Estimating the Height of Mining Induced Connective Fracturing

Longwall mining causes the strata overlying the coal seam to fracture, break and collapse into the void space resulting from the coals extraction. During this dynamic process that initiates at the roof and extends upwards, the fracturing and rotation of roof rock blocks will increase in volume so that with increasing height eventually the support of underlying rock will see the strata above fracture and deform but not completely disintegrate. Above these “zones” if there is sufficient space before the ground surface, elastic deformations and non-transecting abutment fractures may accommodate any necessary movements. These caving processes change the in situ permeability which becomes dominated by the apertures of new fractures and existing defects. Explicitly modelling with a numerical method the initiation and propagation of fractures created during longwall mining and spatially estimating permeability changes for the full overburden column has been the subject of ongoing research at the CSIRO. This paper describes a numerical capability to estimate mining induced permeability changes from first principles, that is, from fracture location, density and aperture. This data provides site-specific input to water impact assessments increasingly important for coal mining license to operate. In addition to permeability assessment we demonstrate differing roof fracture patterns and the variation in roof block formation under massive and bedded strata conditions.