Te Apiti was Meridian Energy’s first New Zealand wind farm and is the largest in the Southern Hemisphere. The wind farm’s total installed capacity of 90MW generates enough clean, sustainable electricity to meet the electricity needs of approximately 45,000 homes. These large structures are now a striking feature on the local landscape. What is not so obvious is the concrete foundation that supports each structure.
Prior to the construction of the wind farm, a comprehensive geotechnical investigation was undertaken by Opus International Consultants Ltd to determine the ultimate foundation bearing capacity and soil shear modulus.
Foundations for wind turbines are low-frequency machine loaded structures subjected to coupled horizontal-rocking vibrations. Therefore, extreme loads, production loads and fatigue loads were all analysed during the design phase. To optimise the diameter and thickness of the pad, preliminary design studies were also undertaken with a simple rigid disk model.
Forces on the foundation pad were analysed using a model that indicated large variations in both bending moment and shear force across the width of the pad and these were averaged for design. Grade 500E reinforcement was used to provide the necessary flexural strength and to maximise fatigue resistance. The transfer of vertical forces from the tower steel shell into the concrete foundation via embedment of the cylinder was also resolved.
Following the exhaustive design process, engineers selected a shallow gravity pad foundation as the most appropriate design for each of the 55 turbine structures at Te Äpiti to rest upon. These concrete pad foundations are suitable for most ground conditions. The pads are a 16m wide octagonal shape with depths varying from 2.55m at the centre to 1.5m at the edges. Each pad contains 375m3 of 30MPa concrete and 28 tonnes of reinforcing steel. This is a total of 22,000m3 of concrete for the wind farm.
Due to the thickness of the foundations (2.55m maximum), they contain a huge volume of concrete. As such, there was a risk that large temperature rises associated with heat-of-hydration effects could result in thermal gradients across the depth of the pad sufficient to cause thermal cracking. The concrete supplier, Higgins Concrete Ltd, therefore designed a concrete mix to control temperature rises. This was achieved primarily by the partial substitution of the highly reactive Type GP cement with fly ash. The use of a larger 30mm aggregate also enhanced the strength properties of the concrete, and minimised thermal gradients.
Te Apiti wind farm embodies a sustainable approach to the construction of infrastructure on many levels. It generates renewable electricity without producing greenhouse gases and therefore protects the environment. Concrete foundations guarantee long-term durability within a demanding environment, while the use of a supplementary cementitious material has reduced the Portland cement content of the concrete and further enhanced the project’s contribution to sustainable development.
*Davey, R. & Green, R. (2006). Te Apiti wind farm turbine foundations: design and construction. Proceedings of The New Zealand Concrete Industry Conference. Christchurch, New Zealand.