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Title: Development and calibration of cyclic loading models for monopile foundations in clays
Author: Balaam, Toby
ISNI:       0000 0005 0291 9632
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2020
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Monopiles are currently the most common foundation for offshore wind turbines. Recent research, such as in the PISA project, have produced new design methods for calculating the ultimate monotonic capacity of monopiles under lateral loading, leading to more efficient designs. However, monopiles are also subject to cyclic lateral loading from wind and waves. This aspect is not yet properly understood or accounted for in design. Better understanding of the cyclic response is important for both serviceability and fatigue design considerations, as cyclic loading can cause permanent foundation rotation, as well as changes to the stiffness and energy dissipation response. A new approach to modelling of cyclic loading, 'Hyperplastic Accelerated Ratcheting Model' (HARM), can capture pile response to cyclic lateral loading on a cycle by-cycle basis for many cycles. Previous research demonstrates that this approach to modelling compares well to experimental data at a macro-scale, such as the full pile moment-rotation response. However, for design, it would be more appropriate for calibration parameters to be developed for site-specific conditions from laboratory element tests. To facilitate this approach, a database of cyclic laboratory tests in fine-grained soils has been acquired from the literature and private communication. Results of the tests are summarised to establish important features of behaviour and inform the formulation of a 0D, multi-surface, kinematic-hardening, rate-dependent, hyperplasticity model in total stress space which is calibrated to each data set. The model is then applied to more complex cases. Firstly it is used to generate synthetic contour diagrams, well used by the industry, to demonstrate prediction of the development of strain across the full constant-amplitude cyclic stress space. Secondly it is used to predict more realistic, multi-amplitude, element tests undertaken at the Norwegian Geotechnical Institute. Both these exercises serve as tests of the underlying mechanisms of the model. At monopile scale, the PISA project resulted in an extended Winkler model with monotonic reaction curves along the length and at the base of the pile; this approach is subsequently extended to incorporate a total stress, 0D hyperplasticity model in place of each reaction curve. The resulting 1D model is capable of predicting the cyclic response along the pile length, including ratcheting. Cyclic tests at Cowden are modelled, with parameters compared to those at element level to investigate the potential mapping of response at different levels of detail. By exploring mobilisation of lateral pressure down the pile for typical operational loading, an element-testing regime is proposed to better reflect the realistic conditions around the pile. The 0D total-stress model predicts the shear behaviour for fine-grained soils well. However, the possible effects of effective stress changes are modelled by softening of the yield surfaces in the total stress model. A better approach would be to capture realistically effective stress changes due to cyclic loading. A preliminary extension of the 0D model to capture effective stresses, based on Modified Cam-Clay, is presented at development stage.
Supervisor: Houlsby, Guy ; Byrne, Byron Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available