Grain-scale characterization of water retention behaviour of sand using X-ray CT

Abstract

This paper introduces X-ray tomography as an experimental method that allows grain-scale measurements for both porosity and degree of saturation. A whole configuration and set-up were developed specifically for the study of unsaturated Hostun sand and its water retention behaviour, using X-ray CT. A “step-by-step” protocol to obtain reconstructed volumes of sufficient quality where the three phases of the specimen can be clearly distinguished (i.e., grain, water and air) was also presented. A post-processing of the images helped the visualization and the characterization of the three phases within the specimen. A region growing separation tool was used to obtain trinarized volumes, allowing a qualitative/quantitative analysis to be performed. A qualitative interpretation of the resulting images has been done focusing on the water retention domains, where images of each different domain were retrieved for different suction values. Later, local measurements of relevant soil variables were conducted for a chosen subvolume of ≈ 3 × D50. This helped to build a map of measurement that covers the entire specimen field. Finally, water retention curve of Hostun sand was plotted and compared to a reference one. An investigation about the relation between the state variables: porosity and degree of saturation, for a constant suction, was performed. A noteworthy trend between porosity and degree of saturation was identified and discussed. The analysis presented in this study could be adapted for other granular materials, combined with pore size distribution and pore shape description, in order to understand the local relation between water retention behaviour characteristics and build a model that covers the whole retention behaviour of unsaturated granular materials.