Experimental Study On The Strength Characteristics Of Vetiver Root Soil Composite

With the rapid development of resource exploitation and infrastructure construction, environmental pollution and ecological destruction are becoming increasingly serious, which has become one of the major social problems faced by all mankind in recent years. Therefore, how to protect the environment and realize the sustainable development of man and nature is an urgent problem to be solved in the construction process. In the slope protection engineering of expressway, the traditional protection technologies such as lattice retaining wall, rubble retaining wall, anchor-plate retaining wall and sprayed concrete slope, are still used in most project at present. In the construction process, these protection technologies are not only expensive, but also seriously damaged the original ecological environment, which will lead to more landslides and debris flow disasters, and affect the effect of slope protection.With the improvement of environmental awareness and the demand for environmental quality of human, ecological protection technology has been came into use in engineering. The essence of ecological protection technology is to combine the plant protection and engineering protection. By using the interaction between plant roots and soil to forme a natural composite material, which enhanced soil strength and the anti-sliding ability of slope, meanwhile increased the stability of soil slope and beautified the environment.This thesis regards root-soil composite which is composed of Changsha local clays and vetiver roots as the research object, the related experimental study on this root-soil composite has been carried out. 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), The "948" Fund Project of The State Forestry Administration, "Technology Import on High Stability of Ecological Slope Protection for Integration of Forest and Grass" (No: 2012-4-76), and Hunan Provincial Natural Science Foundation Fund Project, "Study on Mechanical Characteristics of Plants Slop" (No:12JJ5015) etc. Through a series of indoor tests which included the distribution test on vetiver roots, the drawing-friction characteristics test on single vetiver root, the direct shear and the triaxial shear test on vetiver root-soil composite etc., then the research on the distribution law of vetiver root in soil, and the study on the strength properties of composite are all have been finished. By means of numerical analysis software, the distribution law of roots and the strength characteristics of root-soil composite are fitted and analyzed. Through the research, the main research achievements and the related innovative conclusions are have been made as follows.1. Experimental study on friction characteristics of vetiver roots(1) Under the action of pulling load, the vetiver root which is existed in the composite will be pulled off or be pulled out. The first case is when the root tensile strength is not more than the drawing force and the maximum friction between root and soil, then the root will be pulled off. The second case is when the drawing force is more than the maximum friction between root and soil, but the drawing force is not more than the maximum tensile strength of root, then the root will be pulled out.(2) In the conditions of the same density and different moisture content, root tensile strength is increased with the increasing of root diameter, but with the increasing of moisture content, root tensile strength will be increased first and then will be decreased. When the water content is about 19.5%, the tensile strength of the root reaches the maximum value.(3) In the conditions of the same moisture content and different density, the maximum anti-drawing force of the root-soil interface is increased with the increasing of soil density.2. Experimental study on shear strength characteristics of root-soil composite(1) The different distribution mode of root in soil, the enhancement effect of soil shear strength is completely different. When the root number and root diameter of vertical arranged is equal to oblique arranged, the shear strength of root-soil composite is the highest under the same vertical load. When all roots are oblique arranged, the shear strength of root-soil composite is the second. When all roots are vertical arranged, the shear strength of root-soil composite is the third. When the root number of vertical arranged is equal to horizontal arranged, the shear strength of root-soil composite is the lowest.(2) When the vetiver root number and its arrangement mode is unchanged, with the increasing of root in diameter, the shear strength of root-soil composite is increased first and then is decreased. When root diameter is about lmm, the shear strength of root-soil composite reaches the maximum value. Therefore, for the shear strength of the root-soil composite there may be an effective root diameter. If the root diameter is too large or too small, the effect of root diameter on the shear strength of the soil is not obvious.(3) The effect of vetiver roots diameter on internal friction angle and cohesion of root-soil composite is different. With the increasing of root in diameter, the internal friction angle is increased first and then is decreased. But with the increasing of root in diameter, the cohesive may be increased or be decreased, and the relations are relatively complex. On the other hand, the effect of distribution mode of roots in soil on cohesion of composite is very obvious, but this effect on internal friction angle is not obvious.(4) The reseach results show, with the increasing of axial strain, the distance between the curves of deviatoric stress versus axial strain is increased gradually, This shows that the "reinforcement" effect of root on soil can be appeared only when the axial strain is larger.(5) Under the condition of the same root distribution mode, the cohesion and internal friction angle of root-soil composite are all decreased gradually with the increasing of water content. This influence is in accord with the effect of water content on the shear strength of root soil composite.(6) Through analyzing and fitting the experimental results, the related formulae to calculate the cohesive and the internal friction angle of root-soil composite are obtained respectively, which can consider this two indexes along with the change of moisture content.