Bimodal SWCC model considering multiscale pore structure and volume deformation effects and its validation

Extreme drought exacerbates soil erosion in laterite regions, yet existing research exhibits limitations: (1) The intrinsic relationship between volumetric shrinkage and water retention characteristics in laterites across wide moisture-content ranges remains unclear; (2) There remains significant potential for the development of bimodal predictive models that explicitly integrate volumetric deformation and multi-scale pore structure effects into the soil-water characteristic curve (SWCC) framework. This study established a bimodal SWCC equation integrating volumetric change effects, based on multi-scale pore structure theory (considering inter-/intra-aggregate pore systems) and the Young-Laplace capillarity principle (modeling pore networks as randomly connected capillary bundles), combined with desiccation-induced shrinkage evolution patterns. The physical significance of the model parameters was investigated through parameter sensitivity analysis. Wide-suction-range experiments (including both SWCC and parallel shrinkage tests) were conducted on Kunming laterite. The low-suction range (400 kPa or below) was determined using a pressure plate apparatus, while the high-suction range (10¹～10⁵ kPa) was measured using the filter paper method. The experimental procedures for the filter paper method involved soil sample desiccation, 14-day constant-temperature and humidity curing, and moisture-content determination for both filter paper and soil. Shrinkage tests were performed using the air-drying method, with specimen dimensions and mass-based moisture content measured at 1～4-hour intervals. The model's engineering applicability was validated using data from four typical regional soils, as well as Kunming laterite data corrected for volumetric change. The results show that the "pore ternary classification hypothesis" and the "water-filling critical criterion hypothesis" effectively resolve challenges in defining the boundary between macropores and micropores. Consequently, the pore distribution function was constructed using multi-scale pore superposition. Segmented van Genuchten functions across suction intervals were then transformed, superimposed, and integrated with correction functions and a bimodal calibration function, leading to the development of a volumetric change-integrated bimodal prediction model. Two core parameter sets were identified that modulate curve morphology by regulating desiccation initiation timing (either advancing or delaying), post-air-entry suction desiccation acceleration, and variations in residual moisture. Following Fredlund's classical unimodal model, these parameters are defined as characteristic parameters that control the air-entry value, desaturation rate, and residual moisture content for macropores and micropores. Within the suction overlap region, data trends from both the pressure plate and filter paper methods were consistent, with filter paper data points being slightly higher. The Kunming laterite SWCC exhibited distinct bimodality, featuring two characteristic air-entry points and a plateau within the moderate suction range. A critical suction threshold of 34 MPa was identified: specimens with higher initial dry density demonstrated superior water retention below this threshold, while SWCC converged beyond it. The void ratio evolution during desiccation exhibited a pronounced initial dry-density dependence and followed a clear two-stage pattern: an initial rapid decrease followed by gradual stabilization. Consistently, all shrinkage curves tended to stabilize when the gravimetric water content decreased to approximately 20%, regardless of initial dry density. Volumetric shrinkage had a significant impact on water retention performance. Below a suction of 34 MPa, shrinkage caused an upward shift in the corrected SWCC curve. Once suction reached or exceeded 34 MPa, volumetric change stabilized and the correction effect diminished. Energy dissipation analysis suggests that desiccation essentially involves continuous solid-liquid-gas phase transitions, driven by suction-gradient-induced microstructural reorganization. Model validation revealed three-phase behavior in the volumetric water content versus matric suction relationship: Phase I featured macropore-dominated drainage, Phase III reflected micropore-controlled retention, and Phase II (the transition zone) represented the synergistic interaction of macropores and micropores. Evaluation metrics confirmed exceptional predictive performance (coefficient of determination= 0.987～0.999; root mean square error= 0.3145～1.381). As an extension of the classical unimodal model, this new model combines mathematical conciseness with engineering applicability, providing theoretical support for understanding the water retention evolution mechanism during desiccation shrinkage in extremely arid laterite regions and offering practical implications for disaster prevention and control.