Experimental Study On Mechanical Characteristics Of Root-Soil Composite Based On Elastic Constitutive Relations

With the rapid development of infrastructure construction in our country, a large number of artificial slopes are formed because of deep digging and high filling, which has caused some damage to the original ecological environment. Therefore, how to build an ecological environment-friendly engineering has become a common concern in the whole society. In recent years, a lot of ecological slope protection technology are used in expressway construction, which is just produced under this background. Traditional rigid protection technology not only can’t solute the long-term stability of slopes, but also because of which changed the original ecological environment, a number of new landslides and debris flow disasters may be caused. Therefore, the slope protection technology which combined with engineering protection and ecological protection is received more and more attentions from engineering and academic circles. However, due to the related theoretical research in this field is still lagging behind the engineering application in our county, the study on the mechanism of plant roots reinforce soil and protect slopes isn’t reported widely in the related literatures, and so the consensus on the understanding of the mechanism of root-soil interaction has not yet been formed. Therefore, further researching on the mechanical properties of root-soil composite, exploring the mechanical mechanism of plant roots reinforced soil, and revealing the laws of strength and deformation of the root-soil composite will have important theoretical significance and engineering practical value for the development and application of ecological slope protection technology.Based on this purpose, and reling on the government’s fund projects, the experimental study on mechanical characteristics of root-soil composite has been carried out in this thesis. All the research works are supported by the following government’s fund projects, these are include the National Natural Science Fund Project, "Study on Molding Mechanism of High-Strength Forest-Grass Mixed Roots and Reinforcing Mechanism of Root-Soil Composite in Plants Slope" (No:31270671), and the "948" project fund by The State Forestry Administration, "Technology Import on High Stability of Ecological Slope Protection for Integration of Forest and Grass" (No:2012-4-76) etc. Using GDS unsaturated soil test apparatus, and aimed at root-soil composites which are consisted with four kinds of different root-content, the researches are carried out respectively, through a series of indoor tests. These tests are included drained conventional triaxial compression tests, isotropic consolidation tests, and constant p stress path tests etc. Based on the theories of Duncan-Chang model and K-G model, the influence laws of root-content on the mechanical characteristics of root-soil composite was explored, and the inner mechanism of plant roots reinforcing soil was revealed. Through the research, the main research achievements and the innovative conclusions have been obtained as follows.(A)Triaxial experimental study on root-soil composite based on Duncan-Chang modelThe conventional triaxial compression tests on root-soil composites which are consisted with four kinds of root-content were carried out, the influence laws of root-content on tangent deformation modulus and tangent Poisson’s ratio were researched.The research results are as follows.1. For root-soil composite, the relations between deviatoric stress and axial strain are in the form of hyperbolic curve, which can be described by Duncan-Chang model theory.2. Under the same condition of confining pressure, with the increasing of root-content, the curves of deviatoric stress versus axial strain became higher. This shows that the existence of vetiver roots can improve the strength of root-soil composite.3. Under the same conditions of root-content, with the increasing of confining pressure, the initial deformation modulus and the ultimate deviatoric stress are all enhanced. Similarly, under the same conditions of confining pressure, with the increasing of root-content, the initial deformation modulus and the ultimate deviatoric stress are all enhanced too. The failure ratio of specimens are mainly affected by confining pressure of the test. Along with the increasing of confining pressure, the failure ratio become decreased. On the other hand, the influence of root-content on the failure ratio is not obvious.4. Along with the increasing of root-content, the values of parameters A and n will be increased correspondingly, that’s to say, the initial deformation modulus will be increased. But when the root-content goes beyond a certain range, its effect on the modulus will not be apparent.5. Along with the increasing of root-content, the cohesion of root-soil composite will be increased gradually, but its growth rate will be decreased gradually. Along with the increasing of root-content, the change trend of internal friction angle of root-soil composite is not obvious.6. Along with the increasing of root-content and confining pressure, the value of parameter M will be decreased, and the change of parameter F is not obvious, which means that the initial Poisson ratio of root-soil composite will be decreased.(B)Triaxial experimental study on root-soil composite based on K-G modelThe isotropic consolidation tests for rootless soil and root-soil composite were carried out respectively, and then for different spherical stress, a series of constant p stress path drained triaxial compression tests were also finished. At the same time, the laws of effect of root-content on bulk modulus, volume strains, shear modulus and shear strains of root-soil composite were discussed. The research results are as follows.1.In constant p triaxial compression test, relations between deviatoric stress and shear strain is in accord with K-G model theory.2.In the isotropic consolidation test, under the condition of the same spherical stress, along with the root-content increasing, the initial bulk modulus and parameter αk will be all increased gradually. As a result, the bulk modulus of root-soil composite is enhanced, and the bulk strain is decreased correspondingly. This means that owing to the existence of plant roots, the bulk deformation is blocked, and the compression deformation of root-soil composite is restrained, which eventually lead to improving the strength of root-soil composite.3. In constant p drained triaxial compression test, along with the increasing of root-content, the failure line of root-soil composite is became higher. That is to say, the strength of root-soil composite is enhanced.4. Along with the increasing of root-content, the initial shear modulus is significantly improved, at the same time, the shear strain is decreased correspondingly. Meanwhile, the absolute value of parameters αG and βG are get different degrees of increasing. And so that, the shear modulus of soil-root composite is enhanced, and the shear strain is reduced correspondingly, which eventually leading to the shear strength of root-soil composite is improved.The above research work have laid a foundation for further exploring of the stress-strain relations of root-soil composite and the interaction mechanism of root-soil composite. It has also provided a theoretical basis for the strength and stability analysis of ecological slope protection.