Experiments on shear properties of vetiver root-red clay root-soil composite under different failure modes

To investigate the influence of vetiver roots on the shear properties of red clay, one-year-old vetiver root-red clay composites were selected as the research object. After obtaining the basic parameters of red clay and vetiver roots, the shear strength and shear characteristic parameters of the root-soil composite under different failure modes were obtained through direct shear tests, single root pullout tests, and interfacial shear resistance tests. Furthermore, the variation laws of shear strength under the influences of root segment inclination angle, soil dry density, and water content were analyzed. The results show that the direct shear strength of the root-soil composite under different normal loads is greater than that of plain soil. When the angle between the root growth direction and the shear direction is smaller, the increase in soil shear strength is larger. When the root axis is parallel to the shear direction, the root reinforcement effect is better than that under orthogonal conditions. The addition of roots shows a significant effect on the improvement of soil cohesion. The pullout force increases with increasing pullout displacement and reaches a peak at a displacement of 3-8 mm, after which two stages of rapid decrease and slow decrease are experienced until the root segment is completely pulled out. Both the pullout force and the interfacial shear strength exhibit positive correlations with soil dry density, but peak values appear when the soil water content is approximately 20%, that is, near the optimum water content. With increasing soil dry density, the bonding between the root surface and soil particles is enhanced, while the root-soil contact area and friction effect increase simultaneously. The interfacial shear strength, interfacial cohesion, and interfacial friction coefficient ratio of the root-soil interface all reach maximum values at a water content of approximately 20%. Under a given level of soil compaction, the root-soil friction effect can be improved and root pullout failure can be reduced by regulating the humidity of the root growth environment. The research results provide experimental evidence for the accurate evaluation of the shear strength of root-soil composites under different failure modes and have reference value for the reasonable selection of shear parameters in root-reinforced soil models.