Opencast coal mining: a unique opportunity for Clee Hill Quarry

Abstract

Abstract Clee Hill Quarry is an active hardstone (dolerite) quarry located 5 miles to the east of Ludlow, Shropshire. The quarry is in Carboniferous Coal Measures which have been intruded by a thick ( c. 60m) conformable fine-grained olivine-dolerite sill. The whole sequence is now folded into a broad synclinal structure and is extensively faulted. The area is blanketed by glacial deposits, 2–12m thick. Historically, quarrying operations have been closely allied to the geological structure being confined to the margins of the syncline where the dolerite is devoid of overlying Coal Measures. In 1973 a programme was undertaken to assess the viability of using Coal Measures material for the production of lightweight aggregate. This programme was initiated because virtually all remaining, albeit substantial, planned reserves of dolerite were overlain by considerable thicknesses of Coal Measures and glacial deposits. While initial results from a technical viewpoint were encouraging, the project was abandoned for commercial reasons. The Coal Measures sequence overlying the dolerite sill comprises in excess of 30 m of mudstones, siltstones and sandstones as well as four recognized coal seams. Piecemeal mining of this coal had taken place on the Clee Hills for centuries, and in the area for future quarrying abundant evidence was available at surface of former bell-pit workings. In the early 1980s a decision was taken to evaluate the coal deposits at Clee Hill Quarry. The area for detailed assessment was defined using existing borehole data and evidence from old workings identified from site survey and aerial photographs. Exploration in two separate phases comprised the drilling of 44 open holes with spot coring and borehole geophysical logging. Coal quality was determined by analysing all relevant borehole core samples. Although coal reserve estimates assumed a high degree of past shallow mining activity, the economic viability of working the coal seams by opencast methods was established. In 1986 a planning application was submitted to Shropshire County Council to work and remove all coal overlying the dolerite, over a 3 year period. The proposal would allow exploitation of a valuable mineral asset, release substantial reserves of hardstone (dolerite) and allow restoration at an early date of large areas of former mining and quarrying dereliction. Planning permission was obtained in August 1988. Negotiations with the Opencast Executive of British Coal culminated in the granting of a licence to work both British Coal (vested) and ARC (alienated) coal. Coal mining was commenced in October 1988. Predictions of seam thickness and faulting were consistently accurate and vindicated the site investigation programme. In two aspects more detailed/accurate information would have been useful. Firstly, with regard to coal quality, borehole core samples did not accurately reflect in situ moisture content. Secondly, the extent of old workings in the upper coal seams was significantly overestimated. Neither aspect significantly affected the viability of the project. The mining phase including the major restoration works was completed in July 1992.