AbstractThe Department of the Environment and the Welsh Office jointly sponsored a South Wales Desk Study into the feasibility of producing thematic maps of the Mining subsidence risk for planners. A method was to be developed and tested for a 25 km2 Pilot Area centred on Ebbw Vale.The study was started by collecting geological and mining data for the Pilot Area and compiling this information on a map. It was then intended to prepare the mining subsidence map from this map by back analysing subsidence incidents to determine the hazardous areas. This simple approach could not be followed for two reasons. First, the record of mining was incomplete and it was therefore necessary to define ‘worked’ seams and areas from the total historical record and not only from the surviving mine plans. Secondly, insufficient subsidence incidents were discovered in the Pilot Area to carry out meaningful back analysis.The study was extended to collect all readily available records of subsidence incidents throughout the Coalfield. A total of 388 were traced, mainly dating from 1960. About threequarters were collapses through superficial materials and were either close to the outcrop or above mine entries, the remaining 25% propagated to the ground surface through rock. The upper limit of migration was typically 8 times the extracted height where the dip was shallow, incrasing to as much as 18 times for steep dips. Selected grout contracts and opencast mining sites were also examined to gain information on the present condition of old workings and to study the ‘reputations’ of the various seams for voids.The results of the coalfield study were applied to the Pilot Area to produce a 1:10 000 scale Development Advice Map, ‘Mining Subsidence for use by developers and planners’. The map is zoned to show areas where specialist advice is necessary to support a planning application and it also shows all mine entries traced by the desk study. The method of production is general for the South Wales Coalfield and further maps could be prepared, using the Coalfield Study, quickly and cheaply.
