Explosive destruction

(Figure) explosive destruction

EXPLOSIVE DESTRUCTION (EN: blasting destruction, cutting by blasting; DE: Explosionszerstorung, Explosionszerkleinerung, Explosionszersetzung; FR: facturation à l'aide du tir; ES: rotura por explosion; RU: взрывное разрушение) is the rapidly proceeding process of the partition of the solid medium into the separations under the action of the explosion. As the result of the explosive destruction, on the account of the high speed of the application of the load, and of the deformation of the medium, there is formed the multitude of the cracks, unlike the static destruction, for which there is characteristic the forming of the solitary cracks.

Within the non-limited medium, where the influence of the exposed surface is small (camouflet), the explosive destruction proceeds through two stages. The initial destruction proceeds on the dynamic front of the crushing, namely, on the surface, where is achieved the limiting elastic state of the medium, and are formed the macro-cracks, according to the two mechanisms: with the cleavage, upon the achieving of the limiting shear stresses (within the zone, which is near to the centre of the explosion, where the stresses are compressive), and with the tearing, upon the achieving of the tensile strength (within the far zone, where act the tensile stresses). The prevalent mass of the medium at the front of the crushing is destructed with the tearing, because the shear strength within the brittle media and rocks significantly (by the order of magnitude and more) exceeds the tensile strength. As the result, there is formed the zone of the radial cracks.

The secondary destruction proceeds during the subsequent shear deformation of the disturbed medium. This destruction prevails near the explosive cavity, where the deformations have the greatest magnitude, and is the major cause for the repeated grinding of the medium, and for the useless heat losses of the energy, which are caused by the internal friction within the destructed medium. During the secondary explosive destruction, the cracks are oriented in the non-ordered manner, this is the so-named zone of the crushing. The influence of the secondary crushing grows with the deformation, therefore its greatest contribution will be in the case of the strong (for example, nuclear) explosion, when the degree of the deforming grows substantially in comparison with the chemical explosive substances. The sizes of the zones of the crushing R*, and of the radial cracks R0, are associated with the relation

where s* is the compressive strength, and s0 is the tensile strength.

The size of the zone of crushing depends substantially on the intensity of the explosive source. If the initial pressure of the products of the explosion is near to the compressive strength, then the zone of the crushing may be absent entirely. In this case, there prevails the destruction with the tearing, and the zone of the radial cracks starts from the explosive cavity.

In case of the explosion near the exposed surface of the massif, the described mechanism of the destruction is complemented with the cleavage, namely, with the destruction with the tearing under the action of the tensile radial stresses, which are emerging during the reflection of the blast wave from the exposed surface (see the Figure).

In case of the cleavage, the cracks are oriented mostly parallel to the exposed surface. The maximal depth of the zone of the cleavage destruction does not exceed half of the length of the phase of the compressive stresses of the blast wave.

The quality of the explosive destruction is characterized by the distribution of the pieces throught the size range (the granulometric composition). In case of the explosion within the homogeneous medium (without the initial crackedness), there has been established the precise correlation between the mechanism of the destruction, and the granulometric composition of the medium, which has been destructed. The initial destruction leads to the forming, within the mining mass, of the approximately equal quantity of the pieces of the different sizes, and during the secondary destruction, the major part of the pieces has the approximately equal size.

Within the real rocks, there always exist the defects (the non-uniformities) of the various scale, namely, from the natural crackedness of the massif, to the dislocations within the grains of the minerals, which lead to the origination of the cracks, and determine the granulometric composition of the rock, which has been destructed by the explosion. The prevalent influence of the certain non-uniformities depends on the magnitude of the acting stresses, and manifests itself differentially, depending on the distance from the centre of the explosion, and on the intensity of the explosive source. Near the explosive cavity, where act the maximal stresses, there may undergo the destruction even the most sturdy minerals, which belong to the rocks. There exert the greatest influence onto the forming of the granulometric composition the crackedness (macro, and micro), and the layering. For example, in case of the weak explosion within the rock with the clearly expressed block structure, the granulometric composition is determined mostly by the initial system of the cracks. Onto the explosive destruction, that is, onto the creation of the new surfaces within the massif, there is consumed approximately 1% of the total energy of the explosion.

The control over the destructive action of the explosion comprises the possible regulation of the average size of the piece, and of the set of the fractions. For this purpose, they use the short-delay explosion, buffered explosion, and so on.