Analysis of main factors influencing the apertures of mining-induced horizontal fractures at longwall coal mining

Main factors influencing the wide range of apertures (from 50 to 400 mm) of mining-induced horizontal fractures (Torezko-Snezhnyanskaya coal area, Ukraine) at traditional (without grout injection into bed separation) longwall coal mining were analysed. Apertures of these fractures were directly measured (using an original experimental device) in vertical boreholes drilled from ground surface to active underground coal workings. It is revealed that main factors influencing the apertures of mining-induced horizontal fractures are thickness of extracted coal seam, distance from extracted coal seam to horizontal fracture, uniaxial compressive strengths of rock layers above and below the fracture, thicknesses of rock layers above and below the fracture, and thickness of “layer-bridge” (key strata). It is argued that influence of above-mentioned factors on the aperture is strongly dependent on combinations (Cases 1–5) of values of these factors. The mathematical forms of correlations (and values of involved empirical coefficients) and role (dominant or insignificant) of each factor in each of five Cases are discussed.