622.235.5 In blasting by means of elongated cylindrical charges with annular air gaps, the radii of the zones of fracture rf and overcrushing r c, other conditions being constant, depend on the gap width h.In this article we attempt to determine the pressure Pc at the front of the compression wave in the solid rock corresponding to the boundary of formation of a zone of overcrushing. For this purpose we propose a system of dimensionless model parameters which are equal for coal and the model material so that we can use the model to simulate the processes of coal crushing in laboratory conditions. From the experimental results it will be possible to estimate the dependence of Pc on the external conditions of loading.The pressures developed in the coal and rock during blasting from boreholes are much less than the threshold values for melting and evaporation, which according to Arkhipov and Stepanov [1] reach hundreds of kilobars. Therefore, the model parameters determining the processes of fusion and evaporation, were not taken into acccount. The other parameters were chosen as in [2,3]. We took account of the similarity in blasting with a counterpressure ~, the charge shape, and the energy emission determined by the composition and weight of explosives. The final dimensionless parameters of the model were reduced to the system PoUo 2Po;(Pl ~1!~. co. Gt I ~

By the optimum depth for a long vertical cylindrical explosive charge, h,, we mean the length of the monolithic stemming of homogeneous material which, under given explosion conditions (given mechanical properties of the rock and stemming, natural parting of the rock, quantity and properties of the explosives, presence or absence of air gaps between the charge and the borehole walls, and direct or reverse initiation), will give the maximum utilization of the energy of the explosion in terms of newly formed free surface (crushing) and the total crushed volume.We will consider hz from two points of view.First we calculate the optimum maximum depth hz = h+, which we find by means of the phenomenological theory of puncturing of a barrier Of finite thickness, as developed in [I, 2]. Then we will use another criterion for hl --simultaneity of ejection of the stemming and emergence of the explosion products at the free surface through the clefts in the surrounding rock [3]. The two approaches will be compared by means of an illustrative example. Experimental ResultsWe investigated the commonest stemming materials (sand, gravel, clay, and water) with charges of Ammonite 6ZhV weighing Q = 300 g, with diameter do = 4.2 cm, length ~o = 22 cm, in 1 m boreholes in a granite quarry in the region of the "Kuznechnoe" station of the Leningrad region and in the Bugaevskii sandstone quarry in the Voroshilovgrad region.To record the motion of the stemming and its time of flight from the borehole we constructed a special apparatus, based on recording the changes of inductance in a coil 2 m in diameter in which a magnetic field from ferrite rings pressed into the stemming at distances of 60, 40, and 20 cm from the borehole mouth moved.The coil was placed around the borehole. The emf induced in the transducers was recorded with an oscillograph.The velocities of the transducers were equated with the velocity of the corresponding layers of stemming.The time marks were made with an ionization gage at the opposite end of the cartridge from the detonator. The resulting oscillograms were processed in conformity with calibration graphs taken before the explosion after warming up the apparatus and balancing the measurement bridge.The start of motion of the stemming was monitored with an SKS-IM instrument. *Deceased. All-Union Scientific-Research Institue of Natural Gas.