Study On Improving The Residual Shear Strength Of Freeway Slope With Mixed Root System

With the rapid development of highway transportation,the situation of building highways under complex terrain is increasing.It is inevitable that high filling and deep excavation is easy to occur,the slope is prone to stress redistribution and instability phenomenon,which is not conducive to the safety of expressway operation.It is of great significance to study the mechanical characteristics of typical slope protection plants for the slope stability and the selection and allocation of slope protection plants.The stability of highway slope can be effectively improved by the model of plant slope protection with the combination of shrub plants and herbaceous plants.This paper relies on the 948 project of the State Forestry Administration,"the introduction of key technologies for deep cutting slope landscape restoration"(project number 2015-4-38)and the eighth contract section of Dayue expressway.The root system consisting of herbaceous plants bermudagrass and shrub,mulbera and Robinia pseudoacacia was selected as the research object.Direct shear test and three axis shear test were carried out to study on the dry density,root diameter,layout,shear angle and other factors on the mixed shrub grass roots to improve the soil shear strength effect.The change law of safety factor of slope under different influence factors is analyzed by strength reduction method by ABAQUS software.The main innovation results of the research work are as follows.1.Through the experiment of repeated direct shear test in the root soil complex of mixed root system,it was found that for the test soil,the root system has a vertical load of 50kPa,which is the best soil consolidation effect in the range of buried depth of 2.8m;Under the same shear displacement,the shear rate increases and the residual shear stress of the soil increases correspondingly.When the shear rate is 0.16mm/min,the stable residual strength of soil can be obtained when the shear displacement reaches 32mm.2.Under the same test conditions,the residual strength index of soil increased and then decreased with the increase of root diameter.When root diameter was the root,namely Robinia pseudoacacia 1.3mm-1.6mm,Magnolia 0.8mim-1.2mm and bermudagrass 0.4mm-0.6mm,root system had the best soil fixation effect.3.Through the three axis shear test of the root and soil composite of shrub grass mixture,it is found that when the cutting face angle is 45 degrees,elastoplastic deformation of plain soil specimen when the axial strain is less than 5%,the elastoplastic deformation occurs when the axial strain is less than 8%in the root soil complex;When the angle of the preface is 60 degrees,it is close to the fracture angle of the soil,and the soil is strongly softened.The existence of the root system can increase the deformation stage of the soil,so that the brittle failure of the soil is delayed to 13%from the axial strain of 9%.4.Based on the data obtained from the experiment,the finite element analysis of the slope is carried out by the strength reduction method.It is found that the method of using ABAQUS to calculate safety factor for slope stability is feasible.When the displacement point of plastic zone or characteristic position is taken as criterion,it is more safe and reasonable.5.Under the same experimental conditions,90 +45 degrees can be the combination among the safety coefficient of slope increased by 40.44%;When the root diameter is the representative of the root diameter,the safety factor of the slope is 18.04%higher than that of the plain soil;When the pre shear surface is 60 degrees closer to the fracture surface of soil,the existence of mixed root system of grass irrigation improves the safety factor of slope by 15.68%.