Defoemational and physico-chemical properties of cereain sediments, with particular reference to colliery spoil

Abstract

The present study is placed in the context of the disastrous Aberfan (Wales) colliery tip failure in 1966, particularly with respect to the singular lack of knowledge regarding the consequences of long-term weathering of coal-bearing strata. Initial breakdown of fresh indurated sediments from underground workings (roof and floor measures) is shown to be a function of a) sedimentary structures b) capillarity (air-breakage) c) expandable mixed-layer 10A clay content. A geographical variation in clay mineralogy implies that breakdown should generally be at a minimum in the Durham, Northumberland and Scottish coalfields. An in situ study of a stratigraphic section containing all rock-types likely to be found in British colliery tips shows that chemical (weathering) processes are very restricted. Pyrite is the only mineral species which has completely broken down within the 8ft-deep weathering zone, over a period of about 10,000 years. Cohesion is the strength parameter most susceptible to in situ degradation and the shear strength parameters of the near-surface materials generally conform with those of jointed and fissured rocks, rather than soils The residual (ultimate) strength of weathered- unweathered Coal Measures sediments is shown to be a function of the ratio quartz: clay minerals. Major temperature-time dependent, mineralogical changes in the superficial zone of the very coaly, 100-year-old Brancepeth colliery tip are not matched by a large fall-off in shear strength. The composite internal friction value is similar to shale-fill dams in Britain; there is no statistically significant strength difference between upper slope (younger) and lower slope (older) samples, which are some 27 per cent higher than the residual spoil strength. Convincing chemical, physical and mechanical data from a 50-year-old tip at Yorkshire Main Colliery show that small changes within the heap are more readily attributable to changing colliery practice, rather than to degradation processes. After initially rapid physical breakdowm, the material has changed little after burial. This study has shown that long-term weathering processes have little influence on the overall stability of colliery spoil.