Experimental study on the moisture migration and triaxial mechanical damage mechanisms of water-bearing coal samples

Abstract

To investigate the internal moisture migration patterns and mechanical damage characteristics of water-bearing coal, X-ray diffraction (XRD), scanning electron microscopy (SEM), nuclear magnetic resonance (NMR) spectroscopy, acoustic emission (AE) tests, and conventional triaxial tests were carried out on coal samples saturated at different water pressures. The results showed that the saturated water content of the samples increased exponentially with the water pressure and reached a threshold value, from which the limiting water pressure and limiting water height were approximated. Under non-destructive immersion, the water absorption process of the coal samples was a combination of layer-by-layer saturation and downward water transport. As the water pressure increased, the number of pores indicated by the NMR analysis increased, and the degree of internal damage defined by the integrated area of the T2 curve also increased. These results were used to quantitatively characterize the damage degree of the samples. The triaxial test results showed that the peak strength, residual strength, modulus of elasticity, and Poisson's ratio of the saturated coal samples all decreased linearly or exponentially upon increasing the water pressure. The breaking angle and number of shear cracks increased, and localized deformation damage eventually developed in the form of narrow shear zones. A modified Mohr-Coulomb criterion based on water pressure and water level was proposed after the experimental results were verified by theoretical derivation. This work provides a scientific basis for studying rock weakening mechanisms in water environments.