Effects of root characteristics on the shear characteristics of root-soil complex in wood fiber reconstructed red soil

Red soil is one of the typical soil types in China, which is widely distributed in 17 provinces with a total area of 569,000 km², accounting for about 1/5 of the total land area in China. It is a widely distributed soil type in Yunnan Province. Compared with other natural soils, red soil has high viscosity, high water content, low shear strength, serious loss of organic matter and nutrients, and changes in soil structure and quality, which leads to low root characteristic parameters of root-soil complex and cannot meet the needs of engineering practice. However, the roots of herbaceous plants are mostly dense and fine roots, which have a good reinforcement effect on shallow soil, consolidate soil and prevent erosion, and their root characteristic parameters can better reflect the contribution of roots to soil shear strength. It is of certain significance to study the mechanical properties of root-soil complex of herbs through root characteristic parameters. In order to find out the influence mechanism of root characteristics on the shear characteristics of herb root-soil complex under different mass fractions (0, 0.5%, 2.5% and 5.0%) of wood fiber reconstruction red soil, the shear strength of root-soil complex of white clover and ryegrass was measured by direct shear test, and four root characteristic parameters, such as root surface area index, root volume ratio, root length and underground biomass, were measured by WinRHIZO root analysis system, and their resistance was further revealed. The results showed that proper wood fiber addition was beneficial to the growth of fibrous root herbs. When the wood fiber content is high, the root length of Lolium perenne and white clover are 456.27 and 438.35 cm respectively, and the underground biomass are 0.410 and 0.150 mg respectively. With the increase of root characteristic parameters, the shear strength of ryegrass root-soil composite is greater than that of white clover root-soil composite, and the shear strength of both is better than that of single wood fiber composite soil or root soil, with the increase ranges of 26.55% - 47.20% and 26.21% - 35.07% respectively, but there is a certain change threshold. The Mohr-Coulomb model is used to simulate the shear characteristic parameters of the root-soil complex. When the characteristic parameters of root system reach the change threshold, the change of cohesion is obviously and plays a leading role, but has little influence on internal friction angle. The shear strength of root-soil complex increases first and then decreases. When the root parameter value exceeds the threshold, the shear strength of wood fiber root-soil composite decreases, showing a trend of increasing first and then decreasing. This study has important theoretical and engineering practice value for further study of slope protection, vegetation restoration and land consolidation in red soil region.