Construction method of cast-in-place concrete pile and cast-in-place concrete pile

Abstract

To provide a method for constructing a cast-in-place concrete pile and a cast-in-place concrete pile which can secure bearing strength even when a distance between a pile axial part center and a site boundary is short.SOLUTION: A method for constructing a cast-in-place concrete pile includes a shaft part excavation hole formation step, a coaxial expanded bottom part excavation hole formation step, a lower stage expanded bottom part formation step of inserting an excavation machine for an expanded bottom into an excavation hole 22A of vertical expanded bottom part, decentering and arranging a central axis of the excavation machine for the expanded bottom with respect to a central axis O of a shaft part, excavating a hole wall of the excavation hole 22A of the vertical expanded bottom part while expanding and extending an extended blade of the excavation machine for the expanded bottom, and forming an excavation hole 23A of the lower stage expanded bottom part; and a concrete placing step of placing concrete on each of excavation holes 22A and 23A of the shaft part, the upper stage expanded bottom part, the vertical expanded bottom part and the lower stage expanded bottom part, wherein the method performs the lower stage expanded bottom part formation step a plurality of times, and the excavation holes 23A of all the lower stage expanded bottom parts do not exceed a boundary line L1.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to a method for constructing a cast-in-place concrete pile and to a cast-in-place concrete pile. [Background technology]

[0002] Traditionally, cast-in-place concrete bell-bottom piles have been widely used as pile foundations for heavy buildings such as skyscrapers. These cast-in-place concrete bell-bottom piles (hereinafter referred to as "cast-in-place concrete piles") can have a shaft diameter of up to approximately 3 m, and the construction diameter of the bell-bottom can be up to approximately 5 to 6 m. The outer periphery of the bell-bottom extends outward from the center of the pile shaft by approximately 2 to 3 m.

[0003] Furthermore, by using the eccentric flared excavation technology described in Patent Documents 1 and 2, it is possible to create large piles with a construction diameter of approximately 7 m. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2019-183497 [Patent Document 2] Japanese Patent Application Publication No. 2019-190090 Summary of the Invention [Problem to be solved by the invention]

[0005] However, in order to create a large pile with a construction diameter of approximately 7m, the outer periphery of the expanded base would have to extend outward by more than 3m from the centre of the pile shaft. In urban areas, buildings are built to fill the entire site, and pillars are sometimes located near the site boundary, but constructing expanded base piles to match the pillar positions can pose problems, such as the expanded base extending beyond the site boundary or coming into contact with the earth retaining wall, making construction impossible.

[0006] Therefore, the present invention has been made in consideration of the above circumstances, and provides a method for constructing a cast-in-place concrete pile and a cast-in-place concrete pile that can ensure bearing capacity even if the distance between the center of the pile shaft and the site boundary is short. [Means for solving the problem]

[0007] In order to achieve the above object, the present invention employs the following means. That is, the method for constructing a cast-in-place concrete pile according to the present invention is a method for constructing a cast-in-place concrete pile having a columnar shaft and an expanded bottom portion formed by expanding the tip of the shaft, and includes a shaft excavation hole formation step of pre-excavating a drilling hole for the shaft in the ground using a pile excavator; a step of inserting a bottom expanding machine into the tip of the drilling hole for the shaft and aligning the central axis of the bottom expanding machine with the central axis of the shaft, and excavating the hole wall at the tip of the shaft excavation hole formed by pre-excavation while expanding the expansion wings of the bottom expanding machine to form an upper expanded bottom drilling hole that is coaxial with the shaft excavation hole and an upper expanded bottom drilling hole that is coaxial with the upper expanded bottom drilling hole; a coaxial expansion hole forming step of forming a vertical expansion hole extending downward; a lower expansion hole forming step of inserting the expansion drilling machine into the vertical expansion hole and positioning the central axis of the expansion drilling machine eccentrically relative to the central axis of the shaft, and drilling the hole wall of the vertical expansion hole while expanding the expansion wings of the expansion drilling machine to form a lower expansion hole; and a concrete pouring step of pouring concrete into each of the drill holes in the shaft, the upper expansion hole, the vertical expansion hole and the lower expansion hole, wherein the lower expansion hole forming step is performed multiple times to form a lower expansion hole so that all of the lower expansion hole holes do not cross the boundary line. The boundary line is a line for constructing the cast-in-place concrete pile so as not to cross the boundary line. .

[0008] In the method for constructing a cast-in-place concrete pile according to the present invention, the lower flared base formation step is performed multiple times to construct multiple lower flared bases without crossing the boundary line. This ensures the necessary bearing capacity even if the distance between the center of the pile shaft and the boundary line, such as the site boundary, is short.

[0009] In addition, in the method for constructing a cast-in-place concrete pile according to the present invention, the shape of the bottom surface of the cast-in-place concrete pile may be asymmetrical with respect to a line passing through the center of the shaft portion and parallel to the boundary line in a plan view.

[0010] In the method for constructing a cast-in-place concrete pile according to the present invention, the lower expansion step is performed multiple times to construct the bottom of the cast-in-place concrete pile asymmetrically with respect to a line passing through the center of the shaft and parallel to the boundary line. This ensures the necessary bearing capacity even if the distance between the center of the pile shaft and the boundary line, such as the site boundary, is short.

[0011] In addition, in the method for constructing a cast-in-place concrete pile according to the present invention, the diameter of the excavation hole in the vertical expansion section may be the same as the diameter of the excavation hole in the lower expansion section.

[0012] In the method for constructing a cast-in-place concrete pile according to the present invention, the lower flared section is formed multiple times so that the diameter of the vertical flared section is the same as that of the lower flared section. This ensures the necessary bearing capacity even if the distance between the center of the pile shaft and the boundary line, such as the site boundary, is short. In addition, since the width of the expansion wings of the bottom expansion drilling machine can be made the same during multiple lower bottom expansion formation processes, it is easy to set and adjust the width of the expansion wings, making the work easier.

[0013] In addition, in the method of constructing a cast-in-place concrete pile of the present invention, the center position of the excavation hole of the lower expansion portion formed in at least two of the lower expansion portion formation processes performed multiple times may be on a reference line parallel to the boundary line.

[0014] In the method for constructing a cast-in-place concrete pile according to the present invention, the center of the excavation hole for the lower flared section formed in at least two lower flared section formation steps is constructed so that it is on a reference line parallel to the boundary line. This ensures the necessary bearing capacity even if the distance between the center of the pile shaft and the boundary line, such as the site boundary, is short.

[0015] In addition, in the method for constructing a cast-in-place concrete pile according to the present invention, the diameter of the excavation hole in the lower expansion section may be larger than the diameter of the excavation hole in the vertical expansion section.

[0016] In the method for constructing a cast-in-place concrete pile according to the present invention, the lower flared section is formed multiple times so that the diameter of the lower flared section is larger than the diameter of the vertical flared section. This ensures the necessary bearing capacity even if the distance between the center of the pile shaft and the boundary line, such as the site boundary, is short. It also allows the pile base area to be increased.

[0017] In addition, in the method for constructing a cast-in-place concrete pile according to the present invention, there may be two boundary lines that are perpendicular to each other, and all of the excavation holes in the lower expansion section may not cross the two boundary lines.

[0018] In the method for constructing a cast-in-place concrete pile according to the present invention, the lower flared base formation process is performed multiple times so that all lower flared bases do not cross two boundary lines. This ensures the necessary bearing capacity even if the distance between the center of the pile shaft and a boundary line such as a site boundary is short.

[0019] The cast-in-place concrete pile according to the present invention is a cast-in-place concrete pile having a columnar shank and an expanded bottom portion formed by expanding the tip of the shank, wherein the expanded bottom portion has an upper expanded bottom portion that gradually expands radially outward as it extends downward, a vertical expanded bottom portion extending vertically downward from the upper expanded bottom portion, and a plurality of lower expanded bottom portions that gradually expand radially outward as they extend downward from the vertical expanded bottom portion, wherein the central axes of the upper expanded bottom portion and the vertical expanded bottom portions are arranged coaxially with the central axis of the shank, the central axis of the lower expanded bottom portion is arranged eccentrically from the central axis of the shank, and the plurality of lower expanded bottom portions are arranged so as not to cross the boundary line, and the shape of the bottom surface is asymmetrical with respect to a line that passes through the center of the shank and is parallel to the boundary line in a plan view. The boundary line is a line for constructing the cast-in-place concrete pile so as not to cross the boundary line. .

[0020] In the cast-in-place concrete pile of the present invention, the lower expansion base is constructed so that it does not extend beyond the boundary line. This ensures the necessary bearing capacity even if the distance between the center of the pile shaft and the boundary line, such as the site boundary, is short.

[0021] In addition, in the cast-in-place concrete pile of the present invention, when viewed in a plane, the distance between the center of the central axis of the shaft portion and the center of gravity of the bottom surface of the lower expansion portion may be less than 1 / 2 of the radius of a circle having an area equivalent to that of the bottom surface of the cast-in-place concrete pile.

[0022] In the cast-in-place concrete pile of the present invention configured as described above, the distance between the center of the central axis of the shaft and the center of gravity of the bottom of the lower expanded base is, in plan view, less than half the radius of a circle with an area equivalent to the bottom of the concrete pile. This ensures the necessary bearing capacity even if the distance between the center of the pile shaft and a boundary line such as a site boundary is short. In addition, because the entire expanded base resists the vertical load acting on it, the effect of the center of gravity at the center of the pile shaft and the tip of the pile being different can be mitigated. [Effects of the Invention]

[0023] According to the method for constructing a cast-in-place concrete pile and the cast-in-place concrete pile of the present invention, it is possible to ensure bearing capacity even if the distance between the center of the pile shaft and the site boundary is short. [Brief explanation of the drawings]

[0024] [Figure 1] FIG. 2 is a side view showing the configuration of a borehole for a cast-in-place concrete pile according to an embodiment of the present invention. [Figure 2] This is a side view showing a method for constructing a cast-in-place concrete pile, where (a) is a diagram showing the shaft portion forming process, and (b) is a diagram showing the coaxial expanded bottom portion forming process. [Figure 3] 10(a) and 10(b) are side views showing a method of constructing a cast-in-place concrete pile, showing the state in which a bottom-expanding excavator is positioned in an eccentric position. [Figure 4]This is a side view showing a method of constructing a cast-in-place concrete pile, illustrating the lower expanded bottom forming process. [Figure 5] This is a plan view of a method for constructing a cast-in-place concrete pile at a location where there is one boundary line, showing (a) the coaxial expansion process, (b) the first lower expansion process, (c) the second lower expansion process, and (d) the third lower expansion process. [Figure 6] This is a plan view of a method for constructing a cast-in-place concrete pile at a location where there are two boundary lines, showing (a) the coaxial expansion process, (b) the first lower expansion process, (c) the second lower expansion process, and (d) the third lower expansion process. [Figure 7] This is a plan view showing a method of constructing a cast-in-place concrete pile, showing the case where the diameter of the excavation hole in the lower expansion section is larger than the diameter of the excavation hole in the vertical expansion section. [Figure 8] This is a plan view showing a method of constructing a cast-in-place concrete pile, showing the relationship between the center of the central axis of the shaft portion and the center of gravity of the bottom surface of the cast-in-place concrete pile. [Figure 9] FIG. [Figure 10] FIG. 10 is a diagram showing the positions of the enlarged bottom portion and the stem portion of the experimental model (case A1=0-B, 0.106 to 0.519R1-B). [Figure 11] This figure shows the positions of the enlarged bottom and the axis of the experimental model (case A1 = 0.106, 0.193R1-C). [Figure 12] FIG. 10 is a diagram showing the experimental results when the size of A1 is changed, and shows the relationship between the tip load intensity and the tip sinking rate. [Figure 13] FIG. 10 shows the experimental results when the size of A1 is changed, and shows the change in ultimate bearing capacity. DETAILED DESCRIPTION OF THE INVENTION

[0025] Hereinafter, a method for constructing a cast-in-place concrete pile and a cast-in-place concrete pile according to an embodiment of the present invention will be described with reference to the drawings.

[0026] The method for constructing a cast-in-place concrete pile 1 according to this embodiment is a method for constructing a columnar shaft 10 and an expanded base 2 (21, 22, 23) formed by expanding the tip of the shaft 10, as shown in Figure 1. The cast-in-place concrete pile 1 is placed in the ground G.

[0027] The expanded bottom portion 2 (21, 22, 23) is formed using a well-known expanded bottom drilling machine 3, as will be described later. As the expanded bottom drilling machine 3, it is preferable to use a device that can form the expanded bottom portion 2 by drilling a drilling hole 2A with the shaft portion 10.

[0028] A method for constructing the cast-in-place concrete pile 1 will now be described. First, the shaft drilling hole forming step is carried out. As shown in Figure 2(a), as in the conventional method, a pile drilling machine (not shown) such as an earth drill is used to drill from the ground surface to a predetermined depth in the ground while filling it with stabilizing liquid, thereby forming a first drilling hole 10A for the shaft portion 10 of the cast-in-place concrete pile 1, which has an approximately constant shaft diameter.

[0029] Next, the coaxial expanded bottom drilling hole forming process is carried out. As shown in Figure 2(b), a bottom-expanding drilling machine 3 is used and inserted into the tip of the first drilling hole 10A of the shaft 10. After aligning the central axis (rod axis C1) of the bottom-expanding drilling machine 3 with the central axis O of the shaft 10, drilling is performed while spreading the extension wings 33 (position of the two-dot chain line marked P1 in Figure 2(b)).

[0030] As shown in Figure 2(b), the bottom-expanding excavator 3 comprises a rod 31 rotatably supported, for example, by a rotary support device (not shown) arranged on the ground, a cylindrical excavator body 32 attached to the tip (lower end) of the rod 31, a pair of left and right extension wings 33 arranged to be able to extend and retract radially from the outer surface 32a of the excavator body 32 perpendicular to the rod axis C1 of the rod 31, and a stabilizer 34 protruding downward from the center of the bottom surface (tip) 32b of the excavator body 32 coaxially with the rod axis C1.

[0031] The rod 31 is supported so as to be rotatable about the rod axis C1 with the rod axis C1 oriented vertically, and the lower end of the rod is integrally fixed to the center of the upper surface of the excavator body 32. The excavator body 32 rotates about the rod axis C1 as the rod 31 rotates.

[0032] The expansion blades 33 are shaped so that they extend radially outward from top to bottom in a side view, and a plurality of excavation blades (not shown) are provided along the inclined direction on the inclined portion 33a extending diagonally downward from top to bottom. The outer diameter of the expansion blades 33 when they protrude most from the excavator body 32 is the maximum expanded bottom diameter of the bottom-expanding excavator 3. The pair of expansion blades 33 are provided in opposing positions on both sides of the excavator body 32 when viewed from the direction of the rod axis C1.

[0033] As shown in Figure 3(a), the stabilizer 34 constitutes a positioning protrusion for stabilizing the bottom-expanding drilling machine 3 by striking and penetrating the bottom of each borehole. Note that the stabilizer 34 of this embodiment has a tapered, pointed tip in the protruding direction. It may also be shaped like a drill or auger to make it easier to penetrate into the ground below.

[0034] As shown in Figure 2(b), drilling is performed to expand the hole wall of the tip 10a (see Figure 2(a)) of the first drilling hole 10A of the shaft 10 formed by prior drilling while spreading the expansion wings 33 of the bottom-expanding drilling machine 3. A bottom-expanding section 2 is formed that is coaxial with the first drilling hole 10A of the shaft 10, i.e., the second drilling hole 21A of the upper-stage bottom-expanding section 21 and the third drilling hole 22A of the vertical bottom-expanding section 22 extending vertically downward from the upper-stage bottom-expanding section 21. The second drilling hole 21A of the upper-stage bottom-expanding section 21 and the third drilling hole 22A of the vertical bottom-expanding section 22 are formed with a circular section centered on the central axis of the shaft 10 in a planar cross-sectional view.

[0035] In a specific construction method, the excavator body 32 connected to the lower end of the rod 31 is inserted up to the tip of the first borehole 10A of the shaft 10 (the position indicated by the two-dot chain line indicated by symbol P1 in Figure 2(b)). At this time, it is preferable to insert the stabilizer 34 of the bottom-expanding excavator 3 into the ground at the bottom of the pile in the first borehole 10A of the shaft 10 and position the excavator body 32 so that it does not shift left or right, in order to increase stability.

[0036] Next, the bottom-expanding drilling machine 3 is expanded and rotated around the rod axis C1 to drill the hole wall portion of the first drilled hole 10A of the shaft portion 10, thereby forming a drilled hole with a substantially truncated cone-shaped bottom expansion section 2 at the tip of the first drilled hole 10A of the shaft portion 10. Here, the expansion wings 33 of the bottom-expanding drilling machine 3 are expanded in a maximally expanded state, and the expansion amount can be set according to the expanded bottom diameter dimension of the bottom expansion section 2 set at this time.

[0037] After the approximately truncated cone-shaped expanded bottom section 2 is formed at the tip of the first borehole 10A of the shaft 10, the expanded bottom section 2 is formed by rotating the expanded bottom drilling machine 3 and lifting the excavator body 32 together with the rod 31 to a predetermined depth (here, the position of the upper expanded bottom section 21) while drilling, forming the beer bottle-shaped expanded bottom section 2. In other words, because the inclined portion 33a with the excavation blade of the expansion wing 33 is positioned upward, the excavator body 32 is raised to excavate the first borehole 10A of the shaft 10, thereby forming the third borehole 22A with a vertical expanded bottom section 22 that is coaxial with the central axis O of the shaft 10 and has the maximum expanded bottom diameter when the expansion wing 33 is extended. Then, at the predetermined position where the lifting of the excavator body 32 is stopped (the position indicated by the solid line P3 in FIG. 2(b)), the upper expanded bottom section 21 is formed. In FIG. 2(b), the expanded bottom drilling machine 3, located at the position indicated by the two-dot chain line P2, is shown in the middle of being raised from the position P1 to the position P3.

[0038] The second drilled hole 21A of the upper-stage expanded bottom portion 21 has a shape that gradually widens radially outward as it extends vertically downward. The third drilled hole 22A of the vertical expanded bottom portion 22 has a cylindrical shape that extends vertically downward from the second drilled hole 21A of the upper-stage expanded bottom portion 21. The central axis of the second drilled hole 21A of the upper-stage expanded bottom portion 21 and the central axis of the third drilled hole 22A of the vertical expanded bottom portion 22 are arranged coaxially with the central axis O of the first drilled hole 10A of the shaft portion 10.

[0039] As shown in Figures 3(a) and (b), the outer diameter of the lower end of the second drilling hole 21A in the upper-stage expanded bottom section 21 to be drilled and the outer diameter of the third drilling hole 22A in the vertical expanded bottom section 22 are determined by at least one (in this embodiment, both) of the position where the rod 31 abuts the hole wall of the shaft section 10 when the drilling machine body 32 is positioned within the vertical expanded bottom section 22, and the position where the outer surface 32a of the drilling machine body 32 when the expansion wings 33 are closed abuts the hole wall of the third drilling hole 22A in the vertical expanded bottom section 22.

[0040] As shown in Figures 3(a) and (b), the bottom-expanding drilling machine 3 is inserted into a predetermined depth position (a position near the bottom of the hole, the position of the two-dot chain line Q1 shown in Figure 3(a)) of the third drilling hole 22A of the vertical bottom-expanding section 22, and the rod axis C1 of the bottom-expanding drilling machine 3 is eccentrically positioned by sliding it horizontally at a predetermined eccentricity amount N relative to the central axis O of the shaft section 10. In this embodiment, the eccentric position of the bottom-expanding drilling machine 3 (position Q2 in Figure 3(a) and position Q3 in Figure 3(b)) is a position where the rod 31 abuts the hole wall of the shaft section 10, and where the outer surface 32a of the drilling machine body 32 with the expansion wings 33 closed abuts the hole wall of the third drilling hole 22A of the vertical bottom-expanding section 22. At this time, as shown in Figure 3(b), the stabilizer 34 of the bottom-expanding excavator 3 is inserted into the ground at the bottom of the third excavation hole 22A of the vertical bottom-expanding section 22, and the excavator body 32 is positioned so that it does not shift left or right (state indicated by symbol Q3).

[0041] In this embodiment, when the bottom-expanding excavator 3 is made eccentric, a positioning and holding means (not shown), such as an eccentric bottom-expanding excavation guide, may be used to guide the rod 31 to a predetermined eccentric position. In this case, when the bottom-expanding drilling machine 3 is rotated around the rod axis C1 to excavate the hole wall, the bottom-expanding drilling machine 3 is held in its eccentric position by the positioning and holding means, allowing bottom-expanding drilling to be performed preferably.

[0042] Next, the lower expansion hole drilling process is carried out. As shown in Figure 4, the hole wall of the third drilling hole 22A of the vertical bottom expansion section 22 is expanded by expanding the expansion wings 33 from the drilling machine body 32 arranged in an eccentric position while rotating around the rod axis C1 to form the fourth drilling hole 23A of the lower bottom expansion section 23. At this time, the expansion wings 33 of the bottom expansion drilling machine 3 are expanded in a maximally expanded state, but the amount of expansion is set according to the expanded bottom diameter dimension of the fourth drilling hole 23A of the lower bottom expansion section 23 to be formed.

[0043] The fourth drilling hole 23A of the lower expanded bottom portion 23 has a shape that gradually widens radially outward as it extends downward from the third drilling hole 22A of the vertical expanded bottom portion 22. The central axis of the fourth drilling hole 23A of the lower expanded bottom portion 23 is positioned eccentrically from the central axis O of the first drilling hole 10A of the shaft portion 10.

[0044] As described above, the rod axis C1 of the bottom-expanding drilling machine 3 is positioned eccentrically relative to the central axis O of the shaft portion 10, and the formation of the fourth drilling hole 23A of the lower-stage bottom-expanding portion 23 is repeated. As shown in Figure 5(d), the fourth drilling hole 23A of the lower-stage bottom-expanding portion 23 is constructed with a convex arc portion 23a that protrudes radially outward from the third drilling hole 22A of the vertical bottom-expanding portion 22 in a planar cross-sectional view. The convex arc portion 23a of the lower-stage bottom-expanding portion 23 is located radially outward from the second drilling hole 21A of the upper-stage bottom-expanding portion 21 (see Figure 4) and the third drilling hole 22A of the vertical bottom-expanding portion 22.

[0045] As shown in Figure 5, there is a boundary line L1 such as a site boundary line, and the concrete pile 1 is constructed so as not to cross the boundary line L1. Figure 5 shows an example in which the rod axis C1 of the bottom-expanding excavator 3 is eccentric three times relative to the central axis O of the shaft 10, and the fourth drilling hole 23A of the lower bottom-expanding section 23 is repeatedly formed.

[0046] Figure 5(a) shows a stage in the coaxial bottom expansion formation process shown in Figure 3(b), and shows the state in which the third drilling hole 22A of the vertical bottom expansion section 22 has been formed. The center of the third drilling hole 22A of the vertical bottom expansion section 22 is on the central axis O of the shaft section 10. The diameter of the third drilling hole 22A of the vertical bottom expansion section 22 is defined as r1. A line parallel to the boundary line L1 and passing through the central axis O of the shaft section 10 is defined as the reference line M1.

[0047] Figures 5(b) to (d) show stages in the lower-stage expanded bottom portion forming process shown in Figure 4. As shown in Figure 5(b), in the first eccentric position, the central axis O1 is set on the reference line M1, and the fourth drilling hole 23A of the lower-stage expanded bottom portion 23 having a diameter length r1 is formed. A convex arc portion 23a is formed that protrudes to one side in the extension direction of the boundary line L1 (upper side of the paper in Figure 5(b)) relative to the third drilling hole 22A of the vertical expanded bottom portion 22.

[0048] As shown in Figure 5(c), at the second eccentric position, a central axis O2 is set on an orthogonal line M2 perpendicular to the boundary line L1, and a fourth drilling hole 23A of the lower expanded bottom section 23 with a diameter length of r1 is formed. A convex arc section 23a is formed in the third drilling hole 22A of the vertical expanded bottom section 22, protruding on the opposite side of the boundary line L1 (the left side of the paper in Figure 5(b)).

[0049] As shown in Figure 5(d), at the third eccentric position, a central axis O3 is set on the reference line M1, and a fourth drilling hole 23A of the lower expanded bottom section 23 having a diameter of r1 is formed. A convex arc portion 23a is formed protruding to the other side of the extension direction of the boundary line L1 (the lower side of the paper in Figure 5(b)) relative to the third drilling hole 22A of the vertical expanded bottom section 22. The cast-in-place concrete pile 1 of this embodiment constructed in this manner has a pile bottom surface 1a (see Figure 1) equipped with a convex arc portion protruding radially outward from the circular outer surface of the vertical expanded bottom section 22 (the hole wall of the third drilling hole 22A) centered on the central axis O of the shaft section 10 in a plan cross-sectional view.

[0050] As described above, the fourth drilling holes 23A of all the lower expanded bottom sections 23 are drilled so as not to cross the boundary line L1. In plan view, the shape formed by the fourth drilling holes 23A of all the lower expanded bottom sections 23 and the third drilling holes 22A of the vertical expanded bottom sections 22 (the shape of the pile bottom surface 1a of the concrete pile 1) is asymmetric with respect to the reference line M1.

[0051] The example shown in the upper part of Figure 1 shows an example in which the fourth drilling holes 23A were repeatedly formed by eccentrically moving the shaft 10 five times relative to the central axis O. In this case, too, none of the fourth drilling holes 23A in the lower expanded bottom portion 23 cross the boundary line L1. In plan view, the shapes formed by all of the fourth drilling holes 23A in the lower expanded bottom portion 23 (the shape of the pile bottom surface 1a of the concrete pile 1) are asymmetric with respect to the reference line M1.

[0052] In the example shown in Figure 6, the shaft 10 is eccentric three times with respect to the central axis O, and all of the fourth drilling holes 23A of the lower-stage expanded bottom section 23 are formed so that no two of them cross the boundary lines L1 and L2. The method for forming the fourth drilling holes 23A of the lower-stage expanded bottom section 23 the first time (see Figure 6(b)) is the same as the method shown in Figure 5(b) above. The method for forming the fourth drilling holes 23A of the lower-stage expanded bottom section 23 the second time (see Figure 6(c)) is the same as the method shown in Figure 5(c) above. The distance between the central axis O1 and the central axis O and the distance between the central axis O2 and the central axis O are the same. The central axis O3 is set as shown in Figure 6(d). The angle between the line connecting the central axis O3 and the central axis O and the reference line M1 is 45 degrees, and the angle between the line connecting the central axis O3 and the central axis O and the reference line M2 is 45 degrees. The distance between the central axis O3 and the central axis O is the same as the distance between the central axes O1, O2 and the central axis O. The central axis O3 is the point where a reference line M3 parallel to the reference line M1 intersects with a reference line M4 parallel to the reference line M2. At the third eccentric position, a fourth drilling hole 23A of the lower expansion section 23 having a diameter length r1 is formed around the central axis O3. A convex arc portion 23a is formed for the third drilling hole 22A of the vertical expansion section 22, protruding to one side of the extension direction of the boundary line L1 (the upper side of the paper in Figure 5(d)) and opposite the boundary line L1.

[0053] In the example shown in Figure 7, the diameter r2 of the fourth drilling hole 23A of the lower-stage expanded bottom section 23 is greater than the diameter r1 of the third drilling hole 22A of the vertical expanded bottom section 22. The center axis O11 of the fourth drilling hole 23A of the first lower-stage expanded bottom section 23 and the center axis O13 of the fourth drilling hole 23A of the third lower-stage expanded bottom section 23 are set on a reference line M5 that is a distance r2 from the boundary line L1 and is parallel to the boundary line L1. The center axis O12 of the fourth drilling hole 23A of the second lower-stage expanded bottom section 23 is set on the reference line M2.

[0054] As shown in Figure 8, in a plan view, it is preferable that the distance A1 between the central axis O of the shaft portion 10 and the center of gravity P of the pile bottom surface 1a of the concrete pile 1 (see Figure 1) is less than 1 / 2 the length R1 of the radius of a circle S1 whose area is equivalent to the pile bottom surface 1a of the concrete pile 1 (length R2).

[0055] After the lower expansion hole drilling process, a concrete pouring process is carried out. As shown in Figure 1, concrete is poured into each of the boreholes 10A, 21A, 22A, and 23A in the shaft 10, upper flared bottom 21, vertical flared bottom 22, and lower flared bottom 23 using a tremie pipe to replace the stabilizing liquid, thereby constructing a cast-in-place concrete pile 1. The construction of the cast-in-place concrete flared bottom pile is then completed by inserting a reinforcing bar into each of the boreholes 10A, 21A, 22A, and 23A, backfilling the pile head side as necessary, and constructing a pile head foundation.

[0056] According to the construction method and cast-in-place concrete pile configured in this manner, the lower flared bottom formation process is performed multiple times to construct multiple lower flared bottom sections 23 so that they do not cross the boundary line L1. This ensures the necessary bearing capacity even if the distance between the central axis O of the shaft 10 and the boundary line L1 is short.

[0057] 5, the lower expanded bottom forming process is performed multiple times to construct the shape of the pile bottom surface 1a of the concrete pile 1 asymmetrical with respect to a reference line M1 that passes through the central axis O of the shaft 10 and is parallel to the boundary line L1. This ensures the necessary bearing capacity even if the distance between the central axis O of the shaft 10 and the boundary line L1 is short.

[0058] In addition, the lower flared bottom forming process is performed multiple times to construct the vertical flared bottom 22 so that the diameter r1 is the same as the diameter r1 of the lower flared bottom 23. This ensures the necessary bearing capacity even if the distance between the central axis O of the shaft 10 and the boundary line L1 is short. Furthermore, since the expansion width of the expansion wing 33 of the bottom expansion drilling machine 3 can be made the same during multiple lower bottom expansion formation processes, it is easy to set and adjust the expansion width of the expansion wing 3, resulting in good workability.

[0059] In addition, the positions of the central axes O1 and O3 of the borehole 23A of the lower flared section 23 formed in at least two lower flared section forming processes are constructed so as to be on a reference line M1 parallel to the boundary line L1. This ensures that the necessary bearing capacity can be secured even if the distance between the central axis O of the shaft section 10 and the boundary line L1 is short.

[0060] As shown in Figure 7, the lower flared bottom forming process is performed multiple times to construct the lower flared bottom 23 so that its diameter r2 is greater than the diameter r1 of the vertical flared bottom 22. This ensures the necessary bearing capacity even if the distance between the central axis O of the shaft 10 and the boundary line L1 is short. It also increases the pile base area.

[0061] 6, the lower flared bottom forming process is performed multiple times to construct all lower flared bottoms 23 so that they do not cross the two boundary lines L1 and L2. This ensures the necessary bearing capacity even if the distance between the central axis O of the shaft 10 and the boundary line L1 is short.

[0062] Furthermore, as shown in Figure 8, the distance A1 between the center of the central axis O of the shaft portion 10 and the center of gravity P of the pile bottom surface 1a (see Figure 1) of the concrete pile 1 in plan view is constructed so as to be equal to or less than half the length R1 of the radius of a circle S1 whose area is equivalent to the pile bottom surface 1a of the concrete pile 1. This ensures the necessary bearing capacity even if the distance between the central axis O of the shaft portion 10 and the boundary line L1 is short. In addition, because the entire pile bottom surface 1a resists the applied vertical load, the influence of the difference between the central axis O of the shaft portion 10 and the center of gravity P of the pile tip can be mitigated.

[0063] Model experiments were conducted in which A1 / R1 was varied from 0 (a normal pile with a symmetrically expanded base) to 0.519. Figure 9 is a diagram of the experimental model. Figure 10 is a diagram showing the positions of the expanded base and shaft of the experimental model (case A1 = 0 to 000519R1). Figure 11 is a diagram showing the positions of the expanded base and shaft of the experimental model (case A1 = 0.106R1-T). In Figures 10 to 13, cases where the pile shape is shown in Figure 10 are marked with "-B," and cases where the pile shape is shown in Figure 11 are marked with "-C." As shown in FIG. 12, even if the magnitude of A1 is changed, the relationship between the load and the settlement does not change, and neither the stiffness nor the bearing capacity decreases. As shown in Figure 13, when A1 is changed from 0.106 to 0.519 times R1 for a pile with a circular bottom (see Figure 10, case "-B"), it can be seen that the bearing capacity does not decrease compared to the symmetrically expanded base (A1 = 0-B). Comparing A1 = 0.106R1-B and A1 = 0.106R1-C, where A1 / R1 is the same, it can be seen that the bearing capacity is equivalent for a pile with a circular bottom (case "-B") and the bottom shape of the pile of the present invention (see Figure 11, case "-C"). Therefore, it can be seen that the bearing capacity does not decrease even when A1 is 1 / 2 times R1 for the bottom shape of the pile of the present invention (case "-C").

[0064] The above describes the method for constructing a cast-in-place concrete pile and embodiments of the cast-in-place concrete pile according to the present invention, but the present invention is not limited to the above embodiments and can be modified as appropriate within the scope of the spirit of the present invention.

[0065] For example, as in this embodiment, the expanded bottom diameters of the second drilling hole 21A of the upper-stage expanded bottom section 21 and the fourth drilling hole 23A of the lower-stage expanded bottom section 23 are not limited to being formed by the maximum expanded bottom diameter obtained by maximally expanding the expansion wings 33 of the expanding drilling machine 3. In other words, it is sufficient to construct an expanded bottom section with the required expanded bottom area that is set, and the expansion wings 33 can be expanded to a diameter corresponding to that expanded bottom area.

[0066] In addition, in this embodiment, the bottom-expanding excavator 3 that excavates the second excavation hole 21A in the upper-stage bottom-expanding section 21 and the third excavation hole 22A in the vertical bottom-expanding section 22 is the same excavator, but excavators with different configurations may be used for each. Furthermore, the shape and configuration of the bottom-expanding excavator 3 are not limited to those in the above-mentioned embodiment.

[0067] Furthermore, for example, the entire ground directly below the bottom of the pile and the ground in the area where the stabilizer 34 abuts (the ground directly below the bottom of the pile at an eccentric position) may be improved in advance using a deep ground improvement method or the like. In this case, when excavating the enlarged portion by subsequent excavation, the enlarged excavator can be further stabilized to excavate the hole wall of the borehole, making it possible to more reliably and suitably construct an enlarged pile that exceeds the maximum enlarged diameter of the enlarged excavator.

[0068] In addition, the components in the above-described embodiments can be replaced with well-known components as appropriate, without departing from the spirit of the present invention. [Explanation of symbols]

[0069] 1...Concrete pile 2...Flat bottom 3...Bottom widening excavator 10...Shaft 21...Flat bottom 21...Upper flared section 22...Vertical expansion 23...Lower flared section 33...extended wings G...Ground L1,L2…boundary line M1, M2, M3, M4, M5…baseline O,O1,O2,O3,O11,O12,O13…Center axis P…center of gravity

Claims

1. A method for constructing a cast-in-place concrete pile having a columnar shaft and an expanded base formed by expanding the tip of the shaft, A shaft excavation hole forming process of pre-excavating a shaft excavation hole in the ground using a pile excavator; a coaxial expanded bottom excavation hole forming process in which a bottom expanding drilling machine is inserted into the tip of the excavation hole of the shaft section, and the central axis of the bottom expanding drilling machine is aligned with the central axis of the shaft section, and the hole wall at the tip of the excavation hole of the shaft section formed by preliminary drilling is excavated so as to expand while spreading the expansion wings of the bottom expanding drilling machine, thereby forming an upper expanded bottom excavation hole that is coaxial with the excavation hole of the shaft section and an excavation hole of a vertical expanded bottom section extending downward from the upper expanded bottom section; a lower-stage expanded bottom forming process in which the bottom expanding drilling machine is inserted into the excavation hole of the vertical bottom expanding section, the central axis of the bottom expanding drilling machine is positioned eccentrically with respect to the central axis of the shaft section, and the hole wall of the excavation hole of the vertical bottom expanding section is excavated so as to expand while spreading the expansion wings of the bottom expanding drilling machine, thereby forming an excavation hole of the lower-stage expanded bottom section; A concrete pouring process for pouring concrete into each of the boreholes of the shaft portion, the upper expanded bottom portion, the vertical expanded bottom portion, and the lower expanded bottom portion, The lower-stage expanded bottom forming step is carried out a plurality of times so that the excavation holes of all the lower-stage expanded bottom portions do not cross the boundary line; A method for constructing a cast-in-place concrete pile, wherein the boundary line is a line for constructing the cast-in-place concrete pile so as not to cross the boundary line.

2. 2. The method for constructing a cast-in-place concrete pile according to claim 1, wherein, in a plan view, the shape of the bottom surface of the cast-in-place concrete pile is asymmetric with respect to a line passing through the center of the shaft portion and parallel to the boundary line.

3. 3. A method for constructing a cast-in-place concrete pile according to claim 1 or 2, wherein the diameter of the borehole in the vertical expansion section is the same as the diameter of the borehole in the lower expansion section.

4. A method for constructing a cast-in-place concrete pile described in any one of claims 1 to 3, wherein the center position of the excavation hole for the lower expansion portion formed in at least two of the lower expansion portion formation processes performed multiple times is on a reference line parallel to the boundary line.

5. 3. A method for constructing a cast-in-place concrete pile according to claim 1 or 2, wherein the diameter of the borehole in the lower expanded bottom portion is larger than the diameter of the borehole in the vertical expanded bottom portion.

6. The boundary lines are two lines that are perpendicular to each other, A method for constructing a cast-in-place concrete pile according to any one of claims 1 to 5, wherein none of the excavation holes in the lower expansion section extend beyond the two boundary lines.

7. A cast-in-place concrete pile having a columnar shaft and an expanded base formed by expanding the tip of the shaft, The expanded bottom portion has an upper expanded bottom portion that gradually widens radially outward as it extends downward, a vertical expanded bottom portion that extends vertically downward from the upper expanded bottom portion, and a plurality of lower expanded bottom portions that gradually widen radially outward as they extend downward from the vertical expanded bottom portion, The central axis of the upper-stage expanded bottom portion and the central axis of the vertical expanded bottom portion are arranged coaxially with the central axis of the shaft portion, The central axis of the lower expanded bottom portion is disposed eccentrically from the central axis of the shaft portion, the plurality of lower flared bottom portions are arranged so as not to cross the boundary line; In a plan view, the shape of the bottom surface is asymmetric with respect to a line that passes through the center of the shaft portion and is parallel to the boundary line, The boundary line is a line for constructing the cast-in-place concrete pile so as not to cross the boundary line.

8. A cast-in-place concrete pile as described in claim 7, wherein, in a planar view, the distance between the center of the central axis of the shaft portion and the center of gravity of the bottom surface of the lower expanded bottom portion is less than 1 / 2 of the radius of a circle whose area is equivalent to the bottom surface of the cast-in-place concrete pile.

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