JP2016125271A - Calculation method for pull-out resistance of pile - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a calculation method for pull-out resistance of a pile that clarifies the pull-out resistance at each of widened-diameter parts and enables accurate calculation of the pull-out resistance even when soil quality changes in a middle of the widened-diameter part.SOLUTION: A calculation method is for calculating pull-out resistance of an under-reamed pile 1 having a widened-diameter part 3 widened diagonally downward from a shaft part 2. Specifically, in the method that calculates the pull-out resistance by calculating a side surface area of the shaft part and the widened-diameter part in contact with a ground, multiplying the side surface area by a coefficient of frictional resistance specific to strength of soil in the surrounding and frictional resistance in relation with the soil, and calculating the pull-out resistance based on an integrated value of the calculation results for the shaft part and the widened-diameter part and effective self-weight of the under-reamed pile, the side surface area is calculated at least for each type of soil, and different values of the coefficient of frictional resistance are used for different angles of inclination and types of soil, in a portion with an inclined surface.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for calculating a pulling resistance force of a pile having a diameter-expanded portion that is obliquely enlarged downward from a shaft portion.

  It is known that the pulling resistance force of a pile can be enhanced by providing the pile with a diameter-expanded portion that extends downward in a truncated cone shape from a columnar shaft portion. However, there is currently no clear calculation standard for the method of calculating the drawing resistance of the enlarged diameter portion.

  On the other hand, it is known that a building with a large aspect ratio, such as an elongated building, is subjected to a large pulling force on a support pile when subjected to an earthquake force. Also, when the groundwater level rises and receives buoyancy, or when it receives a horizontal force due to a tsunami, a large pulling force acts on the pile.

  For such a large pull-out force, it can be easily understood that a pile having a diameter-expanded portion is effective qualitatively, so as disclosed in Patent Documents 1 and 2, A method for calculating the drawing resistance has been proposed.

  Patent Documents 1 and 2 are methods for calculating the pulling resistance force in a multi-stage pile on the premise that a plurality of enlarged diameter portions are provided at intervals in the axial direction. For example, in Patent Document 1, the pulling resistance force is calculated on the assumption that the ground included in a range that assumes a certain spread from the enlarged diameter portion becomes an overload of the enlarged diameter portion and becomes a drawing resistance.

  On the other hand, in Patent Document 2, the pulling resistance force is calculated from the sum of the ultimate shear resistance force and the ultimate support pressure received from the ground around the enlarged diameter portion. Here, Patent Document 2 is a literature on so-called piles with knots, and it is a calculation method based on the premise that the soil diameter of the ground in contact with each expanded portion is uniform, with each expanded portion being small. Yes.

JP 2011-174251 A Japanese Patent No. 4658684

  However, when the enlarged diameter portion becomes large, the soil quality may change from the middle of the enlarged diameter portion. Further, it is obvious that even when the diameter-expanded portion is one (bottom-expanded pile), the pulling resistance is larger than that of a normal columnar pile.

  Therefore, the present invention can clarify the pulling resistance for each enlarged diameter portion, and can calculate the pulling resistance force of a pile that can accurately calculate the pulling resistance even when the soil quality changes in the middle of the enlarged diameter portion. The purpose is to provide.

  In order to achieve the above object, the method for calculating the pulling resistance force of the pile of the present invention is a method for calculating the pulling resistance force of the pile having a diameter-expanded portion that is obliquely expanded downward from the shaft portion, A step of calculating a side area in which the shaft portion and the diameter-expanded portion are in contact with the ground, a step of multiplying the side area by a frictional resistance coefficient related to a strength of a surrounding soil and a frictional resistance with the soil; A step of calculating an extraction resistance based on an integrated value of calculation results of the shaft portion and the enlarged diameter portion and an effective dead weight of the pile, and a range of an inclined surface which is obliquely enlarged toward the lower side of the enlarged diameter portion In the method, the side area is calculated at least for each soil, and the frictional resistance coefficient is different depending on the inclination angle of the inclined surface and the soil.

  Here, it is preferable that the frictional resistance coefficient used in the range of the inclined surface is a value that increases as the inclination angle increases. For example, an exponentially increasing value can be used as the frictional resistance coefficient used in the range of the inclined surface.

  Moreover, it is preferable that the said soil strength is N value in sandy ground, and is uniaxial compressive strength in clayey ground.

  In the method of calculating the pullout resistance force of the pile of the present invention configured as described above, in the range of the inclined surface of the enlarged-diameter portion, the side area where the inclined surface of the enlarged-diameter portion contacts at least for each soil is calculated, and the side The area is multiplied by the strength of the soil, and the frictional resistance coefficient, which varies depending on the inclination angle of the enlarged diameter portion and the soil.

  Then, the pulling resistance force of the pile is calculated based on the integrated value of the calculation result of the enlarged diameter portion and the calculation result of the shaft portion calculated for each soil material and the effective weight of the pile.

  In this way, the pullout resistance can be clearly calculated for each enlarged diameter part, so even if the enlarged diameter part is a single bottom expanded pile or a multi-stage pile in which multiple enlarged diameter parts are provided, the resistance to extraction can be accurately determined. Can be calculated.

  Moreover, even if the soil quality changes in the middle of the enlarged diameter portion, the pullout resistance can be calculated for each soil, so the pile pullout resistance can be accurately calculated. Of course, the present invention can also be applied to the case where there is only one kind of soil that the enlarged diameter portion contacts.

  Furthermore, if the frictional resistance coefficient is a coefficient that increases with an increase in the inclination angle, the fact that the inclination angle has been increased can be accurately reflected, so that it is possible to construct a pile economically, such as shortening the pile length. become. In particular, by reflecting the knowledge that the frictional resistance coefficient increases exponentially, more economical pile design and construction can be achieved.

  In addition, when the soil strength used to calculate the pulling resistance of piles is N value in sandy ground and uniaxial compressive strength in clay soil, numerical values related to the strength of each soil should be easily obtained. Can do.

It is explanatory drawing for demonstrating the calculation method of the drawing-out resistance force of the bottom-expanded pile of embodiment of this invention. It is explanatory drawing which showed the structure of the whole building in order to explain the drawing-out force which acts on an expanded pile. It is explanatory drawing for demonstrating the relationship between the drawing-out force and resistance force which act on an expanded bottom pile. It is the figure which showed the relationship between inclination-angle (theta) and frictional resistance coefficient (lambda) in sandy ground. It is the figure which showed the relationship between inclination-angle (theta) and frictional resistance coefficient (micro | micron | mu) in a clayey ground. It is explanatory drawing which showed the structure of the whole building in order to demonstrate the drawing-out force which acts on the multistage pile of an Example.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a diagram showing a configuration of an expanded bottom pile 1 as a pile, which is used for explaining a calculation method of a pulling resistance force of a pile according to the present embodiment.

  FIG. 2 is a diagram schematically illustrating the entire configuration of the building B supported by the plurality of bottom-spreading piles 1 and 1. This building B is an elongated building having a high aspect ratio with a high height with respect to the base (bottom area).

  The building B is supported by the expanded bottom piles 1 and 1 through the foundation B1 serving as the bottom plate. The expanded bottom pile 1 is mainly configured by a cylindrical shaft portion 2 having a constant diameter and a diameter expanded portion 3 that is obliquely expanded downward from the shaft portion 2.

  Such an expanded pile 1 can be constructed by inserting a reinforcing bar into a hole excavated in the ground G and placing concrete. That is, the expanded bottom pile 1 is a cast-in-place concrete pile.

  Normally, the weight of the building B and the foundation B1, the weight of the equipment installed inside the building, the weight of the user, and the like act on the expanded piles 1 and 1 as an overload. That is, it is pushed into the ground G. In addition, in this specification, the description regarding this pushing resistance force (supporting force) is abbreviate | omitted.

  On the other hand, during an earthquake, as shown in FIG. 2, for example, a force (earthquake force Q) due to the earthquake acts from the left side of the building B. As a result, building B leans to the right.

  If it becomes so, the pushing force more than usual will act on the expanded bottom pile 1 on the right side of the building B. As shown in the figure, the pushing force is countered by an upward resistance force generated around the expanded pile 1.

  On the other hand, a pulling force opposite to the normal pushing force acts on the bottom expanded pile 1 on the left side of the building B. As shown in the drawing, the pulling force is countered by the downward resistance force generated around the expanded pile 1.

  The pulling force acts on the expanded pile 1 not only during an earthquake, but also when the groundwater level G1 rises and the buoyancy W received by the foundation B1 increases, for example.

  FIG. 3 is a diagram schematically showing the resistance state of the expanded pile 1 when the pulling force P is acting. Here, the diameter-enlarged portion 3 is mainly configured by a frustoconical inclined portion 31 and a rising portion 32 formed in a columnar shape from the lower end of the inclined portion 31. Further, the tip end portion 33 protrudes downward from the center of the rising portion 32.

For the pulling force P, a frictional resistance force τ ₁ generated between the peripheral surface of the shaft portion 2 and the ground G, and a frictional resistance force τ ₂ generated between the peripheral surface of the inclined portion 31 and the ground G. Then, the frictional resistance τ ₃ generated between the peripheral surface of the rising portion 32 and the ground G and the effective weight S of the expanded pile 1 are countered.

  Therefore, the pulling resistance force that opposes the pulling force P will be described below. Here, the pulling resistance force includes a value calculated for a force acting for a long period of time such as buoyancy W and a value calculated for a force acting for a short period of time such as seismic force Q.

Hereinafter, long-term and long-term acceptable pullout resistance force R _T acceptable values for pull-out force acting, the allowable values for pull-out force acting short term and short-term allowable pull-out resistance R _t.

R _T = 1/3 (λ · N _s · L _s · ψ + μ · q _u · L _c · ψ) + w _p (1)
R _t = 2/3 (λ · N _s · L _s · ψ + μ · q _u · L _c · ψ) + w _p (2)
Here, the frictional resistance coefficient of λ is sandy soil, N _s is the mean value of the number of shots by SPT of sandy soil, L _s is the length of拡底pile 1 are in contact with the sandy soil, [psi is拡底pile 1 Indicates the perimeter of.

Further, μ is a frictional resistance coefficient of the clay ground, q _u is an average value of the uniaxial compressive strength of the clay ground, and L _c is a length of the bottom pile 1 in contact with the clay ground. Further, w _p represents the effective weight of the expanded pile 1.

In short, the pulling-out resistance (R _T , R _t ) of the expanded bottom pile 1 is the side area (L _s · ψ or L _c · ψ) where the shaft portion 2 and the enlarged diameter portion 3 are in contact with the ground G, and the surrounding area It is calculated by multiplying and multiplying the soil strength (N _s or q _u ) and the frictional resistance coefficient (λ or μ) of the surrounding soil.

The above formulas (1) and (2) will be described in detail with reference to FIG. The inclined portion 31 is a truncated cone having an inclination angle of the inclined surface with respect to the axis (vertical line) θ, a diameter of the upper surface D, a diameter of the lower surface D _W , and an overall height H.

The inclined portion 31 can be divided in calculation at the position where the soil changes (for each soil). “Every soil” means not only the soil type such as sandy soil and clay soil, but also the same soil type, if the strength (N _s , q _u ) is different, it can be treated as another soil Say what you can do.

In FIG. 1, the inclined portion 31 is divided into n regions by parallel lines. Here, the position in the height direction, in the upper surface of the inclined portion 31 to the lower _{surface, Z 0, ··· Z i-} 1, Z i, indicated by · · · _{Z n.} In addition, although the division number n can also be made into the number of the soil which appears in the inclination part 31, in order to raise the calculation precision mentioned later, it can also divide | segment within the same soil.

Then, the diameter of the inclined portion 31 of the positions corresponding to the _{height, D 0, ··· D i-} 1, D i, indicated by · · · _{D n.} Also, the height of each divided region _is, L 1, · · · _L i, indicated with · · · _{L n.}

Further, the side areas of the respective divided regions (the peripheral _{area), A 1, ··· A i} , indicated by · · · _{A n.} With thus set code, lateral area A _i of the region between Z _i-1 ~Z i can be expressed by the following equation.

A _i = π / (2 cos θ) (D _i−1 + D _i ) L _i (3)
Then, the length ψ _i around each region of the inclined portion 31 is ψ _i = A _i / L _i . However, since L _s · ψ and L _c · ψ in the above formulas (1) and (2) represent the area (side area) of the inclined surface in contact with the ground, the side area A _i is obtained in each region. It only has to be done.

On the other hand, the shaft portion 2 is a cylindrical body having a diameter D and a length L1. The rising portion 32 is a cylindrical body having a diameter _DW and a height h. For example, the constant is h = 0.5 m.

  The shaft portion 2 and the rising portion 32 are cylindrical bodies as in a normal pile, and this portion can be easily pulled out using the above formulas (1) and (2) as in a normal pile. Can be calculated.

That is, since the peripheral length ψ is constant within the range of the cylindrical body, the length (L _s or L _c ) in contact with each soil material may be obtained. In the shaft portion 2 and the rising portion 32 as well, the calculation can be performed for each soil as in the case of the above-described enlarged diameter portion 3.

Further, the effective dead weight w _p of the expanded bottom pile 1 is a value obtained by subtracting the buoyancy acting on the expanded bottom pile 1 from the sum of the weight of the shaft portion 2 and the weight of the expanded diameter portion 3. In short, multiplied by the specific gravity of the reinforced concrete to the volume of拡底pile 1, by calculating the buoyancy from the volume portion of拡底pile 1 to be less than the groundwater level G1, it is possible to obtain an effective self-weight w _p.

The strength of the sandy ground can be expressed by an average value (N _s ) of the number of hits (N value) by the standard penetration test. Here, for example, the shaft portion 2, and 30 when N _s is greater than 30. On the other hand, the inclined portion 31, and 60 when N _s is greater than 60.

The strength of the clayey ground can be represented by an average value (q _u ) of uniaxial compressive strength. Here, for example, in the shaft portion 2, when q _u exceeds 200 kN / m ² , it is set to 200. On the other hand, in the inclined part 31, when q _u exceeds 1000 kN / m ² , it is set to 1000.

  The frictional resistance coefficient (λ, μ) is a numerical value related to the frictional resistance between the peripheral surface of the expanded pile 1 and the surrounding ground G in contact therewith.

  For example, a conventionally known constant is used for the frictional resistance coefficient (λ, μ) of the shaft portion 2 and the rising portion 32 whose side surfaces are vertical surfaces. For example, 0.8 × 10/3 can be used for the frictional resistance coefficient λ of sandy ground. Moreover, 0.8 × 1/2 can be used for the frictional resistance coefficient μ of the clayey ground.

  On the other hand, in the inclined portion 31 having an inclined surface that extends obliquely downward from the shaft portion 2, different frictional resistance coefficients (λ, μ) are used depending on the soil quality and the inclination angle θ. The frictional resistance coefficient (λ, μ) in the inclined portion 31 will be described with reference to FIGS.

  FIG. 4 is a diagram showing the relationship between the inclination angle θ and the frictional resistance coefficient λ in sandy ground. Legends such as ○ and □ indicate values obtained by the loading test, and curves are created based on the loading test results.

  As can be seen from this curve, in the sandy ground, it can be said that the frictional resistance coefficient λ increases exponentially as the inclination angle θ increases. Therefore, the frictional resistance coefficient λ of the inclined portion 31 used when calculating the allowable pulling force is expressed by the following formula in consideration of safety.

λ = 5.85e ^0.05θ · ζ (4)
Here, ζ represents a reduction coefficient set by the ratio between the pile length L and the maximum bottom expanded diameter D _W of the expanded diameter portion 3. For example, ζ = 0.46 + 0.164 (L / D _W ) can be set in the range of 0 <L / D _W <3.3, and ζ = 1.0 can be set in the range of L / D _W ≧ 3.3.

That is, when the root insertion (pile length L) is shallow, it is assumed that the expanded diameter portion 3 does not have sufficient pulling resistance as in the above formula. Therefore, when the expanded diameter portion 3 is provided shallower than 3.3 _DW , The frictional resistance coefficient λ is reduced by the reduction coefficient ζ.

  As described above, in the method for calculating the pulling resistance force of the expanded pile 1 described in the present embodiment, the frictional resistance coefficient λ changes according to the inclination angle θ of the inclined portion 31.

  On the other hand, FIG. 5 is a diagram showing the relationship between the inclination angle θ and the frictional resistance coefficient μ in the clayey ground. Legends such as ○ and □ indicate values obtained by the loading test, and curves are created based on the loading test results.

  As can be seen from this curve, it can be said that the frictional resistance coefficient μ increases exponentially with an increase in the inclination angle θ even in the clayey ground. Therefore, the frictional resistance coefficient μ of the inclined portion 31 used when calculating the allowable pulling force is expressed by the following formula in consideration of safety.

μ = 0.40e ^0.04θ・ ζ (5)
It should be noted that the above formulas (4) and (5) can be reliably applied in the range of 1.1 ° ≦ θ ≦ 21.1 ° where the loading test results are present. Further, it can be said that the present invention is applicable if the inclination angle θ is in the range of 45 ° or less.

  Next, the effect | action of the calculation method of the drawing-out resistance of the bottom expanded pile 1 of this Embodiment is demonstrated.

  In the calculation method of the pulling-out resistance force of the bottom expanded pile 1 of this Embodiment comprised in this way, in the range of the inclined surface of the inclined part 31 of the enlarged diameter part 3, the inclined surface of the inclined part 31 is at least for every soil quality. Calculate the area of the contact side. Here, “at least for every soil” means that the soil can be divided even within the same range. It is only necessary that the calculated side area is divided at least at the transition (boundary) of the soil.

Furthermore, the frictional resistance coefficient (λ or μ) varies depending on the strength of the soil (N _s or q _u ), the inclination angle θ of the inclined portion 31 of the enlarged diameter portion 3 and the soil (sandy ground or clayey ground). ).

  And based on the integrated value of the calculation result of the enlarged diameter part 3 calculated in this way for every soil and the calculation result of the axial part 2, and the effective dead weight S of the expanded pile 1, the pulling resistance force of the expanded pile 1 is obtained. Calculate.

  Thus, since drawing resistance can be calculated clearly for every enlarged diameter part 3, even if the enlarged diameter part 3 is one bottom expansion pile 1, extraction resistance can be calculated correctly.

  Further, even when the soil quality changes in the middle of the expanded diameter portion 3, the pulling resistance can be calculated for each soil, so that the pulling resistance force of the expanded pile 1 can be accurately calculated. Of course, the present invention can also be applied to the case where there is only one kind of soil that the enlarged diameter portion 3 contacts.

  Furthermore, if the frictional resistance coefficient is a coefficient that increases as the inclination angle θ increases, the fact that the inclination angle θ is increased can be accurately reflected, so that the pile length L can be shortened and the pile is constructed economically. Will be able to.

  That is, even if it becomes possible to construct the enlarged diameter part 3 having a large inclination angle θ, if the increase in the drawing resistance is not recognized in the design, the pile length is increased or the number of piles is increased. For example, the resistance to pulling out must be increased.

  On the other hand, if the increment of the inclination angle θ is properly evaluated, it is not necessary to unnecessarily lengthen the pile length or increase the number of piles, and the pile can be constructed economically.

  Further, when the diameter of the shaft portion 2 is reduced and the inclination angle θ is increased, the amount of excavation may be reduced even with the same pulling resistance force. If the amount of excavation can be reduced in this way, not only the excavation cost can be reduced, but also the amount of concrete pouring and the amount of generated industrial waste soil can be suppressed.

  In particular, when the soil is sandy ground, the ratio of the exponential increase of the frictional resistance coefficient λ to the increase of the inclination angle θ is large, and it becomes possible to design and construct a more economical pile. .

  In addition, when the soil strength used to calculate the pullout resistance of the pile is derived from the N value in sandy ground and from the uniaxial compressive strength in clay soil, numerical values related to the strength of each soil are easily obtained. can do.

  Hereinafter, the calculation method of the pulling-out resistance force of the multistage pile 4 of the form different from the expanded bottom pile 1 of above-described embodiment is demonstrated, referring FIG. Note that the description of the same or equivalent parts as those described in the above embodiment will be described with the same terms and the same reference numerals.

  The multi-stage pile 4 as a pile described in this embodiment is provided with a plurality of enlarged diameter portions 42 and 43 at intervals in the axial direction. That is, the multi-stage pile 4 includes a cylindrical shaft portion 41A having a constant diameter, a first-stage enlarged diameter portion 42 that is obliquely enlarged downward from the axial portion 41A, and a downward direction from the enlarged diameter portion 42. It is mainly comprised by the column-shaped axial part 41B extended, and the diameter expansion part 43 of the lowest step expanded diagonally toward the downward direction from the axial part 41B.

  In the present embodiment, the multi-stage pile 4 having two enlarged diameter portions 42 and 43 will be described. However, the present invention is not limited to this, and the number of steps of the enlarged diameter portion is set according to the necessary pulling force and supporting force. be able to.

  As described in the above embodiment, a force (earthquake force Q) due to the earthquake acts from the left side of the building B, for example, at the time of the earthquake. Further, when a tsunami occurs, as shown in FIG. 6, there is a possibility of receiving a large horizontal force H or a buoyancy W due to an increase in the groundwater level G1.

  In order to economically counter such a large pulling force, it is possible to cope with this by providing a plurality of enlarged diameter portions 42 and 43. And if the increase in drawing-out resistance by increasing the diameter-expanded parts 42 and 43 can be calculated correctly, construction of a safe and economical pile can be realized.

  Also in the calculation method of the pulling-out resistance force of the multistage pile 4 of a present Example, the calculation similar to the calculation method demonstrated in the said embodiment is performed. In short, formulas (1) to (5) can be used in the same way.

  As a difference, in the expanded pile 1 of the above-described embodiment, it is only necessary to calculate the pulling resistance for only one expanded portion 3, but in the multi-stage pile 4 of the present embodiment, a plurality of expanded portions 42, The point which performs calculation of drawing resistance to 43 is mentioned.

  As described above, the drawing resistance can be clearly calculated for each of the enlarged diameter portions 42 and 43. Therefore, even if the multistage pile 4 is provided with a plurality of enlarged diameter portions 42 and 43, the extraction resistance force can be accurately calculated.

  Other configurations and functions and effects are substantially the same as those in the above-described embodiment, and thus description thereof is omitted.

  The embodiment of the present invention has been described in detail above with reference to the drawings. However, the specific configuration is not limited to the embodiment and the example, and the design change is within a range not departing from the gist of the present invention. Are included in the present invention.

  For example, in the said embodiment and Example, although the case where the lower surface of the enlarged diameter part 3,42,43 became a horizontal surface was demonstrated, it is not limited to this, The lower surface of an enlarged diameter part is a lower pile bottom Or the calculation method of this invention is applicable also to the pile provided with the inclined surface reduced (it narrows) toward an axial part. In short, since the inclined surface added to the pulling resistance force is only the inclined surface that is obliquely enlarged downward, it is not limited by the shape of the lower surface.

1 Expanded pile (pile)
2 Shaft part 3 Expanded diameter part 31 Inclined part (range of inclined surface)
4 Multi-stage pile (pile)
41A, 41B Shaft portion 42, 43 Expanded portion G Ground P Pull-out force θ Inclination angle λ, μ Friction resistance coefficient A _i side area S Effective weight

Claims (4)

A method for calculating a pulling resistance force of a pile having a diameter-expanded portion that is obliquely expanded downward from a shaft portion,
Calculate the side area where the shaft part and the enlarged diameter part are in contact with the ground,
Multiply the side area by the frictional resistance coefficient related to the strength of the surrounding soil and the frictional resistance with the soil,
When calculating the pulling resistance based on the integrated value of the calculation result of the shaft part and the enlarged diameter part and the effective dead weight of the pile,
In the range of the inclined surface that is obliquely enlarged toward the lower portion of the diameter-enlarged portion, the side area is calculated at least for each soil, and the frictional resistance coefficient varies depending on the inclination angle of the inclined surface and the soil quality. A method for calculating the pull-out resistance of a pile, characterized in that the above is used.   The calculation method of the pulling-out resistance force of the pile according to claim 1, wherein the frictional resistance coefficient used in the range of the inclined surface increases as the inclination angle increases.   The method according to claim 2, wherein the frictional resistance coefficient used in the range of the inclined surface increases exponentially.   The calculation method of the pulling-out resistance force of the pile according to any one of claims 1 to 3, wherein the soil strength is an N value in sandy ground and uniaxial compressive strength in clayey ground. .