A general framework for the mechanical behaviour of clays

It is suggested that the structure of clays may be distinguished simply as either ‘sedimentation’ or ‘post-sedimentation’, depending on whether gross yield in one-dimensional compression in void ratio-vertical effective stress plane occurs on the original sedimentation compression curve or to the right of this curve. A behavioural framework is proposed for clays having either of these structures, the magnitude of the strength sensitivity S_t when consolidated to the gross yield state (in compression) quantifying the effects of structure. The behaviour of natural clay consolidated to stress states pre-gross yield is related to the undisturbed clay sensitivity S_t; the gross yield curve is generally of arch shape, its size relative to that of the reconstituted clay depending on this sensitivity. Clays having S_t > 1 consolidated to stresses beyond gross yield exhibit post-gross yield behaviour in which values of sensitivity reduce with increasing post-yield strain. It is found that following normalization for composition (using the stress ratio at critical state M), volume (using p*_e) and structure (using the sensitivity at gross yield S_t ), many different clays have the same gross yield curve. A basically frictional linear flow rule is found to operate from small or medium to large strains for both natural and reconstituted clays.