Fines-Induced Shear Band Thickening in Gap-Graded Soils: Evidence from Digital Image and Volume Correlations

Shijin Li, Alexandre Sac-Morane, Jeroen Soete, Hannes Claes, Johan Vanhulst, Hadrien Rattez

Shear band thickness is a critical parameter governing strain localisation and post-peak strength degradation in granular soils. However, its relationship with fines content and the consistency between surface- and volume-based measurement techniques remain insufficiently understood. This study investigates the evolution and residual thickness of shear bands in gap-graded sands with fines contents ranging from 0% to 40%, using triaxial tests equipped with either digital image correlation (DIC) or digital volume correlation (DVC). The stress-strain response and friction angle variations with varying mean particle size and fines content align well with, and further validate, previous experimental and numerical studies. DIC measurements, derived from surface strain fields, are compared with DVC measurements obtained from internal deformation fields reconstructed via X-ray computed tomography. Results show that increasing fines content promotes both the development and thickening of shear bands, with trends consistently captured by both techniques. Shear bands became clearly identifiable at axial strains of 5.0-5.8%, coinciding with the end of strain softening and transition to residual strength. DVC revealed early, non-uniform internal strain localisation and secondary shear plane formation, features not captured by DIC. The residual shear band thickness normalised with mean particle size increased linearly with fines content from ~7.6 to ~11.7, showing that the shear band of sand is not only controlled by the mean particle size but also from fines content. These findings provide new insights into the role of fines in strain localisation, guide the selection of experimental methods for shear band characterisation, and offer valuable data for calibrating and validating numerical models of soil failure mechanisms.