Background to the Invention Traditional wind turbine bases utilise gravity forces provided by large passive foundation resistance to prevent overturning. This passive foundation is a relatively large concrete mass and may be set in the ground at a depth of up to 10 metres. The general steps involved in construction of this passive foundation are as follows: i) a hole is excavated and inner and outer cylindrical casings of different diameters are placed in the hole; ii) the casings are coaxially aligned in a vertical disposition and a bottom and top ring, interconnected by circumferentially spaced foundation bolts, positioned in an annulus space between the inner and outer casings; iii) the annulus space is filled with concrete to partially embed the foundation bolts, and the inner casing is backfilled.

This traditional wind turbine foundation and construction technique suffer from at least the following drawbacks: i) construction of the passive foundation requires significant excavation which is time consuming and expensive; ii) the passive foundation is relatively heavy which contributes not only to material costs but also to transportation costs which may be significant for construction in remote locations; iii) the"footprint"imposed by the passive foundation and the depth at which it is embedded impacts adversely on the surrounding environment.

Summary of the Invention According to one aspect of the present invention there is provided a reinforced concrete foundation comprising: a concrete slab being reinforced with a plurality of reinforcing bars; one or more tensioned tendons being operatively coupled to the concrete slab; and one or more ground anchors each being operatively coupled to the concrete slab and adapted to engage the ground adjacent the concrete slab so that it is effectively anchored.

Preferably the tensioned tendons are post-tensioned tendons. More preferably the post-tensioned tendons each include an elongate duct housing multiple tension strands to be tensioned. Even more preferably the ducts each extend through the concrete slab and the tension strands protrude from the corresponding duct to permit tensioning and grouting of the duct.

Preferably the ground anchors are each in the form of a permanent rock anchor located within an anchor hole formed in the ground and underlying the concrete slab.

More preferably the permanent rock anchor includes an elongate sheath housing one or more stress strands to be stressed, and an internal grout tube located within the sheath for grouting of the sheath. Even more preferably the rock anchor also includes an external grout tube located about the sheath for grouting of an annulus between the sheath and the anchor hole.

Preferably the foundation also comprises a mounting element being partly embedded in the concrete slab, an exposed portion of the mounting element being adapted for connection to a structural member such as a tower of a wind turbine generator. More preferably the mounting element is shaped cylindrical or oval and complementary to a corresponding bottom portion of the tower.

Preferably the plurality of reinforcing bars together form a reinforcement structure which is configured to strengthen the concrete slab in the vicinity of the mounting element.

More preferably the cylindrical mounting element together with the associated reinforcing structure is of a"can design".

According to another aspect of the invention there is provided a method of constructing a reinforced concrete foundation, said method comprising the steps of: assembling a reinforcement structure of reinforcing bars, the reinforcement structure being in the general configuration of the concrete foundation; positioning one or more tendons within the reinforcement structure; erecting formwork about the reinforcement structure, the formwork defining a formwork space; pouring concrete in the formwork space to form a concrete slab; drilling one or more anchor holes in the ground in the vicinity of the reinforcement structure; locating one or more ground anchors in respective of said one or more anchor holes and extending into the reinforcement structure; tensioning the tendons within the concrete slab so that it is stressed.

Preferably the step of tensioning the tendons involves post-tensioning of the tendons after the concrete is cured. More preferably the tendons each include an elongate duct which houses multiple tension strands and wherein the step of post-tensioning of the tendons also involves grouting of the duct to embed the tensioned strands.

Preferably the method also comprises the step of stressing the ground anchors.

More preferably the ground anchors are each in the form of a permanent rock anchor including an elongate sheath housing one or more stress strands and an internal grout tube, and wherein the step of stressing the ground anchors involves grouting of the sheath via the internal grout tube and thereafter stressing the respective strands. Even more preferably the rock anchor also includes an external grout tube located about the sheath, and wherein the step of stressing the ground anchors also involves grouting via the external grout tube an annulus space between the sheath and the anchor hole.

Preferably the method further comprises the step of, prior to pouring of the concrete, positioning a mounting element within the reinforcement structure. More preferably the mounting element is partly embedded in concrete poured into the formwork space.

Preferably the concrete slab is in the form of a concrete anchor cap. More preferably the structural member is a tower or column such as a wind turbine tower mounted to the concrete anchor cap.

Brief Description of the Drawings In order to achieve a better understanding of the nature of the present invention a preferred embodiment of a reinforced concrete foundation and a method of constructing the foundation will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a perspective view of a reinforced concrete foundation according to an embodiment of the invention; Figure 2 is a perspective view of the foundation of figure 1 excluding the tensioned tendons and ground anchors; Figure 3 shows in plan and sectional views the reinforcement structure of the foundation of figures 1 and 2; Figure 4 shows in plan and sectional views the post-tensioning details of the tendons of the foundation of figure 1; and Figure 5 shows in plan and sectional views the ground anchor detail of the foundation of figure 1.

Detailed Description of the Preferred Embodiment As best shown in figure 1 there is a reinforced concrete foundation 10 which in this embodiment is a tower foundation for a wind turbine generator (not shown). The wind turbine generator (WTG) foundation broadly comprises a concrete slab 12, and a plurality of tensioned tendons and ground anchors such as 14A and 16A, respectively. The concrete slab 12 (of which the base only is shown for clarity) includes a reinforcement structure designated generally as 18 of interconnected reinforcing bars (not individually designated).

In this example the WTG foundation 10 is in plan view shaped octagonal. The WTG foundation 10 includes a mounting element in the form of a cylindrical"can"20 which is partly embedded within the concrete slab 12. The can 20 is coaxially aligned with

the concrete slab 12 and is shaped complementary to a corresponding bottom portion of the WTG tower (not illustrated). The WTG tower is in a traditional manner welded to the can 20.

The WTG foundation of this particular construction includes ten (10) of the tensioned tendons 14A to 14J and eight (8) of the ground anchors 16A to 16H. The tendons 14A to 14H are straight and each extend in a horizontal plane between opposing perimeter faces of the concrete slab 12. The other tensioned tendons 14I and 14J are each arcuate or semi-circular in shape and together extend circumferentially about the ground anchors 16A to 16H within the concrete slab 12. The ground anchors 16A to 16H are vertically disposed and equally spaced circumferentially about the concrete slab 12.

As best shown in figure 3, the reinforcement structure 18 primarily includes reinforcement in four (4) layers, namely: i) an"under can"reinforcement structure 30 including a multiplicity of radially extending reinforcing bars such as 32 interconnected by a plurality of circumferentially extending and radially spaced lacers such as 34; ii) a"bottom reinforcement"structure 36 including a multiplicity of radially extending reinforcing bars such as 38 interconnected by a plurality of circumferentially extending and radially spaced lacers such as 40, and a central mesh reinforcement 42; iii) a"top reinforcement"structure 44 including a multiplicity of radially extending reinforcing bars such as 46 interconnected by a plurality of circumferentially extending and radially spaced lacers such as 48 and a central mesh reinforcement 50; and iv) an"above can foot"reinforcement structure 51 including a multiplicity of radially extending pairs of reinforcing bars such as 52 and 54 located inside and outside, respectively, the can 20 and being interconnected by a plurality of circumferentially extending and radially spaced inner and outer lacers 56 and 58, respectively.

The radial reinforcing bars such as 32,38, 46,52 and 54 are typically fabricated as U-bars which interconnect and separate adjacent layers of the

reinforcement structure 18. The can 20 includes a lower flange 60 which is in a horizontal plane and protrudes radially inward and outward of a peripheral wall 62 of the can 20. The can 20 is supported within the reinforcement structure 18 via a plurality of circumferentially spaced adjustable feet 64A to 64C which locate under the flange 60. The reinforcement mesh 42 of the bottom reinforcement structure 36 is octagonal in shape and roughly of the same size as the can 20. The reinforcement mesh 50 of the top reinforcement structure 44 is square-shaped and sized so as to locate within the can 20.

As best shown in figure 4, the WTG foundation 10 includes the ten (10) tensioned tendons 14A to 14J. Although not illustrated each of the tendons such as 14A is in the form of an elongate duct housing multiple tension strands. The straight tendons 14A to 14H extend through the concrete slab 12 in two (2) sets each extending between opposing faces of the octagonal concrete slab 12. Each of the tendons such as 14F is at a"dead end"anchored at a perimeter face of the concrete slab 12 whereas an opposite"live end"of the tendon 14F is connected to an anchorage mounted within an opposing face of the concrete slab 12. The"live end" anchorage can in a conventional manner be connected to a tensioning device such as a multistrand jack for tensioning of the multiple tension strands. In this embodiment the tendons such as 14F are post-tensioned following curing of the concrete slab 12. The arcuate tendons 14I and 14J are each semi-circular being fixed at a dead end and at an opposite live end being connected to a live end anchorage.

The semi-circular tendons 14I and 14J together surround the ground anchors 16A to 16H in a circular shape.

As shown in the detailed sectional view of figure 4, the can 20 includes opposing pairs of openings such as 66F for the passage of the corresponding tensioned tendon 14F. In this example, the duct of the tendons such as 14F are each wrapped with a protective sheeting, such as a HDPE material, in order to isolate the tendon 14F from the can 20. As shown in the detail and sectional views of figure 4, "antiburst"reinforcement designated generally as 68E and 68F is provided at each end of the respective tendon 14E and 14F. This additional reinforcement increases the strength of the concrete slabl2 in the vicinity of the dead and live ends of the tendons such as 14F. This additional reinforcement is interconnected via

reinforcing bars such as 70 and connected via additional reinforcing bars such as 72 to the bottom reinforcement structured 36 of figure 3.

As best shown in figure 5, the WTG foundation 10 includes the eight (8) ground anchors 16A to 16H. Each of the ground anchors is in the form of a permanent rock anchor such as 16A located within a corresponding anchor hole such as 74A drilled in the ground and underlying the concrete slab 12. The permanent rock anchors such 16A each include an elongate sheath 76A housing one or more stress stands 78A. The rock anchor 16A also includes an internal grout tube 80A and an external grout tube 82A. The internal grout tube 80A is wrapped about the stress strands 78A within the sheath 76A whereas the external grout tube 82A is wound about the sheath 76A in an annulus between the sheath 76A and the anchor hole 74A. The rock anchors such as 16A each include a bearing plate 84A connected to an upper end of the sheath 74A, and an anchor head 86A bearing against the bearing plate 84A and within which end portions of the strands 78A are retained after tensioning.

The permanent rock anchors 16A to 16H of this embodiment are circumferentially spaced about the concrete slabl2 approximately midway between the can 20 and a perimeter of the slab 12. The rock anchors 16A to 16H are located adjacent respective of the perimeter edges of the slab 12 formed at the intersection of the adjacent faces. The rock anchors such as 16A are vertically oriented within the concrete slab 12 and the reinforcement structure 18 includes additional reinforcement such as 88 about an upper perimeter portion of the rock anchors such as 16A.

The general steps involved in construction of this WTG foundation 10 are as follows: i) the ground is excavated at the site of the WTG, and a concrete base or blinding slab is poured; ii) foundation markings are setout on the blinding slab and the"under can"reinforcement structure 30 is layed out and tied; iii) the mounting element in the form of the can 20 is located on the adjustable feet 64A to 64C which are positioned on the blinding slab;

iv) the remaining reinforcement structure 18 is assembled around the can 20 and its associated service ducting; v) the duct of the tendons 14A to 14J and the sheath of the ground anchors 16A to 16H are positioned within the reinforcement structure 18; vi) formwork is erected about the reinforcement structure 18; vii) the tendons such as 14A are positioned within their respective ducts; viii) concrete is poured within the formwork so as to form a concrete slab in which the reinforcement structure 18 and part of the can 20 are embedded; ix) the required number of ground holes such as 74A are drilled in the blinding slab and the ground to a precalculated depth; x) the prefabricated ground anchors 16A to 16H are located vertically within the reinforcement structure 18 and positioned vertically within the respective ground holes such as 74A; xi) the ground anchors such as 16A are each grouted; and xii) the tendons 14A to 14J are post-tensioned and grouted, and thereafter the ground anchors 16A to 16H also post-tensioned.

In this embodiment the concrete slab is in the form of a concrete anchor cap 12. The tower of a WTG is positioned on top of and butt welded to the can 20.

The concrete anchor cap 12 with the associated reinforcement structure is of a significantly reduced volume compared to traditional passive foundations. In this embodiment the foundation mass is around 70 percent lighter than that of the traditional passive foundation and minimal excavation is required with the WTG foundation 10 being at a depth of around 1.4 metres and a diameter of approximately 7.0 metres.

Fabrication of the reinforcement structure 18 is configured to strengthen the concrete slab or anchor cap 12 in the vicinity of the can 20. The reinforcement structure 18 is concentrated about those regions of the concrete anchor cap 12 which according to structural design criterion and finite element analysis experience the maximum stress. This area of maximum stress is located about the mounting element or can 20 of this example. The fabrication of the reinforcement structure 18 is otherwise performed using traditional techniques in the field of reinforcing steel tying.

The ground anchor holes such as 74A are drilled to a predetermined depth as disclosed in the following example of an"anchor schedule". Each of the ground anchor holes such as 74A is water pressure tested.

In this example the formwork includes a series of steel panels which interlink and lock-up about the perimeter of the reinforcement structure 18. The concrete is poured and vibrated within the formwork space and about the reinforcement structure 18 and can 20. This pouring and vibration of the concrete is otherwise a known construction technique.

Once the concrete anchor cap 12 has sufficiently cured, the tendons 14A to 14J and the ground anchors 16A to 16H are post-tensioned. The tendons 14A to 14J are stressed or tensioned according to the sequence of the following "post-tensioning schedule". In this example, the jacking force per tension strand is 212.5kN. The ground anchors are then stressed according to the sequence and loads specified in the following"anchor schedule".

The tendons 14A to 14H are grouted once they have been post-tensioned.

The ground anchors are on the other hand internally and externally grouted via grout tubes such as 80A and 82A, respectively, and then post-tensioned. These grouting processes are performed in accordance with known techniques in the art.

ANCHOR SCHEDULE No. Angle of No. of SWL Test Load Lock Off Free Length Bond Length Hold Depth Anchlor Head Stressing Inclination 15.2mm kN kN Load kN (m) (m) (m) Detail (mm) Sequence (1.5xSWL) 1. 90 18 2550 3825 I2900 8.0 8.0 16.5 450x450x70 First 2. 90 18 2550 3825 2900 8.5 8.0 17.0 450x450x70 Sixth 3. 90 18 2550 3825 2900 9.0 8.0 17.5 450x450x70 Third 4. 90 18 2550 3825 2900 9.5 8.0 18.0 450x450x70 Last 5. 90 18 2550 3825 2900 10.0 8.0 18.5 450x450x70 Second 6. 90 18 2550 3825 2900 10.5 8.0 19.5 450x450x70 Fifth 7. 90 18 2550 3825 2900 11.0 8.0 19.5 450x450x70 Fourth 8. 90 18 2550 3825 2900 11.5 8.0 20.0 450x450x70 Seventh POST TENSIONING SCHEDULE No. Angle of No. of Stressing Duct Size Inclination 15. 2mm Sequence & Type Strands 1. 90 7 Seventh 60 ID GI 2.90 8 Third 60 ID GI 3.90 8 Fourth 60 ID GI 4. 90 7 Eighth 60 ID GI 5. 90 7 Ninth 60 ID GI 6.90 8 Fifth 60 ID GI 7.90 8 Sixth 60 ID GI 8.90 7 Last 60 ID GI 9.90 First 60 ID HDPE 10.90 6 Second 60 ID HDPE

Now that a preferred embodiment of the present invention has been described in some detail it will be apparent to those skilled in the art that the reinforced concrete foundation and its method of construction have the following advantages: i) the reinforced concrete foundation provides a significant reduction in the foundation"footprint"as compared to a traditional passive foundation; ii) the reinforced concrete foundation including tensioned tendons and ground anchors has a significantly reduced volume compared to the traditional passive foundation and thus requires reduced excavation; iii) materials and transportation costs are reduced with the reduction in foundation mass;

iv) adverse environmental impact is minimised as a result of the reduced footprint imposed by this foundation compared to the traditional passive foundation; v) the construction methodology reduces the time involved in construction of the reinforced concrete foundation due to the reduced volume of excavation and the particular construction technique adopted; and vi) the reinforced concrete foundation including the post-tensioned tendons and the associated reinforcement structure reduces the number of penetrations to the mounting element or can.

Those skilled in the art should appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. For example, the configuration of the reinforcement structure and the number of tensioned tendons and ground anchors may vary from that described but still remain within the ambit of the present invention. Furthermore, the specific method of constructing the reinforced concrete foundation may vary provided it includes incorporation of ground anchors and tendons as broadly defined in this specification. For example, the reinforced concrete slab may be prefabricated at a workshop/factory and then transported to site for ground anchoring and post-stressing. This may in particular for remote locations be an attractive alternative to the in-situ construction technique described. The reinforced concrete foundation need not be limited to WTG towers but may extend to other structural columns or towers.

All such variations and modifications are to be considered within the scope of the present invention, the nature of which is to be determined from the foregoing description.