Atmosphere-plant-soil interactions: theories and mechanisms

Plants are sophisticated and intelligent natural construction materials. They can be used for enhancing the stability of shallow soil slopes and minimizing surface erosion. It is evident that the use of plants can be low-cost, sustainable (almost maintenance free) and environmentally friendly. Not only can plant roots provide mechanical reinforcement, they can also induce soil suction via evapotranspiration (hydrological effects) to increase soil shear strength and to reduce water permeability for minimizing rainfall infiltration in the ground. Most previous researches have mainly focused on the mechanical effects of roots, while the mechanisms and contributions of induced soil suction to slope stability are often ignored. A multi-disciplinary research team led by the author has carried out an in-depth study on the mechanisms of atmosphere-plant-soil interactions based on the advanced theories of unsaturated soils and plant characteristics. New constitutive models are developed to estimate the water retention ability of vegetated soils and to simulate conjunctive surface and subsurface transient flow considering different root architectures. In addition, a new analytical model is derived to calculate soil suction induced by roots having one of four architectures (i.e., exponential, triangular, uniform and parabolic distributions with depth) and thereby to predict the factor of safety of vegetated soil slopes. Moreover, a novel artificial model root system is developed to simulate both mechanical and hydrological effects of roots in centrifuge. The influences of root architectures on induced suction, slope stability and deformation mechanisms are investigated. The experimental and theoretical results reveal that (1) vegetated soil is able to retain higher suction than bare soil under both drying and wetting conditions; (2) for Schefflera heptaphylla (Ivy tree), a commonly found plant species in many Asian countries, there is a linear relationship between root area index and leaf area index, which in turn has an approximately linear relationship with evapotranspiration-induced soil suction; (3) fungi can significantly increase root tensile strength and therefore enhance the mechanical reinforcement effects of roots; (4) among the four types of roots investigated, the exponential one induces the highest suction and hence is the most effective in stabilizing shallow soil slopes. Through extensive laboratory testing, field monitoring, centrifuge modelling and theoretical analysis, this study has established a theoretical framework, developed a novel testing technique in centrifuge and contributed towards the fundamental understanding of atmosphere-plant-soil interactions. The findings from this study also provide a scientific basis for the design of vegetated soil slopes.