Application of loose materal mechanics to solving some rock mechanics problems

Abstract

i. The picture of movement in a rock mass during development of stoping in a horizontal coal seam is illustrated in Fig. i. Extensive full-scale studies carried out in coal mines and pits have made it possible to detect a series of typical features which accompany movement of the rock mass during recovery of mineral resources. First, it appeared that the inclination angle of the displacement surface to the horizontal ~ (angle of total displacement) depends weakly on depth, thickness of the seam being worked, width of the working, roof structure, and it is 55-65 ~ (Fig. l) [I]. Second, in carrying out mining in the vicinity of a ~orked out space there is a marked change in the original stressed state, and what is particularly important, a zone of increased vertical pressures forms, i.e., a zone of support pressure. Stresses in the support pressure zone markedly exceed those in the intact rock mass, and for different conditions according to a series of estimates they may be from 2 to ii yH (y is rock density, H is working depth [2].If we now turn to Fig. la in [3], in which the initial stage of flow of dense sand is given during discharge from a vessel, then it is possible to observe its overall qualitative similarity with the deformation picture in question in an underworked rock mass. As was shown in [4,5], the slip surface formed here has the minimum shear strength (~mob = ~, ~mob is mo-, bile friction angle ~ is contact friction angle of the particles), forms in all cases in the first instance, and makes it possible for further development of deformation in the loose rock mass. The inclination angle from the vertical of these surfaces for materials with compact particle packing is of the order of 23-35 ~ [3, 5], and consequently it is 67-55 ~ from the horizontal.Thus, inclination angles for "surfaces of minimum strength" forming during discharge of loose materials, almost coincide with inclination angles ~s for surfaces of overall displacement in the underworked rock mass.At first sight this unexpected fact when the main features of the processes in materials with differing structure, strength, and deformation properties coincide not only qualitatively, but also quantitatively, may be explained by considering phenomena accompanying rock displacement and the scale factor. It is well known that displacement in an uuderworked rock mass precedes roof sag. Due to the low strength of rock in tension, even in the absence of cracking in the intact rock mass, sagging leads to crack formation [i]. Following [5][6][7], a cracked rock mass may be represented as a mass of loose material whose size of component "particles" is gov-'ernedby the crack network. One more confirmation of this is the deformation picture being considered.It is noted that angle ~ in the rock mass is independent only of roof structure with horizontal and slightly inclined seam bedding. With an increase in the dip angle the value o~ angle ~ may change. This is connected with the fact that as the dip angle increases, the mode of failure starts to be mo...