The amount of elastic energy released from a rock mass during destruction depends on the rock capacity for stress relaxation [1]. A study analyzing how the speed of intrusion of a working tool into the rock affects this process has shown that if the speed at which the rock mass is uncovered is reduced, the energy influx from relaxation O f stresses declines to values that drastically changethe rock destruction patterns: brittle pulse-type destruction is replaced by viscous quiet destruction. The pulse-like unstable destruction may develop into a hazardous dynamic effect, a rock burst. In ~ffew of this, ways of modifying the destruction pattern deserve close attention of investigators. The energy influx is proportional to squared stress [1] operating at the site of intrusion; the relaxation of stresses in rocks, therefore, affects the pattern of bed destruction substantially.The bed zone near the face is in a complex stressed-strained state. Some parts of it experience uniaxial compression, compressions under lateral pressure and bends (roof rocks, floor rocks), and are in different degrees of deformation -from elastic deformations to deformations at the limit of residual strength. "4'hen a part of the bed is destroyed, the energy supplied to the destruction zone comes from the entire complexly stressed system. For a correct evaluation of the magnitude of the energy influx, a notion of the relaxation capacity of the rock during various types of stressed states and various deformation phases is necessary. The stressed state of a fixed bed element also changes as it approaches the exposure during the course of working or destruction, namely: with the shorter distance from the exposure the lateral component of the load is decreased. It is also essential to know whether this variation of the stressed state affects the relaxation activity of rocks and, if so, to what extent. This is essential for being able to evaluate the energy influx and, therefore, the potential pattern of rock destruction. This paper continues an experimental investigation of stress relaxation in rocks under various types of stressed states at different deformation phases, including the transcritical region. The tests were done in the conditions of uniaxial compression, compression under hydrostatic pressures varying up to values at which the rock strength characteristics attained a plateau, and for bending. The hydrostatic pressures remained constant throughout the tests, which simulated the stressed state of rocks in the interior of thebed, and also for gradually decreasing lateral pressures, which was similar to the variation experienced by the stressed state of an element of the rock mass as it comes closer to the exposed zone. EXPERIMENTAL METHODSAll tests with stress relaxation were done in the laboratory on rock samples. The purpose of the tests was to measure the stress relaxation in the same specimens for different deformation phases. To this end, the specimens were deformed up to a certain value during a continuous growth of the strain;...
The time dependence of the supporting capacity of a material under given pressures is conventionally called the endurance of the given material. Other conditions being equal, a material under constant load breaks more rapidly when the load is greater.The relation between the compressive (tensile) stress in a specimen of a material, o, and the time to breakage (time of existence), t, is exponential:where t o and a are constants of the material.Thus, a graph of log t vs a gives a straight line 9 Equation (1) has been verified for a number of materials. Good agreement with (1) has been found in some metals [1], in silver chloride crystals [2], in polymers [3], and in watery plaster of Paris (Griggs, 1936), carnallite salts (Vodop'yanova and Urazova, 1964), single crystals of rock salt (Manke, 1934), and weak rocks (Fisenko, 1965).As well as experiments on endurance, investigations of creep in materials in which the results are analyzed by means of an equation of type (1) where e0 is a constant of the material, were found to be equal.In this article we give the results of some experiments on creep and long-term strength of specimens of potash salt of the Verkhnekamskaya and Starobin deposits and Cambrian clay from the Leningrad region. The specimens were prisms, 150 • 150 • 300 mm in size.The laboratory uniaxial-compression tests were performed in hundred-ton spring presses of type UDI [5]. Figure I shows a diagram of the press. The specimen 1 is positioned on movable crosspiece 2. By means of hydraulic jack 3 and screw 4, with a set of disk springs 5, the required load is transmitted via movable crosspiece 2 to the specimen 1. Nut 6 is screwed down threaded shaft 4 until it meets fixed crosspiece 7. Hydraulic jack 3 is released and the load is applied by column 8 and fixed crosspieces 7 and 9. During the experiment, the load is kept constant owing to the energy stored in the compressed stack of springs 5.During the experiments we measure the stresses and all the principal deformations of the specimen. The deformations are measured by means of a set of dial gages with 0.01-ram scale divisions. Figure 2 shows a rock specimen with a set of gages and special fixtures for their installation. The total longitudinal deformation of the specimen is measured by gages 1 and 2 (the latter cannot be seen in Fig. 2). The longitudinal deformation in the middle part of the specimen on a base of 100 mm is also measured by gages 3 and 4 (the latter cannot be seen in Fig. 2), fixed on special attachments in the form of plates with knife edges which set the base for the measurements. The transverse deformations in the middle part of the specimen are measured by means of gages 5 and 6fixed on brackets
By studying the influence of atmospheric conditions on the rheological properties of rocks, we can find out more about their deformation and can study the process of fracture in nearly natural conditions. Renzhiglov [1] investigated the influence of moisture in rocks ( argillaceous and arenaceous) on their theological properties, He found that moisture activates their creep. Surfactants strongly alter the mechanical properties of materials [2]. The surfactant may be simply a damp atmosphere. Penetrating the specimen, atmospheric moisture weakens the elementary interatomic bonds, so that creep becomes more rapid. Many investigations have revealed that it is elementary mechanismsof interatomic bond rupture which are responsible for processes occurring during long-term tests.
The marked effect of the circumstances under which a tool penetrates into rock on the features of failure have been noted for a long while, and in the 1950s detailed studies were carried out giving the first quantitative information about the degree of this effect.By controlling rationally the energy of elastic strains released from the material compressed by rock pressure, it is possible on the one hand to reduce the energy consumed in breaking and to facilitate mining of mineral resources and, on the other hand, to reduce, and even exclude, the danger of rock bursts [i, 2].
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