Integrated impacts of vegetation and soil type on slope stability: A case study of Kheyrud Forest, Iran

In this study, an integrated 2D numerical model is applied to investigate the mechanical effects of the vegetation and soil type on slope stability. The developed model can assess the mechanical aspects of vegetation in slope stabilization. For this purpose, a case study of the Kheyrud forest located in northern Iran is considered as a real case site. Different scenarios including; three soil types (fine grain with low-, medium-, and high-strength) and two vegetation types (Maple and Common-ash) are assessed in the stability analysis (based on safety factor). The results confirm that the considered vegetation can prevent shallow landslide occasions, but has a limited impact on deep landslide events. The ratio of root zone depth to the depth of slide is the most important parameter in the contribution of vegetation in the increasing of the slope stability. The simulation results show an improvement in the safety factor up to 25% when the mechanical aspects of vegetation are considered. This improvement leads to stability for slopes with 10% more gradient. The results also demonstrate the reinforcement effect of the roots (stability improvement up to 25%) and negative effect of the surcharge load (Reduction of stability up to 5% at a surcharge of 1.2 kPa) on the slope stability. The extent of these impacts varies by soil properties, tree characteristics, and slope geometry so that the overall result can vary. The sensitivity analysis demonstrated an increase in the soil cohesion grows the negative effect of tree weight. Also, it is observed that the increase of soil unit weight follows a decrease in the stabilization role of vegetation. The role of the soil's friction angle is insignificant. The findings can serve as a foundation for enhancing the scientific understanding of the relationship between the stabilization role of vegetation and soil, vegetation, and slope characteristics. Generally, in the stabilization of a slope by artificial planting forests, the native shrubs or trees with more reinforcing effects and low surcharge are more effective.