JP2011174252A - Multi-stage diameter-enlarged pile and structure - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a multi-stage diameter-enlarged pile capable of acquiring a required vertical bearing capacity and also reducing construction cost, and to obtain a structure. <P>SOLUTION: The multi-stage diameter-enlarged pile 20 comprises a shaft 28 berried in the ground 12 containing a soft layer 22 and a support layer 24; and diameter-enlarged portions 30 (intermediate diameter-enlarged portions 32, 34, and a bottom-enlarged portion 36) formed at a plurality of locations along the axial direction of the shaft 28. The plurality of diameter-enlarged portions 30 are arranged in the respective supporting layers 24, and their diameters are determined so that the range of stress propagation is set within the respective supporting layers 24 according to thicknesses of the respective supporting layers 24. It is thereby possible to acquire the vertical bearing capacity necessary and sufficient for the multi-stage diameter-enlarged pile 20. Since the diameter-enlarged portions 30 larger than necessary are not constructed, it is possible to reduce the construction cost in comparison with a conventional multi-stage diameter-enlarged pile having the same outer diameters of diameter-enlarged portions. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a multistage enlarged diameter pile and a structure.

  As the structure becomes larger and taller, foundation piles are required to have high vertical support performance and pull-out resistance performance. To increase the support capacity of the pile, the piles have been expanded in multiple locations in the vertical direction. A multistage expanded pile is used (see, for example, Patent Document 1). The multistage expanded pile of patent document 1 mainly makes the outer diameter of the intermediate expanded part and the outer diameter of the expanded bottom part equal.

  However, in the site where the structure is constructed, when the support layer thickness for fixing the intermediate enlarged portion is a relatively thin ground structure with respect to the outer diameter of the intermediate enlarged portion, the bottom resistance of the intermediate enlarged portion The force is influenced not only by the support layer but also by the mechanical properties (strength, deformation rigidity, etc.) of the formation deposited directly under the support layer. For this reason, as in Patent Document 1, in a multistage expanded pile in which the outer diameter of the intermediate expanded portion and the outer diameter of the expanded bottom portion are equal, the mechanical properties of the formation deposited immediately below the support layer are inferior to those of the support layer. In some cases (for example, in the case of a soft clay layer), when the outer diameter of some intermediate enlarged portions becomes larger than necessary, the influence range of the ground that contributes to the bottom resistance force of the intermediate enlarged portion is a soft layer. In other words, the bottom resistance force of the intermediate enlarged portion may be reduced, and the required vertical support force may not be obtained. Moreover, since the outer diameter of the intermediate enlarged portion becomes larger than necessary, it is difficult to reduce the construction cost.

JP-A-4-265321

  An object of this invention is to obtain the multistage enlarged diameter pile and structure which can reduce a construction cost while obtaining required vertical supporting force.

  The multistage expanded pile according to claim 1 of the present invention includes a shaft portion embedded in the ground having a plurality of layers including a soft layer and a support layer harder than the soft layer, and a plurality of locations in the axial direction of the shaft portion. A multi-stage enlarged pile having a larger diameter than the outer diameter of the shaft portion, wherein the plurality of enlarged diameter portions are disposed on the support layer and have a thickness of the support layer. The outer diameter is determined accordingly.

  According to the above configuration, when a load acts on the pile head and the multistage enlarged pile is pushed vertically downward, the stress in the ground immediately below the enlarged portion increases, but the stress propagation range is in the support layer. Thus, since the outer diameter of the enlarged-diameter portion is determined, the vertical supporting force exhibited by the ground can be sufficiently extracted. Thereby, a necessary and sufficient vertical supporting force can be obtained as a multistage expanded pile. Furthermore, since the outer diameter of the enlarged diameter portion is determined according to the thickness of the support layer, it is not necessary to construct an enlarged diameter portion larger than necessary, and the outer diameter of each enlarged diameter portion is the same as in the past. The construction cost can be reduced compared to what is being done.

  In the multistage enlarged diameter pile according to claim 2 of the present invention, the outer diameter of the enlarged diameter portion is larger in the thicker portion than in the thinned support layer. According to this configuration, when the support layer is thin, the outer diameter of the enlarged diameter portion is reduced to obtain a small vertical support force, and when the support layer is thick, the outer diameter of the enlarged diameter portion is increased to increase the vertical support force. Get. Therefore, the vertical supporting force exhibited by the ground can be pulled out to the maximum with the minimum material cost of the pile, and the necessary and sufficient vertical supporting force can be obtained as a multistage enlarged diameter pile.

  The multi-stage enlarged pile according to claim 3 of the present invention is configured such that the enlarged portion includes an enlarged bottom portion formed at a lower end of the shaft portion and an intermediate enlarged portion formed in the middle of the shaft portion. And the outer diameter of the said expanded base part is calculated | required from the difference of the vertical support force which the said intermediate | middle expanded diameter part bears, and the vertical support force per one. According to this configuration, when the widened portion is provided in a sufficiently thick support layer, the outer diameter of the widened portion is not determined by the stress propagation depth of the support layer, so that the widened portion is not increased more than necessary. Thereby, cost reduction and construction period shortening are possible.

  In the multistage enlarged diameter pile according to claim 4 of the present invention, the outer diameter of the shaft portion is equal in the vertical direction. According to this configuration, the outer diameter is determined only in accordance with the layer configuration of the ground only in the enlarged diameter portion, and the outer diameter of the shaft portion is not changed.

  In the multistage expanded pile according to claim 5 of the present invention, at least the lower surface of the expanded portion is embedded in the support layer. According to this configuration, since only a portion necessary for support is embedded in the support layer, a vertical support force can be efficiently obtained as a multistage expanded pile.

  A structure according to a sixth aspect of the present invention includes the multistage diameter-expanded pile according to any one of the first to fifth aspects, and a frame constructed on the shaft portion. According to this structure, the vertical supporting force of a multistage enlarged diameter pile can be raised. Furthermore, since the outer diameter of the enlarged diameter portion is determined according to the thickness of the support layer, it is not necessary to construct an enlarged diameter portion larger than necessary, and the outer diameter of each enlarged diameter portion is the same as in the past. The construction cost can be reduced compared to what is being done.

  Since this invention was set as the said structure, it can obtain the multistage enlarged diameter pile and structure which can reduce construction cost while obtaining required vertical supporting force.

It is a block diagram of the whole building which concerns on embodiment of this invention. It is explanatory drawing which shows the construction state of the multistage enlarged diameter pile which concerns on embodiment of this invention. It is a schematic diagram of the intermediate diameter expansion part which concerns on embodiment of this invention. It is a schematic diagram (a top view and a sectional view) showing a stress propagation state of an intermediate diameter expansion part concerning an embodiment of the present invention. It is a schematic diagram for calculating the vertical supporting force of the intermediate diameter enlarged portion according to the embodiment of the present invention. It is a schematic diagram which shows the penetration state of the intermediate diameter expansion part which concerns on embodiment of this invention. (A) It is a schematic diagram of the multistage enlarged diameter pile of a prior art example. (B) It is a schematic diagram of the multistage enlarged diameter pile of this embodiment.

  Embodiments of the multistage enlarged pile and the structure of the present invention will be described with reference to the drawings. FIG. 1 shows a building 10 as an example of a structure. The building 10 includes a plurality of multi-stage enlarged piles 20 (20A, 20B, 20C, 20D) embedded in the ground 12 at intervals in the horizontal direction (arrow X direction), and a multi-stage enlarged pile 20 (on the ground 12). ) Constructed with a plurality of pillars 14 and beams 16 as an example of the housing constructed.

  As an example, the ground 12 has a configuration in which a plurality of soft layers 22 (22A, 22B, 22C) and a plurality of support layers 24 (24A, 24B, 24C) harder than the soft layers 22 are alternately stacked. Yes. Here, as a determination method of being the support layer 24, if a layer having an N value of 50 or more obtained by the standard penetration test method (JIS A 1219) can be confirmed by 5.0 m or more, it is defined as a support layer ( Used by the Architectural Institute of Japan). Since the multistage enlarged piles 20A, 20B, 20C, and 20D have the same configuration, the multistage enlarged pile 20A will be described below, and the explanation of the multistage enlarged piles 20B, 20C, and 20D is omitted.

  The multi-stage diameter-expanded pile 20A includes a columnar shaft portion 28 whose axial direction is the vertical direction (arrow Z direction), and a diameter-expanded portion 30 that is expanded radially outward from the outer peripheral surface of the shaft portion 28. ing. The shaft portion 28 includes a shaft portion 28A formed on the soft layer 22A, a shaft portion 28B formed on the support layer 24A and the soft layer 22B, and a shaft portion 28C formed on the support layer 24B and the soft layer 22C. In this embodiment, the outer diameter of the shaft portion 28A, the outer diameter of the shaft portion 28B, and the outer diameter of the shaft portion 28C are the same. Note that the same outer diameter of the shaft portions 28A, 28B, and 28C means that they are the same in design, and an error from the design center value caused by construction on the outer diameter of the shaft portions 28A, 28B, and 28C. Even if there is a difference, it is regarded as the same thing.

  On the other hand, the enlarged diameter portion 30 includes an intermediate enlarged portion 32 formed on the upper portion of the support layer 24A and an intermediate enlarged portion formed on the upper portion of the support layer 24B and having an outermost outer diameter larger than the outermost diameter of the intermediate enlarged portion 32. The diameter portion 34 and an expanded bottom portion 36 formed on the support layer 24 </ b> C and having an outermost outer diameter smaller than the outermost diameter of the intermediate expanded portion 32. In addition, the determination method of the outermost diameter of the intermediate | middle enlarged diameter part 32, the intermediate | middle enlarged diameter part 34, and the expanded bottom part 36 is mentioned later. Moreover, the number of each enlarged diameter part 30 is not limited to this embodiment, and is suitably selected by two or more.

  Next, the excavator 40 for excavating the multistage enlarged pile 20 will be described.

  As shown in FIG. 2, an excavator 40 is used as an example for the construction of the multistage enlarged diameter pile 20 (see FIG. 1). The excavator 40 includes a crane 42, a turning device 44, and a positioning arm 46. The crane 42 previously suspends the kelly bar 48 in a pile hole 52 formed by excavating the ground 12 downward (arrow DOWN direction) using another excavating means. It is designed to move up and down in the direction.

  Further, a turning device 44 is attached to the tip of the positioning arm 46 protruding from the crane 42. The swivel device 44 is adapted to swivel the kelly bar 48 in the direction of the arrow R, and the lower end portion 48A of the kelly bar 48 has an upper end portion of the enlarged diameter bucket 50 that expands the diameter in the radial direction and excavates the pile hole 52. Are connected by pins (not shown). A stabilizing liquid such as bentonite is injected into the pile hole 52 from a supply pipe (not shown) to prevent the hole wall from collapsing.

  The diameter-enlarged bucket 50 has a central axis 54 that is disposed at the center of the pile hole 52 and serves as a rotation center, and a link mechanism 56 is provided around the central axis 54. The link mechanism 56 has a plurality of arm members 58 that are expanded and contracted in the outer diameter direction of the pile hole 52 by an expansion / contraction operation of a hydraulic cylinder (not shown), and an end of the arm member 58 opposite to the central axis 54. The wing expansion part 60 is welded to the part.

  The expanded blade portion 60 has a flat cross section formed in an arc shape, and is arranged in four directions (every 90 °) about the central axis 54. Further, the wing expansion portion 60 is provided with a plurality of excavation bits (not shown). With such a configuration, when the turning device 44 turns the kelly bar 48, the diameter-enlarged bucket 50 turns integrally with the center shaft 54 and excavates the inner peripheral wall of the pile hole 52 according to the diameter-enlarged amount. It is like that. Note that a stabilizer 62 that contacts the inner wall of the pile hole 52 is provided on the upper portion of the diameter-enlarged bucket 50 so that the centers of the kelly bar 48 and the central shaft 54 are not greatly displaced from the center of the pile hole 52.

  Next, the construction procedure of the multistage expanded pile 20 will be described.

  In the ground 12 of FIG. 2, the shaft portion 28 is excavated in advance in the vertical direction using another excavating means. As described above, the shaft portion 28 is replenished with a stabilizing liquid L such as bentonite to prevent the hole wall from collapsing. Then, the crane 42 lowers the kelly bar 48 in the arrow down direction, and lowers the diameter-expanded bucket 50 in the reduced diameter state to the bottom of the shaft portion 28.

  Subsequently, the turning device 44 is driven, and the kelly bar 48 turns in the direction of arrow R. Here, by operating a hydraulic cylinder (not shown), the arm member 58 of the link mechanism 56 moves in the horizontal direction, and the diameter of the blade expanding portion 60 is increased. The expanded wing portion 60 expands while turning, and the inner wall of the shaft portion 28 is excavated by an excavation bit (not shown) to form the expanded diameter portion 30. In this way, one enlarged diameter portion 30 is formed.

  When constructing the multistage expanded pile 20, the shaft portion 28 is excavated by other excavating means to a preset deepest portion, and then expanded while rotating the blade expansion portion 60 at the set portion of the expanded portion 30. By digging and excavating, once reducing the diameter of the blade expansion section 60 and moving to the location where the next diameter expansion section 30 is installed, a plurality of diameter expansion sections 30 (intermediate diameter expansion sections 32, 34 and the expanded bottom portion 36). And after excavation of each enlarged diameter part 30, the diameter-expanded bucket 50 reduced in diameter is pulled up, a reinforcing steel cage (not shown) is disposed inside the pile hole 52, and piles are passed through a tremy pipe (not shown). Concrete is placed in the hole 52. Thereby, the multistage enlarged diameter pile 20 is completed.

  Next, a method for determining the outer diameter of the enlarged diameter portion 30 will be described. Here, first, the propagation range of stress in the ground 12 will be described.

  FIG. 3 is a schematic diagram showing the state of stress propagation in the direct base plate (the ground 12 in FIG. 1) of the intermediate enlarged diameter portion 34. When the load R acts on the pile head of the multistage expanded pile 20 (the shaft portion 28A and the intermediate expanded section 32 are not shown) and the multistage expanded pile 20 is pushed vertically downward, The stress in the ground just below the enlarged diameter portion 34 increases. Therefore, assuming that the ground 12 is an elastic body, the propagation range of stress in the ground immediately below the intermediate diameter enlarged portion 34 is examined. In addition, since the outer diameter is calculated | required by the same procedure also in the intermediate | middle enlarged diameter part 32, description about the intermediate | middle enlarged diameter part 32 is abbreviate | omitted.

As shown in FIG. 3 (right side) is assumed to indentation load in the intermediate diameter portion 34 and the circular etc. distributed load p _1. Further, σ _{Z1 is} an increase in ground stress immediately below the intermediate enlarged portion 34, D _{1 is} an outer diameter of the intermediate enlarged portion 34, and d is an outer diameter of the shaft portion 28C. Here, when a contour line (isoline) is obtained for σ _Z1 / p ₁ , a stress propagation range (stress bulb) in the ground as shown in FIG. 4 is obtained.

In FIG. 4, the isolines of σ _Z1 / p ₁ = 0.1, 0.2, 0.3, 0.5, 0.7, 0.9 are shown as thick solid lines. The value of σ _Z1 / p ₁ shows that the smaller the influence of the indentation load in the intermediate enlarged portion 34 on the stress distribution in the ground, the smaller the value. For example, the rising portion lower about 1-fold or more distant ground range of the outer diameter _{D 1} of the intermediate diameter portion 34 from the (point E1, E2) to the vertically downward direction of the intermediate diameter portion 34, the sigma _Z1 / _{p 1} The value is smaller than 0.2 (20% of the circular uniform distributed load p ₁ ), and it can be seen that the influence of the propagation of stress in the ground due to the pressing of the intermediate enlarged portion 34 is small. That effect of propagation of the stress in the ground is small, which means that even when increasing the outer diameter D ₁ of the intermediate diameter portion 34 more than necessary is useless.

  Here, the thickness of the support layer fixing the intermediate enlarged portion is relatively smaller than the outer diameter of the intermediate enlarged portion, as in a conventional multistage enlarged pile 200 (see FIG. 7A) described later. In the case of a thin ground configuration, the bottom resistance force of the intermediate expanded portion is affected by not only the support layer but also the mechanical properties (strength, deformation rigidity, etc.) of the layer deposited immediately below the support layer. And when the mechanical properties of the strata deposited directly below the support layer are inferior to those of the support layer (for example, a soft clay layer), increasing the outer diameter of the intermediate enlarged portion will reduce the bottom resistance of the intermediate enlarged portion. Since the propagation range of the contributing stress in the ground extends to the soft layer, the bottom resistance force of the intermediate enlarged portion is reduced.

  Next, the propagation range S of the stress in the ground that contributes to the bottom surface resistance of the intermediate diameter enlarged portion 34 will be described.

FIG. 5 is a schematic diagram of the intermediate enlarged diameter portion 34. Here, assuming that the support layer 24B is a gravel layer, the soft layer 22C is a viscous soil layer, and the intermediate expanded portion 34 is fixed to the gravel layer, the intermediate expanded portion 34 that also considers the viscous soil layer immediately below the gravel layer. Estimate the vertical bearing capacity. When the outer diameter of the intermediate enlarged portion 34 is D1, the outer diameter of the shaft portion 28C is d, the thickness of the support layer 24B is H, and the amount of expansion of the half outer diameter to the soft layer 22 is 2 / H, the soft layer 22C. The dispersion area Ac of the stress in the ground at the upper end is expressed by equation (1). In the formula (1), when D ₁ = 4.0 m, d = 1.8 m, and H = 3.0 m as an example, the dispersion area Ac = 35.9 m ² .

軟弱層２２Ｃ上端における地盤１２の鉛直応力増分△σ_zは、中間拡径部３４の極限底面抵抗力度をｑ_ｍとして、（２）式で表される。ここで、支持層２４Ｂの厚さが十分に厚い場合の多段拡径杭２０先端部における極限先端支持力度に相当する値を７５００ｋＮ／ｍ^２と仮定すると、（１）式の結果と（２）式とから、軟弱層２２Ｃ上端における地盤１２の鉛直応力増分△σ_z＝２０８９ｋＮ／ｍ^２と求められる。

The vertical stress increase Δσ _z of the ground 12 at the upper end of the soft layer 22C is expressed by the equation (2), where q _m is the ultimate bottom surface resistance force degree of the intermediate expanded portion 34. Assuming that the value corresponding to the ultimate tip support force at the tip of the multi-stage expanded pile 20 when the thickness of the support layer 24B is sufficiently thick is 7500 kN / m ² , the result of the equation (1) and (2) From the equation, the vertical stress increment Δσ _z of the ground 12 at the upper end of the soft layer 22C is determined to be 2089 kN / m ² .

一方、多段拡径杭２０の形状係数をα、β、支持力係数をＮ_ｃ、Ｎ_γ、Ｎ_ｑ、地盤１２の粘着力をｃ、多段拡径杭２０の幅をＢ（＝Ｄ_１）、根入れ深さをＤ_ｆ、地盤１２の水中単位体積重量をγ_１、根入れ部分の土の水中単位体積重量をγ_２とすると、軟弱層２２Ｃ上端における極限鉛直支持力度ｑ_ｕは（３）式で表せる（建築基礎構造設計指針 日本建築学会 ｐｐ．１１６−１１７、２００１）。

On the other hand, the shape factor of the multi-stage expanded pile 20 is α, β, the bearing capacity coefficient is N _c , N _γ , N _q , the adhesive strength of the ground 12 is c, and the width of the multi-stage expanded pile 20 is B (= D ₁ ). When the depth of root penetration is D _f , the unit volume weight of the ground 12 in water is γ ₁ , and the unit volume weight of soil in the root portion is γ ₂ , the ultimate vertical bearing strength q _{u at the} upper end of the soft layer 22C is (3 ) Expression (Guidelines for Architectural Foundation Structural Design, Architectural Institute of Japan pp. 116-117, 2001)

多段拡径杭２０が円形であるとすると、形状係数α＝１．２、β＝０．３となる。また、せん断抵抗角φ＝０°の場合、支持力係数Ｎ_ｃ＝５．１４、Ｎ_γ＝０．０、Ｎ_ｑ＝１．０となる。ここで、地盤１２の粘着力ｃ＝２００ｋＮ／ｍ^２、多段拡径杭２０の幅Ｂ＝Ｄ_１＝４．０ｍ、根入れ深さＤ_ｆ＝１３．０ｍ（図５より）、根入れ部分の土の水中単位体積重量をγ_２＝８ｋＮ／ｍ^３と仮定すると、（３）式により、極限鉛直支持力度ｑ_ｕ＝１３３８ｋＮ／ｍ^２と求められる。

If the multistage expanded pile 20 is circular, the shape factors α = 1.2 and β = 0.3. Further, when the shear resistance angle φ = 0 °, the bearing force coefficient N _c = 5.14, N _γ = 0.0, and N _q = 1.0. Here, the adhesive strength c of the ground 12 is c = 200 kN / m ² , the width B of the multistage expanded pile 20 is B = D ₁ = 4.0 m, the depth of root penetration D _f = 13.0 m (from FIG. 5), the root portion Assuming that the unit volume weight of the soil in water is γ ₂ = 8 kN / m ³ , the ultimate vertical bearing strength q _u = 1338 kN / m ² is obtained from the equation (3).

From the calculation results using the formulas (1) to (3), Δσ _z <q _u , and Δσ _z is obtained from the limit bottom surface resistance force q _{m of} the intermediate enlarged portion 34. It can be seen that the ultimate bottom resistance depends on the ultimate vertical support force of the soft layer 22C. The area _{A r} of the intermediate diameter portion 34 is determined by equation (4), ultimate bottom resistance of _{q m} of the intermediate diameter portion 34 has an intrinsic vertical bearing capacity of qu, dispersion area Ac, and the intermediate diameter portion 34 The area _Ar is obtained by the equation (5).

ここで、上記演算により得られた極限鉛直支持力度ｑ_ｕ＝１３３８ｋＮ／ｍ^２、（４）式、及び（５）式を用いて、中間拡径部３４の極限底面抵抗力度ｑ_ｍを逆算すると、ｑ_ｍ＝４８００ｋＮ／ｍ^２となり、支持層２４Ｂの厚さが十分に厚い場合の中間拡径部３４の極限底面抵抗力度ｑ_ｍ＝７５００ｋＮ／ｍ^２（仮定した値）と比較して、小さい値となっていることが分かる。これらの演算結果から、中間拡径部３４を定着する支持層２４Ｂの厚さが中間拡径部３４の外径に対して比較的薄い地盤構成の場合、中間拡径部３４の底面抵抗力は、支持層２４Ｂだけではなく支持層２４Ｂの直下に堆積する軟弱層２２Ｃの力学特性の影響を受け、中間拡径部３４の外径を大きくすると、中間拡径部３４の底面抵抗力が低下してしまうことが確認できた。

Here, using the ultimate vertical support force q _u = 1338 kN / m ² obtained by the above calculation, the equations (4), and (5), the ultimate bottom surface resistance force q _m of the intermediate expanded portion 34 is calculated backward. Q _m = 4800 kN / m ² , which is smaller than the limit bottom surface resistance force q _m = 7500 kN / m ² (assumed value) of the intermediate expanded portion 34 when the thickness of the support layer 24B is sufficiently thick It turns out that it is a value. From these calculation results, when the thickness of the support layer 24B fixing the intermediate enlarged portion 34 is a ground structure relatively thin with respect to the outer diameter of the intermediate enlarged portion 34, the bottom resistance force of the intermediate enlarged portion 34 is When the outer diameter of the intermediate enlarged portion 34 is increased due to the influence of the mechanical characteristics of the soft layer 22C deposited not only on the support layer 24B but also directly below the support layer 24B, the bottom resistance of the intermediate enlarged portion 34 decreases. I was able to confirm.

Based on the above examination results, as shown in FIG. 6, in the multistage enlarged pile 20 of the present embodiment, the stress propagation range S in the ground 12 immediately below the intermediate enlarged portion 34 (σ _Z1 / p _{1 in} FIG. 4). In the support layer 24B, the outer diameter of the intermediate enlarged portion 34 is D ₁ , the depth of penetration of the intermediate enlarged portion 34 into the support layer 24B is t, and the intermediate enlarged portion 34 is fixed. the thickness of the support layer 24B as H _1, and sets the installation depth and the outer diameter of the intermediate diameter portion 34 so as to satisfy the relationship _{D 1 ≦ (H 1 -t)} . And at least the lower surface 34A of the intermediate enlarged diameter portion 34 is embedded in the support layer 24B. The installation depth and the outer diameter are similarly set in the intermediate enlarged portion 32 and the bottom expanded portion 36, but the outer diameter of the bottom expanded portion 36 is the vertical support force borne by the intermediate enlarged portions 32 and 34, and It is calculated | required from the difference with the vertical supporting force required per one multistage expansion pile 20.

  Next, the operation of the embodiment of the present invention will be described.

FIG. 7A schematically shows a conventional multi-stage enlarged pile 200 as a comparison object with the present embodiment. The multi-stage expanded pile 200 includes a columnar shaft portion 202 having an axial direction in the illustrated vertical direction, and a diameter expanded portion 210 that is expanded radially outward from the outer peripheral surface of the shaft portion 202. The shaft portion 202 includes a shaft portion 202A formed on the soft layer 22A, a shaft portion 202B formed on the support layer 24A and the soft layer 22B, and a shaft portion 202C formed on the support layer 24B and the soft layer 22C. The outer diameter of the shaft portion 202A, the outer diameter of the shaft portion 202B, and the outer diameter of the shaft portion 202C are the same size (outer diameter d). Further, the enlarged diameter portion 210 includes an intermediate enlarged diameter portion 204 formed on the support layer 24A, an intermediate enlarged diameter portion 206 formed on the support layer 24B, and an expanded bottom formed on the support layer 24C. Part 208. If the outer diameter of the intermediate enlarged portion 204 is D ₄ , the outer diameter of the intermediate enlarged portion 206 is D ₅ , and the outer diameter of the expanded bottom portion 208 is D ₆ , D ₄ = D ₅ = D ₆ . .

On the other hand, FIG.7 (b) has shown typically the multistage enlarged diameter pile 20 of this embodiment. As described above, the multi-stage expanded-diameter pile 20 is expanded from the outer peripheral surface of the cylindrical portion 28 (28A, 28B, 28C) whose axial direction is the illustrated vertical direction to the outside in the radial direction. The enlarged diameter portion 30 (intermediate enlarged diameter portions 32 and 34 and the expanded bottom portion 36). Each shaft portion 28 has the same outer diameter (outer diameter d). If the outer diameter of the intermediate enlarged portion 32 is D ₁ , the outer diameter of the intermediate enlarged portion 34 is D ₂ , and the outer diameter of the expanded bottom portion 36 is D ₃ , D ₁ <D ₂ and D ₁ > D ₃ It has become.

Here, Table 1 shows an example of a result of a trial calculation in order to compare the multi-stage expanded pile 200 of the conventional example and the multi-stage expanded pile 20 of the present embodiment. Table 1 shows the outer diameter D of each enlarged portion, the outer diameter d of each shaft portion, the layer thickness H of each support layer, the depth of penetration D _f , the unit volume weight γ of soil, the adhesive force c of soil, Each parameter of the area A _r and the dispersion area Ac of each enlarged diameter portion, and the ultimate vertical support obtained by substituting these parameters into the equations (1), (3), (4), and (5) The strength q _u and the ultimate bottom resistance strength q _m are shown. Incidentally, ultimate vertical bearing force Q _m is determined by the product of the area A _r and ultimate vertical bearing capacity of q _m of way, the enlarged diameter portion shown in (6). Moreover, in each support layer, the penetration depth (equivalent to t of FIG. 6) of the enlarged diameter part to the support layer is set to 0.5 m.

表１より、本実施形態の多段拡径杭２０（図７（ｂ）参照）は、従来例の多段拡径杭２００（図７（ａ）参照）に比べて、極限底面抵抗力度ｑ_ｍが等しいか大きくなっており、さらに、全体の体積ΣＶが６５％まで縮減されていることが分かる。ここで、多段拡径杭全体の鉛直支持力ΣＱｍを全体の体積ΣＶで除した支持力効率ΣＱｍ／ΣＶ（多段拡径杭の単位体積当たりの支持力）を求めると、表２に示すように本実施形態の多段拡径杭２０が従来例の多段拡径杭２００に比べて、支持力効率が２５％向上していることがわかる。

From Table 1, the multi-stage diameter pile 20 of the present embodiment (see FIG. 7 (b)), the prior art multi-stage expanded piles 200 as compared to (see FIG. 7 (a)), the ultimate bottom resistance index _{q m} It can be seen that the total volume ΣV is reduced to 65%. Here, when the bearing capacity efficiency ΣQm / ΣV (bearing capacity per unit volume of the multi-stage expanded pile) obtained by dividing the vertical bearing capacity ΣQm of the entire multi-stage expanded pile by the entire volume ΣV is shown in Table 2. It can be seen that the multi-stage enlarged pile 20 of the present embodiment is improved in supporting force efficiency by 25% compared to the conventional multi-stage enlarged pile 200.

図７（ａ）、（ｂ）に示すように、本実施形態の多段拡径杭２０の地盤内応力の伝播範囲Ｓを上から順にＳ１、Ｓ２、Ｓ３とし、従来例の多段拡径杭２００の地盤内応力の伝播範囲Ｓを上から順にＳ４、Ｓ５、Ｓ６とする。ここで、本実施形態の多段拡径杭２０の支持力効率が、従来例の多段拡径杭２００の支持力効率に比べて大きくなるのは、従来例の多段拡径杭２００における伝播範囲Ｓ４が、支持層２４Ａの領域を超えて軟弱層２２Ｂまで広がっているのに対し、本実施形態の多段拡径杭２０における伝播範囲Ｓ１、Ｓ２、Ｓ３がいずれも支持層２４Ａ、２４Ｂ、２４Ｃ内となっているためと考えられる。

As shown in FIGS. 7A and 7B, the propagation range S of the stress in the ground of the multistage enlarged pile 20 of this embodiment is set to S1, S2, S3 in order from the top, and the multistage enlarged pile 200 of the conventional example is used. The ground propagation stress propagation range S is S4, S5, and S6 in order from the top. Here, the supporting force efficiency of the multi-stage expanded pile 20 of the present embodiment is larger than the supporting capacity efficiency of the conventional multi-stage expanded pile 200 in the propagation range S4 in the conventional multi-stage expanded pile 200. However, the propagation ranges S1, S2, and S3 in the multistage expanded pile 20 of the present embodiment are all within the support layers 24A, 24B, and 24C, while extending beyond the region of the support layer 24A to the soft layer 22B. It is thought to be because.

As described above, according to the multistage expanded pile 20 of the present embodiment, the propagation range S of the stress in the ground when a load is applied to the pile head and pushed vertically downward is set to each support layer 24 ( 24A, 24B, 24C), the outer diameter of each of the enlarged diameter portions 30 (intermediate enlarged diameter portions 32, 34, and expanded bottom portion 36) is determined, that is, the thin portion of each support layer 24. Since the outer diameter of the enlarged diameter portion 30 is reduced to obtain a small vertical supporting force, and the thicker support layer 24 is used to increase the outer diameter of the enlarged diameter portion 30 to obtain a larger vertical supporting force. There can be pulled out of the vertical support force Q _m to exert enough. This makes it possible to obtain require sufficient vertical supporting force Q _m as a multi-stage diameter piles 20.

  In addition, the multistage enlarged pile 20 of the present embodiment is configured so that the enlarged diameter portions 30 (intermediate enlarged diameter portions 32 and 34 and the expanded bottom portion 36) have thicknesses corresponding to the thicknesses of the support layers 24 (24A, 24B, and 24C). Since the outer diameter is determined, it is no longer necessary to construct an enlarged portion larger than necessary, and the pile volume is reduced compared to the conventional example in which the outer diameter of each enlarged portion is the same. . As a result, the construction cost of the multistage expanded pile can be reduced, the construction period can be shortened, and construction byproducts (such as sludge generated during pile construction) can be reduced, thereby reducing the environmental load.

  Further, since the outer diameter of the bottom expanded portion 36 is obtained from the difference between the vertical supporting force borne by the intermediate expanded portions 32 and 34 and the vertical supporting force per one multi-stage expanded pile 20, the bottom expanded portion 36 is sufficiently thick. When the support layer 24C is provided, the outer diameter of the bottom expanded portion 36 is not determined by the stress propagation depth of the support layer 24C, so that the bottom expanded portion 36 is not made larger than necessary. Thereby, cost reduction and construction period shortening are possible.

  Moreover, the multi-stage expanded pile 20 has the same outer diameter of the shaft portion 28 (28A, 28B, 28C) in the vertical direction, and only the expanded portion 30 determines the outer diameter according to the layer configuration of the ground 12, Since the outer diameter of the shaft portion 28 is not changed, the construction becomes easy. Furthermore, since the multistage expanded pile 20 has rooted in the support layer 24 only in the part required for support (at least the lower surface of the expanded section 30), the multistage expanded pile 20 can efficiently obtain the vertical support force as the multistage expanded pile 20. Can do.

  Moreover, as the building 10 not only increases the vertical support force of the multistage expanded pile 20 but also the outer diameter of the expanded portion 30 is determined according to the thickness of the support layer 24, the expansion is larger than necessary. The construction of the diameter portion 30 is eliminated, and the construction cost is reduced as compared with the case where the outer diameters of the respective enlarged diameter portions 210 are the same as in the conventional multi-stage enlarged pile 200 (see FIG. 7A). Can be reduced.

  In addition, this invention is not limited to said embodiment.

  The number of the multistage expanded piles 20 may be not only four but also a plurality of two or more. Further, the number of intermediate enlarged portions may be not only two places described in the embodiment but also a plurality of places of three or more places. Furthermore, the number of layers of the support layer 24 and the soft layer 22 may be two or more.

  Depending on the thickness of the support layer 24, the outer diameter of the intermediate enlarged pile 32 may be larger than the outer diameter of the intermediate enlarged pile 34.

10 Building (structure)
14 Pillar (frame)
16 Beam
20 Multi-stage expanded pile 28 Shaft 30 Expanded section 32 Intermediate expanded section (expanded section)
34 Middle diameter expansion part (expansion part)
34A The lower surface 36 of the enlarged diameter part 36 The enlarged bottom part (expanded part)

Claims (6)

A shaft portion embedded in the ground having a plurality of layers including a soft layer and a support layer harder than the soft layer, and formed at a plurality of locations in the axial direction of the shaft portion and having a diameter larger than the outer diameter of the shaft portion A multistage expanded pile having an enlarged diameter portion,
A plurality of the enlarged diameter piles are arranged in the support layer and have an outer diameter determined according to the thickness of the support layer.   The outer diameter of the said enlarged diameter part is a multistage enlarged diameter pile of Claim 1 with the larger one where the thickness of the said support layer is thicker than the place where the thickness is thin. The expanded diameter portion is composed of an expanded bottom portion formed at the lower end of the shaft portion, and an intermediate expanded diameter portion formed in the middle of the shaft portion,
2. The multistage expanded pile according to claim 1, wherein an outer diameter of the expanded bottom portion is obtained from a difference between a vertical supporting force borne by the intermediate expanded portion and a vertical supporting force per one piece.   The multi-stage diameter-expanded pile according to any one of claims 1 to 3, wherein an outer diameter of the shaft portion is equal in a vertical direction.   The multistage expanded pile according to any one of claims 1 to 4, wherein at least a lower surface of the expanded diameter portion is embedded in the support layer. The multistage expanded pile according to any one of claims 1 to 5,
A housing constructed on the shank,
A structure having

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