Hydrogeological aspects of rock mechanics and mining subsidence around longwall extractions
A problem of potential water occurrences exists, whenever coal reserves are worked beneath either surface or sub-surface water bodies. The object of this work is to examine hydrogeological techniques and problems which can be associated with longwall coal extraction. Current U.K. mine drainage problems are discussed along with the various means by which water can enter a working horizon. An area of principal concern is the potential access of water via fracture networks associated with subsidence profile formation. The role of subsidence development with respect to different geological conditions is considered. Similarly, parameters which control the potential yield of an aquifer, as well as the techniques available for assessing them, are also discussed, with particular reference to Coal Measures strata. A reappraisal of existing test site data, collected by Nottingham University Mining Department is undertaken and concludes that permeability changes can be linked to subsidence profile development. Permeability monitoring techniques can be used for strata control investigations and reference is made to monitoring the caving characteristics of a massive sandstone roof. Similarly, problems connected with thick sandstone horizons and potential weight bump conditions are also examined. The effects of subsidence profile formation on the undersea coalfield workings of North-East England are examined with respect to the NCB 1968 guidelines for working under the sea. A geological and hydrogeological reappraisal, concludes that water occurrences can be expected in any area where a potential aquifer horizon has been displaced by faulting into close proximity with a working horizon.