A METHOD AND AN ASSEMBLY FOR CASTING A TOWER FOUNDATION

Abstract: A method and a fixture assembly for casting a foundation for a tower (16), in particular for a wind turbine, providing a high quality contact surface for the bottom flange (15) of the tower (16) is disclosed. The method comprises the positioning a plurality of anchoring rods (2) in a ground excavation and of a fixture assembly at an upper end of the excavation so that the anchoring rods (2) extend through openings in a fixture plate (1), which is fully supported by at least three pull-off means (4) above the fixture plate (1) and each engaging with the upper end of four anchoring rods (2). Liquid concrete (17) is poured into the excavation to a horizontal level so that the fixture plate (1) is partly submerged in the concrete (17), and after hardening, the fixture plate (1) is pulled off from the concrete (17) by means of the pull-off means (4), leaving a recess formed in the concrete (17) being suitable for receiving a bottom flange (15) of the tower (16).

Description: A METHOD AND AN ASSEMBLY FOR CASTING A TOWER FOUNDATION The present invention relates to an improved method of casting a foundation for a tower providing a high quality contact surface for the bottom part of the tower The invention is highly suitable for making foundations for towers for wind turbines with substantially horizontal rotor axes because such towers are subjected to a strong horizontal force near the top and are thus very demanding for the foundations The invention also relates to a fixture assembly for fixing the upper ends of anchoring rods during casting of the foundation and making a recess in the concrete of the foundation for accommodating a bottom flange of the tower Background Tower foundations having anchoring rods protruding upwardly from a concrete block for fastening the tower are commonly made by fixing the upper end of the rods with a fixture plate before the concrete is poured into the excavation in which the anchoring rods are placed The surface of the concrete is levelled by hand and the fixture plate is not in contact with the concrete The fixture plate is removed after hardening of the concrete for reuse The area where the bottom flange of the tower will be placed is covered with a layer of expanding concrete where after the bottom flange is positioned on top of the expanding concrete A proper supporting surface for the bottom flange is provided in this way, but the procedure adds another 2-3 days of work to the making of the foundation A method similar to the above described known method is disclosed in US 5,505,033 Another method is disclosed in FR 2682982, wherein a plate made at least partly from a material which can be deformed is placed between the foundation and the bottom flange The fixture plate may also be cast into the concrete foundation, thereby providing a proper supporting surface for the bottom flange of the tower However, the very precise manufacture of the fixture plate makes this option costly WO 96/16232 discloses a tower foundation in which a template of a shape similar to the bottom flange of the tower is partly submerged by liquid concrete during casting of the foundation, before and during which the template is supported at the lower surface by nuts on anchoring rods extending through the template and by a structure above the template which structure is supported by feet in abutting contact with the ground away from the template and the foundation The template is subsequent to the hardening of the concrete lifted off the foundation leaving a suitable recess for the bottom flange of the tower However, the nuts must be either removed or displaced further down below the level of the bottom flange because the nuts would hinder pre-stretching of the anchoring rods if they are in abutment with the bottom flange This leads to large voids in the horizontal surface of the recess and thus a less support of the flange Furthermore, is has been found that the template is very difficult to detach and lift off without permanently bending the template and hinder the reuse of the template Description of the invention It is an object of the present invention to provide a method of casting a foundation for a tower by which a proper supporting surface for the bottom flange of the tower is provided within the same casting process as the casting of the foundation It is a further object of the invention to provide in which a fixture plate which is partly submerged in the concrete so as to form said supporting surface is suspended from the part of the anchoring rods protruding above the fixture plate so as to avoid the requirement of supporting the fixture plate with extern means, which is less precise, as well as a requirement of voids in the supporting surface for enabling the removal of supporting means, such as nuts It is a yet further object to provide a method and means for detaching the fixture plate after hardening of the concrete in a controlled way so as to prevent stresses from permanently deforming the fixture plate Thus, the present invention relates to a method of casting a foundation for a tower, comprising the steps of positioning a plurality of anchoring rods substantially vertical in a ground excavation, positioning a fixture assembly at an upper end of the excavation, the assembly comprising a fixture plate and at least two, preferably at least three, pull-off means, the fixture plate having a number of openings defined therein though which the plurality of anchoring rods extends each through one of said openings so that an upper end of each anchoring rod protrudes upwardly from an upper surface of the fixture plate, the fixture plate being positioned so that a lower surface thereof is situated substantially horizontal at an upper end of the excavation the fixture plate being fully supported in said position by said pull-off means being positioned above the fixture plate and each engaging with said upper end of at least one anchoring rod, pouring liquid concrete into and filling the excavation to a horizontal level so that the fixture plate is partly submerged in the concrete, and allowing the concrete to harden, where after the fixture plate is pulled off from the concrete, leaving a recess formed in the concrete being suitable for receiving a bottom flange of the tower The term anchoring rods usually means steel rods having a circular cross-section but is understood to include rods of other cross-sections, such as I beams, angle bars , tubes of various shapes, such as round, square, triangular, etc , but also steel wires and wires or ropes made from fibre-reinforces elastic or plastic materials, such as carbon fibres, glass fibres and/or Kevlar fibres The fixture plate is preferably, prior to being pulled off, detached from the hardened concrete by means of the pull-off means so that the force for detaching or loosening the fixture plate only is used over a short vertical travelling distance of the fixture plate so as to prevent the plate from becoming permanently deformed This may be performed if the pull-off means are displaced relatively to the anchoring rods during detachment by means of turning nuts being arranged on a treaded part of said anchoring rods and being in abutting contact with said pull-off means at an upper surface of said nuts, so that an upward force is applied to the pull-off means and thereby the fixture plate According to an alternative and preferred embodiment, the pull-off means are connected to the fixture plate by means of adjustable connecting means that may displace the fixture plate relatively to the individual pull-off means, the detachment being performed by displacing the fixture plate upwardly towards the pull-off means by means of adjusting said connecting means Also, in order to remove air captured under the fixture plate and forming voids in the supporting surface, the fixture plate may advantageously subsequent to the pouring of the concrete and when the concrete is still liquid be displaced from the initial position upwards and downwards at least once by means of the pull-off means, ending up substantially at the initial position In order to diminish the size and number of voids in the concrete around the fixture plate and thus providing an improved contact between the concrete foundation and the bottom flange of the tower, the method advantageously further comprises the step of vibrating the liquid concrete in the excavation The dimensions of the fixture plate as projected on a horizontal plane preferably exceed the corresponding dimensions of the bottom flange of the tower, so that a vertical groove being 5-40 mm, such as 10-30 mm, preferably 15-25 mm wide will be formed between the edges of the recess formed in the concrete and the vertical sides of the bottom flange of the tower when said flange is positioned in the recess This groove may accommodate a sealing material to prevent water from seeping in between the bottom flange of the tower and the support surface Furthermore, in order to control the position of the anchoring rods, to fasten the rods in the foundation and to ensure that the rods may be pre-tensioned, the method may further comprise the step of positioning an anchoring plate at a lower part of the excavation, securing the anchoring plate to another founding structure being positioned below the anchoring plate and securing a lower part of each anchoring rod to the anchoring plate For most purposes it is suitable that the fixture plate has a conical cross-section as seen in a vertical section so as to allow for the fixture plate to be pulled off from the concrete, and at least some of the sides of the fixture plate may therefore be inwardly inclined 5-45 degrees, such as 8-15 degrees and preferably about 10 degrees to vertical It is preferred that at least 5 millimetres of the height of the fixture plate is submerged in the concrete, preferably at least 10 mm and most preferred within the range of 15-50 mm, such as 30-40 mm For most towers being round or substantially round, the fixture plate is preferably ring shaped and suitable dimensions for the application of the present invention are an outer diameter of 1 5-12 meters and an inner diameter of 1 1-9 5 meters, the difference between the inner and the outer diameter being in the range from 0 4 to 2 5 meters The plurality of anchoring rods should be distributed evenly around the fixture plate so that they make take up forces on the tower coming from any side The number of anchoring rods are preferably from 64 to 256, such as from 96 to 200 It is an advantage for symmetry reasons that the number of rods is a multiple of eight The present invention further relates to an assembly comprising a fixture plate having a number of openings defined therein through which a plurality of parallel anchoring rods may extend, each through one of said openings, and at least two, preferably at least three, pull-off means, each having organs for engaging with an upper end of at least one of said anchoring rod extending from an upper surface of the fixture plate, and each having connecting means for connecting to the fixture plate so as to fully support the fixture plate when being arranged in a substantially vertical position This assembly is designed to be used with the method according to the invention as described above and may have the disclosed features Each anchoring rod is advantageously provided with external thread at least at the part protruding above the fixture plate, and the fixture plate may be provided with pull-off means mounted thereon for pulling off the fixture plate from the concrete, said pull-off means being adapted for interacting with some of the anchoring rods during the step in which the fixture plate is pulled off, which step is effected by the turning of nuts being arranged on said anchoring rods and being at an upper surface of said nuts in abutting contact with said pull-off means, so that an upward force is applied to the pull-off means and thereby the fixture plate The anchoring plate may furthermore be fixed to another founding structure being positioned below the anchoring plate, such as a vertical foundation plate made from concrete and placed 1 5-5 meters, such as 2-4 meters, below the surface of the ground and with a side length of 8-25 meters, such as 12-20 meters Alternatively or additionally, the underlying foundation may comprise piles that are driven into the ground. Brief description of the figures An embodiment of the present invention is shown in Figs. 1-8 of which Fig. 1 shows a vertical cross-section of the assembled fixture plate, anchoring rods and anchoring plate, Fig. 2 is a top view of the fixture plate, Fig. 3 shows a cross-section of one side of the fixture plate, Fig. 4 is a top view and Fig. 5 is a side view of a pull-off device as mounted on the fixture plate and the anchoring rods, Fig. 6 is a side view of a pull-off device with measures, Fig. 7 shows a vertical cross-section of one side of the bottom flange of the tower, and Fig. 8 is a vertical cross-section of the bottom flange of the tower as mounted in the recess in the concrete. Measures on the figures are given in millimetres. Detailed description of the figures A vertical cross-section of the assembled fixture plate 1 , anchoring rods 2 and anchoring plate 3 is shown in Fig. 1. A pull-off device 4 mounted on the fixture plate 1 is shown in the circle and details of the pull-off device 4 are shown in Figs. 4-6. The assembling is placed in an excavation and is ready for the concrete to be poured into the excavation. The anchoring plate 3 is fastened with anchoring means 5 to a vertical 14 x 14 meter foundation plate made from concrete and placed 2.5-3 meters below the surface of the ground. Alternatively or additionally, the underlying foundation may comprise piles that are driven into the ground. The anchoring rods 2 are at a lower end fastened to the anchoring plate 3 and are at an upper end provided with external thread so that the fixture plate 1 can be fastened to the rods 2 with nuts 6. The fixture plate 1 is ring shaped as seen from the top as shown in Fig 2 The 144 anchoring rods 2 extend through holes 7 that are evenly distributed in two rows in the fixture plate 1 The fixture plate 1 is further provided with six holes 8 with internal thread for mounting of the pull-off devices 4 A cross-section of one side of the fixture plate is shown in Fig 3 with measures indicating the distance between the given position and the same position on the other side of the fixture ring, so that the outer diameter of the plate 1 is given as 3381 millimetres The height of the fixture plate is 40 millimetres and the plate is submerged 30-40 mm into the concrete during casting of the foundation which has shown to be a suitable dept for the voids in the concrete around the fixture plate 1 to be removed or delimited during vibration of the liquid concrete, and the fixture plate 1 is still possible to remove after hardening of the concrete However, a deeper submersion dept may be chosen for larger diameters of the fixture plate 1 The outer edges 9 of the fixture plate 1 are inwardly inclined 10 degrees to vertical in order to ease the removal of the plate 1 from the hardened concrete Fig 4 is a top view and Fig 5 is a side view of a pull-off device 4 as mounted on the fixture plate 1 and the anchoring rods 2 Fig 6 shows a side view of the pull-off device 4 with measures The pull-of device 4 is mounted with a bolt 10 into a hole 8 in the fixture plate 1 The hole 8 is provided with internal thread and is closed at the lower end with a plug 11 so that the lower surface of the fixture plate 1 is as smooth as possible The pull- off device 4 has four holes 12 giving room for four anchoring rods 2 to pass The pull-off device is connected to the anchoring rods 2 with one nut 13 above the device 4 and one nut 14 below The fixture plate 1 is pulled off from the foundation by removing the upper nuts 13 and turning of the lower nuts 14 so that the pull-off devices 4 and thereby the fixture plate 1 are lifted relative to the anchoring rods 2 Alternatively, the bolt 10 is replaced with a bar having external thread at both ends and a nut situated above the upper surface of the pull-off device 4, and the fixture plate is moved vertically, so as to e g. detach and to pull-off the fixture plate 1 from the foundation, by turning of said nut In this latter embodiment, the vertical movement is easier obtained but demands more angular moment applied to the nut than in the former embodiment in which two or more nuts are turned at each pull-off device 4 to lift the fixture plate 1 Fig 7 shows a vertical cross-section of one side of the bottom flange 15 of the tower 16, and Fig 8 is a vertical cross-section of the bottom flange 15 of the tower 16 as mounted in the recess in the concrete 17 The recess is wider than the bottom flange 15, leaving a vertical groove 18 of about 20 millimetres between the vertical sides of the bottom flange 15 and the sides of the recess The groove 18 is filled with a resilient filler 19 and sealed with a water tight sealing material The anchoring rods 2 are pre-stretched 2 5 millimetres from their original length of 1 5 meters with a hydraulic tool before the nuts are tightened for fastening the bottom flange 15 to the foundation A pre-stretching of 1-2 per thousand of the original length thus providing a tension in the anchoring rods made from steel, has shown to be advantageous for keeping the bottom flange 15 constantly in close contact with the foundation and for reduce the risk of fatigue fracture of the rods 2 The anchoring rods 2 have been provided with a hard sheath 20 made from a plastic material such as polyvinyl chloride polymer or high density polyethylene in which sheaths 20 the rods 2 may slide, thus preventing the concrete 1 7 from sticking to the rods 2 and hinder the pre-stretching of the rods

Claims: CLAIMS 1 A method of casting a foundation for a tower comprising the steps of positioning a plurality of anchoring rods substantially vertical in a ground excavation, positioning a fixture assembly at an upper end of the excavation, the assembly comprising a fixture plate and at least two, preferably at least three, pull-off means, the fixture plate having a number of openings defined therein though which the plurality of anchoring rods extends each through one of said openings so that an upper end of each anchoring rod protrudes upwardly from an upper surface of the fixture plate, the fixture plate being positioned so that a lower surface thereof is situated substantially horizontal at an upper end of the excavation, the fixture plate being fully supported in said position by said pull-off means being positioned above the fixture plate and each engaging with said upper end of at least one anchoring rod, pouring liquid concrete into and filling the excavation to a horizontal level so that the fixture plate is partly submerged in the concrete, and allowing the concrete to harden, where after the fixture plate is pulled off from the concrete, leaving a recess formed in the concrete being suitable for receiving a bottom flange of the tower 2 A method according to claim 1 , wherein the fixture plate prior to being pulled off is detached from the hardened concrete by means of the pull-off means 3 A method according to claim 2, wherein the pull-off means are displaced relatively to the anchoring rods during detachment by means of turning nuts being arranged on a treaded part of said anchoring rods and being in abutting contact with said pull-off means at an upper surface of said nuts, so that an upward force is applied to the pull-off means and thereby the fixture plate 4 A method according to claim 2, wherein the pull-off means are connected to the fixture plate by means of adjustable connecting means that may displace the fixture plate relatively to the individual pull-off means, the detachment being performed by displacing the fixture plate upwardly towards the pull-off means by means of adjusting said connecting means 5 A method according to claim 3 or 4, wherein the fixture plate subsequent to the pouring of the concrete and when the concrete is still liquid is displaced from the initial position upwards and downwards at least once by means of the pull-off means, ending up substantially at the initial position, so as to remove air captured under the fixture plate 5 6 A method according to any of the preceding claims, wherein the extend of the fixture plate as projected on a horizontal plane exceeds the corresponding extend of the bottom flange of the tower, so that a vertical groove being 5-40 mm wide will be formed between the edges of the recess formed in the concrete and the vertical sides of the bottom flange 10 of the tower when said flange is positioned in the recess 7 A method according to any of the preceding claims, further comprising the steps of positioning an anchoring plate at a lower part of the excavation, securing the anchoring plate to another founding structure being positioned below 15 the anchoring plate, and securing a lower part of each anchoring rod to the anchoring plate 8 A method according to any of the preceding claims, wherein the fixture plate has a conical cross-section as seen in a vertical section so as to allow for the fixture plate to be 20 pulled off from the concrete 9. A method according to claim 8, wherein at least some of the sides of the fixture plate are inwardly inclined 5-45 degrees to vertical. 25 10 A method according to any of the preceding claims, wherein the fixture plate is submerged at least 5 millimetres in the concrete 11. A method according to any of the preceding claims, wherein the fixture plate is ring shaped having an outer diameter of 1 5-12 meters and an inner diameter of 1 J -9 5 30 meters, the difference between the inner and the outer diameter being in the range from 0 4 to 2.5 meters, and the plurality of anchoring rods are distributed evenly around the fixture plate 12. A method according to any of the preceding claims, wherein the number of anchoring 35 rods are from 64 to 256 13. An assembly comprising a fixture plate having a number of openings defined therein through which a plurality of parallel anchoring rods may extend, each through one of said openings, and at least two, preferably at least three, pull-off means, each having organs for engaging with an upper end of at least one of said anchoring rod extending from an upper surface of the fixture plate, and each having connecting means for connecting to the fixture plate so as to fully support the fixture plate when being arranged in a substantially vertical position. 14. An assembly according to claim 13, wherein the connecting means connects to an upper part of the fixture plate. 15. An assembly according to claim 13 or 14, wherein the connecting means are adjustable and may displace the fixture plate relatively to the individual pull-off means so as to enable an upwardly displacement of the fixture plate towards the pull-off means by means of adjusting said connecting means. 16. An assembly according to claim 14 or 15, wherein the fixture plate has a conical cross-section as seen in a vertical section so as to allow for the fixture plate to be pulled off from concrete in which it is partly embedded. 17. An assembly according to claim 16, wherein at least some of the sides of the fixture plate are inwardly inclined 5-45 degrees to vertical. 18. An assembly according to any of claims 14-17, wherein the fixture plate is ring shaped of an outer diameter of 1.5-12 meters and an inner diameter of 1 J-9.5 meters, the difference between the inner and the outer diameter being in the range from 0.4 to 2.5 meters, and the openings for anchoring rods are distributed evenly around the fixture plate. 19. An assembly according to any of claims 14-18, wherein the number of openings for anchoring rods are from 64 to 256.

Inventors: NORRE TEDDY, NORRE, TEDDY
Applicants: NEG MICON A S, NORRE TEDDY, NEG MICON A/S, NORRE, TEDDY
Application year: 2000
Main classification: E02D27
IPC codes: E02D27, F03D1