Further Studies of the Properties of London Clay

Abstract

Synopsis A number of physical and mechanical properties of the London Clay are given based on observations made in several tunnel excavations, and from laboratory tests on specimens. These specimens have been carefully prepared by hand trimming from blocks of clay, which were extracted from tunnels over a large area of the London Basin. Earlier studies of someof these properties (Cooling and Skempton, 1942; Bishop, 1947; Skempton and Henkel, 1957)have been restricted to measurements on samples obtained from boreholes or involving tube-sampling operations. Three important structural features are recognized in the clay, namely, fissures, laminations, and “backs”. The latter two features are rarely considered. The laminations render the clay orthotropic with one axis of symmetry perpendicular to the bedding and specimens have been orientated in mechanical tests so as to demonstrate this feature. The “backs” appear to be associated with minor folding (or faulting) of the stratum, but their extent and importance requires wider exploration. Stress-strain characteristics of the block specimens measured in undrained triaxial tests show that the clay is stronger and less compressible than measurements on borehole samples have suggested. Measurements of the longitudinal wave velocity on clay samples readily detect the direction of the laminations, and attenuation of the ultrasonic signal is indicative of the fissuring and particularly of disturbances caused by sampling operations. Consolidation and swelling characteristics were measured in the oedometer and again showed the orthotropic properties of the clay. The measured swelling pressures appear to provide an approximate guide to the existing effective pressures in the ground; these are also indicated by a distinct change in the pressure-void ratio relationship. I1 est donnéun certain nombre de propriétés physiques et mécaniques de l'argile londonnienne basées sur des observations faites dans plusieurs fouilles de tunnels, et d'expériences en laboratoires sur des échantillons. Ces échantillons ont été soigneusement préparés par dégrossissage à la main de blocs d'argile qui furent extraits de tunnels sur une vaste superficie du bassin de Londres. Des études antérieures de quelques unes de ces propriétés (Cooling et Skempton, 1942; Bishop, 1947; Skempton et Henkel, 1957) ont été limitées á des mesures sur des échantillons prélevés au tours de forages ou d'opérations comprenant l'echantillonnage par tube carottier. Trois caractéristiques de structure importantes se reconnaissent dans l'argile, à savoir: les fissures, le feuilletage et les failles “backs”. Les deux derniéres caractéristiques sont rarement prises en considération. Les feuihetages rendent l'argile or thotropique avec un axe de symetrie perpendiculaire à la couche et au tours d'épreuves mécaniques on a orienté des échantillons de mane à faire ressortir cette caractéristique. Les failles “backs” semblent étra associées à de petites failles de la couche, mais leur étendue et importance demande de plus amples recherches. Les caractéristiques de tension-deformation des blocs-échantillons mesurées lors d'essais triaxiaux drainés montrent que l'argile est solide et moins compressible que des mesures faites sur échantillons de forage le sugérent. Les mesures de la vélocité d'ondulation longitudinals sur des échantillons décélent facilement la direction des feuilletages, et l'atténuation du signal ultrasonique indique des fissures et particulièrement des remaniements causés par les opérations d'échantillonnage. Des caractéristiques de consolidation et de gonflement sont mésurées à l'cedometre et de nouveaufont apparaitre les propriétés orthotropiques de l'argile. Les pressions de gonflement semblent donner un guide approximatif sur les pressions existant effectivement dans le sol; celles-ci sont aussirévélées par un changement distinct de la relation du rapport entre la pression et le vide.

The relevance of soil fabric to site investigation practice

“…The influence of the fissured fabric of London clay on undrained and drained strength has received a great deal of attention (Ward et al, 1959;Bishop, 1966;Marsland and Butler, 1967;Skempton and Hutchinson, 1969) and an examination of fissure spacing and orientation is an essential preliminary step towards representative sampling and testing. The very thorough examination of the joints and fissures at Wraysbury by demonstrated an absence of permeable fabric at that location.…”

The analytical method in soil mechanics

“…The stress history to which London Clay has been subjected would be expected to have developed marked anisotropy and we find this to be the case. Ward, Samuels and Butler in their paper (Ward et al, 1959). The sites extend from Hampstead to Stockwell and from Walton to the City.…”

“…There was a good deal of evidence (for London Clay see Skempton and Henkel, 1957;Ward, et al, 1959Ward, et al, , 1965 Two extreme cases can be identified (a) x=0 (b) E(0) =o. The first of these is the ctassical case and would provide a useful check of the general analysis.…”

Discussion: Stress-dependency of elastic moduli for cross-anisotropic soils

“…(Gazetas, 1982;Kirkpatrick & Rennie, 1972;Kirkgard & Lade, 1991). Heavily overconsolidated clays may develop a strong anisotropic behaviour, with reported typical values of n ranging from 1.35 to 2.4 (Ward, Samuels & Gutler, 1959;Atkinson, 1975;Gerrard, 1977) whereas desiccated clays have shown values of I? from 1.8 to 4.2 (Gazetas, 1981).…”