INTRODUCTION

In Japanese coal mines, with the increase in working depth, we have met with the unusual phenomena. Recently, in Miike under-sea coal mine, coal burst has been a important problem together with the supporting of deep level entry. Fortunately, the serious accident has been avoided except a few cases, however a clear explanation of its mechanism has not been made and many technical problems still remain. In this paper, analyzing the results obtained by a series of in-situ measurements in Miike coal mine, we shall investigate the coal seam behavior in connection with coal burst and finally we shall refer to its control.

STRESS STATE OF FLAT COAL SEAM INDUCED BY LONGWALL MINING

The primary approximation of stress distribution is given by the solution for a flat elliptic opening of width 2c in an infinite elastic ground with stress p at infinity perpendicular to its plane as shown in Fig. 1 (a). The vertical normal stresses ay on x-axis are given by and the convergence 2v between the hangingwall and footwall is given by

(Equation in full paper)

The stress distributions, given by eq.(10) and (11), for the case of p/Sc=1, p=0.5 and 0=30°, are shown in Fig. 2(a) where we suppose that E is sufficiently high and the contact does not occur. Always we can find out the peak of ay at x=cp, near the end of opening, and the peak values are in proportion to 2c. Such a stress condition, characterized by the extremely high stress concentration closely near the end of opening, takes place generally in the case of the coal seam sandwiched by the very rigid strata and in front of the working face which is desired to remove smoothly with every cutting but in practice which always removes intermittently and becomes pregnant with the hazard of coal burst. On the other hand, in the case of coal seam sandwiched by comparatively soft strata, the slides among bedding planes in the strata generally make progress and which weaken the restriction on the expansile deformation of plastic coal, so that τxy must become smaller than the one given by eq.(8). where H is the thickness of strata as shown in Fig. 1 (d). This assumption leads to a solution similar to eq. (10). By way of example, in Fig. 2(b) we show the stress distributions for the case of H/M=2. As compared with Fig. 2(a), the widths of plastic region are more wide and the peak values of ay are smaller and which slightly increase with 2c after the contact between hangingwall and footwall. In practice, such a growth of plastic region usually causes a large scale of deformation of level entry in the neighborhood of old working.

(Figure in full paper)

USUAL PROGRESS OF PLASTIC COAL ZONE ALONG LEVEL ENTRY WITH LONGWALL FACE ADVANCE AND SUPPORTING WITH PACK WALL

The stress changes in coal seam observed by in-situ measurements with pressure cells are illustrated in Fig. 3, where the cells have been set with the distance

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