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The mining-induced stress distribution and fracture characteristics of overlying strata in the lower coal seams are different from those in single coal seam mining. Based on the distribution of the mining-induced fracture elliptical parabolic zone in a single coal seam, the dynamic distribution equation is deduced for the mining-induced fracture elliptical parabolic zone in two short coal seams through theoretical analysis. Combined with physical experiment and numerical simulation, a study has been made on the characteristics of overlying strata deformation, displacement, and mining-induced fracture evolution in the single coal seam and its lower coal seam mining. The results show that high stress concentrations occurred near the coal pillars at the upper coal floor on both sides after mining, and the overlying strata subsidence of the upper coal seam is 155.70% of the intermediate strata after superimposed mining, with the upper and lower coal floor showing significant stress relief. Under the influence of the two mining actions, the overlying strata of the upper coal seam have more fully developed fractures. The fracture density increases with the advance distance, the position of caving zone and fracture zone continues to develop upward, and the compaction zone further compacts; the fracture zone presents the distribution of the superimposed mining elliptical paraboloid zone. FLAC3D was used to simulate superposition of two layers of coal mining; the strata displacement and stress distribution nephograms were obtained as well, with the result that the floor stress release could reach up to 14.87 MPa under the influence of mining of two adjacent seams. This indicates that the pressure relief degree of the floor under the influence of two mining operations is obviously higher than that under one mining operation. The fracture distribution obtained by numerical simulation is basically consistent with the physical simulation and the superimposed mining elliptical paraboloid zone.

In order to determine the damage law of narrow coal pillars in the roadway along the goaf, taking the 4-5(06) working face of a mine in Xinjiang as the research object, the 3D finite difference software FLAC3D is used to analyse the stress and displacement distribution characteristics of narrow coal pillars in the mining process. The results show that the internal stress of the coal pillar reaches the peak when the narrow coal pillar advances from 11m to 12m, and the coal pillar is subjected to the pre-supporting pressure, the range of the rupture zone is increased, and the bearing capacity is obviously reduced. When the working face advances 65m, the displacement in the coal column reaches a peak, and the deformation of the coal column is more serious. The field test results show that the damage law of the narrow coal pillars in the working face mining process is basically consistent with the simulation results. After the bolts, the cable-stayed anchor cable and the shotcrete, the deformation of the narrow coal pillar is in a controllable range, and the safety production requirements of the working face are realized.