Wilstedt

Wilstedt-Süd wind farm

Pile foundations using Franki piles

Deep foundations for wind turbines on peatland

The foundations of a wind turbine must withstand high and varying loads. These include the turbine’s own weight, dynamic loads caused by wind, as well as the effects of fluctuating groundwater levels. In the Wilstedt-Süd wind farm, these challenges were compounded by demanding geotechnical conditions. The subsoil consists of peat, mud and silt. These soil layers are highly susceptible to settlement and did not provide a sufficient foundation for shallow foundations. Bearable sands were only encountered in the foundation area at depths of between six and twelve metres. The foundation concept therefore called for deep pile foundations using Frankipiles NG®.

336 Frankipiles NG® for six wind turbines

PORR special civil engineering manufactured a total of 336 Frankipiles NG® for the foundations of the six wind turbines. The cast-in-place concrete driven piles have a diameter of 51 cm and reach lengths of up to 16 m. The Frankipile NG® is suitable for transferring high loads into deeper, load-bearing soil layers. The number, length and pile cap volume were determined in accordance with the ground conditions and the structural requirements of the individual wind turbines. The enlarged pile cap facilitates the transfer of loads into the load-bearing subsoil.

Pre-compacting the gravel improves the bearing capacity of the subsoil

In some areas, the ground still did not exhibit sufficient bearing capacity even at the planned pile depth. Before the actual pile driving commenced, gravel was therefore compacted below and above the planned pile depth. Pre-compacting the gravel specifically compacted the soil in the area of the pile foot. The subsoil improvement complemented the pile foundation and was adapted to the geotechnical conditions of the moorland site.

Pile structural analysis and special civil engineering from a single source

In addition to manufacturing the Frankipiles NG® piles, the special civil engineering planning team undertook the pile structural analysis. This ensured that the design and execution of the foundations were tailored to the loads from the wind turbines as well as to the local ground conditions. The client for the foundation works was the wind energy division of the Max Bögl Group. Nordex N149 wind turbines were used. The project developer, wpd onshore GmbH, already operates nine wind turbines at the Wilstedt site.

Foundation solution for the Wilstedt-Süd wind farm

The combination of Frankipiles NG®, an extended pile base and pre-compacting the gravel enabled a deep foundation tailored to the settlement-prone subsoil. A total of 336 piles formed the foundation base for six wind turbines at the Wilstedt-Süd wind farm. The project demonstrates how pile foundations and supplementary subsoil improvements can be combined when erecting wind turbines on peat soil.