Mechanism of physical actions for a local directional change in state of a rock mass

Abstract

The practice of mining under complex geological conditions shows that traditional methods and technologies for extracting mineral resources do not satisfy completely the complex demands of safety and efficiency of mining production, while their improvement does not yield the expected results. In this connection, qualitatively new actions on rock masses and their associated processes and phenomena have recently attracted the attention of scientists in the interests of their utilization in progressive technologies of underground extraction. New transport sources'with magnetic engines, coal conversion to a fluidized state (hydraulic pulses, microbiological, etc.), coal unloading and degasification (cyclic, vibrational) and others that are discussed in the literature [i], say, should be among these in application to coal mining. The representation of a rock mass as an energetic system and the study of the deformation kinetics of gas-saturated coal during mechanical spoilage of the equilibrium state permitted estabishment of the dynamical nature of the change in rock pressure [2] which can be magnified or attenuated depending on the actions and the geological conditions of developing the coal seam. Substantial results can be achieved here by limited power actions, for instance, weak perturbations of the cyclic breaks of the thrust pressure in hydraulic props or vibrations type [3]. The advantages of weak mechanical perturbations in a directional change of state and properties of a massif are evident since they are not power intensive, are simple to control, easily automated, and can be inscribed in the main techical cycle of mining execution. Meanwhile, weak actions of another nature, physical, especialiy in the solution of the structural strength and fracture of materials and construction elements problems in machine construction are lately widespread: the exposure of materials to electrical, magnetic, electromagnetic, ultrasonic pulses, electron beams to increase the strength and working time, the transfer to a given deformation level and mode, the change in material properties and configuration, rupture. Metals in the limit state display an especially high response to periodically changing loads, i.e., under inelastic strain conditions [4]. In this connection, the possibility of applying a low power action to extract mineral resources under complex geological conditions is of great interest, for example, to magnify the dynamics of rock pressure and to raise the efficiency of mass fracture in a face or its attenuation to reduce the danger of the occurrence of dynamic rock pressure displays (ejections, rock bursts, sudden cave-ins). By nature rocks are are porous saturated media, i.e., possess a high-level of defectiveness. In the natural compressed state they possess a high level of structural element activity near the defect surface that is expressed in stress concentration and defect development. In such a state problem of the controllable change in their structure and properties by assuring conditions for interaction of action pulses with active structural elements or liquidation of the mentioned interaction is in fact fully realizable. Moreover, it should be taken into account that under an impulsive action the interaction of a wave with a defect in the form of a crack is expressed as perturbation concentration at its apex [5]. Therefore, small impulses in energetic respects, for instance, micro-impulses at the micro-level, particularly acoustic, can bring about substantial changes. An objection to microimpulsive exposure (especially acoustic)associated with the "necessary" wave damping and reflection can be made. However, investigations performed in recent years on the build-up of new features and conditions of fracture refute it. A study of 16 earthquakes of magnitude 4.0 ! M < 7.7 at ranges from i0 to I00 km by USA scientists permitted detection of high-frequency impulses in the form of time-limited narrowband white