Study on energy evolution and permeability characteristics of deep coal damage under triaxial cyclic loading and unloading conditions

In order to analyze the energy evolution and permeability characteristics of coal during the coal mining process, the triaxial cyclic loading and unloading tests were carried out by controlling the confining and axial pressures, and the permeability was measured. The evolution characteristics of elastic parameters(elastic modulus and Poisson's ratio) and energy densities with the axial strain were analyzed under different confining pressures. A modified damage variable was introduced for this elastoplastic material and its correlation with the permeability was explored. The results show that, during the loading and unloading processes and after entering the yield phase, the elastic modulus begins to decrease, and the dissipated energy density, dissipated energy ratio and damage variable gradually increase, and that in the post-peak period, changes of these parameters develop rapidly. It is also demonstrated that rock damage is an energy-dissipating damage evolution process. Taking the yield point as the demarcation point, the relationship between the permeability and the damage variable can be described by a logarithmical function and an exponent function before and after yielding, respectively, and the fitting formulas were obtained under different confining pressures. The permeability decreases with increasing the confining pressure, which indicates that the confining pressure has an inhibitory effect on permeation. © 2018, Science Press. All right reserved.