Multi-scale insights into shrink-swell of clayey subgrades under drying-wetting cycles

Expansive clayey subgrades undergo shrinkage, swelling and cracking under moisture changes, whereas conventional laboratory assessments mainly report axial strain. This study presents a comparative multi-scale investigation of three clayey materials: two natural subgrade soils from France (Sarlat and Esnandes) and a calcium-rich Bentonite used as a reference expansive clay. One compacted specimen per material was followed through four fixed-duration drying-wetting cycles. Central axial displacement, top-view projected soil area and projected visible void area were combined to define an apparent volumetric deformation index. Separate air-dried specimens were examined by X-ray microtomography to characterize the locally resolved void structure. Under the tested 50 mm-diameter, 20 mm-high geometry and asymmetric boundary conditions, projected surface deformation exceeded central axial deformation. Bentonite showed the largest apparent volumetric response, reaching 59% swelling and − 36% shrinkage, followed by Esnandes and Sarlat. Crack Surface Ratio co-varied with apparent volumetric shrinkage, but it did not characterize crack width, depth or connectivity. Tomography showed that the CT-resolved void fraction represents voids above an effective detection limit of approximately 12–18 μm rather than the full active pore network. The comparison is consistent with mineralogical activity being a primary influence on deformation magnitude, whereas fabric rearrangement and cracking may modulate attenuation and partial reversibility over the four cycles. Because the study used single specimens, a single local tomography volume and laboratory-specific boundary conditions, the results are interpreted as comparative observations rather than predictive relationships or field-scale performance estimates.