A Study On The Failure Of The Biological Soil Crust Blown By Wind Sand Flux

The biological soil crust(BSC) has a very high coverage in arid and semi-arid desertification areas, which plays an important ecological function in wind-sand engineering. In the area of desertification, the BSC is impacted by wind sand flux. It is very important to understand and analyse of the failure of the BSC impacted by wind sand flux in the protection of BSC and in the wind-sand engineering. So, some properties of the BSC were measured and the failure of the BSC impacted by wind sand flux was analysed thoretically in this paper.Firstly, the particle size, density, moisture content and compress strength of the BSC were measured. And the failure of the BSC impacted by wind sand flux was observed in the wind tunnel. The results show that:the average particle size of the BSC is much smaller than that of the semi-fixed dunes, mobile dunes and the flat sand surface, and slightly less than that of the fixed dunes; some very fine sand is contained in the BSC, which is not contained in the natural mobile sand; the average density of the BSC(1.40×103kg/m3) is less than the average density of the natural sand(1.51×103kg/m3); the moisture content of the BSC is very low and less than0.5%, which has no difference with the moisture content of the natural sand; when the thickness of the BSC is less than5mm, the compressive strength of the BSC increases approximately linearly with the thickness of the BSC. The wind tunnel experiment shows that:in the early stages of the impact by wind sand flux, the shape and thickness of the BSC do not change; while with the continue impact, the particles of the surface of the BSC becomes more loosely and the thickness of the BSC becomes thinner; as a result, the surface of the BSC begins to appear the small holes; with more and more the holes in the surface of the BSC, the BSC eventually fractures.Secondly, according to the elastic collision theory, combined with the Hertz assumption, the collision time of the collision of sand-sand, the collision of grain-bed and the collision of sand-BSC are calculated. Because the surface of the BSC is uneven, and has many pores, according to the results of grain-bed collision, the collision time of sand-BSC is corrected by introducing a correction factor α=5.0. Using the impulse theorem, the impact force is calculated. The impact forces of group sands are equivalent to the uniform static load and the dynamic load. Based on the volume fraction and the saltation cycle of the sand, the frequency of sand impacting the BSC is calculated. The frequency is40～122Hz, and the circular frequency is 251-767Hz.Lastly, taken the BSC as the simply supported plate model, the natural frequency of the BSC is analysed. When the thickness of the BSC is very small, the fundamental natural frequency of the BSC without considering the foundation is smaller than that of the BSC with considering the foundation. Without considering the foundation, the fundamental natural frequency of the BSC increases linearly with the thickness of the BSC, and inverses ratio to the area. With considering the foundation, the fundamental natural frequency of the BSC first decreases and then increases with the thickness of the BSC, and last is closed to the fundamental natural frequency of the BSC without considering the foundation; the fundamental natural frequency of the BSC does not change with geometry size of the BSC. According to the elastic modulus and the tensile strength of the BSC measured by Wang Zhenting, select the first strength theory as the failure criterion. According to the fall speed of sand in wind tunnel measured by Xie Li, the failure of the BSC is analysed. When the BSC is compressed by the static load and the wind speed is16m/s, if without considering the foundation, it is failure when the thickness is less than3mm; if considering the foundation, it is not failure. When the BSC is compressed by the dynamic load and the wind speed is16m/s, if without considering the foundation, the frequency of wind sand flux is larger than the fundamental natural frequency of the BSC of less than10mm, so the BSC is failure; if considering the foundation, it is not failure.