Research on Damage Mechanism and Mechanical Characteristics of Coal Rock under Water Immersion

Abstract

This study aims to reveal the impact of immersion duration on the internal structural damage and mechanical property degradation of coal rocks. Coal rocks from the post-mining area of Liangshuijing Coal Mine were selected as the research subject. Physical and mechanical tests were carried out on these with different immersion durations (0 d, 15 d, 30 d, 60 d, 120 d, and 240 d) using scanning electron microscopy (SEM), nuclear magnetic resonance (NMR), acoustic emission (AE), digital image correlation (DIC), and compression testing, further elucidating the damage degradation mechanisms of water-immersed coal rocks. The research demonstrates that changes in the pore structure of coal rocks can be divided into two stages as the soaking time varies: the stage of water swelling (saturation process) and the stage of soaking damage (long-term immersion process). The water swelling stage of coal rock extends from surface drying and contraction to water swelling, and the soaking damage stage of coal rock extends from expansion to soaking damage. During the stage of soaking damage, the water showed dynamic changes from macropores to mesopores to micropores, with a gradual increase in the number of micropores. The AE count and cumulative count of coal rock decrease first and then increase, and the four stages’ acoustic characteristics and macroscopic characterization phenomena appear. The mechanical properties declined. After 240 d of immersion, the uniaxial compressive strength and elastic modulus decreased by 48.93% and 29.53%, respectively, and the plastic characteristics were enhanced. These research results provide a beneficial reference for understanding and predicting the instability and destruction of water-immersed coal rocks.