A Multiscale Experimental and Constitutive Study of Silt–Clay Transition Soils

Abstract This study investigates the macromechanical behavior and microstructural features of synthetic silt–clay mixtures spanning a plasticity index range of 9.8%–31%, with emphasis on how pore-size distribution and fabric anisotropy provide microstructural insights into the macrobehavior described by constitutive laws. Integrated multiscale experiments—including anisotropically consolidated undrained triaxial compression tests, unloading–reloading tests, constant rate of strain tests, mercury intrusion porosimetry, and scanning electron microscopy (SEM)—were conducted. A unified elastoplastic constitutive model incorporating anisotropy was employed, supplemented by a novel methodology for identifying anisotropy parameters using unloading tests. Quantitative assessment of fabric orientation was performed based on SEM images of different soils. Key findings indicate that as silt content increases, the soil fabric becomes denser, leading to reduced compressibility, higher peak strength, enhanced dilatancy, slower dissipation of overconsolidation, and faster structural degradation. The initial anisotropy was also found to intensify with silt content, and a linear correlation was established between the microscale fabric anisotropy parameter (Δ) and the macroscale initial anisotropy parameter ( ζ 0 ). The integrated micro–macrocorrelations provide a basis for constitutive model selection and parameter determination in the geotechnical numerical analysis of complex soil profiles involving transition soils.