Mechanical properties of slope protection by vetiver root using fractal theory

This study aims to quantitatively evaluate the performance of the slope protection by vetiver root. The vetiver was taken as the research object with different growth periods in Wuhan City, Hubei Province, China. The morphological parameters of the root were obtained using vegetation ecology and fractal theory. The fractal software was used to calculate the fractal dimension of the vetiver root. A systematic analysis was implemented on the evolution between the fractal dimension and the morphological parameters of the root. The laboratory triaxial test was carried out to explore the effect of root content and distribution on the cohesion and internal friction angle of root-soil composite in the fractal dimension. A stability model of the slope was established to consider the fractal dimension of the vetiver root. The results show that the fractal dimension of the vetiver root had a logarithmic relationship with the growth period. The fractal dimension also increased with the increasing growth period. Once the growth period was 12 months, the fractal dimension was 1.669 6, whereas, the fractal dimension tended to be stable in the growth period of more than 12 months. The number of roots increased with the increase of growth period, whereas, the growth rate of root number decreased significantly. The number of roots decreased gradually along the depth of the soil. Most roots were distributed in the range of 0-0.4 m from the ground, which was mainly used to reinforce the slope in the shallow layer. The fractal dimension of roots was linearly correlated with the number of roots. There was an approximate linear relationship between the root surface and the fractal dimension, with the increase of the growth period. The surface and water absorption of the root increased, but the growth rate was gradually decreasing. At the beginning of the growth period, the root content gradually increased with the increase of root fractal dimension, indicating a stronger branch of the root. The root content was about 0.62% when the fractal dimension was 1.669 6. The cohesion of the root-soil composite increased by 41.5% and 37.4% for the intersecting and mixed root distribution, respectively, compared with the rootless soil. But there was a very small difference in the internal friction angle among the vertical, inclined, intersecting and mixed root distributions. The maximum internal friction angle increased only by 5.2%. Once the root content increased from 0 to 1.0% in the root-soil composite, the cohesion and the internal friction angle of the root-soil composite increased by 72.0% and 6.9%, respectively. The root content shared the greater effect on cohesion. While there was a small variation in the internal friction angle. In the intersecting root distribution, when the root content was 1.0%, the stability coefficient of the slope increased by 28.4%, compared with the rootless soil. When the root content of the root-soil composite was 0.6%, the influence of root distribution on slope stability was ranked in the order of the vertical, inclined, intersecting and mixed type. The slope stability coefficients of intersecting and mixed roots increased by 14.7% and 15.8%, respectively, which was close to the effect on the stability of the slope. In the intersecting root distribution, the fractal dimension was a linear relationship with the stability coefficient of slope, and the determination coefficient was 0.958 6. The fractal dimension can be used to evaluate the slope stability. The findings can also provide a strong reference for slope protection by vetiver and vegetation selection.