CHEMICAL AND BIOCHEMICAL WEATHERING OF PYRITIC MUDROCKS IN A SHALE EMBANKMENT

Following the partial collapse of a shale embankment in Derbyshire, a detailed mineralogical, chemical and electron microscopical investigation was carried out to establish whether the crushed mudstone fill had been significantly affected by chemical weathering after emplacement. Samples of unweathered and weathered Namurian mudrocks from the area were also analysed for comparison and laboratory experiments carried out to simulate abiotic and biochemical shale weathering. It is concluded that chemical alteration of the shale is a rapid process and that significant post-emplacement chemical and mineralogical changes occurred in the embankment fill. The main reactions involve oxidation of framboidal pyrite, leaching of carbonates by sulphuric acid, formation of authigenic sulphate and iron hydroxide minerals, and leaching of cations from clay minerals. In parts of the fill forming the outer shoulders of the embankment, up to 10% volume loss may have occurred due to acid leaching.