Coupled hydro-mechanical pore-scale modeling of biopore-coated clods for upscaling soil shrinkage and hydraulic properties

Earthworms and plant roots are vital for macropore formation and stabilization. The organo-mineral coating of biopore surfaces also regulates macropore-matrix mass exchange during preferential flow. The influence of finer-textured burrow coatings on macroscopic soil properties during shrinkage could potentially be assessed by upscaling pore-scale hydraulic and mechanical simulations. The aim was to investigate the influence of micro parameters (particle size, stiffness, and bond strength) on macro parameters (i.e., shrinkage curve and soil hydraulic properties). Drainage experiments and simulations were carried out using biopore-coated clod-size samples compared to those without coating. Simulations were performed using a two-phase pore-scale finite volume coupled with discrete element model (DEM-2PFV). The structural dynamics was characterized by analyzing the pore volume and soil shrinkage curve obtained from numerically determined data. The soil hydraulic parameters were described using uni- and bimodal van Genuchten (vG) functions. The drainage simulations revealed hydro-mechanical dynamics characterized by Braudeau-shrinkage curve subdomains: The matrix-only samples, with lower particle bond strength, exhibited relatively higher shrinkage. The coated samples, with higher particle stiffness and bond strength, displayed greater hydro-mechanical stability. The numerically determined initial value of the saturated hydraulic conductivity (Ks) was about 70 times larger for matrix-only samples than for coated samples. As expected, for the nonrigid soil structures, constant Ks, alpha, and n values for bimodal vG model resulted in prediction errors. Upscaling DEM-2PFV pore-scale model outcomes quantifies micro-coating effects on macro hydro-mechanics. This yields void ratio-based soil water retention and hydraulic conductivity functions, advancing macroscopic soil hydraulic models and enhancing structured soil flow and transport descriptions. Constant hydraulic properties led to prediction errors in nonrigid soil structures. Upscaling pore-scale simulations quantified shrinkage and water retention curves. Coated biopore surfaces increase soil horizontal hydro-structural stability.