Outburst mechanism of tunnelling through coal seams and the safety strategy by using strong-weak coupling circle-layers

The gas outburst potential is typically high when roadways uncover the gassy coal seams associated with outburst hazards. This paper describes the complex stress and displacement evolution that occurs in roadway tunnelling through coal seams under different original stress conditions, which is helpful for better understanding how the stress leads to an outburst accident, and proposing some relevant control measures. The stress concentration is primarily resulted from the strength difference between coal and rock, and the abutment stress increases gradually as the forward residual rock thickness decreases when the roadway excavation enters a coal seam from rock stratum. When the abutment stress is sufficiently high, the forward residual rock will suddenly be broken after the next cycle, and the stress will decrease rapidly to release elastic energy and generate new fractures. Meanwhile, the coal and rock in front of the heading face will move rapidly towards the free space. Therefore, an outburst accident may occur immediately when the gas pressure in front of the heading face is high enough. Once the outburst accident is triggered, the gas expansion energy and elastic energy distant from the heading face will contribute to developing the accident. The stress perpendicular to the roadway is the primary factor causing such outbursts. The abutment stress concentration will be higher when the maximum original stress is perpendicular to the roadway, which will increase the outburst potential. Therefore, it is suggested that arranging roadways parallel to the maximum original stress, if possible, can also reduce the outburst threat. Outbursts are triggered by the elastic energy near the heading face and developed by the gas expansion energy and elastic energy distant from the heading face. In this study, the method of strong-weak coupling circle-layers was proposed to reduce the outburst hazard when roadways excavated through gassy coal seams. This method was proven to be valid by the comparative experiment. The weak circle can be realized by bore holes and hydraulic cutting. It could not only reduce the elastic energy near the heading face but also prevent the distant energy from contributing to the outburst accident. In addition, the strong circle can be made by injecting water slurry and applying anchors. It could help to reduce the strength difference between coal and rock, thereby decreasing the local stress concentration, resisting the residual outburst energy near the heading face, and preventing the outburst from being triggered. The strong-weak coupling structure was successfully applied in Bailongshan coal mine when uncovering the #7,8 coal seam. This structure can effectively promote gas drainage, reduce the coal and gas outburst risk. As a result, the mining efficiency improves a lot, and the uncovering time greatly decreases from the expected 2 years to 5 months.