Study On Deformation Mechanism And Support Mechanical Behavior Of Large-Span Shallow-Buried Tunnel In Expansive Ground

To relieve the increasingly tense transport pressure, the construction of railways and highways has reached a high tide all over our country. As an increasing number of tunnel projects across unfavorable geology and the improvement of techniques standards in the construction of railways and highways, lots of shallow large span expansive soil tunnels are inevitably encountered. Due to the water swelling and drying shrinkage property of expansive surrounding rock, the deterioration of surrounding rock appears in tunneling process most often, large deformation of surrounding rock and support structure is happened, even be destroyed. All these cause a series of safety accident and bring great difficulties to construction quality and progress control. Considering the complication of swelling mechanism and engineering property of expansive soils, mature principles and methods apply to tunneling design and construction in expansive soil have not formed at present, it is still one of the most difficult points in the design of tunnel projects. Therefore, it has important practical significance to study on expansive surrounding rock tunnel, especially for the deformation mechanism and support mechanical behavior of shallow large-span expansive soil tunnel.The dissertation takes the China railway corporation project of key technology research on shallow large-span railway tunnel in expansive soil (major project-18-2011) as the background and relies on the construction of Cheng-Gong tunnel in Yunnan province. In view of the deformation mechanism and support mechanical behavior of shallow large span expansive soil tunnels, on the basis of related previous studies, a systematic research is carried out by combining ways of theoretical analyses, derivation analytic formula, laboratory test, numerical simulation and field test. The main achievements and conclusions are as follows:(1)By adopting lateral constrained-upper loaded experiment, a relationship of expansive force and different initial conditions (different dry densities and different original water content) is established, the mathematical relationship of expansive force with signal dry density and original water content in the conventional formula is perfected, and in the situation of dry density, the relation of expansive ratio with upper load and water content is derived. Using Direct Shear test to analyze the relation of shear strength and moisture content of expansive soil and fit the curve of shear stress with overburden and initial water content.(2) A study of wetting deformation property of the different initial state expansive soil under three-dimensional stress is conducted by introducing modified tri-axial test. The agreement between laboratory test and practical engineering is much more scientific and reasonable to research the mechanical behavior of expansive soil.(3)Based on the linear elasticity assumptions, the analytical solution for the deformation and stress of lining under different expansive ratio is obtained. With the change of expansive ratio of surrounding rock and the stiffness of support structure, the law of the deformation and stress of support structure is obtained. Stress-strain analytical solutions of lining structure based on elastic mechanics theory is deduced on the different expansive surrounding soil condition of deep and shallow tunnel. The stress-strain distributed regularity of support structure is ascertained theoretically with the change of expansive ratio and support rigidity.(4)On the basis of related previous studies, the failure mode of working face for shallow buried tunnel constructed in cohesive soil is established. Based on the upper bound theorem of plastic, the different drainage conditions is discussed combining with the assumed mode above, the solutions of the limit support pressure for the stability of working face is obtained and the stability of working face of shallow large span tunnel is evaluated. The factors that have effects on the horizontal support stress for the stability of working face is analyzed, and the result indicates that buried depth of tunnel is in the first place.(5)Using theoretical analysis and combining with laboratory test, bond behavior of GFRP and the impacts on the bond behavior are studied. The analytical solutions for the shear stiffness of anchor is derived by elastic mechanics theory, and according to test results, the solution is optimized for more practical applications. Based on the existing research, through both theoretical and numerical analysis, strengthening effect of length, number, reinforcement range of GFRP for working face is discussed, and obtains the proper reinforcement parameters.(6)The different tunneling methods are modeled by employing finite-difference simulation software, the deformation and stress of surrounding rock, load distribution form and mechanical properties of support structure and deformation and stability of working face are studied after expansive soil unloading during construction. Advantages and disadvantages of 3 type of construction methods which are pipe roof+bench method, pipe roof+bench method with remained core soil and pipe roof+full face excavation+face bolt are compared, the result shows that the third is the best method for the shallow large span tunnel. And related preventive measures were proposed for the expansive problem.(7)The process of soil expanding after absorbing water is simulated by means of adopting the finite element software ABAQUS with built-in seepage module and introducing the soil-water characteristic curve, the deformation and mechanical characteristic of lining structure is analyzed comparatively before and after surrounding rock inflation to better evaluate the effects of surrounding rock expanding.(8)The deformation and stress state of surrounding rock and support structure in the expansive stratum are studied based on multi-section field tests, the contact stresses between expansive surrounding rock and initial lining, initial lining and permanent lining and the distributed stress of support structure are obtained, the load sharing ratio of permanent lining during construction and operation get clear and the suffering area of soil surrounding the tunnel profile in the process of excavation is confirmed. The design and construction of tunnel are optimized based on on-site monitoring results and timely feedback, and that provides a scientific decision-making basis for the risk control and engineering measures of support structure during construction and establishes a technology guarantee system of long term safety during operation for support structure system of tunnels in expansive soil.