JP7256367B2 - Evaluation method of tip bearing capacity of pile foundation - Google Patents

Description

Application of Article 30, Paragraph 2 of the Patent Act (Leaflet) Publisher: Japan Pile Co., Ltd. Publication name: MAGNUM Construction Method Leaflet Publication date: September 21, 2018 Person: Mikihiko Hashido Publication name: Kenchiku Gijutsu, December issue, No. 827 Publication date: November 17, 2018 (Company website) Posting address: http://www. japanese pile. co. jp/method/buildingtech/magnum/ Publication date: November 27, 2018 (Pamphlet) Publisher: Japan Pile Co., Ltd. Publication name: Pamphlet “MUGNUM construction method” Publication date: November 27, 2018 ( Seminar) Meeting name: MAGNUM construction method seminar Venue: Nihonbashi Hamacho F Tower (3-21-1 Nihonbashi Hamacho, Chuo-ku, Tokyo) Date: December 16, 2018 Other 3 times (4 times in total)

The present disclosure relates to a method for evaluating tip bearing capacity of pile foundations.

The force (tip bearing force) R _pu that the foot protection part of the pile foundation can receive from the ground is generally expressed by the following formula:
R _pu =α·N _ave ·A _p
is represented by (for example, Patent Document 1).
In the above formula, α is the tip bearing capacity coefficient, N _ave is the average N value of the ground around the bottom end of the pile, and A _p is the closed cross-sectional area of the bottom end of the pile.

Japanese Patent No. 6004298

As is clear from the above formula, when calculating the tip supporting force R _pu , the accuracy of the tip supporting force R _pu to be obtained varies greatly depending on the accuracy of the tip supporting force coefficient α. Therefore, the problem is how to set the tip supporting force coefficient α.
In this respect, in the pile foundation design method disclosed in Patent Document 1, characteristic data representing the correlation between various parameters such as the diameter of the existing pile and the tip bearing capacity coefficient is prepared in advance, and the designed specifications are obtained. Accordingly, an appropriate tip bearing force coefficient is selected from the characteristic data, and a highly accurate tip bearing force coefficient α can be obtained.

However, the task of preparing characteristic data for each of various parameters is enormous, and it is desirable to set an approximation formula that can easily obtain the tip bearing force coefficient with high accuracy.
In view of the above circumstances, an object of at least one embodiment of the present invention is to calculate the tip bearing capacity of the pile foundation using an approximation formula capable of calculating the tip bearing capacity coefficient with high accuracy. It is to provide a bearing capacity evaluation method.

(1) A method for evaluating the tip bearing capacity of a pile foundation according to at least one embodiment of the present invention,
In a method for evaluating the tip bearing capacity of a pile foundation having a foot protection part constituted by a foot protection member surrounding the bottom end of a pile buried in a pile hole and the bottom end of the pile,
When the ground around the lower end of the pile is sandy ground or gravel ground,
The tip bearing capacity of the pile foundation at a position 2 m above the bottom end of the pile is R _pu , the tip bearing capacity coefficient is α, the closed cross-sectional area of the bottom end of the pile is A _p , and the bottom end of the pile is When the average N value of the surrounding ground is N _ave , the expansion ratio of the pile hole is ω , and the effective length under the pile of the foot protection part is _LL ' , the following formula:
R _pu =α·N _ave ·A _p (1)
α=240ω ^1.5 +45(2+L _L ′ )ω (2a)
Evaluate the tip bearing capacity R _pu of the pile foundation using
The N _ave is an average value of N values between an upper limit position of 2 m upward from the bottom end of the pile and a lower limit position of a predetermined distance downward from the bottom end of the pile,
When the predetermined distance between the bottom end of the pile and the lower limit position is LNL _{, LNL} is the following formula:
LNL = _LL + D _e + _{D on}
(where D _e is the diameter of the foot protection part and D _on is the nodal diameter of the pile in the foot protection part),
The expansion ratio ω of the pile hole is 1 or more and 2 or less,
The under-pile length LL of the foot protection portion _is more than 0 m and 2 m or less and 3.1 De _or less,
The effective length L _L ′ of the foot protection portion under the pile is set to 0 when the length L _L of the foot protection portion positioned below the pile is 0.5 m or less. When the foot protection portion under the pile length L _L is more than 0.5 m and not more than 2 m , it is set to the same value as the foot protection portion under the pile length L _L .

According to the above configuration (1), when the ground around the lower end of the pile is sandy ground or gravel ground, by obtaining the tip bearing capacity coefficient based on the formula (2a), a highly accurate tip bearing capacity coefficient is obtained. As a result, the tip support force can be obtained with high accuracy based on the formula (1).

(2) A method for evaluating the tip bearing capacity of a pile foundation according to at least one embodiment of the present invention,
In a method for evaluating the tip bearing capacity of a pile foundation provided with a foot protection part composed of a foot protection member surrounding the bottom end of a pile buried in a hole and the bottom end of the pile,
When the ground around the lower end of the pile is clayey ground,
The tip bearing capacity of the pile foundation at a position 2 m above the bottom end of the pile is R _pu , the tip bearing capacity coefficient is α, the closed cross-sectional area of the bottom end of the pile is A _p , and the bottom end of the pile is When the average N value of the surrounding ground is N _ave , the expansion ratio of the pile hole is ω , and the effective length under the pile of the foot protection part is _LL ' , the following formula:
R _pu =α·N _ave ·A _p (1)
α=210ω ^1.25 +45(2+L _L ′ )ω (2b)
Evaluate the tip bearing capacity R _pu of the pile foundation using
The N _ave is an average value of N values between an upper limit position of 2 m upward from the bottom end of the pile and a lower limit position of a predetermined distance downward from the bottom end of the pile,
When the predetermined distance between the bottom end of the pile and the lower limit position is LNL _{, LNL} is the following formula:
LNL = _LL + D _e + _{D on}
(where D _e is the diameter of the foot protection part and D _on is the nodal diameter of the pile in the foot protection part),
The expansion ratio ω of the pile hole is 1 or more and 2 or less,
The under-pile length LL of the foot protection portion _is more than 0 m and 2 m or less and 3.1 De _or less,
The effective length L _L ′ of the foot protection portion under the pile is set to 0 when the length L _L of the foot protection portion positioned below the pile is 0.5 m or less. When the foot protection portion under the pile length L _L is more than 0.5 m and not more than 2 m , it is set to the same value as the foot protection portion under the pile length L _L .

According to the above configuration (2), when the ground around the lower end of the pile is sandy ground or gravel ground, by obtaining the tip bearing capacity coefficient based on the formula (2b), a highly accurate tip bearing capacity coefficient is obtained. As a result, the tip support force can be obtained with high accuracy based on the formula (1).

According to at least one embodiment of the present invention, a method for evaluating the tip bearing capacity of a pile foundation is provided, which can calculate the tip bearing capacity of a pile foundation using an approximate expression that can calculate the tip bearing capacity coefficient with high accuracy. be done.

It is a figure for demonstrating the evaluation method of the tip bearing capacity of the pile foundation which concerns on one Embodiment of this invention, and is a schematic sectional drawing of the foot protection part of a pile foundation. It is a figure for demonstrating the blocking cross-sectional area of a pile. 4 is a graph showing the relationship between the under-pile length _LL of the foot protection portion and the ratio of the ultimate tip axial force to the calculated tip axial force.

Several embodiments of the present invention will now be described with reference to the accompanying drawings. However, the dimensions, materials, shapes, relative arrangements, etc. of the components described as embodiments or shown in the drawings are not intended to limit the scope of the present invention, and are merely illustrative examples. do not have.
For example, expressions that express shapes such as squares and cylinders do not only represent shapes such as squares and cylinders in a geometrically strict sense, but also include irregularities and chamfers to the extent that the same effect can be obtained. The shape including the part etc. shall also be represented.

FIG. 1 is a diagram for explaining an evaluation method (hereinafter simply referred to as an evaluation method) of the force (end bearing force) that can be received by a foot protection portion of a pile foundation according to an embodiment of the present invention, It is a schematic sectional view of the foot protection part of a pile foundation. FIG. 2 is a diagram for explaining the closed cross-sectional area of a pile. FIG. 3 is a graph showing the relationship between the under-pile length _LL of the foot protection portion and the ratio of the ultimate tip axial force to the calculated tip axial force.

As shown in FIG. 1, a pile foundation 1 to be evaluated by the evaluation method is constructed by a pre-boring method, and includes a foot protection portion 2. As shown in FIG. The foot protection part 2 is composed of a lower end of the pile 5 embedded in the bottom of the pile hole 3 and a foot protection member 7 surrounding the lower end of the pile 5 .
The pile 5 is a prefabricated hollow concrete pile having a shaft portion 9 and one or more annular knot portions 11 projecting radially from the shaft portion 9 . For example, the pile 5 is provided with two or more knot portions 11 spaced apart in the longitudinal direction of the shaft portion 9 , and the two knot portions 11 are surrounded by the foot protection member 7 . The pile 5 may be a straight pile having no joints, and the material thereof is not limited to concrete, and may be a steel pipe.
The foot protection member 7 is made of hardening material such as cement milk or soil cement.

In the evaluation method, when the ground 13 around the lower end of the pile 5 is sandy ground or gravel ground, the tip bearing capacity of the pile foundation 1 is _Rpu , the tip bearing capacity coefficient is α, and the foot protection part 2 The closed cross-sectional area of the lower end of the pile 5 located in _Ap , the average N value of the ground around the lower end of the pile 5 _{as Nave} , the expansion ratio of the pile hole 3 as ω, and the position below the pile 5 When the length under the pile of the foot protection part is _LL , the following formula:
R _pu =α·N _ave ·A _p (1)
α=240ω ^1.5 +45(2+L _L )ω (2a)
is used to evaluate the force (end bearing force) R _pu that the foot protection part 2 of the pile foundation 1 can receive from the ground.

The evaluation position of the tip bearing force R _pu is the position 2 m above the lower end of the pile 5 as shown in FIG. 1 . In addition, the closed cross-sectional area _Ap of the lower end of the pile 5 located in the foot protection part 2 is the maximum cross-sectional area of the pile 5 in the foot protection part 2, as shown by hatching in FIG. It is the cross-sectional area defined by the outer edge of the knot portion 11 when the portion 11 is located in the foot protection portion 2 . In addition, when the pile 5 is hollow, the closed cross-sectional area _Ap is the cross-sectional area including the hollow portion of the pile 5 .

The average N value N _ave is the average value of the N values (the number of impacts (times)) of the ground 13 around the lower end of the pile 5 obtained by a standard penetration test. In this embodiment, the average N value N _ave is the upper limit position of the distance _Lu (=2 m) upward from the bottom end of the pile 5 and the lower limit position of the distance L _L + _De + _Don downward from the bottom end of the pile 5. is the average of the N values between For example, a standard penetration test is performed at multiple depths at intervals of 0.5m or 1m, and the average N-value N _ave is the average of the multiple N-values thus obtained.

In addition, the average N value N _ave is the average N value N _u of the standard penetration test in the section of the distance L _u (= 2 m) upward from the bottom end of the pile 5, and the distance L _L +D downward from the bottom end of the pile 5 It can be obtained as the average value of the average value _NL of the N values of the standard penetration test in the interval _e + D _on .

Here, when the average N value N _ave is obtained as the average value of the average value N _u and the average value _NL , weighting may be performed so that the weight of the average value _NL is increased.
For example, when the ground 13 around the lower ends of the piles 2 is sandy ground or gravel ground, the average N value N _ave may be obtained by the following formula (3a).
N _ave =(N _U +3N _L )/4 (3a)
Further, for example, when the ground 13 around the lower ends of the piles 2 is clayey ground, the average N value N _ave may be obtained by the following equation (3b).
N _ave =(N _U +2N _L )/3 (3b)
In addition, when the pile bottom length _LL is short and the standard penetration test is not performed between the bottom end of the pile 5 and the lower limit position, the N value directly above the bottom end of the pile and the N value directly below the lower limit position Let the average value of NL be the average value _NL .

The enlargement ratio ω of the pile hole 3 is the ratio D _e / _{D s of} the diameter D _e of the foot protection part 2 to the reference excavation diameter D s for the pile 5 . The reference excavation diameter D _s is, for example, a value obtained by adding 0.05 (m) to the outer diameter (knot diameter) D _on of the node 11 in the foot protection portion 2 . When there are a plurality of nodal portions 11 in the foot protection portion 2, the largest nodal portion diameter is adopted as the nodal portion diameter _Don . The reference excavation diameter D _s need not be the actual excavation diameter of the pile hole 3 . The pile hole 3 may be an enlarged bottom hole having an enlarged bottom portion with an enlarged excavation diameter at the foot protection portion 2, or may be a straight hole as shown in FIG.
Preferably, the expansion ratio ω of the pile foundation 1 to be evaluated by the evaluation method of the present embodiment is 1 or more and 2 or less.
Further, preferably, the under-pile length _LL of the foot protection portion 2 in the pile foundation 1 to be evaluated by the evaluation method of the present embodiment is 0 m or more and 2 m or less, and the foot protection portion 2 has a diameter D _e 3.1 times or less. Furthermore, preferably, when the under-pile length L _L is 0.5 m or less, the under-pile length L _L to be substituted into the formulas (2a) and (2b) is set to 0.
1, D represents the outer diameter of the shaft portion of the pile 5, and _Do represents the outer diameter of the joint portion 11 of the pile 5 above the foot protection portion 2. As shown in FIG.

According to the evaluation method of the above configuration, when the ground 13 around the lower end of the pile 5 is sandy ground or gravel ground, by obtaining the tip bearing force coefficient α based on the formula (2a), a highly accurate tip A bearing capacity coefficient α is obtained. As a result, it is possible to obtain the tip support force R _pu with high accuracy based on the equation (1).

Next, a method for evaluating tip bearing capacity of a pile foundation according to another embodiment of the present invention will be described. In addition, in the following description, the same reference numerals or symbols are attached to the same or similar configurations as those of the above-described embodiment, and the description is omitted or simplified.

A method for evaluating the tip bearing capacity of a pile foundation according to another embodiment is the following equation when the ground 13 around the lower end of the pile 5 is clayey ground:
R _pu =α·N _ave ·A _p (1)
α=210ω ^1.25 +45(2+L _L )ω (2b)
is used to evaluate the force R _pu that the hardened part of the pile foundation can receive from the ground.

According to the evaluation method with the above configuration, when the ground around the lower end of the pile is clayey ground, a highly accurate tip bearing force coefficient α can be obtained by obtaining the tip bearing force coefficient α based on the formula (2b). be done. As a result, it is possible to obtain the tip support force R _pu with high accuracy based on the equation (1).

<Example>
Examples are described below.
A pile foundation with the specifications shown in Table 1 was constructed on the ground that had been previously subjected to a boring test and the N value was measured. ) was measured at a position 2 m above the bottom end of the pile. Friction-cutting material was applied from the middle to the upper part of the pile so that only the tip bearing capacity could be measured.
Then, using equations (1), (2a) and (2b), the tip bearing force (calculated tip bearing force) of each specification was calculated, and the ratio of the ultimate tip bearing force to the calculated tip bearing force was obtained. The graph in FIG. 3 shows the relationship between the under-pile length _LL and the ratio of the ultimate tip bearing force to the calculated tip bearing force.

As can be seen from FIG. 3, the ratio of the ultimate tip support force to the calculated tip support force is greater than or equal to 1 in all embodiments, and using equations (1), (2a) and (2b), a high accuracy It can be seen that the tip bearing force can be calculated by

Finally, the present invention is not limited to the several embodiments described above, but includes modifications of the above embodiments.

1 Pile foundation 2 Foot protection part 3 Pile hole 5 Pile 7 Foot protection member 9 Shaft part 11 Knot part 13 Ground

Claims (2)

In a method for evaluating the tip bearing capacity of a pile foundation having a foot protection part constituted by a foot protection member surrounding the bottom end of a pile buried in a pile hole and the bottom end of the pile,
When the ground around the lower end of the pile is sandy ground or gravel ground,
The tip bearing capacity of the pile foundation at a position 2 m above the bottom end of the pile is R _pu , the tip bearing capacity coefficient is α, the closed cross-sectional area of the bottom end of the pile is A _p , and the bottom end of the pile is When the average N value of the surrounding ground is N _ave , the expansion ratio of the pile hole is ω , and the effective length under the pile of the foot protection part is _LL ' , the following formula:
R _pu =α·N _ave ·A _p
α=240ω ^1.5 +45(2+ _LL ′ )ω
Evaluate the tip bearing capacity R _pu of the pile foundation using
The N _ave is an average value of N values between an upper limit position of 2 m upward from the bottom end of the pile and a lower limit position of a predetermined distance downward from the bottom end of the pile,
When the predetermined distance between the bottom end of the pile and the lower limit position is LNL _{, LNL} is the following formula:
LNL = _LL + D _e + _{D on}
(where D _e is the diameter of the foot protection part and D _on is the nodal diameter of the pile in the foot protection part),
The expansion ratio ω of the pile hole is 1 or more and 2 or less,
The under-pile length L _L of the foot protection portion is more than 0 m and 2 m or less and 3.1 D _e or less,
The effective length L _L ′ of the foot protection portion under the pile is set to 0 when the length L _L of the foot protection portion positioned below the pile is 0.5 m or less. When the foot protection portion under the pile length LL _is more than 0.5 m and 2 m or less , it is set to the same value as the foot protection portion under the pile length _LL .
A method for evaluating the tip bearing capacity of a pile foundation, characterized by:
In a method for evaluating the tip bearing capacity of a pile foundation having a foot protection part constituted by a foot protection member surrounding the bottom end of a pile buried in a pile hole and the bottom end of the pile,
When the ground around the lower end of the pile is clayey ground,
The tip bearing capacity of the pile foundation at a position 2 m above the bottom end of the pile is R _pu , the tip bearing capacity coefficient is α, the closed cross-sectional area of the bottom end of the pile is A _p , and the bottom end of the pile is When the average N value of the surrounding ground is N _ave , the expansion ratio of the pile hole is ω , and the effective length under the pile of the foot protection part is _LL ' , the following formula:
R _pu =α·N _ave ·A _p
α=210ω ^1.25 +45(2+ _LL ′ )ω
is used to evaluate the tip bearing capacity R _pu of the pile foundation , and the _Nave is between the upper limit position of 2 m upward from the bottom end of the pile and the lower limit position of a predetermined distance downward from the bottom end of the pile is the average value of N values,
When the predetermined distance between the bottom end of the pile and the lower limit position is LNL _{, LNL} is the following formula:
LNL = _LL + D _e + _{D on}
(where D _e is the diameter of the foot protection part and D _on is the nodal diameter of the pile in the foot protection part),
The expansion ratio ω of the pile hole is 1 or more and 2 or less,
The under-pile length L _L of the foot protection portion is more than 0 m and 2 m or less and 3.1 D _e or less,
The effective length L _L ′ of the foot protection portion under the pile is set to 0 when the length L _L of the foot protection portion positioned below the pile is 0.5 m or less. When the foot protection portion under the pile length LL _is more than 0.5 m and 2 m or less , it is set to the same value as the foot protection portion under the pile length _LL .
A method for evaluating tip bearing capacity of a pile foundation characterized by:

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