Influences of coal-rock interface morphology's spatiotemporal evolutionary characteristics on top coal caving law

The work examined how the spatiotemporal evolution of the coal-rock interface morphology affected the top coal caving ratio. The geological conditions of the 3307 fully mechanized caving face at Tang'an Coal Mine served as the research context. PFC^2D simulations were conducted to analyze the spatiotemporal evolution of coal-rock interface morphology during mining advancement. Simulations covered the caving intervals of 0.8, 1.6, and 2.4 m, with a mining height of 3 m and the top coal thickness of 3 m at a mining-to-caving ratio of 1:1. A quantitative relationship was established between the initial and final caving boundaries and coal-rock interface morphology. This revealed the intrinsic connections between the morphology, coal-rock movement, and the distribution of residual coal in the goaf. The influence of differences in coal-rock interface morphology on the top coal caving ratio was determined. The dynamic changes in the initial caving boundary (θ₁) occurred over time and space for caving intervals of 0.8, 1.6, and 2.4 m at a mining-to-caving ratio of 1:1. The initial caving boundary (θ₁) was categorized into the types A_I ~ A_II, B_I ~ B_II, and C_I ~ C_II based on its differences. The initial caving boundary (θ₁) variation pattern determined the final caving boundary (θ₂) and corresponding coal-rock interface morphology. Initial caving boundary angle θ₁ for types I and II decreased when the coal caving interval increased. For the same interval, initial boundary θ₁ for types I and II followed the pattern: θ_I-1 > θ_II-1. The corresponding change pattern for final boundary θ₂ was as follows. When θ₁ > 90°, θ₁ ≈ θ₂; when θ₁ < 90°, θ₁ < θ₂. For initial boundary θ₁ > 90° in A_I, A_II, and B_I, N₁ ∥ N₂, and the morphological differences between types A_I and A_II were minimal. Morphological changes were gradual, which resulted in minor fluctuations in the top coal caving volume. For initial boundary θ₁ < 90° in B_II, C_I, and C_II, θ₁ < θ₂. The difference between types I-II of B_I ~ B_II and C_I ~ C_II became more pronounced as the interval length increased. The difference between the peak and trough corresponding to types I ~ II increased. The sinusoidal periodic change pattern in top coal caving volume became more evident, which decreased the top coal recovery rate. The minimal morphological differences and gentle spatiotemporal distribution of type-A_I and -A_II coal-rock interfaces offered new insights for intelligent coal caving control and applications in this mine or similar mines.