Using global sensitivity analysis to quantify the uncertainty of root reinforcement in vegetated slope stability

Abstract Aims Variabilities of vegetation and soil cause uncertainty to the factor of safety (FoS) of unsaturated vegetated slopes, yet the significance of these variabilities on the uncertainty of FoS is unclear. This study aims to quantify the effect of the uncertainties of root reinforcement and soil hydromechanical properties to the uncertainty of the FoS. Methods The variance‐based global sensitivity analysis was adopted to evaluate how the variance of FoS of vegetated slopes can be apportioned by the variabilities of soil and root parameters. A copula theory was applied to model the correlation amongst the parameters. Results For slip depths shallower than 0.30 m, the major source of the variance of the FoS included the parameters that define root reinforcement, followed by the parameters of soil shear strength. The variation of transpiration‐induced soil suction had limited effect on the FoS variance under heavy rainfall. Taking into account the correlations amongst the parameters had minor influence on their contribution to the variance of the FoS. Conclusions We observed threshold slip depths, where the relative contribution of uncertainties in root and soil parameters on the FoS uncertainty underwent a transition. Root reinforcement for slips as deep as 0.60 m can provide reliable slope stabilisation effects.