A laboratory investigation of some factors affecting the stability of a mine in the Eastern Transvaal coalfield, South Africa

The stability of roofs and floors in pillared coal mines is affected by a number of properties of the strata above and below the coal seam which is worked. These include the lithological character of the rocks, the sedimentary features contained within them, the geomechanical properties of the rocks, notably their strength and deformation moduli, the durability of the rocks on exposure, and the incidence and geometry of discontinuities. Instability also can be associated with the development of excessive methane and water pressures in the strata surrounding a worked coal seam. Accordingly, rock-core material was obtained from a mine in the Eastern Transvaal Coalfield in order to investigate the influence of certain of these properties on the stability of the roof and floor of the mine. The investigation showed the existence of a number of sedimentary facies that had different geological characteristics and geomechanical properties. A large number of compressive and tensile strength tests were carried out on the rock-core material, as was permeability testing. The latter was undertaken using an Ohle cell, a Bernaix cell and a Hoek triaxial cell. The permeating fluids were nitrogen, methane and water, except in the case of the Hoek cell in which only methane was used. The Hoek cell allowed permeability to be determined under different stress conditions. In addition, the geodurability of the rocks of the argillaceous facies also was determined. The compressive and tensile strength of the various facies varied notably and the latter frequently showed significant anisotropy. Permeability tended to increase with increasing grain size in the coarser-grained facies whereas the opposite tended to occur in the fine-grained facies when tested with gas. The application of stress brought about a decrease in permeability. In the Bernaix cell, divergent flow was greater than convergent flow. The tensile stress that developed during divergent flow testing with water caused some specimens to fail. The durability indicated that some of the argillaceous material was of very poor quality.