The study of long term cyclic loading has had increased interest in the last few years as many current and future projects for offshore wind turbines are taking place in the UK. Attention has been given to the changes in the foundation's stiffness as this may alter the natural frequency of the structure. As well, during the lifetime of the structure millions of cycles accumulate defoln1ations which may affect its serviceability. The condition of long term cyclic loading of soils can be idealized as a drained boundary problem, as wind and wave loads are applied at low frequencies. This quasi-static loading allows no development of excess pore water pressure in the soil. The main objective of this research was to simulate an element of soil subjected to quasi-static long term loading by means of drained cyclic triaxial tests and applying a simplified version of the stress paths imposed at in situ conditions. The testing program consisted of a set of drained and undrained monotonic triaxial tests, drained cyclic triaxial tests, and a small set of undrained cyclic triaxial tests. The experimental campaign studied,the elastic stiffness at isotropic and anisotropic stress states t6 quantify the inherent and stress induced anisotropy of Hostun sand. In addition, the evolving elastic properties of sand during progressive cyclic loading was investigated. Accumulated deformations during drained cyclic loading were obtained as well as the parameters that influenced their magnitude, intensity, and direction. Specifically, the influence of initial density, stress ratio, cyclic stress amplitude, state parameter, and confining pressure were explored. The relationship between creep deformations and cyclic deformations was analysed. Special attention was given to the limitations of triaxial testing and the effect of constrained boundaries applied to the specimen. Also, the effect of changing the sample preparation method on the results of accumulated volumetric strain. The results obtained from the testing program show the degree of inherent anisotropy of Hostun sand samples prepared by moist tamping method which is lower when compared with samples prepared by pluviation (Sadek, 2006; Sunyer, 2007). The stress-induced anisotropy of Hostun sand was studied by , means of constant mean' stress and constant radial stress paths. The results indicate the influence of vertical and horizontal effective stresses and the importance of quantifying these variables in the empirical expression for the elastic stiffness Go. The drained cyclic triaxial test results discuss about the influence of the stress ratio on the accumulated strains and that the direction of the deviatoric and volumetric strains depends' almost entirely in this parameter. The normalization of the data by stress ratio, cyclic stress amplitude, and the state parameter revealed that similar volumetric deformations are encountered for tests at different confining pressures. Comparison of the test results with simulations using the Severn Trent sand model (Gajo & Muir Wood, 1999) exposed the good agreement between the model and the tests under drained and undrained monotonic stress conditions. The limitations of the model for simulating drained cyclic loading conditions were revealed .
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:659108 |
| Date | January 2014 |
| Creators | Leiva, Daniella Escribano |
| Publisher | University of Bristol |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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