REAPPRAISAL OF TIME-DEPENDENT SUBSIDENCE DUE TO LONGWALL COAL-MINING

Subsidence due to longwall coal mining may conveniently be considered as taking place in two phases: active subsidence as the face advances, and residual subsidence after it has come to a halt. Published data s how that the dynamic subsidence profile at low face advance rates is i ndependent of the advance rate itself. This finding is incompatible wi th the common practice of modelling subsidence as a simple viscoelasti c process. Qualitative consideration of the mechanical processes takin g place during active and residual subsidence shows that different tim e-dependent behaviour during the two phases of subsidence is to be exp ected. We infer from reported observations of time-dependent subsidenc e that the relaxation times involved during active subsidence are of t he order of days, whereas the relaxation times involved during residua l subsidence are of the order of one year. As different mechanical pro cesses are involved during each phase, one should not attempt to predi ct the duration of residual subsidence from observations of active sub sidence.