The level of deformation of the rock massif of a blasted slab must be planned in advance, depending on the required results of blasting. Thus the energy costs of barren rock overfilling as part of preparing for overburden excavation are inefficient. On the contrary, an increase in the blast energy spent on degrading and breaking the ore mass is an efficient measure of preparing for the excavation of mineral wealth. There are currently two methods used to determine the pre-destruction of a blasted rock massif. The first one is based on determining the number of strain waves passing through locations of borehole charges. However, this method fails to determine the preliminary rock destruction level. The second method is based on determining coefficients of the pre-destruction of a rock massif by these strain waves. The merit of this method is that it allows evaluating the quality pattern of the pre-destruction of a rock massif. The procedure of considering the fraction of energy of the strain waves, reflected by the shielding rock mass to the destructive amount of blasting charges and refracted to this destroyed rock, is proposed.

The article deals with improving the quality of rock blast crushing due to increase in delay intervals. The recent decades change in understanding of the rock destruction mechanism under the blast loads influence is shown. The blast mechanical effect reveal itself in rock crushing as well as in the loss of strength away from a charge. The blast waves are acting as compressive and tensile loads in this area. Existing micro-defects and micro-cracks grow as a result. The rock mass strength properties changes. A new condition of the rock mass is called pre-destroyed. The blasts time delay increasing causes cracks merging and a gradual size decrease of the rocks structural elements.

The article deals with improving the quality of fragmentation of rocks by blast due to increase in intervals of delay. It shows the change in understanding of the mechanism of rock destruction under the influence of blast loads in recent decades. The mechanical effect of blast loads is manifested not only in crushing of rocks, but also in the weakening of their strength in remote distances from a charge. In the area of the blast wave compressive and tensile loads lead to the development of existing micro-defects and micro-cracks. The rock mass changes its strength properties, passes into a new state, called pre-destruction. The increase of delay time between blasts of charges increases the overall time of the blast, contributes to the merging of cracks and a gradual decrease in the sizes of the structural elements of the rocks. Impact on rocks alternating loads up to 20 or more times allows to increase the distance between the charges.

The dynamics of the development of an industrial mass explosion at deceleration intervals of 150x200 ms on a frame-by-frame video recording on a consumer video camera with a shooting frequency of 25 frames per second is studied. Possibilities of visual assessment of the explosion site of individual borehole charges are evaluated. The main difficulties in recording processing are caused by the difference in the intervals between frames (40 ms) and decelerations between charges (50 ms). A significant deviation of the actual deceleration intervals from the nominal value was revealed, which significantly changes the actual picture of the development of a mass explosion, including an increase in the actual development time of a mass explosion against the calculated one. The passage through the zones of the location of individual borehole charges up to 40 stress waves was established. Such multiple impacts of stress waves are expressed in a decrease in the magnitude of the interval velocities of dust and gas emissions as the number of impacts of stress waves increases. Due to the multiple effects of stress waves in the extension stage, the fracturing of the rock mass increases. This is expressed in a decrease in the dynamics, and then in the termination of the outburst of bottom hole material as the disturbance of rocks in the prefracture zone increases. There is no breakdown of rocks; the “ore-rock” contacts have retained their original position, which makes it possible to reduce the dilution of ore to a minimum.

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