Modeling of Strain Accumulation in Clays under Cyclic Loading
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The stiffness of the soil at the foundation level plays an important role in the global stiffness of the offshore structure. Beside the static loads, the offshore foundation need also suffer cyclic loads from both wind and waves. Stiffness and strength degradation in the soil have been observed under cyclic loading. As the availability of the existing p-y method suggested by the API regulation has not approved for the application for the large-diameter offshore monopole foundation. A lot of constitutive numerical models have been developed in order to take the behavior of the soil under cyclic loading into account. In this thesis, constitutive models APS-I, APS-II and UCASC are developed to account for the stiffness and strength degradation of clays under cyclic loading. Instead of following the cyclic behavior of the clay in the scale of time, the models are based on semi-explicit method in which the cyclic behavior of clays are described as a function of cyclic number. In this way, the computational time is more economic and the better precision is reached. In APS-I model, an accumulated yield surface is used. An accumulated plastic strain hardening rule and a cyclic transfer rule are formulated to describe the anisotropic accumulated plastic strain developed in clays under cyclic loading as a function of cyclic stress ratio, average stress ratio and equivalent cyclic number. Besides, NGI-ADP yield surface is used in the model to account for the static plastic strain developed. In this way, the average strength reduction can also be captured as a by-product of the model when the failure strain is reached. The model is implemented with a backward Euler integration scheme. The validity of the model is proved by the comparison between the test results and calculation results of the Drammen clay and Moum clay. Furthermore, in order to improve the precision of the capture of the average strength reduction, an improved APS-II model is formulated with more accurate accumulated plastic strain hardening rule and cyclic transfer rule. The improvement of the APS-II model is also shown from the comparison between the tests results and calculated results of the Drammen clay and the Moum clay. In addition, a new UCASC-AV model is proposed. One yield surface is used in the UCASCAV model to account for both static strain and accumulated plastic strain. An average strength reduction rule is formulated. Equivalent cyclic number Neq can be updated automatically with the cyclic stress level and the cycle number increment ΔN. Average stiffness degradation is predicted as a by-product of the average strength degradation. Thus, the average strain developed in the clay under cyclic loading can be predicted. In addition, a new UCASC-CY model is also proposed. A cyclic strength reduction rule is formulated to relate the cyclic strength reduction to the cyclic number. Nonlinear elasticity is adopted in the model. Thus, the cyclic strain developed in the clay under cyclic loading can be calculated. The validity of the UCASC models is proved through the comparison between the test results and calculated results of the Drammen clay. Finally, a calculation procedure for the combination of the UCASC-AV model and the UCASC-CY model is proposed for the offshore foundation calculation. At the end of the thesis, accommodations of the future work are given, including the inclusion of the excess pore pressure generated during the cyclic loading and the inclusion of the small strain stiffness.