Influence of the spatial variability of the root cohesion on a slope-scale stability model: a case study of residual soil slope in Thailand

The bioengineering method using vegetation is an ecological approach for slope stabilisation. However, due to a large variability of vegetation root patterns, a precise quantification of root reinforcement is relatively difficult, leading to a reluctance to use such a technique in practice. This paper presents a probabilistic framework for slope stability analysis considering the spatial variability of root reinforcement. A residual soil slope under a heavy rainfall event was used to model the seepage and stability analysis. The effect of root reinforcement was considered through an additional soil shear strength or root cohesion. Typical characteristics of the root reinforcement of vetiver grass (Chrysopogon zizanioides) in Thailand were assumed in the analysis. A probabilistic analysis was performed considering both stationary and non-stationary random fields of root cohesion. The results indicated that the failure of the vegetated slope could occur when the variance coefficient of the root cohesion was more than a critical value (a critical cov = 0.45 for the uniformly distributed root cohesion case and a critical cov = 0.32 for the case of linear decrease of root cohesion in this particular slope). In practice, the efficiency of the bioengineering method can be improved by controlling the variation of root cohesion within such limits.