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Analysis of an advanced dewatering scheme at an opencast coal site in Northumberland

Abstract A triangular area of Coal Measures rocks bounded to the south by the major WSW-ENE trending Causey Park Dyke, to the north and west by the limit of outcrop and the east by the North Sea, has been extensively worked by opencast methods over the past twenty years. Due to the close concentration of opencast workings it has been feasible to maintain a regional dewatering scheme over a number of years from Hauxley Shaft. However, during prospecting operations for an opencast coal site it was found that substantial quantities of groundwater existed in old workings horizons, ponded against the Causey Park Dyke in a broad anticlinal structural trap. It was decided by the Contractor that the area should be dewatered before excavations took place, to avoid the danger of flooding and water induced slope failures, and a submersible borehole pump was, thus, installed. This provided an opportunity to undertake a pumping test in mining disturbed strata. Due to the high permeability of the old underground workings it was found that the data did not conform to any standard analysis methods. However, it was found that the system could be modelled by treating it as a lagoon of triangular wedge shape. A mass balance calculation was performed and it was predicted that it would take approximately 350 days to dewater the site. The field data conforms closely to the predicted drawdowns and it is concluded that this method may be applicable to many regions with a mining history.