Abstract:
The Xining Basin, located in the transition region between the northeastern Qinghai-Tibet Plateau and the west of the Loess Plateau, has been chosen as the testing area. C. korshinskii (Caragana korshinskii Kom.) and Z. xanthoxylon (Zygophyllum xanthoxylon (Bunge) Maxim.), which are planted in a self-established plot and have a growth period of 150 d, have been taken as the tested samples. Tensile test of single root and shear test have been conducted on the roots of C. korshinskii and Z. xanthoxylon and the root-soil composite systems (root of C. korshinskii and soil, root of Z. xanthoxylon and soil) respectively. By tensile test of single root, tensile strength of single root for the 2 kinds of shrubs has been obtained. Meanwhile, shear test to root-soil composite systems with the inclination between roots and shear plane of 45, 60, 75 and 90° has been carried out, and the shear strength of the 2 kinds of root-soil composite systems under the 4 different inclinations has been obtained. On this basis, the relationship between the tensile strength of single root of 2 kinds of shrubs and the cohesion force of 2 kinds of root-soil composite systems has been discussed, and the contribution of shrub roots to soil shear strength has been quantitatively analyzed. The results of tensile test of single root and shear test of root-soil composite system show that the tensile strength of single root has a relatively close relationship with the root diameter, and the root tensile strength of single root for C. korshinskii (44.84 MPa) is slightly greater than that for Z. xanthoxylon (37.52 MPa). The cohesion forces of root-soil composite systems for C. korshinskii and Z. xanthoxylon with the inclination of 45, 60, 75 and 90° are 9.90, 11.68, 12.97, 13.92 and 8.73, 11.27, 12.61, 13.96 kPa respectively, and all of them are greater than that of soil without roots which is 6.41 kPa. The cohesion force of 2 kinds of root-soil composite systems shows an increasing trend with the inclination increasing from 45 to 90°, and the incremental rate of cohesion force exhibits the same increasing trend. Moreover, the cohesion force of root-soil composite systems is positively correlated with the tensile strength of single root, which indicates that the cohesion force of root-soil composite system increases with the root tensile strength increasing. Variation of cohesion force of root-soil composite system reflects that roots play a key role in increasing shear strength of slope soil in the testing area, and another conclusion has also been obtained that increasing the tensile strength of single root significantly increases the shear strength of root-soil composite system. And the contribution of roots in increasing shear strength of slope soil varies with the variation in inclination; the incremental rate of cohesion force of root-soil composite system for C. korshinskii increases from 54.45% to 117.16%, and for Z. xanthoxylon increases from 36.19% to 117.78% with the inclination increasing from 45 to 90°. Therefore, as the roots are perpendicular to the shear plane, they play the most significant role in increasing the shear strength of roots. The results can be used as an instruction in quantitatively assessing the contribution of roots in increasing the shear strength of slope soil, and are helpful in the further investigation of the mechanism of the roots in increasing the shear strength of slope soil.