Pile foundation structure and method for constructing a pile foundation structure

Abstract

This invention proposes a pile foundation structure and a method for constructing this pile foundation structure that ensures bending strength at the pile head joint in the case of a rigid connection, and allows for a wide range of control over the degree of fixation α at the pile head in the case of a semi-rigid connection, while also enabling rational design. [Solution] A pile foundation structure 2 consisting of a pile 3 and a base plate 4 of a structure placed on top of the pile 3, wherein the pile 3 is a pre-fabricated pile 6 having an enlarged diameter portion 63 at its upper end, and is formed such that the diameter of the pile head is larger than the diameter of the smallest part of the pile 3.

Description

Technical Field

[0001] The present invention relates to a pile foundation structure of a structure and a method for constructing the pile foundation structure.

Background Art

[0002] In building structures and civil engineering structures, when the surface ground is soft ground, pile foundations are often adopted to stably support the load of the structure. The types of piles used in pile foundations include precast piles (concrete piles and steel pipe piles) manufactured in factories and transported and buried at the construction site, and cast-in-place concrete piles manufactured at the construction site. Among these, precast concrete piles are cylindrical with a pile diameter of about 300 to 1200 mm, and the pile length per piece is about 5 to 15 m, and they are connected and used as needed. In a pile foundation, the head of the pile is joined to the bottom slab of the structure (including the pile cap).

[0003] In the design of a pile foundation, first, the pile length, pile diameter, number of piles, and grade of the pile for safely supporting the vertical load of the structure are considered. Next, the safety against the horizontal load acting on the pile during an earthquake is examined, and the specifications of the pile that can ensure the safety of the pile most reasonably are determined. Note that the joining method between the pile head and the bottom slab of the structure in a pile foundation includes rigid joining and semi-rigid joining. Rigid joining is a pile head joining method in which the fixing bars of the pile are fixed to the bottom slab of the structure and firmly joined, and in design, the bending moment acting on the pile head is transmitted to the structure. On the other hand, semi-rigid joining is a pile head joining method in which the bending moment transmitted from the pile head to the structure is reduced by adding deformation performance to the pile head joint (see, for example, Patent Document 1).

[0004] Patent Document 2 shows that the degree of pile head fixation α in a semi-rigid connection increases with respect to the rotational stiffness Kθ of the pile head connection, that the rotational stiffness Kθ of the pile head connection increases with respect to the pile head bending moment M0 (Mf in the cited document) based on a hyperbolic function, and that the pile head bending moment M0 is represented by a hyperbolic function whose initial stiffness is K0 and whose maximum value asymptotically approaches M0max (=N*D / 2, N: axial force, D: axial diameter, Mmax in the cited document) (see Figure 2 (Equation 1-3), Figure 5 (Equation 3-1, Equation 3-2-1, Equation 3-2-2)). In other words, it is known that α in a semi-rigid connection increases with respect to the axial force N and the axial diameter D. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2023-056207 [Patent Document 2] Japanese Patent Publication No. 2007-247287 [Overview of the project] [Problems that the invention aims to solve]

[0006] In rigid pile foundation structures, a large bending moment acts on the pile head, while the bending moment in the ground is smaller. In rigid pile foundation structures, it is necessary to transmit the bending moment acting on the pile head to the base slab of the structure, so anchoring reinforcement must be fixed to the base slab. In recent years, the axial force borne by a single pile has been increasing, so there are more cases where the bending moment acting on the pile head during an earthquake becomes excessive. In this case, the amount of reinforcement in the pile head increases, so it is necessary to increase the pile diameter, but increasing the pile diameter is uneconomical.

[0007] In a semi-rigid pile foundation structure, the bending moment at the pile head is reduced and the bending moment in the subsurface increases compared to a rigid connection. Generally, the reduction rate of the bending moment at the pile head is expressed by the pile head fixity α. α=1.0 indicates a rigid connection with a reduction rate of zero. α=0.5 indicates a semi-rigid connection with a reduction rate of 50%. α=0.0 indicates a pin connection with a reduction rate of 100%. For a semi-rigid connection, the pile head fixity α is preferably a value such that the bending moment at the pile head and the bending moment in the subsurface are of similar magnitude (for example, α=0.41). The pile head fixity α depends not only on the structure (specifications) of the pile head connection, but also mainly on the axial force at the pile head, the horizontal force acting on the pile head, and the hardness of the ground. Of these, the axial force at the pile head depends on the weight of the structure, and the magnitude of the horizontal force is determined by the target seismic standards. Furthermore, the hardness of the ground depends on the site conditions. Therefore, it is difficult to control the pile head fixity α through design ingenuity.

[0008] Furthermore, while semi-rigid connections reduce the bending moment at the pile head compared to rigid connections by providing deformation capabilities to the pile head connection, there is a limit (allowable value) to this deformation capability. This limit depends on the compressive stress acting on the contact surface between the base slab of the structure and the pile head, and decreases as the compressive axial force increases. Since this compressive stress is proportional to the axial force at the pile head, the limit becomes smaller for piles on the compression side during earthquakes. In recent years, the axial force borne by a single pile has tended to increase, so the deformation capability of semi-rigid connections is limited by the limit (allowable value), making rational design difficult in some cases.

[0009] The present invention aims to propose a pile foundation structure and a method for constructing this pile foundation structure that ensures bending strength at the pile head joint in the case of a rigid joint, and that allows for a wide range of control over the degree of fixation α at the pile head in the case of a semi-rigid joint, while also enabling rational design. [Means for solving the problem]

[0010] The pile foundation structure of the present invention for solving the aforementioned problems comprises a pile and a base plate of a structure supported by the pile, wherein the pile has an enlarged diameter precast pile having an enlarged diameter portion, and the pile diameter of the pile head is the same as the pile diameter of the enlarged diameter portion. Preferably, the pile head of the pile is formed by the enlarged diameter portion. Furthermore, the pile may be semi-rigidly connected to the base plate or rigidly connected to the base plate. In addition, a frustoconical recess may be formed in the bottom surface of the base plate into which the upper end of the pile is inserted, creating a gap between the recess and the outer surface of the upper end of the pile.

[0011] Furthermore, the method for constructing a pile foundation structure according to the present invention includes a pile driving step of forming a pile by placing an enlarged-diameter precast pile having an enlarged-diameter portion at its upper end in the ground, and a base plate forming step of forming the base plate on top of the pile. If the pile is made up of a general precast pile with a constant outer diameter and the enlarged-diameter precast pile connected together, it is desirable to place the enlarged-diameter precast pile at the top in the pile driving step.

[0012] With this pile foundation structure, the degree of fixation α of the semi-rigid connection type pile head can be rationally controlled by changing the pile diameter at the pile head without changing the diameter of the pile shaft (general section diameter) determined from the standpoint of safety and economics. Furthermore, it is possible to broaden the control range of α by adjusting the diameter expansion ratio of the prefabricated pile. In addition, the axial stress at the pile head joint can be reduced by increasing the contact area of ​​the joint surface between the pile and the base slab of the structure. This improves the allowable rotation angle (deformation performance), and consequently enables rational design. Furthermore, in a rigid connection type pile foundation structure, the axial stress at the pile head joint can be reduced by increasing the contact area between the pile and the base slab, and the amount of reinforcement that can be placed at the pile head can be increased. This improves the bending strength of the pile head.

[0013] In the case of a pile foundation structure consisting of multiple piles and a base slab of a structure supported by the multiple piles, it is desirable that the piles other than those arranged on the outer periphery of the base slab have an enlarged diameter precast pile with an enlarged diameter section, and that the pile diameter at the pile head is the same as the pile diameter of the enlarged diameter section.

[0014] Here, the degree of pile head fixation α in a semi-rigid connection depends on the axial force at the pile head, and increases as the axial force increases. During an earthquake, the axial force at the pile head is not only due to the weight of the structure, but also to the fluctuating axial force caused by the overturning moment of the structure. Therefore, during an earthquake, depending on the direction of the seismic force, there will be piles with larger axial forces (compression-side piles) and piles with smaller axial forces (pull-side piles). In this case, since the degree of pile head fixation α depends on the axial force, the horizontal force will concentrate on the piles with larger axial forces. However, since seismic forces act not in one direction but in alternating positive and negative directions, as a result, the piles on the outer perimeter of the structure will have to bear a larger seismic force compared to the piles in the center. Thus, in a semi-rigid connection type pile foundation, the axial force dependence of the degree of pile head fixation α causes an imbalance in the seismic force borne by each pile, and in some cases, rational design may not be possible depending on the conditions. On the other hand, with the aforementioned pile foundation structure, the uneven distribution of load on the outer perimeter piles can be reduced by enlarging the diameter of the pile heads of the central piles. [Effects of the Invention]

[0015] According to the pile foundation structure and method for constructing the pile foundation structure of the present invention, in the case of a rigid connection, the bending strength of the pile head connection can be secured, and in the case of a semi-rigid connection, the range over which the degree of fixation α of the pile head can be controlled can be increased, and a rational design is possible. [Brief explanation of the drawing]

[0016] [Figure 1] This is a cross-sectional view showing the pile foundation structure of the first embodiment. [Figure 2] (a) is a cross-sectional view showing a straight precast pile, and (b) is a cross-sectional view showing an enlarged diameter precast pile. [Figure 3] This is a perspective view showing the end plate. [Figure 4] This is a perspective view showing the formwork for the pile head. [Figure 5] This is a flowchart showing the procedure for constructing the pile foundation structure according to the first embodiment. [Figure 6] This is a cross-sectional view showing the work process of the construction method for the pile foundation structure of the first embodiment, where (a) is the pile driving process and (b) is the ground excavation process and the crushed stone layer formation process. [Figure 7] It is a cross-sectional view showing the working conditions of the construction method of the pile foundation structure following FIG. 6, where (a) is the formwork installation step, (b) is the concrete layer formation step, and (c) is the bottom plate formation step. [Figure 8] (a) is the bending moment distribution of the rigidly connected pile foundation structure and the semi-rigidly connected pile foundation structure, and (b) is the bending moment distribution of the rigidly connected pile foundation structure and the semi-rigidly connected structure with an enlarged pile head diameter. [Figure 9] It is an elevation view showing an example of the structure of the second embodiment. [Figure 10] It is a cross-sectional view showing the pile foundation structure of the second embodiment, where (a) is the pile foundation structure with peripheral piles, and (b) is the pile foundation structure with a central pile. [Figure 11] It is a graph showing the relationship between the bending moment and the rotation angle of the pile head in the pile head semi-rigid connection. [Figure 12] It is an explanatory diagram of the relationship between the bending moment and the rotation angle of the pile head in the pile head semi-rigid connection, where (a) is the initial state, (b) is the elastic region, (c) is the uplift boundary, (d) is the non-linear region, and (e) is the maximum bending moment. [Figure 13] (a) is an elevation view showing a structure with an earthquake force acting on it, (b) is a graph showing the horizontal force acting on the piles of the conventional pile foundation structure, and (c) is a graph showing the horizontal force acting on the piles of the pile foundation structure of the second embodiment. [Figure 14] It is a graph showing the bending moment distribution of the conventional pile and the pile of the second embodiment. [Figure 15] It is a cross-sectional view showing the pile foundation structure of the third embodiment. [Figure 16] It is a flowchart showing the procedure of the construction method of the pile foundation structure of the third embodiment. [Figure 17] It is a cross-sectional view showing the working conditions of the construction method of the pile foundation structure of the third embodiment, where (a) is the concrete layer formation step, and (b) is the bottom plate formation step. [Figure 18] (a) is an elevation view of a structure with a mixture of high-rise and low-rise parts having a conventional pile foundation structure, and (b) is an elevation view of a structure with a mixture of high-rise and low-rise parts having the pile foundation structure of the fourth embodiment. [Figure 19] This is a cross-sectional view showing an example of a pile foundation structure in a form other than the present invention. [Modes for carrying out the invention]

[0017] <First Embodiment> A pile foundation structure 2 according to the first embodiment will now be described. Figure 1 shows the pile foundation structure 2 of this embodiment. As shown in Figure 1, the pile foundation structure 2 consists of a pile 3 and a base plate 4 of a structure placed on top of the pile 3. The pile 3 has an enlarged diameter precast pile 62, which is a precast pile 6 having an enlarged diameter portion 63 at its upper end, provided at the very top. A straight precast pile 61 with a constant pile diameter is connected to the lower end of the enlarged diameter precast pile 62.

[0018] Figure 2(a) shows a straight precast pile 61. As shown in Figure 2(a), the straight precast pile 61 is a cylindrical concrete member in which reinforcing materials such as PC steel bars 65 and spiral reinforcing bars 66 are embedded. Annular end plates 7 are provided at both ends of the straight precast pile 61. The precast piles 6 are connected to each other via the end plates 7.

[0019] Figure 2(b) shows the enlarged diameter precast pile 62. As shown in Figure 2(b), the enlarged diameter precast pile 62 is a cylindrical concrete member in which reinforcing materials such as PC steel bars 65 and spiral reinforcing bars 66 are embedded. The general part 64 of the enlarged diameter precast pile 62 (the part other than the enlarged diameter section 63) has the same outer diameter as the straight precast pile 61, and has a smaller outer diameter than the enlarged diameter section 63. Annular end plates 7 are provided at both ends of the enlarged diameter precast pile 62.

[0020] Figure 3 shows the end plate 7. The end plate 7 is made of an annular steel plate whose outer and inner diameters are the same as those of the precast pile 6. Multiple bolt holes 71, 71, ... are formed in the end plate 7 at intervals in the circumferential direction. The arrangement of the bolt holes 71 formed in the end plate 7 (including the number and spacing) is determined as appropriate. A reinforcing band 72 is provided around the joint between the end plate 7 and the precast pile 6.

[0021] As shown in Figure 1, the upper end (end plate 7) of the pile 3 is in contact with the base plate 4. The base plate 4 is a reinforced concrete structure, and a recess 41 is formed at a position on the bottom surface corresponding to the pile 3 into which the upper end of the pile 3 is inserted. The recess 41 has a frustoconical shape, creating a gap between it and the outer surface of the upper end of the pile 3. The recess 41 is formed by pouring concrete into the base plate 4 with the pile head formwork 5 covering the pile head of the pile 3.

[0022] Figure 4 shows the pile head formwork 5. As shown in Figure 4, the pile head formwork 5 is formed by an upper part 51 that rests on the upper surface of the pile 3 and a side part 52 that extends downward from the edge of the upper part 51. The diameter of the side part 52 increases as it goes downward so that it moves away from the side surface of the pile 3. That is, the pile head formwork 5 has a frustoconical shape, and a gap E is formed between the side surface of the pile 3 and the inner surface of the side part 52, which expands as it goes downward (see Figure 1).

[0023] Multiple (two in this embodiment) through holes 53, 53 are formed in the upper part 51. Bolts that are screwed into bolt holes 71 formed in the end plate 7 can be inserted through the through holes 53. In addition, a ring-shaped flange 54 extending outward is formed at the lower end of the side part 52. By providing a flange 54 on the bottom surface of the pile head formwork 5, the risk of workers injuring their hands during transportation or installation of the pile head formwork 5 can be reduced. Furthermore, the flange 54 improves rigidity without increasing the thickness of the steel plate, and since there is no increase in weight due to the increase in thickness, it is possible to improve work efficiency and reduce material costs and transportation costs.

[0024] Next, the method for constructing the pile foundation structure will be explained. Figure 5 shows the procedure for constructing the pile foundation structure. As shown in Figure 5, the method for constructing the pile foundation structure comprises a pile driving process S11, a ground excavation process S12, a crushed stone layer formation process S13, a pile head formwork installation process S14, a concrete layer formation process S15, and a base slab formation process S16. Figures 6 and 7 show the work status of the pile foundation structure construction method.

[0025] In pile driving process S11, pre-fabricated piles 6 are placed in the ground to form a pile. In pile driving process S11, as shown in Figure 6(a), the piles are embedded in the ground G using a rotary press-in device (not shown). Pile 3 consists of a straight pre-fabricated pile 61 with a constant outer diameter and an enlarged pre-fabricated pile 62 connected together, and is placed in the ground with the enlarged pre-fabricated pile 62 positioned at the top (see Figure 1). When rotary press-in the pile 3, bolts B1 are screwed into the bolt holes 71 of the end plate 7 of the enlarged pre-fabricated pile 62 positioned at the top to prevent soil and other debris from clogging the bolt holes 71.

[0026] In the ground excavation process S12, as shown in Figure 6(b), the ground G surrounding the pile 3 is excavated to expose the pile head. Once the pile head is exposed, the bolt B1 that was screwed into the bolt hole 71 is also removed. In the crushed stone layer formation process S13, the unevenness of the base surface G1 is leveled, and then crushed stone is laid and compacted on the base surface G1 to form the crushed stone layer S. Depending on the ground conditions, the crushed stone layer S (crushed stone layer formation process) may be omitted.

[0027] In the pile head formwork installation process S14, as shown in Figure 7(a), the pile head formwork 5 is placed on the upper end surface of the pile 3, and the pile head is covered with the pile head formwork 5. This creates a gap E between the side surface of the pile head and the inner surface of the pile head formwork 5 that expands downwards. Once the pile head formwork 5 is placed over the pile head of the pile 3, bolts B2 inserted through the through holes 53 (see Figure 4) are screwed into the bolt holes 71 to temporarily fix the pile head formwork 5 to the pile 3 (pile head). After the pile head formwork 5 is temporarily fixed to the pile head of the pile 3, the gap between the flange 54 (lower edge) of the pile head formwork 5 and the upper surface of the crushed stone layer S is closed by piling ground material such as crushed stone or soil around the outer circumference of the lower end (flange 54) of the pile head formwork 5. In this embodiment, the pile head formwork 5 was placed over the pile head after the crushed stone layer S was formed, but the pile head formwork 5 may be placed over the pile head before the crushed stone layer S is formed.

[0028] In the concrete layer formation process S15, as shown in Figure 7(b), leveling concrete is poured around the pile head formwork 5 to form a leveling concrete layer C. After the leveling concrete has cured, the bolts B2 are removed. After removing the bolts B2 from the bolt holes 71, a pull-out resistance member (not shown) may be attached to the bolt holes 71 if necessary.

[0029] In the base slab formation process S16, first, reinforcing bars and formwork (not shown) are placed on top of the leveling concrete layer C. The reinforcing bars are placed around and above the pile head. Next, concrete for the base slab is poured into the formwork to form the base slab 4 on top of the leveling concrete layer C, as shown in Figure 7(c).

[0030] According to the pile foundation structure 2 of this embodiment, the connection between the pile 3 and the base slab 4 can be made into a semi-rigid connection state as anticipated in the design. In other words, because a gap is formed around the pile head of the pile 3 by the pile head formwork 5, rotation of the pile head is permitted, and even if horizontal forces (shear forces) acting on the structure due to earthquakes, wind, etc., act on the pile 3, a large bending moment will not be generated at the pile head.

[0031] Furthermore, according to pile foundation structure 2, the balance of bending moment between the pile head and the underground portion can be improved by enlarging the diameter of the pile head. Figure 8 and Table 1 illustrate the calculation results for the cases of rigid connection (Comparative Example 1), semi-rigid connection (Comparative Example 2), and semi-rigid connection with enlarged pile head (Example). The calculation conditions were that the ground was cohesive soil with an N value of 5 (uniform ground), the total length of the pile was 30m, and the shaft diameter of the pile was 600mm, with the ground and pile being elastic. The axial force was set to a long-term axial force of 1000kN and a fluctuating axial force during an earthquake of -700kN, and the bending moment distribution of the rigid connection (Comparative Example 1), semi-rigid connection (Comparative Example 2), and semi-rigid connection with enlarged pile head (Example) was compared when a horizontal force of 300kN was applied to the pile head. The diameter of the enlarged section was 800mm.

[0032] [Table 1]

[0033] As shown in Table 1 and Figure 8(a), comparing rigid and semi-rigid connections, the pile head bending moment M0 is 515 kNm for rigid connections, while it is 142 kNm for semi-rigid connections, indicating a significant reduction in M0. However, the degree of fixity α at the pile head of semi-rigid connections is 0.28, which is greater than M0. max The (maximum bending moment in the underground section) becomes quite large, resulting in poor balance (M0 / M max =0.58). As shown in Table 1 and Figure 8(b), in the pile foundation structure of this embodiment, the pile head fixation degree α = 0.38, and M0 and M max It can be seen that the balance is improved (M0 / M max =0.92). Thus, the pile foundation structure of this embodiment improves the balance of the bending moment distribution, making it possible to construct a rational pile foundation.

[0034] <Second Embodiment> A pile foundation structure according to the second embodiment will now be described. Figure 9 shows structure 1 of this embodiment. As shown in Figure 9, structure 1 is supported by a pile foundation structure 2 having a plurality of piles 3, 3, ... Figure 10 shows pile foundation structure 2. The pile foundation structure 2 of this embodiment is a semi-rigid joint type pile foundation structure that reduces the bending moment applied to the pile head while ensuring shear force transmission capacity, and as shown in Figures 10(a) and (b), a base plate (pile cap in this embodiment) 4 of the structure is placed on the pile heads of piles 3 embedded in the ground G. The pile heads of piles 3 are covered with a top-shaped cylindrical pile head formwork 5 with an inclined inner surface. Furthermore, a crushed stone layer S and a leveling concrete layer C are stacked between the ground G and the base plate 4. The piles 3 are formed to a predetermined length by connecting a plurality of pre-fabricated piles 6, 6, ... The number of pre-fabricated piles 6 that make up the pile 3 is not limited, and for example, the pile 3 may be formed by a single pre-fabricated pile 6.

[0035] The outer perimeter piles 31 (see Figure 9), which are piles 3 provided on the outer perimeter of structure 1, are formed by connecting straight precast piles 61, which are precast piles 6 with a constant pile diameter along their entire length, as shown in Figure 10(a). In other words, the outer perimeter piles 31 have a constant pile diameter along their entire length.

[0036] On the other hand, the central pile 32 (see Figure 9), which is a pile other than the outer perimeter piles 31 (a pile located in the center of the base slab), has an enlarged diameter precast pile 62 at its uppermost part, as shown in Figure 10(b). A straight precast pile 61 with a constant diameter is connected to the lower end of the enlarged diameter precast pile 62. In other words, the pile diameter at the top of the central pile 32 is larger than the pile diameters of the other parts (general section).

[0037] The details of the straight precast pile 61 and the enlarged diameter precast pile 62 are the same as those described in the first embodiment, so a detailed explanation will be omitted. Similarly, the details of the end plate 7, base plate 4, and pile head formwork 5 are the same as those described in the first embodiment, so a detailed explanation will be omitted. Furthermore, since the method for constructing the pile foundation structure in the second embodiment is the same as the method for constructing the pile foundation structure in the first embodiment, a detailed explanation will be omitted.

[0038] According to the pile foundation structure 2 of this embodiment, the connection between the pile 3 and the base slab 4 can be made into a semi-rigid connection state as anticipated in the design. In other words, because a gap is formed around the pile head of the pile 3 by the pile head formwork 5, rotation of the pile head is permitted, and even if horizontal forces (shear forces) acting on the structure due to earthquakes, wind, etc., act on the pile 3, a large bending moment will not be generated at the pile head.

[0039] Furthermore, by using enlarged-diameter piles to enlarge the pile head, the contact area between the base slab, the pile 3, and the joint surface is increased, thereby reducing the axial stress at the pile head joint. Figures 11 and 12 show the relationship between the bending moment M and the rotation angle θ at the pile head in a semi-rigid pile head joint. As shown in Figure 11, in the initial state, only the axial force necessary to support the structure is generated at the pile head, and axial stress acts uniformly on the contact surface between the pile head and the foundation base slab (see Figure 12(a)). During an earthquake, when a horizontal force acts on the structure, a bending moment is generated at the pile head, and the stress distribution at the contact surface between the pile head and the foundation base slab becomes trapezoidal (see Figure 12(b)). As the bending moment increases, the stress distribution at the contact surface becomes triangular (see Figure 12(c)). Furthermore, as the bending moment increases, uplift occurs at the end of the pile head, the contact area between the pile head and the foundation base slab decreases, and the axial stress at the contact surface increases (see Figures 12(d) and (e)). If this axial stress reaches a limit value (for example, the bearing strength of the concrete in the foundation slab), damage will occur at the pile head joint, so it is necessary to keep the rotation of the pile head during an earthquake within the allowable rotation angle. In this invention, the axial stress at the pile head joint can be reduced by increasing the contact area between the base slab, the pile 3, and the joint surface, thereby further improving the allowable rotation angle.

[0040] According to the pile foundation structure of this embodiment, by enlarging the diameter of the pile head of the central pile 3 (central pile 32), the uneven distribution of load on the outer piles 3 (outer piles 31) can be reduced. Here, the degree of pile head fixation α in a semi-rigid connection depends on the axial force at the pile head, and increases as the axial force increases. During an earthquake, the axial force at the pile head is not only due to the weight of the structure, but also to the fluctuating axial force caused by the overturning moment of the structure. Therefore, during an earthquake, depending on the direction of the seismic force, there will be piles 3 with a large axial force (compression-side piles) and piles 3 with a small axial force (pull-outside piles) (see Figures 13(a) and (b)). At this time, since the degree of pile head fixation α depends on the axial force, the horizontal force will concentrate on the pile with the large axial force. However, since seismic forces act not in one direction but in alternating positive and negative directions, the piles on the outer perimeter of the structure will end up bearing a larger seismic force compared to the central piles. Thus, in semi-rigid pile foundations, the axial force dependence of the pile head fixation degree α causes an imbalance in the seismic forces borne by each pile, and under certain conditions, rational design may not be possible. On the other hand, in pile foundation structures, the diameter of the pile heads is partially enlarged, thus addressing this imbalance (see Figure 13(c)).

[0041] Furthermore, with the pile foundation structure, it is possible to rationally control the degree of fixation α of the semi-rigid joint type pile head by changing the pile head diameter without changing the pile shaft diameter (general pile diameter) determined from the standpoint of safety and economics. In addition, it is possible to broaden the control range of α by adjusting the diameter expansion ratio of the prefabricated pile. Moreover, by increasing the contact area of ​​the joint surface between the pile and the base slab of the structure, the axial stress at the pile head joint can be reduced. This improves the allowable rotation angle (deformation performance), and consequently enables rational design (see Figure 14).

[0042] In this embodiment, for the sake of simplicity, we focused on the outer perimeter piles 31 of a building with a large axial force during an earthquake, using four piles. However, the present invention aims to reduce the horizontal force borne by piles with a large axial force during an earthquake. Therefore, it is not limited to applying enlarged diameter precast piles to piles other than those on the outer perimeter of a building. Rather, it is crucial to apply enlarged diameter precast piles to piles with relatively small maximum axial forces during earthquakes, thereby achieving the effects of the present invention.

[0043] <Third Embodiment> In the third embodiment, a pile foundation structure 20 consisting of a pile 3 and a base plate 4 of a structure placed on the pile 3 will be described, similar to the first embodiment. Figure 15 shows the pile foundation structure 20 of the third embodiment. In the pile foundation structure 20 of the third embodiment, as shown in Figure 15, the pile 3 and the base plate 4 are rigidly connected.

[0044] Pile 3 is formed to a predetermined length by connecting multiple pre-fabricated piles 6, 6, ... . The number of pre-fabricated piles 6 that make up pile 3 is not limited; for example, the pile may be formed from a single pre-fabricated pile 6.

[0045] Pile 3 has a pre-fabricated pile 62, which is a pre-fabricated pile 6 having an enlarged diameter section 63 at its upper end, at its top. A straight pre-fabricated pile 61 with a constant diameter is connected to the lower end of the enlarged diameter pre-fabricated pile 62. That is, the diameter of the pile head of pile 3 is larger than the diameter of the rest of the pile (general section). The general section 64 of the enlarged diameter pre-fabricated pile 62 (the part other than the enlarged diameter section 63) has the same outer diameter as the straight pre-fabricated pile 61 and a smaller outer diameter than the enlarged diameter section 63. Annular end plates 7 are provided at both ends of the enlarged diameter pre-fabricated pile 62. The configuration of the end plates 7 is the same as that of the end plates 7 in the first embodiment, so a detailed explanation is omitted.

[0046] The pile 3 is rigidly connected to the base plate 4 via anchoring bars 8. The anchoring bars 8 are erected on the upper end of the pile 3, with their lower ends fixed to bolt holes 71 in the end plate 7. The base slab 4 is a reinforced concrete structure. The base slab 4 is formed by pouring concrete while incorporating the upper ends of the piles 3 and the anchoring reinforcement bars 8.

[0047] Next, the method for constructing the pile foundation structure will be explained. Figure 16 shows the procedure for constructing the pile foundation structure. As shown in Figure 16, the method for constructing the pile foundation structure comprises a pile driving process S21, a ground excavation process S22, a crushed stone layer formation process S23, a concrete layer formation process S24, and a base slab formation process S25. Figure 17 shows the work status of the pile foundation structure construction method.

[0048] In the pile driving process S21, pre-fabricated piles 6 are placed in the ground to form the pile. In the pile driving process S21, the piles are embedded in the ground G using a rotary press-in device (see Figure 6(a)). The pile 3 consists of a straight pre-fabricated pile 61 with a constant outer diameter and an enlarged diameter pre-fabricated pile 62 connected together, with the enlarged diameter pre-fabricated pile 62 placed at the top.

[0049] In the ground excavation process S22, the ground G surrounding pile 3 is excavated to expose the pile head. Once the pile head is exposed, the bolt B1 screwed into the bolt hole 71 is also removed (see Figure 6(b)). In the crushed stone layer formation process S23, the unevenness of the base surface G1 is leveled, and then crushed stone is laid and compacted on the base surface G1 to form the crushed stone layer S (see Figure 6(b)). Depending on the ground conditions, the crushed stone layer S (crushed stone layer formation process) may be omitted.

[0050] In the concrete layer formation process S24, as shown in Figure 17(a), leveling concrete is poured around the pile 3 to form a leveling concrete layer C. In the base slab formation process S25, first, reinforcing bars and formwork (not shown) are placed on top of the leveling concrete layer C. The reinforcing bars are placed around the pile head and above 51. At this time, anchoring bars 8 are installed at the pile head (end plate). Next, concrete for the base slab is poured into the formwork to form the base slab 4 on top of the leveling concrete layer C, as shown in Figure 17(b).

[0051] According to the pile foundation structure 2 of this embodiment, by providing an enlarged diameter section at the pile head of the pile 3, the contact area between the pile 3 and the base plate 4 is increased, which in turn reduces the axial stress at the pile head joint and increases the amount of reinforcement that can be placed at the pile head. This improves the bending strength of the pile head.

[0052] <Fourth Embodiment> A pile foundation structure according to the fourth embodiment will now be described. Figure 18 shows a structure 10 of the fourth embodiment. In the fourth embodiment, a high-rise section 11 and a low-rise section 12 are mixed in the same structure 10. As shown in Figure 18(a), in this case, the piles 3a directly below the high-rise section are larger in diameter than the piles 3b directly below the low-rise section, depending on the weight of the structure 10. Therefore, in the conventional technology, the load is disproportionately borne by the piles 3a of the high-rise section 11, which are larger in diameter and have greater axial force during an earthquake. In this embodiment, as shown in Figure 18(b), by applying enlarged-diameter precast piles 62 to the piles 3 of the low-rise section 12, it is possible to increase the load on the piles 3 of the low-rise section 12 during an earthquake. As a result, the load on the piles 3 of the high-rise section 11 is reduced, and a well-balanced (high seismic performance) design for the entire pile foundation becomes possible.

[0053] Although embodiments of the present invention have been described above, the present invention is not limited to the embodiments described above, and each of the above-mentioned components can be modified as appropriate without departing from the spirit of the present invention. For example, the enlarged diameter precast pile 62 can also be made by enlarging the ends of node piles or by enlarging steel pipe concrete piles.

[0054] The diameter expansion ratio (expanded diameter / shaft diameter) of the enlarged portion 63 is generally around 1.09 to 1.25, but the diameter expansion ratio of the enlarged portion 63 is not limited. The range of the enlarged diameter portion 63 of pile 3 is not limited, but from an economic standpoint, it is preferable to keep it at or below half the length of the precast pile 6 at the upper end. The present invention is not intended to improve the seismic resistance of the pile body itself, and does not envision the use of an enlarged diameter precast pile 62 in the lower precast pile 6 of the precast pile 6 (straight precast pile 61) at the upper end, as shown in Figure 19, to increase the pile diameter of the precast pile 6 at the upper end along its entire length. [Explanation of Symbols]

[0055] 1 structure 2 Pile foundation structure 3 stakes 31 Perimeter piles 32 Central pile 4 bottom plate 41 Recess 5. Pile head formwork 6. Precast piles 61 Straight Precast Stakes 62 Enlarged diameter precast piles 63 Expanded diameter part 64 General section 7 End plate

Claims

1. A pile foundation structure comprising a pile and a base slab of a structure placed on the pile, The pile foundation structure is characterized in that the pile is a pre-fabricated pile having an enlarged diameter portion at its upper end, and the pile diameter at the pile head is larger than the diameter of the smallest portion of the pile.

2. A pile foundation structure comprising a plurality of piles and a base slab of a structure placed on the plurality of piles, A pile foundation structure characterized in that, apart from the piles arranged on the outer periphery of the base slab, the piles are pre-fabricated piles having an enlarged diameter portion at their upper end, and are formed such that the diameter of the pile head is larger than the diameter of the smallest part of the pile.

3. The pile foundation structure according to claim 1 or claim 2, characterized in that the upper end of the enlarged diameter portion is in contact with the base plate.

4. The pile foundation structure according to claim 1 or claim 2, characterized in that the pile and the base plate are rigidly connected.

5. The pile foundation structure according to claim 1 or claim 2, characterized in that a frustoconical recess is formed on the bottom surface of the base plate, into which the upper end of the pile is inserted, and a gap is formed between the recess and the outer surface of the upper end of the pile.

6. A method for constructing a pile foundation structure, wherein a base slab of a reinforced concrete structure is placed on the pile head of a pile installed in the ground, A pile driving process in which a pre-fabricated pile with an enlarged diameter section at its upper end is placed in the ground to form a pile, A method for constructing a pile foundation structure, characterized by including a base plate forming step of forming the base plate on the pile.

7. The aforementioned pile is formed by connecting a straight pre-fabricated pile with a constant outer diameter and the aforementioned enlarged pre-fabricated pile. The method for constructing a pile foundation structure according to claim 6, characterized in that the pile driving step involves driving the enlarged diameter pre-fabricated pile with the enlarged diameter pre-fabricated pile positioned at the top.

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