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

To explore the influence mechanism of confining pressure and stress amplitude on the mechanical properties and permeability characteristics of coal samples, loading and unloading experiments with different confining pressures and amplitudes were performed with a triaxial test system. The stress‒strain, permeability and energy changes of the 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 reveal that with increasing 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. High confining pressure effectively inhibits fracture expansion, making the structure of the coal sample more regular after destruction; as a whole, the fractal dimension decreases, and the structure tends to be simplified and densified. In contrast, the increase in loading and unloading amplitude accelerates the structural failure and fatigue damage process of coal samples, resulting in a decrease in peak strength, an increase in irreversible strain, crack expansion and penetration, an increase in the fracture proportion and fractal dimension, and a significant increase in permeability. The results of three-dimensional fracture reconstruction further confirm the significant effect of confining pressure and amplitude on the evolution of the fracture spatial structure. An increase in confining pressure helps to close microcracks and compact pore channels, whereas a large-amplitude disturbance induces fracture penetration and the formation of a flow network.