The estimation of inflows to underground coal mines using longwall extraction (1985)

Abstract
An underground coal mine is planned in the Southern Transvaal. The method of extraction will be longwall mining. This causes considerable disruption to the hydrogeological regime. The project attempts to assess the surface and ground water resources in the mine concession, estimate inflows into the working and establish a base line water quality for the area prior to the start of mining.
The surface water resources are examined as a desk study. Estimates of mean annual run-off across the mine concession are made and the amount of storage in the dams on the concession is calculated.
Ground water resources were examined during a field programme which included the mapping of the structural features that control ground water flow and occurrence. Packer tests were used to establish hydraulic conductivities representative of the dolerite, fractured dolerite, shales, sandstones and coal measures.
In general the sediments can be described as relatively impermeable. The main resources of the ground water are found as isolated linear features and as minor sources perched on the dolerite or shale. Joints and fissures in the dolerite, above 180mbgl are the most important source of ground water. Ground water is also found stored in the weathered mantle.
Four analytical methods have been used in the project to estimate total inflows to avoid equivalent volumes to an extracted panel of coal. A range of permeability values was used in the equations and a wide range of inflow-estimates was obtained. The estimates are considered to be several orders of magnitude larger than inflows experienced in reality under similar conditions. The disadvantages of the simple analytical approach are discussed.
The project therefore uses a qualitative method of assessing inflows. This involved the production of a hazard plan. The hazard plan is directly applicable to the planned mine as ground water is controlled by the geological structures in and around the mining area.
The plan combines four hydrogeological indices which are the head above coal seam, depth of weathered overburden, proximity to surface water and proximity to dykes, joints and faults. This results in five classes of hazard. The plan can be used as a tool in mine planning, the volume of inflow associated with each class of hazard can be assessed during mining operations and extrapolated throughout the concession. In the absence of sophisticated analytical models the hazard plan is considered the best approach available.
The project constructs a water baseline for the concession prior to mining through the collection and analysis of twenty water samples. A comparison of quality with Government standards is made and the water quality for the area is defined as good. The expected changes to water, should pollution control be unsuccessful, are outlined.

Morton, K.L. (1985) The estimation of inflows to underground coal mines using longwall extraction. MSc thesis, [Unpublished].