A Regional Landslide Stability Analysis Method under the Combined Impact of Rainfall and Vegetation Roots in South China

The aim of this study was to develop a regional landslide stability analysis method considering the combined impact of rainfall and the roots of vegetation in densely vegetated areas. A typical mountainous watershed in the Nanling National Nature Reserve of South China was chosen as the study area. First, the unmanned aerial vehicle (UAV) method was used to obtain surface element information including topography, vegetation, and landslides. Five main plant species were identified. The RipRoot model was then used to calculate the additional cohesion of these five plant species, and the relationship between the root systems of the different plant species and the soil shear strength was subsequently revealed. Finally, the root cohesion was introduced into the stability index mapping model (SINMAP), and the receiver operating characteristic curve (ROC) method was used to calculate the accuracy of slope stability when considering only soil cohesion as well as the composite cohesion of both soil and roots. The results showed significant differences in the root cohesion of different plants in the study area and a significant increase in the calculation accuracy (from 90% to 95.6%) when root cohesion was considered in the landslide stability calculation. These study results not only enrich theoretical studies on the impact of vegetation roots on landslide stability but also provide a scientific support for preventing disasters in mountainous landslide-prone areas.

Scaling the Roots Mechanical Reinforcement in Plantation of Cunninghamia R. Br in Southwest China

The degree of mechanical reinforcement provided by plants depends upon its roots distribution in the soil and mechanical properties of the roots. The mechanical properties and distribution of root traits (root diameter and number) in the soil of the standing forest depends on the tree stem diameter. This variation of root traits with tree stem diameter is rarely investigated. Therefore, this research presents the effect of tree stem diameter on the distribution of roots within the standing forest of Cunninghamia in the Longchi forest area, Sichuan province, China. In this area, shallow landslides take place frequently. We investigated the root traits distribution for trees with different stem diameters, i.e., 220 mm, 320 mm, 450 mm, and 468 mm, to show the variation of roots distribution in the soil with stem diameter. The root architecture of the selected trees was studied by step excavation method of the root zone accompanied by measurement of roots physical parameters (roots number and roots diameter) and indices (roots area ratio (RAR), roots biomass (RB), and roots distribution (RD)). We measured the root’s maximum tensile strength by performing root tensile tests in the laboratory. The field and laboratory-measured data were used to estimate the root cohesion by both the commonly used model Wu and Waldron Model (WWM) and Fiber Bundle Model (FBM). The results indicate that the tree stem diameter correlates with both the root distribution and the tensile strength. The roots indices and root cohesion increase with an increase in the diameter of the tree. Further, RAR decreases with depth and lateral distance from the tree stem, while the maximum values were observed in 10 cm depth. The relationship between roots diameter and roots tensile strength is established through power function. The average root cohesion estimated for a tree with stem diameter 220 mm is 23 kPa, 29 kPa for 320 mm, 54 kPa for 450 mm, and 63 kPa for 460 mm. This effect of stem diameter on the increase of soil shear resistance should be considered while evaluating the stability of slopes in standing forests. The comparison between WWM and FBM for investigated species suggests that WWM estimates the cohesion values greater than FBM by 65%.

The Effect of Root Native Tree Species on Soil Shear Strength on Hillslopes of Sierra Madre Oriental, Mexico

Background: The presence of vegetation reduces soil erosion and shallow slope failure both by reinforcing soil shear resistance and influencing the geo-mechanic conditions of soil. For this reason, vegetation strategies in areas vulnerable to erosion are considered to be an effective control measure for soil erosion. Method: The tree species used in this research are widespread in the slopes of Chipinque mountain of Sierra Madre Oriental and belong to four native species: Cercis canadensis, Celtis laevigata, Quercus rysophylla and Ligustrum lucidum. In order to investigate the mechanical characteristics of roots, single roots specimens were sampled and tested for tensile strength. The tests were conducted with the Universal Testing Machine Shimadzu type SLFL-100KN to evaluate the influence of root shear strength on the soil using the Wu Model (Wu et al., 1979) as well as to analyze root cohesion and Root Area Ratio (RAR). The latter was calculated by taking both direct (field) and indirect measurements on image processing.Results: The results show that C. laevigata roots have the strongest tensile strenght, followed by Q. rysophylla > C. Canadensis > L. lucidum. RAR ranges from C. laevigata (0.0587%) > C. Canadensis (0.0585%) >L. lucidum (0.0504%) > Q. rysophylla (0.0441%). L. lucidum provides the less increase soil shear strength through root cohesion (16.12 kN/m2) > C. canadensis (53.70 kN/m2) > Q. rysophylla (89.07 kN/m2) to C. laevigata (97.41 kN/m2)

SLOPE STABILITY ANALYSIS CONSIDERING WEIGHT OF TREES AND ROOT REINFORCEMENT

We study the effect of roots of alder trees on soil reinforcement and slope stabilization. Two types of soil, i.e. Marl and Clayey soils and alders of three ages are considered. The slope stability is studied according to the tree indices based on tree age and soil type. The effect of root reinforcement on slope stability is considered by an additional cohesion. The stability analyses are carried out by the FEM. We perform parameter studies considering tree age, soil type and surcharge. The results indicate that soil type is effective on cohesion. The results also showed that with increasing age of trees from 7 to 15 years, the amount of additional root cohesion increased and with the increase of the age of trees to 20 years this amount slightly decreased. Also, with regard to a constant slope geometry, the type of soil and the uniform surcharge pressure, 7-year-old trees have shown better performance in slope stabilization. It has been observed that as the age of alder trees grows, although the amount of additional root cohesion increases, however, due to increased surcharge pressure, the overall slope stability factor decreases.