Architecture, stress state and permeability of a fault zone in Jiulishan coal mine, China: Implication for coal and gas outbursts

The Mafangquan (MFQ) fault zone, transecting a coal seam in Jiulishan coal mine in Jiaozuo coalfield, Henan Province, China, was investigated in detail in view of coal mining safety, including its architecture, stress and permeability features and implication for coal and gas outburst. 10 boreholes have been drilled for formation testing as well as coal sample retrieval for laboratory analysis. Gas content, Protodyakonov strength, gas emission index and pore volume are tested and analysed at different distances from the MFQ fault. A fault structure that can be separated into three distinct zones, including the fault core, the mylonitized zone and the granulated/cataclastic zone, was identified. In-situ stress tests, numerical simulations and model predictions show a principal stress rotation and fault zone stress regime. It is found that the maximum principal stress has shifted to a lower angle pent to the fault zone extend direction gradually with a destress zone from fault core to the damage zone. The permeability of the fault zone has a correlation to fault zone architecture and stress regimes in the coal seam with a high permeability in the mylonitized coal zone. The coal weakened due to deformation, high pore pressure and stress superimposition have a negative impact on coal and gas outburst. In particular, the stress rotation limits the formation of hydrofractures in the case of high pore pressure and reduces fault zone strength, which may have an orthogonal relation to the mining galleries, favouring coal and gas outburst.