Predicting vertical subsidence above chain pillars

A significant component of surface subsidence above longwall coal mines develops above the pillars between the extraction panels. The prediction of this subsidence faces both aleatoric and epistemic uncertainties. Foundation engineering concepts can be utilised provided there are appropriate estimates of the deformation moduli of the rock mass. For the bedded rock masses under consideration the deformation moduli are dependent on the spacings of bedding discontinuities about which there is inadequate knowledge. There is an additional complication in that often the site investigation does not extend far enough below the target coal seam to adequately characterise the rock mass. An empirically derived system stiffness can be used to estimate elastic deformations. Longwall mining results in very rapid loading of chain pillars (possible in the order of 3.5 MPa/day). In the vicinity of the coal seams there may be low strength clay-rich layers that can undergo undrained loading conditions such that bearing capacity failures result in larger but unpredictable surface deformations. Longwall mine layouts often allow yielding of the pillar system. However, pillar strength is empirically determined from a database of failed pillars with a width to height ratio much smaller than typical chain pillars and there is much uncertainty about how chain pillars deform after yield.