NO20171562A1

NO20171562A1 - A foundation for a tower structure and a method for constructing a foundation for a tower structure

Info

Publication number: NO20171562A1
Application number: NO20171562A
Authority: NO
Inventors: Svein Kvernstuen; Oddvar Lerbrekk; Christian Lindgren Jensen
Original assignee: Grenen Maskf Aps
Priority date: 2017-09-29
Filing date: 2017-09-29
Publication date: 2019-04-01

Description

A FOUNDATION FOR A TOWER STRUCTURE AND A METHOD FOR CONSTRUCTING A FOUNDATION FOR A TOWER STRUCTURE

The invention concerns a foundation for a tower structure as well as a method for constructing a foundation for a tower structure.

A tower structure, such as a wind turbine or transmission tower etc., needs to be securely fastened in the ground to avoid tipping over. A typical foundation for an onshore wind turbine in rock comprises a plurality of solid rods, typically 42 mm in diameter and 12 to 15 m in length, which anchors a base plate to the ground. The solid rods are placed in boreholes drilled in the ground. In order to increase the anchoring volume of the solids rods, the boreholes are drilled at a non-vertical angle, and the rods are grouted in place by pumping grout from the surface prior to insertion of the rods. An upper end portion of the solid rods are threaded, and nuts are used to secure the base plate, typically a large metal ring, to the ground. This method has a few drawbacks though.

Since the boreholes are drilled at a non-vertical angle from the surface, the top of the rods will not be perpendicular to the base plate, which is typically arranged horizontally. Therefore, a wedgeshaped spacer must be inserted between the nut and the top surface of the base plate, and the drilling angle must be very precise. Drilling of large, non-vertical boreholes is cumbersome and therefore expensive. Additionally, the long solid rods are not easy to handle during the installation process, and it is costly to machine the threads onto such long rods. Finally, when filling grout from the top there is a risk of contaminating the grout, as well as minimal control of which depth the grout ends up at. A contaminated grout may result in insufficient bonding between the rod and the wall of the borehole and could lead to an unstable structure.

DE102014006581 A1 discloses a foundation for a tower structure, where the foundation comprises a spherical cavity, and where solid rods extend radially out from said spherical cavity, thus avoiding the use of wedge-shaped spacers. However, even though drilling is performed perpendicularly to the surface of the spherical cavity, the boreholes must still be drilled at a non-vertical angle.

US3526069 A discloses an anchoring device where a single pile member is driven into the ground, and bended rods are extruded from this pile member to serve as hooks. The rods may be hollow to allow a liquid, such as concrete, to be pumped through said rods. The single pile member and relatively small hooks of this anchoring device makes it unsuitable for a large tower structure.

The invention disclosed herein has for its object to remedy or to reduce at least one of the drawbacks of the prior art, or at least provide a useful alternative to prior art. The object is achieved through features, which are specified in the description below and in the claims that follow. The invention is defined by the independent patent claims, while the dependent claims define advantageous embodiments of the invention.

A first aspect of the invention is related to a foundation for a tower structure, said foundation comprising a baseplate for supporting the tower structure and anchoring means for fastening the base plate to the ground, wherein the anchoring means are flexible anchoring means for extending into boreholes having a nonlinear portion. The foundation is particularly suitable for a tower structure such as an onshore wind turbine, a transmission tower, a pylon, a radio mast, a bridge requiring rock anchors, or any structure requiring being anchored to the ground. Such a structure will herein be referred to as a tower structure.

The flexible anchoring means may be sufficiently flexible to follow the curvature of the nonlinear portion of the boreholes. Thus, the boreholes may be drilled with any method capable of drilling boreholes having a nonlinear portion. For example, curved holes may be drilled using directional drilling, which is a well-known method from the oil and gas industry. Each borehole may have at least one corresponding flexible anchoring means. The nonlinear portion of the boreholes may also increase the friction between the anchoring means and said boreholes.

The flexible anchoring means may comprise a tube designed for withstanding tension forces from the tower structure. For example, the tubes may be steel braided tubes. The flexible anchoring means typically has a stiffness sufficient to avoid buckling when being inserted into the borehole having a nonlinear portion. The end of the anchoring means may be provided with a guiding means to guide the anchoring means to the bottom of the borehole. The guiding means may comprise e.g. at least one wheel near the bottom end portion of the flexible anchoring means, whereby the flexible anchoring means may easily reach the bottom portion of the borehole by virtue of its own weight. The guiding means may also comprise e.g. a heavy portion near the bottom end portion the flexible anchoring means which will cause said flexible anchoring means to fall into the borehole. The flexible anchoring means and/or the guiding means may comprise a surface material with low friction towards the wall of the borehole for easy transfer of said flexible anchoring means into said borehole.

The flexible anchoring means may be fastened in the borehole using a solidifying paste that hardens after being injected into the borehole. The solidifying paste may typically be e.g. grout or other types of concrete. The use of solidifying paste may ensure that the flexible anchoring means is fixed to the borehole. The solidifying paste may be pumped through the hollow tube, which may secure controlled placement of the grout, as the borehole will be filled from the bottom. If the boreholes are filled from the top, the solidifying paste may not reach the bottom. Thus, by pumping the solidifying paste through the hollow tube, the solidifying paste may ensure a strong bond between the hollow tube and the wall of the borehole along the entire length of the borehole.

The upper portion of the borehole, i.e. the portion closest to the surface, may be substantially straight and vertical. This has the advantage that no wedge-shaped spacer may be needed for securing the base plate to the anchoring means. Additionally, the vertical portion of the borehole may assure that the flexible anchoring means extends deep into the ground before any nonlinear regions begin, whereby the layer of ground closest to the surface will be less destabilized than if the nonlinear portion would begin immediately below the surface. The nonlinear portion below the vertical portion will ensure that the flexible anchoring means anchor a large volume of the ground. The top end portion of the flexible anchoring means may be provided with a spelter socket, and a solid threaded rod may be connected to said spelter socket at its topside. The threaded rod will therefore be perpendicular to the base plate, which may be secured with a nut without any need for a wedge-shaped spacer. Threads will therefore only need to be machined onto short, solid rods, and not onto 12-15 m long rods. Thus, the use of flexible anchoring means may result in an easier installation, reduced risk of point loads, and less bending forces compared to traditional foundations.

The tower structure to be connected to the foundation may typically have a bottom portion complementary to the foundation base plate, and the tower structure may be fastened to the foundation by fastening said tower structure to the base plate by any suitable connection means. The base plate may have any suitable shape depending on the shape of the bottom portion of the corresponding tower structure. The bottom portion of the tower structure may for example comprise holes which are perpendicular to the plane of the base plate at positions corresponding to holes in said base plate, and substantially vertical bolts or threaded rods may extend through said holes and be tightened from one or both sides, for example using nuts. A novel connection means for fastening the tower structure to base plate of the foundation may be wire mounts specifically adapted for this purpose. Instead of merely fastening the tower structure to the base plate, the technical effect of the wire mounts may be to connect the tower structure to both the base plate and a region below the base plate. This region may typically comprise of solidifying paste. The base plate can thereby be relatively thin, as some of the tension from the tower structure may be transferred to a larger part of the foundation than just the base plate.

The wire mounts may thus comprise a part which is connectable to the tower structure, for example one or more substantially vertical rods, or threaded rods screwed into the wire mounts. The substantially vertical rods may extend through holes in the base plate and the bottom portion of the tower structure, and nuts may be used to fasten the tower structure to the rods. Fastening of the nuts will also fasten the wire mounts, and thereby the tower structure, to the bottom of the base plate. The wire mounts may also comprise a part which connects the wire mounts, and thereby the tower structure, to the region of the foundation below the base plate, a region typically comprising solidifying paste. This part of the wire mounts may for example comprise a wire or any other elongated part attached to or going through said wire mounts. The wire may typically be oriented with its axes in the directions substantially parallel to the plane of the base plate to increase the resistance towards tension from the tower structure. The wires from multiple wire mounts may be oriented in a way to cover a large region below the base plate, for example by extending outward from the centre of the base plate.

A second aspect of the invention is related to a method for constructing foundations, wherein the method comprises the steps of:

- drilling boreholes having a nonlinear portion,

- inserting flexible anchoring means into the boreholes,

- fastening the flexible anchoring means in the borehole using solidifying paste, and

- fastening a base plate of the foundation to the flexible anchoring means.

The flexible anchoring means may be tubes, and the step of fastening the flexible anchoring means in the borehole using solidifying paste may be performed by pumping the solidifying paste through the tubes. The step of drilling boreholes having a nonlinear portion may comprise drilling curved boreholes using directional drilling. The method according to the second aspect of the invention may comprise constructing foundations according to the first aspect of the invention.

In the following is described an example of a preferred embodiment illustrated in the accompanying drawings, wherein:

Fig.1 Shows a flexible anchoring means forming part of a foundation according to the invention, viewed in perspective;

Fig.2 Shows in a smaller scale a foundation according to the invention comprising a plurality of flexible anchoring means, viewed in perspective;

Fig.3 Shows the foundation from figure 2 viewed from above;

Fig.4 Shows in a larger scale a wire mount according to the invention, viewed in perspective;

Fig.5 Shows in a larger scale the wire mounts from figure 4 viewed from the side;

Fig.6 Shows the foundation from figure 2-3 additionally comprising wire mounts, viewed in perspective; and

Fig.7 Shows the foundation from figure 6 viewed from above.

In the drawings, reference numeral 1 indicates a flexible anchoring means according to the invention. Identical reference numerals indicate identical or similar features. The drawings are schematic, and the features in the drawings are not necessarily drawn to scale.

Figure 1 shows a flexible anchoring means 1 forming part of the foundation according to the invention. The flexible anchoring means 1 is adapted to be able to follow the course of a borehole 2 comprising a nonlinear portion 3. The nonlinear portion 3 of the borehole 2 may for example be drilled using directional drilling. The borehole 2 has a diameter which is larger than the diameter of the flexible anchoring means 1. In the shown embodiment, the flexible anchoring means 1 is a tube, where through a solidifying paste 12 (hatched region) has been injected to fill the borehole 2, such that the anchoring means 1 is firmly secured in said borehole 2. The inside region of the tube may also contain solidifying paste 12. A spelter socket 7 is connected to the upper end portion 6 of the flexible anchoring means 1, and a threaded rod 9 is screwed into the spelter socket 7. A nut 11 fastens a base plate 13 to the flexible anchoring means 1. The threaded rod 9 and nut 11 may alternatively be a bolt or other similar fastening means. The borehole 2 comprises a substantially vertical upper portion 5. The substantially vertical upper portion 5 has several advantages. For example, it is easier to drill vertically than at a non-vertical angle, a nut 11 can secure the base plate 13 without the need of a wedge-shaped spacer, and finally the nonlinear portion 3 of the borehole 2 may increase the anchoring volume of the flexible anchoring means 1 deep in the ground without destabilizing the upper layer of ground close to the base plate 13.

Figures 2 and 3 show a foundation 4 comprising a base plate 13 and a plurality of flexible anchoring means 1. For simplicity, the boreholes 2 housing the flexible anchoring means 1 are not shown in this and the following drawings, but said boreholes 2 have substantially the same shape as the flexible anchoring means 1 (see e.g. figure 1). In the shown embodiment, the nonlinear portions 3 of the boreholes 2 have been drilled in different directions in order to maximize the volume of ground gripped by the foundation 4. The foundation 4 comprises two concentric, circular rows of flexible anchoring means 1, where the flexible anchoring means 1 in the outermost row are bended away from an axis through the center of and normal to the base plate 13, while the flexible anchoring means 1 in the innermost row are bended towards said axis. The base plate 13 is fastened to the flexible anchoring means 1 using spelter sockets 7 and threaded rods 9 with nuts 11 as best seen in figure 1.

Figures 4 and 5 show a wire mount 15 comprising a wire 17 and substantially vertical rods 10 with nuts 11. The substantially vertical rods 10 may be threaded rods screwed into corresponding sockets 19 in the wire mount 15. The substantially vertical rods 10 are intended to extend through holes in the base plate 13 of the foundation 4 and in the corresponding bottom portion of the tower structure. The wire 17 serves to connect the tower structure to the region of the foundation 4 below the base plate 13. This region around the wire mount 15 may typically comprise solidifying paste 12, whereby the wire 17 will connect the tower structure to said region of solidifying paste 12.

Figures 6 and 7 show the foundation 4 from figures 2 and 3 including the wire mounts 15 (not visible) from figure 4 and 5. The wire mounts 15 are positioned just below the base plate 13 and are therefore not visible in figures 6 and 7. The innermost and outermost row of threaded rods 9 extend through the base plate 13 and are connected to the flexible anchoring means 1, while the middle two rows of substantially vertical rods 10 are connected to the wire mounts 15. The wires 17 extend through the wire mounts 15 and extend on both inner and outer side of the base plate 13 of the foundation 4. Figure 6 shows how the middle two concentric, circular rows of substantially vertical rods 10, which are connected to the wire mounts 15, extend to a higher elevation than the innermost and outermost rows of threaded rods 9, which are connected to the flexible anchoring means 1. The substantially vertical rods 10 are intended to extend through corresponding holes in a bottom portion of the tower structure (not shown), and the nuts 11 are thereafter fastened onto said substantially vertical rods 10. Thus, the bottom portion of the tower structure is secured to the base plate 13 of the foundation 4 and to the upper part of the ground through the wire mounts 15, while the base plate 13 of the foundation 4 is fastened further in the ground by the flexible anchoring means 1.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.

The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

1. A foundation (4) for a tower structure, said foundation (4) comprising:

- a base plate (13) for supporting the tower structure; and

- anchoring means (1) for fastening the base plate (13) to the ground,

c h a r a c t e r i z e d i n that the anchoring means (1) are flexible anchoring means (1) for extending into boreholes (2) having a nonlinear portion (3).

2. The foundation (4) according to claim 1, wherein the flexible anchoring means (1) comprises a tube.

3. The foundation (4) according to any of the previous claims, wherein the foundation (4) comprises a solidifying paste (12) for fastening the flexible anchoring means (1) in the boreholes (2).

4. The foundation (4) according to any of the previous claims, wherein the foundation (4) comprises a connection means for connecting the tower structure to the base plate (13) and to a region of the foundation (4) below the paste plate (13).

5. The foundation (4) according to claim 4, wherein the connection means is wire mounts (15).

6. The foundation (4) according to any of the previous claims, wherein the flexible anchoring means (1) comprises a guiding means at its bottom end portion (8).

7. Method for constructing a foundation (4) for a tower structure, c h a r a c t e r i z e d i n that the method comprises the steps of:

- drilling boreholes (2) having a nonlinear portion (3);

- inserting flexible anchoring means (1) into the boreholes (2);

- fastening the flexible anchoring means (1) in the boreholes (2) using solidifying paste (12); and

- fastening a base plate (13) of the foundation (4) to the flexible anchoring means (1).

8. The method according to claim 7, wherein the flexible anchoring means (1) are tubes and the step of fastening the flexible anchoring means (1) in the boreholes (2) using solidifying paste (12) is performed by injecting the solidifying paste (12) through the tubes.

9. The method according to any of the claims 7-8, wherein the step of drilling boreholes (2) having a nonlinear portion (3) comprises drilling curved boreholes (2) using directional drilling.

10. The method according to any of the claims 7-9 for constructing a foundation (4) according to any of the claims 1-6.