Evaluation of monopile embedment using frequency, damping and modal shape analysis

The design of monopiles foundation for Offshore Wind Farms requires the characterisation of the rigidity between the
soil and the shaft of the pile. While soil investigation methods are normally used, return of experience with measured
data is rarely available. This paper presents a series of simple dynamic tests, performed on two 1:6 scaled monopiles
foundation installed in rock as part of a lateral pile test campaign. Results of the dynamic tests can be compared with
the theoretical fixed beam using frequency, damping and modal shape parameters with the aim of assessing to rigidity
of the soil-structure interaction. The tests were carried out onshore France in 2016 by FUGRO and EDF-RE for offshore
wind farms development projects as an add-on to a large-scale pile testing program. Two foundation piles of 1.2 m
diameter, partially installed into Calcarenite, received a range of lateral dynamic strokes. The dynamic response of each
pile was recorded at high frequency by accelerometers installed along the stick-up length of piles. The results were
analysed to identify the lowest resonant frequency and subsequent vibration patterns. In addition, logarithmic
decrement analyses and modal deformation analyses were performed to better understand the piles dynamic response.
Comparing those results to the theoretical values of a fixed beam provides valuable information and help to assess the
as-built connection between the rock and the piles. Such information can be used at design stage to quantify the rigidity
of interaction between the soil and the pile. It can also be used during the lifetime of the foundation to monitor the time
evolution of this connexion.