Pore network structure linked by X-ray CT to particle characteristics and transport parameters

Abstract

Mass transport in soils occurs through pore networks that are highly affected by basic physical properties such as the degree of compaction, and particle size and shape. In this study, micro-focus X-ray computed tomography (CT) was used to obtain information on the pore network structure at different compaction levels for repacked columns of sands and glass beads representing different size fractions and particle shapes. Mass transport parameters, including gas diffusion coefficient (Dp) and air permeability (ka) at variably saturated conditions, were measured on the same columns using standard methods, and literature data on saturated hydraulic conductivity (Ksat) for the same materials were analyzed. A comparison of X-ray CT derived pore network structure and physical parameters showed that the round sands and glass beads exhibited larger pores, a higher pore coordination number, and a lower volumetric surface area than that of angular sands at the same particle size, resulting in higher Ksat as well as higher Dp and ka under relatively dry conditions. The X-ray CT derived the mean pore diameter (d), and the pore coordination number (Cn) for each material correlated well with key gas transport properties such as percolation thresholds and pore network connectivity. A predictive Dp model from wet to dry conditions based fully on X-ray CT derived parameters (d and Cn) was developed and showed good agreement with measured Dp for both round and angular sands.