RLT on prefab concrete piles of wind turbine foundations in tidal sands in the Netherlands

During pile driving for wind turbine foundations in pre-dominantly tidal sands, unexpected low blow counts as low as one blow per 0.25 m were observed. To restore confidence in the foundation various aspects were investigated. Within the scope of this investigation several rapid load tests (RLT) have been executed in accordance with NPR7201:2017 and ISO22477-10:2016. From these tests it was concluded that the pile bearing capacity was sufficient. The maximum applied test loads, without geotechnical failure occurring, exceeded the maximum pile bearing capacity predicted by the calculation methods of the Dutch geotechnical standard NEN9997-1. Alongside bearing capacity, the dynamic axial stiffness of the pile in unloading – reloading is of great importance in the design of piled wind turbine foundations. A new simple method has been proposed to directly extract the unloading-reloading stiffness from the RLT test