Experimental Study On The Mechanical Properties Of Root-soil Complex Which Is Mixed With Forest And Grass

The instability and failure of the slope is a common and multiple natural geological disasters. Highways, high-speed rails and other infrastructure constructions have been developed rapidly, which lead to occurrence of a large number of subgrade slope failure in recent20years, especially in mountainous southern of China. Though the traditional rigid slope protection can significantly improve the stability of slopes, it not only costs highly but also seriously damages to the ecological environment. Thus the low-cost vegetation restoration techniques are increasingly concerned in ecological slope protection. The early ecological slope protections, such as planting turf or shallower roots plants, are only suitable for soil and water conservation or soil protection. When the slope failure surface is larger and deeper, this protection effect is not obvious. Using protection method of the combination of engineering technology and ecological technology of integration of forest and grass, some plants such as shrubs and herbs which are suitable planting in the slope can be used for protective and control slope surface landslide. And for deeper slope they can be used play a role as " plant soil nail" to anchor the slope. These plants are not only must for improving the overall slope stability, but also are must for eliminating the noise. Furthermore, for the improving of optical and thermal environment, and for the improving of ecological environment surrounding the roads, they are also play a very important role. Therefore, it has important theoretical value and engineering significance to study on the ecological slope protection technology of high stability of forest-grass integration, and it will make the slope protection engineering and ecological effects are organically together.Based on the theory of ecological slope and advanced technology at home and abroad, the research on the high stability of forest-grass integration ecological slope protection technology is carried out in this thesis. The study is funded by the following research projects. The project supported by National Natural Science Foundation,"Study on the forming mechanism of High strength of mixed roots of forest-grass and the mechanism of root-soil reinforced slope"(No.,31270671). The ’948project’ supported by State Forestry Administration,"The import of technology of high stability ecological slope protection of forest-grass integration"(No., 2012-4-76). The project supported by the Natural Science Foundation of Hunan Province,"Research on mechanical characteristics of plant slope protection"(No.,12JJ5015). And the project supported by Central South University of Forestry and Technology,"The project of science and technology innovation fund for graduate students"(No., CX2012B24), etc. Through experimental study and theoretical analysis, the mechanical properties of root-soil complex which is mixed with forest and grass are discussed. The main contents of research work and the creative achievements are as follows.The main contents of this research work1、According to the designed slopes which consist of different plant density and different gradient, herbs and shrubs, ie. Ligustrum quihoui and Vetiveria zizanioides have been cultivated in outdoor model box.2、After being cultivated for a year, a series of direct shear tests on root-soil composite which is mixed with roots of Vetiveria zizanioides and Ligustrum quihoui are carried out, under the conditions of different root content and different root ratio as well as different water content. The laws of shear strength of root-soil composite which is mixed with these two kinds of plants roots are have been explored, under different conditions.3、Through triaxial tests on root-soil composite which is mixed with roots of forest and grass, the characteristics of shear strength of mixed root-soil composite are analyzed Further.4、On the basis of experimental research, by analyzing the model and parameter selection, the strength laws of mixed root-soil composite are have been researched, and its theoretical constitutive models are have been set up too.The main innovative achievements of this research work1、The root system of Vetiveria zizanioides is more developed, its diameters are about0.8-0.9mm,80%of the roots are distributed within the depth of60cm beneath the slope surface. Ligustrum quihoui have an obvious taproot which is vertically into the deeper soil, its diameter is larger. Its lateral roots are grown thickly, and is extended to horizontal direction within the upper or middle layer of the earth surface.2、When the root content and root ratio of soil samples is constant, the direct shear test results showed that, the higher the water content is, the lower the shear strength of soil-root composite is, under the same vertical stress. With the increase of moisture content, the shear strengths are decreased in different degree, whether un-root-soil or root-soil, but the root-soil’s shear strength is decreased less.3、The study found that, include the uniformity of soil particle size, and the root content are all have effects on shear strengths of root-soil complex. When its root content is the same, the higher the uniform of soil particle size is, the higher the shear strength is. On the other hand, when the soil particle size is the same, the higher the root content is, the higher the shear strength is.4、For herbaceous or woody plant, the shear strength curves of soil-root composite can be fitted using least squares method. The first and secondary fitting curves are all closed to linear relationship. This indicated that the shear strength of single root-soil composite is in accord with Coulomb’s strength theory. Similarly, for root-soil composite which mixed with forest and grass roots, its first and secondary fitting curves are also showed a linear relationship. Therefore, it can be considered that, the shear strength of mixed roots-soil composite is also in accord with Coulomb’ strength theory.5、A series of direct shear tests show, vetiver and lobular privet roots can significantly improve the shear strength of soil-root composites. But for the shear strength index of this complex, the contributions of these two kinds of plant roots are different. Vetiver’ roots are mainly to improve the cohesion of soil-root complex, but Ligustrum quihoui’ roots are mainly to improve its internal friction angle.6、Nonlinear fitting method is applied to describe the relation between soil-root complex’ internal friction angle and its lobular privet’ root content ratio. It is found that, with the increasing of lobular privet’ root content ratio, the friction angle is increased in a power function form. Accordingly, a power function form relationship between internal friction angle of soil-root complex and lobular privet’ root content ratio is set up. Furthermore, the coefficient of power function is a constant which is related with moisture content and root content.7、Through a series of triaxial shear tests on forest and grass mixed soil-root complex, the stress-strain curves of mixed soil-root complex have been obtaned under different confining pressure. By analysing these stress-strain curves, it is found that the relationship between deviatoric stress and axial strain is consistent with the hyperbolic function under different confining pressure. Moreover, the relationship between strain-stress ratio and axial strain is consistent with the linear function, and it has the characteristics of the Duncan-zhang model. Based on the Duncan-zhang model, a new constitutive model which has the form of the Duncan-Zhang E-B model and can be used for mixed root-soil complex is established in this thesis, and the specific methods to determine the new model parameters have been put forward.For the exploration of mechanical properties of mixed soil root complex, and in order to develop and to use high stability forest and grass slope protection technology, the above research has important theoretical value and broad application prospect in engineering.