Water Transfer in Granite Residual Soil: A Dual-Porosity SWCC Model in Stage-Divided Drying

Evaporative water movement through structured soils, e.g., granite residual soil (GRS), is significantly influenced by the intricate soil fabric and the interconnectedness of its pores. This phenomenon, known as desiccation, poses a substantial challenge in geo-environmental engineering. Elucidating the water transfer mechanisms within granite residual soil under evaporative conditions is crucial for comprehending the behavior of structured soils during drying. The microstructure and characteristics of water migration in a dual-pore system of GRS were investigated using nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), and soil water characteristic curve (SWCC) measuring devices (HYPROP and WP4C). The findings suggest that the extensive aggregation of cemented interfaces between iron oxide particles and kaolinite flakes is the microscopic underpinning of micropore formation within the GRS. The dual-pore network, comprised of micropores and macropores, exhibits complete connectivity at high degrees of saturation. However, this transforms into a system of partially isolated pores at low saturation levels. Water movement within the dual-porosity system adheres to the principle of preferential drainage from macropores, resulting in the persistence of saturation within micropores. During the initial stages of drying, internal fluid redistribution from macropores can replenish water within the microaggregated domains (micropore-rich regions). Furthermore, a dual-porosity model was constructed to represent the natural drying process. This model incorporates synchronous drying from both micropores and macropores, along with stage-divided drying. The internal fluid flow within the model is optimized using parameters derived from the NMR data, establishing a valuable tool for elucidating the drying mechanism in structured soils.