Finite element numerical simulation of Black Locust and Arborvitae roots on slope stability on Loess Plateau of China

Abstract: To investigate the effects of major forestry species roots on slope stability on Loess Plateau of China, we used monospecific stands of Robinia pseudoacacia and Platycladus orientalis as a case study. Tree roots provide positive mechanical influence (i.e. additional cohesion) on slope stability. We used two different methods to determine root additional cohesion in this research, i.e. Wu and Waldron's Model (WM) and revised WM (RWM). WM was developed based on limit equilibrium theory and assumed that all root in soil clods were mobilized in tension and fail simultaneously. Although WM approach was considered as a powerful and widely used method, it overestimated root additional cohesion due to all roots breakage simultaneously hypothesis. Therefore, based on many shear tests, a reduction factor for WM is introduced, which is RWM. The most critical parameters for WM and RWM were root area ratio (RAR) and root tensile strength. In this research, RAR was recorded on the soil trench profile, while root tensile strength was obtained by individual root tensile test. To evaluate tree roots effects on slope stability, a 2-D finite element model with terraced and contrast rectilinear surface shape of slope stability was developed and used to calculate the increase in factor of safety (FoS) due to root additional cohesion. Results showed that whether the land was prepared or not, afforestation can significantly increase slope stability. Moreover Robinia pseudoacacia roots were better that Platycladus orientalis roots on soil reinforcement. Terraced slopes were more stable than rectilinear slopes, regardless of the differences in hydrological regimes between these two terrain morphologies. It was also found that the percentage of FoS increase was larger when considering root additional cohesion simulated by RWM and virtual bare slope than root additional cohesion simulated by WM and RWM for both stands. Numerical sensitivity analyses for root additional cohesion illustrated that the relationship between FoS and additional cohesion was not linear, but exhibited as an asymptotic behavior. In detail, FoS value was stable when root additional cohesion reached the threshold value, which indicated that FoS was not sensitive to root additional cohesion calculation method. In addition, although root additional cohesion varied with the slope location, it was hard to find clear pattern to follow in our stands. However, roots in bottom part of slope always had stronger mechanical effects on slope stability. Therefore, more attention should be paid on the toe of slope and fully exerted its positive role for afforestation managers. This research can provide a basic theory of afforestation mode in spatial distribution and hence control shallow landslide.