The latest theoretical and experimental study of physical processes initiated by explosions in solid media have made it possible to obtain a deeper insight into the mechanism of the energy transfer to ambient rock from charges of explosives interspersed with various air gaps. As shown in (1), high explosive action causes excessive fragmentation of medium in the vicinity of the charge, while expansion of the process into the massif is negligible. A theoretical outline of a more rational application of explosion energy was given with a view of obtaining more uniform fragmentation of rock convenient for loading and transportation. The proposed idea proven by experimental study consisted of the possibility of explosion control based on redistribution of explosion energy. Practical realization of the above idea was proposed (1) involving charges with air gaps for rock fragmentation.
Since the Rolla Symposiumon Mining Research (USA, 1961) (2), borehole charges with air gaps have become common practice. This method of rock blasting is applied in open cast and underground mines working the hardest and viscous, hard, medium hard and soft rocks (ore, quartzites, granites, limestones, sandstones, coals, etc.) as well as at enterprises for extraction of building materials and raw chemical minerals of any strength.
Application of charges with air gaps results in an. increase of the degree and uniformity of fragmentation (up to 94 percent of the blasted rock mass consists of fragments not exceeding 400 mm), the average fragment size decreases by 1.5-2 times and the amount of lumps is reduced by 2-10 times (in some cases there are no lumps at all). Besides, the amount of operation for filling boreholes with tamping material is reduced, the specific consumption of explosives reduced by 10-30 percent, rock mass output increases, cutter breaks inside the massif decrease, the seismic effect of the blast also decreases. At the same time the productivity of mine transport rises by 10-30 percent, and sometimes it even doubles.
Fig. 1 shows integral curves of granulometric composition of various rocks as a function of charge structure (Available in full paper), Integral curves of granulometric composition of various rocks as a function of charge structure.
Application of charges with air gaps underground results in reduction of specific consumption of explosives by 15-30 per cent, an increase of coefficient of blast hole utilization, a decrease of dilution and reduction of the seismic effect. In addition, the grading of minerals mined at coal and shale mines improve. Use of charges with air gaps is practiced at more than 100 enterprises mining ore, coal and non-metallic deposits by open cast or underground method with high productivity (3,4,5,).
Further theoretical and laboratory studies of explosion energy transfer to a solid medium carried out by engineer I.F. Zharikov under the supervision of the authors made it possible to study the inner structure of the compression wave front and determine the dependence of shock impulse transformation on the distance passed by the wave. For this purpose the optical polarization method was applied to study strains (dynamic photoelastic method) using an electromagnetic probe (6) for recording displacement speeds of the medium without any noticeable distortions.