The role of pore pressure on the mechanical behavior of coal under undrained cyclic triaxial loading

Geofluids widely exist in natural rocks, and the fluid overpressure affects the mechanical behavior of rocks, triggering dynamic instability events. To investigate the role of pore fluid in low-permeability coal away from excavation boundary but still influenced by the periodic excavation disturbance, a series of undrained cyclic triaxial compressive tests were conducted on saturated coal. The results show that pore water has little effect on the axial strain evolution of coal, but has a significant effect on the radial and volumetric strain evolution. Different from dry coal, the saturated coal samples (except the coal samples SUC 1-1 and SUC 1-2 which show a slight volume shrinkage stage) show dilation deformation directly without any volume shrinkage, due to regional overpressure caused by the undrained condition. With this constraint against crack closure, a low frictional strength of crack surfaces was produced, favoring instability. Moreover, the saturated coal cracks cannot close tightly at the loading stage due to water propping effect, resulting in that the crack could not keep stationary, instead it slips throughout the loading and unloading processes. Also, the pore overpressure provides tensile stress, producing a large number of macro-tensile cracks in the final failure. This tensile cracking process is influenced by the loading frequency, as the pore pressure decay lags behind the unloaded stress, producing pore overpressure to tensile cracking in the unloading process. This lag effect becomes more prominent with the increase of cyclic loading frequency. Consequently, the number of tensile cracks of saturated coal at the loading frequency of 0.4 Hz is larger than that at the loading frequency of 0.1 Hz.