Civil Engineering Association

Design of Foundations For Wind Turbines

This thesis is examining the foundations for onshore wind turbines where both the more convential method with a large concrete slab are investigated, but also alternative foundation methods are studied, mainly piled foundations. Different types of foundations is presented and discussed in which the design procedure consists of both manual calculations and numerical analyses. A case study of an 80 meter high wind turbine with realistic loads is presented. The study includes geotechnical and structural design for three different soil profiles, in which three different foundation methods are used.

The three cases are:
1. Strong and stiff moraine soil in which the most common foundation method with a spread
foundation is used.
2. A 20 m thick layer of clay that overlay the strong bedrock in which toe-bearing precast
concrete piles are used. In this case only the piles are assumed to bear the load.
3. Clay soil with the bedrock at considerable depth in which precast concrete piles are used as
cohesion piles. Both piles and the concrete slab are assumed to bear load in a so-called
piled-raft foundation.

For the three cases above, the same foundation slab is used, but for case 2 and 3 the slab is cast
on piles. The results of this study show that all three above-mentioned foundation methods are feasible, but for the third case the differential settlements are significantly big resulting in a horizontal displacement of the tower's top of 155 mm. The first case is the cheapest and easiest to perform, and is preferred if the geotechnical conditions permit that. The second case results in a relative small total pile length of 680 m, while the third case results in 3720 m in total pile length.

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DESIGN OF FOUNDATIONS FOR WIND TURBINES