Effect of desiccation cracking on the fluid transfer process in agricultural soil

The natural soil structure can be strongly modified and generate heterogeneities during wetting and drying processes. This significantly affects the transfer of fluids and nutrients between the atmosphere, the subsoil, the hydrosphere and the biosphere. Experimental observations on a Cutanic Luvisol from agricultural field in Gembloux, Belgium, by using X-ray microtomography coupled with 3D image analysis have shown the cracking phenomenon occurring and leading to preferential flows in the soil sample during a drainage process. In order to better understand the impact of cracks on the behaviour of this soil type, in this study, we have proposed a numerical modelling of soil evaporation process by using the constitutive models implemented in the finite element code LAGAMINE. Considering that the soil we study is a loamy soil, we have chosen to fit the dual model of Durner (Durner, 1994) for the water retention capacity. The drying kinetics is modelled using the boundary layer model (Gerard et al., 2010), assuming that the vapour and heat transfers take place in a boundary layer at the surface of the porous medium. The embedded fracture model is chosen to represent the development of the fractures in porous medium in which fracture opening is activated by a threshold strain parameter (Olivella et Alonso, 2008). The results obtained have shown that an increase in permeability in the fracture zones makes the permeability tensor anisotropic up to one order and thus strongly modifies the drying kinetics of the soil core (e.g., evaporation rate). The results also have suggested that using a simple concept of cracking development, a continuum model is capable of modelling preferential flows developed in a fractured porous medium such as agricultural soil.