ABSTRACT
Reliable prediction of transpiration-induced soil moisture and changes in soil suction is important for transient seepage analysis in unsaturated vegetated soils and hence for estimating soil hydrological changes and determining slope stability. There are two broad approaches to modelling transpiration and root water uptake, namely macroscopic and microscopic approaches. The former integrates a so-called macroscopic sink term into the Darcy?Richards equation (Feddes et al., 1976; Hopmans and Bristow, 2002), and this sink term is a function of climate and the ability of plant roots to take up water (Casaroli et al., 2010; Fatahi et al., 2010; Świtała et al., 2018). The latter approach views the soil?plant?water continuum as analogous to an electric circuit (Roose and Fowler, 2004; Janott et al., 2011) and uses a number of resistance (or conductivity) terms to describe the ease of water flow in different components of a plant such as a leaf, the stem and the root. Because of its simplicity and the relative ease in calibrating the input parameters, the macroscopic approach has been more commonly used to analyse plant effects on the engineering performance of earthen structures, including slopes and landfill covers (Fatahi et al., 2010; Briggs et al., 2016; Ng et al., 2018c). Although this modelling approach appears to be appealing, it has a number of major limitations. For example, it ignores the effects of (1) the root architecture on the magnitude and distribution of plant-induced suction and (2) root-induced changes in soil hydraulic properties, including soil water retention ability and water permeability. All these parameters have significant effects on the soil hydrology (see Chapter 2). This chapter presents advanced theoretical analyses that capture these previously ignored hydrological effects of plants on changes in soil suction and consequently the stability of unsaturated vegetated soil slopes. Closed-formed, analytical steady- and transient-state solutions are derived to estimate the magnitude and distribution of plant-induced soil suction and hence the stability of an infinite vegetated slope with different root architectures.
