Assessing the mechanical effects of vegetation on the stability of slopes with different geometries and soil types

This study evaluated the mechanical effects of vegetation, both root reinforcement and trees’ weight, on slope stability using the Morgenstern–Price method. Root reinforcement was equivalently expressed as root additional cohesion, which varied upon vegetation type and the depth of the root zone. The gravity of trees was simplified as a uniformly distributed load on the slope surface. Three types of variables, including vegetation type, slope geometry, and soil type, were considered. Parametric analysis shows that the response of the factor of safety (FoS) of slopes with different soil textures to the mechanical effects of vegetation significantly differs. The mature forest-covered sand slope with the smallest length and the lowest slope angle exhibits the largest increase in FoS of 34.3% relative to the plain soil slope. With the same configuration, the relative improvement in FoS of silt slopes is only 1/8 to 1/4 of that of sand slopes. The stability enhancement for clay slopes by vegetation is very limited (< 1.5%), and in certain situations, the stability even decreases. Moreover, we find that the Morgenstern–Price method can accurately estimate the additional destabilizing and stabilizing forces due to the mechanical effects, which allows its contribution to the improved FoS to be quantitatively divided into three components: additional cohesion from root system, trees’ surcharge, and variation in depth of slip surface. Generally, the findings of this paper deepen the understanding of the mechanical effects of vegetation on soil slope stability and provide a baseline for future eco-geotechnical engineering design.