The Effects of Controlled Destructuring on the Small Strain Shear STiffness G₀ of Bothkennar Clay

Abstract

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 G ₀ 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 G₀, but with time, the clay regains much of its original small strain stiffness on reconsolidation to the initial stress state. While small changes of G₀ 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.