Hydraulic properties of unsaturated calcarenites by means of a new integrated approach

Most infiltration and water retention models for unsaturated porous media proposed in the literature often are based on simplified representations of the pore system and use the matric head as the main state variable. An integrated study on the hydraulic properties of homogeneous calcarenites with a complex pore network is presented, aimed at assessing water infiltration and retention mechanisms, starting from dry to wet conditions under high water inflow. Medium-grained grainstones and fine-grained packstones were used as material for laboratory testing and model set up. Petrophysical characterization and bimodal distribution of pore volume was obtained by standard and unconventional procedures. Permeability test under saturated conditions was performed with constant and falling head tests. Falling head infiltration tests at bench scale were carried out to estimate the hydraulic properties of the materials under unsaturated conditions. Unsaturated hydraulic relationships such as the soil water retention curve (SWRC) and hydraulic conductivity function were examined. The advection–diffusion form of Richard's equation, together with the pore bundle model and dual porosity concept under equilibrium conditions represent the theoretical framework used to investigate the unsaturated hydraulic behavior of the calcarenites in the falling head infiltration test. The SWRC, relative hydraulic conductivity and celerity were determined through the capillary theory and pore bundle concept considering the bi-modal nature of the pore-size distribution. Theoretical solutions and model predictions were compared with experimental data, which demonstrated the dominant control of macro-porosity on water flow and infiltration rate. In particular, our experimental results indicate that the wetting front propagates mainly through the macropores with a gravity driven flow velocity higher than the diffusive flow through the micropores. The effect of texture, degree of saturation and boundary conditions on infiltration dynamics is discussed.