Damage evolution mechanism of coal rock under long-term soaking

The deterioration of the mechanical properties of water-rich coal rock is a predisposition to many mine geologic hazards. To explore the progressive damage characteristics of coal rock under long-term soaking, the weakly caking coals of the Shenmu region were selected as the research objects. Firstly, a long-term soaking test of coal rock was carried on without pressure for 15, 30, 60, 120, and 240 days. Then, uniaxial compression tests with acoustic emission (AE) monitoring were carried out on dry and soaked coal rock to analyze its water-softening characteristics. Finally, the evolution of pore structure of coal rock under different soaking durations was studied using scanning electron microscope (SEM) and nuclear magnetic resonance (NMR). The results showed that: 1) the water saturation of coal rock increased with the duration of soaking, and the increase rate was continuously decreased, and tended to a constant; 2) In the process of saturation, the mechanical properties of coal rock deteriorate, and the mechanical properties of saturated coal rock still deteriorate with the increase of soaking duration, manifested as the decrease of compressive strength and elastic modulus, and an increase in cumulative AE ringing numbers; 3) A surface shrink may occur after the drying process, and during the soaking test, the small pores of samples gradually increased, and the pore structure became more complex with the increase of soaking duration. The research results can provide useful prevention for the instability and failure of coal rock in water-rick conditions.