Vegetation plays an important role in improving slope stability. It is crucial to develop simple and effective methods for assessing the stability of vegetated slopes. Based on upper bound limit analysis, a method was proposed to analyse the stability of a two-dimensional vegetated slope with uniform root architectures under steady transpiration state. The effects of water absorption and reinforcement by vegetation roots on slope stability were considered using this method. Parametric studies were performed to investigate the effects of the soil type, root depth, plant transpiration rate, root tensile strength, slope angle and internal friction angle on slope stability. Several generic stability plots were provided. The results showed that roots significantly improved soil cohesion but slightly affected the internal friction angle. Root systems could provide additional soil cohesion. Horizontally and vertically distributed roots imposed the best mechanical reinforcement effect on the soil. The shear strength increases by 1.78 times. Compared with that of plain soils, the critical state line (CSL) of the root-soil composite moved upwards. The soil type strongly influences the pore water pressure. With increasing plant transpiration rate, root tensile strength and root depth, vegetated slope stability can increase by 58 %. The slope stability decreases by 50 % with increasing slope angle. The stability number (Ns) decreases with increasing internal friction angle. The effects of water absorption and reinforcement by roots on slope stability decrease with increasing desaturation coefficient and saturated permeability coefficient. Compared with that of loess and sand slopes, the reinforcement effect of vegetation roots is more significant for the stability of clay slopes.

Vegetation is natural and environment-friendly material for slope reinforcement. To simple and effective analysis of vegetated slopes, a new method is proposed to consider the tensile strength cutting criterion (C-F criterion) of unsaturated root-soil composites. The proposed method incorporates the hydrological and mechanical effects of vegetation roots. A 1D stability model is developed to calculate the safety factors of vegetated slopes under steady transpiration state. Parametric studies are performed to investigate the effects of shrub root depth, slope angle, rainfall intensity, transpiration rate, and tensile strength on pore-water pressure (uw) and slope safety factors (Fs). uw and Fs are calculated using both the C-F criterion and the Fredlund strength criterion. The results demonstrate that Fs and uw decreases with increasing slope angle and rainfall intensity. Slope angle and rainfall intensity of vegetated slopes has a negative impacts on the slope stability. Moreover, Fs increases with increasing tensile strength. Furthermore, the transpiration rate and root depth increases, Fs and uw increases. Root depth, tensile strength, and transpiration rate are adverse for slope stability. Increasing slope angle and rainfall have detrimental effects on slope stability. Shallow slopes are more sensitive to rainfall than deep slopes. Fs for vegetated slopes with tensile strength cut-off are reduced compared to those based on the Fredlund strength equation. The C-F criterion is best suited for evaluating the shallow slope stability. Overall, the proposed method is a simple and practical approach to assess the vegetated slopes stability.