Numerical and experimental study of fracture evolution in layered sandstone and mudstone above the cavity

In accordance with the distinctive characteristics observed in the sandstone-mudstone composite roof within Zhaogu coal mine, Jiaozuo coalfield, this paper delves into the influence of the non-uniform key stratum of Dazhan sandstone, characterized by varying thicknesses, on the overall strength of the composite roof. A mechanical model for the sand-mudstone composite body, considering different sandstone thickness ratios, was established. The study employed ESLS DIC and AE systems to investigate the mechanical behavior. The findings reveal a general decline in compressive strength, with a stepwise reduction in post-peak stress correlating with diminishing sandstone thickness ratios in the combined samples. Specifically, when the sandstone thickness ratio is 25%, the elastic strain energy accumulated in the sample is discharged before the initial peak, resulting in a distinctive "double-peak" shape in the stress–strain curve. Notably, deformation coordination between sandstone and mudstone is evident during loading, wherein mudstone horizontal deformation near the interface is constrained by sandstone. This constraint induces circumferential inward shear stress, gradually diminishing with increasing distance from the sand-mudstone interface. Post-failure, the strain concentration degree and range of the sample undergo significant changes, reflected in fluctuations in the corresponding fractal dimension. Acoustic emission (AE) events predominantly manifest in the upper section of samples as the sandstone thickness ratio decreases. Throughout the loading process, multiple peak periods of counts concentration are observed, accompanied by noticeable fluctuations in the cumulative counts curve.