Friction characteristics of root-soil interface during branches root pull-out of Indigofera amblyantha and Cassia bicapsulafis

Abstract: The front stage of root system can be the root branch morphology and different angle of release in the process of pull force of root stabilization and strengthening soil in soil mechanics properties. In this study, the pull-out experiment was carried out to clarify the effect of branch angle on the frictional mechanical properties of root pulling using SK-WDW-2 microcomputer. The experimental subjects were taken as the slope greening shrubs Indigofera amblyantha and Cassia bicapsulafis. The results show that there were three failure modes of roots with the branches: pullout failure, breakage failure on the taproot fracture, breakage failure on the branch root. The main failure modes were the pullout failure, accounting for 82.61% and 86.05% of the total samples, respectively. The maximum pullout force increased with the increase of the root diameter. There were the similar initial stages of the pull-out force and the relative displacement curve (F-S curve) under the three failure modes. The pull-out force was generated at the zero relative displacement of roots. There was a significant positive correlation between the branch angle and the initial sliding pulling force, indicating that the root with the branch first released a part of the force during the tension. However, the correlation between the bifurcation angle and initial force was more significant in the Indigofera amblyantha than in the Cassia bicapsulafis (Indigofera amblyantha P=0.042, Cassia bicapsulafis P=0.003). Specifically, the peak displacements were 19.7, and 6.56mm, respectively, when the root diameters were1-2, and ≥4-5 mm, with the angle of Indigofera amblyantha of 60°-90°. Once the angle of Cassia bicapsulafis was 60°-90 °degrees, and the root diameters were 1-2, and ≥4-5 mm, the peak displacements were 17.32, and 7.28 mm, respectively. Therefore, the peak displacement corresponding to the maximum pull-out force also increased with the increase of branch angle under the same root diameter range. Under the same range of branch angle, the peak displacement corresponding to the maximum pull-out force also increased with the increase of root diameter. The smaller the root diameter was, the larger the branch angle and the larger the peak displacement were. The branch angle of the average maximum pulling force for Indigofera amblyantha was ranked by ≥30°-60° , ≥60°-90°, 0°-30°, whereas, the branch angle of the average maximum pull-out force of Cassia bicapsulafis was ranked by ≥60°-90°, ≥30°-60°, 0°-30°. The root soil interface friction coefficients of both species increased with the increase of branch angle. The mean maximum static friction coefficient increased greatly from 0-30 to 30-60, while the mean maximum static friction coefficient increased slightly from ≥30-60 to ≥60-90. In the same range of angles, the mean maximum static friction coefficient of Indigofera amblyantha was smaller than that of Cassia bicapsulafis. The part of the force that released by the branch root in the initial stage of tension was calculated by the soil pressure, according to the soil pressure on the branch root. When the branch angle was less than 90°, the soil pressure under the root system increased with the increase of the branch angle under the same soil depth. Therefore, the quantitative analysis was realized to determine the influence of branching angles on frictional mechanical properties. The finding can provide a strong reference to further understanding the soil fixation and anchorage mechanism of roots.