Research On Root Distribution Characteristics Of Different Slope Protection Grass Species And Mechanical Properties Of Root-soil Comple

The construction of the engineering slope has damaged the original ecological structure,resulting in serious instability of the slope and threatening the safety of human life and property.The mechanical properties of root-soil complex of herbaceous plants play a decisive role in slope protection.It is of great scientific significance and practical value to study the mechanical properties of root-soil complex for slope protection to reveal the soil fixation mechanism of grass roots and prevent shallow landslides.Ryegrass,alfalfa and setaria were selected as research objects,root parameters of herbaceous plants(root length,root surface area,root volume,root diameter,number of root tips,number of root branches,and number of crosses)were measured.The distribution characteristics of root parameters were studied under different planting densities,soil depths and planting methods(netted planting,net-free planting).Hydrostatic disintegration test was used to study the disintegration properties of root-soil complex.The effects of planting density,soil depth,root parameters and netted planting on disintegration characteristics of root soil complex were analyzed.The shear behavior of root soil complex was studied by direct shear test in laboratory,the effects of planting density,vertical load,root parameters and planting methods on soil shear properties were investigated.The main research conclusions are as follows:(1)The root length,root surface area,root tip number,root branch number and cross number of alfalfa and ryegrass showed an increasing trend with the increase of planting density,and show a decreasing trend with the increase of soil depth.The root parameters in the range of 20-30cm soil depth were much smaller than those in the range of 0-10cm under netted planting condition,which indicated that laying fiber net in the topsoil layer had a certain effect on the root extension down,and some roots were enclosed in the shallow soil.The fiber net had great inhibition on the branching ability of alfalfa,the number of root branches was greatly reduced.(2)The presence of grass roots in soil can inhibit the disintegration process of soil.The disintegration quantity of root-soil complex compared with Ck,the disintegration quantity decreased by 22.25%～93.75%due to the root system.Compared with net-free condition,the disintegration quantity decreased by 42.60%～86.52%under netted planting,which indicated that laying fiber network in the topsoil layer can assist the root system to strengthen the anti-disintegration performance of soil.The disintegration amount,soil disintegration rate and soil cumulative disintegration index increased with the decrease of planting density and the increase of soil depth.(3)There was a significant(P=0.05)or extremely significant(P=0.01)negative correlation between disintegration quantity and root length,and a significant(P=0.05)or extremely significant(P=0.01)positive correlation between disintegration amount and root diameter.(4)The shear resistance increased linearly at the initial shearing stage.The growth rate of shear resistance increased with the increase of planting density at the initial shearing stage.Shear resistance and vertical load obey Mol Coulomb’s theorem(R~2=0.85～0.96).(5)The presence of grass roots could significantly improve the cohesion and internal friction Angle.The shear strength increased with the increase of planting density.Compared with plain soil(S),Under no-net and netting conditions,the internal friction Angle of ryegrass,alfalfa and setaria increased by 60.76%,23.50%,26.76%,49.07%,21.79%and 15.86%,respectively,and the cohesion increased by 71.58%,9.14%,53.76%,96.62%,91.59%,63.32%,respectively.Under netted planting conditions,the cohesion of root-soil complex was better than that of net free planting.(6)The shear strength ratio(F)initially increased and then decreased with the increase of planting density.The root length,root surface area and the internal friction Angle(φ)increased exponentially,and were linearly correlated with soil cohesion(c)(R~2=0.79-0.89).In conclusion,roots of herbaceous vegetation can effectively improve the anti-disintegration ability and shear performance of soil.While netting in the topsoil layer had an obstructive effect on root growth and distribution,which also had a certain reinforcement effect on slope soil stability.The comprehensive effect of ryegrass on soil corrosion resistance was better than that of alfalfa and setaria,so ryegrass could be used as the preferred grass for slope greening.