Mechanical degradation model of porous coal with water intrusion

To study the mechanical degradation of deep coal seam due to geothermal and groundwater effects, a nonlinear constitutive model of porous coal in humid environments, considering water-coal interaction, is proposed from the perspective of non-equilibrium thermodynamics. Wet coal damage is attributed to interfacial degradation of coal matrix and dissolution of inorganic salt, which can be described together by the product of an internal variable and a degradation parameter. A linear correlation is identified between the fixed carbon content and the degradation parameter, indicating that coal is more prone to mechanical degradation after water absorption if it contains a higher amount of carbon. After water saturation, the elastic modulus of coal with a fixed carbon content of 56.59% is significantly reduced by 46%, while that of coal with fixed carbon content of 40.81% is reduced by 29%. This study also confirms that the water content in porous coal first increases rapidly, by more than 60% during the first 25% water intrusion time, and then stabilizes gradually. The accuracy and the applicability of the model are validated by comparing the experimental data with the theoretical predictions of the changes in elastic modulus and porosity during coal water absorption.