Designing a longwall layout in the face of stringent subsidence constraints - the Mandalong experience

Following the granting of the consent to conduct longwall mining at Mandalong, a low-risk option that was identified by the mine owner (Centennial Coal) was to have a longwall layout that restricted tilts to less than 7mm/m, strains to less than 4mm/m, and vertical subsidence to less than 500mm under the flood plain. The depth of cover under the flood plain is of the order of 155m. The overburden geology at the site includes a massive conglomerate unit about 100m above the seam and this has allowed the successful design and implementation of sub-critical extraction panels of 125m width. As of mid 2007, 4 such panels have been extracted and the sag that has developed above them is in the order of 50mm-100mm and the maximum subsidence under the flood plain is approximately 240mm; the associated maximum tilt is 4.5mm/m, and the maximum strains are less than 1.8 mm/m. The larger component of the vertical subsidence was correctly predicted to be associated with the compression of the roof/coal pillar/floor system. A foundation engineering approach was taken to predict this pillar subsidence and the results for the first 4 panels indicate that recently published research on the deformation modulus of rock mass is critical to predicting the magnitude of pillar subsidence. The adverse impact of thrust faults in destroying the spanning capability of the conglomerate beam was correctly identified.