Characterization of fissure distribution of overburden rock under roof cutting and entry retaining based on key strata theory

Aiming at the problem of changing the distribution of overburden fissures in the overhead mining area due to the cutting and retaining roadway process, the stress distribution, overburden transport and mining fissure distribution law in the mining zone under the roof cutting and retaining roadway process were analyzed by means of a combination of physical similarity simulation and numerical simulation. On this basis, based on the key strata theory and the cumulative effect of unloading and expansion of mining overburden, the changes in the height of overburden fissure development and the width of fissure zones on the roof cutting and un-cutting sides were investigated, the extraction effect of the directional unloading gas extraction boreholes in different strata was analyzed, and the overburden fissure distribution law under the roof cutting and retaining roadway process was verified and inverted. The results shown that, the roof cutting and retaining roadway process can effectively reduce the stress concentration of the top and bottom plates on the roof cutting side, but the stress will still be transferred to the deeper part of coal. Roof cutting caused changes in the thickness of the collapsed rock layer and the form of roof breakage, which led to the changes in the height of the fissure zone development, and the width of the off-strata fissure zones within the sub-critical strata control range. The height of the collapsed zone on the roof cutting side was twice as much as that on the un-cutting side, and the height of the fissure zone was 0.87 times as much as that on the un-cutting side. In the middle and lower part of the fissure zone, the amount of off- strata and the number of penetrating fissures on the cut top side are larger than those on the uncut top side; the amount of fissure development on the roof cutting side were larger than those on the un-cutting side. The amount of fissure development on the coal seam roof of roof cutting side increased with the increasing distance in the range of 8−30 m, and decreased with the increasing distance in the range of 30−48 m, The fissures were mainly distributed in the middle and lower part of the fissure zone. The “staggered” gas concentration and flow rate of the extraction boreholes in different layers validated the above conclusions. The study results have certain reference value for the decompression gas management in high gas mines under the process of roof cutting and retaining roadway.