Mechanisms of root reinforcement in soils: an experimental methodology using four-dimensional X-ray computed tomography and digital volume correlation

Abstract

Vegetation on railway or highway slopes can improve slope stability through the generation of soil pore water suctions by plant transpiration and mechanical soil reinforcement by the roots. To incorporate the enhanced shearing resistance and stiffness of root-reinforced soils in stability calculations, it is necessary to understand and quantify its effectiveness. This requires integrated and sophisticated experimental and multi-scale modelling approaches to develop an understanding of the processes at different length scales, from individual root–soil interaction through to full soil-profile or slope scale. One of the challenges with multi-scale models is ensuring that they sufficiently closely represent real behaviour. This requires calibration against detailed high-quality and data-rich experiments. This study presents a novel experimental methodology, which combines in situ direct shear loading of a willow root-reinforced soil with X-ray computed tomography to capture the three-dimensional chronology of soil and root deformation within the shear zone. Digital volume correlation (DVC) analysis was applied to the computed tomography dataset to obtain full-field three-dimensional displacement and strain information. This paper demonstrates the feasibility and discusses the challenges associated with DVC experiments on root-reinforced soils.

Modelling of stress transfer in root-reinforced soils informed by four-dimensional X-ray computed tomography and digital volume correlation data

“…After DVC processing, full-field data containing displacements, normal strain and shear strain components were post-processed using a custom MATLAB script. A study was carried out to optimize the DVC sub-volume size for reliable correlation, error caused by noise, strain measurement accuracy and spatial resolution [21]. This found that it was necessary to correlate adjacent scans in the XCT dataset (rather than e.g.…”

“…The minimum subset size achievable was 32 pixels cubed, and for subsets of this size noise was found to accumulate linearly with the number of displacement steps in the sequence, with the largest strain standard deviation reaching 38 millistrain at the end of the test. The subset size and noise sensitivity study is described in detail in [21]. The 75% overlap in the subset size meant that displacement was calculated on a three-dimensional grid of eight pixels or 0.37 mm.…”

“…The limiting soil-root interface stress (τ in the Waldron model) can be taken as purely frictional, based on the normal stress within the soil and an estimate of the interface friction angle. However, this neglects root branching and lateral roots that anchor the root system into the soil and are likely to increase τ [21,22]. Appropriate values of τ that take into account these effects will vary with plant root morphology and soil type, and are not well characterized.…”

A generic form of fibre bundle models for root reinforcement of soil

“…The first set of data consisted of laboratory direct shear tests on soil reinforced by juvenile willow, gorse and festulolium grass grown under laboratory conditions (Liang et al 2017), Bull et al (2020). In the following, first author initials 'TL' and 'DB' are used to differentiate tests conducted by Liang et al and Bull et al.…”

2D Kinematic Quantification of Soil Particles around Growing Plant Root based on Optical Mechanics

“…To capture the behavior of soil and soil-root interactions, it is necessary to obtain deformation information from the bulk of the soil (Bull et al, 2020). Advanced experimental methods using imaging techniques (e.g., X-ray Computed Tomography (XCT) or time lapse synchrotron X-ray Computed Tomography (4D CT)) have been shown to work well for capturing three-dimensional information of soils and roots (Perret et al, 2007;Peth et al, 2010;Keyes et al, 2017).…”