The air-permeable boundaries of deformed rocks in the zone of influence of an extraction working in a thick steep seam

Abstract

A n urgent problem for safety conditions in working thick steep seams is that of spontaneous fires, most of which arise in the worked-out areas in working and isolated districts.Factors governing the formation of heat emission centers and heat propagation in the worked-out area include not only the physical properties of the coal and the percentage oxygen content but also the flow rates of the air filtration currents. Present methods of isolating worked-out areas (various types of stoppings and antifire pillars) only partly prevent access of air to isolated districts, because they do not prevent leaks through deformed rocks between seams. The activity of the aerodynamic, thermal, and diffusion processes in underworked or overworked strata is largely governed by their degree of disturbance, and determination of the boundaries of the air-permeable sections of the deformed rock reduces to calculating the boundaries of fissure formation in the disturbed strata.By observations in underworked strata and investigations on models [1, 2] it was found that the depth and size of the crack formation cones in systems with caving are primarily governed by the dimensions of the working, the thickness of the worked seam, and the structure and strength properties of the surrounding rocks.Calculations of the displacements around an extraction working are based [3] on the unit influence function of the worked-out area affecting the displacement of any point in the rune of influence; this takes the following form:, where H is the distance from the center of the working m the point along the line of maximum subsidence, in meters, B is the width of the worked-out area (m the dip), in meters, and n is an index representing the influence of the lithological composition and strength properties of the country rocks on the displacement process.The lithologieal composition index is determined from the graphs in Fig. 1 as the mean value of the percentage sandstone and argiIlite contents of the strata, with allowance for their strength characteristics.Investigations with models and analydcal calculations [2,3] show that the displacement process in underworked strata develops in the form of closed isolines of complex shape and variable deformation. In general form the character of the displacement isolines was determined as elliptical. The values of the contours and their deformations decrease with increasing distance from the working.We also found that from the qualitative view point the attenuation of the displacements foliows the same law in any direction away from the center of the working as given by Eq. (1).The maximum displacement in the underworked strata is determined quantitatively from the angle of m aximum subsidence, which is given by VostNII, Kemerovo.