Effect of sand grain size on simulated mining-induced overburden failure in physical model tests

To investigate the influence of sand grain size (SGS) on mining-induced overburden failure in physical model tests, uniaxial compressive experiments on rock-like material with various SGSs were performed, and two SGSs (0-0.5 and 0.5-1 mm) were selected. The Young's modulus, uniaxial compressive strength, and tangent modulus were larger in rock-like material samples with finer SGS and the failure mode changed from mostly shear failure to tensile failure with rising SGS. As the mechanical characteristics of coarser sand samples are weaker, the overburden breakage distance was shorter in the coarser-sand model and the ratio of the average breakage distance between the two models was 1.688. The extent of the overburden fracture was heavier in the coarser sand model and closer pattern to that of the actual coal mine. A large database of the heights of the overburden caved zone (H-cz) and fractured zone (H-fz) values with various mining height (H-m) based on previous studies was compiled to obtain best-fit parabolic empirical formulas. The H-cz/H-m and H-fz/H-m ratios measured in the field were similar to those estimated by the proposed empirical formulas. The discrepancy between the physical model test and field measurement may be explained by the fracture pattern in the overburden of the caved zone in the form of layered rectangles, unlike the polygonal forms in the coal mine. We propose four recommendations to optimize the physical model for such tests, from the perspective of SGS. The results and recommendations presented here can provide a useful guide for coal mining scientists and engineers designing models for testing of coal mining conditions.