Characteristics of coal and gas outburst and controlling mechanism of stress field in the hanging wall of normal faults

The fault zone is the main geological unit where coal and gas outbursts occur. Statistics of many coal and gas outbursts show that the number and intensity of the outbursts in the hanging wall are significantly greater than those in the foot wall for normal faults. However, there are few studies on the geological mechanism of this phenomenon, especially the variation of the in-situ stress field in the hanging wall of normal faults before and after mining, and its controlling mechanism of coal and gas outbursts that has not been completely demonstrated. In view of this, the normal fault DF₄ in Zhongmacun Coal Mine in Jiaozuo Mining Area was taken as a geological model, and the FLAC^3D software was applied to simulate and study the variation law of the in-situ stress field on hanging and foot walls of the normal fault before mining and during the process of tunneling to approach the fault plane of the heading face under the conditions of the burial depths of the coal seams of 660 m, 800 m and 1 000 m, respectively, corresponding to the three kinds of stress fields with σ₁=σ_H, σ₁=σ_v and σ_v=σ_H =σ_h(σ₁, σ_H, σ_h and σ_v are the maximum principal stress, maximum horizontal principal stress, minimum horizontal principal stress and vertical principal stress, respectively). The effect of in-situ stress distribution on coal and gas outbursts was investigated. The results show that the original in-situ stress of the hanging wall of normal faults is greater than that of the foot wall before mining. The in-situ stress of the hanging wall is always greater than that of the foot wall in the process of driving face approaching the fault plane. In particular, the original in-situ stress and the mining stress accumulate in the fault zone and then the in-situ stress of the hanging wall increases significantly when the roadway is 10 m near the fault plane. In summary, the in-situ stress concentrates and increases in the hanging wall before and after mining, which is deduced as the decisive factor for the occurrence of coal and gas outbursts in the hanging wall of normal faults. This research can provide a theoretical basis for preventing the coal and gas outburst disaster.