Method for determining the specifications of ground improvement bodies for seismic reinforcement of pile foundations

Abstract

The width of the ground improvement body used to seismically reinforce pile foundations is set to a reasonable value. [Solution] In a construction method for reinforcing seismic resistance by forming a ground improvement body 12 around a pile foundation 10, the ultimate ground reaction force of the pile foundation is calculated for multiple depths, and when multiple types of ground improvement bodies 12 with different width dimensions are formed around the pile foundation 10, the ultimate ground reaction force of each ground improvement body 12 is calculated for multiple depths, the depth at which a plastic hinge occurs when a horizontal force is applied to the top of the pile foundation 10 is calculated, and among the multiple ground improvement bodies 12, a ground improvement body 12 is selected in which the depth at which its ultimate ground reaction force exceeds the ultimate bearing capacity required for seismic reinforcement is shallower than the depth at which a plastic hinge occurs, and the width dimension of the said ground improvement body 12 is determined to be the width dimension of the ground improvement body required for seismic reinforcement of the pile foundation 10.

Description

[Technical Field]

[0001] The present invention relates to a seismic reinforcement method for pile foundations, and particularly relates to a technology for improving seismic performance by forming a ground improvement body around the pile foundation of a building. [Background Art]

[0002] Attempts to secure seismic performance by forming a ground improvement body around the pile foundation of a building have been made heretofore. For example, the following Patent Document 1 discloses a technology for seismically reinforcing a pile foundation and an upper foundation by improving the ground around an existing upper foundation (such as footing) and forming two upper and lower layers of solidified improvement bodies. Further, in the following Patent Document 2, a technology for improving the shear resistance of a pile foundation is disclosed, in which a hole is drilled at a predetermined position on the outer periphery of the pile foundation, and an improvement body is created by pulling up the hole while injecting a slurry-like solidifying material along a fan-shaped locus, and the improvement body surrounds the footing including the lower surface thereof. [Patent Document 1] Japanese Unexamined Patent Publication No. 2013-177741 [Patent Document 2] Japanese Unexamined Patent Publication No. 2021-050580 [Summary of the Invention] [Problem to be Solved by the Invention]

[0003] By using these technologies, it can be expected that existing pile foundations are reinforced by the ground improvement body, and the seismic performance of the building is improved. However, since there is a lack of theoretical consideration on the width dimension of the ground improvement body to be created, in actual construction, a ground improvement body with an excessively large width dimension tends to be created from the perspective of ensuring safety, which has been a factor hindering shortening of the construction period and cost reduction.

[0004] The present invention has been devised in view of such circumstances, and aims to provide a technology capable of shortening the construction period and reducing costs by setting the width dimension of a ground improvement body for seismic reinforcement of pile foundations to a reasonable value. [Means for Solving the Problem]

[0005] In order to achieve the above object, a method for determining specifications of a ground improvement body for seismic reinforcement of a pile foundation according to the present invention is a construction method for reinforcing seismic performance by forming a ground improvement body around a pile foundation, the method comprising: a step of calculating ultimate ground reaction force of the pile foundation to be subjected to seismic reinforcement for each of a plurality of depths; a step of calculating, for each of the plurality of depths, ultimate ground reaction force in a case where a plurality of types of ground improvement bodies having different width dimensions are formed around the pile foundation; a step of calculating a plastic hinge occurrence depth when a horizontal force is applied to a head of the pile foundation; a step of selecting, from among the plurality of ground improvement bodies, a ground improvement body for which a depth at which the ultimate ground reaction force thereof is equal to or greater than the ultimate strength required for seismic reinforcement is shallower than the plastic hinge occurrence depth; and a step of accrediting the width dimension of the selected ground improvement body as the width dimension of the ground improvement body required for seismic reinforcement of the pile foundation. The above-mentioned "ultimate ground reaction force" is theoretically derived using a mathematical formula that reflects the size (diameter and width dimension) of the pile foundation and the ground improvement body, and the characteristics of the ground (unit weight, internal friction angle, adhesive force, etc.).

[0006] In the step of calculating the ultimate ground reaction force when a plurality of types of ground improvement bodies having different width dimensions are formed around the pile foundation, on the premise that wedge-shaped failure occurs in the surface ground through the ground improvement body due to the horizontal force acting on the head of the pile foundation, the following processes are performed: calculating the weight of a soil wedge in front of the ground improvement body caused by the wedge-shaped failure; calculating resistance between the soil wedge and the ground generated when the soil wedge is pushed up; calculating the weight of a partial region on the loading front face of the ground improvement body; calculating resistance acting between both side faces of the partial region on the loading front face of the ground improvement body and the ground; and summing up each of the above calculation results.

[0007] If multiple ground improvement bodies are selected, it is desirable to operate in a way that ensures the ground improvement body with the narrowest width is selected.

[0008] Furthermore, it is desirable to include a step of provisionally setting the design standard strength of the selected ground improvement body to a predetermined value, a step of calculating the thrust shear strength of the ground improvement body based on the design standard strength when a horizontal force is applied to the head of the pile foundation, and a step of certifying the design standard strength as the appropriate design standard strength of the ground improvement body if this thrust shear strength is equal to or greater than the ultimate strength required for seismic reinforcement. [Effects of the Invention]

[0009] According to this invention, the required width dimension for ground improvement bodies used for seismic reinforcement of pile foundations can be theoretically calculated. This effectively prevents the construction of ground improvement bodies with excessively large widths, as has been done in the past, thereby enabling shorter construction periods and cost reductions. [Brief explanation of the drawing]

[0010] [Figure 1] This is a perspective view showing the state of seismic reinforcement by forming a ground improvement body around the pile foundation. [Figure 2] This is a conceptual plan diagram illustrating a construction method for forming a ground improvement body around a pile foundation. [Figure 3] This flowchart shows the procedure for determining the width and design strength of the ground improvement body. [Figure 4] This graph shows the relationship between the depth of pile foundations and the ultimate ground reaction force under certain ground conditions. [Figure 5] This is a conceptual diagram illustrating wedge-shaped failure in the surface soil resulting from a horizontal force acting on the pile head of a pile foundation. [Figure 6] This is a conceptual diagram illustrating how the soil wedge at the front of a pile is pushed up when the pile foundation is subjected to horizontal force. [Figure 7] This graph includes a curve showing the relationship between the depth of the pile foundation and the ultimate ground reaction force, and multiple curves showing the relationship between the depth of multiple ground improvement bodies formed around the pile foundation with different widths and the ultimate ground reaction force. [Figure 8] This is a conceptual diagram illustrating wedge-shaped fracture that occurs in the surface soil via a ground improvement body formed around the pile foundation, as a result of a horizontal force acting on the pile head of the pile foundation. [Figure 9] This is a conceptual diagram showing soil wedges being pushed up by a ground improvement body formed around the pile foundation. [Figure 10] This is a conceptual diagram showing the portion of the ground improvement body formed around the pile foundation that is related to the calculation of the ultimate ground reaction force. [Figure 11] Figure 7 is a graph with the depth Lp at which a plastic hinge occurs when a horizontal force acts on the pile head of a pile foundation. [Figure 12] Assuming that the ultimate bearing capacity required for seismic reinforcement is 400kN, Figure 11 shows the depth Li at which the ultimate ground reaction force of each improved ground body exceeds 400kN, with the depth Li added to the original graph. [Figure 13] Assuming that the ultimate bearing capacity required for seismic reinforcement is 350 kN, the graph in Figure 11 shows the depth Li at which the ultimate ground reaction force of each improved ground body exceeds 350 kN. [Figure 14] This is a schematic diagram illustrating the procedure for verifying the thrust shear resistance of a ground improvement structure. [Best Mode for Carrying Out the Invention]

[0011] As shown in Figure 1, the present invention is based on a construction method that reinforces the ground against earthquakes by forming a rectangular prism-shaped ground improvement body 12 around a cylindrical pile foundation 10 that is the target of seismic reinforcement, and aims to optimize the width dimension (B or D) of the ground improvement body 12. In the figure, reference numeral 14 indicates a footing connected to the top of the pile foundation 10.

[0012] The ground improvement body 12 is formed by a procedure similar to the construction method disclosed in the above-mentioned Patent Document 2. Specifically, as shown in Figure 2, the ground is drilled from near two points (20a, 20b) on the diagonal of the rectangle 20, which corresponds to the cross-section of a rectangular prism, and a rod (not shown) equipped with a monitor at its tip is inserted into the borehole 22. Next, a slurry-like solidifying agent and compressed air are injected from a monitor (not shown) to cut the ground around the pile foundation 10 while mixing it with the solidifying agent, thereby creating a ground improvement body 12.

[0013] When compressed air is injected, the slurry-like solidifying material is surrounded by the compressed air, and the reach of the slurry-like solidifying material (the distance the solidifying material cuts into the ground) can be adjusted by adjusting the amount of compressed air injected. When spraying the slurry-like solidifying material and compressed air, the rod is repeatedly moved back and forth or oscillated at the required angle to improve the ground so that the trajectory 24 of the solidifying material and compressed air sprayed from the monitor forms a fan shape, and the improved ground body 12 is created by pulling up the rod.

[0014] There are no particular limitations on the depth of the ground improvement body 12 to be constructed; it is sufficient if it is at least the effective depth (so-called "1 / β") to which the pile foundation 10 can resist horizontal forces in the relevant ground. However, if damage is found at a depth greater than this as a result of the pile soundness survey, the depth may be set to include that section. Furthermore, although Figure 2 illustrates a square cross-section where the widths of B and D are equal, the ground improvement body 12 can also be formed in a rectangular cross-section where the widths of B and D are different.

[0015] The method for determining the width dimension and design strength of the ground improvement body 12 will be explained below, following the flowchart in Figure 3. First, let's consider the ultimate ground reaction force P of a single pile foundation (pile diameter: 0.6m) that is subject to seismic reinforcement. _p Multiple values ​​are calculated for each arbitrary depth (Z) and graphed (S10). Figure 4 illustrates this graph, plotting multiple values ​​of the ultimate ground reaction force for each calculated depth on a plane with depth (m) on the vertical axis and ultimate ground reaction force (kN) on the horizontal axis, and connecting them with curves.

[0016] This extreme ground reaction force P _p This is calculated by applying the theory disclosed in the following paper. [Non-Patent Document 1] Horizontal Resistance of Piles Considering Ground Failure / Hideaki Kishida, Shoichi Nakai / Transactions of the Architectural Institute of Japan, No. 281 / July 1979 This paper presents the idea that, as shown in Figure 5, "when a horizontal force is applied to the head of the pile foundation 10, it is assumed that the soil wedge 30 in front of the pile foundation 10 is pushed up, and the ultimate ground reaction force of the target pile foundation 10 can be determined by adding the weight of the soil mass of the soil wedge 30, the shear resistance of the inclined front surface 30a of the soil wedge 30, and the shear resistance acting on the sides 30b and 30c of the soil wedge 30."

[0017] Figure 6 shows the soil wedge 30 as observed from the rear side, illustrating that the portion corresponding to height Z on the front side 10a, when the pile foundation 10 is divided vertically into two parts, constitutes a part of the soil wedge 30. In the figure, "θ1" indicates the angle between the side surface 30b or 30c of the soil wedge 30 and the plane 30d or 30e that is perpendicular to the cross-section 10b of the pile foundation 10. Furthermore, "θ2" in the figure indicates the angle between the cross-section 10b of the pile foundation 10 and the sloping front surface 30a of the soil wedge 30.

[0018] Ultimate ground reaction force P _p The specific calculation procedure is as follows: First, the horizontal force applied to the top of the pile foundation 10 and the ultimate ground reaction force of the pile foundation 10 can be expressed by the following equations. (1) H·sinθ²=Ws _p ·cosθ²+Fb _p +Fs …[Equation 1] H: Pile head weight Ws _p :Weight of soil clod (kN) of soil wedge 30 F brand _p Shear resistance (kN) of the inclined front surface 30a of the soil wedge 30 Fs: Shear resistance (kN) of sides 30b and 30c of the soil wedge. θ2:45°+φ / 2 φ: Angle of internal friction of the ground (°)

[0019] Divide both sides of this Equation 1 by sinθ₂, and express H on the left side as the ultimate ground reaction p _p , the resulting expression is given by the following formula. (2) p _p ={Ws _p ·cosθ₂+Fb _p +Fs} / sinθ₂ …[Formula 2] p _p : Ultimate ground reaction (ground) Hereinafter, the method for obtaining Ws _p , Fb _p and Fs will be described in detail.

[0020] [Method for obtaining Ws _p (weight of the soil mass of soil wedge 30)] (3) Ws _p =γ×{(Ztanθ₂×Z / 2×d)+ 2×(1 / 2×Ztanθ₂×Ztanθ₂·tanθ₁×Z×1 / 3)} …[Formula 3] γ: Unit weight of ground (kN / m 3 ) Z: Arbitrary depth (m) d: Pile diameter (m) θ₁:φ / 2 * When Z is located below the groundwater level, the effect of buoyancy shall be considered for γ.

[0021] [Method for obtaining Fb _p (shear resistance on the inclined front face 30a of the soil wedge)] (4) Fb _p =c×A2 _p +Ws _p sinθ₂tanφ …[Formula 4] c: Cohesion of ground (kN / m 2 ) A2 _p : Area of the inclined front face of soil wedge 30 (m 2 ) * See Formula 5 below (5) A2 _p =(d+d+2Ztanθ₂·tanθ₁)×Z / cosθ₂×1 / 2 …[Formula 5]

[0022] [Method for obtaining Fs (shear resistance on side faces 30b and 30c of the soil wedge)] (6) Fs=2×(c+1 / 3K0·γ·Z·tanφ)×A3×cosθ1 …[Formula 6] K0: Coefficient of earth pressure at rest in the ground (-) A3: Surface area of ​​soil wedge (m²) 2 ) *See formula 7 below. (7) A3 = Z × Z tanθ 2 / cosθ 1 × 1 / 2 …[Equation 7]

[0023] Next, the ultimate ground reaction force P of multiple ground improvement bodies 12, each formed with a different width around the pile foundation 10 that is subject to seismic reinforcement. _i Multiple values ​​are calculated for each arbitrary depth (Z) and graphed (S12). Figure 7 illustrates this graph, plotting multiple values ​​of the ultimate ground reaction force for each depth of each of the 12 ground improvement bodies on a plane similar to Figure 4, with depth (m) on the vertical axis and ultimate ground reaction force (kN) on the horizontal axis, and connecting them with curves. The graph shows the calculation results for three ground improvement bodies 12A, 12B, and 12C, with widths set to 1.4m, 1.8m, and 2.2m respectively. However, other widths may also be used.

[0024] The ultimate ground reaction force P when a ground improvement body 12 is formed around the pile foundation 10 to be reinforced. _i This can be determined based on the following, for example. (i) In accordance with the above theory, as shown in Figure 8, assuming that the horizontal force acting on the top of the pile foundation 10 pushes up the soil wedge 40 in front of the soil improvement body via the soil improvement body, first, as shown in Figure 9(a), the weight Ws of this soil wedge 40 _i Calculate. (b) Next, as shown in Figure 9(b), the shear resistance Fb of the inclined front surface 40a of the trapezoidal soil wedge 40. _i Calculate. (h) Next, as shown in Figures 9(c) and (d), the shear resistance Fs of the sides 40b and 40c of the triangular soil wedge 40 _i Calculate each of them. (ii) Next, as shown in Figure 10(a), the weight Wi of a portion area 12a in front of the load applied to the ground improvement body 12 is calculated. This partial region 12a refers to the triangular prism-shaped portion connecting the triangular regions X1 and X2 on both sides of the ground improvement body 12. The width of this triangular prism is defined as the width dimension B of the ground improvement body 12, the base of each triangular region X1 and X2 is defined as D / 2, and the height is defined as Z (an arbitrary depth). (e) Next, as shown in Figures 10(b) and (c), the shear resistance Fis of each triangular region X1 and X2 is calculated. (f) Finally, add up the values ​​from (i) to (e).

[0025] In Figure 8, "θ1" indicates the angle between the two sides 40b or 40c of the soil wedge 40 and the extended surfaces 40d or 40e of both sides of the ground improvement body 12. Furthermore, "θ2" in the same figure indicates the angle between the front surface 12b of the ground improvement body 12 and the sloping front surface 40a of the soil wedge 40.

[0026] Ultimate ground reaction force P _i The specific calculation procedure is as follows: First, the horizontal force acting on the pile head and the ultimate ground reaction force have the following relationship: (8) H·sinθ2=Wi·cosθ2+Fis+Ws _i ·cosβ +Fb _i +Fs …[Equation 8] H: Horizontal force acting on the pile head Wi: Weight (kN) of a portion of area 12a of the ground improvement body 12 Fis: Shear resistance force (kN) of triangular regions X1 and X2 on both sides of the ground improvement body 12. Ws _i :Weight of soil wedge 40 (kN) F brand _i Shear resistance (kN) of the 40a front surface of the soil wedge. Fs: Shear resistance (kN) of sides 40b and 40c of soil wedge 40 θ2:45°+φ / 2 φ: Angle of internal friction of the ground (°)

[0027] Divide both sides of equation 8 by sinθ², and change the left side H to the limiting ground reaction force p _i The expression obtained is as follows: (9) p _i ={Wi·cosθ²+fis +Ws _i * cosθ² + Fb_i + Fs} / sinθ² …[Equation 9] p _i : Extreme ground reaction force (ground + ground improvement body) Below, Wi, Fis, Ws _i Fb _i Next, we will explain in detail how to find Fs.

[0028] [How to calculate Wi (weight of a portion of the ground improvement body, area 12a)] (10) Wi = γ × {(D / 2 × Z) × 1 / 2 × B} …[Equation 10] γ: Unit weight of soil (kN / m³) 3 ) B: Width of the ground improvement structure (in the direction perpendicular to the direction of horizontal force application) (m) Z: Any depth (m) D: Depth of the ground improvement structure (parallel to the direction of horizontal force application) (m)

[0029] [How to calculate Fis (shear resistance force of triangular regions X1 and X2 of the ground improvement body 12)] (11) Fis=2×(c+1 / 3K0·γ·Z·tanφ)×A4 …[Formula 11] c: Cohesion of the ground (kN / m 2 ) K0: Coefficient of earth pressure at rest in the ground (-) A4: Area of ​​the triangular region X of the ground improvement body (m²) 2 ) *See formula 12 below. (12) A4 = (D / 2 × Z) × 1 / 2 …[Formula 12]

[0030] [Ws _i [How to calculate the weight of a soil wedge (40)] (13) Ws _i =γ×{(Ztanθ2×Z / 2×B)+ 2 × (1 / 2 × Z tanθ² × Z tanθ²·tanθ¹ × Z × 1 / 3)} …[Equation 13] θ1:φ / 2

[0031] [Fb _i (How to determine the shear resistance of the 40a front surface of the soil wedge) (14) Fb _i = c × A2 _i +Ws _i sinθ2tanφ···[Equation 14] A2 _i : Area of ​​soil wedge slope (m 2 ) *See formula 15 below. (15) A2 _i =(B+B+2Ztanθ2·tanθ1)×Z / cosθ2×1 / 2 …[Equation 15]

[0032] [How to calculate Fs (shear resistance of sides 40b and 40c of the soil wedge)] See formulas 6 and 7 above.

[0033] Next, the depth Lp at which a plastic hinge occurs in the pile foundation 10 subject to seismic reinforcement when a horizontal force is applied to the pile head is calculated using static analysis (beam-spring model), 2D finite element method, 3D finite element method, etc. (S14).

[0034] As an example, the following shows a method in which the "point of maximum bending moment generation lm" according to the Chang equation of elastic bearing beam theory is defined as the plastic hinge generation depth Lp. (16) lm=π / 2β …[Formula 16] *Pile head free condition (17) lm = π / 4β …[Equation 17] *Constraint condition for pile head rotation lm: Point where the maximum bending moment occurs underground. β: Pile characteristic value (1 / m) *See formula 18 below (18) β=(k h d / 4EI) 1 / 4 …[Formula 18] k h : Horizontal ground reaction coefficient (kN / m 3 ) d: Pile diameter (m) EI: Bending stiffness of pile (kN / m 2 ) Figure 11 shows the state where the plastic hinge initiation depth Lp is set to a depth of 2m.

[0035] Next, based on the following criteria, the ground improvement body 12 with the most reasonable width dimension is selected from among the ground improvement bodies 12A, 12B, and 12C (S16). (a) The ultimate ground reaction force p of the ground improvement body 12 _i A ground improvement body 12 is selected such that the depth Li at which the ultimate bearing capacity Hreq required for seismic reinforcement is greater than or equal to the depth Lp at which the plastic hinge occurs in the pile foundation 10. (If Li ≥ Lp, the rigidity of the ground around the pile increases, so the location of the plastic hinge will not increase in depth; therefore, ground improvement bodies 12 of the corresponding width should be excluded.) (b) If there are multiple selected ground improvement bodies 12, the one with the narrowest width will be selected.

[0036] Here, we will explain in detail based on the graph in Figure 12. For example, assuming the required ultimate bearing capacity Hreq is 400 (kN), in both ground improvement bodies 12A and 12B, with widths of 1.4m and 1.8m, Li will be deeper than the plastic hinge occurrence depth Lp. However, in ground improvement body 12C, with a width of 2.2m, Li is shallower than Lp, so in this scenario, ground improvement body 12C is selected.

[0037] In contrast, as shown in Figure 13, assuming that the required ultimate bearing capacity Hreq is 350 (kN), the Li of the ground improvement body 12B with a width of 1.8m and the Li of the ground improvement body 12C with a width of 2.2m are both shallower than the plastic hinge occurrence depth Lp. Therefore, the ground improvement body 12B with a width of 1.8m, which requires a narrower width, is ultimately selected.

[0038] Next, the design standard strength Fc of the selected ground improvement body 12 is provisionally set to a predetermined value (S18), and the value of Fc is determined by checking the thrust shear. In other words, as shown in Figure 14(a), when a horizontal force equivalent to the required ultimate bearing capacity Hreq is applied to the pile head, it is confirmed by the following formula 19 that the ground improvement body 12 will not be pushed out and shear-fractured by the pile foundation 10 (S20). (19) Hreq ≤ 2 × τ × b × Lp + p …[Equation 19] τ: Shear strength of ground improvement body 12 (kN / m 2 ) *See formula 20 below. b: Length (m) of the ground improvement body 12 in front of the pile in the direction of load application. Lp: Depth of plastic hinge formation (= position where maximum bending moment occurs) p: Horizontal ground reaction force (kN) corresponding to the pile diameter *p represents the ultimate ground reaction force P for the pile foundation alone as described above. _p It is equal to. (20) τ = 0.3 × Fc …[Equation 20] Fc: Design strength of the ground improvement body (kN / m 2 )

[0039] In equation 19, "b × Lp" on the right-hand side represents the area of ​​the rectangular extrusion shear surface Y, as shown in Figure 14(b), and the "2 ×" at the beginning of the right-hand side indicates that there are two such extrusion shear surfaces Y, one on the left and one on the right (Figure 14(a)). The shear strength at these two thrust shear sections Y, plus the horizontal ground reaction force p corresponding to the pile diameter, is the sum of these shear strengths and the thrust shear strength of the ground improvement body 12.

[0040] Here, Hreq: 400 (kN), Fc: 1,000 (kN / m 2 Substituting the values ​​b:1.1 (m) and Lp:2 (m) into equations 19 and 20, we obtain the following: (21) 400≦2×0.3×1,000×1.1×2+p ≤ 1,320 + p …[Equation 21]

[0041] In other words, even without considering the horizontal ground reaction force p equal to the pile diameter, the ground improvement body 12 alone can sufficiently exceed the required ultimate bearing capacity of 400 (kN), and it can be confirmed that thrust-out shear failure will not occur. Based on this, the width dimension (2.2m) and design strength (1,000kN) of the ground improvement body 12 are certified as appropriate specifications (S22).

[0042] If the calculation using formula 21 results in a value below the required ultimate bearing capacity, the calculation is repeated by adjusting the design standard strength Fc of the ground improvement body 12, or by increasing the width dimension of the ground improvement, or both (S24 and S22). This adjustment of the design strength Fc is repeated until it is confirmed that the ground improvement body 12 does not undergo push-out shear failure due to the action of a horizontal force equivalent to the ultimate bearing capacity Hreq.

[0043] The above example shows the formation of a ground improvement body 12 around a single pile foundation (single pile) 10, but this invention is not limited to this. For example, in the case of a pile foundation where multiple pile foundations are connected to a single footing, seismic reinforcement is carried out by forming a ground improvement body around it. In such construction work, the ultimate ground reaction force can be derived using the same calculation method as described above, and the width dimension of the ground improvement body and the design standard strength can be optimized. In this case, the ultimate ground reaction force P in S10 described above. _p This is calculated as the sum of the ultimate ground reactions of each pile foundation. It is important to note that the ultimate ground reaction of each pile foundation will be different depending on the arrangement shape, spacing, and number of piles. Also, the plastic hinge occurrence depth Lp of the pile foundation in S14 above will differ significantly depending on the arrangement shape of each pile, especially before and after the direction of application of the load, similar to the value of the ultimate ground reaction. Taking this into appropriate consideration, the shallowest plastic hinge location among each pile can be determined as a reference when selecting the ground improvement body, on the safe side.

[0044] Furthermore, by forming a ground improvement body around pile foundations damaged by earthquakes or other events, this invention can be applied when repairing and reinforcing pile foundations, making it possible to calculate the optimal width dimension of the ground improvement body and the design standard strength. [Explanation of Symbols]

[0045] 10. Pile foundation 12 Ground Improvement Body 14 Footing 20. Rectangle (cross-sectional shape of the ground improvement body) 22 boreholes 30 Earth wedges 40 Earth wedges X1 triangular area X2 triangular area Y-shaped extrusion shear surface

Claims

1. In a construction method that reinforces seismic resistance by forming a ground improvement body around the pile foundation, The process involves calculating the ultimate ground reaction force of pile foundations subject to seismic reinforcement at multiple depths, The process involves calculating the ultimate ground reaction force for each of several depths when multiple types of ground improvement bodies with different widths are formed around the above-mentioned pile foundation, The process of calculating the depth at which a plastic hinge occurs when a horizontal force is applied to the top of the pile foundation, The process of selecting a ground improvement body from among the above multiple ground improvement bodies such that the depth at which its ultimate ground reaction force exceeds the ultimate bearing capacity required for seismic reinforcement is shallower than the depth at which the plastic hinge occurs, The process involves determining the width dimension of the selected ground improvement body as the width dimension of the ground improvement body necessary for seismic reinforcement of the pile foundation mentioned above, A method for determining the specifications of a ground improvement body for seismic reinforcement of pile foundations consisting of the following.

2. In the process of calculating the ultimate ground reaction force when multiple types of ground improvement bodies with different width dimensions are formed around the pile foundation described above, Assuming that the horizontal force applied to the top of the pile foundation causes wedge-shaped fracture in the surface soil via the ground improvement body, A process to calculate the weight of the soil wedge in front of the ground improvement body caused by the above wedge-shaped fracture, This process calculates the resistance between the soil wedge and the ground that occurs when it is pushed up, A process to calculate the weight of the triangular prism region connecting the lower end position of the soil wedge at the loading front of the ground improvement body and the center position of the top of the improved body, A process to calculate the resistance acting between the ground and both sides of the triangular prism region connecting the lower end position of the soil wedge at the loading front of the ground improvement body and the center position of the top of the improved body, and the ground, The method for determining the specifications of a ground improvement body for seismic reinforcement of a pile foundation according to claim 1, characterized in that the above calculation results are summed up.

3. A method for determining the specifications of a ground improvement body for seismic reinforcement of a pile foundation, as described in claim 1 or 2, characterized in that, when there are multiple selected ground improvement bodies, the ground improvement body with the narrowest width dimension is ultimately selected.

4. The process involves provisionally setting the design strength of the selected ground improvement body to a predetermined value, A process for calculating the push-out shear strength of the ground improvement body based on the design standard strength when a horizontal force is applied to the head of the pile foundation, If this piercing shear strength is greater than or equal to the ultimate strength required for seismic reinforcement, the above design standard strength is certified as the appropriate design standard strength for the ground improvement body. A method for determining the specifications of a ground improvement body for seismic reinforcement of a pile foundation according to claim 1 or 2, comprising:

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