Cyclic loading and rate effects for monopile foundations in saturated sand

Thesis

Monopile foundations remain the most popular fixing solution of offshore wind turbines. As turbines and monopile sizes continue to increase in size, refined design solutions are required to ensure optimisation and cost efficiency. Instantaneous drainage of excess pore pressures is currently assumed in design approaches, disregarding the possibility of partial drainage. In medium-dense to dense sands, partially drained loading conditions are investigated as part of this work through 1g model experiments in two different saturated sand set-ups. 1g model tests conducted in dry and oil-saturated 14/25 Leighton Buzzard sand and dry and water-saturated M4 Silica Flour provide a wide range of permeabilities to test rate dependent monotonic behaviour and different aspects of cyclic response. Scaling laws implemented for both set-ups are outlined, where the scaling of diffusion relative to dynamic time is prioritised over strength parameters in the saturated tests. Extended characterisation of silica flour as a testing material is provided, and a full description of the experimental apparatus previously developed by Wu (2022) is outlined. Test results show strong loading rate effects in both saturated set-ups during monotonic push-over tests. A series of constant and variable rate tests are reported in both materials. Pore pressure measurements show strong negative induced excess pore pressures during monotonic loading, resulting in enhanced pile capacity. Cyclic tests investigate a wide range of parameters including loading amplitude, asymmetry, frequency and post-cyclic response. In addition to the 1g tests, some theoretical development using Rate Process Theory and Hyperplasticity Accelerated Ratcheting Model (HARM) is provided. A brief comparison to the medium-scale field tests conducted at a sand site in Cuxhaven, Germany is made. The main outcome of this research is to further inform the revision of design guidance of offshore piled foundations, as part of the PICASO project, in medium-dense to dense sands.

Authors: Keane, R

• DOI: 10.5287/ora-00wzaor60
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: sand; partial drainage; cyclic loading; rate effects; monopile; PICASO
• Subjects: Design and construction; Offshore structures; Geotechnical engineering