Slurry flow in mining-induced overburden fractures is an important theoretical concept for the grouting design of longwall overburden grout injection engineering. In this study, a visual experimental simulation system of longwall overburden grouting was designed to study the flow, pressure distribution, consolidation, and fill thickness of fly ash slurry in overburden bedding separation. Experiments showed that the slurry generates a radial and bidirectional flow during nonpressure grouting and presents itself as an approximately elliptical dominant flow channel under pressure injection. This channel expanded horizontally along the strike direction and gradually became tabular. The slurry pressure increased as the grouting time increased. Although the pressure curves at different locations exhibited similar trends, their values did not decrease as the distance from the borehole center decreased during observations. Bleeding and consolidation occurred in the slurry as soon as it flowed out of the borehole to the fracture, and the degree of consolidation increased as a function of the distance from the injection borehole. The bleeding water gathered continually to the boundary of the bedding separation fracture and was then seeped to and stored by the underlying strata based on the injection pressure. The final injection fill is manifested as a half pace with a large thickness at the center. This research provides a theoretical basis for the design and optimization of overburden grout injection in underground longwall mining.

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The mining-induced overburden bed separation grouting technique can control surface subsidence through the high-pressure grouting and filling into the bed separation during mining. The physical simulation method can be used to objectively reproduce the dynamic migration process of filling slurry in the bed separation but the traditional similar-simulation materials are not suitable for the simulation of bed separation grouting. Considering the water disintegration, weak water storage capacity, and poor permeability of traditional simulation materials, the existing similar-simulation materials were modified in this study. The improved similar-simulation materials have adjustable physical and mechanical parameters, stable properties in a water-filled environment, and high water storage and permeability, and the reasonable ratio of similar-simulation materials was determined for hard rock, medium-hard rock, and soft rock. The similarity simulation function suitable for bed separation grouting was deduced and the time similarity coefficient and permeability similarity coefficient of the bed separation grouting simulation were obtained to judge the similarity and applicability of similar-simulation materials with specific proportions. This study provides a reliable experimental simulation scheme for the physical simulation of mining-induced bed separation grouting and provides a theoretical basis for the improvement of similar-simulation materials with fluid–solid characteristics.