An interpretation of structural degradation for three natural clays

Abstract

The mechanical behaviour of natural clays is influenced by the degradation of the microstructure induced by the applied loads. The consequence of progressive damage to the natural microstructure is that it is not possible to describe the state of the soil in terms of effective stress components and void ratio only, and therefore it is not possible to locate a unique state boundary surface by normalizing stresses with respect to the equivalent pressure. In this paper it is shown that, by defining a normalizing pressure that accounts for the progressive loss of structure occurring during a test, the state of the soil during loading can be completely described in a normalized stress space. The laws linking the normalizing pressure to the degree of microstructure disruption are derived from the isotropic hardening laws of two constitutive models recently proposed for natural soils. The normalizing procedure is applied to the experimental results obtained on three natural clays of different depositional origin and with different mechanical properties. For the three clays, the parameters describing structure degradation permit the assessment of the rate of loss of structure with plastic strains. For the clays examined, the volumetric plastic strains are seen to play a major role, compared to the deviatoric plastic strains, in causing progressive disruption of the microstructure. Lastly, it is shown that the initial degree of structure and the rate of structure degradation can be related to the depositional conditions and the stress history of the clay deposits.Key words: clays, compressibility, fabric–structure of soils, laboratory tests, plasticity, shear strength.