The role of pore water plays in coal under uniaxial cyclic loading

Geofluids are common in underground coal mines and have a significant influence on the deformation of coal rock, where the pore pressure of geofluids varies during swelling or shrinkage processes. In order to characterize the role of pore water in coal, the uniaxial behavior of saturated coal under cyclic compression was investigated considering different loading rates, amplitudes, and numbers of cycles. The results show that Young's modulus (E)increased in the first cycle and then stabilized in the remainder of loading cycles. Also, the increment of E in the first cycle was inversely correlated with the loading rate. For the first time, it is found that the radial residual strain gradually decreased in each subsequent loading cycle. This is different from that of water weakening property of coal by immersion of the coal in water without pressurization. In the unloading phase, the pore pressure decayed more slowly than the stress, leading to a high level of deviatoric stress. As a result, the acoustic emission events appeared even at the minimum of cyclic loading. The transverse relaxation time (T₂) distribution shows that the pore water pressure mainly changed the primary pores, while its influence on the secondary pores was negligible. From a poromechanical perspective, high pore pressure developed in pores of poor connectivity during initial cyclic loading, causing stress orientation rotation and tensile damage. With further cyclic loading, the pore water was continuously discharged as the pores were compacted into flat cracks. As a result, the onset stress of dilatancy gradually increased in the subsequent loading cycles due to the enhanced frictional resistance to crack slip.