Supporting structure of pit in pit of elevator shaft in super deep foundation pit in soft soil area and construction method thereof

Abstract

This invention provides a support structure and construction method for an ultra-deep elevator shaft pit in soft soil areas. The support structure includes a pit-within-a-pit retaining structure using bored piles, capping beams with tie-down cables distributed along each edge of the elevator shaft pit, connecting strips at each inner corner of the retaining structure, and a truss between each capping beam tie-down cable and each edge of the retaining structure. Each long and short support axis of the retaining structure arches towards the outside of the elevator shaft pit, with a straight section in the middle. The capping beam tie-down cables are distributed along the long and short support axes before the arching of the retaining structure, and the truss is located in the straight section. This invention improves the overall bending resistance of the support system and effectively solves the problem of displacement control of the cantilever section of the elevator shaft pit support piles.

Description

Technical Field

[0001] This invention relates to the field of foundation pit support, specifically to a support structure for an elevator shaft pit in an ultra-deep foundation pit in soft soil areas and its construction method. Technical Background

[0002] In the support of ultra-deep foundation pits in soft soil areas, the soft soil layer has poor soil quality and exhibits the "three highs" characteristics of high water content, high compressibility, high rheology, and high creep, resulting in large deformation of the support structure of the elevator shaft pit. Therefore, it is necessary to consider taking necessary measures to strengthen the elevator shaft pit.

[0003] In existing engineering projects, the commonly used reinforcement schemes for controlling the displacement of the elevator shaft pit support structure are: ① using cast-in-place piles + internal bracing; ② using caissons + bracing; ③ using a semi-reverse construction method. In scheme ①, the internal bracing method significantly increases the construction period and also results in excessively high earthwork costs. In scheme ②, the caisson + bracing method increases construction measures costs and earthwork costs, while also increasing the difficulty of earthwork excavation, significantly extending the construction period and delaying progress. In scheme ③, the semi-reverse construction method significantly increases earthwork costs, significantly extending the construction period and delaying progress. Summary of the Invention

[0004] This invention addresses the shortcomings of existing technologies by providing a support structure and construction method for ultra-deep foundation pit elevator shafts in soft soil areas. This method controls the deformation and stability of the foundation pit through bored piles without adding internal supports. The bored piles are distributed in a parabolic arch shape, with a large-sized capping beam at the top. Prestress is applied to the parabolic arch capping beam using multiple anchor cables, fully utilizing the stress characteristics of the tie-rod-parabolic arch system to improve the bending resistance of the entire support system, control the displacement of the pile tops, and achieve overall stress balance in the foundation pit.

[0005] To address the aforementioned technical problems, this invention provides a support structure for ultra-deep elevator shaft pits in soft soil areas. It is suitable for elevator shaft pits with a regular quadrilateral shape and soft soil on the sidewalls and base. The depth of the elevator shaft pit is between 3.8 and 5.5 meters. The support structure comprises a pit-within-a-pit retaining structure formed around the elevator shaft pit using bored piles, capping beams with tie-down cables distributed along each edge of the elevator shaft pit, connecting plates at each inner corner of the pit-within-a-pit retaining structure, and trusses between each capping beam tie-down cable and each edge of the pit-within-a-pit retaining structure. Each long and short support axis of the pit-within-a-pit retaining structure arches towards the outside of the elevator shaft pit, and the arched long and short support axes... All sections are transformed from straight lines into parabolas with an arching degree of 3 / 100 to 6 / 100. The middle 1 / 4 to 1 / 3 of each parabola is set as a straight section, with the distance between this straight section and the support axis before arching being 0.5 to 2 meters. The cap beam tie-up anchors are distributed at the long and short support axes of the pit-in-pit retaining structure before arching. Both ends of each set of cap beam tie-up anchors are anchored to the cap beams of the bored piles at two adjacent corners of the pit-in-pit retaining structure. The truss is installed between the straight section of the pit-in-pit retaining structure after arching and the cap beam tie-up anchors. The connecting plate is installed between the long and short cap beams of the pit-in-pit retaining structure, with both sides overlapping and connecting to the long and short cap beams, with an overlap length of 1.6 to 2.0 meters.

[0006] The preferred technical solution of this invention is as follows: the long side and short side of the elevator pit are close in length; the camber of each side support axis of the pit-in-pit retaining structure is 3 / 100; the length of the straight section in the middle of the parabola after camber is 1 / 3 of the original pit side length; the truss is located in the middle of the straight section; two reinforced bored piles are added to the outside of each corner of the pit-in-pit retaining structure, and the two reinforced bored piles are located on the extension lines of the long side and short side of the retaining structure before camber; the two ends of the tie anchor cables of each set of cap beams are anchored to the top cap beams of the two reinforced bored piles on the same long side or the same short side extension line through anchors and anchor plates; the connecting strip is a triangular strip, and a 10-15mm wide and 20-25mm deep gap is chiseled at the overlapping part with the long side cap beam and the short side cap beam, and the gap is filled with asphalt hemp rope.

[0007] The preferred technical solution of this invention is as follows: the cap beam tie anchor cables are made of 1860MPa steel strands. Each set of cap beam tie anchor cables includes multiple anchor cables, each anchor cable consisting of at least four steel strands. The end of each anchor cable is tensioned and locked by an OVM anchor and an anchor plate. The end of each set of cap beam tie anchor cables passes through the cap beam of the pit-within-pit retaining structure. The portion of the cap beam tie anchor cable passing through the cap beam of the pit-within-pit retaining structure is separated from the cap beam by a pre-embedded corrugated pipe. The steel strand is covered with a plastic sheath for rust and corrosion protection; the capping beam tie anchor cables at the long side and the short side of the elevator shaft pit intersect in the capping beam of the bored pile at the corner of the pit-in-pit retaining structure, and the two sets of capping beam tie anchor cables are arranged symmetrically up and down; the stirrups are densely arranged in the area where the capping beam and the capping beam tie anchor cables overlap on each side of the pit-in-pit retaining structure, and spiral stirrups with a diameter of 12mm, a pitch of 30mm, and a length of 300mm are set at the anchor cable pre-reserved holes in the capping beam.

[0008] The preferred technical solution of the present invention is as follows: the pit-in-pit retaining structure (1) includes bored cast-in-place piles (100) and pile top capping beams (101). The spacing between two adjacent bored cast-in-place piles (100) on each side is 1.2 to 1.5 times the pile diameter, and the pile top capping beams (101) on each side are connected as a whole to form the capping beam of the pit-in-pit retaining structure. The long side capping beam (3) and the short side capping beam (4) of the pit-in-pit retaining structure (1) are constructed along the long side support axis and the short side support axis after arching, respectively.

[0009] To achieve the above-mentioned technical objectives, the present invention also provides a construction method for the support structure of the ultra-deep foundation pit elevator shaft in soft soil areas, characterized in that: the construction method is for the ultra-deep foundation pit elevator shaft in soft soil areas with a depth of 3.8 to 5.5 m and a certain thickness of peat soil layer distributed on the sidewalls and base, and the specific construction steps are as follows:

[0010] (1) Based on the geological survey, determine the surrounding environmental conditions, excavation depth and top load of the elevator shaft pit. The excavation depth of the elevator shaft pit is h, and it is excavated in two stages, with each excavation depth being h / 2. The short and long support axes of the pit-in-pit retaining structure are arched along the direction perpendicular to each side towards the outside of the design edge line of the elevator shaft pit. The arching degree is controlled between 3 / 100 and 6 / 100. The middle 1 / 4 to 1 / 3 of the parabola formed after arching of each side is set as a straight section, and the distance between the straight section and the support axis before arching is 0.5 to 2m.

[0011] (2) The soil pressure values ​​q1 and q2 at the bottom of the elevator shaft pit during the two excavations were calculated using the Lizheng Deep Foundation Pit software. Based on the soil pressure values ​​q1 and q2 at the bottom of the two excavations, the equivalent uniformly distributed loads Q1 and Q2 of the cap beam during the two excavations of the elevator shaft pit were calculated according to formulas ① and ② respectively.

[0012]

[0013]

[0014] In the formula, h1 and h2 are the depths (m) of the two excavations of the pit within the pit.

[0015] Q1, Q2 — Equivalent uniformly distributed load values ​​of the cap beam (kN / m) when the excavation depth of the pit-in-pit is h1, h2, respectively;

[0016] q1, q2 — Earth pressure values ​​q1, q2 (kN / m / m) at the bottom of the pit when the excavation depth of the pit-within-a-pit is h1, h2;

[0017] α—Equivalent uniformly distributed load factor for the cap beam, typically taken as 1.8;

[0018] (3) Based on the equivalent uniformly distributed loads Q1 and Q2 of the cap beam during the two excavations of the elevator pit in step (2), the calculation model of the tie rod double hinge arch is used to calculate the internal force values ​​P1 and P2 of the cap beam tie anchor cable of the long side and short side of the elevator pit during the two excavations, and distribute them evenly to each anchor cable.

[0019] (4) After determining the support axis of each side of the pit and the straight section in the middle in step (1), extract the coordinates, diameter, reinforcement and pile length information of each pile position from the design and construction drawings to form the support pile construction parameters.

[0020] (5) During the construction phase, according to the construction parameters extracted in step (4), after the foundation pit is excavated to the top elevation of the elevator pit, the support piles of the elevator pit retaining structure are constructed first; after the support piles of the elevator pit retaining structure reach the curing period, the pile top capping beam is constructed, and its excavation width does not exceed the width of the capping beam plus 200mm. The excavation elevation of the capping beam of the pit retaining structure is the same as the bottom elevation of the capping beam; within 5-6m of the end of the capping beam, the corrugated pipe is used to reserve anchor cable holes, and the corrugated pipe is fixed to the stirrups; determine the position of each foundation pit side truss, and then reserve the pressure-bearing interface component at the corresponding position of the straight section in the middle of the capping beam, and construct the corner connecting plate strip of the capping beam of the pit retaining structure simultaneously;

[0021] (6) After the cap beam of the pit-in-pit retaining structure in step (5) reaches the curing period, the completed anchor cables, anchors and anchor plates are put in, and the middle truss is constructed. The reserved interface at the connection between the truss and the cap beam is connected. Then, the cap beam is subjected to the first prestressing on the tie anchor cable. The applied value is P1 calculated in step (3). The prestressing of the cap beam tie anchor cable on the four sides of the elevator pit is applied synchronously. The cap beam tie anchor cable on each side is symmetrically subjected to prestressing to form a preliminary inverted bow-shaped support structure system.

[0022] (7) After completing step (6), the first layer of soil in the elevator pit is excavated to a depth of h / 2. The soil around the piles is excavated in the elevator pit using a long-arm excavator. The soil between the piles is sprayed with fine stone concrete and steel mesh is hung as it is excavated. During the excavation of the foundation pit, the vertical displacement constraint is applied to the support side through the connection between the truss and the cap beam to control the displacement of the middle part of the cap beam.

[0023] (8) After step (7) is completed, the prestress is applied to the anchor cable of the crown beam for the second time. The applied value is P2 calculated in step (3), and the application method is the same as in step (6).

[0024] (9) After step (8) is completed, the second layer of soil in the elevator shaft pit is excavated to the base elevation of the elevator shaft pit. The soil around the piles is excavated in the pit using a long-arm excavator. The soil between the piles is sprayed with fine stone concrete and steel mesh is hung as it is excavated. The basement floor slab of the pit is constructed immediately after the elevator shaft pit is excavated to the bottom.

[0025] The preferred technical solution of the present invention is as follows: In steps (4) to (9), the displacement of the support piles and the internal force of the anchor cables are monitored during the earthwork excavation and fed back to the construction of the pit-in-pit support and earthwork excavation; In step (2), the pile calculation module of the Lizheng deep foundation pit software is used to input the stratum parameters, the foundation pit excavation depth and the pile body parameters, and the earth pressure values ​​q1 and q2 at the bottom of the pit under the corresponding excavation conditions are calculated by using the Rankine earth pressure theory formula.

[0026] A further technical solution of the present invention: In step (3), the internal force values ​​P1 and P2 of the cap beam tie anchor cable of the long and short sides of the elevator shaft pit during the two excavations are calculated according to the "Handbook of Static Calculation of Building Structures" using a tie rod double hinged arch calculation model. Each arching side of the pit-in-pit retaining structure and the cap beam tie anchor cable are regarded as a whole tie rod double hinged arch. The specific calculation process is as follows:

[0027]

[0028]

[0029]

[0030]

[0031]

[0032]

[0033] In the formula:

[0034] K1 and K2 are the axial force deformation correction coefficients for the double-hinged arch with anchor cable tie rod of the cap beam when the excavation depth of the pit is h1 and h2, respectively.

[0035] H1 and H2 are the total internal forces (kN) of the anchor cables of each group of cap beams when the excavation depths of the pit-in-pit are h1 and h2, respectively.

[0036] P1 and P2 are the internal force values ​​(kN) of each anchor cable in each group of cap beam tie anchor cables when the excavation depth of the pit-in-pit is h1 and h2, respectively.

[0037] m—the number of anchor cables in each group of cap beam tie anchor cables, generally taken as 6 to 12;

[0038] l—The length (m) of the tie anchor cable of the cap beam on each side of the pit-within-a-pit retaining structure;

[0039] f—The arch height (m) of each arched side of the pit-within-a-pit retaining structure;

[0040] A c — Cross-sectional area of ​​the crown beam (m²) 2 );

[0041] n—Axial force deformation correction factor, usually taken as 1.67;

[0042] E—Elastic modulus of the crowning arch material (kN / m) 2 );

[0043] I c —Moment of inertia of the crown beam arch section (m) 4 );

[0044] E1—Elastic modulus of the capping beam for the anchor cable (kN / );

[0045] A1—Cross-sectional area of ​​the capping beam and anchor cables (m²) 2 ).

[0046] A further technical solution of the present invention: In step (5), the truss positioning is arranged according to the principle of minimizing the mid-span deflection of the capping beam. The distance between the two vertical sides of the truss is a1, and the distance from the left side of the truss to the midpoint of the capping beam is a2, where a1 = 4a2; then the truss can effectively control the deflection value of the middle part of the capping beam to be:

[0047]

[0048]

[0049]

[0050] In the formula, ν—deflection at the middle of the cap beam (m);

[0051] M x —Maximum bending moment at the center of the capping beam (kN / m);

[0052] P0—Vertical pressure value on both sides of the truss (kN);

[0053] a1—Horizontal distance between vertical members on both sides of the truss (m), generally taken as 1.5 to 2.0m;

[0054] a2—Horizontal distance (m) between the left-side member of the truss and the midpoint of the capping beam, generally taken as 0.0 to 1.0m;

[0055] δ—The vertical displacement increment (m) of the middle truss of the cap beam, not exceeding 1.0m.

[0056] The preferred technical solution of the present invention is as follows: the truss is a rectangular truss, the long side of which is 1 to 2 times the short side, and the long side of the truss is parallel to the anchor cable; when constructing the middle truss in step (6), first install the two vertical short sides of the truss, then install the long side of the truss parallel to the anchor cable, and finally install the two intersecting oblique sides. Each side is preferentially installed with the embedded parts that are connected to the reserved pressure-bearing interface.

[0057] The preferred technical solution of the present invention is as follows: In step (5), when constructing the capping beam of the elevator shaft pit, the stirrups are densely arranged within 500mm of its end, and spiral stirrups with a diameter of 12mm, a pitch of 30mm, and a length of 300mm are set at the anchor cable reserved hole; the connecting plate is set between the long side capping beam and the short side capping beam of the pit-in-pit retaining structure, the thickness of the connecting plate is 240-360mm, it is triangular, the overlap length of its long side capping beam and the short side capping beam is 1.6-2.0m, and a gap with a width of 10mm and a depth of 20mm is chiseled at the overlapping part of the connecting plate and the capping beam, and then filled with asphalt hemp rope.

[0058] In this invention, the displacement of the support piles and the internal force of the anchor cables are monitored during earthwork excavation, and feedback is provided to guide the construction of the pit-in-pit support and earthwork excavation. Based on the development of the displacement at the top of the support piles, displacement constraints are applied to the vertical direction of the truss by using the interface reserved at the connection between the truss and the cap beam, so as to further effectively control the displacement of the middle part of the cap beam and further improve the stability and safety of the support system during construction.

[0059] The elevator shaft pit support structure of this invention establishes a calculation model along the long or short side of the pit, within the plane containing the cross-section of the support pile cap beam, to theoretically analyze the internal force distribution of the support structure system. The established model is a tie-rod double-hinged arch, with the arch body composed of the cap beam cross-section, the tie rods composed of prestressed anchor cables, and the arch foot support composed of the cap beam within the range of three support piles at the end of the cap beam and a haunch triangular plate band. The external load it bears is the horizontal earth pressure at the depth of the cap beam.

[0060] The ultra-deep foundation pit elevator shaft in this invention is located in a soft soil area, with poor soil quality on the sidewalls and bottom. To ensure the progress of the main tower, the elevator shaft pit must be constructed first, while avoiding impacting the already completed main tower engineering piles. The commonly used support pile + internal bracing system is no longer sufficient to meet the construction schedule. This invention, without using internal bracing, rationally arranges the support piles and creatively adopts a tie-arch-truss "recurved bow" support system, simultaneously deploying support piles, anchor cables, and trusses. Through theoretical derivation and actual working condition analysis, a method for calculating the prestress value of the anchor cables applied twice is provided, thus addressing the issue of the support piles.

[0061] This invention addresses the issue of deep elevator shaft pits in soft soil areas. Without using internal supports, it utilizes the stress and deformation characteristics of the two-hinged arches of the tie rods. Through the rational arrangement of support piles and the use of anchor cables and trusses on the capping beams on each side of the elevator shaft pit, and by theoretical derivation based on the principle of linear superposition and practical working condition analysis, a method for calculating the prestress value applied twice by the anchor cables is provided. This improves the overall bending resistance of the support system, effectively solving the problem of displacement control in deep soft soil areas and ensuring the overall stress balance of the foundation pit. Practical engineering cases demonstrate the effectiveness and applicability of the proposed method, shortening the construction period, reducing project costs, and offering simple and convenient construction operations, making it highly valuable for widespread application. Attached Figure Description

[0062] Figure 1 This is a planar schematic diagram of the present invention;

[0063] Figure 2 This is a cross-sectional view of the pit-in-pit retaining structure and anchor cable of the present invention;

[0064] Figure 3 This invention provides an internal force analysis model for a double-hinged arch with a foundation pit tie rod.

[0065] Figure 4 This is a diagram of the internal force analysis and calculation model for the double-hinged arch with a foundation pit tie rod according to the present invention;

[0066] Figure 5 This invention provides an analysis model for earth pressure on the sidewalls of foundation pit excavation.

[0067] Figure 6 This invention provides an equivalent crown beam uniformly distributed earth pressure analysis model for foundation pit excavation.

[0068] Figure 7 This is the truss layout point analysis model of the present invention;

[0069] Figure 8 This is a diagram showing the arrangement of tie anchor cables for the long side cap beam in the pit-within-a-pit embodiment of the present invention;

[0070] Figure 9This is a diagram showing the arrangement of tie anchor cables for the short side cap beam in the pit-within-a-pit configuration according to an embodiment of the present invention.

[0071] Figure 10 This is a flowchart illustrating the implementation of the present invention.

[0072] In the figure: 1—Pit-within-pit retaining structure, 100—Drilled pile, 101—Pile top capping beam, 2—Capping beam tie anchor cable, 3—Long side capping beam, 4—Short side capping beam, 5—Connecting plate strip, 6—Reinforced drilled pile, 7—Anchor and anchor plate, 8—Elevator shaft pit-within-pit, 9—Main structure outline of elevator shaft pit-within-pit, 10—Truss. Detailed Implementation

[0073] The present invention will be further described below with reference to the accompanying drawings and embodiments. Figures 1 to 9 The accompanying drawings for the construction structure in the embodiments are simplified and are only used to clearly and concisely illustrate the structure of the embodiments of the present invention. Figure 9 This is a simplified diagram of the construction process in this invention. The technical solutions shown in the accompanying drawings are specific embodiments of this invention and are not intended to limit the scope of the claimed invention. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without inventive effort are within the scope of protection of this invention.

[0074] In the description of this invention, it should be understood that the terms "upper," "lower," "inner," "outer," "left," "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 only used to facilitate the description of this invention and to simplify the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0075] The embodiment provides a support structure for an ultra-deep elevator shaft pit in a soft soil area, such as... Figure 1 and Figure 2As shown, the elevator pit is rectangular with soft soil on its sidewalls and base, and its depth is between 3.8 and 5.5 meters. The elevator pit support structure includes the elevator pit 8, comprising a pit-in-pit retaining structure 1 formed around the elevator pit using bored piles, capping beam tie-up anchors 2 distributed along each edge of the elevator pit, connecting strips 5 installed at each inner corner of the pit-in-pit retaining structure 1, and the pit-in-pit retaining structure installed between each capping beam tie-up anchor 2 and each edge of the pit. The truss 10 between the 1st and 2nd trusses, the long side and short side of the pit within the elevator shaft are approximately the same length, the pit-within-a-pit retaining structure 1 includes bored piles 100 and pile cap beams 101, the spacing between two adjacent bored piles 100 on each side is 1.2 to 1.5 times the pile diameter, and the pile cap beams 101 on each side are connected as a whole to form the cap beam of the pit-within-a-pit retaining structure; the long side cap beam 3 and the short side cap beam 4 of the pit-within-a-pit retaining structure 1 are constructed along the long side support axis and the short side support axis after arching, respectively. Two reinforcing bored piles 6 are added to the outside of each corner of the pit-within-a-pit retaining structure 1, and the two reinforcing bored piles 4 are located on the extension lines of the long side and the short side before the pit retaining structure is arched, respectively, and the cap beams of the reinforcing bored piles 6 are also connected as a whole with the pile cap beams 101 of the pit retaining structure. The long and short support axes of the pit-in-pit retaining structure 1 are arched towards the outside of the elevator shaft pit. After arching, the long and short support axes change from straight lines to an arch of 3 / 100. A straight section is set in the middle of each parabola. The distance between the straight section and the support axis before arching is 0.5 to 2m. The length of the straight section 100 in the middle of each arched parabola is 1 / 3 of the original pit side length. The truss 10 is set between the straight section 100 of the pit-in-pit retaining structure 1 after arching and the tie anchor cable 2 of the cap beam. The truss 10 is located in the middle of the straight section 100. Through the connection between the truss and the cap beam, displacement constraints are applied in the vertical direction of the truss to effectively control the displacement of the middle part of the cap beam and further improve the stability and safety of the support system during construction. The truss is a rectangular truss, with its long side being 1 to 2 times the length of its short side, and the long side of the truss is parallel to the anchor cable; the connecting plate strip 5 is installed between the long side cap beam 3 and the short side cap beam 4 of the pit-in-pit retaining structure 1, with both sides of the connecting plate strip overlapping with the long side cap beam 3 and the short side cap beam 4 respectively, forming a single unit, with an overlap length of 1.6 to 2.0 m; the connecting plate strip 5 is a triangular plate strip, with a 10 to 15 mm wide and 20 to 25 mm deep gap chiseled at the overlapping part with the long side cap beam 3 and the short side cap beam 4, and the gap is filled with asphalt hemp rope.

[0076] The support structure for the elevator shaft pit in the ultra-deep foundation pit in the soft soil area in the embodiment is as follows: Figure 1 and Figure 2As shown, the cap beam tie anchor cables 2 are distributed at the long and short support axes before the arching of the pit-in-pit retaining structure 1. The two ends of each set of cap beam tie anchor cables 2 are anchored to the top cap beam of two reinforced bored piles 3 on the same long or short extension line via anchor plates and anchors 7. The cap beam tie anchor cables 2 are made of 1860MPa steel strand. Each set of cap beam tie anchor cables 2 includes multiple anchor cables, each consisting of at least four steel strands. The end of each anchor cable is tensioned and locked by OVM anchors and anchor plates. The end of each set of cap beam tie anchor cables 2 passes through the cap beam of the pit-in-pit retaining structure 1. The portion of the cap beam tie anchor cable 2 passing through the cap beam of the pit-in-pit retaining structure 1 is separated from the cap beam by a pre-embedded corrugated pipe, and the outer surface of the anchor cable steel strands... The elevator shaft is encased in a plastic sheath for rust and corrosion protection. The tie-bar anchors on the long and short sides of the elevator shaft pit intersect within the capping beam of the bored pile at the corner of the pit-in-pit retaining structure 1, with the two intersecting sets of tie-bar anchors arranged symmetrically vertically. In the overlapping area between the capping beam and the tie-bar anchor 2 on each side of the pit-in-pit retaining structure 1, the stirrups are densely arranged, and spiral stirrups with a diameter of 12mm, a pitch of 30mm, and a length of 300mm are installed at the pre-reserved holes for the anchors within the capping beam. The excavation of the elevator shaft pit is carried out in layers and sections, with concrete sprayed onto the soil between the piles as it is excavated.

[0077] The elevator shaft pit support structure of this invention applies the principle of linear superposition of structures. A calculation model is established along the long or short side of the pit within the pit, within the plane containing the cross-section of the support pile cap beam, to theoretically analyze the internal force distribution of the support structure system. The established model is a "recurve arch" tie-rod double-hinged arch. The arch body is composed of the cap beam cross-section, the tie rods are composed of prestressed anchor cables, and the arch foot support is composed of the cap beam and connecting plate strip within the range of three support piles at the end of the cap beam. The external load it bears is the horizontal earth pressure at the depth of the cap beam. In the calculation model of the "recurve arch" tie-rod double-hinged arch of this invention, it is considered that the arch axes on both sides are quadratic parabolas, and the middle section is a straight section, such as... Figure 3 , Figure 4 As shown, according to Table 6-9 and related sections of the "Handbook of Static Calculation of Building Structures", a double-hinged arch calculation model with tie rods is adopted for the long and short side capping beams of the pit-within-a-pit structure. Under the action of a uniformly distributed external load q, the internal force of the anchor cable tie rod is...

[0078]

[0079]

[0080] In the formula, H represents the internal force (kN) of the anchor cable tie rod in the two-hinged arch;

[0081] Q—Design value of uniformly distributed horizontal load on the crown beam arch (kN / m);

[0082] l—Length of the tie rod in the two hinged arches of the anchor cable tie rod (m);

[0083] f—Arch height (m) in the two-hinged arch of the cap beam anchor cable tie rod;

[0084] K—Axial force deformation correction coefficient of the two hinged arches of the cap beam anchor cable tie rod;

[0085] A c — Cross-sectional area of ​​the crown beam (m²) 2 );

[0086] n—Axial force deformation correction factor, usually taken as 1.67;

[0087] E—Elastic modulus of the crowning arch material (kN / m) 2 );

[0088] I c —Moment of inertia of the crown beam arch section (m) 4 );

[0089] E1—Elastic modulus of anchor cable tie rod material (kN / m) 2 );

[0090] A1—Cross-sectional area of ​​the anchor cable rod (m²) 2 );

[0091] In this invention, under the conditions of half-depth excavation and bottom excavation, the earth pressures on the excavation surface are uniformly distributed loads q1 and q2, respectively, and the equivalent uniformly distributed earth pressures on the capping beam are Q1 and Q2. A calculation model is established along the long or short side of the pit to theoretically analyze the equivalent uniformly distributed loads on the capping beam within the excavation surface of the support piles. The established calculation model is a cantilever beam model, where the beam body represents the excavation depth corresponding to the support piles, the beam supports represent the excavation surface and the lower, unexposed support piles, the external loads borne are the earth pressures distributed along the depth of the support piles, and the equivalent loads are the concentrated loads at the capping beam location, such as... Figure 5 , Figure 6 As shown. According to the load equivalence principle, taking the same moment at support O, the equivalent uniformly distributed load of the capping beam can be obtained. Then, in the next step of determining the first and second prestresses of the capping beam anchor cables, the internal forces P1 and P2 of each anchor cable tie rod are calculated according to the following formula:

[0092]

[0093]

[0094]

[0095]

[0096]

[0097]

[0098] In the formula, h1 and h2 are the excavation depths (m) of the two pits within the pit.

[0099] Q1, Q2 — Equivalent uniformly distributed load values ​​of the cap beam (kN / m) when the excavation depth of the pit-in-pit is h1, h2, respectively;

[0100] q1, q2 — Excavation surface load values ​​(kN / m / m) when the excavation depth of the pit-in-pit is h1, h2, respectively;

[0101] α—Equivalent uniformly distributed load factor for the cap beam, typically taken as 1.8;

[0102] K1, K2—Axial force deformation correction coefficients for the two hinged arches when the excavation depth of the pit-in-pit is h1, h2;

[0103] H1, H2 — Internal force values ​​(kN) of the tie rod in the two hinged arches of the tie rod when the excavation depth of the pit-in-pit is h1 and h2, respectively;

[0104] P1, P2 — Internal force values ​​(kN) of each anchor cable when the excavation depth of the pit-in-pit is h1, h2, respectively;

[0105] m—Number of anchor cables inside the cap beam, generally taken as 9 to 12.

[0106] Formulas ① and ② above can be used to calculate the internal force values ​​of the tie rods in a double-hinged arch under a uniformly distributed external load q. Formulas ③ and ④ above can be used to calculate the equivalent capping beam loads for the pit-within-a-pit excavation at half depth and the excavation to the bottom. Formulas ①, ②, ③, ④, ⑤, and ⑥ above can be used to calculate the overall internal force values ​​of the anchor cable tie rods for the pit-within-a-pit excavation at half depth and the excavation to the bottom. Combining these with formulas ⑦ and ⑧, the internal force values ​​of each capping beam on the anchor cables for the pit-within-a-pit excavation at half depth and the excavation to the bottom can be calculated.

[0107] The truss positioning in this invention is arranged according to the principle of minimizing the mid-span deflection of the cap beam, such as... Figure 7 The distance between the two vertical sides of the truss shown is a1, and the distance from the left side of the truss to the midpoint of the capping beam is a2, where a1 = 4a2; then the truss can effectively control the deflection value at the middle of the capping beam:

[0108]

[0109]

[0110] In the formula, ν—deflection at the middle of the cap beam (m);

[0111] M x —Maximum bending moment at the center of the capping beam (kN / m);

[0112] P0—Vertical pressure value on both sides of the truss (kN);

[0113] a1—Horizontal distance between vertical members on both sides of the truss (m), generally taken as 1.5 to 2.0m;

[0114] a2—Horizontal distance (m) between the left-side member of the truss and the midpoint of the capping beam, generally taken as 0.0 to 1.0m;

[0115] Based on the principle of linear superposition of structures, the horizontal distance between the truss installation point and the center of the cap beam can be calculated using equations ⑨ and ⑩. At the same time, the sum of the internal forces in the vertical direction of the truss as a whole can be obtained, so as to adjust the internal forces in a timely manner according to the displacement of the foundation pit.

[0116] The construction process of the present invention will be further described below with reference to specific embodiments. The embodiments specifically address a foundation pit project, the general overview of which is as follows: The site includes a main tower approximately 330m high and an apartment building approximately 188m high, with a four-story (partially three-story) basement. A cast-in-place pile-raft foundation is used. The foundation pit area is approximately 37285.3m². 2 The vertical excavation perimeter of the foundation pit is approximately 830.5m. According to the data provided by the construction party and the detailed survey report, the excavation depth of the foundation pit is between 12.0m and 23.1m, classifying it as an ultra-large and ultra-deep foundation pit. The rectangular elevator shaft pit of the main commercial tower has dimensions of 33.0m × 16.0m, with an excavation depth of 4.8m, and the closest point to the edge of the foundation pit is approximately 25.0m. The specific implementation method of this invention is as follows:

[0117] (1) The ultra-deep foundation pit adopts an internal support system. The long side of the rectangular elevator shaft pit is 31.5m and the short side is 17.7m. It adopts a drilled grouting + cap beam tie anchor cable support system. The diameter of the C30 drilled grouting piles in the pit is 600mm, the center distance of the piles is 1200mm, and the net spacing between the piles is 600mm. The long side and the short side of the pit are arched 3 / 100 outward from the pit along the direction perpendicular to the side length, with arch heights of 0.96m and 0.54m respectively. The cap beam on the long side is equipped with 12 tie anchor cables, and the cap beam on the short side is equipped with 9 tie anchor cables. Each tie anchor cable is composed of 4 steel strands.

[0118] According to the geological survey report, detailed geological conditions information for the area of ​​the foundation pit can be obtained in Table 1.

[0119] Table 1. Geological shear strength parameters of soil layers in the examples.

[0120]

[0121] (2) Using the pile calculation module of the Lizheng deep foundation pit software, inputting soil parameters, foundation pit excavation depth, pile parameters, etc., and using Rankine's earth pressure theory formula, the excavation surface earth pressure values ​​under the following conditions are calculated: q1 = 35.7 kPa / m and q2 = 55.4 kPa / m, respectively, when the pit is excavated to half its depth (h1 = 2.4 m) and when it is excavated to the bottom (h2 = 4.8 m). Therefore, the equivalent cap beam load value is:

[0122]

[0123]

[0124] The arching begins from the second support pile at the corner of the capping beam, with a trench width of 0.7m. The lengths of the tie anchor cables on both sides of the capping beam within the pit are l. 长 =32.0m, l 短 =18.2m, the arch height of the cap beam is f 长 =0.96m, f 短 =0.54m, the cross-sectional area of ​​the arch is A c =0.7m 2 The moment of inertia of the arch crown section is I. c =0.05833m 4 The elastic modulus of the arch ring material is E = 3 × 10⁻⁶. 4 MPa, the elastic modulus of the tie rod material is E1=1.95×10 5 MPa, the cross-sectional area of ​​the tie rod is

[0125] The axial force deformation correction coefficients for the two hinged arches of the long and short side tie rods in the pit-within-a-pit:

[0126]

[0127]

[0128] Under the conditions of excavation depths h1 = 2.4m and h2 = 4.8m in the pit-within-a-pit, the internal force values ​​of the tie rods of the two hinged arches with tie rods on the long and short sides are as follows:

[0129]

[0130]

[0131]

[0132]

[0133] Under the conditions of pit-within-a-pit excavation depths h1 = 2.4m and h2 = 4.8m, 12 anchor cables (m) are installed on the long side respectively. 长=12) Nine anchor cables are installed on the short side (m). 短 =9, the internal force value of each anchor cable:

[0134]

[0135]

[0136]

[0137]

[0138] (3) After determining the support axis of each side of the pit and the straight section in the middle in step (1), extract the coordinates, diameter, reinforcement, pile length and other information of each pile position from the design and construction drawings to form the support pile construction parameters.

[0139] (4) During the construction phase, following the construction parameters extracted in step (3), after the foundation pit is excavated to the top elevation of the elevator shaft pit, the support piles for the elevator shaft pit retaining structure are constructed first. After the support piles in the elevator shaft pit reach the curing period, the capping beam at the top of the piles is constructed, and its excavation width does not exceed the width of the capping beam plus 200mm. The excavation elevation of the capping beam in the pit is the same as the bottom elevation of the capping beam. The capping beam is used within approximately 5.5m of its end. The corrugated pipe is pre-drilled with anchor cable holes and fixed to the stirrups; the layout points of each foundation pit truss are determined, and then the pressure-bearing interface components are pre-drilled at the corresponding positions of the straight section in the middle of the cap beam. The construction of the corner connecting plate strip of the cap beam of the pit-in-pit retaining structure is carried out simultaneously.

[0140] Truss positioning: The dimensions of the middle truss of the cap beam are a1 1 =2.0m, a1 2 =1.6m; a2 1 =0.5m, a2 2 =0.4m; the initial displacement condition is δ=0.3m; therefore, under the general depth of foundation pit excavation and the condition of excavation to the bottom, the vertical load on the truss is:

[0141]

[0142]

[0143]

[0144]

[0145] Therefore, the theoretical inverse control value of the deflection at the middle of the crown beam can be obtained:

[0146]

[0147]

[0148]

[0149]

[0150] The truss is installed in a position that minimizes the deflection in the middle of the straight section of the capping beam. The connection between the truss and the capping beam provides an outward reaction force to the capping beam, which can produce a pre-arching effect on the capping beam towards the outside of the foundation pit. When the soil pressure on the outside of the capping beam increases, it can offset part of the deflection in the middle of the capping beam, thereby achieving the purpose of controlling the deflection in the middle of the capping beam.

[0151] (5) After the capping beam in step (4) reaches the curing period, the completed anchor cables, anchors, and anchor plates are placed in, and the truss is constructed. When constructing the middle truss, the two vertical short sides of the truss are installed first, then the long side of the truss parallel to the anchor cables is installed, and finally the two intersecting oblique sides are installed. Each side is preferentially connected to the pre-embedded parts of the reserved pressure-bearing interface. After the truss construction is completed, the first prestress is applied. The long and short capping beams are subjected to the first prestress for the anchor cables: 242.8kN and 137.9kN respectively. The prestress of the anchor cables on the four sides of the pit is applied simultaneously, and the prestress of each anchor cable is applied symmetrically.

[0152] (6) After completing step (5), the first layer of soil in the pit is excavated to a depth of 2.4m. The soil around the piles is excavated in the pit using a long-arm excavator, and a dedicated person is on duty. The soil between the piles is excavated and fine stone concrete is poured and steel mesh is hung.

[0153] (7) After step (6) is completed, the long side and short side cap beams are subjected to the first prestressing of the anchor cables: 753.78kN and 428.4kN respectively; the prestressing of the anchor cables on the four sides of the pit is applied simultaneously, and the prestressing of each anchor cable is applied symmetrically.

[0154] (8) Excavate the second layer of soil in the pit-in-pit, excavate the foundation elevation of the pit-in-pit, use a long-arm excavator to excavate the soil around the piles in the pit-in-pit, and have a dedicated person on duty. As the soil between the piles is excavated, spray fine stone concrete and hang steel mesh. After the foundation pit is excavated to the bottom, immediately construct the basement floor slab of the pit-in-pit and the elevator shaft structure.

[0155] During the excavation of the foundation pit, this invention applies vertical displacement constraints to the truss by using the pre-reserved interface at the connection between the truss and the capping beam, based on the displacement development of the top of the support piles, in order to effectively control the displacement of the middle part of the capping beam and further improve the stability and safety of the support system during construction.

[0156] The inventors of this application also calculated the cost of using two different support schemes within the construction area of ​​the pit-in-pit support:

[0157] ① After adopting the tie-arch-truss "recurved bow" scheme provided by this invention, the concrete usage is 546.6m³.3 Based on the current market price of C30 concrete, which is 410 yuan / m³ 3 The initial cost is 224,000 yuan; the amount of steel reinforcement used is 72.0 tons, and at the current market price of 5,000 yuan / ton, the cost is 360,000 yuan; the amount of anchor cable (4S15.2 specification) used is 1335.6 meters, and at the current market price of 135 yuan / meter, the cost is 180,000 yuan. Therefore, the total cost is 764,000 yuan.

[0158] ② After adopting the reinforcement scheme of bored cast-in-place piles + internal bracing (one steel internal bracing is installed at the elevation of the capping beam) + passive zone triaxial mixing piles, the concrete usage is 551.4 m³. 3 Based on the current market price of C30 concrete, which is 410 yuan / m³ 3 The initial cost is 226,000 yuan; the amount of reinforcing steel used is 72.7 tons, at the current market price of 5,000 yuan / ton, costing 364,000 yuan; the amount of structural steel (double-section H500×300 specification) used is 232.8m, weighing approximately 43.1 tons, at the current market price of 5,200 yuan / ton, and considering an installation cost coefficient of 1.5, costing 336,000 yuan; the construction period for dismantling supports and other delays is approximately 7 days, calculated at 100,000 yuan / day, costing 700,000 yuan. Therefore, the total cost is 1,626,000 yuan.

[0159] Through the above calculations and comparisons, the reinforcement solution of the present invention has obvious economic advantages, and saves construction time and is convenient to construct.

[0160] As described above, the support structure and construction method for ultra-deep foundation pits in soft soil areas provided by this invention control the deformation and stability of the foundation pit through bored piles when internal supports cannot be added. The bored piles are distributed in a parabolic arch shape at both ends and a straight line shape in the middle. A large-sized capping beam is set on the top of the bored piles, and prestress is applied to the parabolic arch capping beam through multiple anchor cables. A displacement adjustment truss is set in the vertical direction in the middle of the capping beam, which fully utilizes the force characteristics of the tie rod-arch-truss "inverted bow" support system to improve the bending resistance of the entire support system, control the displacement of the pile top, and meet the overall force balance of the foundation pit. Through engineering case practice, the effectiveness and applicability of the method proposed in the patent are demonstrated: the method proposed in the patent can maintain the stability of the foundation pit and meet the overall force balance of the foundation pit by reasonably arranging the support piles, capping beam, tie cables, truss and other structures; it can also reduce the project cost, facilitate construction, and save construction time, and has high promotion and application value.

[0161] The above description is merely one embodiment of the present invention, and while it is detailed and specific, it should not be construed as limiting the scope of the invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the appended claims.

Claims

1. A support structure for ultra-deep elevator shaft pits in soft soil areas, suitable for situations where the elevator shaft pit is a regular quadrilateral and soft soil exists on the sidewalls and base, wherein the depth of the elevator shaft pit is between 3.8 and 5.5 m, characterized in that: The support structure includes a pit-in-pit retaining structure (1) formed around the inner pit of the elevator shaft pit using bored piles, cap beam tie anchors (2) distributed along each pit side of the inner pit of the elevator shaft pit, connecting strips (5) set at each inner corner of the pit-in-pit retaining structure (1), and trusses (10) set between each cap beam tie anchor (2) and each pit side of the pit-in-pit retaining structure (1). Each long side support axis and short side support axis of the pit-in-pit retaining structure (1) arches towards the outside of the inner pit of the elevator shaft pit, and after arching, the long side support axis and short side support axis change from the original straight line to a parabola with an arch of 3 / 100 to 6 / 100, and the middle 1 / 4 to 1 / 3 of the length of each parabola is set as a straight section. The distance between the straight section and the support axis before arching is 0.5~2m; the cap beam tie anchor cable (2) is distributed at the long side support axis and short side support axis of the pit-in-pit retaining structure (1) before arching. The two ends of each set of cap beam tie anchor cables (2) are respectively anchored on the bored pile cap beam at the two adjacent corners of the pit-in-pit retaining structure (1). The truss (10) is set between the straight section of the pit-in-pit retaining structure (1) after arching and the cap beam tie anchor cable (2); the connecting plate strip (5) is set between the long side cap beam (3) and the short side cap beam (4) of the pit-in-pit retaining structure (1). Its two sides are respectively connected to the long side cap beam (3) and the short side cap beam (4) and the overlap length is 1.6~2.0m; The truss positions are arranged to minimize the mid-span deflection of the capping beam. The distance between the two vertical sides of the truss is a1, and the distance from the left side of the truss to the midpoint of the capping beam is a2, where a1 = 4a2. Therefore, the truss can effectively control the deflection at the midpoint of the capping beam by: ； ； In the formula, —Deflection at the midpoint of the capping beam, in meters; —The maximum bending moment at the middle of the capping beam, in kN·m; —Vertical pressure values ​​on both sides of the truss, in kN; —The horizontal distance between the vertical members on both sides of the truss is generally taken as 1.5~2.0m; —The horizontal distance between the left-side member of the truss and the midpoint of the capping beam is generally 0.0~1.0m; —The vertical displacement increment of the central truss of the capping beam is adjustable in meters and does not exceed 1.0m; —Length of the tie rod in the two hinged arches of the anchor cable tie rod, in meters; —Moment of inertia of the crown beam arch section, in meters. 4 ; E - modulus of elasticity of the crown ring material in kN / m 2 ; H—The internal force value of the anchor cable tie rod in the two-hinged arch, in kN.

2. The support structure for an ultra-deep foundation pit elevator shaft in soft soil area according to claim 1, characterized in that: The long side and short side of the pit within the elevator shaft are close in length. The camber of each side support axis of the pit-within-pit retaining structure (1) is 3 / 100. The length of the straight section in the middle of the parabola after camber is 1 / 3 of the original pit side length. The truss (10) is located in the middle of the straight section. Two reinforced bored piles (6) are added to the outside of each corner of the pit-within-pit retaining structure (1), and the two reinforced bored piles (6) are located at the pit retaining structure. Before the arch is formed, on the extended lines of the long and short sides; the two ends of the tie anchor cable (2) of each set of crown beams are respectively anchored to the top crown beam of two reinforced bored piles (6) on the same long or short side extension line through anchors and anchor plates (7); the connecting strip (5) is a triangular strip, and a gap of 10-15mm wide and 20-25mm deep is chiseled at the overlapping part with the long side crown beam (3) and the short side crown beam (4), and the gap is filled with asphalt hemp rope.

3. A support structure for an ultra-deep foundation pit elevator shaft in soft soil areas according to claim 1 or 2, characterized in that: The cap beam tie anchor cable (2) is made of 1860MPa steel strand. Each set of cap beam tie anchor cables (2) includes multiple anchor cables, each anchor cable consisting of at least four steel strands. The end of each anchor cable is tensioned and locked by OVM anchor and anchor plate. The end of each set of cap beam tie anchor cables (2) passes through the cap beam of the pit-in-pit retaining structure (1). The part of the cap beam tie anchor cable (2) that passes through the cap beam of the pit-in-pit retaining structure (1) is separated from the cap beam of the pit-in-pit retaining structure (1) by a pre-embedded corrugated pipe, and the anchor cable... The steel strand is wrapped with a plastic sheath for rust and corrosion prevention; the cap beam tie anchor cable at the long side of the elevator pit and the cap beam tie anchor cable at the short side intersect in the cap beam of the bored pile at the corner of the pit-in-pit retaining structure (1), and the two sets of cap beam tie anchor cables are arranged symmetrically up and down; the stirrups are densely set in the area where the cap beam and cap beam tie anchor cable (2) overlap on each side of the pit-in-pit retaining structure (1), and spiral stirrups with a diameter of 12mm, a pitch of 30mm, and a length of 300mm are set at the anchor cable reserved hole in the cap beam.

4. A support structure for an ultra-deep foundation pit elevator shaft in soft soil areas according to claim 1 or 2, characterized in that: The pit-in-pit retaining structure (1) includes bored cast-in-place piles (100) and pile top capping beams (101). The spacing between two adjacent bored cast-in-place piles (100) on each side is 1.2 to 1.5 times the pile diameter, and the pile top capping beams (101) on each side are connected as a whole to form the capping beam of the pit-in-pit retaining structure. The long side capping beam (3) and the short side capping beam (4) of the pit-in-pit retaining structure (1) are constructed along the long side support axis and the short side support axis after arching, respectively.

5. A construction method for the support structure of an ultra-deep foundation pit elevator shaft in soft soil area as described in any one of claims 1 to 4, characterized in that: The construction method described herein is for ultra-deep foundation pits, elevator shafts, and sub-pits in soft soil areas with a depth of 3.8 to 5.5 meters and a certain thickness of peat soil layer distributed on the sidewalls and base. The specific construction steps are as follows: S1. Based on the geological survey, determine the surrounding environmental conditions, excavation depth, and top load of the elevator shaft pit. The excavation depth of the elevator shaft pit is h, and it is excavated in two stages, with each excavation depth being h / 2. The short and long support axes of the pit-in-pit retaining structure are all arched along the direction perpendicular to each side towards the outside of the design edge line of the elevator shaft pit. The arching degree is controlled between 3 / 100 and 6 / 100. The middle 1 / 4 to 1 / 3 of the length of the parabola formed after arching of each side is set as a straight section, and the distance between the straight section and the support axis before arching is 0.5 to 2m. S2. The earth pressure values ​​at the bottom of the elevator shaft pit during the two excavations were calculated using the Lizheng Deep Foundation Pit software. Based on the earth pressure values ​​at the bottom surface during the two excavations The equivalent uniformly distributed load on the cap beam during the two excavations of the elevator shaft pit is calculated using formulas ① and ②, respectively. : ① ② In the formula, —Depth of the two excavations within the pit (m); —The depth of the pit-within-a-pit excavation is Equivalent uniformly distributed load value of the cap beam (kN / m); —The depth of the pit-within-a-pit excavation is Earth pressure value at the bottom of the excavation (kN / m / m); —The equivalent uniformly distributed load factor for the cap beam is generally taken as 1.8; S3. Based on the equivalent uniformly distributed load on the capping beam during the two excavations of the elevator shaft pit calculated in step S2. Using a calculation model of a tie-rod double-hinged arch, the internal force values ​​P of the cap beam on the anchor cables of the elevator shaft pit on the long and short sides during two excavations were calculated. 1、 P2, and evenly distributed to each anchor cable; S4. After determining the support axis of each side of the pit and the straight section in the middle in step S1, extract the coordinates, diameter, reinforcement and pile length information of each pile position from the design and construction drawings to form the construction parameters of the support pile. S5. During the construction phase, following the construction parameters extracted in step S4, after the foundation pit is excavated to the top elevation of the elevator shaft pit, the support piles for the elevator shaft pit retaining structure are constructed first. After the support piles for the elevator shaft pit retaining structure reach the curing period, the top capping beam is constructed, with an excavation width not exceeding the width of the capping beam plus 200mm. The excavation elevation of the capping beam for the pit retaining structure is the same as the bottom elevation of the capping beam. Corrugated pipes are used to pre-reserve anchor cable holes within 5-6m of the capping beam ends, and the corrugated pipes are fixed to the stirrups. The position of each foundation pit side truss is determined, and then pressure-bearing interface components are reserved at the corresponding positions of the straight sections in the middle of the capping beam. The corner connecting plates of the capping beam for the pit retaining structure are constructed simultaneously. The truss positions are arranged according to the principle of minimizing the mid-span deflection of the capping beam. The distance between the two vertical sides of the truss is a1, and the distance from the left side of the truss to the midpoint of the capping beam is a2, where a1=4a2. Therefore, the truss can effectively control the deflection value in the middle of the capping beam. ； ； In the formula, —Deflection at the midpoint of the capping beam, in meters; —The maximum bending moment at the middle of the capping beam, in kN·m; —Vertical pressure values ​​on both sides of the truss, in kN; —The horizontal distance between the vertical members on both sides of the truss is generally taken as 1.5~2.0m; —The horizontal distance between the left-side member of the truss and the midpoint of the capping beam is generally 0.0~1.0m; —The vertical displacement increment of the central truss of the cap beam is adjustable in meters and does not exceed 1.0m; —Length of the tie rod in the two hinged arches of the anchor cable tie rod, in meters; —Moment of inertia of the crown beam arch section, in meters. 4 ; E - modulus of elasticity of the crown ring material in kN / m 2 ; H—Internal force value of the anchor cable tie rod in the two-hinged arch, in kN; S6. After the capping beam of the pit-in-pit retaining structure in step S5 reaches the curing period, the completed anchor cables, anchors and anchor plates are placed in the pit, and the middle truss is constructed. The reserved interface at the connection between the truss and the capping beam is connected. Then, the capping beam is subjected to the first prestressing on the tie anchor cables. The applied value is P1 calculated in step S3. The prestressing of the capping beam tie anchor cables on the four sides of the elevator shaft pit is applied simultaneously. The capping beam tie anchor cables on each side are symmetrically subjected to prestressing to form a preliminary inverted bow-shaped support structure system. S7. After completing step S6, the first layer of soil excavation in the inner pit of the elevator shaft is carried out, with an excavation depth of h / 2. The soil around the piles is excavated in the inner pit of the elevator shaft using a long-arm excavator. The soil between the piles is sprayed with fine stone concrete and steel mesh is hung as it is excavated. During the excavation of the foundation pit, the vertical displacement constraint is applied to the support edge through the connection between the truss and the capping beam to control the displacement of the middle part of the capping beam. S8. After step S7 is completed, the crown beam counter-pulling anchor cable is prestressed for the second time, and the applied value is P calculated in step S3 2， The application mode is the same as step S6; S9. After step S8 is completed, the second layer of soil excavation in the inner pit of the elevator shaft is carried out. The excavation is carried out to the base elevation of the inner pit of the elevator shaft. The soil around the piles is excavated in the inner pit using a long-arm excavator. The soil between the piles is sprayed with fine stone concrete and steel mesh is hung as it is excavated. The basement floor slab of the inner pit is constructed immediately after the inner pit of the elevator shaft is excavated to the bottom.

6. The construction method of the elevator shaft support structure for ultra-deep foundation pits in soft soil areas according to claim 5, characterized in that: In steps S4-S9, during earthwork excavation, the displacement of the support piles and the internal force of the anchor cables are monitored and fed back into the construction of the pit-in-pit support and earthwork excavation. In step S2, the pile calculation module of the Lizheng deep foundation pit software is used. The stratum parameters, foundation pit excavation depth, and pile parameters are input, and the earth pressure value at the bottom of the pit under the corresponding excavation condition is calculated using the Rankine earth pressure theory formula. .

7. The construction method of the elevator shaft support structure for ultra-deep foundation pits in soft soil areas according to claim 6, characterized in that... In step S3, during the two excavations, the internal force values ​​P of the anchor cables of the capping beams on the long and short sides of the elevator shaft pit are as follows: 1、 P2 uses a tie-rod double-hinged arch calculation model according to the "Handbook of Static Calculation of Building Structures". Each arching edge of the pit-within-a-pit retaining structure and the tie anchor cables of the cap beam are regarded as a whole tie-rod double-hinged arch. The specific calculation process is as follows: ③ ④ ⑤ ⑥ ⑦ ⑧ In the formula: —representing the excavation depth of the pit within a pit The axial force deformation correction coefficient of the double-hinged arch with anchor cable tie rod of the crown beam; —representing the excavation depth of the pit within a pit The total internal force of the anchor cables of each group of cap beams, in kN; —representing the excavation depth of the pit within a pit The internal force value (kN) of each anchor cable in each group of cap beam tie anchor cables; —The number of anchor cables in each set of cap beam tie anchor cables is generally 6 to 12; —The length of the tie-anchor cable of the cap beam on each side of the pit-within-a-pit retaining structure, in meters; —The arch height of each arched side of the pit-within-a-pit retaining structure, in meters; —Cross-sectional area of ​​the crown beam arch, in m² 2 ; —Axial force deformation correction factor, usually taken as 1.67; —The elastic modulus of the crown beam arch material, in kN / m 2 ; —Moment of inertia of the crown beam arch section, in meters. 4 ; —The elastic modulus of the anchor cables for the capping beam, in kN / m 2 ; —Cross-sectional area of ​​the anchor cables for the capping beam, in m² 2 .

8. The construction method of the elevator shaft support structure for ultra-deep foundation pits in soft soil areas according to claim 5, characterized in that: The truss is a rectangular truss, with its long side being 1 to 2 times the length of its short side, and the long side of the truss is parallel to the anchor cable. When constructing the middle truss in step S6, first install the two vertical short sides of the truss, then install the long side of the truss parallel to the anchor cable, and finally install the two intersecting oblique sides. Each side is preferentially connected to the embedded part of the reserved pressure-bearing interface.

9. The construction method of the elevator shaft support structure for ultra-deep foundation pits in soft soil areas according to claim 5, characterized in that: In step S5, during the construction of the elevator shaft pit capping beam, the stirrups are densely arranged within 500mm of its end, and spiral stirrups with a diameter of 12mm, a pitch of 30mm, and a length of 300mm are installed at the anchor cable pre-reserved holes; the connecting plate strip is set between the long side capping beam and the short side capping beam of the pit-in-pit retaining structure, the thickness of the connecting plate strip is 240-360mm, it is triangular, and the overlap length of its long side capping beam and short side capping beam is 1.6-2.0m, and a gap with a width of 10mm and a depth of 20mm is chiseled at the overlapping part of the connecting plate strip and the capping beam, and then filled with asphalt hemp rope.

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