The Deformation Characteristics Of Wuhan Soft Clay And Its Dynamic Responses Subjected To Cyclic Loading

To study the deformation characteristics and dynamic responses subjected to traffic loading, a series of laboratory tests are conducted on saturated soft clay in Wuhan, and theoretical derivation and numerical analysis are performed. The main conclusions are as follows:Firstly, the basic engineering characteristics of Wuhan soft clay are evaluated, and the consolidation test is performed, then the effect of soil structure to its consolidation characteristics (consolidation coefficient, compressibility coefficient, permeability coefficient) is analyzed. According to the one-dimensional Terzaghi's consolidation theory, the relation between the consolidation velocity and time is deduced, proposing a new method to evaluate the analytical solution of the consolidation coefficient. This is a new way of getting the consolidation coefficient, which avoids the defect of traditional graphical method and can eliminate the effects of the initial subsidence and the secondary consolidation.Secondly, the consolidated undrained triaxial shear tests are conducted on soil samples in two different depths at undisturbed and corresponding remoulded states, and the effect of soil structure to its static strength parameters is studied. In order to normalize the stress-strain relationship, based on hyperbolic function, a new normalization condition is derived theoretically, which the ultimate value of principal stress (σ1-σ3)ulti should be proportional to the initial tangent modulus E0i. In this condition, the normalization stress-strain relationship of Wuhan soft clay under CU test condition is established, while the ultimate value of principal stress is used as the normalization factor.Thirdly, the rail and the underlying foundation are solved respectively when studying the dynamic responses subjected to moving loads. When building the motion equation of this system, the rail is simplified to an infinite Euler-Bernoullic beam, resting on a two-parameter foundation, and the foundation is divided into an elastic layer and a saturated soil layer with the dividing underground water level as their border. They are coupled with the stress and the displacement boundary condition. The analysis solutions of the 2-D and 3-D motion equation of the foundation subjected to moving loads are derived. Then, in 2-D the influence law is studied using numerical analysis, which concerns the influence of the factors( velocity, frequency of moving loads, permeability coefficient, elastic parameter and shear parameter of the foundation) to the distribution of the foundation's vertical displacement and pore pressure.Fourthly, a series of unidirectional cyclic triaxial tests are conducted on Wuhan soft clay samples at undisturbed and corresponding remoulded states, then the effects of the amplitude values of dynamic stress, static deviator stress, vibration frequency, consolidated confining pressure and load repetition on the dynamic behaviors of the saturated soft clay are analyzed. A new mathematical model is proposed to fit the stable curve of accumulative plastic strain of soft clay versus load repetition subjected to cyclic loading. The empirical models of accumulative strain and double amplitude strain are established respectively, based on the dynamic triaxial test results of undisturbed and remoulded soil samples, concerning the amplitude values of dynamic stress, consolidated confining pressure, static deviatoric stress and load repetition. Furthermore, a numeric method of solving critical dynamic stress is proposed according to the evolution law of the parameters in the new mathematical model, which gives a new way to solve the critical dynamic stress. Dynamic backbone curve models concerning the load repetition are constructed to consider the stiffness softening features of undisturbed and remoulded soil subjected to cyclic loading. A new failure criterion for soil structure that the inflection point on the maximum stain curve meaning failure is presented. And then a dynamic strength equation for remoulded soil is built. Further discussion is performed on the effect of soil structure to the dynamic properties of soil.Finally, combining the dynamic backbone curve model, accumulation plastic strain model and damping ratio model, a nonlinear dynamic constitutive model of soil is built, which can reflect the stiffness softening and the strain accumulation. This idea abandons the traditional method of building the hysteretic curve based on the backbone curve and hence overcomes the defect that it can not reflect the phenomenon of strain accumulation and the stiffness softening in the traditional method. The simulative results computed from the model agree well with the experimental data.