The use of in-situ site investigation techniques for the axial design of offshore piles

Offshore wind turbines are typically founded on driven steel pipe piles, using either a single large monopile or multiple piles anchoring a tripod or jacket-type structure. Recent design methods for offshore driven piles have been developed based on the results of field tests using instrumented closedended model piles that allowed the measurement of the radial effective stress at a number of locations along the pile shaft. These measured radial stresses were then directly related to in-situ soil properties such as CPT cone resistance. This paper investigates the use of in-situ site investigation techniques, in particular the CPT, for the design of open-ended piles. A recent CPT-based design method, the UCD-11 (Igoe et al. 2011), which is based on field tests using an instrumented open-ended model pile, is applied in the present study to predict the axial tension capacity of two open-ended piles driven into a medium dense sand deposit. The accuracy of this recent method is compared with both traditional earth-pressure and other CPT based methods in order to assess its predictive performance.