Some geotechnical and sedimentary aspects of ballclays from Devon

Abstract

Summary Some geotechnical properties of the ballclays of Devon have been related to their mineralogical composition, sedimentary depositional environment and tectonic setting. Small scale sedimentological structures of these predominantly disordered' kaolinitic clays indicate shallow water deposition in a fluvio-lacustrine environment which existed during Oligocene times in the Lustleigh-Sticklepath fault zone. The two principal types of ballclay, the stoneware and whiteware ceramic varieties, have consistently different negative liquidity indices which imply different degrees of overconsolidation. The differences of consistency between the stoneware and whiteware ballclays are considered to have arisen as a result of variations in the intensity of desiccation to which the individual beds of ballclay were subjected shortly after deposition, when receding flood waters allowed drying of the fresh sediment. The stoneware ballclays are typically stiff to hard, apparently 'intact' clays found inter-bedded with silts and sands, whilst the whiteware clays are hard or very hard, highly fissured and slickensided slightly lignitic clays which occur in close association with lignite seams and in many respects resemble underclays from coal-bearing sequences. Overconsolidation is considered to result principally from desiccation of the individual beds of clay, hence estimates of the former maximum thickness of the ballclay deposits within the Lustleigh-Sticklepath fault zone from one-dimensional consolidation tests using undisturbed and remoulded specimens are precluded.