Substrate Constraint Effects on Internal Deformation and Desiccation Cracking in Drying Fine Sediments: An X-ray CT Study

Desiccation cracks in fine sediments increase hydraulic conductivity and weaken soil strength, posing risks to the stability of geotechnical structures. While substrate constraint is known to influence crack patterns, its internal mechanism remained unclear. This study employs X-ray computed tomography (CT) with embedded micron-sized markers to quantify internal deformation during drying of bentonite paste under smooth and rough substrate conditions. Sequential scans were analysed using a hybrid particle tracking algorithm to reconstruct three-dimensional displacement at micron resolution and derive corresponding strain fields. Results show that under smooth substrate conditions, shrinkage is nearly uniform across the sample, with only slightly higher volumetric strain near the evaporation surface, and no cracks were observed during drying. In contrast, rough substrates strongly restrict horizontal shrinkage at the base, producing highly non-uniform strain distribution along both depth and surface regions, where cracks eventually initiate at the surface central area with smaller horizontal shrinkage. Based on the 3D experimental observations, a conceptual model is proposed to elucidate how substrate constraints govern non-uniform shrinkage and desiccation cracking formation. The findings offer new mechanistic insights into substrate-controlled desiccation cracking and support future efforts to assess and mitigate cracking risks relevant to geotechnical infrastructures.