Extreme drought has exacerbated soil erosion in laterite regions. Yet existing research exhibits two limitations: (1) It remains unclear on the relationship between volumetric shrinkage and water retention in laterites under the wide moisture-content ranges; (2) The bimodal prediction models are often required to explicitly integrate the volumetric deformation and multi-scale pore structure into the soil-water characteristic curve (SWCC) framework. In this study, a bimodal SWCC equation was established to integrate the volumetric variation, the multi-scale pore structure (considering inter-/intra-aggregate pore systems), and the Young-Laplace capillarity (pore networks as randomly connected capillary bundles). The desiccation also induced the shrinkage evolution patterns. Parameter sensitivity analysis was used to investigate the physical significance of the model parameters. 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 involved the 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, where the specimen dimensions and mass-based moisture content were measured at 1-4-hour intervals. The engineering applicability was validated using data from four typical regional soils. Kunming laterite data were corrected for the volumetric variation. The results show that the pore ternary classification and the water-filling critical criterion hypothesis effectively defined the boundary between macropores and micropores. Consequently, the pore distribution function was constructed using multi-scale pore superposition. Segmented van Genuchten functions at the suction intervals were then transformed, superimposed, and integrated with the correction functions. A bimodal adjustment function was extended for the volumetric change-integrated bimodal prediction model. Two-parameter datasets were identified to modulate the curve morphology. Desiccation initiation timing (either advancing or delaying) was regulated to accelerate the post-air-entry suction desiccation and residual moisture. According to Fredlund's classical unimodal model, these parameters were defined as the characteristic parameters to control the air-entry value, desaturation rate, and residual moisture content for the macropores and micropores. Data trends from both the pressure plate and filter papers were consistent in the suction overlap region. The filter paper data points were slightly higher than before. The Kunming laterite SWCC exhibited the bimodality with the two air-entry points and a plateau in the moderate suction range. A critical suction threshold of 34 MPa was identified, where the specimens with the higher initial dry density shared the superior water retention below this threshold, while the SWCC converged beyond it. The void ratio evolution during desiccation exhibited an initial dry-density dependence, followed by a 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 the initial dry density. Volumetric shrinkage had a significant impact on water retention performance. Below a suction of 34 MPa, the shrinkage caused an upward shift in the SWCC curve after correction. Once the suction reached or exceeded 34 MPa, the volume was stabilized to avoid the correction. Energy dissipation analysis showed that the desiccation essentially involved the continuous solid-liquid-gas phase transitions under suction-gradient-induced microstructure. Model validation revealed that there was a 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 that the excellent prediction performance was achieved (R²= 0.987-0.999; root mean square error= 0.3145-1.381). The classical unimodal model was also extended into an engineering application. The finding can also provide the theoretical support for the water retention evolution during desiccation shrinkage in the extremely arid laterite regions, particularly for disaster prevention and control.

The periodic fluctuations of reservoir water level can often cause the soil in the hydro-fluctuation belt to the upward-downward seepage and dry-wet cycles. The soil structure damage can also be induced by erosion and dry cracking. This study aims to investigate the evolution and synergistic effect of suffusion and dry cracking during soil erosion in the reservoir area. A simulation experimental system was self-developed to separately perform the multiple cycles of upward-downward seepage and dry wet cycles. Their alternating combined effects were also utilized to obtain the poorly graded soils. A systematic investigation was carried out to monitor the fine particle loss in the soil and the development of surface cracks during simulation. An analysis was also made to explore the influence of surface dry cracking on the suffusion and the influence of suffusion during dry shrinkage cracking. A summary was finally given on the synergistic effect of suffusion and dry cracking on soil structure damage. The results showed that there was the random and persistent development of suffusion channels in soil under the individual action of upward-downward seepage. Multiple periodic upward-downward seepage was used to wash away the originally blocked or deposited fine particles, indicating the "repeated washing" behavior. A stronger degree of particle loss was also found than that in the persistence of one-way seepage. The suffusion damage was attributed to the overall pore structure of the soil. The development of soil surface cracks shared the memory and gradual increase under the single action of dry-wet cycles. Once new cracks occurred at the old cracks, there was a slow increase in the width and gradual blunting of openings. There was an increase in the loss ratio of fine particles in the soil and the ratio of soil surface cracks, as the number of cycles gradually stabilized. The cracking damage was characterized by the destruction of soil integrity. The suffusion in soil was induced to form the seepage holes, and then disrupt the soil continuity under the alternating combined action of the two. Thereby, there was an increase in the content of fine particles in the surface layer, leading to the intensified development of cracks. Soil surface cracking was accelerated to detach the fine particles and the expansion of dominant channels, leading to the high particle loss ratio in subsequent cycles. The location and quantity of crack development were attributed to the non-uniform distribution of fine particles on the soil surface. A crack ratio was only stabilized after three cycles. At the same time, the presence of cracks also expanded the advantageous seepage channels, leading to an increase in the differences between the upper and lower layers of the soil sample and the degree of fine particle loss. As such, there was the coupled suffusion and shrinkage cracking. Furthermore, the cumulative particle loss ratio under the combined action was 116.67% higher than that under the single action of upward-downward seepage after five cycles. The surface crack ratio was 73.33% higher than that under the single action of the dry-wet cycle, indicating the significant synergistic effect on the soil structure damage. The finding can provide a strong reference for the mechanism of soil structure damage in the hydro-fluctuation belt. The evolution of soil erosion and shoreline retreat can also offer important theoretical support to soil erosion prevention and ecological governance in the Three Gorges Reservoir.