Influence of confining pressure and stress amplitude on mechanical properties and permeability characteristics of coal

In order to explore the influence mechanism of confining pressure and stress amplitude on the mechanical properties and permeability characteristics of coal samples, the loading and unloading experiments of different confining pressures and amplitudes were carried out by triaxial test system. The stress-strain, permeability and energy changes of coal samples were monitored in real time, and the stress response, energy evolution, creep deformation and fracture distribution characteristics were systematically analyzed. The experimental results show that with the increase of confining pressure, the peak stress and elastic modulus of coal samples increase significantly, the input energy and elastic energy increase synchronously, and the permeability and fracture ratio decrease significantly. The high confining pressure effectively inhibits the fracture expansion, makes the structure of the coal sample more regular after the destruction, the fractal dimension decreases as a whole, and the structure tends to be simplified and densified. On the contrary, the increase of loading and unloading amplitude will aggravate the structural failure and fatigue damage process of coal samples, resulting in the decrease of peak strength, the increase of irreversible strain, the expansion and penetration of cracks, the increase of fracture proportion and fractal dimension, and the significant increase of permeability. The results of three-dimensional fracture reconstruction further confirm the significant influence of confining pressure and amplitude on the evolution of fracture spatial structure. The increase of confining pressure helps to close micro-cracks and compact pore channels, while the large amplitude disturbance promotes fracture penetration and forms a flow network.