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A large proportion of the southern UK is underlain by stiff clays. Improving their geotechnical characterisation is important for many current and future infrastructure projects. This paper presents an integrated study of the complex stiffness behaviour of four key medium-plasticity, highly overconsolidated strata: the Gault, Kimmeridge, Oxford and London clays. The latter were deposited between the Jurassic and the Eocene under broadly similar marine conditions. Coordinated programmes of advanced static and dynamic laboratory measurements have been undertaken on high-quality samples, concentrating on samples taken from similar depths at inland sites and including triaxial and hollow cylinder stress path experiments employing high-resolution local strain, multi-axial bender element and resonant column techniques. A new approach was employed to interpret the hollow cylinder experiments and the laboratory measurements are examined in combination with independent field shear wave data. The clays' stiffness characteristics are shown to be markedly anisotropic, pressure dependent and highly non-linear. Synthesis allows key conclusions to be drawn regarding: the relative reliability of alternative measurement approaches; the potential spread of stiffness behaviours between the clays; and whether the clays' varying geological ages and burial depths have any systematic influence on their stiffness characteristics. The results have important geotechnical engineering implications.

The importance of local measurement of small strains is well recognised. Without such measurements, accurate stiffness determination is not possible and can result in overestimations of strains resulting from stress changes in the ground. Axial strain is often considered to be the primary strain to be measured under triaxial conditions and there has been a focus on its measurement. Radial strain is also important, for example for determining Poisson's ratio and bulk modulus, or maintaining K0 conditions during consolidation. There are a number of devices available that enable its determination, using both direct and indirect measurement approaches. This note describes a system for measuring radial strains directly using linear variable differential transformers. It is shown to be simple, reliable, accurate and robust.