The Study Of Mechanical Properties Of Herb Plants' Root Reinforcing Soil In Alluvial Fan Of Banggang

To study the soil-reinforcement mechanism of herbs' root system in the soil of collapsing gully alluvial fan,this article takes Pennisetum sp.and Paspalum wettsteinii as research objects.This article analyses soil properties,distribution characteristics of root system in the field,tensile properties of root system,and the effects of herbs' root systems on soil shear strength in alluvial fan through the pull resistance tests of roots,the in-situ shearing tests,and direct shear tests.The results show that:(1)With the increase of soil depth,the soil moisture content of herbaceous and bare lands shows a tendency of decline,while the soil bulk density increases.Root systems of two herb plants are mainly distributed in O—5 cm soil layer,in which the root parameters of Pennisetum sp.and Paspalum wettsteinii are 3.1 to 4.39 times and 2.23 to 2.57 times respectively as many as the average of their species.Root parameters of Pennisetum sp.and Paspalum wettsteinii show logarithmic and linear decrease respectively with the increased soil depth.The variation of root length density of all diameter classes is similar,the root length density decreases with the increase of soil depth and the diameter class.The diameter of two herb plants' root systems are both mainly less than 0.5 mm.(2)Gauge length and root diameter have significant correlations with root ultimate tensile resistance and tensile strength,but insignificant correlation can be observed between tensile rate and tensile strength.Root tensile resistance of Pennisetum sp.and Paspalum wettsteinii increases by power function with the increase of root diameter.However,root tensile strength decreases by power function with the increase of root diameter.Meanwhile,root tensile resistance and root tensile strength of Paspalum wettsteinii decrease with the increase of gauge length.Root tensile strength of Pennisetum sp.is the strongest when the gauge length is 5 cm.In the same tensile condition,the mechanical properties of single root of Pennisetum sp.are better than those of Paspalum wettsteinii,which shows the anti-tensile ability of Pennisetum sp.is stronger than that of Paspalum wettsteinii.(3)The results of in-situ shearing tests show that,root systems of both herb vegetation can improve the soil shear strength in collapsing gully alluvial fan,and Pennisetum sp.is more effective than Paspalum wettsteinii.Soil shear strength of Pennisetum sp.and Paspalum wettsteinii decreases logarithmically and exponentially with the increase of soil depth.In 0-20cm soil layer,shear strength of soil with two herb plants is significantly larger than that of bare land.Soil shear strength has an extremely significantly positive correlation with biomass density,root surface area density and fork density(P<0.01),as well as an significantly positive correlation with root length density(P<0.05).The dominant factor of Pennisetum sp.to characterize soil shear strength is biomass density,while that of Paspalum wettsteinii is root length density.The results of direct shear tests show that there is no obvious influence of root diameter on shear strength of roots-soil composite.The shear strength of roots-soil composite increases with the increase of root weight density.When root weight density exceeds a certain value,the effects of root systems on improving soil shear strength are significant.The soil cohesive force and internal frictional angle of roots-soil composite go up with the increase of root weight density.Root area ratio is an indicator to investigate the intensity of shear strength of roots-soil composite.The above results show that Root systems of Pennisetum sp.and Paspalum wettsteinii.can improve the soil shear strength in collapsing gully alluvial fan,and Pennisetum sp.is more effective than Paspalum wettsteinii.Therefore,both two herb vegetation are worthy of popularization in Benggang erosion areas.