Tracking spatial heterogeneity of local degree of saturation in unsaturated soils at different scales via X-ray computed tomography
Zhenliang Jiang, Jianbin Liu, Anthony Kwan Leung
Abstract
Correctly tracking the evolution of spatial heterogeneity of local degree of saturation ( S r ) in unsaturated soils is essential to explain the seepage phenomenon, which is crucial to assessing slope stability. Several methods exist for quantifying the heterogeneity of local S r . However, a comprehensive comparison of these methods in terms of accuracy, relative advantages, and disadvantages is currently lacking. This paper presents a comparative analysis of local S r obtained at multiple scales, ranging from the element scale to the slice, representative element volume (REV), pore, and voxel scales. The spatial heterogeneity of S r in an unsaturated glass beads specimen at different matric suctions was visualised and quantified by multiscale X-ray micro-focus computed tomography image-based analysis methods. Local S r obtained at different scales displayed a comparable trend along the sample depth, yet the REV-scale method showed a much scattered and discontinuous distribution. In contrast, the pore-scale method detected a distinct two-clustered, bimodal distribution of S r . The pore-scale method has the highest integrated resolution, as it has the highest spatial resolution (i.e. number of data points) and provides more information (i.e. number of extractable physical parameters). This method thus provides a more effective approach for tracking the spatial heterogeneity of S r . Based on this method, pore-scale water retention curves were determined, offering new quantitative means to characterise pore water heterogeneity and explain water drainage processes such as hysteresis at the pore scale. • In-situ X-ray CT scanning was performed during the drying of glass beads. • Spatial heterogeneity of local degree of saturation ( S r ) was quantified by multiscale methods. • Significantly different heterogeneous distribution of S r was identified by different methods. • Pore-scale water retention curve can characterise pore water heterogeneity. • Pore-scale method presented the highest spatial resolution and information availability.