Most research into clay behaviour deals with reconstituted specimens that have been isotropically compressed. While there are many ways of interpreting the test results, some of the most widely used approaches are based on the ideas of elasto-plasticity incorporated, for example, in the Modified Cam Clay model which was developed specifically for isotropic material. Real clays, that were initially anisotropically compressed, exhibit various forms of elastic-plastic components but differ in detail from those of reconstituted clays. In particular, pre-yield volume changes and yield loci are different from those proposed in Modified Cam Clay. The work reported in this paper examines the estimation of insitu stresses and the pre-yield behaviour of overconsolidated natural clays. Four consolidated undrained tests and one consolidated drained test were carried out on samples of Belfast Upper Boulder Clay obtained at various depths and locations. The effective stress in the specimens p— k was measured using a pressure plate apparatus for each sample and it was used to estimate the insitu stresses in the ground. The measured effective stress in the sample p''k in combination with specific volume vk and the ratio of the cross anisotropic parameters J/3G* enable the prediction of the pre yield stress path during drained shearing. Comparison between predicted and measured stress paths is very good.
There are many ways of interpreting the behaviour of clays, some of the most effective being based on the ideas of elasto-plasticity. These suggest that concepts such as elasticity, yielding and plastic hardening can be applied to clay soils. One common elastic-plastic model is the Cam Clay Model (Roscoe and Burland 1968) which is based on evidence obtained for reconstituted materials. Natural clays differ in many ways from reconstituted clays. Natural clays are structured and reflect the conditions under which the materials were deposited.
