Advanced laboratory characterisation of London clay

Abstract

New findings about the geology of London Clay (King, 1981) have highlighted the importance of investigating the relationship between geology and engineering behaviour for stratified soils. Recent events, such as the Heathrow tunnel collapse in 1994 and the poorly predicted ground movements at St. James Park during the construction of the Jubilee line extension have also highlighted a local need to revise the general proprieties of the material with which engineers in London deal. This research aimed at finding a framework for the London Clay relating the engineering proprieties of this material to its geological features. High quality samples from different depths in London Clay were tested in their intact and reconstituted states using oedometer and advanced triaxial apparatus. The lithological units of the London Clay at the site have been accounted for in analysing the mechanical response of the clay. The structure and the nature of the clay from different strata were investigated microscopically and correlated with its large and small strain mechanical response. Shallower units showed a more open structure and higher clay content than deeper units. Samples from the same units had the same mechanical behaviour and engineering parameters, regardless their depth within the stratum, but differences were found between the different units, which reflected the differences in the nature and structure of clay from each stratum. The behaviour in both compression and shearing seemed to be dominated by the structure of the clay as well as by its nature, so that clay from units having a more packed and orientated structure showed a stiffer response and higher strengths than the clay from units with a more open structure. The behaviour of the clay was also investigated in the elastic region and the elastic parameters confirmed the effects of lithology on sample behaviour.