US20210214909A1

US20210214909A1 - Pile foundation

Info

Publication number: US20210214909A1
Application number: US17/272,900
Authority: US
Inventors: Hideharu Nakamura; Shigeru Tanabe
Original assignee: Tokyo Electric Power Services Co Ltd
Current assignee: Tokyo Electric Power Services Co Ltd
Priority date: 2018-09-03
Filing date: 2019-08-22
Publication date: 2021-07-15
Also published as: JP6905496B2; TW202012747A; JP2020037780A; WO2020050049A1

Abstract

The present invention relates to a pile foundation including: a pile that extends in a vertical direction, and whose pile head protrudes above the ground, and that supports a tower-type structure, and ribs that are provided at the pile in a vicinity of a surface of the ground, and that protrude outwardly in radial directions from an outer circumferential surface of the pile.

Description

TECHNICAL FIELD

The present disclosure relates to a pile foundation.

BACKGROUND ART

Japanese Patent Application Laid-Open (JP-A) No. 2005-232694 discloses a structure in which, in a pile foundation that is provided with a screwed steel pile in which spiral blades are provided at a distal end of a cylindrical pile main body, a cast-in-place concrete pile is formed so as to cover a pile head and an outer circumferential surface of this steel pile. Further, Japanese Patent Application Laid-Open (JP-A) No. H7-127053 discloses a pile that penetrates a ground layer portion where liquefaction is generated. A ground agitator is formed on a circumferential surface of this pile that corresponds to the liquefaction portion.

SUMMARY OF INVENTION

Technical Problem

In a pile foundation that supports a tower type of structure, a structure that ensures sufficient resistance to horizontal force without increasing the diameter of the pile is required. As described in JP-A No. 2005-232694, a structure in which a concrete pile is formed around a periphery of a pile head of a screwed steel pile is as one example of such a structure. However, large-scale equipment is required in order to screw in a screwed steel pile. Moreover, it is also necessary to firstly excavate the ground around the periphery of the pile head in order to place the concrete so that, from the standpoint of construction time, there is considerable room for improvement.
The present disclosure provides a pile foundation that ensures sufficient resistance to horizontal force while enabling the construction time to be shortened.

Solution to the Problem

A first aspect of the present disclosure is a pile foundation including: a pile that extends in a vertical direction, and whose pile head protrudes above the ground, and that supports a tower-type structure, and ribs that are provided on the pile in a vicinity of a ground surface of the ground, and that protrude outwardly in radial directions from an outer circumferential surface of the pile.
In the pile foundation according to the first aspect of the present disclosure, a pile extends in a vertical direction, and a head portion of this pile protrudes above the ground. Further, ribs are provided on the pile in a vicinity of a ground surface of the ground, and these ribs protrude outwardly in radial directions from an outer circumferential surface of the pile. Due thereto, in a case in which horizontal force is input into the pile, it is possible to shift a slip plane of the pile from the circumferential surface of the pile to a distal end side of the ribs. As a result, it is possible to increase resistance received from the ground, so that sufficient resistance to horizontal force may be ensured.
Moreover, because it is possible to increase resistance to horizontal force simply by forming these ribs, construction may be performed in the same manner as when ribs are not provided so that the construction time may be shortened.
In a pile foundation according to a second aspect of the present disclosure, in the first aspect, a plurality of the ribs are provided at equidistant intervals in a circumferential direction of the pile.
In the pile foundation according to the second aspect of the present disclosure, even in a case in which horizontal forces are input into a pile from various different directions, it is still possible to ensure sufficient resistance to these horizontal forces.
In a pile foundation according to a third aspect of the present disclosure, in the first aspect or second aspect, upper end portions of the ribs are positioned on a lower side of the ground surface of the ground.
In the pile foundation according to the third aspect of the present disclosure, because the ribs extends above the ground surface of the ground, a floor slab or the like may be provided on this ground surface.
In a pile foundation according to a fourth aspect of the present disclosure, in anyone of the first through third aspects, the pile is formed by a steel pipe, and the ribs are formed from a steel material.
In the pile foundation according to the fourth aspect of the present disclosure, because the pile and the ribs are formed from a steel material, when fixing the ribs to the pile, in addition to a method in which the ribs are fixed by being mechanically fastened thereto using nuts and bolts and the like, it is also possible to fix the ribs to the pile using a method such as welding or the like.
In a pile foundation according to a fifth aspect of the present disclosure, in anyone of the first through fourth aspects, the tower-type structure forms a leg portion of a wind power generator.
In the pile foundation according to the fifth aspect of the present disclosure, even in a case in which an external force from a heavy object such as a wind power generator that is acting in a direction that might cause the pile to topple over is input into the pile, the ribs make it possible to ensure sufficient resistance to such a horizontal force.
In a pile foundation according to a sixth aspect of the present disclosure, in any one of the first through fifth aspects, the pile is formed in a tubular shape, and internal ribs that protrude inwardly in radial directions from an inner circumferential surface of a portion of the pile that is embedded in the ground are provided.
In the pile foundation according to the sixth aspect of the present disclosure, the rigidity of the pile may be further improved by providing internal ribs on the inner circumferential surface of the pile in addition to the ribs on the outer circumferential surface of the pile. As a result of this, even if the size of the pile is reduced, it is still possible to ensure the required resistance.

Advantageous Effects of the Invention

As has been described above, according to the pile foundation and method of constructing a pile foundation of the present disclosure, it is possible to ensure sufficient resistance to horizontal force while enabling the construction time to be shortened.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view illustrating an overall view of a wind power generator in which a pile foundation according to a first exemplary embodiment has been applied.

FIG. 2A is an elevational view of the pile foundation according to the first exemplary embodiment.

FIG. 2B is a plan cross-sectional view illustrating a state across a line 2A-2A illustrated in FIG. 2A.

FIG. 3A is a schematic view illustrating a slip plane when an external force is acting on a pile of a comparative example.

FIG. 3B is a schematic view illustrating a slip plane when an external force is acting on the pile of the first exemplary embodiment.

FIG. 4A is an elevational view of a pile foundation according to a first modified example of the first exemplary embodiment.

FIG. 4B is a plan cross-sectional view illustrating a state across a line 4B-4B illustrated in FIG. 4A.

FIG. 5A is a plan cross-sectional view of a pile foundation according to a second modified example of the first exemplary embodiment.

FIG. 5B is a plan cross-sectional view of a pile foundation according to a third modified example of the first exemplary embodiment.

FIG. 5C is a plan cross-sectional view of a pile foundation according to a fourth modified example of the first exemplary embodiment.

FIG. 6A is an elevational cross-sectional view of a pile foundation according to a fifth modified example of the first exemplary embodiment.

FIG. 6B is an elevational cross-sectional view of a pile foundation according to a sixth modified example of the first exemplary embodiment.

FIG. 7A is an elevational cross-sectional view of a pile foundation according to a seventh modified example of the first exemplary embodiment.

FIG. 7B is an elevational cross-sectional view of a pile foundation according to an eighth modified example of the first exemplary embodiment.

FIG. 8A is an elevational view of a pile foundation according to a second exemplary embodiment.

FIG. 8B is a plan view of the pile foundation according to the second exemplary embodiment.

DESCRIPTION OF THE EMBODIMENTS

First Exemplary Embodiment

A pile foundation 10 according to a first exemplary embodiment will now be described with reference to the drawings. As is illustrated in FIG. 1, the pile foundation 10 of the present exemplary embodiment serves as a foundation to support a wind power generator 12.
The wind power generator 12 is structured to include a leg portion (i.e., a tower) 14 serving as a tower-type structure that extends in a vertical direction from the pile foundation 10, and a wind turbine portion 16 that is provided on an upper end portion of the leg portion 14. The wind turbine portion 16 is structured to include a nacelle 18, a hub 20, and blades 22.
The leg portion 14 is formed so as to become progressively smaller in diameter approaching the upper portion thereof, and a lower end of this leg portion 14 is connected to the pile foundation 10. The nacelle 18 that forms part of the wind turbine 16 is mounted on the upper end portion of the leg portion 14 so as to be able to rotate freely around this upper end portion, and an electricity generator and an amplifier and the like (not illustrated in the drawings) are housed within this nacelle 18.
The nacelle 18 is connected to the hub 20 via a rotor shaft (not illustrated in the drawings). A plural the rotating blades 22 are attached to the hub 20 and, in the present exemplary embodiment, as an example, three blades 22 are attached to a circumferential surface of the hub 20.
The leg portion 14 of the wind power generator 12 that is formed in the above-described manner is supported on the pile foundation 10. Here, the pile foundation 10 of the present exemplary embodiment is structured to include a pile 24 and ribs 28.
The pile 24 is formed by a steel pipe whose axial direction extends in a vertical direction, and is disposed on substantially the same axis as the leg portion 14 of the wind power generator 12. Portions of the pile 24 other than a pile head 24A that is provided in an upper portion thereof are driven into the ground 26 using a pile-driving construction method. Here, in the present exemplary embodiment, since the pile 24 is used in the pile foundation 10 of an offshore wind power generator 12, the pile 24 is driven into the ocean bed, and is pile-driven to a depth of approximately 4 to 6 times the pile diameter of the pile 24 from the ground surface of the ground 26. In the present exemplary embodiment, as an example, a pile 24 having a pile diameter of 8 meters is used, so that the pile 24 is driven to a depth of approximately 40 meters from the surface of the ground 26.
As is illustrated in FIG. 2A, the pile head 24A protrudes above the ground 26. The plural ribs 28 are provided on the pile 24 in a vicinity of a ground surface of the ground 26. Each one of the ribs 28 extends in the axial direction of the pile 24 (i.e., in a vertical direction) and, in the present exemplary embodiment, extends downwards from the position of the ground surface of the ground 26. In other words, an upper end portion of each rib 28 is positioned at the ground surface of the ground 26, and is disposed so as to not protrude above this ground surface.
As is illustrated in FIG. 2B, eight ribs 28 are provided at equidistant intervals from each other in the circumferential direction of the pile 24. Each one of the ribs 28 protrudes outwardly in a radial direction from an outer circumferential surface of the pile 24.
Here, the ribs 28 are formed from a steel material, and are fixed using a method such as welding or the like to the outer circumferential surface of the pile 24 before the pile 24 is driven into the ground. Once the ribs 28 have been fixed to the pile 24, the pile 24 is driven into the ground 26 using a pile-driving construction method. Note that, the method employed to fix the ribs 28 may be a method such as mechanically fastening the ribs 28 using nuts and bolts.
(Actions)
Next, actions of the present exemplary embodiment will be described.
In the pile foundation 10 of the present exemplary embodiment, because the plural ribs 28 protrude outwardly in a radial direction from the outer circumferential surface of the pile 24, even in a case in which external force (i.e., a horizontal force) acting in a direction that might cause the pile 24 to topple over is input from the wind power generator 12 to the pile 24, it is possible to shift a slip plane of the pile 24 from the circumferential surface of the pile 24 to a distal end side of the ribs 28. As a result, sufficient resistance to horizontal force may be ensured. This action will now be described in detail with reference to FIG. 3.
As is illustrated in FIG. 3A, a case of a pile 100 of a comparative example in which the ribs 28 are not provided will now be considered. In this structure, in a case in which horizontal force is input into the pile 24, a slip plane P1 is formed at a boundary portion between the circumferential surface of this pile 24 and the ground 26.
In contrast to this, as is illustrated in FIG. 31, in the pile 24 that forms part of the pile foundation 10 of the present exemplary embodiment, by providing the ribs 28, a slip plane P2 is formed at a boundary portion between the distal end portion of the ribs 28 and the ground 26. In this way, in the pile foundation 10 of the present exemplary embodiment, compared to the comparative example in which the ribs 28 are not provided, it is possible for a greater degree of resistance to be received from the ground 26. Note that in FIG. 3B, for reasons of convenience, only the ribs 28 that receive resistance from the ground 26 are illustrated, and the remaining six ribs 28 have been omitted from the drawing.
Moreover, in a case in which the pile 24 is being pile-driven using a pile-driving construction method, the ground 26 in the vicinity of the circumferential surface of the pile 24 becomes softer as a result of the pile-driving performed on the pile 26. In other words, there is a reduction in the friction force thereof. In a structure such as this, in a case in which the ribs 28 are not provided, as the case in the comparative example, the slip plane P1 is generated in the portion of the circumferential surface of the pile 100 where there is little friction force. Because of this, there is a possibility that the resistance thereof will be reduced.
In the present exemplary embodiment, because the slip plane P2 is further to the outer side than the circumferential surface of the pile 24, it is possible to obtain resistance from the ground 26 in portions where the friction force thereof is greater, so that the required resistance to horizontal force may be ensured.
Furthermore, in the present exemplary embodiment, because it is possible to increase resistance to horizontal force simply by forming the ribs 28, it is not necessary to increase the diameter of the pile 24. Further, by extending the ribs 28 in the vertical direction, when the pile 24 is being driven into the ground using a pile-driving construction method, it is possible to reduce receiving resistance from the ribs 28. As a result, construction may be performed in the same way as in a case in which the ribs 28 are not provided, so that it is possible to shorten the construction time.
Furthermore, in the present exemplary embodiment, as is illustrated in FIG. 2B, a plural (i.e., eight) the ribs 28 are provided at equidistant intervals in the circumferential direction of the pile 24. Due to the above, even in a case in which horizontal forces are input from various different directions into the pile 24, it is still possible to ensure sufficient resistance to these horizontal forces.
Additionally, in the present exemplary embodiment, the upper end side of each rib 28 is positioned on the lower side of the ground surface. In other words, the ribs 28 do not protrude above the surface of the ground. Consequently, it is possible to provide a floor slab or the like on the ground surface, and to thereby increase resistance to horizontal force (see FIG. 8). In contrast, since the portion of the pile 24 that is located in the vicinity of the ground surface of the ground 26 is a portion where moment is increased, by providing the ribs 28 in this portion, it is possible to secure sufficient resistance to horizontal force.
Moreover, in the present exemplary embodiment, the pile 24 is formed by a steel pipe, and the ribs 28 are also formed from a steel material. Due to the above, when the ribs 28 are being fixed to the pile 24, in addition to a method in which the ribs 28 are fixed to the pile 24 by being mechanically fastened thereto using nuts and bolts and the like, it is also possible to fix the ribs 28 to the pile 24 using a method such as welding or the like.
Note that, in the above-described exemplary embodiment, the interior of the metal pipe used to form the pile 24 is a hollow cavity, however, the present disclosure is not limited to this, and it is also possible, for example, to employ the structure of a first modified example which is illustrated in FIG. 4. Moreover, the structures of a second modified example through an eighth modified example, which are illustrated in FIG. 5 through FIG. 7, may also be employed for the shape and number of the ribs.

First Modified Example

As is illustrated in FIG. 4A, in the present modified example, what is known as a CFT (Concrete-Filled Steel Tube) structure in which the interior of the pile 24, which is formed by a steel pipe, is filled with concrete 32 is employed. As is illustrated in FIG. 4B, as a result of the interior of the pile 24 being filled with concrete 32, the concrete 32 adheres to the inner circumferential surface of the steel pipe.
As is illustrated in FIG. 4A, the concrete 32 fills the pile 24 from the pile head 24A, which is positioned on the upper side of the ground surface of the ground 26, as far as the lower side of a lower end portion of the ribs 28. By filling this portion where the pile 24 requires superior yield strength with the concrete 32 in this way, it is possible to reduce the thickness of the pile 24, or to reduce the diameter of the pile 24.

Second Modified Example

As is illustrated in FIG. 5A, in the present modified example, 16 ribs 28 are provided at equidistant intervals from each other in the circumferential direction of the pile 24. Each one of the ribs 28 protrudes outwardly in a radial direction from the outer circumferential surface of the pile 24. Additionally, the lengths of the ribs 28 from the circumferential surface of the pile 24 are all the same length.

Third Modified Example

As is illustrated in FIG. 5B, in the present modified example, internal ribs 34 that protrude inwardly in radial directions from an inner circumferential surface of a portion of the pile 24 that is embedded in the ground 26 are provided. As a result of the internal ribs 34 being provided in this way on the inner circumferential surface of the pile 24 in addition to the ribs 28 on the outer circumferential surface of the pile 24, the rigidity of the pile 24 may be further improved. As a result of this, even if the size of the pile 24 is reduced, it is still possible to ensure the required resistance.
Note that, in FIG. 5B, the interior of the pile 24 is illustrated as a hollow cavity, however, because the pile 24 is driven into the ground 26 by a pile-driving construction method, the interior of the pile 24 actually contains dirt and sand.

Fourth Modified Example

As is illustrated in FIG. 5C, in the present modified example, the length of each rib 28 from the circumferential surface of the pile 24 is made shorter than in the first exemplary embodiment. For example, in a case in which the ground 26 is comparatively hard, by shortening the length of the ribs 28, it becomes possible to pile-drive the pile 24 more easily.

Fifth Modified Example

As is illustrated in FIG. 6A, in the present modified example, ribs 40 protrude outwardly in radial directions from the circumferential surface of the pile 24, and a lower end portion of each rib 40 is formed such that a length thereof in the radial direction becomes progressively shorter the closer it is to the lower side. Further, the lower end portion of each rib 40 in the present modified example is inclined so as to be located further to the outer side in a radial direction the closer it is to the lower side.

Sixth Modified Example

As is illustrated in FIG. 6B, in the present modified example, ribs 42 protrude outwardly in radial directions from the circumferential surface of the pile 24, and a lower end portion of each rib 42 is inclined so as to be located further to the inner side in a radial direction the closer it is to the lower side. By forming the lower end portion of each rib such that they are inclined in the manners illustrated in the fifth modified example and the sixth modified example, it is possible to pile-drive the pile 24 more easily.

Seventh Modified Example

As is illustrated in FIG. 7A, in the present modified example, ribs 44 protrude outwardly in radial directions from the circumferential surface of the pile 24, and a lower end portion of each rib 44 is curved so as to be located further to the inner side in a radial direction the closer it is to the lower side.

Eighth Modified Example

As is illustrated in FIG. 7B, ribs 46 of the present modified example are curved in the opposite direction from those in the seventh modified example. In other words, a lower end portion of each rib 46 is curved so as to be located further to the outer side in a radial direction the closer it is to the lower side.

Second Exemplary Embodiment

Next, a pile foundation 50 according to a second exemplary embodiment will be described with reference to the drawings. Note that component elements that are similar to those of the first exemplary embodiment are given the same descriptive symbols and any description thereof is omitted when this is appropriate.
The foundation pile 50 of the present exemplary embodiment is formed so as to include the pile 24, the ribs 28, and a floor slab 52, and the pile 24 and ribs 28 have a similar structure as those of the first exemplary embodiment. The floor slab 52 is provided on a pile head 24A of the pile 24, and is structured to include a base 54 and triangular plates 56.
The base 54 is formed such that a thickness direction thereof extends in the axial direction of the pile 24 (i.e., in the vertical direction), and the base 54 is installed on the ground 26. As is illustrated in FIG. 8B, the base 54 is formed in a substantially circular shape so as to be concentric with the pile 24 when looked at in plan view. In the present exemplary embodiment, as an example, the base 54 is formed from a steel material, and is fixed to a circumferential surface of the pile head 24A. A method employing mechanical fastening using nuts and bolts or the like may be employed as the method used to fix the base 54 to the pile head 24A.
A plural the triangular plates 56 are provided on an upper surface side of the base 54. Eight of the triangular plates 56 are provided at equidistant intervals from each other in a circumferential direction of the pile 24, and each of the triangular plates 56 is formed substantially in a triangular shape such that one rectilinear portion thereof extends in a direction along the pile 24, and another rectilinear portion thereof extends in a direction along the base 30. Moreover, in the present exemplary embodiment, the position of each triangular plate 56 coincides with the position of the corresponding rib 28. Due to the above, when looked at in plan view, the triangular plates 56 and the ribs 28 are positioned so as to mutually overlap each other vertically.
A lower end surface of each triangular plate 56 extends in a radial direction of the pile 24 along the base 54, and is fixed to the upper surface of the base 54. A side surface of each triangular plate 56 that is positioned closest to the center of the pile 24 extends in the vertical direction along the pile head 24A, and is fixed to the pile head 24A. A method employing mechanical fastening using nuts and bolts or the like may be employed, in the same way as for the base 54, as the method used to fix the triangular plates 56 to the base 54 and the pile head 24A.
As is described above, the floor slab 52 is laid on top of the ground 26, and is fixed to the pile head 24A. Due to the above, a structure is created in which external force acting on the pile 24 is transmitted to the ground 26 via the floor slab 52.
(Method of Constructing a Pile Foundation)
Next, an example of a method of constructing the pile foundation 50 of the present exemplary embodiment will be described. Firstly, while the pile 24 and the floor slab 52 are still in a state of mutual separation from each other, the ribs 28 are fixed in advance to the pile 24. Next, the pile 24 is driven to a predetermined depth into the ground 26 using a pile-driving construction method. If this type of pile-driving construction method is employed, then the ground 26 is not restricted to being sandy ground or comparatively soft gravel ground, and the pile 24 may even be constructed in (i.e., pile-driven into) soft rock.
Next, the floor slab 52 is fixed to the driven pile 24. Here, in the present exemplary embodiment, because this operation involves fixing the floor slab 52 to the pile head 24A under the sea, a method may be employed in which the floor slab 52 is formed with the triangular plates 56 attached in advance to the base 54, and in this state, the floor slab 52 is fitted over the pile head 24A from the upper side of the pile 24, and is then installed on the ground 26.
After the floor slab 52 has been installed on the ground 26, the base 54 and the triangular plates 56 are fixed to the pile head 24A using a predetermined method. In this manner the pile foundation 50 is constructed.
(Actions)
Next, actions of the present exemplary embodiment will be described.
In the pile foundation 50 of the present exemplary embodiment, the floor slab 52 is installed on the ground 26, and this floor slab 52 is fixed to the pile 24 and is formed so as to transmit any force acting on the pile 24 to the ground 26. As a result, even in a case in which external force acting in a direction that might cause the pile 24 to topple over is input from the leg portion 14 of the wind power generator 12, which is a tower-type structure, to the pile 24, at least a portion of this external force may be transmitted to the ground 26 via the floor slab 52, so that the ability of the pile 24 to withstand horizontal force may be secured. The remaining actions are similar to those of the first exemplary embodiment.
First and second exemplary embodiments of the present disclosure as well as modified examples thereof have been described above, however, it should be understood that various modifications and the like may be made thereto insofar as they do not depart from the spirit or scope of the present disclosure. For example, in the above-described exemplary embodiments, a description is given of a pile foundation that supports a wind power generator that is serving as a tower-type structure, however, the present disclosure is not limited to this. In other words, the present disclosure may instead be applied to a pile foundation that supports another tower-type structure, or to a pile foundation that supports a tower-type structure such as a steel tower. In this case, by pile-driving a plural piles into the ground, it is possible to support a tower-type structure such as a steel tower.
Moreover, in the above-described exemplary embodiments, a monopile foundation that supports a wind power generator by a single pile is described, however, the present disclosure is not limited to this and may be applied to other types of foundations. For example, the present disclosure may also be applied to a tripod type of foundation in which three piles are pile-driven into the ground, and these three piles are linked together so as to support a wind power generator. In this case, by providing ribs respectively for each of the piles, the same type of actions as those demonstrated in the above-described exemplary embodiments may be obtained.
Furthermore, in the above-described exemplary embodiments, the piles are formed by steel pipes, however, the material used to form the piles is not limited to this, and the piles may instead be formed from another type of material. For example, wooden piles made from wood and concrete piles made from concrete may also be used. It is also possible to use a combination of these materials.
Moreover, in the above-described exemplary embodiments, the upper end portion of each rib is located at the ground surface of the ground 26, however, the present disclosure is not limited to this, and it is also possible to employ a structure in which the upper end portion of each rib protrudes above the ground surface.
In the fifth through eighth modified examples illustrated in FIG. 6 and FIG. 7, a structure in which the shape of the lower end portion of the ribs is altered is described, however, it is also possible to additionally form the distal end portion of each rib as a sharp point. For example, in the ribs 28 illustrated in FIG. 2A, it is also possible to form the lower end portion of each rib 28 such that the thickness thereof becomes progressively thinner the closer it is to the lower end side thereof. By employing this type of structure, because the lower end portion is sharpened, it is possible to reduce the resistance from the ribs 28 when the pile 24 is being pile-driven.
Priority is claimed on Japanese Patent Application No. 2018-164264, filed Sep. 3, 2018, the disclosure of which is incorporated herein by reference.
All references, patent applications and technical specifications cited in the present specification are incorporated by reference into the present specification to the same extent as if the individual references, patent applications and technical specifications were specifically and individually recited as being incorporated by reference.

Claims

1. A pile foundation comprising:

a pile that extends in a vertical direction, that has a pile head protruding above the ground, and that supports a tower-type structure; and

ribs that are provided at the pile in a vicinity of a surface of the ground, and that protrude outwardly in radial directions from an outer circumferential surface of the pile.

2. The pile foundation according to claim 1, wherein a plurality of the ribs are provided at equidistant intervals in a circumferential direction of the pile.

3. The pile foundation according to claim 1, wherein upper end portions of the ribs are positioned on a lower side of the surface of the ground.

4. The pile foundation according to claim 1, wherein:

the pile is formed by a steel pipe, and

the ribs are formed from a steel material.

5. The pile foundation according to claim 1, wherein the tower-type structure forms a leg portion of a wind power generator.

6. The pile foundation according to claim 1, wherein:

the pile is formed in a tubular shape, and

internal ribs that protrude inwardly in radial directions from an inner circumferential surface of a portion of the pile that is embedded in the ground are provided.