Effects Of Spatial Variation Of Tree Root Characteristics And Terraces On Slope Stability

Vegetation is widely used for controlling shallow landslides. The mechanisms by which roots increase apparent soil cohesion was well documented and many values of root additional cohesion are available in the literature for different plant species. However, less information was given about the spatial variation of soil reinforcement by roots at a slope scale and its influence on slope stability, in particular in forest areas.The goal of this paper was to investigate the effect of spatial variability of root characteristics on slope stability, with a case study on two monospecific 19-y-old stands of Robinia pseudoacacia and 21-y-old stands of Platycladus orientalis grown on slopes in the semiarid Loess Plateau of China. Vertical profile walls were dug at different distance and directions around trees situated at three different slope locations, i.e. up-, mid-and down-slope. Grids with a 10cm×10cm mesh were placed on vertical walls. Roots were counted within each grid cell and split according to their diameter class. Root area ratio (RAR) was estimated and compared among different positions around the trees and at three different locations along the slope. Roots tensile strength and Young's modulus were determined with laboratory mechanical tests. RAR, root tensile strength and Young's modulus were used as inputs in six different root reinforcement models to calculate root additional cohesion. A 2D finite element model of slope stability was developed and used to calculate the increase in factor of safety (FoS) due to root additional cohesion.Main conclusions are present as follows:(1) Scale effects were found on biomechanical characteristics of Robinia pseudoacacia roots and Platycladus orientalis roots. Root tensile strength and Young's modulus decrease with root diameter increasing. Both root tensile strength and Young's modulus of Robinia pseudoacacia was about two times higher than tensile strength of Platycladus orientalis.(2) The mean RAR of Robinia pseudoacacia was significant higher than that of Platycladus orientalis. RAR distribution was significantly different with regards to tree location on the slope. General RAR of Robinia pseudoacacia was higher in downslope, while general RAR of Platycladus orientalis was higher in midslope. Furthermore, RAR was increasing first and then decreasing with depth, and decreasing with distance increasing from the stem at lateral level.(3) The six theoretical models used to estimate the root additional cohesion gave different vertical profiles according to the underlying hypothesis on how forces are transferred to the roots. Results found WM gave the maximum root additional cohesion, while WM-Hbis gave the minimum values. The results of FBMs were between two extreme. In addition, root additional cohesion distribution on slope was relative to RAR distribution.(4) Theoretical analyses of slope stability showed that terraced slopes were 20% more stable than rectilinear slopes, disregarding the differences in hydrological regimes between these two terrain morphologies. Numerical sensitivity analyses also showed that the FoS reached an asymptotic value when increasing root additional cohesion. Actual additional cohesions of the two studied sites corresponded to FoS that were already close to this asymptotic values. Consequently variations of these actual root cohesions would not much affect slope stability. However, it was showed that actual root reinforcement was significantly more effective down-slope than up-nor mid-slope.Original points of the thesis are:(1) The study on roots reinforcement distribution at slope scale was firstly carried out on loess plateau of China.(2) Compared all the methods of root additional cohesion calculation available.(3) In short-term, root architecture may not change a lot. In past, we more focused on looking for an optimized root additional cohesion method to increase slope stability analysis. However, numerical sensitivity analyses results indicated that a threshold appeared on roots reinforce slope stability. When root additional cohesion is higher than this threshold value, FoS could not change significantly. In conclusion, it is more interesting to look for a threshold value of root density instead of paying much attention on more accurate root additional cohesion calculation method.