Calculating the load on movable guard-type supports in flat-lying or inclined seams

Abstract

An urgent problem in mining practice is to determine the loads on guard-type supports in flat-lylng and inclined (semlsteep) seams.In this article we will discuss one approach to the construction of load calculation schemes based on investigations of the deformation mechanisms of loose materials beyond the support and of the surrounding rocks [i, 2].i. Experiments performed by the method in [3] for seems dipping at ~ < 60 = have revealed that for certain displacements of the support, the material (dry sand) beyond the guard is separated into blocks by global sllp planes (lines) (Fig. i), As a result, the guard is acted on not by the whole column of loose material but only by part of it, AxAsA3, intersected by some sllp plane and directly linked ~rith the support. This fact forms the basis for our scheme of calculations.Thus, one of the principal parameters determining the pressure on the support is the angle of inclination of the slip plane to the floor of the seam, which we call B. The angle ~ is found experimentally and depends on the angle of dip a of the seem and the angle ~ between the guard part of the support and the floor. We will not consider how B depends on the properties of the material.Figure 2 is a plot of 8 and 8 vs a (where e is the angle of inclination of the slip plane to the horizontal) for ~ = 90 = . In the construction of both curves the sllp curves were approximated by straight lines. We see that the curves of 8 and ~ vs r have characteristic points A, B, C, and D. The slope of the sllp lines is a consequence of all the conditlons of deformation of the material in its limiting state. Therefore we can expect that the singular points on the diagrams plotting the slopes of the sllp lines vs the angle of dip of the seam will mark changes in the conditions of deformations of the material beyond the support. These changes can be followed by various experiments.Suppose that the angle of dip of the seam is 0 < a < 30 ~ " ~@ (where @o = 32_34 @ is the angle of internal friction of the material).In this case the roof of the seam exerts no influence on the formation of the region AxA=As, because from the onset of movement of the guard the material loses contact with it (see Fig. la-d). The slope of llne A,As depends on the properties of the material, the slope of the free surface of the loose material (the surface "unstuck" from the roof) to the horizontal, and the friction conditions at the floor of the seam.On further increase in ~, the picture changes (see Fig. le-f).The free space formed between the roof and the block AxA=A4 is continually filled with loose material (the roof comes down) and the formation of the sllp planes involves friction of the material both on the floor and on the roof. However, in this case also the floor continues to play the principal role.If the dip is steep (a Z 60 = = ~/4 + @/2), the deformation process begins to involve not only friction of the loose material on the floor and roof (the latter increasing with a) but also the dilation properties of the material [3].Instit...