WO2004057113A1

WO2004057113A1 - Foundation for a transversally stressed tower

Info

Publication number: WO2004057113A1
Application number: PCT/FI2003/000924
Authority: WO
Inventors: Olli Juhani SÄÄKSVUORI; Hannu Henrik Jokiniemi
Original assignee: Rautaruukki Oyj
Priority date: 2002-12-19
Filing date: 2003-12-03
Publication date: 2004-07-08
Also published as: AU2003303228A1; FI20022235A

Abstract

The invention relates to the foundation of a transversely stressed tower in the soil (M). The foundation comprises a foundation case (2) consisting of a substantially circular steel sheet wall (5), a mounting ring (4) fixed to the area of the upper edge of the wall by means of welds (15), and a steel sheet bottom (6) fixed to the area of the lower edge of the wall by means of welds (16). A plurality of tubular steel piles (3a, 3b, 3c…) extends from the vicinity of the steel sheet wall through the steel sheet bottom and is fixed by welds (13) to the steel sheet bottom. In addition, the foundation case and the tubular steel piles are filed with concrete (B).

Description

Foundation for a transversally stressed tower

The invention relates to the foundation of a transversally stressed tower in a soil, the foundation comprising a mounting ring for attaching said tower and anchor bars, which are connected to said mounting ring in a load-supporting manner, and at least part of which extend into the soil in an inclined position.

A transversely stressed tower-like construction, such as a wind power plant, a mast comprising large- sized antennas or the like, which is subjected to strong horizontal forces during use, is conventionally founded on piling performed in advance by casting a bedplate of concrete connected to piles in a dismountable mould, a flange- like mounting ring, i.e. adapter equipped with concrete bonds being positioned on the bedplate before casting, and the actual tower being subsequently fixed to the adapter. Wind power plants are exposed to particularly strong transverse forces, and for this reason, the tower supporting a wind generator at high heights, where the desired wind speeds are higher and more regular than near the earth surface, comprises a steel jacket, which is fixed to the mounting ring by means of bolts. In this conjunction, besides the weight of the structures, the foundation should withstand a very high bending moment exerted on the building, the bending moment generating strong tensile stresses and bending stresses in the tower foundation, unlike normal buildings, where the foundation is subjected to compression stresses alone.

US patent specification 2002/0124502 describes a tower foundation embedded in the soil and comprising a cylindrical central vertical portion of reinforced concrete of smaller diameter, the tower being fixed to the adapter provided on the upper surface of the foundation, and a vertical portion of reinforced concrete with greater diameter forming a cylindrical outer circumference, and a flange-like spreading portion connecting these two. All these parts are embedded in the soil so that the weights of the outermost cylinder portion and the spreading portion and the weight of the soil on top of these resist with the circumference the tilting moment generated by the tower. US patent specification 2002/0066255 describes a tower foundation embedded very deep in the soil and comprising one single cylindrical portion, which is at least partly filled and surrounded with concrete, the tower being fixed to the adapter on the upper surface of this portion. Both of these tower foundations require considerable soil excavation and earth moving, which are time-consuming and costly operations. Especially the foundation of the former reference is most compli- cated and requires much labour for manufacture on the site. Hence both of these proposed foundations are slow and costly to install, especially at sites with poor means of communication.

EP patent specification 1 074 663 depicts a foundation for a tower construction comprising a central portion for supporting the tower, three or more stabilizing means, i.e. three horizontal long branches projecting radially from the central portion and having ground anchor means at their outermost peaks. The stabilizing branches are equally long and they are disposed at regular intervals around the cen- tral portion. The ground anchor means comprise at least one substantially vertical ground anchor bar and at least one tilted ground anchor bar, which preferably extends radially downward, in the same way as the associated branch, and beyond this branch. In this specification, the foundation may be manufactured from reinforced concrete, wood, aluminium, steel, or a mixture of these, and the foundation is posi- tioned in place with only the upper surface of the central portion including the tower fixing plate placed above the ground surface. The primary purpose is to avoid the use of heavy transport equipment in the transportation of the foundation to the installation site, and also to avoid other environmental impacts caused by the foundation work. Despite these appreciable efforts, this foundation is also bulky and im- practical in shape during transportation, requiring a good deal of earth moving and being thus slow and costly to install, especially at sites having poor means of communication.

The purpose of the invention is to provide a foundation for a transversely stressed tower-like construction, especially a wind power plant, requiring optimally small and light parts for its construction, which are thus readily transported to the building site even under hard ground and weather conditions. Another purpose of the invention is to provide such a foundation requiring a minimum of earth moving operations. Yet the finished foundation should be such that is capable of supporting im- peccably the strong tensile stresses exerted on it especially by a wind power plant.

The shortcomings described above are eliminated and the purposes defined above are reached by means of the tower foundation in accordance with the invention, which is characterized by the features defined in the characterizing part of claim 1.

It has now been surprisingly found that the foundation can be dimensioned in a relatively small size by using a special foundation case made of steel, through which steel pile pipes are drilled or passed in some other manner, the foundation case in itself forming together with the steel pile pipes a concrete casting mould, whose size typically approaches that of the tower mounting ring, while imparting the foundation high resistance to tensile stresses and any other stresses. Excellent other endur- ance properties are also attained by means of appropriate reinforcement and by consequently filling the foundation case with concrete at the same time as the steel pile pipes are filled and surrounded with concrete. Since such a small-sized and lightweight foundation case is easy to transport and given the easy transport of the light steel piles, especially by sections, and also the carefree transport of the necessary concrete raw materials, the installation of the foundation of the invention will in no situation be a restricting factor in terms of the construction costs of a wind power plant or any similar self-supported assembly or in selecting the site for it.

The foundation of the invention and its construction are explained in greater detail below with reference to the accompanying drawings.

Figure 1 shows a first embodiment of the foundation of the invention in vertical section along plane I-I of figure 2.

Figure 2 is a top view of the foundation of figure 1 from direction II of figure 1.

Figure 3 shows a part of a second embodiment of the foundation of the invention in vertical section and in the same projection as figure 1.

Figure 4 shows part of a third embodiment of the foundation of the invention in vertical section and in the same projection as figures 1 and 3.

The foundation 1 in soil M of a transversely stressed tower, such as a wind power plant or any similar tower-like structure, which cannot be supported by guys, at least not to any significant extent, comprises a mounting ring 4 for fixing said tower and also anchor bars, which are connected to the mounting ring in a load-supporting manner, and at least part of which extend to the soil in a tilted position. Such a tower subjects the foundation to considerably stronger moments F_M than do conventional tower constructions, in generating tensile stresses that are often directed even straight upward, whereas the foundation of conventional buildings is always subjected to compression stresses. In addition, especially in wind power plants, the vanes catching the wind should be allowed to rotate freely about the tower as the wind direction changes, and this prevents support of the tower by means of long guys typically formed of wires and extending obliquely from the top to the ground, as are commonly used to support conventional antenna masts. The foundation of the invention explained below independently resists moment stresses F_M effective in respect to the ground surface, preferably without such additional supports or the like placed at horizontal distances from the foundation case which are not permanently fixed to the foundation case or to the piles fixed to the foundation case. Hence the foundation of the invention is a complete tower foundation in its optimal embodiment.

The foundation of the invention for supporting a tower completely or at least substantially without guys comprises firstly a single foundation case 2, which consists of a substantially round steel sheet wall 5, a mounting ring 4 attached to it by welds 15 in the area of its upper edge, and a steel bottom plate 6 attached to it by welds 16 in the area of its lower edge. This substantially round foundation case 2 means that the horizontal cross-sectional surface of the foundation case accounts for at least 80% or preferably for at least 90% of the horizontal cross-sectional surface of a circle drawn around it at the same height. This foundation also comprises a plurality of tubular steel piles 3a, 3b, 3c... extending adjacent the steel sheet wall 5, naturally within the foundation case, through the steel sheet bottom 6 to the soil, the piles being fixed by welds 13 to the steel sheet bottom, and the foundation also comprises concrete B within the foundation case and said steel pile pipes 3a, 3b, 3c ... In accordance with the invention, the foundation case 2 further has a height H and the tubular steel piles 3a, 3b, 3c. have an inner length Ls above the steel sheet bottom 6, which is substantially smaller than said height, in other words, the steel pile pipes terminate within the foundation case at least at the location of the steel sheet bottom, yet preferably only just above the steel base plate. This enables the concrete reinforcements 9 incorporated in the foundation to extend from within the tubular steel piles over said inner length Ls into the foundation case, and if desired, also from adjacent and/or opposed tubular steel piles to each other, as shown in figures 1 and 2. The concrete reinforcements 9 mentioned above are located within the cast concrete B in a conventional manner, however, the construction may further include other anchor means, which are of a type known er se or of a new type, between the tubular steel piles and the concrete. The foundation case has a diameter Dl, which equals the diameter D2 of the mounting ring 4 or which, in the vicinity of the steel bottom plate, is at the most one and a half times the diameter D2 of the mounting ring. The height H of the foundation case is at least 10% or at the most 100% of the diameter Dl of the foundation case, and preferably 30% to 60% of the diameter Dl of the foundation case.

The foundation case 2 used may be straight or cylindrical, as can be understood from figures 1 and 2, or in the shape of a truncated cone or a truncated dome, as understood from figure 3, or in the shape of a doughnut, i.e. toroidal, and in that case, viewed in the projection of figure 3, the outer surface of the foundation case would be concave instead of convex. In this case, the truncation is consequently done in a direction transverse or perpendicular to the line of symmetry. The steel sheet wall 5 and/or steel sheet bottom 6 may be made of an even or planar sheet, such as shown in figures 1 and 2, or corrugated sheet, such as exemplified in figure 4. The corrugated shape may occur in the steel sheet bottom either in one direction alone or in two cross directions. The corrugated shape of the steel sheet wall 5 may be vertical or horizontal. It should be understood that any sheet material, which has a surface shape known per se or a new surface shape and is suitable for this purpose, can be used as steel sheet wall 5 and/or steel sheet bottom 6, provided that the surface shapes have smaller dimensions than the main dimensions of the foundation case 2. The steel sheet wall 5 typically comprises inner stiffeners 20, such as horizontal ribs or ribs comprising horizontal sections, which also serve to increase the adhesive strength to the concrete B and hence the moment-bearing capacity of the entire foundation 1. Normally inner stiffeners 20 are provided e.g. below half of the height H of the foundation case, as shown in figure 1.

The diameter Dl of the foundation case 2 or the diameter D2 of the mounting ring 4 is typically at least 2 mm or at the most 10 m. For this purpose, the diameter D3 of the tubular steel piles 3a, 3b, 3c .. is typically at least 250 mm or at the most 1200 mm. Such a foundation can receive a tower, say the tower of a wind power plant, whose height is of the order of 40 m to 120 m.

In its preferred embodiment, the foundation, especially the foundation case, also comprises a plurality of adjusting legs 8, by means of which the foundation case can be fitted in its correct position. The adjusting legs have screw shanks 10 and typically support flanges 7 bearing against the ground M at the lower ends of the screw shanks, and also nut parts 9a or 9b attached to the steel sheet bottom 6 of the foun- dation case or bearing against the steel sheet bottom 6 at least from below. Figure 1 shows one embodiment, which helps to understand that rotating of the screw shanks 10, rotation of the screw shank 10 will shift the support flanges 7 closer to or further away from the steel bottom plate with the lower nut part 9b non-rotatable relative to the steel bottom plate, while the position of the foundation case relative to the soil changes. Optionally, the screw shank 10 can be made non-rotatable, while rotating the lower nut part 9b bearing against the steel bottom plate, so that the support flanges 7 move closer to or further away from the steel bottom plate, while the position of the foundation case relative to the soil changes.

In accordance with the invention, the foundation case 2, or more specifically its steel sheet bottom 6, may be located above the ground M equaling a vertical dimen- sion E, as shown in figure 1, the vertical dimension being as small as possible, yet such as allows the foundation case and especially its mounting ring 4 to be fitted in horizontal position by means of the adjusting legs 8 described above. The bottom part of the foundation case can also be embedded in the soil M, yet only partly at the most, as shown in figure 3, even if this does not yield any advantage. In this case as well, the steel sheet bottom 6 is above the bottom of the soil M, i.e. the pile hole, equaling a vertical dimension E. The foundation 1 of the invention can be placed on the ground, i.e. on "dry" soil M, or in water areas, and then the soil M is the bottom of the sea or a lake or the like.

The foundation of the invention described above can be mounted in the following manner when "bored piles" are used as tubular steel piles, such as, for instance, piles disclosed in US patent specification 6,368,021 or a pile drilled into the soil in a similar manner: la} The foundation case 2 is fitted at the correct location and approximately in the correct plane.

2a} Unless already provided, holes are drilled through the steel sheet bottom 6.

3a} The tubular steel piles 3a, 3b, 3c... are driven into the soil M through the holes in the steel sheet bottom.

3 a} The piles are equipped with concrete reinforcements 9 and concrete B is cast into the piles.

4a} The foundation case and thus the mounting ring 4 are adjusted by means of the adjusting legs 8 into the correct plane.

5a} The foundation case is fixed to the piles by welding.

6a} The inside of the foundation case is equipped with concrete reinforcements 9 and concrete B is cast into the foundation case.

7a} The tower is fixed to the mounting ring 4 of the foundation case. It is, of course, also possible to perform steps 3a and 6a above in combination or simultaneously, and to perform the positioning of the foundation at an earlier stage, and then the steps are the following: lb} The foundation case 2 is mounted at the correct location and approximately in the correct plane.

2b} Unless already provided, holes are drilled through the steel sheet bottom 6.

3b} The tubular steel piles 3a, 3b, 3c .. are driven into the soil M through the holes in the steel sheet bottom.

4b} The foundation case and thus the mounting ring 4 are adjusted by means of the adjusting legs 8 into the correct plane.

5b} The foundation case is fixed to the piles by welding.

6b} The piles and the foundation case are equipped with concrete reinforcements 9 while concrete B is cast into the piles and the foundation case.

7b} The tower is fixed to the mounting ring 4 of the foundation case.

The foundation of the invention described above can be installed as follows when

"rammed piles" are used as tubular steel piles, e.g. piles as disclosed by WO

00/36226 or other piles rammed into the soil in a similar manner: lc} The tubular steel piles 3a, 3b, 3c .. are driven into the soil M in an area defined by the periphery of the foundation case 2 at the site intended for it.

2c} Holes are drilled through the steel sheet bottom 6 in consistency with the location of the upper ends of the piles driven into the soil.

3c} The foundation case 2 is fitted into position with the ends of the tubular steel piles emerging from the soil passing through the holes in the steel sheet bottom. 4c} The piles are equipped with concrete reinforcements 9 and concrete B is cast into the piles.

5c} The foundation case and thus the mounting ring 4 are adjusted by means of the adjusting legs 8 into the correct plane.

6c} The foundation case is fixed to the piles by welding. 7c} The inside of the foundation case is equipped with concrete reinforcements 9 and concrete B is cast into the foundation case.

8c} The tower is fixed to the mounting ring 4 of the foundation case.

Exactly as in the optional embodiment described above, steps 4c and 7c mentioned above can be performed in combination or simultaneously, and the positioning of the foundation can be carried out at an earlier stage, and then the steps are as follows: la} The tubular steel piles 3a, 3b, 3c .. are driven into the soil M in an area defined by the periphery of the foundation case 2 at the site intended for it. 2d} Holes are drilled through the steel sheet bottom 6 in consistency with the location of the upper ends of the piles driven into the soil. 3d} The foundation case 2 is fitted into position with the ends of the tubular steel piles emerging from the soil passing through the holes in the steel sheet bottom. 4d} The foundation case and thus the mounting ring 4 are adjusted by means of the adjusting legs 8 in the correct plane. 5d} The foundation case is fixed to the piles by welding. 6d} The piles and the foundation case are equipped with concrete reinforcements 9 while concrete B is cast into the piles and the foundation case. 7d} The tower is fixed to the mounting ring 4 of the foundation case.

Welding of the tubular steel piles 3a, 3b, 3c... to the steel bottom plate 6 provides not only increased strength of the foundation 1 during use but also its load-bearing capacity during the concrete casting, as the concrete B has not yet hardened, so as to minimize any deformations and inclinations of the foundation even when the soil M under the adjusting legs 8 is soft. Concrete B is cast into the foundation case 2 to a thickness S such that also the concrete reinforcements 9 extending from the upper ends of the tubular steel piles to other locations inside the foundation case, and also the stiffeners 20 within the foundation, or a number of their horizontal sections, are covered, yet preferably leaving a section H-S of the mounting ring 4 free from concrete downward.

Claims

1. Foundation (1) of a transversally stressed tower in the soil (M), the foundation comprising a mounting ring (4) for fixing said tower and anchor bars, which are connected in a load-supporting manner to the mounting ring and at least part of which extends to the soil in an inclined position, characterized in that the foundation comprises:

- one foundation case (2), which consists of a substantially round steel sheet wall (5), the mounting ring (4) attached to the area of the upper edge of the wall by means of welds (15), and a steel sheet bottom (6) attached to the area of the lower edge of the wall by means of welds (16);

- a plurality of tubular steel piles (3a, 3b, 3c ..), which extend from the vicinity of the steel sheet wall through the steel sheet bottom to the soil and are fixed by welds (13) to the steel sheet bottom; - concrete (B) within said foundation case and said tubular steel piles.

2. A tower foundation as defined in claim 1, characterized in that the foundation case (2) has a height (H) and the tubular steel piles (3a, 3b, 3c) have an inner length (Ls) above the steel sheet bottom which length is substantially smaller than said height; and in that the foundation further comprises concrete reinforcements (9), which extend from within the tubular steel piles over said inner length into the inside of the foundation case.

3. A tower foundation as defined in claim 1, characterized in that said foundation case (2) has the shape of a cylinder, or a truncated cone, or a truncated dome, or a truncated doughnut.

4. A tower foundation as defined in claim 1, characterized in that it further comprises a number of adjusting legs (8), which comprise screw shanks (10), support flanges (7) supported on the ground (M), and nut parts (9a or 9b) fixed to the steel bottom plate (6) of the foundation case or bearing against the steel bottom plate (6) at least from below.

5. A tower foundation as defined in any of the preceding claims, characterized in that the foundation case (2) is located above the ground (M) equaling a vertical dimension (E), or at the most partly embedded in the ground (M).

6. A tower foundation as defined in any of the preceding claims, characterized in that the steel sheet wall (5) and/or the steel sheet bottom (6) are made of flat sheet or corrugated sheet.

7. A tower foundation as defined in any of the preceding claims, characterized in that the foundation case has a diameter (Dl) equaling the diameter of the mounting ring (4) and a height (H) accounting for 10% to 100% of the diameter (Dl) of the foundation case.