Evolution characteristics of strain and permeability of coal samples under loading and unloading conditions

With the increase of the mining depth,rockburst or composite dynamic disasters with both rockburst and coal and gas outburst have occurred in the high-gas mines and coal and gas-outburst mines.The vertical stress on coal beam near the coal face increases significantly,while the horizontal stress is degraded in the process of coal mining.The superposition of gas internal energy,in-situ stresses and abutment pressure of the coal-rock system provide the mechanical conditions for the occurrence of compound dynamic disasters.Therefore,in order to study the influence mechanism of gas pressure and mining stress on coal deformation and gas flow,the strains and gas flow of coal samples during axial loading and radial unloading were measured to simulate the change characteristics of bearing pressure and horizontal stress in the mining process by coal stain-permeability synchronous testing system.It shows that coal samples shrank and the permeability of coal samples decreased under the low differential stress at the initial stage of loading and unloading because coal cracks closed.When the differential stress reached a certain value,the turning point of gas flow from down to up and the dilatancy of coal samples appeared successively.More gas flew from the coal samples due to the increasing plastic deformation and permeability of coal samples after the dilatancy.The gas flow inflections of most coal samples have been spotted before coal dilatancy so that the micro-fractures in coal samples have begun to extend before the dilatancy.With the increase of gas pressure,the differential stress of the coal dilatancy onset and the gas flow inflection decreased.Therefore,high gas pressure can bring down the differential stress threshold corresponding to of the micro-crack propagation in coal under mining disturbance.When the gas pressure was high (2.0 MPa),the average of stress difference corresponding to the coal dilatancy onset and the gas flow inflection was only 0.102 MPa.The coal dilatancy onset and the gas flow inflection occurred almost simultaneously.It can be inferred that under the influence of mining stress,the elastic deformation zone which stores a lot of energy with high gas pressure is very likely to quickly transform into the plastic deformation zone before a large amount of gas emission.Once the coal and rock system lost stability,the strain energy stored in the elastic zone and the gas in coal beam can release at the same time and the damage characteristics of compound dynamic disaster will occur.