Effect of Stress Level on the Microstructural Evolution of Clay under Creep

Creep in clay can significantly affect long-term deformation evolution and therefore impact the safety of geotechnical structures. To improve our understanding of the mechanism of creep, we have examined the microstructural evolution of a kaolin clay sample submitted to creep under three-dimensional or axisymmetric loading conditions, focusing on the effect of the stress level. This experimental study identifies the local mechanisms in normally consolidated and overconsolidated remolded clay samples during creep under triaxial conditions at different stress levels. The results show that the macro and micro behaviors of the kaolin clay are predominantly governed by the contractancy or dilatancy mechanism activated along stress paths at constant p′. Within the contractancy domain, the scanning electron microscopy (SEM) observations showed that the microstructural anisotropy increased with the augmentation of the stress level. Microstructural evolution during creep can be attributed to changing patterns in particle reorientation and pore geometry, resulting in plastic strain hardening or softening as well as in viscous fluid flow. The evolution of the clay microstructure therefore depends on both the stress level and the over consolidation ratio (OCR). The differences in the orientation pattern under creep appeared to be enhanced according to the contractancy or dilatancy mechanism. The dilative specimens exhibited particle orientations that were relatively random. The flattening or expansion of micropores under creep corresponded to the contraction or dilation mechanism at the specimen scale. An attempt based on the analysis of the SEM photographs was made to evaluate the evolution of anisotropy during the different loading phases.