Unlocking changing clay behaviour: insights from nano-tomography of evolving soil fabric

Cyclic loading in normally consolidated clay generates excess pore pressure that, upon dissipation, produces post-cyclic volumetric strains consistently larger than those predicted by the classical unload–reload line. Although this macroscopic behaviour has been well documented, its microstructural origin remains unresolved. This study combines direct simple shear tests on reconstituted Onsøy clay with three-dimensional x-ray nano-tomography to directly observe clay fabric variation. Fabric measurements revealed that the normally consolidated specimen exhibited a predominantly horizontal platelet orientation (θ≈86°), which was only modestly disturbed after undrained cyclic loading (θ≈77°). In contrast, the over-consolidated specimen showed a markedly different fabric orientation (θ≈60°), reflecting a greater degree of platelet rotation relative to the loading direction. Upon post-cyclic recompression, the fabric angle recovered towards θ≈81°, indicating a tendency to realign with the principal stress direction. These observations suggest that, consistent with established micromechanical frameworks, the near-horizontal fabric retained after cyclic loading is interpreted as promoting the loss of edge-to-face contacts and the development of a face-to-face platelet configuration during consolidation, an irreversible mechanism that may drive strains beyond the elastic recompression line.