The response of coarse and fine coal-mine discards under controlled load triaxial testing

Abstract

The aim of the project was to investigate the behaviour of coarse and fine colliery discards with respect to liquefaction potential, using controlled load triaxial testing. It was hoped to relate the behaviour in these tests to measured material properties such as strength, shape, chemistry and mineralogy. The investigation of coarse discards contrasted the behaviour of a weak, seatearth-rich discard from Gedling Colliery with that of a strong shale-rich discard from Abernant Colliery. The former showed some liquefaction potential at low confining pressures, while the latter showed no liquefaction potential. A seatearth and a shale from County Durham were used to demonstrate say possible links between these basic materials and the behaviours observed. However, both showed responses similar to the Gedling discard. Investigation of material properties indicated that the strength of material, its stability in water and its facility for pore pressure equalisation were the most important factors influencing response during controlled load testing. The strength and stability of the material are probably related to its organic carbon content. The Abernant discard had a high strength and stability in water and showed good potential for equalisation of pore pressures during testing, in contrast to the other three materials. Tests on mixtures of Abernant discard and the Durham seatearth showed a transition between the behaviours of the end materials for a mixture containing between 20 and 25 per cent seatearth. Tests on fine discard from Peckfield Colliery showed significant differences in behaviour between samples from the lagoon and those fabricated in the laboratory. The former showed strong dilatant behaviour, while the latter showed some potential for liquefaction. This is probably due to differences in soil structure (organisation of particles). Tests on fine discard from Abernant showed a high potential for liquefaction. The most significant difference, possibly affecting liquefaction potential, between this discard and that from Peckfield was the uniformity of grading, the Abernant discard being considerably more uniform. The results for the Abernant fine discard showed no correlation with those for the coarse discard from the same colliery.