A gravity-based structure (GBS) includes central and protruding parts having a common base slab. For the central part, outer and inner walls and a top slab are cast in concrete so that the GBS forms a rectangular prism. As casting of individual outer wall sections is completed, a reinforcement cage is assembled, formwork is installed, and protruding part outer and inner walls are cast, the outer walls being formed along the base slab perimeter with a height lower than the central part outer wall height. As separate protruding part wall sections are cast, a reinforcing cage is assembled, formwork is installed for a protruding part top slab, and then cast. Post-tensioning of central part base and top slabs and central and protruding part outer and inner walls is done as central and protruding part top slab casting is completed and the concrete is cured, increasing buoyancy and reducing submersion.

The present invention provides a foundation for a wind turbine. To reduce set-up time and to allow complete alignment of the platform which carries the base flange of the tower construction, the invention provides a foundation with a cage structure having an upper stress distribution flange connected by a plurality of tensioned anchor bolts to a lower flange. The flanges are separated by a number of distance elements whereby the shape of the cage structure becomes fixed by the combination between tensioned bolts and distance elements. Since the cage structure has a fixed shape, the upper stress distribution flange can be aligned before the cage structure is embedded in concrete, and it becomes unnecessary to wait for the concrete to harden.

A manufacturing method of a concrete pile is shown. The method includes the following, that is, pouring concrete in the pile molding space, reducing the pile molding space to compress and mold the concrete, draining water drained from the concrete by compression and molding outside of the formwork from the drainage hole, and holding the concrete a predetermined amount of time to harden the concrete. The formwork includes an outer formwork that molds an outer wall surface of the concrete pile, an inner formwork that molds an inner wall surface of a hollow space of the concrete pile, and a pair of end formwork that mold upper and lower end surfaces of the concrete pile. The drainage hole is a gap between adjacent components when the mold is tightened, and the gap is configured to be capable of being opened larger during cleaning than when the mold is tightened.

A post-tensioning system that is configured for use with concrete wind turbine foundation designs to post-tension the foundations and reduce the amount of reinforcement steel that is needed compared to conventional non-post-tensioned, steel reinforced concrete foundation designs. In the case of an inverted T gravity foundation, the post-tensioning system described herein reduces the amount of reinforcement steel that is required by 30-40 tons.