Experimental Study on the Effect of Root Content on the Shear Strength of Root–Soil Composite with Thick and Fine Roots of Cryptomeria japonica (Thunb. ex L.f.) D.Don

Abstract

The current research on slope protection with plants mainly focuses on herbs and shrubs. In order to investigate the difference in shear strength of root–soil composite with thick and fine roots under different root content conditions, Cryptomeria japonica (Thunb. ex L.f.) D.Don was selected as the research object, and the distribution characteristics of its roots with the increase of buried depth were studied using the longitudinal profile method. Based on the distribution range of root area ratio (RAR) in field investigation, the modified large-scale direct shear tests were executed on the root–soil composite samples with thick and fine roots of five RAR grades under four normal stress levels, and the variation rule of the shear strength of the root–soil composites with thick and fine roots under different root contents was analyzed. The influence mechanism of RAR was briefly discussed. The results show that the RAR of Cryptomeria japonica (Thunb. ex L.f.) D.Don increases first and then decreases with the increase of buried depth, and decreases with the increase of horizontal distance from the excavation point to the trunk. Both the thick and fine roots can increase the soil shear strength, but the effect of thick roots is greater than that of fine roots. The shear strengths of root–soil composites with thick and fine roots both increase first and then decrease with the increase of RAR, which means that there exists the optimal root content for the roots with the best reinforcement effect on soil, and the optimal RAR is 0.1% and 0.2%, respectively. The cohesive and internal friction angle of the root–soil composite can be improved by the roots of Cryptomeria japonica (Thunb. ex L.f.) D.Don, and the thick root is better than the fine root. Meanwhile, the enhancement effect on the cohesion is greater than that on the internal friction angle. The results are of great significance for understanding the effect of roots on soil shear strength and enriching the existing theory of slope protection with arbors.