Micro- and macro-scale water retention properties of granular soils: contribution of the X-Ray CT-based voxel percolation method

Abstract

Water retention in granular soils is a key mechanism for understanding transport processes in the vadose zone for various applications from agronomy to hydrological and environmental sciences. The macroscopic pattern of water entrapment is mainly driven by the pore-scale morphology and capillary and gravity forces. In the present study, the drainage water retention curve (WRC) was measured for three different granular materials using a miniaturised hanging column apparatus. The samples were scanned using X-ray micro-computed tomography during the experiment. A segmentation procedure was applied to identify air, water and solid phases in 3D at the pore-scale. A representative elementary volume analysis based on volume and surface properties validated the experimental setup size. A morphological approach, the voxel percolation method (VPM) was used to model the drainage experiment under the assumption of capillary-dominated quasi-static flow. At the macro-scale, the VPM showed a good capability to predict the WRC when compared with direct experimental measurements. An in-depth comparison with image data also revealed a satisfactory agreement concerning both the average volumetric distributions and the pore-scale local topology. Image voxelisation and the quasi-static assumption of VPM are likely to explain minor discrepancies observed at low suctions and for coarser materials.

Landmark Papers: No. 9 Jarvis, N.J. 2007. A review of non‐equilibrium water flow and solute transport in soil macropores: Principles, controlling factors and consequences for water quality. European Journal of Soil Science, 58, 523–546

“…Samec et al (2013) demonstrated the potential to reproduce the scalefree topology of macropore networks by network models. Additionally, the percolation theory and non-equilibrium thermodynamics are considered promising for modelling the connectivity and genesis of the organized heterogeneity of macropore networks (Jarvis et al, 2016;Jarvis, Larsbo, & Koestel, 2017;Lin, 2010;Shiota, Mukunoki, Oxarango, Tinet, & Golfier, 2019).…”

Experimental study on the microscopic swelling-shrinking mechanisms of clay-sand mixtures from wetting to drying

“…Due to their non-destructive, intuitive, fast and accurate nature, 1 H nuclear magnetic resonance (NMR) and X-ray computed tomography (CT) techniques have been widely applied to characterize the morphology and structure of various porous materials such as shale, sandstone, cement, silica, zeolite and composite fibers [25][26][27]. So far, milestones have been achieved in the observation of soil structure, quantitative description of particles and extraction of pore features by CT scanning [28][29][30][31], but there are still some challenges in reconstructing a 3-D soil-water model. In particular, this is especially critical to clarify the geometric shape, quantitative ratio and spatial distribution of liquid water inside the pores during a wetting-drying process.…”

Comparison of various 3D pore space reconstruction methods and implications on transport properties of nanoporous rocks

“…There is therefore a very serious hope to significantly improve the characterization of these properties digitally by taking full advantage of advances in imaging at different scales [Andrä et al, 2013a,b]. This is the aim of the so-called Digital Rock Physics (DRP) approach based on the use of high resolution digital images of rocks so that the material properties are evaluated numerically at the pore scale [Blunt et al, 2013, Guibert et al, 2015, Tahmasebi et al, 2017c, Shiota et al, 2019.…”