Experimental And Modeling Research On Shear Characteristics Of Sand Subjected To Wave-induced Principal Stress Axes Rotation

Wave loading is one of the most important loadings in ocean engineering, which causes a cyclical wave pressure on the surface of seabed. Under the wave pressure, the normal stresses and shear stress change cyclically of a soil element in the seabed, which makes the principal stress rotates continuously. According to the classic plastic mechanics theory, plastic strain cannot be produced when the principal stresses keep constant but the principal stress axes rotates progressively, which belongs to the neutral loading. However, a lot of experimental results have proven that the principal stress axes rotation significantly affects the stress-strain behavior, cyclic strength and pore pressure characteristics. The seabed instability by wave-induced principal stress axes rotation is one of the main reasons resulting in the destruction of some ocean structures. Therefore, the experimental and modelling researches on the effects of wave-induced principal stress rotation on shear characteristics have an important theory value and practical meaning.The previous experimental researches on wave-induced principal stress axes rotation are mostly concentrated on the cyclic principal stress axes rotation shear tests under the circular rotation stress path, which is because based on the analytical solution for the seabed of infinite thickness, the amplitude of deviator stress (?z'-?x')/2 and shear stress ?zx by wave-induced is equal but the initial phase difference between them is 90°. However, the thickness of seabed is almost finite, and the thickness of the seabed plays a dominant role in the evaluation of the wave-induced response of the seabed in the literature, therefore, the stress path of a soil element in the seabed of finite thickness should be quite different from the seabed of infinite thickness. The dilatancy which should be considered is another important material characteristic of sand. Though the theory of state-dependent dilatancy can reflect the dilatancy of sand well, most of these state-dependent models cannot reflect the effects of principal stress axes orientation.Surrounding the effects of wave-indcued principal stress axes rotation on the shear characteristics and combined with the engineering characteristics of sand, a series of experimental and modeling researches are carried out. On the one hand, the results and observations of this research overcomes that the shear characteristics obtained from the circular rotation stress path in the literature are not applicable for analyzing the stability of the finite seabed, and on the other hand, a method to analyze the effects of wave-induced principal stress axes rotation on the deformation of sand is applied.The main research contents and achievements are as follows:(1) Nonstandard elliptical, i.e., non-circular, rotation stress path is proven to be a more common stress state of a soil element in the seabed subjected to the linear regular wave-induced principal stress axes rotation. Three parameters-amplitude of deviator stress, amplitude of shear stress and the initial phase difference, which determine the size and shape of the nonstandard elliptical rotation stress path, are deduced(2) The effects of the amplitude of deviator stress, amplitude of shear stress and the initial phase difference on cyclic strength of sand are investigated respectively in detail by a series of the undrained cyclic principal stress axes rotation tests under the nonstandard elliptical rotation stress path. The experimental results indicate that:the logarithm of failure cycles is decreased linearly with intial phase difference; For the same failure cycles, due to the initial phase difference, the cyclic strength of sand obtained by cyclic principal stress axes rotation test may be more than the cyclic strength obtained by cyclic triaxial shear test or cyclic torsional shear test.(3) Based on the relationship on cyclic strength between the nonstandard elliptical rotation stress path and the standard elliptical rotation stress path, and a series of undrained cyclic principal stress axes rotation tests on the standard elliptical rotation stress path, a function between failure cycles and three characteristic parameter of nonstandard elliptical rotational stress path is summarized. Based on this function, a new method to measure the cyclic strength of sand under the cyclic principal stress axes rotation tests on the nonstandard elliptical rotation stress path is proposed.(4) Through a series of undrained monotonic shear test in different fixed principal stress axes orientation, the effects of principal stress orientation on the dilatancy are studied. And combined with the theory of state-dependent dilatancy, a new state parameter containing the angle of principal stress axes orientation is proposed. By simulating serverl undrained monotonic shear tests in different fixed principal stress axes orientation, it is proven that the effects of the principal stress axes orientation can be reflected accurately by introducing the proposed state parameter into model.(5) A new constitutive model which can consider the effects of principal stress axes orientation and the dilatancy of sand is built based on the generalized plastic mechanics theory by introducing the angle of principal stress axes orientation and state-parameter independently, and the calibration method of model parameters is proposed. The feature of deformation due to the principal stress axes rotation can be reasonably simulated, and the proposed model overcomes the disadvantage in that other constitutive models need to use different sets of parameters to predict the shearing characteristics of the same kind of sand at different initial confining pressure or initial void ratio. In accordance with different static and dynamic kinds of the experiment results, the proposed model is proved to be correct and accurate.