JP3680952B2 - Cast-in-place concrete pile - Google Patents

Description

[0001]
[Industrial application fields]
The present invention relates to a cast-in-place concrete pile. More specifically, the present invention relates to a cast-in-place concrete pile capable of obtaining a high vertical support force by improving the tip shape.
[0002]
[Prior art]
As a foundation pile of a building structure, a cast-in-place concrete pile with less noise and vibration during construction is known. In particular, as a foundation pile used for a large building structure, a bottom-span cast-in-place concrete pile is frequently used. The bottom of the pile tip is expanded by a reverse circulation method or an earth drill method.
[0003]
Conventionally, cast-in-place concrete piles including expanded piles have been constructed with a flat tip shape. Alternatively, the shape of the excavator excavator blade may be applied to a conical shape (conical surface) having a tip shape exceeding 120 degrees.
[0004]
By the way, in the case of bottom-span cast-in-place concrete piles, regarding the failure mechanism of the tip pile body, the building foundation structure design standard (Architectural Institute of Japan) assumes a broken vertical cylindrical surface in the pile bottom as shown in FIG. Therefore, it is necessary to consider that the shear strength is sufficient for the ground reaction force acting on the AB and CD portions. There are no other studies that mention the failure mechanism of the bottom expansion.
[0005]
The inventors of the present invention have attempted to elucidate the failure mechanism of the tip pile body of a conventional shape bottomed cast-in-place concrete pile by a numerical experiment using a finite element model. As a result, it was found that the tip of the pile was destroyed by tensile stress in the circumferential direction and the radial direction, and that the failure occurred before the vertical load acting on the pile body reached the ultimate load of the ground. This is a destruction mechanism that has not been considered in the past.
[0006]
As a preventive measure against the failure due to the tensile stress at the tip of the pile, it is conceivable to arrange a tensile rebar. However, there is a problem in construction such as being deep underground and it is difficult to arrange a reinforcing bar for reinforcing the bottom expanded tip at the tip through the pile shaft.
[0007]
Therefore, the inventors of the present invention have further intensively analyzed by the finite element method, and found that the breakage can be prevented by making the pile tip shape a predetermined shape.
[0008]
[Problems to be solved by the invention]
The present invention has been made on the basis of the knowledge as described above, and achieves the following object.
[0009]
An object of the present invention is to provide a cast-in-place concrete pile in which the vertical bearing force is increased and the tip pile body is prevented from being destroyed.
[0010]
[Means for Solving the Problems]
In order to achieve the above object, the present invention employs the following means.
[0011]
That is, the present invention resides in a cast-in-place concrete pile characterized in that the tip shape is conical and the apex angle is not less than 90 degrees and not more than 120 degrees.
[0012]
[0013]
[0014]
Furthermore, the present invention resides in a cast-in-place concrete pile having the shape described above, wherein the tip portion is widened.
[0015]
[Action]
According to the finite element analysis results, in the cast-in-place concrete pile having the tip shape as described above, no fracture area was observed at the tip of the pile. This is because the direct component of the ground reaction force acting as a compressive stress on the pile tip keeps the inside of the pile tip in a compressive stress state and no tensile stress is generated. Therefore, it is possible to prevent a fracture phenomenon due to tensile stress at the tip of the pile, which can occur in a conventional cast-in-place concrete pile.
[0016]
【Example】
Embodiments of the present invention will be described below with reference to the drawings.
[0017]
FIG. 1 is a sectional view showing an embodiment of the present invention. This embodiment and the following reference example show a bottom-up cast-in-place concrete pile 1, and a bottom-up part 3 is formed at the tip of a shaft part 2.
[0018]
In the embodiment, the shape of the tip portion 4 of the widened portion 3 is a conical shape. The tip 4 is formed so that the apex angle θ is 120 degrees or less (90 degrees in this embodiment). It has been confirmed by numerical experiments described later that the apex angle θ is preferably in this range.
[0019]
FIG. 2 is a cross-sectional view showing a reference example. In this reference example, the shape of the tip 4 is a truncated cone. Also in this reference example, it is desirable that the apex angle θ is 120 degrees or less.
[0020]
FIG. 3 is a cross-sectional view showing another reference example. In this reference example, the shape of the tip 4 is substantially hemispherical.
[0021]
<Results of numerical analysis>
The results of analyzing the deformation behavior of the above-mentioned bottom-shaped cast-in-place concrete piles using a finite element model are shown in FIGS. FIG. 4 shows a conical shape at the tip of the widened portion, and the apex angles θ of (a) and (b) are 90 degrees and 120 degrees, respectively. 5 and 6 show the tip shape of the widened portion as a truncated cone and a hemisphere, respectively. In the analysis, the pile diameter is 1500 mm, the expanded bottom diameter is 2700 mm, and the concrete design standard strength is 320 kg / cm ² .
[0022]
In the legend shown in FIG. 4, α _c is the yield strength ratio of concrete, defined as the ratio of the fracture criterion to the octahedral shear stress inside the pile, and α _c = 1 is the state of failure. This legend also applies to FIGS. 5, 6 and 7. All of the loads are for 4500 tons.
[0023]
In any of the cases shown in FIGS. 4, 5, and 6, a fracture zone of α _c = 1 is seen around the shaft portion, but no fracture zone is seen in the widened portion, and the yield strength ratio is a small value. Yes.
[0024]
<Comparative example>
FIG. 7 shows the result of a numerical analysis performed on the conventional bottom-in-place cast-in-place concrete pile under the same conditions as those of the present invention. (A) is a shape in which the tip shape of the widened portion is a flat surface (θ = 180 degrees), and (b) is a shape in which the tip shape of the widened portion is a conical shape and the apex angle θ is 160 degrees.
[0025]
When the tip shape of the widened part is a flat surface, it is recognized that a fracture area appears not only around the shaft part of the pile body but also at the tip part. Even when the tip shape is conical as shown in (b), when the apex angle exceeds 120 degrees, it is recognized that a region having a high yield ratio appears at the tip portion. In this way, the reason why the fracture tip or the region with a high yield ratio occurs at the tip of the pile is that circumferential and radial tensile stress is generated at the tip of the pile. I understood the result.
[0026]
On the other hand, in the shape shown in FIG. 1, FIG. 2, and FIG. 3, no tensile stress is generated at the tip of the pile. This is nothing but the direct component of the ground reaction force acting on the pile tip as a compression reaction force to keep the inside of the pile tip in a compressive stress state. As a result of the analysis, it has been confirmed that there is almost no influence on the ground supporting force by adopting such a shape.
[0027]
The excavation hole corresponding to the tip shape of the pile according to the present invention can be easily formed by changing the shape of the excavation blade of a conventional reverse circulation excavator or earth drill excavator.
[0028]
[Other Examples]
In the above-described embodiment, the case where the present invention is applied to a cast-in-place concrete pile is explained. However, it is needless to say that the present invention can also be applied to a cast-in-place concrete pile that is not expanded.
[0029]
【The invention's effect】
As described above, according to the present invention, it is possible to prevent breakage due to tensile stress that may occur at the tip of a conventional cast-in-place concrete pile. Thereby, since the pile body is healthy even after the ground is destroyed, the structural strength of the pile body is remarkably improved, and a higher vertical support force than the conventional one can be obtained.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view showing an embodiment of the present invention.
FIG. 2 is a cross-sectional view showing a reference example .
FIG. 3 is a cross-sectional view showing another reference example .
FIG. 4 is a diagram showing a numerical analysis result of deformation behavior of a cast-in-place concrete pile corresponding to the embodiment of the present invention.
FIG. 5 is a diagram showing a numerical analysis result of deformation behavior of a cast-in-place concrete pile corresponding to the reference example shown in FIG . 2 ;
6 is a diagram showing a numerical analysis result of deformation behavior of a cast-in-place concrete pile corresponding to the reference example shown in FIG . 3 ;
FIG. 7 is a diagram showing a numerical analysis result of deformation behavior of a conventional cast-in-place concrete pile.
FIG. 8 is a diagram for explaining a structural standard that has been conventionally required.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Bottom expansion cast-in-place concrete pile 2 ... Shaft part 3 ... Bottom expansion part 4 ... Tip part

Claims (2)

A cast-in-place concrete pile characterized in that the tip shape is conical and the apex angle is not less than 90 degrees and not more than 120 degrees. The cast-in-place concrete pile according to claim 1, wherein the tip portion is widened.

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