Stress sensitivity of permeability in the loading and unloading process of broken coal-rock masses under different influencing factors

Protective layer mining combined with pressure-relief gas extraction is the most effective technology to prevent and control coal and gas outbursts, and has been widely applied to coal seam groups with high gas content and low permeability in China. However, broken coal-rock masses in the goaf undergo loading and unloading during subsequent mining, affecting permeability. A self-developed triaxial seepage test system was utilized to carry out seepage tests during loading and unloading on broken coal-rock masses under conditions of different particle sizes, coal-rock mixing ratios, coal-rock mixing modes, and loading and unloading times. Research results show that the permeability of broken coal-rock masses increases with increasing particle size, decreases with growing proportion of coal in the broken coal-rock samples, and decreases with increasing loading and unloading times, while it is not dependent on the two mixing modes. The absolute stress sensitivity coefficients of permeability of broken coal-rock samples in various stages during loading are always greater than those during unloading. The stress sensitivity of permeability of broken coal-rock masses in the loading and unloading processes increases with increasing particle size, grows with increasing proportion of coal in the broken coal-rock samples, and decreases with increasing loading and unloading times, while it is not dependent on the two mixing modes. The research results provide guidance for the layout of gas drainage boreholes in the goaf of pressure-relief mining of coal seam groups with high gas content and low permeability, contributing to the prevention and control of coal and gas outburst disasters.