E. J. Wilson writes: Goulty & Al-Rawahy conclude that, since different mechanical processes are involved in active and residual subsidence, one should not attempt to predict the duration of residual subsidence from observations of active subsidence. Whilst this is obviously true in a literal sense, the possibility that active subsidence may result in delayed and possibly prolonged residual subsidence should be considered.
The authors quote two cases by Orchard & Allen (1975) in the Durham Coalfield where the presence of thick competent strata prolonged residual subsidence to six years (Peterlee) and delayed the onset of residual subsidence by four years (North Durham). It is significant that both cases showed broadened subsidence profiles. They go on to quote Pine & Randon (1986) who recorded bedding separation below the thick Lower Magnesian limestone in the Selby Coalfield, where active subsidence at the Wistow Mine had produced less than half the predicted settlement, and concluded that bridging in the Permo-Trias can persist until the adjoining panels are worked out. Reference is made to further examples presented by Orchard & Allen (1975) and Collins (1978). In one of the cases quoted by Collins at Pengam, residual subsidence came to a stop after 2.5 years, and was noted by Collins to form a large proportion of the active subsidence. In that case however the active subsidence had only been about half of that predicted. At Pengam, as also at Bedwas, prolonged residual subsidence was ascribed to the presence of very thick Pennant Sandstone in the