Influence characteristics and macro-meso mechanism of pressure immersion time on tensile properties for coal materials

Abstract

In order to further investigate the deterioration of the tensile properties of the coal pillars supported by the abandoned mine legacy due to the duration of water immersion, this research using a self-developed coal rock pressure water immersion test device to investigate tensile properties' deterioration of coal pillars supported by abandoned mine legacy due to the duration of water immersion, in combination with other equipment (XTDIC 3D full-field strain measurement system, scanning electron microscope, fully automatic mercury compression instrument and X-ray diffractometer), five sets of uniaxial tensile tests on coal samples immersed in pressure water for 0, 30, 60, 90, and 120 days were conducted to investigate the deterioration of tensile properties under different pressure water immersion durations. Longer immersion durations corresponded with decreased uniaxial tensile strength, enhanced plastic damage, poorer integrity of the sample end faces, greater pore probability entropy at the fracture, and increased porosity, average pore size, and median pore size; however, the rate of uniaxial tensile strength tended to decrease, as did the degradation magnitude. The evolution of the deformed localisation zone's displacement misalignment involved the localised zone's penetration, expansion, and development. The immersion time of pressure water is prolongated to improve water-coal rocks interaction and increases the likelihood of dissolution of the Hydrophilic clay minerals and cement within the coal sample's pores. Thus, cracks are more likely to expand and penetrate, promoting the number of channels for pressure water to enter the coal sample, weakening its solid bearing structure, and reducing its tensile strength.