Abstract
In this paper a new model for the hydro-mechanical behaviour of rooted soils is developed. It is a physically-based model that couples finite strain soil deformation with unsaturated water and air flow, while improving on existing cohesion-based approaches to mechanical root reinforcement and empirical soil water-uptake approaches typically used to deal with rooted slopes. The model is used to show that the dynamics of soil-water pressure and soil deformation depend strongly on the physics of the root-water uptake and the elasto-plastic soil mechanics. Root water uptake can cause suctions and corresponding soil shrinkage sufficiently large to necessitate a finite-strain approach. Although this deformation can change the intrinsic permeability, hydraulic conductivity remains dominated by the water content. The model incorporates simultaneous air-flow, but this is shown to be unimportant for soil-water dynamics under the conditions assumed in example simulations. The mechanical action of roots is incorporated via a root stress tensor and a simulation is used to show how root tension is mobilised within a swelling soil. The developed model may be used to simulate both laboratory experiments and full-scale vegetated slopes.
Highlights
The role of plant roots is increasingly being included in geotechnical assessment of unsaturated or partlysaturated1 vegetated slopes [1]; [2]; [3]; [4], and is incorporated into ‘bio-engineering’ design for slope performance [5]; [6]; [7]; [8]; [9]; [10]
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This paper describes the development of a coupled physically-based soil deformation model that accounts for the influence of plant roots and is parameterised by independently measurable quantities
Summary
The role of plant roots is increasingly being included in geotechnical assessment of unsaturated or partlysaturated vegetated slopes [1]; [2]; [3]; [4], and is incorporated into ‘bio-engineering’ design for slope performance [5]; [6]; [7]; [8]; [9]; [10]. For the purposes here of demonstrating the mechanical coupling under 1D loading conditions, a very simple root model is assumed as follows: there is a rigid root-soil interface (i.e. the root strains are equal to the soil strains, εεrr = εε); the roots provide only a vertical tensional force; root stress occurs only in tension; the roots act after time ttrr, otherwise hold no tension; the cross-sectional area of the roots is unchanged by stretching (i.e. Poisson’s ratio for the roots is zero); there is a linear relationship between tensional stress and strain, cast in an intermediate reference frame that defines the initial state of the roots; see Supplementary Information A; and the roots do not reach their breaking strain. Given the potentially significant changes in effective stress due to RWU it may be possible that the deformation of the solid skeleton is sufficient to affect the permeability, the coupled fluid flow and mechanical response This has been shown to affect consolidation in saturated media at large strains [87]; [70]. The root tension resists re-expansion of the wetting soil, giving rise to a smaller upward movement of the soil surface over the final 120 days of the simulation
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