium during blasting and the relation to the stress field. The models consisted of rosin cubes, 200 x 200 x 200 mm in size. The fissures were simulated by interlayers of materials which corresponded to the two main types of material which fill cracks in rock. Plastic incompressible fillers such as dense clay or water were simulated by modeling clay; compressible fillers like dust or fine rock fragments were simulated by dense cardboard. Hollow cylinders of modeling clay with wall thicknesses of 0.5, 1.0, and 1.5 cm and equal in height to the models were placed on the bottoms of the molds to coincide with the axis of the blast hole. Cardboard baffles were placed parallel to the cha...