Abstract:
An improved method of making a foundation for a wind turbine comprises: 
     a) forming a pit, 
     b) providing a cylindrical open-bottomed steel can, 
     c) lowering the can into the pit, 
     d) accurately aligning the steel can, 
     e) concreting the steel can in position in the pit, and 
     f) back-filling the interior of the can.

Description:
FIELD OF THE INVENTION 
     This invention relates to a method of making a foundation and is specifically concerned with the making of a foundation for a wind turbine. 
     A currently accepted method of making a foundation for a wind turbine is described in U.S. Pat. No. 5,586,417, to which reference should be made. The foundation is constructed of cementitious material poured in situ between inner and outer cylindrical corrugated metal pipe shells. The foundation is formed within a ground pit which is externally and internally back-filled. 
     An anchor ring is embedded in the lower end of the foundation and sets of inner and outer circumferentially spaced bolts have their lower ends attached to the anchor ring and their upper ends projecting outwardly of the top of the foundation. The upper ends of the bolts pass through holes in a base flange of a tubular tower resting on the foundation and nuts are threaded downwardly upon the upper ends of the bolts and against the base flange. 
     An essential feature of this foundation is that the bolts are all pre-stressed. This type of foundation is accordingly often referred to as a “tensionless” tube. 
     It is an object of the present invention to provide a method of making a foundation, particularly a foundation for a wind turbine, which is more economical than the method of U.S. Pat. No. 5,586,417. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided a method of making a foundation, particularly a foundation for a wind turbine, said method comprising: 
     a) forming a pit, 
     b) providing a cylindrical open-bottomed steel can, 
     c) lowering the can into the pit, 
     d) accurately aligning the steel can, 
     e) concreting the steel can in position in the pit, and 
     f) back-filling the interior of the can. 
     Concreting of the can in position in the pit is preferably effected in a first stage and a second stage, the first stage involving the introduction of concrete into the bottom of the pit to a depth sufficient to form a concrete layer in the open lower end of the can and around the lower end of the can, and the second stage involving the subsequent introduction of concrete around the can to substantially ground level. The second stage will preferably be carried out at least twenty four hours after the first stage to allow time for setting of the concrete introduced in the first stage. 
     The open-ended steel can preferably includes a plurality of sections of different wall thickness, with the lowermost section being of the lowest wall thickness and with the upper section of greatest wall thickness. The steel can may, for example, comprise three sections of different wall thickness. For a steel can having a depth of eight metres, there may be an upper section having a depth of 2 metres and a wall thickness of 23 mm., an intermediate section having a depth of 3 metres and a wall thickness of 18 mm., and a lowermost section having a depth of 3 metres and a wall thickness of 16 mm. It will be appreciated that the depths of the sections and the wall thickness can be varied depending on the required parameters for the finished foundation. 
     The pit is preferably formed by excavation and is preferably of substantially square cross-section. However, as opposed to excavating a pit of substantially square cross-section using a conventional excavating machine, it is possible to drill a circular hole, of greater diameter than the external diameter of the steel can. 
     Back-filling of the interior of the can may be effected using the as-dug excavated material. Compaction of the as-dug excavated material will not normally be required. Once the interior of the can has been filled to the required level, a layer of concrete is preferably laid on top of the in-fill material. 
     The can is preferably supported, during lowering thereof, from a three-point levelling support frame. 
     The upper end of the steel can is provided with fixing means designed to facilitate connection of the structure being supported by the foundation to the steel can. Such fixing means may comprise, for example, an inwardly extending flange at the upper end of the steel can, which flange is formed with a plurality of angularly spaced apertures to receive fixing bolts for connection of the flange at the upper end of the steel can to a corresponding flange at the lower end of the structure. 
     Other connection means may, of course, be provided, particularly if the structure is a wind turbine. For example, the upper end of the steel can may, when installed, extend above ground level to an extent such as to facilitate welding of the upper end of the steel can to the base of the wind turbine. 
     The foundation will thus function as a monopile and, in a typical example, the steel can will have a depth of 8 metres and an outside diameter of 3 metres. The pit which is formed to receive the steel can will then be of square configuration with minimum dimensions of 3.3×3.3 metres square and 7.85 metres deep and maximum dimensions of 4×4 metres square and 8 metres deep. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of a completed monopile foundation, and 
     FIG. 2 is a plan view showing a support and levelling frame used in producing the foundation. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows a cylindrical steel can  10  installed in a pit  11  ready for attachment of the base of the tower of a wind turbine (not shown) to an internally directed flange  12  at the upper end of the can  10 . As can be seen from FIG. 1, the upper end portion of the can  10  is above ground level so that access can readily be obtained to the flange  12  which is formed with a plurality of angularly spaced apertures to enable the flange  12  to be bolted to a corresponding inwardly directed flange or flanges at the base of the tower of the wind turbine. For example, there may be a double flange at the base of the tower with a double row of bolts. Alternatively, the upper end portion of the can  10  may be welded to the base of the tower, the upper end portion of the can  10  then extending above ground level to an extent such as to facilitate welding of the uppermost portion of the can  10  to the base of the tower. The steel can  10  is open at both top and bottom. 
     The pit  11  is formed by means of an excavator and, to receive a can  10  which has a height of 8 metres and an external diameter of 3 metres, the pit  11  will have a depth which is between 7.85 and 8 metres and will be from 3.3×3.3 metres square to 4×4 metres square. As an alternative to forming the pit  11  by excavation, it may be formed by drilling and will then have a depth within the range indicated above and a diameter within the range of from 3.5 to 4 metres. 
     The steel can  10  is in the form of a shell of varying thickness, with the thickness of the shell being greatest at the top of the can  10 . Thus, for a can  10  having a height of 8 metres, the can  10  may be formed as three interconnected sections, with the uppermost section having a height of 2 metres and a wall thickness of 23 mm., a middle section having a height of 3 metres and a wall thickness of 18 mm., and a lowermost section having a height of 3 metres and a wall thickness of 16 mm. 
     After the pit  11  has been formed, a three-point levelling support frame  13  is positioned above the open mouth of the pit  11 . The frame  13  has three feet  14  which are each positioned at least 1 metre from the adjacent edge of the pit  11  and each foot  14  includes a screw jack and a bearing pad so that the feet  14  can be adjusted independently of one another. At the top of the frame  13 , suspension means (not shown) is provided for supporting the steel can  10  during lowering thereof into the pit  11 . Such suspension means is of known construction and standard control means is provided for controlling lowering of the steel can  10  into the pit  11 . 
     The steel can  10  is lowered into a position in which the lower edge of the can is spaced a nominal 0.1 metres above the base of the pit  11 . Adjustment of the position of the can  10  is then effected to ensure that the central longitudinal axis of the can  10  is accurately aligned with the vertical. Once the alignment of the can  10  has been verified, a layer  15  of C7.5/H7.5 concrete is introduced into the base of the pit  11 , such concrete being so introduced that it is situated both within the lower end portion of the can  10  and outside the lower end portion of the can  10 . The layer  15  has a minimum depth of 1 metre and is such that the lowermost portion of the steel can  10  is embedded in the concrete layer  15  with the concrete layer  15  extending upwardly from the lower edge of the can  10  by a distance of at least about 0.75 metres. 
     The initial layer  15  of C7.5/H7.5 concrete is allowed to set for a minimum of twenty four hours before the annular space around the steel can  10  is filled with C7.5/H7.5 concrete to form a concrete sleeve surrounding the can  10  and extending upwardly from the concrete layer  15  to substantially ground level. The interior of the can  10  is back-filled using the as-dug excavated material. Compaction is not normally required. 
     It is to be noted that there is a 200 mm. diameter hole in the wall of the can  10  about 1 metre from the upper edge of the can  10  and that, before all the concrete has been introduced around the can  10  and before back-filling of the interior of the can  10  has been completed, a cable duct  16  is passed through this hole. The cable duct  16  is for the provision of services to the interior of the turbine. 
     After the interior of the can  10  has been filled to the required depth with the as-dug excavated material, a nominally 150 mm. thick slab  17  of C25/H25 concrete incorporating a steel mesh is laid on top of the in-fill material. The cable duct  16  passes through the slab  17 , which has its upper surface a nominal 160 mm. below the upper edge of the can  10 . 
     The arrangement shown in the drawings is given purely by way of example and many modifications thereof are possible. The foundation loads for the specific construction having the dimensions set out above are as follows: 
     Moment=14400 kN. 
     Shear force=305 kN. And 
     Vertical=866 kN. 
     Important advantages of the present invention as compared to that disclosed in U.S. Pat. No. 5,586,417 are as follows: 
     a) reduced overall foundation costs, 
     b) minimum usage of concrete and therefore reduced costs of handling of concrete to (remote) sites, 
     c) it is possible to use low-strength concrete rather than high-strength concrete and therefore obtaining a cost saving, 
     d) off-site pre-fabrication of the monopile steel can provides further cost savings, and 
     e) installation of the foundation involves few elements and no complex procedures, thereby speeding up installation which again leads to reduced costs. 
     The following details and/or options may be included in the specific method described above depending on the site conditions and/or performance criteria: 
     a) The use of shear studs, welded seams or any other form of shear connector on the outside of the steel can  10  to improve the shear resistance between the shear can  10  and the concrete sleeve, 
     b) Back-filling of the inside of the steel can  10  with concrete and/or cement-bound hardcore or selected back-fill in lieu of soil, 
     c) The use of a reinforced concrete plug at the base of the steel can  10  and a reinforced concrete slab at the top of the steel can  10  to limit distortion of the steel can  10 , lateral deflection of the steel can  10  and stress concentration in the steel can  10 , 
     d) The use of flange stiffeners and/or gussets to limit distortion of the steel can  10 , lateral deflection of the steel can  10  and stress concentration in the steel can  10 , 
     e) The use of compacted hardcore, granular material, selected back-fill, cement-bound hardcore or cement-bound selected backfill around the outside of the steel can  10  to fill the excavation, 
     f) The internal slab  17  may be set lower in the steel can  10  to provide additional space for location of a transformer and/or a control panel and/or other equipment. The floor over the internal slab  17  may be a steel grating or the like, 
     g) The use of a reinforcement in the concrete sleeve around the steel can  10  to control cracking, distribute stresses and the like, and 
     h) The use of a nominal flange at the bottom of the steel can  10  to improve the end bearing resistance.