Study on permeability evolution mechanism of aquifer coal seam roof sandstone under plastic flow

The plastic zone of the roof surrounding rock continues to expand after the coal seam mining. The permeability of the surrounding rock under plastic flow is a basic project for water inrush prevention in coal mine. In this work, the permeability experiments were carried out on roof sandstone from Xiaojihan No.2 coal seam under three loading paths. The variations of permeability affected by the confining pressure and axial strain were obtained. In the plastic flow stage, the permeability of roof sandstone was a multivalued function of volumetric strain and confining pressure. The permeability decreased with the increasing confining pressure. As the loading and unloading times increased, permeability was less affected by the confining pressure. and the permeability recovery coefficient increased. Based on the above analysis, the impacts of volume deformation and shear deformation on permeability were analyzed. A permeability model with two coefficients was proposed which was implemented by taking the normal strain and shear strain on the shear plane as independent variables. Through a permeability test with a simple loading path, the optimal estimated value of the permeability influence coefficient was determined by using the Monte Carlo method. It was applied to calculate the permeability under the other two complex loading paths. The calculation results show that the permeability coefficient is the basic attribute of rock materials and irrelevant to the loading path. Essential parameters obtained in this research are useful to analyze the coupled dynamic system's stable structure of the surrounding rock, and also could serve as theoretical foundations for the water inflow prediction in a coal mine.