Effects of root morphologies on shearing characteristics of the root-soil composite: An experimental case study of Ficus virens in Chongqing, China

Vegetation reinforcement is considered to be an environmentally friendly measure for slope improvement, which helps to prevent shallow landslides through roots' mechanical and hydrologic reinforcements. This study focuses on the mechanical reinforcement of the seedling roots of Ficus virens, a rich root system that is widely grown in the southern parts of China. Triaxial tests on the root-soil composite were carried out in silty clay and mixed soils to investigate the effects of root morphologies, including its layout and distribution angle of the main and lateral roots, on the stress-strain relationship, build-up of excess pore water pressure, and stress path. The test results indicate that two types of the soil reinforced by a curved main root and horizontal lateral root (CH) are proved to be the optimal scheme for yielding the best root reinforcement effect. Based on this scheme, the shear strength parameters are determined by fitting a straight-line tangent to Mohr circles under different cell pressures. It is found that soil shear strength is significantly improved by root reinforcement, with the root effect principally on soil cohesion, increasing up to 10 kPa, and negligible on internal friction angle. A modified equation is proposed for characterizing the critical state line of the rooted soil, and an additional cohesion term is proven to be valid for representing the root reinforcement. Different types of failure mechanism are observed in silty clay and mixed soils with/without roots. The findings provide novel insights into the shearing behavior of rooted soil and theoretical evidence for the improvement of slopes reinforced by Ficus virens.