The wide employment of blasting energy in the solution of various problems in the national economy is prompting increasingly deep studies, not only of the parameters of crushing and ejection of rock by blasting, but also of the parameters of ~he holes blasted out. In certain specific problems in the creation of underground oil and gas storage cavities, pumped-storage hydroelectric power stations, and underground mineral leaching, it is even more important ~o determine the parameters of the blasted-out holes than ~ find the crashing and ejection parameters.It is natural ~o find theoretical [1, 2] and experimental [3][4][5][6][7][8][9][10][11] papers studying the parameters of the internal action of an explosion in relation ~ the radius or weight of the charge. The common basis of these investigations is the mechanical pattern of action of blasting in a solid medium, which, in the present state of research on the subject, can be interpreted as follows [10].After detonation of the explosive charge, a powerful compression wave steads out on all sides of the charge chamber, the pressure at its front being about l0 s kg/cm z. The pressure applied to the walls of the charge chamber is transmitted ~ other points in the medium in the form of a wave motion, which in a limited volume has a velocity greater than that of sound. In this volume, crushing of the rock takes place in a state of omnidirectional" compression, so that the medium undergoes plastic deformation rather than brittle fracture. This zone is called the zone of plastic deformation; experiments on models and in the field show that its radius is not more than 3-5 times that of uhe charge. Further fracture of the medium daring passage of the direct compression wave is possible onIy under the resultant action of compressive and tensile forces. In this case, an elementary volume AV will be compressed in the normal direction by a force o r and stretched in the tangential direction by a force 0r (Fig. 1). Radial cracks will spread through the elementary volume AV (Fig. 2, 1). If the medium Is incompressible, the volumeof the fracture dae ~o the direct compression wave is Limited only by the formation of a zone of plastic deformation.On further propagation of the direct compression wave through the solid rock, fracture is possible only at the surface of a half-space, in fact. here o r becomes zero; as a result of the action of tangential stresses in the mass, a system of cracks perpendicular to the free surface arises (Fig. 2o 2). Their number and depth depend mainly onthe stress Or formed at the front of the direct compression wave at the limiting distance h equal ~o the depth of thecharge below the free surface. During the formation of this system of cracks, the direct compression wave is usually ~eflect-ed from the boundary of the half-space and is converted ~ a tensile wave, and as a result a system of cleavage (scabbing) cracks arises near the boundary. As the reflected Wave spreads (Fig. 2, 3) the messes on its front decrease, and as a result the amount of fra...
Present methods of blasting control are mainly based on statistical and empirical relations. However, the great variety of geological conditions and mining requirements means that we need some general approach to the problem of blasting. This could be based on our present knowledge of the mechanism of rock fracture by blasting.It is known [1] that when commercial explosives are detonated the pressure in the charge hole iswhere w is the detonation velocity of the explosives in m/sec, and PE is the density of the explosives in kg/m s.For common explosives used in industrial conditions (Ammonite No. 6, "Zernogranulity,'"Detonity," "Akvatoly," etc.), this pressure reaches about l0 s kg/cm z. The presst~e at the charge-medium contact is given in terms of the characteristics of the rock and explosives by the formula [2]:where p is the density of the medium deformed by the explosion in kg/m s, Cp is the velocity of the longitudinal wave in m/sec, and g is the acceleration due to gravity in m/sec z.This presst~e, applied to the sides of the charge chamber, is transmitted to other points in themedium in the form of a wave which is faster than sound under some conditions. According to Adushkin and Sukhotin [3], this region is about 5-7 times the charge radius. In the more general case, the volume in which the elastic waves are supersonic is given by Shemyakin [4,5], who states that ultrasonic velocity is found only when ~r ~ 1, E ~+pG) --Kc] (4)where /i is Poisson's ratio and C s is the velocity of transverse waves in m/sec.
Contemporary ore mining is one of the most active fields in the use of blasting in the national economy, in which theoretical solution and practical realization have been effected for two very important scientific-technical problems -to attain a given degree of crashing of rocks by blasting and to protect engineering constructions and exposed rocks from file seismic action of the blast. The solution of problems in the seismic effects of mine blasts is playing an important part in the successful development of contemporary mining production.Institute of Mining, Moscow, Novosibirsk.
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