Physical and mechanical properties and damage and fracture mechanisms of coal under hydrothermal coupling

Background Deep coal and rock masses are subjected to long-term complex hydrothermal environments, and their physical and mechanical properties are closely related to the surrounding conditions.

Methods To investigate the hydrothermal coupling effect on the physical and mechanical properties of deep coal, coal samples were selected from the 3⁻¹302 fully mechanized mining face of the Hongqinghe Coal Mine. Density, P-wave velocity, and CT tests were conducted for the coal samples before and after hydrothermal coupling treatment. Uniaxial compression acoustic emission (AE) tests were conducted under hydrothermal coupling by a universal testing machine system and an AE monitoring system. The effects of different immersion durations (0, 3, 5, 7 d) and heat treatment temperatures (25, 50, 75, 100  ℃) on the density, P-wave velocity, mechanical properties, and AE evolution of the coal samples were analyzed. Furthermore, the damage and fracture mechanisms of coal under hydrothermal coupling were revealed based on three-dimensional CT reconstruction method.

Results and Conclusions  (1) The physical and mechanical properties are deteriorated after hydrothermal coupling. Internal damage becomes more significant with the increasing immersion duration and heat-treatment temperature. Compared with the natural-state and room-temperature samples, the density of the samples immersed for 7 d and heat-treated at 100 ℃ decreases by 5.29%, the P-wave velocity decreases by 32.70%, and the strength deterioration degree (D_H-C) increases by 49.08%, whereas the cumulative AE energy and maximum hit rate increase by 174.60% and 270.52%, respectively. Moreover, the cumulative AE energy shows a logarithmic relationship with D_H-C. (2) The physical, mechanical, and AE characteristics of the coal samples exhibit the most significant changes at 3–5 d immersion durations and 50–75 ℃ heat-treatment temperatures. However, the variation amplitudes were relatively low at 5–7 d immersion durations and 75–100 ℃ heat-treatment temperatures, generally remaining below 10%. (3) Hydrothermal coupling mainly promotes the initiation and propagation of microfractures and cracks through expansion pressure induced by water absorption of clay minerals, weakening of interparticle bonding, and deformation and dislocation compression caused by differential thermal expansion. These processes ultimately result in the physical and mechanical properties deterioration of the coal samples. The findings provide a theoretical basis for understanding the mechanisms of deep coal and rock dynamic disasters.