The post-yield behaviour of four Eocene-to-Jurassic UK stiff clays

Abstract

A detailed study is described of the post-yield behaviour of four medium-plasticity heavily overconsolidated UK stiff clays. Sub-layers of the stiff-to-hard Gault, Kimmeridge and Oxford clays were identified, sampled and tested; these, along with facies investigated in an earlier London Clay study, had broadly similar depositional histories. The intention in considering a spread of similar sediments from the Jurassic to the Eocene was to allow any strong effects of geological age, or burial depth, to be identified. A strongly fissured meso-structure was present in three of the four clays, which had a controlling influence on their effective shear strengths, considering that the representative element volume is of paramount importance in measuring the strengths of such soils. All four soils were brittle in shear and, when sheared to sufficient displacements, developed low residual shear strengths. The stiff clays were investigated further through comparisons between natural and reconstituted behaviour, using the latter to normalise the effective stress data for volume and also considering the clays' oedometer swell sensitivities. Normal compression tests, when normalised for void index, implied different degrees of ‘structure' than undrained shear tests, showing that a more elaborate micro- and meso-fabric framework is needed to capture the behaviour of highly overconsolidated and aged geomaterials. This paper focuses on describing the study sites' geotechnical profiles and the stiff clays' yielding behaviours under one-dimensional compression and in triaxial compressive shear.