Influences of three typical trees on slope deformation and stability under wind load

The influencing mechanism of vegetation on slope stability has drawn much attention in recent years. Most studies focus on the soil reinforcement by roots, and the effectiveness on the slope stability, or the dynamic impacting mechanism that is caused by root hydrological coupling effects (i.e. rainfall infiltration or evapotranspiration conditions). It is still lacking root-soil plate deformation and slope stability under wind load conditions. This study aims to clarify the slope stress field and stability influence under wind load. Three typical trees (according to the root structures and crown pattens, C. lanceolata, M. nanmu, and G. acuminata) were selected from the early regional survey as the research objects. And then nine numerical vegetated slope models were established with three slope gradients of 20°, 30°, and 45° under continuously increasing wind velocity from 0-30 m/s using Geo-studio software. Among them, the plant root structure was defined as the virtual 1-dimensional beam units that assigned values, according to the actual root-soil plate geometric features. A comparison was made on the stress, displacement, and the factor of safety in each slope model. Results showed that: 1) The stress concentration occurred at the root-soil plate under the action of wind load. There were the total pressure and tension stress on the upper and lower soil of the root-soil plate at the windward side zone, respectively, whereas, the opposite trend was found at the leeward side. The total stress deviation and the overall rotation trend were intensified with the increase of wind velocity and slope gradient. The ranking order of the average total stress was the Machilus nanmu> Cunninghamia lanceolata> Gordonia acuminata under the same wind velocity and slope. 2) The slope mainly suffered from the local deformation with the continued increase in wind velocity, where the maximum total displacement was concentrated at the root-soil plate zone. The total displacement of the root-soil plate was ranked in the descending order of the middle-slope > upslope > downslope position, which was positively related to the slope gradient with the wind velocity. The order of displacement was consistent with that of average stress under the same wind velocity and slope gradient condition. 3) The factor of safety increased slightly and then decreased rapidly on the steep slope gradient with the increase in wind speed. By contrast, the factor of safety decreased monotonically on the gentle slope gradient. This trend was attributed to the transformation of deep and shallow critical sliding surfaces with wind velocity. The stability of Gordonia acuminata slope was the least affected by the wind and followed by the Cunninghamia lanceolata and Machilus nanmu. It was also found that the factor of safety under the wind load was more sensitive to the crown width and root-soil plate diameter than the root-soil plate depth or crow height, especially at high wind velocity. It implies that the vegetated slopes with the large crown and narrow lateral root-spreading trees were vulnerable to the wind load. To sum up, less influence of wind load was found on the slope that was protected by trees (like G. acuminata) with a large root-soil plate diameter and smaller crown height. But the trees protected slopes (like M. nanmu and C. lanceolata) were characterized by the large crown height, and smaller root-soil plate diameter. The large root depth was dramatically affected by the wind. Thus, M. nanmu and C. lanceolata cannot be recommended as the high-slope protection projects in windy areas.