Stiff sedimentary clays: geological origins and engineering properties

This Keynote Paper† summarises and extends earlier work on the characterisation of clays, with particular reference to stiff sedimentary clays. Examples of clay characterisation are given that include the various main geological events, from sedimentation to erosion and weathering, and other phenomena such as diagenesis and brecciation. The examples demonstrate the value of plotting basic data, such as the current stress state and undrained strength, in terms of the void index. These three parameters permit an immediate, simple categorisation, in terms of geological history, of sedimentary clays. Further examples illustrate the engineering consequences of weathering of stiff clays, and the associated phenomenon of brecciation produced by the processes of cambering and valley bulging. It is emphasised that, at all stages in a clay's geological history, it is the bond strength between the soil particles, not the geological preconsolidation stress, that controls the mechanical behaviour of sedimentary clays. At all geological stages the bond strength contributes to sensitivity in terms of both stress state and strength. These sensitivities are measured against two intrinsic behaviour patterns, the intrinsic compression line and the intrinsic strength line, both of which are defined, and both of which have considerable value for the characterisation of clays. In particular, the relations between yield stress in oedometer compression and triaxial undrained strength enable the quality of laboratory test data to be assessed.