Influence of Moisture on Mechanical Properties and Energy Dissipation Characteristics of Coal–Rock Combined Body

Focusing on underground reservoir coal pillar dams subjected to long-term water immersion, this study employed a self-developed non-destructive water saturation apparatus to prepare monolithic and composite coal–rock specimens with varying moisture conditions. Through uniaxial compression tests combined with acoustic emission (AE) monitoring technology, the mechanical failure characteristics and energy dissipation behavior of these specimens were systematically investigated. The results indicated that both the UCS and elastic modulus (E) of the single-rock specimens decreased with increasing water content. Conversely, the mechanical properties of the composite specimens were significantly influenced by the properties and water saturation state of the rock components within the composite. When the rocks within the composite specimens share identical moisture conditions, higher rock strength correlates with greater specimen strength and strain. Under identical lithological conditions, the peak stress (σc), peak strain (εc), and elastic modulus (E) of the composite specimens decreased with increasing rock moisture content, which exhibited reductions of 45%, 21.8%, and 13.5% in σc, εc, and E, respectively, under saturated conditions. Acoustic emission (AE) monitoring data revealed that AE events in coal–rock composite specimens under uniaxial loading exhibited distinct spatial distribution patterns. Furthermore, as the rock moisture content increased, the ultimate failure mode of the composite specimen progressively shifted from shear failure within the coal matrix toward tensile failure of the composite as a whole. An analysis of the energy characteristics of coal–rock composite specimens under uniaxial compression revealed that rock properties and moisture content significantly influence energy absorption and conversion during loading. With increasing rock moisture content, the total energy, elastic strain energy, and dissipated energy at the peak load exhibited decreasing trends, reflecting the weakening effect of water on energy dissipation in coal–rock composites. This study systematically investigated the instability mechanisms of coal–rock composites from three perspectives—mechanical properties, failure modes, and energy dissipation—thereby providing valuable insights for evaluating the long-term stability of underground reservoir coal pillar dams subjected to prolonged water immersion.