Influence of seam-level bedding plane shears on overburden behaviour around underground coal mines

Bedding plane shears are horizons within horizontally bedded strata along which differential sliding movements occur on bedding planes. These sliding movements may occur as a result of natural processes or as a result of mining-induced subsidence movements. The low strength in shear and lateral persistence of bedding plane shears has a profound effect on ground movements around extracted longwall panels and gives rise to a broad range of phenomena that aren’t widely understood. ACARP Project C33015 supported a desktop compilation of four decades of monitoring experience to characterise the nature, mechanical and hydrogeological properties of recognised types of bedding plane shears. This paper describes observations of movements on one of these types; bedding plane shears located close to the mining horizon. Bedding plane shears that form at or near the mining horizon contribute to phenomena that include far-field horizontal movements, horizontal stress relief effects, mining-induced seismic events, and challenging mining conditions at monoclines and geological faults. High-resolution subsidence monitoring from two mines in the Western Coalfield of NSW indicates these types of shears have residual friction angles as low as 8° allowing horizontal stress relief movements to extend 2-3 km from the edge of longwall mining generating low-level horizontal strains of 0.5 (0.3-0.7) mm/m to a distance of three times overburden depth from the goaf edge.