Experimental Study On The Effect Of Slope Gradient On The Property Of Vetiver Grass Strengthening Soil And Slope Protection

lope instability and soil erosion are common natural geological disasters,which bring serious harm to people's production and life.For this reason,it is essential to take appropriate protective measures for the slope.Traditional protection technology of slope engineering can greatly improve the stability of the slope in its early stage,and plays the effect of strengthening soil and slope protection.However,as time increases,its protective effect will be gradually weakened.There are some problems such as high cost and serious destruction of ecological environment are existed in traditional protection technology of slope engineering.Therefore,on the basis of the traditional slope protection technology,it is necessary to seek an "active and ecotypic"slope protecting method which organically combines engineering construction with ecological environment protection.Plant slope protection technology has significant effect on improving the strength,deformation stability and scour-erosion resistance of slope soil,and has the advantages of low cost and maintaining ecological balance.Among all kinds of factors affecting slope stability,slope gradient is one of the most important natural influence factors.Therefore,the study on the influence of slope gradient on plants slope protection is not only of great theoretical significance,but also is of broad engineering application prospect.Based on the item " Research on Molding Mechanism of High-strength Forest-Grass-Mixed Roots and Reinforcing Mechanism of Root-Soil Composite in Plant Slope " supported by the National Natural Science Foundation of China(Grant No.31270671),Experimental Study on the Effect of Slope Gradient on the Property of Vetiver Grass Strengthening Soil and Slope Protection has been carried out in this thesis.Rely on the demonstration engineering of Ecological Slope Protection of east loop of Chongqing railway hub as research background,the application study on "Protection Technology of Ecological Slope in East Ring Line Project of Chongqing Railway Hub"(Grant No.YF1900SD07B)has been carried out,which is supported by Chongqing railway hub east loop construction command.Through the method combined with experimental study and theoretical analysis,the influence law of the variation of different root morphological parameters with slope gradient is proved,and related influence mechanism is revealed.On the base of experimental study,the influence laws of slope gradient on the shear strength indices of Vetiver' root-soil composite have been found out,and related influence mechanisms have been revealed.The analysis model and analytical theory for the cohesion and internal friction angle of Vetiver' root-soil composite have been put forward under different slope conditions.Through numerical simulation analysis,the influence law of slope gradient on the stability of Vetiver' ecological slope protection has been explored.Through numerical simulation analysis,the influence law of slope gradient on the stability of ecological slope protection with Vetiver grass has been explored.Main research work carried out and innovative conclusions achieved in this thesis are as follows.1.Main research work(1)Through the "cross artificial total excavation method",the roots of Vetiver grass grown under different slope conditions are excavated.The root diameter,root biomass,root group growth range and root diameter ratio of Vetiver grass grown in different slopes and spatial distribution patterns have been studied.the change laws of the influence of root diameter,root biomass,growth range of root group and root diameter ratio with soil depth,slope gradient and slope direction are investigated,and the related influence mechanism are revealed.(2)Through direct shear tests of undisturbed root-contained-soil and undisturbed rootless-soil under different slope conditions,the influence law of slope gradient on shear strength of root-soil composite has been explored,and related influence mechanism has been revealed.(3)The models for analyzing shear strength indices of Vetiver root-soil composite have been established,and the relevant model parameters have been determined.(4)Using finite element method,the stability of slope protected by plants is analyzed by numerical simulation.The influence of slope gradient on spatial distribution morphology of plant roots in slope soil is discussed.The influence law of slope gradient on safety coefficient for the stability of slope protected by plants has been explored,and the related influence mechanism has been revealed.Through the analyzing by numerical simulation,the rationality of the proposed analytical model and theory are have been verified.2.Main research advances and innovative conclusions are obtained(1)Through the analyzing for root morphological parameters of Vetiver grass under different slope conditions,it has been found that slope gradient and slope direction have significant effects on root biomass of Vetiver grass.When root system is located in the uphill direction,the larger the slope gradient,the less the biomass of root system.On the contrary,when the root system is located in the downhill direction,the larger the slope gradient is,the more root biomass is located in shallow soil.Through the fitting on tested results of root biomass in the form of a function,the analysis model of root biomass with the change of soil depth and slope gradient has been established,and the related model parameters have been determined.(2)Based on the analysis of morphological parameters of root system under different slope conditions,it is found that the growth range of root group is changed regularly with the changing of slope gradient and soil depth.With the increasing of depth of soil layer,the growth range of root group of Vetiver grass in the uphill and downhill direction are all shown the trend of are increased firstly and are all shown the trend of are decreased then.The larger of slope gradient,the smaller the growth range of the root group in the uphill direction,but the larger relatively the growth range of the root group in the downhill direction.Based on the analysis of measured results,an analytical model has been established for the variation of growth range of root group with soil depth and slope gradient,and the related model parameters have been determined.This model can be used to analyze the growth range of root group,and it also can be used to analyze the distribution morphology of roots group in the slope space with different soil depth and slope gradient.(3)Through the analysis for morphology parameters of Vetiver root under different slope conditions,it is found that there is a significant effect of slope gradient on root diameter and the ratio of root diameter of Vetiver grass.Among Vetiver grass roots are grown for a year,with the increasing of slope gradient,the proportion of root which with the diameter in the range of 0.5?1.5 mm is shows an increasing trend.With the changing of slope gradient,the size of root diameter in the same soil depth will be also changed.(4)Through direct shear tests on undisturbed root-contained-soil and undisturbed rootless-soil under the conditions of different slope gradient and different soil depth,it is found that the influence of slope gradient on shear strength of root-soil composite is mainly reflected in the improving the cohesion,but has little effect on its internal friction angle.Meanwhile,the research found that because of roots are existed in soil,the cohesion of soil can be improved significantly.With the increasing of slope gradient,the cohesion of root-soil composite in the same soil depth is increased gradually,but the increased extent of cohesion will be decreased gradually.(5)Though the analyzing on tested results of direct shear tests,the model of relation between root content and cohesion has been established,and the related model parameters have been determined.Furthermore,under the conditions of different slope gradient and different soil depth,the model of relation between root number and root content has been established,and the related model parameters have been determined.Combined with the proposed analysis model which is the relation of root biomass and soil depth as well as slope gradient,an analytical model for the calculation of shear strength indices of root-soil composite has been established,and the related model parameters have been determined.(6)According to analysis results by numerical simulation,it is found that root group has the most obvious effect on reducing horizontal displacement.For different slope gradient,the magnitude of displacement of root-contained-slope and rootless-slope are all different.When the change range of slope gradient is within 30°?45°,the research results show that the horizontal displacement,vertical displacement and total displacement of root-contained-slope and rootless-slope are all reached the minimum value when the slope gradient is 30°.Accordingly,it shows that the overall stability of the slope is effected obviously by slope gradient.(7)According to analysis results by numerical simulation,it is found that with the existing of plant roots within slope soil,plastic deformation of the slope can be decreased.However,this effect will be affected by slope gradient.Whether rootless-slope or root-contained-slope,plastic deformation at the foot of slope will be increased with the increasing of slope gradient.(8)According to the results of numerical simulation analysis,it is found that the safety factor of stability of rootless-slope and root-contained-slope are all decreased with the increasing of slope gradient.Although safety factor of slope stability will be decreased with the increasing of slope gradient.But the extent of reduction of safety factor of slope stability is also decreased with the increasing of slope gradient,on account of plant roots are existed in slope soil.Under the same condition of slope gradient,the stability safety factor of rootless-slope is the smallest,but the stability safety system of root-contained-slope is all larger.Therefore,it reflects that slope stability safety factor is improved obviously by the effecting of roots.Through experimental study,theoretical exploration and numerical model analysis,the influence law of slope gradient on root morphology distribution and the strength of root-soil composite of Vetiver grass,have been investigated,and related influence mechanisms have been revealed.Through the study,the influence law of slope gradient on the stability of the slope protected by plants is also explored,and the influence mechanism of slope gradient on the stability of the slope protected by plants is revealed.On this basis,an analysis model for the shear strength indices of root-soil composite has been established under the condition of different slope.Furthermore,an analysis method for the stability of plant slope protection has been proposed,in which the influence of slope gradient is considered.The above research results provide a theoretical basis in order to consider the influence of slope gradient on the strength and stability of soil strengthening and slope protection by Vetiver grass,and also laid a related theoretical basis for the engineering application of plant slope protection technology.