Morphology-dependent behavior of micaceous residual soils: A micro-macro experimental investigation into size and exfoliation effects

Micaceous residual soils (MRS) in tropical granitic regions exhibit pronounced hydro-mechanical sensitivity, primarily governed by the presence of mica minerals and their weathering-induced morphological evolution. However, the quantitative relationship between mica morphological evolution and engineering behavior remains poorly understood. This study quantitatively decouples the effects of mica microstructure, specifically weathering-induced particle size variation and interlayer exfoliation, on MRS behavior through integrating controlled physical exfoliation of commercial mica to simulate natural weathering with multiscale characterization techniques (scanning electron microscopy, X-ray diffraction, energy-dispersive spectroscopy, nitrogen adsorption) and laboratory testing. Results reveal that particle size reduction densifies the soil matrix through pore-filling, reducing permeability and delaying disintegration, while elevating plasticity. Conversely, delamination generates high diameter-to-thickness ratio flakes, significantly increasing compressibility and brittleness while paradoxically decreasing water retention due to the disappearance of interlaminar voids. A particle-scale mechanism, linking morphology (size and layering) to distinct contact modes (bridging, ordering, and pore-filling) and altered interfacial properties, is established to explain the divergent responses in swelling, strength, and disintegration. This work provides insights into how weathering-driven mineralogical transformations in mica affect the behavior of MRSs and offers a reference framework for infrastructure and engineering applications of MRSs in tropical weathering zones.