Abstract
Vegetation enhances soil shearing resistance through water uptake and root reinforcement. Analytical models for soils reinforced with roots rely on input parameters that are difficult to measure, leading to widely varying predictions of behaviour. The opaque heterogeneous nature of rooted soils results in complex soil–root interaction mechanisms that cannot easily be quantified. The authors measured, for the first time, the shear resistance and deformations of fallow, willow-rooted and gorse-rooted soils during direct shear using X-ray computed tomography and digital volume correlation. Both species caused an increase in shear zone thickness, both initially and as shear progressed. Shear zone thickness peaked at up to 35 mm, often close to the thickest roots and towards the centre of the column. Root extension during shear was 10–30% less than the tri-linear root profile assumed in a Waldron-type model, owing to root curvature. Root analogues used to explore the root–soil interface behaviour suggested that root lateral branches play an important role in anchoring the roots. The Waldron-type model was modified to incorporate non-uniform shear zone thickness and growth, and accurately predicted the observed, up to sevenfold, increase in shear resistance of root-reinforced soil.
Highlights
Vegetation can offer a low-cost solution for improving soil stability on slopes through root reinforcement mechanisms [1,2,3,4,5]
The results presented earlier show that the shear zone thickness varies within the cross section of the tube
X-ray computed tomography (XCT) and digital volume correlation (DVC) have shown that the shear zone in direct shear experiments varies in thickness, both spatially depending on the proximity of roots and specimen boundaries and with relative displacement across the shear plane
Summary
Vegetation can offer a low-cost solution for improving soil stability on slopes through root reinforcement mechanisms [1,2,3,4,5]. Root diameters and root area fractions can be measured from roots that cross the shear plane surface and root stiffness obtained from uniaxial tensile testing [9] Other parameters, such as shear zone thickness and soil–root interface shear stress, are much more difficult to determine [17]. Little is known about shear zone thickness behaviour within the bulk of the soil and local effects near the presence of roots Because of these unknowns, coupled with the difficulty in measuring shear zone thickness, many analytical models use empirical approaches to estimate the shear zone thickness and make assumptions about the shear zone, e.g. that it remains constant in size. The experimental aspects of this research are assessed in the context of the Waldron analytical model to understand the influence of certain mechanisms on root reinforcement performance and assess the model’s predictive capability for root-reinforced soils
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