Consolidation and other geotechnical properties of shales with respect to age and composition

Abstract

Studies of a wide selection of over consolidated, weak argillaceous rocks from major formations in the United Kingdom and North America have shown that the compaction history, coupled with the mineralogical composition have a decisive bearing on the nature of the material, both at depth and in the near surface zone. Current evidence indicates that maximum depths of burial of North American sediments are generally much greater than their British counterparts; infearing that overburden does not increase systematically with age. Furthermore major differences have also been observed in the mineralogy and geochemistry of these two groups. In particular, recalculated smectite formulae indicate the onset of the montmorillonite to illite transformation in the former sediments. Preferred orientation studies and electron microscopy have been used/to elucidate the clay microstructure, whereas exchangeable cations and pore water chemistry indicate possible interactions between clay, minerals. Consolidation studies to a pressure of 35000KN/m(^2) on both undisturbed and remoulded materials have led to a new interpretation of the stress-strain response, and in addition these tests have indicated the presence of diagenetic bonding in unweathered materials the strength of which is dependent upon the maximum depth of burial and mineral species present. Furthermore, since slaking and suction experiments frequently only detect this bonding in the Caroniferous materials, it has been inferred that mineral-mineral welding is present in these and that cat ionic bonding predominates in younger sediments. Consequently to avoid unforeseen engineering complications in the field caused by the subsequent destruction of the latter bond type by weathering agents it has been suggested that a combination of suction and consolidation tests should be performed on the shales in question.