JP7787682B2 - Tip support structure for hollow concrete piles - Google Patents

Description

The present invention relates to a tip support structure for hollow concrete piles, such as PHC piles, SC piles, and hollow RC piles, which are supported by a foot protection section.

For buried piles, an expanded base protection section is created at the tip of the pile using cast-in-place concrete or similar, and the load acting on the buried pile from the superstructure is transmitted to the supporting ground via the pile body and base protection section.

High-strength hollow concrete piles, such as PHC piles, SC piles, or centrifugally formed hollow reinforced concrete piles, are often used for these buried piles. Hollow concrete piles in particular can be efficiently constructed using the core excavation method, in which the shaft of an excavator is passed through the hollow part to excavate.

In the past, calculations of the tip bearing capacity of hollow concrete piles assumed that the load was transmitted to the base protection section via the bearing pressure at the tip surface of the hollow concrete pile. However, as the bearing capacity of embedded piles has increased, the bearing pressure at the tip surface of the hollow concrete pile alone is no longer sufficient to transmit the load. Therefore, for high-bearing-capacity piles, when the tip of the hollow concrete pile is blocked, for example, when the concrete in the base protection section seeps into the inside of the tip of the pile during construction, blocking the tip of the hollow concrete pile, the tip bearing capacity is generally calculated by assuming that the load is transmitted from the pile body to the base protection section via the bearing pressure of the entire cross-sectional area of the hollow concrete pile, including the blocked section at the tip surface.

In the case of high-bearing-capacity piles, whether sufficient bearing pressure is obtained across the entire cross section, including the inside of the tip surface of the hollow concrete pile, is often examined by comparing the bearing capacity obtained through FEM analysis, model tests, and full-scale pile compression tests.

In addition, in order to use the entire cross section of the hollow concrete pile, including the inside of its tip surface, as the basis for calculation, it is necessary to increase the unity between the tip of the hollow concrete pile and the base protection section and ensure as smooth a load transfer as possible, so the inner surface of the tip of the hollow concrete pile is usually made uneven.

For example, Patent Document 1 describes a tip-foot protection pile in which an enlarged diameter section made of hardening material is formed at the tip of a hollow precast pile, and the tip-foot protection pile has protrusions on the outer surface, or on both the outer and inner surfaces of the precast pile.

Patent Document 2 also describes a precast concrete pile with an internal protrusion, in which a protrusion that projects inward is formed on the inner peripheral surface of the tip of the pile body, and only this protrusion protrudes above the concrete surface, thereby enhancing the integrity of the base protection section with the concrete.

Japanese Patent Application Publication No. 06-280261 Japanese Patent Application Laid-Open No. 2007-247335

As mentioned above, when calculating the end bearing capacity of a high-bearing-capacity hollow concrete pile, it is common to calculate the end bearing capacity by assuming that the bearing pressure is applied over the area including the closed portion of the tip face of the hollow pile.

Figure 2 shows an example of the relationship between the tip of a PHC pile and the base protection section when constructed using a conventional high-bearing-capacity pile excavation method. In this example, a shear-stopping reinforcing bar 22 is provided on the outer surface of the tip 2 of the PHC pile, and a shear-stopping plate 21, which serves as a protrusion as described in Patent Document 2 mentioned above, protrudes from the inner surface to enhance the unity with the base protection section 3.

A friction cutter is attached via a hollow disk-shaped end plate to the outer periphery of the tip 2 of the PHC pile. In this case, concrete with a water-cement ratio (W/C) of about 60% and a strength of about 15 to 20 N/ ^mm2 is generally used for the foot protection section 3, and the embedded length L of the PHC pile into the foot protection section 3 is designed to be about 0.5 to 2D relative to the pile diameter D, and the diameter of the foot protection section is designed to be about 1.2 to 2D.

In this case, the bearing pressure at the tip of the hollow PHC pile is calculated assuming that the bearing pressure acts on the entire cross section when the hollow is blocked. Therefore, in order to block the hollow, it is necessary to install a shear stop plate on the inner surface of the pile, as mentioned above, and it is also necessary to install shear stop rebar on the outer surface, which results in problems such as increased pile manufacturing costs and reduced workability.

The present invention aims to solve the above-mentioned problems with conventional technology, and aims to provide an economical tip support structure for hollow concrete piles that allows for rational design when calculating the tip bearing capacity of hollow concrete piles.

The tip support structure for a hollow concrete pile of the present invention is a tip support structure for a hollow concrete pile in which the tip of a hollow concrete pile erected in the ground is supported by a base fixing portion whose diameter is larger than the outer diameter of the hollow concrete pile, characterized in that the strength of the base fixing portion is 15 to 30 N/ ^mm2 , more preferably 20 to 30 N/ ^mm2 , the tip of the hollow concrete pile has an open cross-section, and stress transmission in the pile axial direction from the tip of the hollow concrete pile to the base fixing portion is essentially carried out by the bearing pressure at the tip surface of the hollow concrete pile.

The hollow concrete pile referred to in this invention does not mean a pile whose body is formed solely from concrete, but also includes, for example, PHC piles, SC piles, or hollow RC piles formed by centrifugation.

By increasing the strength of the base fixing portion to 15 to 30 N/mm ² , and more preferably 20 to 30 N/mm ² , the load from the lower end surface of the hollow concrete pile can be directly borne by the part of the base fixing portion that contacts the lower end surface of the hollow concrete pile, making it possible to calculate the load transmission to the base fixing portion based solely on the bearing pressure at the lower end surface of the pile.

This eliminates the need to provide protrusions on the outer or inner surfaces of the tip of the conventional pile.

Furthermore, with this invention, there is no need to provide protrusions on the outer or inner surfaces of the pile tip; instead, calculations can be made using the bearing pressure at the bottom end surface of the hollow concrete pile, allowing commercially available prefabricated hollow concrete piles to be used as is.

The material for the base protection section can be high-strength soil cement, which is made by mixing a rich cement milk with soil and sand.Although this will vary depending on the type of cement and the admixtures used, in order to achieve a strength of 15 to 30 N/ ^mm2 , more preferably 20 to 30 N/ ^mm2 , it is desirable to increase the water-cement ratio of the cement milk to 40 to 50%, thereby increasing the concentration.

The strength of the base protection area can also be increased by using admixtures such as high-performance air-entraining water-reducing agents, or by blending various admixtures used in high-strength concrete.

Furthermore, in the tip support structure for hollow concrete piles of the present invention, adding magnesium carbonate to the concrete improves the homogeneity of the concrete, and the bleeding suppression effect prevents the occurrence of weak spots in areas such as where the base protection portion comes into contact with the tip surface of the hollow concrete pile.

The preferred amount of magnesium carbonate is approximately 0.1 to 4.0% by mass relative to the cement.

With the above-described configuration, stress transmission in the axial direction of the pile from the tip of the hollow concrete pile to the base protection section can be achieved by a bearing pressure at the tip surface that is essentially equivalent to the width of the wall thickness of the hollow concrete pile.

Since the tip bearing capacity of a pile is determined by the bearing capacity of the ground at the bottom of the base, it can be said that in order to increase the tip bearing capacity, it is better to have a larger diameter base, but in that case, the premise is that the base will not break when the load is transmitted over a donut-shaped area equivalent to the width of the wall thickness of the hollow concrete pile as described above.

Due to the upper limit of the bearing capacity of the donut-shaped base protection section, which has an area equivalent to the width of the hollow concrete pile's wall thickness, the diameter of the base protection section must be approximately 1.5 times the outer diameter of the hollow concrete pile.

On the other hand, in order to increase the tip bearing capacity, from the perspective of increasing the diameter of the base protection section, it is possible to fill the filler material from the tip of the pile upward to an area at least four times the pile's inner diameter without installing a shear stopper plate or the like inside the hollow concrete pile. This closes the open cross-section at the tip of the hollow concrete pile, making it possible to transmit load across the closed cross-sectional area at the tip of the hollow concrete pile and improving the bearing strength of the base protection section.

In this case, the diameter of the base protection section can be greater than 1.5 times the outer diameter of the hollow concrete pile, up to approximately 1.75 times, or even larger depending on the conditions.

The filling material can be soil cement or other materials with substantially the same composition and strength as the base protection section, but it does not necessarily have to be the same composition and strength as the base protection section; soil cement or cement milk with higher strength than the base protection section can also be used.

Filling material may also be filled only in a specified section at the tip of the hollow concrete pile.

The construction method for the hollow concrete piles in the hollow concrete pile tip support structure of the present invention is primarily the core excavation pile construction method, but this is not necessarily limited to the core excavation pile construction method, and the hollow concrete piles can also be erected later using the pre-boring method.

In particular, in the case of hollow concrete piles constructed using the core pile construction method, as in the past, it is desirable to provide friction cutters around the outer periphery of the tip of the hollow concrete pile to reduce frictional resistance from the ground when the hollow concrete pile is erected into the ground.

In this case, there is the problem of soil remaining stuck to the inside of the friction cutter, but as with hollow concrete piles constructed using the conventional core-drilling pile method, the stuck soil can be removed using an expandable soil removal plate attached to the shaft of the earth auger, or by spraying water or other high-pressure material from the tip of the earth auger, as with hollow concrete piles constructed using the conventional core-drilling pile method.

In addition, it is possible to provide a ring-shaped member with an upwardly tapered surface on the inside of the friction cutter to prevent soil and sand from adhering. For example, the ring-shaped member can be a metal piece that has been machined or molded into a tapered ring and welded, but there are no particular restrictions on the material, manufacturing method, or installation method.

Alternatively, a friction cutter and a ring-shaped member may be processed or molded as a single unit.

By increasing the strength of the base fixing portion to 15 to 30 N/mm ² , and more preferably 20 to 30 N/mm ² , the stress from the lower end surface of the hollow concrete pile can be directly borne by the part of the base fixing portion that contacts the lower end surface of the hollow concrete pile, making it possible to calculate the load transmission to the base fixing portion based solely on the bearing pressure at the lower end surface of the pile.

In this case, since the calculation only requires the bearing pressure at the bottom end surface of the hollow concrete pile, even if no protrusions are provided on the outer or inner surfaces of the pile tip, commercially available prefabricated hollow concrete piles can be used as is.

If the diameter of the base protection portion is up to about 1.5 times the pile diameter, setting the strength of the base protection portion to 15 to 30 N/mm ² , preferably 20 N/mm ² or more, will have significant benefits in terms of reducing pile manufacturing costs and improving workability.

Furthermore, if the strength of the base fixing portion is set to 15 to 30 N/mm ² , preferably 20 N/mm ² or more, and if a filler material is filled inside the hollow concrete pile to block the open cross section at the tip of the hollow concrete pile, the diameter of the base fixing portion can be increased to about 1.75 times the pile diameter, thereby increasing the tip bearing capacity.

FIG. 1 is a vertical cross-sectional view showing the basic concept of the tip support structure of a hollow concrete pile of the present invention. FIG. 1 is a vertical cross-sectional view showing an example of the relationship between the tip and the base of a PHC pile constructed using a conventional excavation method. 2A and 2B are explanatory diagrams of a method for removing soil from the bottom end surface of a hollow concrete pile in the form of FIG. 1, in which (a) is a vertical cross-sectional view showing soil adhering to the bottom end surface of the pile, (b) is a vertical cross-sectional view showing a method for removing soil using a soil removal plate, and (c) is a vertical cross-sectional view showing a method for removing soil using a high-pressure jet. 1A and 1B show modified tip shapes of the lower end of the pile as another embodiment of the tip support structure of the hollow concrete pile of the present invention, where (a) is an explanatory diagram of the concern of soil being left behind in the form of FIG. 1, (b) is an explanatory diagram of the concern of weak parts due to bleeding in the form of FIG. 1, and (c) is a vertical cross-sectional view showing modified tip shapes of the lower end of the pile. 4(c) shows examples of the tip shape of the lower end of the pile for the embodiment of FIG. 4(c), where (a) is a cross-sectional view when the friction cutter and the ring-shaped member having a tapered portion are separate, (b) is a cross-sectional view when the ring-shaped member having a tapered portion also serves as the friction cutter, (c) is a cross-sectional view when a tapered portion integral with the pile body is provided at the tip of the hollow concrete pile, and (d) is a cross-sectional view when a tapered portion integral with the pile body is provided at the tip of the hollow concrete pile, and the outer periphery of the tapered portion is further extended to also serve as a friction cutter. 4(a) and 4(b) are conceptual diagrams showing the stress direction from the tip of the hollow concrete pile to the base reinforcement part and the fracture line of the fracture image in the tip support structure of the hollow concrete pile of the present invention, where (a) is a conceptual diagram corresponding to the embodiment of FIG. 1, and (b) is a conceptual diagram corresponding to the embodiment of FIG. 4(c). 1(a) and 1(b) are explanatory diagrams showing the embedding length of the tip of a hollow concrete pile into the base protection portion in the tip support structure of a hollow concrete pile of the present invention, where (a) is a conceptual diagram corresponding to the embodiment of FIG. 1, and (b) is a conceptual diagram corresponding to the embodiment of FIG. 4(c). FIG. 2 is an explanatory diagram of the bearing capacity mechanism of the entire pile foundation in the tip support structure of the hollow concrete pile of the present invention. This shows the relationship between the load transfer mechanism and the diameter of the foot reinforcement pile in the tip support structure of the hollow concrete pile of the present invention, where (a) is an explanatory diagram of the case where the tip of the hollow concrete pile is not blocked, and (b) is an explanatory diagram of the case where the tip of the hollow concrete pile is blocked by a filler material.

Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings.
FIG. 1 is a vertical cross-sectional view showing the basic concept of the tip support structure of a hollow concrete pile according to the present invention.

In the tip support structure of the hollow concrete pile of the present invention, the tip 2 of the hollow concrete pile 1 erected in the ground is supported by a base fixing portion 3 having a diameter larger than the outer diameter of the hollow concrete pile 1, and the strength of the base fixing portion 3 is set to 15 to 30 N/mm ² , more preferably 20 to 30 N/mm ² .

In addition, the diameter of the base protection section 3 is set to within 1.5 times the outer diameter of the hollow concrete pile, and the tip 2 of the hollow concrete pile 1 has an open cross-section that is not blocked. In this case, the excavation diameter of the base protection section 3 is smaller, so the amount of concrete used can be reduced.

By making the base protection section 3 strong in this way, stress transmission in the axial direction from the tip 2 of the hollow concrete pile 1 to the base protection section 3 can be achieved by the bearing pressure at the tip surface, which is essentially equivalent to the width of the wall thickness of the hollow concrete pile 1.

Figure 3 shows a modified tip shape at the bottom end of the pile as another embodiment of the tip support structure for a hollow concrete pile of the present invention. However, as with hollow concrete piles constructed using the conventional core pile construction method, there is a potential problem of soil and sand (s) adhering to and remaining inside the friction cutter (5), as shown in Figure 3(a).

To address this issue, for example, as shown in Figure 3(b), the adhering sediment s can be removed using the upper protrusion of an expandable sediment removal plate 24 attached to the shaft of the earth auger 23, or, as in the case of hollow concrete piles constructed using the conventional core pile construction method, as shown in Figure 3(c), water or other high-pressure spray can be used to remove the adhering sediment s from the nozzle 25 at the tip of the earth auger 23.

Figure 4 shows a modified tip shape of the lower end of the pile as another embodiment of the tip support structure for a hollow concrete pile of the present invention. Specifically, the configuration shown in Figure 4(c) is an embodiment of the modified tip.

Figure 4(a) shows the condition of remaining soil 31 due to the soil s being left behind, which is a concern in the configuration of Figure 1, and Figure 4(b) shows the condition of weak parts 32 due to bleeding, which is a concern in the configuration of Figure 1.

To address these issues, as shown in Figure 4(c), a ring-shaped member 6 with an upwardly tapered surface is provided inside the friction cutter 5. This prevents the generation of residual soil 31 as shown in Figure 4(a), and also prevents the creation of weak spots 32 due to bleeding as shown in Figure 4(b).

Figure 5 shows an example of the shape of the tip of the lower end of the pile for the embodiment shown in Figure 4(c) above. Figure 5(a) shows a case in which the friction cutter 5 and the metal fittings in the form of a ring-shaped member 6 with a tapered portion are separate and attached by welding.

Figure 5(b) shows a case in which a ring-shaped member 6a with a tapered portion doubles as a friction cutter, and the outer surface of the ring-shaped member 6a protrudes outward beyond the tip 2 of the hollow concrete pile.

Figure 5(c) shows a case where the ring-shaped member with a tapered portion is not made of metal, but is formed from concrete or other material that is integral with the main body of the hollow concrete pile. In other words, the tip 2 of the hollow concrete pile is formed with a tapered portion that is integral with the main body of the pile, and has a tapered surface that tapers upward, as the ring-shaped member 6b.

Figure 5(d) shows a ring-shaped member 6c that also functions as a friction cutter, with the outer periphery of the ring-shaped member 6b in Figure 5(c) extending radially outward from the pile, forming a tapered portion integral with the pile body.

Figure 6 is a conceptual diagram showing the stress direction from the tip of the hollow concrete pile to the base protection section and the fracture line of the fracture image in the tip support structure of the hollow concrete pile of the present invention.

Figure 6(a) shows the stress direction from the tip 2 to the base protection section 3 of the hollow concrete pile 1 corresponding to the embodiment in Figure 1, as well as the fracture line of the fracture image. Stress is transmitted almost perpendicularly from the end plate 4 on the underside of the tip 2 to the base protection section 3.

In contrast, Figure 6(b) shows the stress direction from the tip 2 to the base protection section 3 of the hollow concrete pile 1 corresponding to the embodiment in Figure 4(c), as well as the fracture line of the fracture image. By providing a ring-shaped member 6 at the tip 2, stress is transmitted from the tapered surface of the ring-shaped member 6 to the base protection section 3 at an almost right angle. In this case, a slight confining effect from the tapered surface on the base protection section 3 can also be expected, improving the bearing capacity.

Figure 7 is an explanatory diagram showing the embedding length of the tip of a hollow concrete pile into the base protection section in the tip support structure of a hollow concrete pile of the present invention.

In the case of a hollow concrete pile constructed using the conventional excavation method, as shown in Figure 2, the embedded length L must be set to a certain length. However, in the case of the present invention, the stress from the lower end surface of the tip 2 of the hollow concrete pile 1 is transmitted essentially by bearing pressure alone. As shown in Figures 7(a) and (b), the embedded length L can be kept short, and the pile fixing process after constructing the foot protection section 3 can be omitted, shortening construction time and reducing the risk of the pile remaining high.

Figure 7(a) is a conceptual diagram corresponding to the embodiment of Figure 1, and Figure 7(b) is a conceptual diagram corresponding to the embodiment of Figure 4(c). In the case of Figure 7(b), as explained above in Figure 6(b), a ring-shaped member 6 is provided at the tip 2, and stress is transmitted from the tapered surface of the ring-shaped member 6 to the root protection portion 3 at a nearly right angle, making it possible to further shorten the embedded length L, and it is even possible to make the embedded length essentially close to zero.

Figure 8 is an explanatory diagram of the bearing capacity mechanism of the entire pile foundation in the tip support structure of the hollow concrete pile of the present invention, and the tip bearing capacity of the entire pile foundation is determined by the bearing capacity _Re of the ground at the bottom surface of the foot protection portion 3.

For this reason, it can be said that a larger diameter for the base protection section 3 is better for increasing tip bearing capacity, but in that case, the premise is that the base protection section 3 will not break when load is transmitted (bearing pressure) over a doughnut-shaped area equivalent to the width of the wall thickness of the tip section 2 of the hollow concrete pile 1.

Figure 9 is an explanatory diagram of the relationship between the load transfer mechanism in the foot protection section 3 and the diameter of the foot protection pile 3 when taking into account the bearing capacity mechanism in Figure 8.

Figure 9(a) shows the case where the tip 2 of the hollow concrete pile 1 is not blocked. Since the design is based only on the bearing pressure at the tip surface, it is desirable that the diameter _Df1 of the base protection section 3 be approximately 1.5 times the outer diameter D of the hollow concrete pile 1, provided that the load transfer section T does not break.

Figure 9(b) shows the case where the tip 2 of the hollow concrete pile 1 is blocked by filling material 7. By improving the strength of the load transmission section T, the diameter D _f2 of the base fixing section 3 can be made to accommodate up to approximately 1.75 times the outer diameter of the hollow concrete pile 1.

1... Hollow concrete pile, 2... Pile tip, 3... Foot protection portion, 4... End plate, 5... Friction cutter, 6... Ring-shaped member, 7... Filling material,
21... Shear stop plate, 22... Shear stop rebar, 23... Earth auger, 24... Soil removal plate, 25... Injection nozzle,
31... Leftover soil and sand, 32... Weak areas due to bleeding,
L: embedded length,
p...bearing pressure, P...load, R _e ...bearing capacity of the ground, T...load transmission part,
s...adherent soil, h...high pressure injection

Claims (6)

a hollow concrete pile having a tip support structure in which the tip of the hollow concrete pile erected in the ground is supported by a base protection section with a diameter larger than the outer diameter of the hollow concrete pile, the strength of the base protection section is 20 to 30 N/ ^mm2 , the tip of the hollow concrete pile has an open cross-section, and the inner and outer surfaces of the tip of the hollow concrete pile are free of protrusions, irregularities, or stoppers for integrating it with the base protection section made of soil cement , a friction cutter is provided on the outer periphery of the tip of the hollow concrete pile to reduce frictional resistance from the ground when the hollow concrete pile is erected in the ground, and a metal ring-shaped member with a tapered surface that narrows upward is provided on the inside of the friction cutter, so that stress in the pile axial direction from the tip of the hollow concrete pile to the base protection section is substantially carried out by the bearing pressure of the tip surface of the hollow concrete pile. 2. The tip support structure for a hollow concrete pile according to claim 1 , wherein the friction cutter and the ring-shaped member are integral with each other. The tip support structure for a hollow concrete pile according to claim 1, wherein the hollow concrete pile is a PHC pile, a SC pile, or a centrifugally formed hollow RC pile. 2. The tip support structure of a hollow concrete pile according to claim 1 , wherein the base fixing portion is formed of soil cement, which is a mixture of cement milk having a water-cement ratio of 40 to 50% and soil and sand. 5. The tip support structure for a hollow concrete pile according to claim 4 , wherein the cement milk contains magnesium carbonate. 6. The tip support structure for a hollow concrete pile according to claim 1 , wherein the diameter of the base fixing portion is within 1.5 times the outer diameter of the hollow concrete pile.