Study On The Mechanical Properties And Slope Protection Of Root-soil Complex At Highway Slope

Most of China’s land is located in mountainous or hilly areas,making a large number of artificial slopes produced in highway construction.At present,the highway construction is in the period of rapid development,and the artificial slope is constantly unstable.In order to find a low-carbon ecological,efficient and appropriate slope protection method under the background of carbon neutrality,and to improve the methods and theories of maintaining slope stability,these problems have become particularly important in highway engineering construction.In view of the above problems,a test research area was established in the expressway,three common typical shrubs(AFL,LBT and SAO)were selected,and two slopes(30 and 60)were selected.Based on the ecological characteristics of shrubs,the tensile resistance,shear resistance,anchorage,permeability,water stability and erosion resistance,and the root fixation of shrubs with different slopes was studied.(1)The physical indexes of shallow slope soil,shrub root structure and root configuration were studied.The characteristics of soil and shrub roots were revealed in the study area through soil physical index test,root topology analysis and root structure type analysis.The test results show that the shallow soil of the slope is silty clay with good grading and uneven density and permeability coefficient.In terms of root topology,AFL all have high branching rate and large branch angles,and AFL has abundant lateral roots,which can control the wide shear band.The root topology of AFL is more beneficial to plant roots for soil fixation and slope protection.In terms of the root structure type,AFL belongs to the R-type root structure,which shows good properties for the reinforcement properties,anchorage characteristics,shear resistance characteristics and infiltration characteristics of the root-bearing soil.(2)The tensile properties of two root shrubs and five root diameter material were studied.Through the tensile force test of single root,cell wall chemical composition determination test,root tissue anatomy test,the chemical composition of single root cell wall,root tissue anatomy and the tensile resistance of shrub single root material were revealed.The results show that the root diameter,root cell wall content and root anatomy influence the mechanical properties of root.The cellulose factor is the decisive influencing factor of single root.For conspecific shrubs,single root tensile and single root tensile strength decrease with increasing slope.The stress-strain curves of the three shrubs all met the cubic function or the power function relationship,and the stress-strain curves of the root system of the shrubs all showed the elastic-plastic material characteristics.When the slope is the same,the maximum value of the stress-strain gradually decreases as the root diameter increases.When the root diameter is the same or similar,the maximum stress-strain gradually decreases as the slope increases.In terms of single-root ractigraphy properties,AFL has a stronger root soil fixation effect than LBT and SAO.(3)The mechanical reinforcement effect of shrub roots was studied.The direct shear test of root traits,the determination test of root traits,the triaxial shear test and the analytical model were revealed.The test results show that the cohesion and internal friction Angle of rootless soil are significantly higher than that of rootless soil.Root systems in soil can significantly improve the cohesion of soil,but the influence of internal friction angle is minimal.For the same shrub,the cohesion and internal friction angle increase with the soil depth.The cohesion and internal friction angles of different shrubs differ spatially and are related to the distribution of roots within the soil depth.For the shear mechanical properties of root-bearing soil,AFL has a stronger root-bearing soil fixation effect than LBT and SAO.Root length density and root mass density are the factors of cohesion and internal friction angle.When the root length density and root mass density are the same,the root and soil composite shear strength index(cohesion and internal friction Angle)is the highest.When lower than the optimal water content,the soil cohesion shows a positive correlation with the water content,and reaches a maximum near the optimal water content.When the optimal water content is exceeded,soil cohesion and water content are negatively correlation,this rule is independent of whether the soil contains roots.The higher the soil compactness,the more favorable the plant root increases the cohesion.(4)The mechanical anchoring of shrub roots was studied.Through the development of the analysis model,the influence of the different root structures and different plant root structures was revealed on the interface between soil and roots.The test results show that the AFL has a strengthened root soil fixation effect of LBT than the anchorage mechanics and SAO.Root diameter,embedding length,and load speed are the factors affecting single root anchorage performance and failure mode.Branch root diameter and branch angle are the influencing performance and failure mode of branch root.The single root pull out failure mode shows pull out failure and pull out failure,while the branch root pull out failure mode shows pull out failure,branch root fracture failure and branch node fracture failure.When a single root meets the characteristics of large root diameter,long embedding length and large loading rate,the maximum pulling force is large,and the root anchorage effect is strong.It is not easy to meet the characteristics of large root diameter,low embedding length and low loading rate.When the branch root meets the characteristics of the branch root diameter and the branch Angle,the maximum pulling force is greater,and the anchorage effect of the root system is strong.(5)The permeability,water stability of aggregates and root water absorption of root soil in two slopes and three shrubs under rainfall were studied.Through soil permeability test,aggregate water stability index test,balloon cellulose content test,soil moisture content test,establish the analysis model,reveal the influence of soil balloon cellulose on root soil soil aggregate hydrolysis resistance,reveal the influence of root content,root diameter,cloth root state on root soil permeability,and the influence of root water absorption on the shear strength of root soil.The test results show that adding plant roots can increase the soil permeability,and then inhibit the ground runoff,protect the slope,weaken the erosion effect of the ground runoff on the slope after rainfall,enhance the soil erosion resistance,and achieve the effect of stable slope.Root content,root diameter,and cloth root status are the influencing factors of root-bearing soil penetration.T-GRSP and EE-GRSP are the factors affecting the water stability of soil aggregates.For soil aggregate water stability,AFL has a stronger root fixation than LBT and SAO.Under the same conditions,the shrub slope protection can reduce the soil moisture content of the slope more effectively than the natural exposed slope.Bush root system has water absorption effect,which can reduce the water content of root-containing soil to about12%-16%.(6)The performance of 2 slopes and 2 rain strength was studied.Through the test of simulated scour,root biomass and root morphology characteristics,the influence of shrub root morphology on root-bearing soil was revealed.The test results show that the root biomass,total root length,total root volume and total root surface area are the influencing factors of the scour performance of root soil.Through the principal component analysis,it is found that the root morphological factor is the decisive influencing factor of soil scour performance.For the same shrub,the soil antiscour index decreases with increasing slope.Rainfall intensity is the influencing factor of soil impulse resistance,and the impulse resistance index under heavy rain erosion is significantly lower than that under light rain erosion.Under two kinds of slope and two kinds of rainfall conditions,the root-containing soil anti-erosion index of the three individual shrubs was classified as: SAO> AFL> LBT.When root biomass and root morphological characteristics were the same,AFL had the highest antiscour index and the best effect.The planting density of AFL can be appropriately increased to achieve a better slope protection effect.In conclusion,the shrub root increases the shear resistance of the root soil and prevents the deformation of the slope soil;improves the anchorage strength of the root soil by increasing the tensile strength of the soil;improves the water permeability of the soil and improves the permeability of the root soil.This study revealed the effect of root fixation from the tensile resistance,shear resistance,anchorage,anchorage,permeability,water stability and erosion resistance,studied the factors affecting soil fixation effect,improved the analysis method of plant root fixation,and screened the shrubs suitable for slope protection.