JP7447652B2 - Evaluation method of pull-out resistance - Google Patents

Evaluation method of pull-out resistance Download PDF

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JP7447652B2
JP7447652B2 JP2020071521A JP2020071521A JP7447652B2 JP 7447652 B2 JP7447652 B2 JP 7447652B2 JP 2020071521 A JP2020071521 A JP 2020071521A JP 2020071521 A JP2020071521 A JP 2020071521A JP 7447652 B2 JP7447652 B2 JP 7447652B2
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out resistance
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JP2021167539A (en
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直子 鈴木
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Obayashi Corp
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Description

本発明は、節付き杭の引抜き時に負担する節部の引抜き抵抗力を評価するための、引抜き抵抗力の評価方法に関する。 TECHNICAL FIELD The present invention relates to a method for evaluating a pull-out resistance force for evaluating the pull-out resistance force of a knot portion that is borne when a knotted pile is pulled out.

従来より、建物の大型化及び高層化に対応する基礎杭として、杭長の長大化を抑えつつ高い鉛直支持力を確保することの可能な節付き杭を採用する場合が多い。例えば特許文献1で示すように、節付き杭には、鉛直支持力を増大させるため、軸部の下端部に拡底部を有するとともに、軸部の中間部に1つもしくは複数の節部が設けられている。 BACKGROUND ART Conventionally, knotted piles have often been used as foundation piles to accommodate larger and higher-rise buildings, as they can ensure high vertical bearing capacity while suppressing increases in pile length. For example, as shown in Patent Document 1, a knotted pile has an enlarged bottom at the lower end of the shaft and one or more knots at the middle of the shaft in order to increase the vertical supporting force. It is being

特許4856903号公報Patent No. 4856903

節付き杭に設けられた節部は、鉛直支持力だけでなく引抜き抵抗力を負担可能であるが、その根入れ長によって負担できる引抜き抵抗力が異なる。具体的には、節付き杭に所定の引抜き力が作用されると、節部の根入れ長が十分確保されている場合、もしくは節部全体が硬質な中間層や支持層等に埋設されている場合には、節部が地盤中で引抜き力に抵抗し続け、長期にわたって引抜き抵抗力を負担する。 The knots provided in the knotted pile can bear not only the vertical support force but also the pull-out resistance force, but the pull-out resistance force that can be borne differs depending on the embedment length. Specifically, when a predetermined pulling force is applied to a knotted pile, if the knot has sufficient penetration length, or if the knot is entirely buried in a hard intermediate layer or support layer, etc. If the joint is in the ground, it continues to resist the pull-out force in the ground and bears the pull-out resistance force for a long period of time.

一方、節部の根入れ長が浅い場合には、節部が地盤中で引抜き力に抵抗するものの、地盤中に節部から地表面に達する破壊面が発生し、引抜き抵抗力を失う。このように、根入れ長の浅い節部は、地盤に破壊面が発生するまで引抜き抵抗力を有しているにもかかわらず、設計に反映させない場合が多い。 On the other hand, when the penetration length of the joint is shallow, although the joint resists the pull-out force in the ground, a fracture surface reaching the ground surface from the joint is generated in the ground, and the pull-out resistance is lost. In this way, knots with a shallow penetration length are often not reflected in the design, even though they have pull-out resistance until a fracture surface occurs in the ground.

このため、節付き杭に引抜き抵抗力を要求される場合には、その大小によらず節部を根入れ長が十分確保できる深度位置に設けるよう設計するなど、施工性および経済性の面で不利な設計計画となっていた。 For this reason, when knotted piles are required to have pull-out resistance, regardless of their size, the knots should be designed at a depth that ensures sufficient penetration length, in order to improve workability and economy. This resulted in an unfavorable design plan.

本発明は、かかる課題に鑑みなされたものであって、その主な目的は、場所打ちコンクリート造の節付き杭において、節部の引抜き抵抗力を適切に評価することの可能な、引抜き抵抗力の評価方法を提供する。 The present invention has been made in view of such problems, and its main purpose is to improve the pull-out resistance of the joints in cast-in-place concrete knotted piles. provide an evaluation method.

かかる目的を達成するため、本発明の引抜き抵抗力の評価方法は、軸部と、該軸部に設けた節部とを備える節付き杭の、引抜き時における前記節部の引抜き抵抗力を評価するための、引抜き抵抗力の評価方法であって、前記節付き杭に引抜き力が作用した際の、前記節部の周辺地盤における地中応力の影響範囲を推定する推定工程と、前記地中応力の影響範囲と前記節部の根入れ長とを比較し、前記地中応力の影響範囲が地表面に達するか否かを検証する検証工程と、前記地中応力の影響範囲が地表面に達する場合に、前記節部の引抜き抵抗力を、前記節部から地表面にかけて発生する破壊面と前記節付き杭の軸部との間の土塊の重量と、該土塊の重量と前記破壊面のせん断抵抗に基づいて算定する評価工程と、を備えることを特徴とする。
In order to achieve such an object, the method for evaluating the pullout resistance of the present invention includes evaluating the pullout resistance of the knots during pulling out of a knotted pile that includes a shaft portion and a knot portion provided on the shaft portion. A method for evaluating pull-out resistance for the purpose of A verification step of comparing the range of influence of stress and the penetration length of the joint to verify whether the range of influence of underground stress reaches the ground surface; In the case where the joint reaches the ground surface, the pull-out resistance force of the joint is determined by the weight of the soil clod between the fracture surface that occurs from the joint to the ground surface and the shaft of the knotted pile, the weight of the soil mass, and the mass of the fracture surface. An evaluation step of calculating based on shear resistance.

また、本発明の引抜き抵抗力の評価方法は、前記評価工程では、前記破壊面が円筒形状を想定して算出した引抜き抵抗力と、コーン状を推定した算出した引抜き抵抗力とを比較し、数値の小さい方を前記節部の引抜き抵抗力として採用することを特徴とする。
Further, in the evaluation method of the pullout resistance force of the present invention, in the evaluation step, a pullout resistance force calculated assuming that the fracture surface has a cylindrical shape and a pullout resistance force calculated assuming a cone shape are compared, It is characterized in that the smaller numerical value is adopted as the pull-out resistance force of the knot .

本発明の引抜き抵抗力の評価方法によれば、節部の根入れ長と地中応力の影響範囲に基づいて地中応力の影響範囲が地表面に達するか否かを検証し、検証した結果に基づいて節部の引抜き抵抗力を適切に評価することができる。これにより、従来では引抜き抵抗力を設計に考慮していなかった根入れ長の浅い節部について、鉛直支持力だけでなく引抜き抵抗力を併せて設計に反映することが可能となる。 According to the pull-out resistance evaluation method of the present invention, it is verified whether or not the range of influence of underground stress reaches the ground surface based on the penetration length of the knot and the range of influence of underground stress. The pull-out resistance of the knot can be appropriately evaluated based on the following. This makes it possible to reflect not only the vertical support force but also the pull-out resistance force in the design of joints with a shallow penetration length, where pull-out resistance force has not been considered in the design in the past.

また、節部を、節つき杭に作用する引抜き力に抵抗することを目的として設ける場合に、節部の根入れ長を、要求される引抜き抵抗力の大きさに応じて設定でき、安全性と経済性を兼ね備えた合理的な節付き杭の設計を行うことが可能となる。 In addition, when knots are provided for the purpose of resisting the pull-out force that acts on a knotted pile, the penetration length of the knot can be set according to the required pull-out resistance force, which improves safety. This makes it possible to design rational knotted piles that are both economical and economical.

本発明によれば、場所打ちコンクリート造の節付き杭に設ける節部の引抜き抵抗力を適切に評価することが可能となる。 According to the present invention, it is possible to appropriately evaluate the pull-out resistance of a joint provided in a cast-in-place concrete joint pile.

本発明の実施の形態における節付き杭の概略を示す図である。It is a figure showing an outline of a knotted pile in an embodiment of the present invention. 本発明の実施の形態における節付き杭に設けた節部の詳細を示す図である。It is a figure which shows the detail of the joint part provided in the knotted pile in embodiment of this invention. 本発明の実施の形態における節部の上向き円錐台部を表層に根入れした節付き杭を示す図である。It is a figure showing the knotted pile in which the upward truncated conical part of the knot is embedded in the surface layer according to an embodiment of the present invention. 本発明の実施の形態における節部の根入れ長と地中応力の影響範囲を示す図である。FIG. 3 is a diagram showing the penetration length of a joint and the influence range of underground stress in an embodiment of the present invention. 本発明の実施の形態における節部における引抜き抵抗力の評価方法の流れを示す図である。It is a figure showing the flow of the evaluation method of the pull-out resistance force in a joint part in an embodiment of the present invention. 本発明の実施の形態における模型杭の引抜き実験の様子を示す図である。It is a figure which shows the state of the pulling-out experiment of the model pile in embodiment of this invention. 本発明の実施の形態における模型杭の引抜き実験より得た杭頭変位と杭頭荷重の関係を示すグラフである。It is a graph which shows the relationship between the pile head displacement and the pile head load obtained from the pull-out experiment of the model pile in embodiment of this invention. 本発明の実施の形態における節付き杭の引抜き抵抗機構及び破壊形態をす図である。It is a figure which shows the pull-out resistance mechanism and destruction form of the knotted pile in embodiment of this invention. 本発明の実施の形態における(地中応力の影響長さLs/突出幅Dn)と拡径比Dcとの関係を示すグラフである。It is a graph which shows the relationship between (length Ls affected by underground stress/protrusion width Dn) and diameter expansion ratio Dc in an embodiment of the present invention. 本発明の実施の形態における地盤に破壊面が発生した際に節部の直上に形成される土塊を示す図である。It is a figure showing the clod of soil formed right above a node when a fracture surface occurs in the ground in an embodiment of the present invention.

本発明は、場所打ちコンクリート造の節付き杭について、節付き杭の引抜き時に節部が負担する引抜き抵抗力を評価するものである。以下に、図1~10を参照しつつ、節付き杭における引抜き抵抗力の評価方法の詳細を説明する。 The present invention evaluates the pull-out resistance force exerted by the knots when the knotted pile is pulled out, with respect to cast-in-place concrete knotted piles. The details of the method for evaluating the pull-out resistance of knotted piles will be explained below with reference to FIGS. 1 to 10.

図1(a)(b)で示すように、建物を支持する節付き杭1は、支持層G3に到達する杭長を有し、軸部2と、軸部2の下端に設けられる拡底部3と、軸部2の中間部に設けられる節部4とを備える。 As shown in FIGS. 1(a) and 1(b), the knotted pile 1 that supports the building has a pile length that reaches the support layer G3, and has a shaft portion 2 and an expanded bottom portion provided at the lower end of the shaft portion 2. 3, and a joint portion 4 provided at the intermediate portion of the shaft portion 2.

節部4は、図2で示すように、軸部2より径の大きい円筒部41と、円筒部41の上側に位置する上向き円錐台部42と、円筒部41の下側に位置する下向き円錐台部43とを組み合わせた形状を有している。 As shown in FIG. 2, the joint portion 4 includes a cylindrical portion 41 having a larger diameter than the shaft portion 2, an upward truncated conical portion 42 located above the cylindrical portion 41, and a downward conical portion located below the cylindrical portion 41. It has a shape that combines the base portion 43.

このような形状の節部4は、図1(a)(b)で示すように、少なくとも下向き円錐台部43が支持層G3中もしくは砂礫層等の中間層G2に根入れされており、節付き杭1に押込み力が作用された場合には、拡底部3と節部4とでこれに抵抗する。一方、節付き杭1に引抜き力が作用された場合には、節部4の上向き円錐台部42が地盤中でどのように埋設されているかにより、その引抜き抵抗機構が異なる。 As shown in FIGS. 1(a) and 1(b), the knot portion 4 having such a shape has at least the downward truncated conical portion 43 rooted in the supporting layer G3 or an intermediate layer G2 such as a gravel layer, and is a knot. When a pushing force is applied to the attached pile 1, the expanded bottom portion 3 and the knot portion 4 resist this force. On the other hand, when a pullout force is applied to the knotted pile 1, the pullout resistance mechanism differs depending on how the upward truncated conical portion 42 of the knotted portion 4 is buried in the ground.

例えば、図3で示すような、節部4の上向き円錐台部42が表層G1に設けられている状態において、節部4の根入れ長Hが十分確保されている場合、節部4近傍の地盤に破壊が生じることなく常時、節部4は地盤中で引抜き力に抵抗する。 For example, in a state where the upward truncated conical portion 42 of the knot 4 is provided on the surface layer G1 as shown in FIG. 3, if the penetration length H of the knot 4 is sufficiently secured, the The joint portion 4 always resists the pulling force in the ground without causing any damage to the ground.

節部4の根入れ長Hが十分確保された場合とは、図4(a)で示すように、節付き杭1に引抜き力が作用されて上向き円錐台部42が地盤を押圧した際に、地中応力の影響範囲Aが地盤内に収まる場合をいう。なお、地中応力の影響範囲Aとは、上向き円錐台部42が地盤を押圧した際の支圧効果が及ぶ範囲を指す。 When the penetration length H of the joint 4 is sufficiently secured, as shown in FIG. , refers to the case where the influence range A of underground stress falls within the ground. Note that the range A of influence of underground stress refers to the range in which the bearing effect is exerted when the upward truncated conical portion 42 presses the ground.

一方、節部4の根入れ長Hが十分でない、つまり図4(b)で示すような、地中応力の影響範囲Aが地盤内に収まらない場合には、節部4は地盤中で引抜き力に抵抗するものの、やがて図10(a)(b)で示すように、地盤に地表面に達する破壊面Fsが発生して、抵抗する力は失われていく。つまり、根入れ長Hが十分でない節部4であっても、地盤に破壊面Fsが発生するまでの期間は、引抜き抵抗力を負担する。 On the other hand, if the penetration length H of the knot 4 is not sufficient, that is, if the influence range A of the underground stress does not fit within the ground as shown in Fig. 4(b), the knot 4 will be pulled out in the ground. Although it resists the force, as shown in FIGS. 10(a) and 10(b), a fracture surface Fs reaching the ground surface is generated in the ground, and the resisting force is lost. In other words, even if the joint portion 4 does not have a sufficient penetration length H, it bears the pull-out resistance force until the fracture surface Fs occurs in the ground.

<模型杭を用いた引抜き実験>
上記の節付き杭1に引抜き力が作用した際の節部4の引抜き抵抗機構(地中応力の影響範囲が地表面に到達するか否か)と、根入れ長Hが浅い場合に、節部4の周辺地盤に発生する破壊形態(破壊面Fsの形状)を確認するべく、以下のような実験を行った。
<Extraction experiment using model pile>
The pull-out resistance mechanism of the knots 4 when a pull-out force is applied to the above-mentioned knotted pile 1 (whether or not the range of influence of underground stress reaches the ground surface) and the knots when the penetration length H is shallow. In order to confirm the fracture form (shape of fracture surface Fs) occurring in the surrounding ground of section 4, the following experiment was conducted.

実験は、図6(a)~(c)で示すように、半割の模型杭1’を使って遠心力模型実験を実施し、引抜き時の節部4’の抵抗機構を確認した。地盤材料には乾燥した豊浦砂を採用し、容器内で所定の相対密度の模型地盤G’を作成した。 As shown in FIGS. 6(a) to 6(c), a centrifugal force model experiment was conducted using a half-split model pile 1' to confirm the resistance mechanism of the joint 4' during extraction. Dry Toyoura sand was used as the ground material, and a model ground G' with a predetermined relative density was created in a container.

一方、模型杭1’の寸法は、図2を参照し、軸部径D0=48mm、節部径D=68mm、節部突出幅Dn=10mm、上部傾斜角θn=20°とした。この模型杭1’を、図6(a)~(c)で示すような3通り(Case1~Case3)の根入れ長Hで模型地盤G’に貫入した。 On the other hand, with reference to FIG. 2, the dimensions of the model pile 1' were as follows: shaft diameter D0 = 48 mm, node diameter D = 68 mm, node protrusion width Dn = 10 mm, and upper inclination angle θn = 20°. This model pile 1' was penetrated into the model ground G' at three penetration lengths H (Case 1 to Case 3) as shown in FIGS. 6(a) to 6(c).

Case1は、根入れ長H=20Dnに設定され、節部4の根入れ長Hが十分な場合に相当する(比較例)。一方、Case2及びCase3は、節部4の根入れ長Hが浅い場合に相当し、Case2は、根入れ長H=12Dnに設定され、Case3は、根入れ長H=6Dnに設定されている。なお、模型杭1’には、杭頭部に変位計DGを設置している。 Case 1 corresponds to a case where the penetration length H is set to 20Dn, and the penetration length H of the joint portion 4 is sufficient (comparative example). On the other hand, Case 2 and Case 3 correspond to the case where the penetration length H of the joint portion 4 is shallow; in Case 2, the penetration length H is set to 12Dn, and in Case 3, the penetration length H is set to 6Dn. In addition, a displacement meter DG is installed at the top of the model pile 1'.

このような模型杭1’を模型地盤G’に埋設したのち、模型杭1’に引抜き力を作用させたところ、変位計DGの計測結果から、図7で示すような、杭頭変位と杭頭荷重の関係を得た。また、図8(a)~(c)は、これらCase1~Case3について、模型杭1’の周辺地盤における変位を画像解析によりコンターで表したものである。 After burying such a model pile 1' in the model ground G', a pulling force was applied to the model pile 1', and from the measurement results of the displacement meter DG, the displacement of the pile head and the pile as shown in Fig. 7 were obtained. The relationship of head load was obtained. In addition, FIGS. 8(a) to 8(c) are contours representing displacements in the ground around the model pile 1' for these Cases 1 to 3 through image analysis.

図8(a)の節部4を十分に根入れしたCase1では、節部4近傍の模型地盤G’中に地表面に達するような破壊面は認められない。図7をみると、杭頭変位が増加するに伴って杭頭荷重も増加しており、常時節部4が引抜き力に抵抗している様子がわかる。 In Case 1 in which the knot 4 in FIG. 8(a) is fully embedded, no fracture surface that reaches the ground surface is observed in the model ground G' near the knot 4. Looking at FIG. 7, it can be seen that as the pile head displacement increases, the pile head load also increases, and the joint portion 4 constantly resists the pulling force.

図8(b)(c)の根入れ長Hが十分ではないCase2およびCase3ではいずれも、節部4近傍の模型地盤G’中に地表面まで到達する破壊面Fsが形成されている。図7をみると、Case1より根入れ長Hの浅いCase2では、杭頭変位が増加するに伴って杭頭荷重が徐々に低下し、節部4が引抜き力に抵抗する力を徐々に失われていく様子がわかる。 In both Cases 2 and 3 in which the penetration length H in FIGS. 8(b) and 8(c) is insufficient, a fracture surface Fs reaching the ground surface is formed in the model ground G' near the joint 4. Looking at Figure 7, in Case 2 where the penetration length H is shallower than in Case 1, the pile cap load gradually decreases as the pile cap displacement increases, and the joint 4 gradually loses its ability to resist the pulling force. I can see how it goes.

また、最も根入れ長Hの浅いCase3では、杭頭変位が小さい時点で杭頭荷重が低下し、その後横ばいとなっている。つまり、模型杭1’に引抜き力を作用させた後の早い時点で、節部4が引抜き力に抵抗する力を失っている様子がわかる。 Furthermore, in Case 3 where the penetration length H is the shallowest, the pile cap load decreases when the pile cap displacement is small, and then levels off. In other words, it can be seen that the knot 4 loses its ability to resist the pulling force at an early point after the pulling force is applied to the model pile 1'.

さらに、根入れ長Hが浅い場合の破壊形態を見ると、Case2では、節部4近傍の模型地盤G’中に、円筒状の破壊面Fsを生じている。そして、Case3では、節部4’近傍の模型地盤G’中に、コーン状の破壊面Fsが生じている様子が見てとれる。 Furthermore, looking at the failure form when the penetration length H is shallow, in Case 2, a cylindrical failure surface Fs is generated in the model ground G' near the joint 4. In Case 3, it can be seen that a cone-shaped fracture surface Fs is generated in the model ground G' near the node 4'.

上記の実験結果から、節付き杭1の引抜き時における節部Aの引抜き抵抗機構(を検証し、この検証結果に基づいて選択した最適な評価式を用いて、節部4の引抜き抵抗力を評価することとした。 Based on the above experimental results, we verified the pull-out resistance mechanism of the knot A when the knotted pile 1 is pulled out, and calculated the pull-out resistance of the knot 4 using the optimal evaluation formula selected based on the verification results. I decided to evaluate it.

≪≪引抜き抵抗力の評価方法≫≫
以下に、節付き杭1に設けた節部4による引抜き抵抗力を評価する手順を、図5のフローを参照しつつ説明する。
≪≪Evaluation method of pull-out resistance force≫≫
Below, the procedure for evaluating the pull-out resistance force due to the knots 4 provided on the knotted pile 1 will be explained with reference to the flowchart of FIG. 5.

≪節付き杭の寸法確認:STEP1≫
まず、節付き杭1の寸法を確認する。具体的には、図2で示すように、軸部径D0、節部径D、軸部2に対する節部4の突出幅Dn、節上部角度θn、及び節部4の根入れ長Hを確認する。
≪Check dimensions of knotted pile: STEP 1≫
First, check the dimensions of the knotted pile 1. Specifically, as shown in FIG. 2, check the shaft diameter D0, knot diameter D, protrusion width Dn of the knot 4 with respect to the shaft 2, knot upper angle θn, and penetration length H of the knot 4. do.

ここで、突出幅Dnは、節部径Dと軸部径D0との差の1/2であり、節上部角度θnは、節付き杭1を縦方向に半割した際の上向き円錐台部42の傾斜面と軸部2の外周面とにより形成される内角である。また、節部径Dを軸部径D0で除することにより算定される拡径比Dcを算定しておくと良い。 Here, the protrusion width Dn is 1/2 of the difference between the knot diameter D and the shaft diameter D0, and the knot upper angle θn is the upward truncated conical part when the knotted pile 1 is vertically halved. 42 and the outer circumferential surface of the shaft portion 2. Further, it is preferable to calculate the diameter expansion ratio Dc, which is calculated by dividing the node diameter D by the shaft diameter D0.

≪地中応力の影響範囲の推定及び検証:STEP2≫
次に、節付き杭1に引抜き力が作用した際の節部4の引抜き抵抗機構を推定するべく、節部4の周辺地盤における地中応力の影響範囲Aと節部4の根入れ長Hとを比較し、地中応力の影響範囲Aが地盤中に収まるか否かを検証する。
≪Estimating and verifying the range of influence of underground stress: STEP 2≫
Next, in order to estimate the pull-out resistance mechanism of the knot 4 when a pull-out force is applied to the knotted pile 1, we will examine the influence range A of underground stress in the surrounding ground of the knot 4 and the embedment length H of the knot 4. , and verify whether the influence range A of underground stress is within the ground.

<地中応力の影響長さLsの推定方法>
比較するにあたって本実施の形態では、地中応力の影響長さLsを採用する。地中応力の影響長さLsは、図4(a)(b)で示すように、地中応力の影響範囲Aにおける鉛直方向の最長部に相当する。この地中応力の影響長さLsについて、発明者は以下の解析に基づいて、(地中応力の影響長さLs/突出幅Dn)と拡径比Dcとの間に、図9(a)で示すような関係があることを見出した。
<Method for estimating the length Ls affected by underground stress>
In this embodiment, the length Ls affected by underground stress is used for comparison. The underground stress influence length Ls corresponds to the longest part in the vertical direction in the underground stress influence range A, as shown in FIGS. 4(a) and 4(b). Regarding the length Ls affected by underground stress, the inventor calculated the difference between (length Ls affected by underground stress/protrusion width Dn) and the diameter expansion ratio Dc based on the following analysis, as shown in FIG. 9(a). We found that there is a relationship as shown in

解析は、図9(b)で示すような、拡径比D0の異なる4種類の節付き杭1が固結シルト中に埋設されている場合を想定し、節付き杭1における引抜き時の挙動についてFEMによるシミュレーション解析を行った。なお、上向き円錐台部42における傾斜面の極限摩擦力度は、無限大と仮定している。 The analysis assumes that four types of knotted piles 1 with different diameter expansion ratios D0 are buried in consolidated silt, as shown in Fig. 9(b), and the behavior of the knotted piles 1 when pulled out is analyzed. We conducted a simulation analysis using FEM. It is assumed that the ultimate degree of frictional force of the inclined surface of the upward truncated conical portion 42 is infinite.

まず、節付き杭1に引抜き力を作用させ、節付き杭1の変位が節部径Dの10%に達するときの節部4周辺地盤に生じる地盤の鉛直応力分布を求めた。次に、この分布に基づいて、鉛直応力が節部4の支圧力度の10%に低下する位置までの距離を求め、この距離を地中応力の影響長さLsとした。 First, a pulling force was applied to the knotted pile 1, and the vertical stress distribution of the ground generated in the ground around the knot 4 when the displacement of the knotted pile 1 reached 10% of the knot diameter D was determined. Next, based on this distribution, the distance to the position where the vertical stress decreases to 10% of the bearing pressure strength of the node 4 was determined, and this distance was defined as the underground stress influence length Ls.

その結果、拡径比Dcが大きいほど地盤に対する節部4の載荷面積が大きくなり、地中応力の影響範囲Aが広くなる傾向が認められた。そこで、地中応力の影響長さLsを突出幅Dnで除した値と、拡径比Dcとの関係をプロットしたところ、図9(a)で示すように、地中応力の影響長さLsを突出幅Dnで除した値は、拡径比Dcの増加に伴って減少し、ある一定値に収束することを見出した。 As a result, it was found that the larger the diameter expansion ratio Dc, the larger the loading area of the joint 4 on the ground, and the wider the influence range A of underground stress. Therefore, when we plotted the relationship between the length Ls affected by underground stress divided by the protrusion width Dn and the diameter expansion ratio Dc, we found that the length Ls affected by underground stress It has been found that the value obtained by dividing by the protrusion width Dn decreases as the diameter expansion ratio Dc increases, and converges to a certain constant value.

この解析結果に基づき回帰分析を行って近似曲線を作成し、この近所曲線に、STEP1で確認した節付き杭1の突出幅Dn及び拡径比Dcを代入することにより、節付き杭1各々に対応した地中応力の影響長さLsを推定することとした。 Based on this analysis result, regression analysis is performed to create an approximate curve, and by substituting the protrusion width Dn and diameter expansion ratio Dc of the knotted pile 1 confirmed in STEP 1 to this neighborhood curve, each knotted pile 1 is It was decided to estimate the length Ls affected by the corresponding underground stress.

こうして推定した地中応力の影響長さLsと節部4の根入れ長Hとを比較して、地中応力の影響範囲Aが地盤内に収まるか否かを検証する。 The influence length Ls of the underground stress estimated in this way is compared with the penetration length H of the joint 4 to verify whether the influence range A of the underground stress falls within the ground.

≪引抜き抵抗力の評価工程:STEP3-1≫
図4(a)で示すように、根入れ長Hが地中応力の影響長さLsより長い場合、地中応力の影響範囲Aは地盤内に収まる。つまり、節付き杭1の引抜き時において、節部4がこれに抵抗する際、節部4近傍の地盤に節部4から地表面に到達するような破壊は発生しないものと推定できる。
≪Evaluation process of pull-out resistance: STEP 3-1≫
As shown in FIG. 4(a), when the penetration length H is longer than the underground stress influence length Ls, the influence range A of underground stress falls within the ground. In other words, when the knotted pile 1 is pulled out and the knotted portion 4 resists this, it can be estimated that no damage will occur in the ground near the knotted portion 4 that reaches the ground surface from the knotted portion 4.

したがって、従来より節部4の根入れ長Hが十分確保されている場合に採用されている評価式を適宜採用し、節部4の引抜き抵抗力F(Fa)を算定すればよい。なお、引抜き抵抗力F(Fa)は、例えば以下の(1)式により算定できる。 Therefore, the pull-out resistance force F (Fa) of the knot 4 may be calculated by appropriately employing an evaluation formula that has conventionally been used when the penetration length H of the knot 4 is sufficiently secured. Note that the pull-out resistance force F (Fa) can be calculated, for example, using the following equation (1).

事例として挙げる(1)式は、節部4における円筒部41の最外縁より鉛直上方向に、円筒面状のせん断面が形成されることを想定している。そして、このせん断面付近の地盤の極限せん断抵抗力度に基づいて、節部4が負担する引抜抵抗力F(Fa)を評価する方法である。なお、(1)式の詳細は、特許4856903号公報に譲る。 Equation (1) given as an example assumes that a cylindrical sheared surface is formed vertically upward from the outermost edge of the cylindrical portion 41 in the joint portion 4. This method evaluates the pull-out resistance force F (Fa) borne by the knot 4 based on the ultimate shear resistance of the ground near the shear plane. The details of equation (1) are given in Japanese Patent No. 4856903.

Fa=fsi×ANP・・・・・・・・・・・・・・・・・・・・・・(1)
NP:せん断力を受ける円筒面の面積(m2
si:せん断面付近の地盤の極限せん断抵抗力度(kN/m2
Fa=f si ×A NP・・・・・・・・・・・・・・・・・・・・・(1)
A NP : Area of cylindrical surface receiving shear force (m 2 )
f si : Ultimate shear resistance of the ground near the shear plane (kN/m 2 )

≪引抜き抵抗力の評価工程:STEP3-2≫
一方、地中応力の影響長さLsが根入れ長Hより長い場合、地中応力の影響範囲Aが地盤内に収まらず、地表面に到達する。つまり、節付き杭1の引抜き時において、節部4がこれに抵抗する際、節部4近傍の地盤に節部4から地表面に到達するような破壊面Fsが発生するものと推定できる。
≪Evaluation process of pull-out resistance: STEP 3-2≫
On the other hand, when the underground stress influence length Ls is longer than the penetration length H, the underground stress influence range A does not fall within the ground and reaches the ground surface. That is, when the knotted pile 1 is pulled out, when the knotted portion 4 resists this, it can be estimated that a fracture surface Fs that reaches the ground surface from the knotted portion 4 is generated in the ground near the knotted portion 4.

したがって、節部4の引抜き抵抗力Fは、破壊面Fsが形成されたことにより節部4上に形成された抵抗土塊の重量と、抵抗土塊と破壊面Frのせん断抵抗力との和となる。ところで、前述した模型杭を用いた引抜き実験により、節部4から地表面に達する破壊面Fsが発生した際の破壊形態は、図6(b)(c)で示すように、円筒形状もしくはコーン状の2通りがある。 Therefore, the pull-out resistance force F of the joint 4 is the sum of the weight of the resistance soil clod formed on the joint 4 due to the formation of the fracture surface Fs, and the shear resistance force of the resistance soil mass and the fracture surface Fr. . By the way, in the pull-out experiment using the model pile described above, when the fracture surface Fs that reaches the ground surface from the node 4 occurs, the fracture form is cylindrical or conical, as shown in FIGS. 6(b) and 6(c). There are two ways.

したがって、地中応力の影響範囲Aが地表面に到達する場合の引抜き抵抗力Fは、図10(a)で示すような、節部4上に節部径Dに基づく円筒形状の破壊土塊C1が形成される場合と、図10(a)で示すような、コーン状の破壊土塊C2が形成される場合の各々で算定を行う。なお、コーン状の破壊土塊C2は、破壊面Frと軸部2と節部4の上向き円錐台部42とにより囲まれた土塊である。 Therefore, when the influence range A of the underground stress reaches the ground surface, the pull-out resistance force F is as shown in FIG. 10(a). Calculations are performed for each of the cases in which a broken soil mass C2 is formed and a cone-shaped broken soil mass C2 as shown in FIG. 10(a) is formed. Note that the cone-shaped fractured soil mass C2 is a soil mass surrounded by the fracture surface Fr, the shaft portion 2, and the upward truncated conical portion 42 of the joint portion 4.

節部径Dに基づく円筒形状の破壊土塊C1を想定した場合の引抜き抵抗力F(Fr)は、以下の(2)式で算定できる。また、コーン状の破壊土塊C2を想定した場合の引抜き抵抗力F(Fc)は、以下の(3)式で算定できる。 The pulling resistance force F (Fr) when assuming a cylindrical broken earth mass C1 based on the node diameter D can be calculated using the following equation (2). Moreover, the pull-out resistance force F (Fc) when assuming a cone-shaped broken soil mass C2 can be calculated using the following equation (3).

Fr=Wg+Rf・・・・・・・・・・・・・・・・・・・・・・(2)
Wg:節部直上の円筒形の土塊重量 (kN)
Rf:円筒状破壊時の円筒面のせん断抵抗力 (kN)
Fr=Wg+Rf・・・・・・・・・・・・・・・・・・・・・(2)
Wg: Weight of cylindrical soil just above the joint (kN)
Rf: Shear resistance force of cylindrical surface during cylindrical fracture (kN)

Fc=Wg+Fv・・・・・・・・・・・・・・・・・・・・・・(3)
Wg:コーン状の土塊重量(kN)
Fv:コーン状破壊面のせん断抵抗力の鉛直成分 (kN)
Fc=Wg+Fv・・・・・・・・・・・・・・・・・・・・・(3)
Wg: Weight of cone-shaped soil (kN)
Fv: Vertical component of shear resistance force of cone-shaped fracture surface (kN)

こうして上記の(2)式で算定した引抜き抵抗力F(Fr)と(3)式で算定した引抜き抵抗力F(Fc)とを比較し、数値の小さいものを節付き杭1の引抜き抵抗力Fとして採用する。 In this way, the pull-out resistance force F (Fr) calculated by the above formula (2) is compared with the pull-out resistance force F (Fc) calculated by the formula (3), and the smaller value is determined as the pull-out resistance force of the knotted pile 1. Adopted as F.

上記のとおり、節部4の根入れ長Hと地中応力の影響範囲Aに基づいて地中応力の影響範囲Aが地表面に達するか否かを検証し、検証した結果に基づいて節部4の引抜き抵抗力Fを適切に評価することができる。これにより、従来では引抜き抵抗力Fを設計に考慮していなかった根入れ長Hの浅い節部4について、鉛直支持力だけでなく引抜き抵抗力を併せて設計に反映することが可能となる。 As mentioned above, based on the penetration length H of the joint 4 and the influence range A of the underground stress, it is verified whether or not the influence range A of the underground stress reaches the ground surface, and based on the verified result, the joint The pull-out resistance force F of No. 4 can be appropriately evaluated. This makes it possible to reflect not only the vertical support force but also the pull-out resistance force in the design of the shallow joint portion 4 with the penetration length H, for which the pull-out resistance force F has not been considered in the design in the past.

また、節部4を、節つき杭1に作用する引抜き力に抵抗することを目的として設ける場合に、節部4の根入れ長Hを、要求される引抜き抵抗力Fの大きさに応じて設定でき、安全性と経済性を兼ね備えた合理的な節付き杭1の設計を行うことが可能となる。 In addition, when the knot 4 is provided for the purpose of resisting the pull-out force acting on the knotted pile 1, the penetration length H of the knot 4 can be adjusted depending on the magnitude of the required pull-out resistance force F. It becomes possible to design a rational knotted pile 1 that is both safe and economical.

なお、本発明の引抜き抵抗力の評価方法は、上記の実施形態に限定されるものではなく、本発明の趣旨を逸脱しない範囲で、種々の変更が可能であることはいうまでもない。 It goes without saying that the method for evaluating the pull-out resistance of the present invention is not limited to the above-described embodiments, and that various changes can be made without departing from the spirit of the present invention.

例えば、本実施の形態では節部4について、円筒部41と上向き円錐台部42と下向き円錐台部43とを組みあわせた形状のものを採用した。しかし、その形状は、これに限定されるものではなく、いずれの形状の節部4であっても採用することが可能である。 For example, in the present embodiment, the knot portion 4 has a shape that is a combination of a cylindrical portion 41, an upward truncated conical portion 42, and a downward truncated conical portion 43. However, the shape is not limited to this, and any shape of the joint portion 4 can be employed.

また、本実施の形態では、地中応力の影響長さLsを推定するにあたり、地盤が固結シルトである場合を事例に挙げて実験を行い、拡径比Dcとの関係に係る近似曲線を示した。これら近似曲線は、節部4を設ける地盤の種類に対応させて適宜同様の実験を行って作成すればよい。こうすると、地盤が固結シルトではない場合にも、地中応力の影響長さLsを推定することが可能となる。 In addition, in this embodiment, when estimating the length Ls affected by underground stress, an experiment is conducted using the case where the ground is solidified silt, and an approximate curve related to the relationship with the diameter expansion ratio Dc is calculated. Indicated. These approximate curves may be created by suitably conducting similar experiments in accordance with the type of ground on which the joint portion 4 is to be provided. In this way, even when the ground is not solidified silt, it is possible to estimate the length Ls affected by underground stress.

1 節付き杭
2 軸部
3 拡底部
4 節部
41 円筒部
42 上向き円錐台部
43 下向き円錐台部
G1 表層
G2 中間層
G3 支持層
C1 破壊土塊(円筒形状)
C2 破壊土塊(コーン状)
1’ 模型杭
2’ 軸部
4’ 節部
G’ 模型地盤
1 Knotted pile 2 Shaft part 3 Expanded bottom part 4 Knot part 41 Cylindrical part 42 Upward truncated conical part 43 Downward truncated conical part G1 Surface layer G2 Intermediate layer G3 Support layer C1 Destruction clod (cylindrical shape)
C2 Destruction clod (cone shape)
1' Model pile 2' Shaft 4' Joint G' Model ground

Claims (2)

軸部と、該軸部に設けた節部とを備える節付き杭の、引抜き時における前記節部の引抜き抵抗力を評価するための、引抜き抵抗力の評価方法であって、
前記節付き杭に引抜き力が作用した際の、前記節部の周辺地盤における地中応力の影響範囲を推定する推定工程と、
前記地中応力の影響範囲と前記節部の根入れ長とを比較し、前記地中応力の影響範囲が地表面に達するか否かを検証する検証工程と、
前記地中応力の影響範囲が地表面に達する場合に、前記節部の引抜き抵抗力を、前記節部から地表面にかけて発生する破壊面と前記節付き杭の軸部との間の土塊の重量と、該土塊の重量と前記破壊面のせん断抵抗に基づいて算定する評価工程と、
を備えることを特徴とする引抜き抵抗力の評価方法。
A method for evaluating the pulling resistance of a knotted pile including a shaft and a joint provided on the shaft, the method comprising:
an estimation step of estimating the range of influence of underground stress in the ground surrounding the joint when a pull-out force is applied to the joint;
a verification step of comparing the range of influence of the underground stress and the penetration length of the joint, and verifying whether the range of influence of the underground stress reaches the ground surface;
When the range of influence of the underground stress reaches the ground surface, the pull-out resistance of the knot is determined by the weight of the soil mass between the fracture surface that occurs from the knot to the ground surface and the shaft of the knotted pile. and an evaluation step of calculating based on the weight of the soil clod and the shear resistance of the fracture surface,
A method for evaluating pull-out resistance, comprising:
請求項1に記載の引抜き抵抗力の評価方法において、
前記評価工程では、前記破壊面が円筒形状を想定して算出した引抜き抵抗力と、コーン状を推定した算出した引抜き抵抗力とを比較し、数値の小さい方を前記節部の引抜き抵抗力として採用することを特徴とする引抜き抵抗力の評価方法。
In the method for evaluating pullout resistance according to claim 1,
In the evaluation step, the pull-out resistance force calculated assuming that the fracture surface has a cylindrical shape is compared with the pull-out resistance force calculated assuming a cone shape, and the smaller value is determined as the pull-out resistance force of the knot. A method for evaluating pull-out resistance, which is characterized in that it is adopted .
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