Longwall top coal fall index from an integrated numerical and statistical analysis

Assessment of top coal fall potential is of great importance for sustainable longwall caving mining. However, available assessment tools/indices for the fall are applicable to roof rock only, and their use for top coal (whose geological structures may be different) can be inappropriate. This paper presents a new index for top coal fall in longwall mining where the fall is controlled by the cantilever effect. The index is developed from an integrated numerical and statistical analysis using the database from Ha Lam coal mine in Vietnam. The numerical analysis reveals that the strength and stiffness of in-seam discrete fractures and coal’s elastic modulus are inversely proportional to top coal fall. Meanwhile, the density of discrete fractures and seam depth are found to be directly proportional to the fall. A procedure for the development of assessment equation for top coal is established using single and multiple regressions and model transformation technique. A new assessment index for longwall top coal fall named Fall Index (FI) is proposed, taking coal elastic modulus, fracture density, fracture friction angle, fracture stiffness and seam depth as input parameters. The study also reveals that statistically seam depth has the most significant effect while fracture density and fracture strength show the least significant effect on top coal fall. At the same time, coal’s elastic modulus and fracture stiffness play similar roles in the fall. The results from this paper assist engineers in better assessing top coal fall potential and subsequently better controlling longwall stability for various geological conditions in mine design.