The effects of controlled destructuring on the small strain shear stiffness G0 of Bothkennar clay

The important differences between natural and reconstituted soils are well, recognised. In a natural structured clay, the microstructure enables it to exist at states outside the state boundary surface for the reconstituted soil, resulting in greater peak undrained strength and yield stress at a given void ratio. The structure of soft clays is gradually destroyed by strain; understanding the process of destructuring is important both in developing constitutive models and in understanding the differences between field and laboratory behaviour. A systematic investigation of the effects of destructuring on properties of the Bothkennar clay has been carried out, using changes of small strain shear stiffness Go as an indicator of damage. Tests were conducted on both natural and reconstituted material, so that the effects of microstructure could be isolated. After initial reconsolidation under in-situ stresses to establish a baseline condition, samples were subjected to controlled cycles of undrained compression/extension strain. These have shown that such strains result in significant temporary reduction of Go, but with time, the clay regains much of its original small strain stiffness on reconsolidation to the initial stress state. While small changes of Go after reconsolidation appear to be consistent with small changes to the peak strength, they do not reflect the damage that affects the medium-strain stiffness. To correctly identify effects of microstructure, it proved important to normalise the data to a common void ratio. Drained compression tests at constant stress ratio (approx 1-D) revealed that the normalised stiffness of the reconstituted clay does not form a lower bound to that of the natural clay. These findings appear to have important implications for the formulation of constitutive models of structured clays.