Memory deformation and safety characteristics of hydrated coal rock assemblages under cyclic loading

This study provides an in-depth analysis of the memory deformation characteristics of water-bearing coal-rock assemblages under cyclic loading and its impact on mine safety. With the continuous growth of global coal demand, especially in China, mining activities increasingly involve water-rich environment mines. This trend requires a more comprehensive understanding of the mechanical properties of coal-rock systems and water-insulating coal pillars under water-rock action and their stability. In this study, we conducted cyclic loading and unloading tests under different water saturation rates by constructing specimens of "rock-coal-rock" with different top and bottom lithologies, aiming to analyze the specimens' deformation memory effect, mechanical damage characteristics, and degradation mechanism. The experimental results show that. With the increase in water saturation rate, the peak strength of the coal-rock assemblage gradually decreased, showing a noticeable deterioration effect. In addition, the specimens under cyclic loading and unloading conditions exhibited a more significant deterioration magnitude than those under uniaxial compression conditions. It was also found that N20 (mudstone-coal-mudstone), S20 (sandstone-coal-sandstone), and SH20 (limestone-coal-limestone) assemblages exhibited different patterns of mechanical strength degradation under water-rock action, with N20 showing the most severe degradation. The damage accumulation of the coal-rock assemblage was monitored by acoustic emission (AE) technology, and an AE damage model based on different water saturation rates was established to quantitatively analyze the damage evolution law of the water-bearing coal-rock assemblage. The findings establish a scientific foundation for predicting coal pillar behavior under cyclic loading, optimizing pillar and support structure design, and highlighting the necessity of drainage, drying, and enhanced reinforcement in long-term water-rich rock systems.