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Due to the rapid advancement of urbanization, traffic–related pollutants in street canyons have emerged as the primary source of PM2.5, adversely impacting residents' health. Therefore, it is necessary to reduce PM2.5 concentrations. In this study, a three–dimensional steady–state simulation was conducted using Computational Fluid Dynamics (CFD). Three representative wind directions (θ = 0°, 45°, and 90°, corresponding to parallel, oblique, and perpendicular winds) and five different building height ratios (BHR = 0.25, 0.5, 1, 2, and 4) were used to explore the effect of building height variations on PM2.5 dispersion within street canyons. The results indicated that wind direction significantly influenced PM2.5 dispersion (p < 0.001). As θ increased (θ = 0°, 45°, and 90°), PM2.5 concentration in the canyon increased, reaching the most severe pollution under perpendicular wind. Building height variations had a minor impact compared to wind direction, but differences in PM2.5 concentration were still observed among various BHRs. Specifically, under parallel wind, the influence of BHR on PM2.5 dispersion was relatively small as compared to oblique and perpendicular winds. For oblique wind, PM2.5 concentrations varied based on BHR. Street canyons composed of low–rise or multi–story buildings (BHR = 0.25 or 4) slightly increased PM2.5 concentrations within the canyon, while the lowest PM2.5 concentration was observed at a BHR of 0.5. Under perpendicular wind, symmetrical (BHR = 1) and step–down canyons (BHR = 2 and 4) exhibited comparable peak concentrations of PM2.5, whereas step–up canyons (BHR = 0.25 and 0.5) showed relatively lower concentrations. [ABSTRACT FROM AUTHOR]

Plant roots have considerable impact on the shear properties of soil, but to date the underlying mechanisms have been poorly quantified. In order to understand the fundamental mechanisms of soil reinforcement by Hedysarum scoparium roots and reduce the cost of the testing and relieve the destruction of environment due to digging roots, five-year plant specimens were collected from the Gaoshawo forest field (Northwest China) by in-situ excavation in this study. The shear properties of root-soil composite of Hedysarum scoparium were studied by the finite-element numerical simulation software. The influence of the root area ratio (RAR) and the vertical load on the shear strength of root-soil composite of Hedysarum scoparium was discussed in this study. The laboratory direct shear test was used to prove the reliability of the numerical simulation under the condition of the 7% soil moisture content and the RAR of 0.0034. The results showed that the shear strength of roots-soil composite conformed to the Mohr-Coulomb's yield criterion and the roots of Hedysarum scoparium could notably enhance the soil shear strength. It was also found that a strong correlation between the RAR and the root apparent cohesion. The root apparent cohesion increased with the increasing of the RAR according to a linear function (R2>0.9). Under the same RAR, the capacity of soil reinforcement by the roots was weakened with the increase of the vertical load, and a logarithmic function (R2>0.9) could be used to describe the relationship between the shear strength growth rate of root-soil composite to pure soil and the vertical load. Under the same vertical load, the growth rate of shear strength of root-soil composite to pure soil decreased linearly with the decreasing of the RAR (R2>0.9). The roots of Hedysarum scoparium played an obvious role to reinforce soil under the low vertical loads. The results of the study indicated that the peak value of the shear stress of root-soil composite of Hedysarum scoparium appeared later compared with that of pure soil. It implied that the root reinforcement did not occur until the significant plastic deformation appeared. Therefore, the roots seemed to have little influence on soil reinforcement for small strains acting on soil-root composite. The numerical simulation results were consistent with the results of laboratory test (the maximum relative error was only 4.26%). It was found that an increase in the vertical load of root-soil composite of Hedysarum scoparium made the contribution of roots to the shear strength increment of root-soil composite decrease. The differences of the cohesion stress and friction angle of root-soil composite of Hedysarum scoparium were only 0.6179 kPa and 0.0039° respectively based on the fitting equation between the vertical load and the shear strength. The fitting equation was developed from the numerical simulation and the laboratory direct shear test results. This paper presented a numerical simulation model capable of simulating the direct shear of root-soil composite of Hedysarum scoparium. The numerical simulation results could serve as the basis and reference for further studies on shear characteristics of root-soil composite. [ABSTRACT FROM AUTHOR]

Hedysarum scoparium and Salix psammophila have obvious effect on the fixation of mobile and semi-mobile dunes in the Mu Us Desert. In order to explore the friction characteristics between root and soil, 5-year-old Hedysarum scoparium and Salix psammophila roots were measured in the laboratory. The influences of different conditions such as species, soil moisture and vertical load were examined by using direct shear friction tests in the study. The finite element software was used to simulate the process of the laboratory experiments. The results showed that the cohesion stress of the root-soil interface of Hedysarum scoparium and Salix psammophila had a significant difference (P < 0.05). However, there was no significant difference between Hedysarum scoparium root-soil interface and soil-soil interface (P > 0.05). The cohesion stress of the root-soil interface of Salix psammophila and soil-soil interface had a significant difference (P < 0.05). The cohesion stress of the root-soil interface of Hedysarum scoparium ((1.51±0.65) kPa) was higher than that of Salix psammophila ((-0.92±0.50) kPa), and the cohesion stress of soil-soil interface ((3.22±0.55)kPa) was also higher than that of Salix psammophila. The friction angle of the root-soil interface of Hedysarum scoparium had a significant difference with Salix psammophila (P < 0.05) and it also had a significant difference with soil-soil interface (P < 0.05). However, there was no significant difference between Salix psammophila root-soil interface and soil-soil interface (P > 0.05). The friction angle of the root-soil interface of Hedysarum scoparium ((31.00±0.14)°) was higher than that of soil-soil interface ((30.30±0.25)°) and Salix psammophila((30.20±0.17)°). There was a significant difference between the cohesion stress and the friction angle of the root soil interface (P < 0.05) under 2% (dry season) and 22% (rainy season) soil moisture. However, there was no significant difference under 7%-17% soil moisture (P > 0.05). The cohesion stress of 2% soil moisture ((0.0021±0.34) kPa) was lower than that of 22% soil moisture ((3.16±0.57) kPa). The friction angle of 2% soil moisture ((29.80±0.38)°) was lower than that of 22% soil moisture ((30.92±0.59)°). The relationship between vertical load and shear strength of the root-soil interface obeyed the Mohr-Coulomb theory and the constitutive relation was hyperbola. The maximum relative error of the shear strength simulated by the finite element software was 9.54%. The results of the study indicated that the improvement of shear strength of the root-soil composite was not related to the cohesion stress but related to the friction angle. The shear strength of the root-soil interface of Hedysarum scoparium was stronger than that of soil-soil interface and Salix psammophila (P < 0.01). The change of soil moisture had a similar influence on the cohesion stress and the friction angle of the root-soil interface. The capacity of improving shear strength of soil by root reinforcement was significantly affected by the dry and rainy season (P < 0.05). The process of the direct shear friction tests of root-soil and soil-soil interface could be simulated by the finite element model established in the study. The simulation results were consistent with the laboratory tests. The results of this research can serve as a basis for the further studies on the friction characteristics of root-soil interface and root reinforcement. This study also can provide a reference for the selection of windbreak and sand-fixation tree species. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: The role of vegetation in preventing shallow soil mass movement is now fairly well understood, particularly at the individual plant level. However, how soil is reinforced on a larger scale and the influence of changes in vegetation over time has rarely been investigated. Therefore we carried out a study on the temporal and spatial changes within stands of Cryptomeria japonica D. Don, growing in the Sichuan province of China, an area where shallow landslides are frequent. Soil cores were taken from three neighbouring stands of C. japonica aged 9, 20 and 30 years old and growing on steep slopes. Cores were taken from around trees and the root (<10mm in diameter) biomass density (root density (RD)) present in each core was measured at different depths. The spatial position of trees at each site was noted and soil shear strength was measured. The tensile strength of a sample of roots from each stand was measured. Using the RD data, the root area ratio (RAR) could be estimated. RAR and root tensile strength were used as input to a model of root reinforcement which determines the additional cohesion, c r, or contribution of vegetation to soil. Data were then incorporated into a two-dimensional model of slope stability developed in the finite element (FE) code, Plaxis, which calculates the safety factor (FOS), or likelihood of a slope to fail under certain circumstances. We calculated the FOS of slopes with and without C. japonica, taking into account the spatial position of trees at each stand. Results showed that RD was highest in the 9-year-old stand, but that root tensile strength was lowest. In the 30-year-old stand, RD was low but a higher root tensile strength compensated for the decrease in RAR. The FOS increased by only 15–27% when vegetation was present, with the greatest augmentation in the 9-year-old stand. The older stands had been thinned over the years, resulting in large gaps between trees, which would be prone to local soil slippage. This spatial effect was reflected in the FE analysis, which showed a significant relationship between the number of trees and distance between groups of trees in the 20- and 30-year-old stands only. Therefore, when managing fragile slopes, care should be taken when thinning, so that large gaps do not exist between trees, the influence of which is accrued over time. [Copyright &y& Elsevier]