Study on mechanical properties and damage characteristics of prefabricated crack specimens under water–lithology interaction

• Investigated the mechanical behavior and failure modes of prefabricated fracture specimens under the coupled effects of moisture content and lithology, and quantitatively characterized their strength weakening using specific parameters. • Employed XRD, SEM, and CT techniques to reveal mineralogical variations, fracture surface micro-morphology, and internal dynamic damage evolution of prefabricated fracture specimens before and after water immersion. • Elucidated the microscopic failure mechanisms of prefabricated fracture specimens under water–rock interaction, clarifying the underlying process of strength degradation and damage weakening. To investigate the influence of moisture content and lithology on the pre-splitting and pressure-relief effectiveness of the thick overlying roof above coal seams, laboratory uniaxial compression, Brazilian splitting, and shear tests were conducted under three lithological types and four moisture content conditions, utilizing acoustic emission (AE) monitoring, scanning electron microscopy (SEM), computed tomography (CT) scanning, and X-ray diffraction (XRD) techniques. The experimental results indicate that: (1) The increase in moisture content enhances the plasticity of the rock mass while reducing its brittleness and post-failure fracture toughness. The saturated sandy mudstone exhibits the greatest reduction in uniaxial compressive strength and shear strength, by 46.85 % and 67.6 %, respectively, whereas the saturated siltstone shows the largest decrease in tensile strength, by 36.6 %. (2) With increasing moisture content, all lithological specimens exhibited a higher proportion of shear failure and a lower proportion of tensile failure. The proportion of shear cracks in saturated siltstone and saturated sandy mudstone increased by 8.4 % and 6.2 %, respectively, while in saturated coarse sandstone, the proportions of shear cracks and total pore volume increased by 13.3 % and 78.82 %, respectively. (3) The strength degradation mechanism of the prefabricated crack models during water immersion was jointly governed by electrostatic attraction, hydration repulsion, friction, and external load. The interaction and competition among these four factors resulted in the observed strength degradation characteristics of the prefabricated crack models. These findings provide a theoretical reference for evaluating overburden stability in engineering under water–rock interaction.