The Small Strain Stiffness of a Carbonate Stiff Clay

Abstract

ABSTRACT Knowledge of the small strain stiffness of the heavily overconsolidated Gault Clay, which has up to 30% calcium carbonate content, is rather limited. This has resulted in some difficulties in the analysis and design of structures constructed in the Gault. In this Technical note, the small strain stiffness of the Gault Clay is examined based on results from triaxial tests with internal small strain measurements, published geophysical data, and values deduced from the back-analysis of full-scale field observations of an excavation at Lion Yard Cambridge, U.K. Comparisons of stiffness values have also been made between the Gault Clay and the non-carbonate heavily overconsolidated London Clay. The results of these examinations have led to the conclusion that the stiffness-strain characteristic of Gault Clay is highly non-linear and exhibits first yield at a threshold shear strain of about 10− 5, beyond which the stiffness deteriorates dramatically from an initially very high value. After modest straining the stiffness reduces to values comparable to those for London Clay. The Gault Clay behaves like a low plasticity clay at small strains but as a high plasticity clay at medium to large strains. This behaviour is probably due to the breakdown of the weakly cemented bonding caused by the calcium carbonate content.