A laboratory investigation into the behaviour of sand at low confining stresses

White, JRF

Thesis

A laboratory investigation into the behaviour of sand at low confining stresses

Abstract:: The mechanical behaviour of sands at low confining stresses may have important implications for the design of offshore wind turbine (OWT) foundations. However, previous research investigating this behaviour is limited and conflicting, and cyclic testing campaigns have historically adopted load characteristics that may be unrepresentative of those sustained by OWT foundations. This thesis endeavours to fill the gap in the literature through creation of a new experimental database of soil element tests, that explores the constitutive response of sand at low stress levels subjected to monotonic and cyclic loading.

Results of the monotonic experimental campaign indicate that while the shear strength properties (η_max and ϕ'_max) tend to increase with decreasing effective confining stress, σ'_c,0), this effect becomes increasingly minor for σ'_c,0≤ 50 kN/m². Furthermore, tests undertaken at lower confining stresses exhibit a comparatively stiffer response than those tests sheared at higher pressures. Results of the cyclic triaxial testing campaign exemplify the complexities of the response of sands subjected to cyclic loading. However, clear and systematic trends are found that can be robustly captured by the new cyclic modelling framework adopted in this study. Modern design methods for OWT foundations increasingly rely on finite element analysis (FEA); however, results depend critically on the constitutive models used, and the model parameters adopted in the numerical analyses.

This thesis reviews the state-of-the-art with regard to constitutive modelling frameworks developed for predicting sand behaviour. Evaluation of four sophisticated constitutive models found that it was not possible to determine a single set of calibrated model parameters that yield satisfactory model simulations for both drained and undrained load cases. Furthermore, only the general trends could be predicted when simulating sustained undrained cyclic loading. Further model development is therefore required if such models are to be relied upon for predicting the long-term behaviour of OWT foundations.

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APA Style

White, J. R. F. (2020). A laboratory investigation into the behaviour of sand at low confining stresses [PhD thesis]. University of Oxford.

MLA Style

White, J. RF. A Laboratory Investigation into the Behaviour of Sand at Low Confining Stresses. University of Oxford, 2020.

Chicago Style

White, JRF. 2020. “A Laboratory Investigation into the Behaviour of Sand at Low Confining Stresses.” PhD thesis, University of Oxford.
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