Characterization of damage deterioration of pressure waterlogged coal rock under cyclic loading and unloading effects

The water-preventing coal pillar is not only affected by the water immersion due to the pressure from the mined-out area but also by the cyclic loading and unloading effects generated by the surrounding mining activities. This can exacerbate the damage and deterioration of the coal pillar, potentially leading to its destruction and instability, and may even trigger sudden water inrush disasters in the goaf area. To study the evolution of coal and rock damage and the deterioration mechanism under the combined action of pressure water immersion and cyclic loading and unloading, a self-developed coal and rock pressure water immersion testing device was used in conjunction with a full-field strain measurement system, an acoustic emission system, an X-ray diffractometer, and a scanning electron microscope. Uniaxial cyclic loading and unloading tests were conducted on dry and coal samples with different water pressures (immersion pressures of 0, 3, and 5 MPa, with an immersion duration of 20 days, and 8 cycles of loading and unloading). The strength, deformation and damage, acoustic emission, and energy evolution characteristics of the coal samples were studied to reveal the deterioration mechanism of the mechanical properties of the coal samples under the combined action of pressure water immersion and cyclic loading and unloading. The results indicate: ① Under the combined action of pressure water immersion and cyclic loading and unloading, the internal damage of the coal samples is intensified, weakening their solid load-bearing structure. Compared to dry coal samples, the uniaxial compressive strength of the coal samples after cyclic loading and unloading decreased by 18.04%, 27.12%, and 42.82% respectively; after the first cycle of loading and unloading, the plastic hysteresis loop area of the coal samples increased with the increase of water immersion pressure, and the elastic modulus decreased; as the number of cycles of loading and unloading increased, the plastic hysteresis loop area of the coal samples gradually decreased, and the elastic modulus of the loading and unloading segments showed a “wavy” decrease. ② With the increase of water immersion pressure, the evolution of deformation localization zones during the cyclic loading and unloading process of the coal samples becomes more intense, and as the number of cycles of loading and unloading increases, the stress concentration phenomenon at the defects of the coal samples becomes more pronounced, forming multiple deformation localization zones, leading to tensile damage, accompanied by peeling damage, and an increase in the degree of fragmentation. ③ Pressure water immersion promotes the water-coal and rock interaction, with the internal clay minerals softening and dissolving upon contact with water. Compared to dry coal samples, with the increase of water immersion pressure, the damage variable of the coal samples increased by 4.03%, 7.26%, and 24.19% respectively, and the dissipated energy increased by 20.50%, 38.22%, and 98.14% after the first cycle of loading and unloading; at the same time, as the number of cycles of loading and unloading increased, the internal damage of the coal samples continued to accumulate, with the damage variable and dissipated energy showing an increasing trend. After all cycles of loading and unloading, the average damage variable of the coal samples increased by 3.53%, 6.94%, and 25.47% respectively, and the average dissipated energy increased by 19.18%, 61.87%, and 138.73% respectively; after the cyclic loading and unloading, the dissipated energy of the coal samples during the uniaxial compression stage increased by 62.51%, 89.20%, and 106.93% respectively, and the average porosity and probability entropy at the fracture surface showed an increasing trend. The combined action of pressure water immersion and cyclic loading and unloading intensifies the internal damage of the coal samples, weakens their solid load-bearing structure, reduces the effective bearing area of the coal samples, and leads to the deterioration of the mechanical properties of the coal samples. The aforementioned research has significant theoretical implications for the rational design and stability assurance of water-preventing coal pillars.