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Dynamic effects of rock pressure present difficulties in mechanizing the working of coal seams with hard-to-collapse monolithic roofs. Periodic collapses of panels of the main roof overhanging the worked-out space are typically accompanied by increased load on the face part of the workings, which frequently destroy the hydraulic support and other elements of the mechanized support system.According to the Institute of Geologic Surveying, the number of mines with hard-to-collapse monolithic roofs on gently dipping coal seams in the USSR accounts for about 20% of the total workings [i].Since in the past the most convenient segments of mining fields were worked before any other segments, the number of such mines will be increasing in the future years and the depth of mining operations will also grow.For active prevention of dynamic rock pressure effects in faces with hard-to-collapse roofs, conventional techniques for overburden loosening are used, including advance shooting, hydraulic processing, and combinations of these two methods [2][3][4][5][6]. The purpose is to reduce the collapse step of the overburden in the worked-out space.Advanced shooting is done by drilling deep sloping holes from the preparatory workings into the roof in which explosive charges (so-called torpedos) are placed and blasted. The rock loosening with these methods is uneven: near the hole the rock is overcrushed, while at a distance it is destroyed by isolated random cracks. The effective range of the blast is small --15-25 radii of the explosive charge [2].Hydraulic treatment is conducted through holes by hydraulic moistening and hydraulic rupture. With hydraulic moistening of the main roof, uniform and adequate saturation of the bed is difficult to attain because of the low permeability of constituent rocks. The permeability is reduced with the increasing depth of the working. As liquid is pumped into the hole in the hydraulic rupture regime, the rock bed is also loosened unevenly. The hydraulic rupture occurs uncontrollably on random segments of the hole, mainly through existing natural cracks (e.g. when hydraulic processing is combined with advanced shooting). Starting to move through randomly oriented cracks with the widest openings, the liquid opens them even more and develops only these cracks.With the existing methods it is thus impossible to control the rock-loosening process, leading to uneven changes in the mechanical characteristics of rocks and uneven redistribution of stresses in the roof and therefore in the coal seam. This can have negative effects on breakage operations, especially when working seams are prone to dynamic and gas-dynamic effects.A comparison of easy-and hard-to-collapse rocks showed that the main structural difference is the stratification of easily collapsible rocks, with relatively weak contact between" layers. The hard-to-collapse rocks, even though sedimentary, are monolithic and have no distinct cleavages. This leads to a natural conclusion that producing artificially directed cracks in a bed of hard-t...

Coal oxidizes even at low temperatures, emitting heat. If this heat is dispersed more slowly than it is generated, the coal may ultimately become so hot that it catches fire. The generation of heat by the oxidation of coal depends on the properties of the coal and on the amount of oxygen available to it. The dispersion of the heat depends on the properties of the coal, and also on the size and shape of the volume of coal being oxidized and on the conditions of heat exchange at its boundaries. In particular, the properties of the coalwill depend on its water content.We will call the temperature above which the coal can catch fire the critical temperature. We will call the lowest volume for which this temperature can be reached the critical volume.In this article we establish an approximate relation between the critical volume for spontaneous ignition of the coal and the water content. For this purpose we will consider the conditions of stability of the temperature field in a volume of oxidizing coal and the relation between the critical volume and the thermal characteristics of the coal. On the basis of the relation between the thermal characteristics and water content of the coal, we will investigate how the water content affects the critical volume.In our mathematical study of the temperature field we will make the following assumptions: i) the coal in the volume initially has homogeneous properties; ii) the oxygen in the mine air can reach any point in the volume equally easily, and does so in excess. With these postulates we can formulate the problem.In a region r bounded by a surface r~ we have to find a function u(x, y, z, t) which satisfies the equationon the conditions .J,:0 ~f(x,y,z) 0.; u+/el 0~_u +k2=0 on E, OnHere u is the temperature, a is the thermal diffusivity, c is the specific heat of the coal, p is the density of the coal, t is the time, x, y, and z are Cartesian coordinates, g(x, y, z, t, u) is a function which determines the rate of heat emission, k 1 and kz are quantities (in general not constant) which characterize heat exchange at the boundary surface, and n is the normal to the surface E.In the general case, the solution to this problem is very complicated. For this reason we will make some additional assumptions. Let us assume that the region r is a rectangular parallelepiped with its faces parallel to the coordinate planes, and that some fixed temperature is maintained on each face. Let us also assume that the rate of heat emission, expressed by the function g, is proportional to the rate of oxidation of the coal.According to Veselovskii [1], before the coal catches fire the temperature dependence of the rate of oxidation can be expressed to a first approximation by a straight-line segment. We therefore put g= ~'u + b,where ~' and b are constants.-~ "* Deceased. 1" We will consider this relation from the initial natural temperature of the coal to its ignition point.Eastern Scientific Research Institute.