Abstract
Abstract. Centrifugation provides a fast method to measure soil water retention curves over a wide moisture range. However, deformation of soil structure may occur at high angular velocities in the centrifuge. The objective of this study was to capture these changes in soil structure with X-ray microtomography and to measure local deformations via digital volume correlation. Two samples were investigated that differ in texture and rock content. A detailed analysis of the pore space reveals an interplay between shrinkage due to drying and soil compaction due to compression. Macroporosity increases at moderate angular velocity because of crack formation due to moisture release. At higher angular velocities, corresponding to capillary pressure of ψ < −100 kPa, macroporosity decreases again because of structure deformation due to compression. While volume changes due to swelling clay minerals are immanent in any drying process, the compaction of soil is a specific drawback of the centrifugation method. A new protocol for digital volume correlation was developed to analyze the spatial heterogeneity of deformation. In both samples the displacement of soil constituents is highest in the top part of the sample and exhibits high lateral variability explained by the spatial distribution of macropores in the sample. Centrifugation should therefore only be applied after the completion of all other hydraulic or thermal experiments, or any other analysis that depends on the integrity of soil structure.
Highlights
Soils, rocks and sediments are assumed to be rigid bodies in many modeling applications
X-ray microtomography has turned into a standard technique to fill this gap and measure the threedimensional internal structure of porous media (Ketcham and Carlson, 2001; Cnudde and Boone, 2013; Wildenschild and Sheppard, 2013)
We have developed a workflow for the automatic detection of soil structure deformation by means of free image regiswww.solid-earth.net/7/129/2016/
Summary
Rocks and sediments are assumed to be rigid bodies in many modeling applications. The internal structure of these porous media is modified through a variety of technical and natural processes. There is a huge variety of image processing and image analysis methods that are all tailored for the ultimate goal to quantify the complex, structural heterogeneity based on a few meaningful parameters (Kaestner et al, 2008; Vogel et al, 2010; Schlüter et al, 2014). Spatially explicit information about the internal displacement of particles or aggregates is excluded from analysis in such an approach. This local deformation information is of particular interest, e.g., in soil mechanics (Terzaghi et al, 1996).
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