Multi-cable support structure and construction method for arc-shaped deep foundation pits in old city renovation

Abstract

This invention provides a multi-cable support structure and construction method for arc-shaped deep foundation pits in urban renewal projects. The construction method is applicable to deep foundation pits with a depth of 5-8m and strict surrounding environmental conditions. The support structure includes support piles, a capping beam, cables, and a rear row of precast piles. The support piles are arranged in an arc shape, leaving a wide trench. Multiple sets of cables are symmetrically arranged about the support pile axes, passing through the capping beam and anchored at both ends to the sides of the capping beam, forming a multi-cable system. The rear row of precast piles is spaced out on the outside of the front row of support piles. This invention utilizes the increased spatial stiffness effect of multiple cables, combined with the force principle and displacement distribution law of an "arch" structure, and uses rear-mounted piles at the "arch foot" nodes to enhance node stiffness, strictly limiting the displacement at the top of the pit. This solves the shortcomings of conventional pile-anchor structures, which are limited by conflicts with the surrounding environment, and single cantilever pile schemes, which suffer from insufficient displacement control and high costs, ensuring the safety of the foundation pit and its surrounding environment.

Description

Technical Field

[0001] This invention relates to the field of building foundation pit support, specifically to a multi-cable support structure and construction method for arc-shaped deep foundation pits in old city renovation. Technical Background

[0002] As cities develop and construction become more mature, urban space utilization is increasing. Renovation and construction are becoming more frequent in urban centers and old urban areas where land resources are scarce. To fully utilize underground space, basement excavation often results in deep, large foundation pits, with existing roads, municipal pipelines, and other structures adjacent to the site. Especially in old urban redevelopment, the constraints of surrounding residential buildings, roads, and other structures often lead to more complex site boundaries and planning, resulting in increasingly irregularly shaped basements, even curved ones. Furthermore, the proximity of existing building foundations or road utilities restricts the use of anchor cables. In such cases, even slight mishaps during basement excavation and support can lead to slope displacement, cracks, or even pit collapse, posing significant risks to pit safety, the lives of nearby residents, and public safety. Therefore, stricter and more stringent requirements are placed on the safety and economy of foundation pit support schemes.

[0003] Existing support schemes for deep, arc-shaped foundation pits in urban redevelopment projects are limited in large-scale application due to strict environmental protection requirements. Conventional pile-anchor support schemes cannot avoid the foundations of buildings or other underground obstacles adjacent to the pit top. Currently, commonly used support schemes are generally double-row pile support or large-diameter single-row pile support. The former is limited in layout due to the generally narrow usable space between the basement and the land boundary line, and it is also more expensive. The latter is more commonly used, but to increase support rigidity, the pile diameter usually reaches more than 1m. In soft strata and deeper foundation pits, the pile diameter can reach 1.2m or even larger. However, single cantilever pile support schemes often cannot effectively control the displacement at the top of the slope, and the buildings, roads, pipelines, etc., adjacent to the top of the foundation pit are very sensitive to displacement. Therefore, this scheme is prone to safety hazards during the implementation of foundation pit excavation and support.

[0004] Currently, the question of which support method to adopt for deep arc-shaped foundation pits used in urban renewal projects, while ensuring the safety and economy of the solution, has become a research topic worthy of study. Summary of the Invention

[0005] This invention addresses the aforementioned shortcomings and deficiencies of existing technologies by providing a multi-cable support structure and construction method for arc-shaped deep foundation pits in urban renewal projects. This method utilizes the force principle and displacement distribution law of the "arch" structure, combined with the stiffness enhancement effect of the cables, and uses post-tensioned piles at the "arch foot" nodes to enhance node stiffness, strictly limiting the displacement of the pit top. It can solve the defects of conventional pile-anchor structures being limited by conflicts with the surrounding environment, and the shortcomings of single cantilever pile schemes having insufficient displacement control and high costs. It ensures the safety of the foundation pit and the surrounding environment, and can significantly reduce project costs and lower construction costs.

[0006] To achieve the above objectives, this invention provides a multi-cable support structure for arc-shaped deep foundation pits in urban renewal projects. This support structure is suitable for arc-shaped deep foundation pits with depths of 5-8m. The structure includes support piles, a capping beam, cables, and a rear row of precast piles. The central axis of the support piles is arranged in an arc shape, parallel to the outer edge of the arc-shaped basement, with a pre-reserved groove width between the support piles and the outer edge of the arc-shaped basement. The capping beam connects the tops of multiple arc-shaped support piles. Multiple cables are provided, arranged on the inner arc surface of the arc-shaped support piles, and symmetrically intersecting about the center line of the arc-shaped support pile segments. Adjacent areas of adjacent cables intersect. Both ends of each cable pass through the capping beam and are anchored to the outer edge of the capping beam, forming multiple cable systems. The rear row of precast piles is spaced apart on the outer arc surface of the arc-shaped support piles, and the capping beam at the top of each rear row of precast piles is connected to the capping beam at the top of the front row of support piles.

[0007] The preferred technical solution of the present invention is as follows: the support piles are reinforced concrete cast-in-place piles, and the spacing between adjacent piles along the arc axis is 1.8 to 2.2 times the pile diameter; the rear row of precast piles are square or round piles, and the pile length and pile diameter are smaller than those of the front row of support piles; the capping beam has an enlarged portion at the position corresponding to each rear row of precast piles, and the enlarged portion connects each rear row of precast piles with the front row of support piles to form multiple reinforcing nodes.

[0008] The preferred technical solution of this invention is that the length of the capping beam segment anchored by each cable is 9 to 15 times the pile diameter.

[0009] The preferred technical solution of the present invention is as follows: the rear row of precast piles are set as inclined piles, with the pile axis inclined at 5 to 15° to the outside of the foundation pit, and the pile body of the rear row of precast piles avoids the foundation of adjacent buildings and does not exceed the construction boundary.

[0010] To achieve the above technical objectives, the present invention also provides a construction method for a multi-cable support structure for an arc-shaped deep foundation pit in old city renovation, characterized in that the construction method includes the following construction steps: S1, determining the position of the cables, the cables are arranged from both ends of the arc segment of the foundation pit support in the inner arc of the arc segment, and are arranged symmetrically about the midline of the arc segment, and the length of the capping beam segment anchored by each cable is 9 to 15 times the pile diameter;

[0011] S2. Draw a plan view of the cable structure based on the cable layout, and number the cables from 1 to N, where the numbers are odd and N = 2m + 1. Determine the length of each cable as L1 to L2. N And set according to symmetry: L i =L j Where i+j=m+2 or i+j=3m+3, m=(N-1) / 2, the axial preload T of each cable is determined by analysis and calculation. 1预 ~T N预 ;

[0012] S3. Measure and lay out the design plane position of the support piles on site, and construct the rear row of precast piles;

[0013] S4. Measure and lay out the planar curvature, position, and spacing on-site, and construct the first row of cast-in-place piles;

[0014] S5. Construct the capping beam and connecting beam, and reserve cable ducts;

[0015] S6. After the capping beam and connecting beams reach their design strength, install the cables according to the positions determined in step S1. The end of each cable passes through the pre-reserved cable channel in the capping beam and is anchored to the outer side of the capping beam. The prestressing process of the anchor cables is carried out symmetrically and in stages, from both ends to the center.

[0016] S7. After the prestressing of the cables is completed and tested to meet the design value, the foundation pit excavation and basement construction will proceed.

[0017] A further technical solution of the present invention: The calculation process of cable prestress in step S2 is as follows:

[0018] (1) Measure and verify the radius R and arc β of the arc segment of the foundation pit support; and determine the corresponding arc angles β1 to β1 of cables numbered 1 to N according to the cable numbers. N The angle between cables numbered 1 to N and the horizontal plane The arch height f and span length l of the unit arc segment anchored by the cable, and the height-to-span ratio ρ = f / l;

[0019] (2) Take the arch isolation body corresponding to the numbered first cable as the basic analysis unit; its earth pressure load is q, and the horizontal and vertical concentrated forces at the top of the arch are determined by the internal force T of the (m+2)th cable. m+2 Caused by, respectively, the horizontal concentrated force p 1(m+2)x Vertical concentrated force p 1(m+2)y And from the force analysis, we can see that:

[0020]

[0021]

[0022]

[0023] Among them, the angle θ between the central load and the central axis is 0. q: Earth pressure distributed along the arc segment of the foundation pit support; q1: Longitudinal component of the uniformly distributed force along the arc segment of the foundation pit support, q1 is parallel to the central axis of the entire arc segment;

[0024] By referring to Table 6-3 of the "Handbook of Static Calculation of Building Structures (Second Edition)" and substituting θ = 0, the axial forces in each direction of the numbered first cable are obtained as follows:

[0025] Horizontal force P 1x Axial force caused

[0026] Vertical force P 1y The resulting axial force Z 1(m+2)y =A2P 1(m+2)y K2

[0027] Axial force caused by uniformly distributed force q1 By applying the principle of linear superposition, the total axial force T1 of the first cable can be calculated as follows:

[0028] T1 = Z 1(m+2)x +Z 1(m+2)y +Z 1q ④

[0029] T1 and T were established m+2 The relationship between q

[0030] The calculation process for A1, A2, K1, and K2 in the above formula is as follows:

[0031]

[0032]

[0033]

[0034]

[0035] Φ1=(3-8ρ 2 +48ρ 4 )α-12ρ(1-4ρ 2 )

[0036] Φ2=(1+4ρ 2 )α 2 +4ρ(1-4ρ 2 )α-32ρ 2

[0037] In the above formula: E: elastic modulus of the arch material; E1: elastic modulus of the tie rod material;

[0038] A s : Cross-sectional area of ​​the arch material; A s1 : Cross-sectional area of ​​the tie rod material;

[0039] I: Moment of inertia of the arch section; K1, K2: Axial force deformation correction coefficients;

[0040] f: Arch height of the unit arc segment anchored by the cable; L1: Length of the first cable (numbered 1);

[0041] n1, n2: constants, where

[0042]

[0043]

[0044] The values ​​of n1 and n2 can be initially seen in Table 6-4 of the "Handbook of Static Calculation of Building Structures (Second Edition)";

[0045] (3) Integrate and restore the N basic analysis units of the cables into the arc-shaped foundation pit:

[0046]

[0047] in

[0048]

[0049]

[0050] In the above formula: Let m be the angle between the (m+2)th cable and the horizontal plane, where m = (N-1) / 2; when m is an even number:

[0051]

[0052] When m is odd:

[0053]

[0054] In the formula:

[0055]

[0056]

[0057]

[0058]

[0059]

[0060] Due to symmetry, we obtain:

[0061] L i =L j (Where i+j=m+2 or i+j=3m+3) ⑧

[0062] Solving equations ⑥, ⑦, and ⑧ simultaneously, we obtain the axial forces of cables numbered 1 to N as T1 to T... N And it was finally determined that the total axial force of each cable is proportional to the earth pressure q distributed along the arc segment of the foundation pit support, and the earth pressure is the direct factor causing the axial force of the cable;

[0063] (4) Establish the relationship between the internal force and deflection of the cables, and analyze the effect of cable prestress on reducing the displacement of the foundation pit support structure; according to the existing formula, the relationship between the displacement Δ at the top of the pit and the earth pressure q distributed along the arc segment of the foundation pit support is as follows:

[0064]

[0065] Therefore, Δ∝q, and the beam deflection, i.e., displacement, is proportional to the intensity of the uniformly distributed load.

[0066] It can be seen that the reduction rate ω of the displacement of the arc-shaped foundation pit by each cable is...

[0067]

[0068] At this point, Δ0 represents the displacement of the foundation pit caused by the partial offset of the external earth pressure generated by the prestressing of the cables; Δ max The displacement of the foundation pit caused by external earth pressure when the cables are not prestressed;

[0069] Therefore, the prestress values ​​of cables numbered 1 to N are respectively T 1预 ~T N预 ;

[0070] Among them, the theoretical value of prestress T for each cable n预 for:

[0071] T n预 =ωT n

[0072] Where n = 1 to N.

[0073] The preferred technical solution of the present invention is as follows: In step S1, the designed arched section of the foundation pit support is determined based on the location of the arc segment of the basement and the section of the anchor cable restricted by the surrounding environment of the foundation pit. The construction pile positions of the front row of cast-in-place piles are preset along the arched section of the foundation pit support, and the spacing between adjacent piles along the arc axis is 1.8 to 2.2 times the pile diameter. The pile diameter and pile spacing of the support piles can be preset based on the stress analysis calculation and engineering experience in the existing support design scheme.

[0074] The preferred technical solution of the present invention is as follows: In step S6, the excavation of the foundation pit shall follow the principle of “segmented, layered, and block symmetrical excavation, and over-excavation is strictly prohibited”. During the excavation of the foundation pit, the pit wall shall be sealed with wire mesh and shotcrete or steel mesh in a timely manner according to the design requirements, and wire mesh and soil nails shall be installed between the piles according to the design spacing.

[0075] The preferred technical solution of this invention is as follows: In step S6, after the cap beam and connecting beam reach their design strength, the cables are set and installed. According to the above analysis and design parameters, the prestressing of the cables needs to be applied symmetrically and in stages from both ends to the center. The first tensioning cycle follows i+j=m+2, with cable numbers (1, m+1)→(2, m)→...; the second tensioning cycle follows i+j=3m+3, and after the last group of numbers in the first tensioning cycle, the number is increased by 1, continuing with cable numbers (i, j)→(i+1, 3m+2-i)→...; until the cable in the middle of the last arc segment is i=j, i+j=m+2 or i+j=3m+3.

[0076] The preferred technical solution of the present invention is as follows: In step S7, from the excavation of the foundation pit to the completion of the basement construction and backfilling to the ±0 elevation, the displacement, settlement, deep displacement and internal forces of the foundation pit top should be continuously monitored. When the prestress of the cable is detected to be reduced, it should be re-tensioned in time to ensure that the cable can play its normal and effective role.

[0077] Compared with the prior art, the present invention has the following beneficial effects:

[0078] (1) The planar layout of the support structure of the present invention actively adapts to the irregular shape of the basement or even the curved shape of the basement, and does not use underground support structures such as anchor cables other than piles, thus not occupying the space around the foundation pit, and avoiding the damage to the foundation of the surrounding buildings and the pollution of the underground environment caused by the use of anchor cable structures.

[0079] (2) This invention utilizes the spatial stiffness increase effect of multiple cables, combined with the force principle and displacement distribution law of the "arch" structure, to apply passive resistance in advance, which can withstand more soil pressure. Furthermore, it utilizes the post-tensioned piles at the "arch foot" node to enhance the node stiffness, which can strictly limit the displacement of the pit top, avoid the risk of insufficient stiffness of the cantilever pile leading to inadequate control of the slope top displacement, and ensure the safe operation of surrounding roads, pipelines and buildings during the construction of the foundation pit.

[0080] (3) The rear tie piles at the "arch foot" node of this invention are creatively designed in the form of inclined piles. The rear row of inclined piles, together with the front row of upright piles and the connecting beam, form a "stool" model with a certain angle, which has better stability and stronger resistance to deformation.

[0081] (4) This invention utilizes the spatial stiffness increase effect of multiple cables, combined with the force principle and displacement distribution law of the "arch" structure, and uses post-tensioned piles at the "arch foot" node to enhance the node stiffness. Compared with the traditional pile anchor scheme or cantilever pile scheme, the spacing of the support piles can be increased, and no anchor cables are used at all, which greatly reduces the project cost, shortens the construction period, improves economic benefits, and reflects low-carbon economy and green environmental protection.

[0082] This invention utilizes the spatial stiffness enhancement effect of multiple cables, combined with the force principle and displacement distribution law of the "arch" structure, and uses post-tensioned piles at the "arch foot" node to enhance the node stiffness, strictly limiting the displacement of the pit top. It solves the defects of conventional pile anchor structure which is limited by conflict with the surrounding environment, and single cantilever pile scheme which has insufficient displacement control and high cost. It ensures the safety of the foundation pit and the surrounding environment, and can significantly reduce engineering costs and construction costs. Attached Figure Description

[0083] Figure 1 This is a schematic diagram of the arc-shaped deep foundation pit multi-cable support structure of the present invention;

[0084] Figure 2 This is a schematic cross-sectional view of the present invention;

[0085] Figure 3 This is a schematic cross-sectional view of the present invention.

[0086] Figure 4 This is a detailed drawing of node A of the present invention;

[0087] Figure 5 This is a model diagram of the arc-shaped deep foundation pit multi-cable support structure of the present invention;

[0088] Figure 6 This is a structural model diagram of a single arc segment unit of the support structure of the present invention;

[0089] Figure 7 This is a schematic diagram of the load on the 01 arc segment (cable) unit of the support structure of the present invention;

[0090] Figure 8 This is a detailed assembly and positioning diagram of the arc-shaped deep foundation pit multi-cable support structure of the present invention.

[0091] In the diagram, 1—reinforced concrete support pile, 2—capping beam, 2-1—enlarged capping beam, 3—rear row of precast piles, 4—cable, 5—basement exterior wall line, 6—construction boundary line, 7—bottom of foundation pit, 8—top of foundation pit, 9—foundation of adjacent building or underground obstacle, 10—center line of support pile segment. Detailed Implementation

[0092] The present invention will be further described below with reference to the accompanying drawings and embodiments. Figures 1 to 8All accompanying drawings are simplified versions of embodiments and are intended only to clearly and concisely illustrate the embodiments of the present invention. The technical solutions shown in the drawings below are specific solutions of embodiments of the present invention and are not intended to limit the scope of the claimed invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0093] In the description of this invention, it should be understood that the terms "upper," "lower," "inner," "outer," "left," and "right," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship commonly used when the product of this invention is in use, or the orientation or positional relationship commonly understood by those skilled in the art. They are used only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this invention. In addition, the terms "first," "second," etc., are used only to distinguish descriptions and should not be construed as indicating or implying relative importance.

[0094] The present invention will be further described below with reference to the accompanying drawings and embodiments; for example Figures 1-3 The diagram illustrates a multi-cable support structure and construction method for arc-shaped deep foundation pits in urban renewal projects. It is suitable for deep foundation pits with ordinary geological conditions and a depth of 5-8m, where surrounding environmental conditions are stringent. The support structure includes support piles 1, a capping beam 2, cables 4, and a row of precast piles 3. The central axis of the support piles 1 is arranged in an arc shape and parallel to the outer edge 5 of the arc-shaped basement. A groove width is reserved between the support piles 1 and the outer edge 5 of the arc-shaped basement. The capping beam 2 connects the tops of multiple arc-shaped support piles 1 into a single unit. Multiple cables 4 are provided. The cable 4 is arranged on the inner arc surface of the arc-shaped support piles 1, and is symmetrically arranged about the center line of the arc-shaped support pile segment. The adjacent areas of two adjacent cables 4 intersect. The two ends of each cable 4 pass through the capping beam 2 and are anchored to the outer side of the capping beam 2, forming a system of multiple cable 4 sets. The rear row of precast piles 3 are arranged at intervals on the outer arc surface of the arc-shaped support piles 1. The capping beam at the top of each rear row of precast piles 3 is connected to the capping beam at the top of the front row of support piles 1. The rear row of precast piles 3 are arranged at intervals on the outer side of the front row of support piles 1, forming multiple "reinforcing nodes". The support piles 1 are reinforced concrete cast-in-place piles, and the spacing between adjacent piles along the axis is about 1.8 to 2.2 times the pile diameter. The length of the capping beam 2 segment to which the cable 4 is anchored is about 9 to 15 times the pile diameter. The rear row of precast piles 3 can be square or round piles, with a pile length shorter than the front row of support piles 1 and a pile diameter smaller than the front row of support piles 1; the rear row of precast piles 3 are set as inclined piles, with the pile axis inclined at 5 to 15° to the outside of the foundation pit, and the pile body should avoid the foundation 9 of the adjacent building and not exceed the red line 6.

[0095] This example demonstrates the implementation of a construction foundation pit for an old city renovation project. The project overview is as follows: the ±0.000 level is 37.800m, with a pit depth of approximately 6.5–7.5m. The project includes a single basement level, with the basement foundation slab elevation at the excavation edge ranging from -5.6 to -6.6m. The basement perimeter wall has an 800mm hidden beam and a 100mm thick cushion layer. The main strata within the pit wall area are: plain fill, silty clay, clay, red clay, and limestone. The main groundwater in the area is perched water. The basement boundary line 5 on the south side of the site is designed as a relatively regular arc shape. The distance between the basement foundation edge and the land boundary line 6 is approximately 3.5m, resulting in very limited usable space for excavation and support. The slope top is adjacent to surrounding residential roads and pipelines, and in some sections, it is adjacent to transformers and other structures. The total length of this support section is approximately 70m, and the excavation depth is approximately 7.0m.

[0096] Due to the strict protection of the surrounding environment of the foundation pit, conventional pile-anchor support schemes cannot avoid the foundations of buildings or other underground obstacles adjacent to the pit top, thus limiting their large-scale application. Currently, commonly used support schemes are generally double-row pile support or large-diameter single-row pile support. The former is limited in terms of layout due to the generally narrow usable space between the basement and the land boundary line, and it is also more expensive. The latter is more commonly used, but to increase support stiffness, the pile diameter usually reaches over 1m, and a single cantilever pile support scheme often cannot effectively control the displacement at the top of the slope. Furthermore, buildings, roads, pipelines, etc., adjacent to the pit top are highly sensitive to displacement, making this scheme prone to safety hazards during foundation pit excavation and support implementation. Therefore, for support schemes for arc-shaped deep foundation pits used in urban renewal projects, determining the appropriate support form while ensuring the safety and economy of the scheme has become a worthy research issue.

[0097] For the above embodiments, the designers of this project proposed two foundation pit support schemes, namely: (1) a large-diameter, closely spaced cantilever pile support scheme (C30 concrete piles with a diameter of 1.2m, a pile spacing of 1.5m, and a pile length of approximately 16m); and (2) a common-diameter, sparsely spaced cantilever pile + multiple prestressed anchor support scheme (C30 concrete piles with a diameter of 0.8m, a pile spacing of 1.6m, and a pile length of approximately 13m). Comparing the foundation pit stability, displacement control effect, and economy of the schemes, the common-diameter, sparsely spaced cantilever pile + multiple prestressed anchor support scheme of this patent was finally selected as the foundation pit support construction scheme.

[0098] The principle and steps for determining the prestress of the cables in this embodiment are as follows:

[0099] Step 1: Measure and verify the geometric characteristics of the arc-shaped foundation pit, with an arc radius R = 40m and a total arc β0 = 108°;

[0100] Step 2: Design the cable structure, create plan and section drawings, number the cables 01 to 05, N=5, m=2, determine the length of each cable L1 to L5, and set the cable lengths symmetrically: L1=L3=20m, L4=L5=26m, L2=25m. The axial preload of each cable T1 to T5 needs to be analyzed and calculated.

[0101] Step 3: After completing the above parameter design, the basic parameters of the support structure can be obtained:

[0102] Each cable corresponds to an arc angle β1 to β5, where β1 = β3 = 29.0°, β4 = β5 = 38.0°, and β2 = 36°.

[0103] The angle between each cable and the horizontal plane

[0104] The height-to-span ratio ρ = f / l is 0.077.

[0105] Step 4: Take the isolated arch corresponding to the 01st cable as the basic analysis unit; its earth pressure load is taken as q, and the horizontal and vertical concentrated forces at the top of the arch are caused by the internal force T4 of the 04th cable, respectively, p 1x (Horizontal Concentration Force), p 1y (Vertical concentrated force)

[0106] From the force analysis, we can see that: (1)

[0107] (2)

[0108] (3)

[0109] The angle θ between the central load and the central axis is 0.

[0110] Referring to Table 6-3 of the "Handbook of Static Calculation of Building Structures", substituting θ = 0, the axial forces of the cables are as follows:

[0111] (Horizontal force P) 1x cause)

[0112] Z2 = Z 1y =A2P 1y K2 (vertical force P) 1y cause)

[0113] (caused by uniformly distributed force q)

[0114] In the above formula, A1, A2, K1, and K2 all have well-established formulas for calculation.

[0115] By applying the principle of linear superposition, the total axial force of cable 01 is:

[0116] T1 = Z1 + Z2 + Z3

[0117] This formula establishes the relationship between T1, T4, and q.

[0118] Note: Parameters in the formula

[0119]

[0120]

[0121]

[0122]

[0123] Φ1=(3-8ρ 2 +48ρ 4 )α-12ρ(1-4ρ 2 )

[0124] Φ2=(1+4ρ 2 )α 2 +4ρ(1-4ρ 2 )α-32ρ 2

[0125] In the formula, E: elastic modulus of the arch material; E1: elastic modulus of the tie rod material.

[0126] A s : Cross-sectional area of ​​the arch material; A s1 : Cross-sectional area of ​​tie rod material

[0127] I: Moment of inertia of the arch section; K1, K2: Axial force deformation correction factors

[0128] n1, n2: constants; formulas are shown on the following page.

[0129] constant:

[0130]

[0131]

[0132] The values ​​of n1 and n2 can be seen in Table 6-4 of the manual;

[0133] Earth pressure and surrounding loads are converted to uniformly distributed loads, taking q1 = 150kN;

[0134] Basic parameters of the arched cap beam: Elastic modulus E = 3 × 104 MPa; Cross-sectional area A s =1.2m 2 Moment of inertia I = 1.2m 4 ;

[0135] Basic parameters of the cable material: Elastic modulus E1 = 19.5 × 10⁻⁶ 4 MPa; Cross-sectional area A s1 =840mm 2 ;

[0136] The parameters Φ1, Φ2, K1, K2, n1, n2, ε, ξ, etc. can all be calculated using the aforementioned formulas.

[0137] Step 5: Integrate and restore the 5 basic cable analysis units into the arc-shaped foundation pit;

[0138] Depend on

[0139]

[0140] in

[0141]

[0142]

[0143] And so on:

[0144] T2=ε2(T4+T5)+ξ2q ②

[0145] T3=ε3T5+ξ3q ③

[0146] T4=ε 41 T1+ε 42 T2+ξ4q ④

[0147] T5 = ε 51 T2+ε 52 T3+ξ5q ⑤

[0148] Due to symmetry:

[0149] T1 = T3 ⑥

[0150] Solve the simultaneous equations ①②③④⑤⑥ in step 5 above to obtain...

[0151] and analogy equations

[0152] We obtain T1 = T3 = 249.8 kN

[0153] T4 = T5 = 185.3 kN

[0154] T2 = 39.5 kN

[0155] It was ultimately determined that the total axial force T of each cable is proportional to the earth pressure q outside the pit, and the earth pressure is the direct factor causing the axial force of the cable.

[0156] Step 6: Establish the relationship between cable internal forces and deflection, and analyze the effect of cable prestressing on reducing the displacement of the foundation pit support structure; according to existing formulas, the relationship between the pit top displacement Δ and the external earth pressure q is as follows:

[0157] Therefore, Δ∝q, and the beam deflection (displacement) is proportional to the intensity of the uniformly distributed load.

[0158] It can be seen that the reduction ratio ω of the cable to the displacement of the arc-shaped foundation pit is

[0159]

[0160] In the formula: Δ0 is the displacement of the foundation pit that is offset by the portion of the external earth pressure generated by the prestressing of the cables; Δ max The amount of displacement of the foundation pit caused by external earth pressure when the cables are not prestressed.

[0161] At this time, the theoretical prestress value T of cables numbered 1 to N is... 1预 ~T 5预 ;

[0162] The prestresses of cables numbered 1 to N are as follows:

[0163] T 1预 =ωT1;T 2预 =ωT2;T 3预 =ωT3;

[0164] T 4预 =ωT4;T 5预 =ωT5

[0165] Based on software analysis and theoretical calculations, the maximum displacement of the foundation pit support structure without prestressed cables was determined to be approximately 90 mm. The foundation pit's safety level is Level 1, requiring the maximum displacement to be controlled within 30 mm. This necessitates prestressing the cables with at least approximately 2 / 3 of their axial force. Therefore, the prestressing values ​​for the cables were determined as follows:

[0166] T 1预 =T 3预 =180.0kN

[0167] T 4预 =T 5预 =135kN

[0168] T 2预 =75.0kN.

[0169] The specific construction steps after determining the prestressing of the cables according to this patented solution are as follows:

[0170] S1. Determine the location of the cables. The cables are arranged from both ends of the arched section of the foundation pit support on the inner arc of the arched section, and are symmetrically arranged about the midline of the arched section. The length of the capping beam section to which each cable is anchored is 9 to 15 times the pile diameter.

[0171] S2: Draw a plan view of the cable structure based on the cable layout, and number the cables 1 to 5; the prestress of the five cables are as follows: T 1预 =T 3预 =180.0kN; T 4预 =T 5预 =135kN; T 2预

[0172] =75.0kN.

[0173] S3. Measure and lay out the design plane position of the support piles on site, and construct the rear row of precast piles;

[0174] S4. Measure and lay out the planar curvature, position, and spacing on-site, and construct the first row of cast-in-place piles;

[0175] S5. Construct the capping beam and connecting beam, and reserve cable ducts;

[0176] S6. After the capping beam and connecting beams reach the design strength, install the cables according to the positions determined in step S1. The end of each cable passes through the reserved cable channel in the capping beam and is anchored to the outer side of the capping beam. Tension the cables according to the prestressing in step S2. The prestressing of the cables should be applied in the order of 1, 3→4, 5→2, and the principle of "from both ends to the middle, symmetrical tensioning, and graded tensioning" should be strictly followed.

[0177] S7. After the prestressing of the cables is completed and the test shows that the design value has been achieved, the foundation pit excavation and basement construction shall be carried out. During the foundation pit excavation, the principle of "segmented, layered, and block symmetrical excavation, and over-excavation is strictly prohibited" shall be followed. During the foundation pit excavation, the pit wall shall be sealed with wire mesh and shotcrete or steel mesh in a timely manner according to the design requirements. Wire mesh and soil nails shall be installed between the piles according to the design spacing.

[0178] S8. From the start of foundation pit construction to the completion of basement construction and backfilling to ±0 elevation, the entire process should continuously monitor indicators such as the displacement of the pit top, settlement, deep displacement, and internal forces of the structure. In particular, when the prestress of the cables is detected to be reduced, additional tensioning should be carried out in a timely manner to ensure that the cables function normally and effectively.

[0179] The structure and construction method of this invention achieved good support results. Long-term monitoring revealed that the displacement or deformation values ​​of the foundation pit in the implemented project were all within the allowable range of the design and specifications. Using a 70m long support section as an example, the arc-shaped deep foundation pit multi-cable support structure and construction method of this invention are compared with the traditional large-diameter, closely spaced cantilever pile support structure and construction method:

[0180] (1) In terms of major engineering quantities, the project adopts the traditional large-diameter, closely spaced cantilever pile support method, requiring approximately 50 support piles and approximately 905.0 m³ of C30 concrete. 3 The project utilizes the arc-shaped deep foundation pit multi-cable support structure and construction method of this invention, requiring approximately 45 support piles and about 294.5m³ of C30 concrete. 3 The total length of the cable is about 120m. Based on the calculation of 500 yuan per cubic meter of C30 concrete and 280 yuan per meter of anchor, the project using this invention will save at least 271,650 yuan in project cost compared to the traditional support method.

[0181] (2) In terms of construction period, since the implementation of the project adopts the present invention to reduce the number of piles and pile diameter compared with the traditional support method, it is easier to operate and allocate construction machinery, saving at least 20 days of construction period. Based on the average daily cost of 5,000 yuan / day for workers and construction machinery, the implementation of the project adopts the present invention to reduce the total cost of construction period by 100,000 yuan compared with the traditional support method.

[0182] (3) A comprehensive comparison shows that the implementation of the project using the present invention saves at least 371,650 yuan in project cost and more than 20 days in construction period compared with the traditional support method. The overall benefits of using the present invention are significantly improved.

[0183] This invention utilizes the increased spatial stiffness effect of multiple cables, combined with the force principle and displacement distribution law of an "arch" structure, to strictly limit the displacement at the top of the pit. It overcomes the limitations of conventional pile-anchor structures due to conflicts with the surrounding environment, and the shortcomings of single cantilever pile solutions, which suffer from insufficient displacement control and high costs. This ensures the safety of the foundation pit and its surrounding environment while significantly reducing engineering costs and construction expenses. It has strong promotional value for similar arc-shaped deep foundation pit support projects, and is particularly suitable for arc-shaped deep foundation pits with more complex surrounding environments, longer arc segments, and irregular arc segments, demonstrating greater practicality, economic efficiency, and social benefits.

[0184] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the structural relationships and principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A multi-cable support structure for arc-shaped deep foundation pits in old city renovation, characterized in that: The support structure is suitable for arc-shaped deep foundation pits with a depth of 5-8m. The support structure includes support piles (1), capping beams (2), cables (4), and rear row of precast piles (3). The axes of the support piles (1) are arranged in an arc shape and are parallel to the outer edge (5) of the arc-shaped basement. A groove width is reserved between the support piles (1) and the outer edge (5) of the arc-shaped basement. The capping beam (2) connects the tops of multiple arc-shaped support piles (1) into one unit. Multiple cables (4) are provided, and the multiple cables (4) are arranged on the inner arc surface of the arc-shaped support piles (1), and are symmetrically arranged about the dividing line (10) of the arc-shaped support pile segment. The adjacent areas of two adjacent cables (4) intersect. The two ends of each cable (4) pass through the cap beam (2) and are anchored to the outer side of the cap beam (2), forming a multi-set cable (4) system. The length of the cap beam (2) segment anchored by each cable (4) is 9 to 15 times the pile diameter. The rear row of precast piles (3) are arranged at intervals on the outer arc surface of the arc-shaped support piles (1). The top cap beam of each rear row of precast piles (3) is connected to the top cap beam of the front row of support piles (1). The cap beam (2) is provided with an enlarged part (2-1) at the position corresponding to each rear row of precast piles (3), and the enlarged part (2-1) connects each rear row of precast piles (3) to the front row of support piles (1) to form multiple reinforcing nodes. The spatial stiffness increase effect of multiple cables is utilized, combined with the force principle and displacement distribution law of the arch structure, and the stiffness of the nodes is enhanced by the back-pull piles at the arch foot nodes. The back-pull piles at the arch foot nodes are in the form of inclined piles. The rear row of precast piles (3) are set as inclined piles, with the pile axis inclined to the outside of the foundation pit at 5-15°. The pile body of the rear row of precast piles (3) avoids the adjacent building foundation (9) and does not exceed the construction red line (6).

2. The multi-cable support structure for arc-shaped deep foundation pits in old city renovation as described in claim 1, characterized in that: The support piles (1) are reinforced concrete cast-in-place piles, and the spacing between adjacent piles along the arc axis is 1.8 to 2.2 times the pile diameter; the rear row of precast piles (3) are square or round piles, and the pile length and pile diameter are smaller than those of the front row of support piles (1).

3. A construction method for the multi-cable support structure for arc-shaped deep foundation pits in old city renovation as described in claim 1, characterized in that, The construction method includes the following construction steps: S1. Determine the location of the cables. The cables are arranged from both ends of the arched section of the foundation pit support on the inner arc of the arched section, and are symmetrically arranged about the midline of the arched section. The length of the capping beam section to which each cable is anchored is 9 to 15 times the pile diameter. S2. Draw a plan view of the cable structure based on the cable layout, and number the cables from 1 to N, where the numbers are odd and N = 2m + 1. Determine the length of each cable as L1 to L2. N And set according to symmetry: L i =L j Where i+j=m+2 or i+j=3m+3, m=(N-1) / 2, the axial preload T of each cable is determined by analysis and calculation. 1预 ~T N预 ; S3. Measure and lay out the design plane position of the support piles on site, and construct the rear row of precast piles; S4. Measure and lay out the planar curvature, position, and spacing on-site, and construct the first row of cast-in-place piles; S5. Construct the capping beam and connecting beam, and reserve cable ducts; S6. After the capping beam and connecting beams reach their design strength, install the cables according to the positions determined in step S1. The end of each cable passes through the pre-reserved cable channel in the capping beam and is anchored to the outer surface of the capping beam. The cable prestressing process is carried out symmetrically and in stages, from both ends to the center. S7. After the prestressing of the cables is completed and tested to meet the design value, the foundation pit excavation and basement construction will proceed.

4. The construction method for a multi-cable support structure for an arc-shaped deep foundation pit in old city renovation according to claim 3, characterized in that... The calculation process for cable prestress in step S2 is as follows: (1) Measure and verify the radius R and arc β of the arc segment of the foundation pit support; and determine the corresponding arc angles β1 to β1 of cables numbered 1 to N according to the cable numbers. N The angle between cables numbered 1 to N and the horizontal plane The arch height f and span length l of the unit arc segment anchored by the cable, and the height-to-span ratio ρ = f / l; (2) Take the arch isolation body corresponding to the numbered first cable as the basic analysis unit; its earth pressure load is q, and the horizontal and vertical concentrated forces at the top of the arch are determined by the internal force T of the (m+2)th cable. m+2 Caused by, respectively, the horizontal concentrated force p 1(m+2)x Vertical concentrated force p 1(m+2)y And from the force analysis, we can see that: Among them, the angle θ between the central load and the central axis is 0. q: Earth pressure distributed along the arc segment of the foundation pit support; q1: Longitudinal component of the uniformly distributed force along the arc segment of the foundation pit support, q1 is parallel to the central axis of the entire arc segment; By referring to Table 6-3 of the "Handbook of Static Calculation of Building Structures" and substituting θ = 0, the axial forces in each direction of the numbered first cable are obtained as follows: Horizontal force P 1x Axial force caused Vertical force P 1y The resulting axial force Z 1(m+2)y =A2P 1(m+2)y K2 Axial force caused by uniformly distributed force q1 By applying the principle of linear superposition, the total axial force T1 of the first cable can be calculated as follows: T1=Z 1(m+2)x +Z 1(m+2)y +Z 1q ④ T1 and T were established m+2 The relationship between q The calculation process for A1, A2, K1, and K2 in the above formula is as follows: Φ1=(3-8ρ 2 +48p 4 )a-12p(1-4p 2 ) Φ2=(1+4ρ 2 )a+4p(1-4p 2 )a-32p 2 In the above formula: E: elastic modulus of the arch material; E1: elastic modulus of the tie rod material; A s : Cross-sectional area of ​​the arch material; A s1 : Cross-sectional area of ​​the tie rod material; I: Moment of inertia of the arch section; K1, K2: Axial force deformation correction coefficients; f: Arch height of the unit arc segment anchored by the cable; L1: Length of the first cable (numbered 1); n1, n2: constants, where The values ​​of n1 and n2 can be initially seen in Table 6-4 of the "Handbook of Static Calculation of Building Structures"; (3) Integrate and restore the N basic analysis units of the cables into the arc-shaped foundation pit: in In the above formula: Let m be the angle between the (m+2)th cable and the horizontal plane, where m = (N-1) / 2; When m is even: When m is odd: In the formula: Due to symmetry, we obtain: L i =L j Where i+j=m+2 or i+j=3m+3 ⑧ Solving equations ⑥, ⑦, and ⑧ simultaneously, we obtain the axial forces of cables numbered 1 to N as T1 to T... N And it was finally determined that the total axial force of each cable is proportional to the earth pressure q distributed along the arc segment of the foundation pit support, and the earth pressure is the direct factor causing the axial force of the cable; (4) Establish the relationship between the internal force and deflection of the cables, and analyze the effect of cable prestress on reducing the displacement of the foundation pit support structure; according to the existing formula, the relationship between the displacement Δ at the top of the pit and the earth pressure q distributed along the arc segment of the foundation pit support is as follows: Therefore, Δ∝q, and the beam deflection, i.e., displacement, is proportional to the intensity of the uniformly distributed load. It can be seen that the reduction rate ω of the displacement of the arc-shaped foundation pit by each cable is... At this point, Δ0 represents the displacement of the foundation pit caused by the partial offset of the external earth pressure generated by the prestressing of the cables; Δ max The displacement of the foundation pit caused by external earth pressure when the cables are not prestressed; Therefore, the prestress values ​​of cables numbered 1 to N are respectively T 1预 ~T N预 ; Among them, the theoretical value of prestress T for each cable n预 for: T n预 =ωT n Where n = 1 to N.

5. The construction method for a multi-cable support structure for an arc-shaped deep foundation pit in old city renovation according to claim 3, characterized in that: In step S1, the design of the foundation pit support arc segment is determined based on the location of the arc segment in the basement and the section where the cable is restricted by the surrounding environment of the foundation pit. The construction pile positions of the front row of cast-in-place piles are preset along the arc segment of the foundation pit support. The spacing between adjacent piles along the arc axis is 1.8 to 2.2 times the pile diameter. The pile diameter and pile spacing of the support piles can be preset based on the stress analysis calculation and engineering experience in the existing support design scheme.

6. The construction method for a multi-cable support structure for an arc-shaped deep foundation pit in old city renovation according to claim 3, characterized in that: In step S6, the excavation of the foundation pit shall follow the principle of "segmented, layered, and block symmetrical excavation, and over-excavation is strictly prohibited". During the excavation of the foundation pit, the pit wall shall be sealed with wire mesh and shotcrete or steel mesh in a timely manner according to the design requirements. Wire mesh and soil nails shall be installed between the piles at the design spacing.

7. The construction method for a multi-cable support structure for an arc-shaped deep foundation pit in old city renovation according to claim 3, characterized in that: In step S6, after the cap beam and connecting beam reach their design strength, the cables are set and installed. According to the above analysis and design parameters, the prestress of the cables should be applied symmetrically and in stages from both ends to the center. The first tensioning cycle follows i+j=m+2, with cable numbers (1, m+1)→(2, m)→…; the second tensioning cycle follows i+j=3m+3. After the last group of numbers in the first tensioning cycle is completed, the number is increased by 1, and the cable numbers (i, j)→(i+1, 3m+2-i)→… continue until the cable in the middle of the last arc segment is i=j, i+j=m+2 or i+j=3m+3.

8. The construction method for a multi-cable support structure for an arc-shaped deep foundation pit in old city renovation according to claim 3, characterized in that: In step S7, from the excavation of the foundation pit to the completion of the basement construction and backfilling to ±0 elevation, the displacement, settlement, deep displacement and internal forces of the foundation pit top should be continuously monitored. When the prestress of the cable is detected to be reduced, it should be re-tensioned in time to ensure that the cable can play its normal and effective role.

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