Mechanical Degradation and Acoustic Emission in Coal–rock under Hydrochemical Scouring-Dissolution: Damage and Fracture Mechanism

As coal mining progresses to greater depths, the mechanical behavior and fracture mechanisms of composited roadway under complex hydrogeological conditions become critical for ensuring deep mining safety. This paper investigates the mechanical degradation and acoustic emission (AE) characteristics of coal–rock combined body under hydrochemical scouring-dissolution. Uniaxial compression tests combined with digital image correlation (DIC), nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and X-ray diffraction (XRD) characterize the distinct fracture mechanics and microstructural responses of coal–sandstone combined body to varying pH levels and flow rates. Key findings include: (1) Under more acidic solutions and higher flow rates, coal exhibits a transition from brittle to ductile failure, characterized by microcrack propagation and localized deformation, while sandstone undergoes significant fracture network development due to mineral dissolution and cementation degradation. (2) AE analysis demonstrates coal exhibits uniform microcrack propagation through dense low-energy emissions and stable b-values driven by flow forces, whereas sandstone displays localized high-energy AE bursts with unstable b-values during acid-induced macrofracture development. (3) Microstructural analysis highlights that sandstone is more sensitive to pH-induced dissolution, with rapid pore-fracture expansion, while coal exhibits greater susceptibility to flow rate-driven scouring, causing interlayer delamination and surface erosion. These findings underscore the critical role of groundwater chemistry in shaping the mechanical behavior of coal–rock systems, providing a foundation for safer and more efficient deep mining practices.