The design of a large diameter monopile depends heavily on soil strength and so certainty of knowledge of this property is critical to cost-effective design. Monopiles are often designed based solely on the measurements of a single CPT positioned at their center, and the soil property within the zone of influence of the pile is normally assumed to be laterally homogeneous and equivalent to the CPT measurement at the same depth. The objective of this paper is to assess the whether this design approach is robust by quantifying the reliability (annual probability of failure) of the tensile capacity of a single large diameter open-ended pile in spatially variability siliceous sand. This paper provides a preliminary understanding of how the reliability of the uplift capacity of a large diameter monopile in different spatially-variable soil conditions could vary from the assessed capacity using data from a single CPT. The UWA05 method combined with random field theory is employed to explore the uplift capacity of piles embedded in random soil with a typical scale of fluctuation and different coefficient of variation of cone tip resistance. Approximately 500 random field realizations were performed for each set of soil conditions in order to characterize the results statistically. For each realization, the pile size was selected to fulfil the design requirements based on the CPT data and the 'real' capacity for that pile size was calculated for comparison. The results show that using this design approach to quantify capacity may result in lower than code-tolerated foundation reliabilities even for typical soil conditions.
Y. Cai, M. F. Bransby and C. Gaudin
Keywords: Offshore monopiles, Probabilistic design methodologies, Site characterization
Publisher: DFI