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More than 50 % of Belgium’s greenhouse gases are caused by power generation. To meet the 7.5 % cut in CO2 emissions by 2010 to which Belgium committed in the Kyoto Protocol, the country is betting heavily on wind power for generating electricity. One of the biggest windfarm projects anywhere in the European Union is currently taking shape 30 km off the Belgian coast. Its 60 turbines will generate approx. 1000 GW/h of electricity a year, saving 450,000 tonnes of CO2. This will be enough to cover 600,000 people’s annual electricity consumption. The formwork solution for casting the 44 m tall gravity base foundations of the wind power plants is being supplied by Doka Belgium.
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The Belgian project development company C-Power N.V. is in overall charge of the realisation of this windfarm project. Main contractor T.H.V. Seawind (Dredging International and Fabricom) appointed the Belgian construction firm M.B.G to carry out the forming operations on the wind power plants’ gravity base foundations. Preparations for putting the first 6 wind turbines into service in September 2008 are in full swing. The seafloor of the Thornton Bank, a sandbank off the Belgian coast, has been smoothed and prepared with strip footings on bored piles. This will serve as the base for the 44 m tall gravity base foundations (GBF), which are being pre-cast on land.
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After completion, the hollow in-situ concrete structures are hauled to the offshore site by floating cranes, and lowered onto the seabed at a depth of down to 27 m. Once filled with approx. 2000 m³ of sand, they reach a total weight of 5800 t and so are able to compensate for the forces and moments from the 184 m tall wind turbines.
Owing to their special shape, the GBF present some very tough formwork-technology requirements. Up to a height of 17 m, the 50 cm thick in-situ concrete casing rises in a conical shape. After a 1 m high transition zone with an inside reinforcement ring, the casing merges into a 23 m tall cylinder. Furthermore, at the top of this cylinder a reinforced edge and a 6 m long horizontal cantilever slab have to be cast.
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Before work can begin on the wall-forming operations, the site crew pour the 23.50 m diam. base slab. In the course of five successive casting steps, the cone rising from this base-slab tapers from 17 m to 5.5 m in diameter. For this, contractors M.B.G are using a formwork solution supplied by Doka Belgium in collaboration with the Ready-to-Use Service of Deutsche Doka’s Düsseldorf Branch. It consists of Climbing formwork MF 240 fitted with custom elements of Large-area formwork Top 50. As every single casting section of the conical zone has a different diameter, there are five separate sets of formwork in service here. However, since all the gravity base foundations are of identical construction, each of these formwork-sets can be re-used for the same casting section on every GBF tower.
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At a height of 17 m, the in-situ concrete casing finally merges into a cylindrical cross-section of diam. 5.50 m. M.B.G is forming this cylinder with three sets of Climbing formwork MF 240, in 4.50 m high casting steps. When it came to the 6 m long horizontal cantilever slab at a height of more than 40 m, the Doka Formwork Experts delved deep into their “box of tricks”: To make platforms with extremely high load-bearing capacity, they mounted 8 m high “Supporting construction frames” in the horizontal. The actual formwork solution, consisting of Staxo load-bearing towers and Top 50 large-area formwork, is then set up on these platforms. The triangular shape of the “Supporting construction frames” reliably transfers the vertical loads from the fresh concrete into the previously hardened casting section.
Project Superintendent Arnold Roos has every reason to be satisfied with the smooth workflow on this out-of-the-ordinary project: “Thanks to the high-calibre technical support we are getting from Doka Belgium, and to the just-in-time formwork deliveries, we shall be able to pre-cast the gravity base foundations onshore within the original timetable.”
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The conical-cylindrical gravity base foundations for the wind turbines on Belgium’s Thornton Bank are being efficiently pre-cast onshore by the team from M.B.G. using Doka climbing formwork MF 240.
Five separate sets of formwork are in service for the conical zone. Since all the gravity base foundations are of identical construction, each of these formwork-sets can be re-used for the same casting section on every GBF tower. |
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