Experimental Study On The Effect Of Stress Path On Mechanical Characteristics Of Root-soil Composites

As a branch of engineering technology, Ecological slope protection technology has been widely used in slope protection of highway engineering. However, in this engineering area, the relevant theoretical research lags far behind the engineering practice so far, especially is research on the mechanism of ecological slope protection. Based on research projects funded by national and government, the effects of stress path on mechanical properties of root-soil composite are carried out in this thesis. These funded projects are included, the State Forestry Administration funded "948" project "Technology Import of High Stability of Ecological Slope Protection for Integration of Forest and Grass" (No: 2012-4-76), and the National Natural Science Foundation funded project "Research on Molding Mechanism of High-Strength Forest-Grass Mixed Roots and Reinforcing Mechanism of Root-Soil Composite in Plant Slope" (No:31270671) as well as Hunan Provincial Natural Science Foundation funded project "Study on Mechanical Properties of Plant Slop" (No:12JJ5015) and so on.In this dissertation by means of analysis the pertinent literatures at home and abroad, some technical problems on ecological slope protection are explored. Such as, the distribution characteristics of roots, the morphology of roots, the density of roots, the tensile strength of roots, and the effect laws of these factors on ecological slope stability etc.. According to multidisciplinary principles and methods, the distribution law of Vetiver root system were investigated. The related knowledges used in this reaearch are including soil mechanics, elastic-plastic mechanics, soil and water conservation, and mathematics etc..Through indoor and outdoor tests, and based on combining experimental research and theoretical analysis, a distribution model of Vetiver roots is constructed. Based on the existed researches at home and abroad, a series of experimental studies on root-soil composites which are composed of Vetiver roots and cohesive soil are carried out, under the conditions of different consolidation methods and different stress paths. Furthermore, the effects of different stress path on the rule of strength and deformation of root-soil composite are studied, under different consolidation conditions. Through modifying the fitted critical state line, a new constitutive model which will be to meet the mechanical properties of root-soil composite has been constructed on the basis of the modified Cambridge model. The main contents of this research work and the related innovations are as follows.(1) Study on the distribution regularity of vetiveria root systemThrough the investigation of four strains representative of vetiver grass roots that are grow in a model box for two years, it is found that the vertical roots are the most deepest, and its heart-roots are very developed. The skeleton structure of vetiver is mainly composed of taproot and heart-root, so that this kind of grass belong to the vertical root plant. The diameter of vetiver root is decreased with the length increasing. Using the self similarity theory and the tubular model, Some biological characteristics of vetiver are predicted, such as the root length, diameter of root link, root biomass etc.. A roots morphology distribution model of Vetiveria root is established, which is based on the FracRoot model and is according with the characteristics of Vetiveria. The growth morphology of Vetiveria roots are measured by using micro tube root zone imaging method. And so that the reliability of this dynamic growth model of roots in whole life cycle is verified.(2) Experimental study on mechanical properties of root-soil compositeA series of triaxial compression tests on root-soil composites that the root densities are in the same have been carried out, under the conditions of different consolidation and different stress paths. Furthermore, the mechanical properties of root-soil composite are have been studied, and the innovative conclusions obtained are as follows.① It is found that the shear strength of root-soil composite is enhanced obviously when vetiveria roots are existed in soil. This shows that the roots play a very important role in reinforcing soil, so that the stability of slope is improved. Under the conditions of different consolidation and different stress paths, the shear strengths of the root-soil composites are improved to different degrees than the rootless soils. For the root-soil composite, vetiveria roots provided contributions to cohesion force and internal friction angle are affected by stress path.② Under the conditions of the same confining pressure, no matter the root-soil composite or rootless soil, the shear strengths of samples that are undergone by KO consolidation are higher than that are undergone by isotropic consolidation. This conclusion is in good agreement with that obtained by conventional triaxial compression test or reduced triaxial compression test. This shows that, it is necessary to consider the influence of consolidation mode, when the shear strength parameters of slope soil will be determined.③ For a same kind of triaxial compression test, the critical state line that is obtained from root-soil composite is similar to that is obtained from rootless soil, under the same consolidation conditions. In a lower stress level, it is shown that the critical state line is nonlinear, and that its nonlinear degree is decreased gradually with the increasing of stress level. The critical state lines of root-soil composite adhere to a power-function to change.④ No matter which kind of triaxial compression test is performed (except for q uniform test), the effective stress path that is obtained from root-soil composite is similar to that is obtained from rootless soil, under the same consolidation path. Their effective stress path is in parallel with each orther mainly, and do not exceed their critical state lines. In other words, it follows limit state theory.⑤ Through modifying the fitted equation of critical state line, a new equation of critical state line which is in the form of a power function is obtained. Using this new equation combined with modified Cambridge model, a new constitutive model is constructed, which can reflect the mechanical properties of root-soil composite better. Furthermore, a new constitutive model which is in the form of elastic-plastic matrix is proposed. This new constitutive model is clear in concept and simple in form. Moreover, it is convenient for practical application.