Deep foundation pit high-low span supporting structure close to existing subway line and construction method

Abstract

The invention relates to a deep foundation pit high-low span supporting structure close to an existing subway line and a construction method, and relates to the technical field of civil construction. The deep foundation pit high-low span supporting structure comprises slope protection piles located on the periphery of a deep foundation pit and on the two sides of an existing subway line penetrating through the deep foundation pit. Upward-pressing earthwork is reserved on the upper side of the existing subway line penetrating through the deep foundation pit, and soil nailing wall protection slopes are arranged on the two sides of the upward-pressing earthwork. The support composed of the slope protection piles comprises first pile anchor supports which are arranged on the two sides in the deep foundation pit and located above the existing subway line in a penetrating mode. The second pile anchor support, the first single-row cantilever pile support, the steel support cofferdam support and the like are arranged in the deep foundation pit and located on the two sides of the existing subway line penetrating through the deep foundation pit. The invention further provides a construction method for constructing the high-low span support structure of the deep foundation pit. The subway structure is effectively protected, the adverse effect of construction on the existing subway is reduced, and the safety and reliability of the deep foundation pit are ensured.

Description

Technical Field

[0001] The present application relates to the field of civil construction technology, and in particular to a high-low span support structure and a construction method for a deep foundation pit adjacent to an existing subway line. Background Art

[0002] With the acceleration of urbanization, more and more cities have begun to pay attention to the development and utilization of underground space, especially for metropolises with limited land resources. Underground space has become one of the important ways to alleviate the pressure of ground traffic. With the continuous improvement of my country's urban modernization and motorization levels and the gradual improvement of rail transit systems and planning layouts, the number of new urban projects located in subway protection zones has also increased. However, in this case, how to effectively protect existing facilities (such as subway lines) from damage has become a major issue facing construction units. In the case of large deep foundation pits adjacent to existing subways, most of the deep foundation pits are adjacent to the existing subway on one side and excavation is carried out with one-side support, or the foundation pit is located on the upper part of the existing subway and excavation and unloading and protective measures are used to ensure operational safety. Although this type of deep foundation pit construction technology is relatively mature, with the increasing complexity of the construction environment, it still faces many uncertainties and risk factors when facing complex geological conditions and working close to multiple subway lines. In particular, there is not only a subway tunnel in the middle of the foundation pit, but also other subway lines beside the foundation pit. That is, the foundation pit involves both subway tunnels and transfer passages. Under such complex working conditions, it is very challenging to ensure the safe and stable operation of the two subway lines and complete the construction with high standards and high quality. There is currently no mature construction technology. Summary of the invention

[0003] In order to ensure the safety and reliability of the deep foundation pit under the complex working conditions of an existing subway line running through the middle of the deep foundation pit and an existing subway line on one side of the deep foundation pit, the present application provides a high-low span support structure and a construction method for a deep foundation pit adjacent to an existing subway line.

[0004] In the first aspect, the present application provides a deep foundation pit high-low span support structure near an existing subway line, which adopts the following technical solution: A high-low span support structure for a deep foundation pit adjacent to an existing subway line, comprising slope protection piles located around the deep foundation pit and on both sides of the existing subway line running through the deep foundation pit; the slope protection piles are formed by a full sleeve construction process; An upper pressure earthwork is reserved on the upper side of the existing subway line running through the deep foundation pit, and soil nail wall slope protection is arranged on both sides of the upper pressure earthwork; The support composed of the slope protection piles includes: The first pile-anchor support is arranged on both sides of the deep foundation pit and is located above the existing subway line that passes through it; the first pile-anchor support includes 20 first support piles arranged at intervals and a first anchor rod, the first support pile has a length of 5m, a pile diameter of 0.8m, and a pile spacing of 1.45m. The first support pile is provided with a first crown beam at the top of the pile, the size of the first crown beam is 900X600mm, and the concrete strength grade of the first support pile and the first crown beam is C25; The second pile-anchor support, the first single-row cantilever pile support and the first steel-supported cofferdam support are arranged on both sides of the existing subway line in the deep foundation pit; the second pile-anchor support includes 127 second support piles and three second anchor rods arranged at intervals; the second support piles have a length of 12.0m, a pile diameter of 1m, and a pile spacing of 1.3m or 1.5m. The second support piles are arranged with second crown beams on the top of the piles, and the size of the second crown beam is 1100X700mm or 1100X800mm. The concrete strength grade of the second support piles and the second crown beam is C25; the first single-row cantilever pile support includes 67 third support piles; the third support piles have a length of 9.0m, a pile diameter of 1m, and a pile spacing of 1.5m. The third support piles are arranged with third crown beams on the top of the piles, and the size of the third crown beam is 1100X700mm , the concrete strength grade of the third support pile and the third crown beam is C25; the first steel support cofferdam support includes 51 fourth support piles and 2 inner support steel pipes; the length of the fourth support pile is 13.5m, the pile diameter is 1m, and the pile spacing is 1.3m; the fourth crown beam is arranged on the top of the fourth support pile, and the size of the fourth crown beam is 1100X800mm. The concrete strength grade of the fourth support pile and the fourth crown beam is C25, and the inner support steel pipe and steel surrounding purlin are made of Q235B material; The third pile-anchor support, the second single-row cantilever pile support, the first double-row cantilever pile support and the second steel support cofferdam support are set up on one side of the deep foundation pit near the existing subway line; the third pile-anchor support includes 28 fifth supporting piles and four third anchor rods arranged at intervals, the fifth supporting pile has a length of 16m, a pile diameter of 0.8m, a pile spacing of 1.5m, a fifth crown beam is arranged on the top of the fifth supporting pile, the size of the fifth crown beam is 900X600mm, and the concrete strength grade of the fifth supporting pile and the fifth crown beam is C25; the second single-row cantilever pile support includes 44 sixth supporting piles, the sixth supporting pile has a length of 12m, a pile diameter of 1m, a pile spacing of 1.5m, a sixth crown beam is arranged on the top of the sixth supporting pile, the size of the sixth crown beam is 1100X700mm, and the concrete strength grade of the sixth supporting pile and the sixth crown beam is C25; A double row of cantilever pile support includes a rigid frame beam and 70 seventh support piles. The length of the front and rear rows of the seventh support piles is 20m, the pile diameter is 1m, and the pile spacing is 1.4m. The top of the seventh support pile is arranged with a seventh crown beam, the size of the seventh crown beam is 1100X800mm, the size of the rigid frame beam is 1100X800mm, and the concrete strength grade of the seventh support pile, the rigid frame beam and the seventh crown beam is C25; the second steel support cofferdam support includes 72 eighth support piles and six internal support steel pipes; the length of the eighth support pile is 16m, 21m or 28.5m, the pile diameter is 1m, and the pile spacing is 1.5m; the top of the eighth support pile is arranged with an eighth crown beam, the size of the eighth crown beam is 1100X800mm, the concrete strength grade of the eighth support pile and the eighth crown beam is C25, and the internal support steel pipe and steel surrounding purlin are made of Q235B material; A fourth pile-anchor support is arranged on one side of the deep foundation pit near the high-rise building; a second double-row cantilever pile support is arranged on the other side of the deep foundation pit near the high-rise building; the fourth pile-anchor support includes 16 ninth support piles arranged at intervals and two fourth anchor rods; the ninth support pile has a length of 16.0m, a pile diameter of 0.8m, a pile spacing of 1.5m, a ninth crown beam is arranged on the top of the ninth support pile, the size of the ninth crown beam is 900X600mm, and the ninth support pile and the ninth crown beam are 1.5m long, 1.5m wide, and 1.5m wide. The concrete strength grade of the beams is C25; the second double-row cantilever pile support includes a rigid frame beam and 34 tenth support piles. The length of the front tenth support piles is 20m, the pile diameter is 0.8m, and the pile spacing is 1.4m. The top of the tenth support pile is arranged with a tenth crown beam. The rear row support is reinforced by grouting between piles using the shaft structure. The size of the rigid frame beam is 900X700mm. The concrete strength grade of the tenth support pile, rigid frame beam and tenth crown beam is C25; The side walls around the deep foundation pit and the side walls on both sides of the existing subway line running through the deep foundation pit are formed into protective slopes by full-section grouting.

[0005] In this application, composite support forms such as full-casing bored piles, soil nail wall slope protection, anchor cables and steel supports are used to provide all-round protection for the entire deep foundation pit. By adopting the above technical solution, it is possible to smoothly carry out operations under complex working conditions where there is an existing subway line running through the middle of the deep foundation pit and an existing subway line on one side of the deep foundation pit, ensuring the safe and stable operation of the two subway lines and the safety and reliability of the deep foundation pit.

[0006] Optionally, a masonry retaining wall is arranged on the surface around the deep foundation pit, and the surface of the masonry retaining wall is plastered with mortar; Longitudinal and transverse blind ditches are set in the deep foundation pit to drain the seepage and rainwater in the deep foundation pit. A water collection well is also set in the deep foundation pit.

[0007] By adopting the above technical solution, the ground around the deep foundation pit is waterproofed and drained, which can strictly prevent ground water from invading the soil around the deep foundation pit and realize timely drainage of rainwater in the deep foundation pit, further ensuring the safety and reliability of the deep foundation pit.

[0008] In the second aspect, the present application provides a construction method for a high-low span support structure of a deep foundation pit near an existing subway line, which adopts the following technical solution: A construction method for a high-low span support structure of a deep foundation pit adjacent to an existing subway line, the construction method comprising the following steps: S1: Construction preparation: including site leveling, site inspection, surveying and stake delivery, and neighbor building; S2: Zoning construction: The entire foundation pit is divided into the East Zone located on the east side of the existing subway line, the West Zone located on the west side of the existing subway line, and the Central Zone located above the existing subway line, with the existing subway line running through the foundation pit as the dividing line; Carry out support construction on the support piles and crown beams on the north and south sides of the east and west areas; excavate the east side of the east area, and carry out soil nail wall construction on the upper part of the east side; excavate the west side of the west area, and carry out anchor spraying and backfill construction on the upper part of the west side; Then, the earthwork outside the safety protection zone of the existing subway line running through the foundation pit in the east and west areas was excavated, and the anchor spraying and anchor cable construction between piles was carried out; Afterwards, the upper earthwork of the existing subway line that runs through the foundation pit in the central area was excavated in layers and sections, and support construction was carried out on both sides; S3: Construction of remaining earthwork: The remaining earthwork is constructed in layers and sections, and anchor spraying and anchor cable construction are carried out between piles; S4: Finishing and acceptance: finishing of the bridleway and earthwork abandonment construction, final acceptance and data handover.

[0009] By adopting the above-mentioned technical scheme, the present application realizes the high and low span support structure of the deep foundation pit near the existing subway line. During the excavation of the deep foundation pit, the present application adopts comprehensive support forms such as pile rows, anchor cables, steel supports and full-section grouting according to local conditions, which significantly improves the safety and reliability of the deep foundation pit construction, while ensuring the safety and stability of the deep foundation pit construction near the subway. While achieving the dual goals of construction and protection, the work efficiency is improved, the risk factor of the entire project is reduced, and unnecessary economic losses and social negative impacts are reduced.

[0010] Optionally, in the above step S2, after the earthwork on the upper side of the existing subway line passing through the foundation pit is excavated to the explosion-proof layer area, the explosion-proof layer is broken; then the earthwork is excavated to the elevation of the supporting piles adjacent to the subway area; then supporting piles are constructed on both sides of the existing subway line passing through the foundation pit, and the cap beams are manufactured after the pile heads of the supporting piles are broken.

[0011] By adopting the above technical solution, it is possible to maintain a safe operating environment for the subway line under complex working conditions where an existing subway line runs through the middle of a deep foundation pit; comprehensively improve the overall efficiency of the deep foundation pit support system, and ensure the safety and reliability of the subway line and the entire foundation pit during construction.

[0012] Optionally, in the above step S2, at the intersection of the existing subway line running through the foundation pit and the existing subway line adjacent to one side of the foundation pit, the earth on the upper side of the existing subway line running through the foundation pit is excavated to the explosion-proof layer area, and then the explosion-proof layer is broken; and the excavation is carried out to the elevation of the supporting piles adjacent to the subway area, and then the anchor spraying, anchor rods and soil nails are constructed between the piles; then the low-elevation supporting piles on the side adjacent to the subway area close to the foundation pit are constructed; and the crown beams are manufactured after the pile heads of the supporting piles are broken.

[0013] By adopting the above-mentioned technical solution, special treatment is carried out on the intersection of the existing subway line running through the foundation pit and the adjacent subway line, including layered excavation of earth, removal of the explosion-proof layer, construction of support piles and production of crown beams, etc., which ensures the safety of this key node area and effectively avoids damage to the subway line structure caused by improper construction.

[0014] Optionally, skip driving can be adopted during the construction of supporting piles, with the piles driven every two or three times. The safe distance between the newly concreted piles and the temporary pile holes should be no less than 4 times the pile diameter, the interval time should be no less than 24 hours, and the strength of adjacent bored piles should be no less than 50% of the design strength; and the mud density during the supporting pile construction should be 1.15-1.25.

[0015] By adopting the above technical solution, the skipping construction method is adopted during the construction of the support piles, which can effectively reduce the mutual interference between adjacent pile foundations and ensure the quality and stability of each pile foundation. The construction strategy of hitting one every two or three pile foundations helps to reduce the ground disturbance and potential hole collapse risk caused by continuous construction. It is stipulated that the safe distance between the newly completed concrete pouring pile and the temporary pile hole should not be less than 4 times the pile diameter, and the interval time should be no less than 24 hours. This not only ensures the initial solidification of the completed pile body, but also provides a sufficient time window for the recovery of the stratum. In addition, the strength of the adjacent bored piles is required to be no less than 50% of the design strength, which further improves the safety margin of the overall support structure. The mud density is controlled in the range of 1.15 to 1.25, which optimizes the wall protection effect and prevents the occurrence of hole collapse, thereby ensuring the safety and efficiency of the construction process.

[0016] Optionally, the anchor cable construction spacing is one pile and one anchor, and a full-track anchor hydraulic drilling rig is used for construction. During the construction process, the "jump-drilling" method is used to drill holes to avoid mutual influence between adjacent anchors; When adjacent anchor cables are crossed at the positive corners of the foundation pit, the normal use function of the anchor cables is met by adjusting the anchor cable positions or the anchor cable construction angles. The angle between adjacent anchor rods is 15 to 20 degrees.

[0017] By adopting the above technical solution, the anchor cable construction adopts reasonable spacing arrangement and advanced full-track anchor hydraulic drilling rig for operation. The "jumping" hole-making method effectively avoids interference between adjacent anchor rods and improves construction efficiency and safety. In view of the special working conditions of the positive corner of the foundation pit, the normal use function of the anchor cable system is ensured by adjusting and optimizing the anchor cable position or construction angle. At the same time, the angle range of adjacent anchor rods is limited to 15-20°. This precise design enhances the overall stability of the support, thereby significantly improving the reliability and safety of deep foundation pits in complex environments.

[0018] Optionally, the earth excavation is carried out in layers and sections from top to bottom in sequence using the strip excavation method. The span of the section excavation is not more than 15m, the layer height is not more than 2.5m, and the time interval between the excavation of two adjacent warehouses and the upper and lower layers is not less than 24 hours. During the earth excavation on both sides of the existing subway line that runs through the foundation pit, a symmetrical excavation method is adopted. When the excavation is 2m or 3m away from the existing subway line tunnel, manual cleaning is adopted, and 2m or 3m high soil slopes are left on both sides of the existing subway line tunnel structure to avoid damage to the subway structure tunnel and ensure lateral displacement of the tunnel structure.

[0019] The full sleeve follow-up construction technology is used when constructing the support piles within 6.0m outside the structural edge line on both sides of the subway tunnel to prevent the collapse of the hole from affecting the subway structure. The earthwork on both sides of the subway is "excavated symmetrically in sections and layers" to ensure the deformation stability of the exposed subway section. By adopting the above technical solutions, the safety and reliability of the foundation pit construction process is further improved.

[0020] Optionally, the soil nail wall construction working surface is excavated in sections and layers, with each section being 15 to 20 m long and the excavation depth of each layer matching the vertical spacing of the soil nails to ensure that the over-excavation of each layer does not exceed 500 mm.

[0021] By adopting the above technical solution, the soil nail wall construction can more accurately control the excavation depth and length of each section and each layer, effectively limiting the over-excavation, thereby improving the construction accuracy and reducing unnecessary soil disturbance. This measure helps to maintain the stability of the deep foundation pit slope, prevent landslides or other safety hazards caused by excessive excavation, and thus ensure the overall safety and reliability of the high and low span support structure of the deep foundation pit near the existing subway line.

[0022] Optionally, soil nail wall construction and earth excavation are carried out alternately. The next earth excavation is carried out after each step of soil nail wall slope protection construction is completed and reaches 70% of the design strength. During the next earth excavation, the upper concrete surface layer and soil nails need to be protected to avoid touching the concrete surface layer and soil nails of the slope protection surface that have been constructed.

[0023] By adopting the above technical solution, alternating soil nail wall construction with earth excavation can reasonably arrange the construction process, reduce construction interference, and improve construction efficiency. After each step of soil nail wall slope protection construction is completed and reaches 70% of the design strength, the next step of earth excavation will be carried out. This measure effectively ensures the structural stability of the completed part and prevents the slope protection from failure or deformation due to premature excavation. In the next step of earth excavation, the upper concrete surface layer and soil nails are protected to avoid touching the concrete surface layer and soil nails of the completed slope protection surface, thereby ensuring the integrity and safety of the entire slope protection system, and further improving the overall reliability and construction quality of deep foundation pit support.

[0024] In summary, the present application includes at least one of the following beneficial technical effects: 1. In this application, composite support forms such as full-casing bored piles, soil nail wall slope protection, anchor cables and steel supports are used to form all-round protection for the entire deep foundation pit, so that operations can be carried out smoothly under complex working conditions where there is an existing subway line running through the middle of the deep foundation pit and an existing subway line on one side of the deep foundation pit, thereby ensuring the safe and stable operation of the two subway lines and the safety and reliability of the deep foundation pit.

[0025] 2. The design of upper earthwork and soil nail wall slope protection is used in this application to provide additional protection for the existing subway line during the deep foundation pit excavation process, thereby reducing the impact of ground movement caused by excavation on the subway tunnel.

[0026] 3. In this application, the full-section grouting method is combined with other composite support forms to comprehensively improve the overall stability of the side walls around the deep foundation pit, thereby significantly improving the safety and reliability during the construction process. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a distribution diagram of the support structure in this application.

[0028] Figure 2 It is a process control schematic diagram of the support structure construction method in this application. DETAILED DESCRIPTION

[0029] The following will be combined with the attached Figure 1 and attached Figure 2 , the technical solutions in the embodiments of the present invention are clearly and completely described. The described embodiments are only possible technical implementations of the present invention, not all possible implementations. Those skilled in the art can completely combine the embodiments of the present invention to obtain other embodiments without creative work, and these embodiments are also within the scope of protection of the present invention. Example 1

[0030] Reference Figure 1As shown, in this embodiment, the excavation depth of the foundation pit is 10.72m~21.80m. Taking the example that the east and west sides of the deep foundation pit are existing high-rise buildings, the north side is an existing road, the south side is an existing subway line, and the middle part also has an existing subway line running north-south, for the purpose of description, the existing subway line running north-south through the foundation pit is defined as the first subway line, the existing subway line on the south side of the foundation pit is defined as the second subway line, the first subway line and the second subway line intersect and there is a transfer passage between the two, and the buried depth of the second subway line is greater than the buried depth of the first subway line. From the top view of the foundation pit, the entire foundation pit is divided by the first subway line, the foundation pit area to the east of the first subway line protection zone is the east zone, the foundation pit area to the west of the first subway line protection zone is the west zone, and the first subway line protection zone is the middle zone. In this embodiment, the high-low span support structure of the deep foundation pit adjacent to the existing subway line includes slope protection piles located around the deep foundation pit and on both sides of the existing subway line running through the deep foundation pit; the slope protection piles are formed by a full sleeve construction process; upper pressure earth is reserved on the upper side of the existing subway line running through the deep foundation pit, and soil nail wall slope protection is arranged on both sides of the upper pressure earth; the support composed of the slope protection piles includes: a first pile anchor support arranged on both sides of the deep foundation pit and located above the existing subway line running through; The second pile-anchor support, the first single-row cantilever pile support and the first steel-supported cofferdam support; the third pile-anchor support, the second single-row cantilever pile support, the first double-row cantilever pile support and the second steel-supported cofferdam support set up on one side of the deep foundation pit close to the existing subway line; the fourth pile-anchor support set up on one side of the deep foundation pit close to the high-rise building; the second double-row cantilever pile support set up on the other side of the deep foundation pit close to the high-rise building; the side walls around the deep foundation pit and the side walls on both sides of the existing subway line running through the deep foundation pit are formed by full-section grouting.

[0031] Reference Figure 1 As shown in the figure, specifically, the entire deep foundation pit is divided into 26 support sections; the intersection of the first subway line and the second subway line on the south side of the foundation pit and the intersection of the first subway line and the road side on the north side of the foundation pit are all 1A-1A sections, the east and west sides of the first subway line have 1B-1B sections, 1C-1C sections, 1D-1D sections, 1E-1E sections and 1F-1F sections, and the west side of the foundation pit has West A-West A sections, West B-West A sections, and West B-West B sections from north to south. -West B section and West C-West C section, the East A-East A section and East B-East B section are set on the east side of the foundation pit, the south side of the foundation pit is adjacent to the second subway line and is divided into 6A-6A section, 6B-6B section, 6C-6C section, 6D-6D section, 6D′-6D′ section, 6E-6E section, 6F-6F section and 6F′-6F′ section, the 6G-6G section, 6H-6H section and 6J-6J section are set on the east side of the foundation pit.

[0032] The specific parameters of the 26 support profiles are: 1) 1A-1A section: The excavation depth of the foundation pit is 5.3m, and a pile-anchor support system is adopted; the diameter of the supporting pile is 800mm, the pile spacing is 1.45m, and the pile length is 5.0m; a crown beam with a size of 900X600mm is arranged on the top of the pile. The concrete strength grade of the supporting pile and the crown beam is C25, and a total of 1 anchor rod is arranged.

[0033] 2) 1B-1B section: The excavation depth of the foundation pit is 4.5m, and a cantilever pile support system is adopted; the diameter of the supporting pile is 1000mm, the pile spacing is 1.5m, and the pile length is 9.0m; a cap beam is arranged on the top of the pile with a size of 1100X700mm, and the concrete strength grade of the supporting pile and the cap beam is C25.

[0034] 3) 1C-1C section: The excavation depth of the foundation pit is 6.74m, and a pile-anchor support system is adopted; the diameter of the supporting piles is 1000mm, the pile spacing is 1.5m, and the pile length is 12.0m; a cap beam is arranged on the top of the pile, with a size of 1100X700mm. The concrete strength grade of the supporting piles and the cap beam is C25, and a total of 1 anchor cable is arranged.

[0035] 4) 1D-1D section: The excavation depth of the foundation pit is 6.74m, and a pile support system is adopted; the diameter of the supporting piles is 1000mm, and the pile spacing is 1.3m; a crown beam is arranged on the top of the pile, with a size of 1100X800mm. The concrete strength grade of the supporting piles and crown beams is C25, and a total of 2 609x16 steel pipes are arranged; the internal support steel pipe support and steel perimeter purlin are made of Q235B.

[0036] 5) 1E-1E section: The excavation depth of the foundation pit is 16.02m, and a pile-anchor support system is adopted; the diameter of the supporting piles is 1000mm, the pile spacing is 1.5m, and the pile length is 12.0m; a cap beam is arranged on the top of the pile, with a size of 1100X700mm. The concrete strength grade of the supporting piles and the cap beam is C25, and a total of 1 anchor cable is arranged.

[0037] 6) 1F-1F section: The excavation depth of the foundation pit is 16.02m, and the pile anchor + soil nail wall support system is adopted; the diameter of the supporting pile is 1000mm, the pile spacing is 1.5m, and the pile length is 12.0m; a crown beam is arranged on the top of the pile, with a size of 1100X700mm, and the concrete strength grade of the supporting pile and the crown beam is C25, and a total of 1 anchor cable is arranged. The flower tube soil nail wall adopts a 1:1 slope, and a total of 6 flower tube soil nails are arranged.

[0038] 7) 6A-6A section: the excavation depth of the foundation pit is 15.22m, and a pile support system is adopted; the diameter of the supporting piles is 1000mm, and the pile spacing is 1.5m; a crown beam is arranged on the top of the pile, with a size of 1100X800mm, and the concrete strength grade of the supporting piles and crown beams is C25. A total of 2 609x16 steel pipes are arranged; the internal supporting steel pipe supports and steel perimeter purlins are made of Q235B.

[0039] 8) 6B-6B section: the excavation depth of the foundation pit is 21.80m, and the pile support + anchor cable support system is adopted; the diameter of the supporting pile is 1000mm, and the pile spacing is 1.5m; a crown beam with a size of 1100X800mm is arranged on the top of the pile, and the concrete strength grade of the supporting pile and the crown beam is C25. A total of 2 609x16 steel pipes are arranged; the internal support steel pipe support and steel perimeter purlin are made of Q235B.

[0040] 9) 6C-6C section: the excavation depth of the foundation pit is 21.80m, and a pile support system is adopted; the diameter of the supporting piles is 1000mm, and the pile spacing is 1.5m; a crown beam is arranged on the top of the pile, with a size of 1100X800mm, and the concrete strength grade of the supporting piles and crown beams is C25. A total of 2 609x16 steel pipes are arranged; the internal supporting steel pipe supports and steel perimeter purlins are made of Q235B.

[0041] 10) 6D-6D section: The excavation depth of the foundation pit is 10.22m, and a double-row pile support system is adopted; the diameter of the front row supporting piles is 1000mm, the pile spacing is 1.4m, and the pile length is 20m; a cap beam is arranged on the top of the pile; the diameter of the rear row supporting piles is 1000mm, the pile spacing is 1.4m, and the pile length is 20m; a cap beam is arranged on the top of the pile, with a size of 1100X800mm; the rigid frame beam is 1100X800mm; the supporting piles, rigid frame beam and cap beam are C25.

[0042] 11) 6D'-6D' section: The excavation depth of the foundation pit is 10.22m, and a support system combining double rows of cantilever piles and soil nail walls on the top of the existing structure is adopted.

[0043] 12) 6E-6E section: The excavation depth of the foundation pit of this support section is 4.5m, and a cantilever pile support system is adopted; the diameter of the support pile is 1000mm, the pile spacing is 1.5m, and the pile length is 12.0m; a cap beam is arranged on the top of the pile with a size of 1100X700mm, and the concrete strength grade of the support pile and the cap beam is C25.

[0044] 13) 6F-6F section: The excavation depth of the foundation pit of this support section is 10.22m, and a cantilever pile support system is adopted; the diameter of the support pile is 1000mm, the pile spacing is 1.3m, and the pile length is 20.0m; a cap beam is arranged on the top of the pile with a size of 1100X700mm, and the concrete strength grade of the support pile and the cap beam is C25.

[0045] 14) 6F'-6F' section: The excavation depth of the foundation pit of this support section is 10.22m, and a double-row pile support system is adopted; the diameter of the front row support piles is 1000mm, the pile spacing is 1.4m, and the pile length is 20m; a cap beam is arranged on the pile top; the diameter of the rear row support piles is 1000mm, the pile spacing is 1.4m, and the pile length is 20m; a cap beam is arranged on the pile top, with a size of 1100X800mm; the rigid frame beam is 1100X800mm; the support piles, rigid frame beam and cap beam are C25.

[0046] 15) 6G-6G profile: The excavation depth of the foundation pit is 10.22m. A pile-anchor support system is adopted. The diameter of the supporting piles is 800mm, the pile spacing is 1.5m, and the pile length is 16m. A cap beam with a size of 900X600mm is arranged on the top of the pile. The concrete strength grade of the supporting piles and the cap beam is C25, and a total of 1 anchor cable is arranged.

[0047] 16) 6H-6H section: The excavation depth of the foundation pit is 15.22m. A pile-anchor support system is adopted. The diameter of the supporting piles is 800mm, the pile spacing is 1.5m, and the pile length is 16m. A crown beam with a size of 900X600mm is arranged on the top of the pile. The concrete strength grade of the supporting piles and crown beams is C25, and a total of 2 anchor cables are arranged.

[0048] 17) 6J-6J section: The excavation depth of the foundation pit is 10.32m. A pile-anchor support system is adopted. The diameter of the supporting piles is 800mm, the pile spacing is 1.5m, and the pile length is 16m. A cap beam with a size of 900X600mm is arranged on the top of the pile. The concrete strength grade of the supporting piles and the cap beam is C25, and a total of 1 anchor cable is arranged.

[0049] 18) West A-West A section: The excavation depth of the foundation pit of this support section is 15.22m, and a cantilever pile support system is adopted; existing support piles are used.

[0050] 19) West B-West B Section: The excavation depth of the foundation pit of this support section is 15.22m, and a cantilever pile support system is adopted; existing support piles are used.

[0051] 20) West C-West C Profile: The excavation depth of the foundation pit of this support profile is 8.02m, and a double-row pile support system is adopted; the diameter of the front row of support piles is 800mm, the pile spacing is 1.4m, and the pile length is 20m; a cap beam is arranged on the top of the pile; the rear row is reinforced by grouting between piles using the vertical shaft structure; the rigid frame beam is 900X700mm; the support piles, rigid frame beam and cap beam are C25.

[0052] 21) North A-North A section: The excavation depth of the foundation pit is 16.02m. A pile-anchor support system is adopted. The diameter of the supporting piles is 800mm, the pile spacing is 1.5m, and the pile length is 22m. A cap beam with a size of 900X600mm is arranged on the top of the pile. The concrete strength grade of the supporting piles and the cap beam is C25. A total of 3 anchor cables are arranged.

[0053] 22) North B-North B section: The excavation depth of the foundation pit is 12.22m, and a pile-anchor support system is adopted. The diameter of the supporting piles is 800mm, the pile spacing is 1.5m, and the pile length is 16m. A cap beam with a size of 900X600mm is arranged on the top of the pile. The concrete strength grade of the supporting piles and the cap beam is C25, and a total of 2 anchor cables are arranged.

[0054] 23) East A-East A section: The excavation depth of the foundation pit is 10.82m, and a pile-anchor support system is adopted. The diameter of the supporting piles is 800mm, the pile spacing is 1.5m, and the pile length is 16m. A cap beam with a size of 900X600mm is arranged on the top of the pile. The concrete strength grade of the supporting piles and the cap beam is C25, and a total of 2 anchor cables are arranged.

[0055] 24) East B-East B section: The excavation depth of the foundation pit is 4.90m, and a decorative tube soil nail wall support system is adopted; a 1:0.5 slope is adopted, and a total of 3 decorative tube soil nail walls are arranged.

[0056] 25) Rear A-Rear A section: The excavation depth of the foundation pit is 5.40m, and a decorative tube soil nail wall support system is adopted; a 1:0.5 slope is adopted, and a total of 3 decorative tube soil nail walls are arranged.

[0057] 26) Rear B-Rear B section: The excavation depth of the foundation pit is 11.42m, and a pile-anchor support system is adopted. The diameter of the supporting piles is 800mm, the pile spacing is 1.5m, and the pile length is 16m. A cap beam with a size of 900X600mm is arranged on the top of the pile. The concrete strength grade of the supporting piles and the cap beam is C25, and a total of 2 anchor cables are arranged.

[0058] Slope protection pile type statistics table Serial number Section location Pile length Pile diameter quantity Support form Fertilizer trough width Excavation depth of foundation pit 1 Section 6A 21m 1.0m 17 roots Pile + Steel Support 800mm 15.22m 2 Section 6B 28.5m 1.0m 49 roots Pile+anchor cable+steel support 800mm 21.80m 3 6C Section 16m 1.0m 6 roots Pile+anchor cable+steel support 200mm 21.80m 4 6D Section 20m 1.0m 51 roots Double row cantilever piles 800mm 10.22m 5 6D' section 20m 1.0m 51 roots Double row cantilever piles + soil nail wall 800mm 10.22m 6 Section 6E 12m 1.0m 44 roots Cantilever pile 200mm 4.5m 7 6F Section 20m 1.0m 6 roots Cantilever pile 500mm 10.22m 8 6F' section 20m 1.0m 19 roots Double row cantilever piles 800mm 10.22m 9 6G Profile 16m 0.8m 5 Pile + anchor cable 1000mm 10.22m 10 6H Section 16m 0.8m 7 roots Pile + anchor cable 1000mm 15.22m 11 6J Section 16m 0.8m 16 roots Pile + anchor cable 1000mm 10.32m 12 Section 1A 5m 0.8m 20 pieces Pile + anchor cable 800mm 5.3m 13 Section 1B 9m 1.0m 67 roots Cantilever pile 200mm 4.5m 14 1C Section 12m 1.0m 43 roots Pile + anchor cable 200mm 6.74m 15 1D Section 13.5m 1.0m 51 roots Pile + Steel Support 200mm 6.74m 16 Section 1E 12m 1.0m 78 roots Pile + anchor cable 200mm 16.02m 17 1F Section 12m 1.0m 6 roots Pile + anchor cable + soil nail wall 200mm 16.02m 18 North A Section 20m 0.8m 138 roots Pile + anchor cable 940mm 16.02m 19 North B Section 16m 0.8m 11 roots Pile + anchor cable 940mm 12.22m 20 East A Section 16m 0.8m 13 roots Pile + anchor cable 940mm 10.82m 21 East B Section 20m 0.8m 30 pieces Flower pipe soil nail wall 940mm 4.90m 22 West A Section 24m 1.2m 18 roots Cantilever pile 1000mm 15.22m 23 West B Section 24m 1.2m 30 pieces Cantilever pile 1000mm 15.22m 24 West C Section 20m 0.8m 34 roots Double row cantilever piles 1287mm 8.02m 25 Rear A-section 14m 0.8m 13 roots Flower pipe soil nail wall 940mm 5.40m 26 Rear B section 16m 0.8m 13 roots Pile + anchor cable 940mm 11.42m In this application, a masonry retaining wall is set up around the surface of the deep foundation pit. The size of the masonry retaining wall can be 250mm high and 120mm wide, and the surface of the masonry retaining wall is plastered with mortar; longitudinal and transverse blind ditches are set up in the deep foundation pit to remove seepage and rainwater in the deep foundation pit. A water collection well is also set up in the deep foundation pit, and the water collection well is used to centrally pump out and eliminate water pressure seepage during the bottom plate construction. The setting of the water collection well is properly determined according to the construction segmentation and the amount of water. If upper stagnant water (temporary accumulation of water above the groundwater level, which may come from rainwater infiltration or pipeline leakage, etc.) is encountered during construction, drainage holes can also be installed for drainage. By adopting the above technical solution, the ground around the deep foundation pit is waterproofed and drained, which can strictly prevent ground water from invading the surrounding soil of the deep foundation pit, and realize the timely removal of rainwater in the deep foundation pit, further ensuring the safety and reliability of the deep foundation pit.

[0059] The support structure in this application is a temporary structure and its service life is 1 year after construction is completed. If it needs to continue to be used after the period expires, the safety of the support system must be evaluated and demonstrated by an expert group and confirmed by the original design unit.

[0060] In this application, composite support forms such as full-casing bored piles, soil nail wall slope protection, anchor cables and steel supports are used to provide all-round protection for the entire deep foundation pit. By adopting the above technical solution, it is possible to smoothly carry out operations under complex working conditions where there is an existing subway line running through the middle of the deep foundation pit and an existing subway line on one side of the deep foundation pit, ensuring the safe and stable operation of the two subway lines and the safety and reliability of the deep foundation pit. Example 2

[0061] Reference Figure 2 As shown, in this embodiment, a construction method of a high-low span support structure of a deep foundation pit adjacent to an existing subway line is provided, which is used to construct the high-low span support structure of a deep foundation pit adjacent to an existing subway line in the above-mentioned embodiment 1. The construction method comprises the following steps: S1: Construction preparation: including site leveling, site inspection, surveying and stake delivery, and neighbor building; specifically, site leveling, construction of temporary roads, steel bar processing yard, temporary water and electricity, fire fighting facilities, enclosure gates and temporary construction facilities; S2: Zoning construction: The entire foundation pit is divided into the East Zone located on the east side of the existing subway line, the West Zone located on the west side of the existing subway line, and the Central Zone located above the existing subway line, with the existing subway line running through the foundation pit as the dividing line; Carry out support construction on the support piles and crown beams on the north and south sides of the east and west areas; excavate the east side of the east area, and carry out soil nail wall construction on the upper part of the east side; excavate the west side of the west area, and carry out anchor spraying and backfill construction on the upper part of the west side; Then, the earthwork outside the safety protection zone of the existing subway line running through the foundation pit in the east and west areas was excavated, and the anchor spraying and anchor cable construction between piles was carried out; Afterwards, the upper earthwork of the existing subway line that runs through the foundation pit in the central area was excavated in layers and sections, and support construction was carried out on both sides; Specifically, in this step, after the earthwork on the upper side of the existing subway line that runs through the foundation pit is excavated to the explosion-proof layer area, the explosion-proof layer is broken; then the earthwork is excavated to the support pile elevation near the subway area; then the support piles are constructed on both sides of the existing subway line that runs through the foundation pit, and the top beams are made after the pile heads of the support piles are broken; At the intersection of the existing subway line running through the foundation pit and the existing subway line adjacent to one side of the foundation pit, the soil on the upper side of the existing subway line running through the foundation pit is excavated to the explosion-proof layer area, and the explosion-proof layer is broken; and the excavation is carried out to the elevation of the supporting piles adjacent to the subway area, and then the anchor spraying, anchor rod and soil nailing are carried out; then the low-elevation supporting piles on the side of the foundation pit adjacent to the subway area are constructed; and the crown beam is made after the pile heads of the supporting piles are broken; S3: Construction of remaining earthwork: The remaining earthwork is constructed in layers and sections, and anchor spraying and anchor cable construction are carried out between piles; S4: Finishing and acceptance: finishing of the bridleway and earthwork abandonment construction, final acceptance and data handover.

[0062] In this application, the support pile construction process adopts skipping construction, with one pile driven every two or three times. The safe distance between the newly concreted piles and the temporary pile holes should not be less than 4 times the pile diameter, the interval time should not be less than 24 hours, and the strength of adjacent bored piles should not be less than 50% of the design strength; and the mud density during the support pile construction process is 1.15~1.25.

[0063] In this application, the anchor cable construction spacing is one pile and one anchor, and a full-track anchor hydraulic drilling rig is used for construction. During the construction process, a "skip hitting" method is used to drill holes to avoid mutual influence between adjacent anchor cables. For the anchor cables at the positive corners of the foundation pit, when adjacent anchor cables are cross-constructed, the normal use function of the anchor cables is met by adjusting the anchor cable position or the anchor cable construction angle; at the same time, the angle between adjacent anchor cables is 15 to 20 degrees.

[0064] In this application, the earthwork excavation adopts the strip excavation method from top to bottom in layers and sections in sequence. The span of the segmented excavation shall not be greater than 15m, the layer height shall not be greater than 2.5m, and the time interval between the excavation of two adjacent warehouses and the upper and lower layers of earthwork shall not be less than 24 hours; in the process of earthwork excavation on both sides of the existing subway line that runs through the foundation pit, a symmetrical excavation method is adopted; when the excavation is 2m or 3m away from the existing subway line tunnel, manual cleaning is adopted, and 2m or 3m high soil slopes are left on both sides of the existing subway line tunnel structure to avoid damage to the subway structure tunnel and ensure lateral displacement of the tunnel structure. When constructing the support piles within 6.0m outside the structural edge line on both sides of the subway tunnel, a full sleeve follow-up construction process is adopted to prevent the collapse of the hole from affecting the subway structure. The earthwork on both sides of the subway is "excavated symmetrically in sections and layers" to ensure the deformation stability of the exposed subway section. By adopting the above technical scheme, the safety and reliability of the foundation pit construction process is further improved.

[0065] In this application, the soil nail wall construction work surface is excavated in sections and layers, with each section length of 15 to 20m. The excavation depth of each layer matches the vertical spacing of the soil nails to ensure that the over-excavation of each layer does not exceed 500mm. The soil nail wall construction and earth excavation are carried out alternately. After each step of the soil nail wall slope protection construction is completed and reaches 70% of the design strength, the next step of earth excavation is carried out. During the next step of earth excavation, the upper concrete surface layer and soil nails need to be protected to avoid touching the concrete surface layer and soil nails of the slope protection surface that have been constructed.

[0066] During the construction of this application, the maximum load around the deep foundation pit is 30kPa. Excessive loading is strictly prohibited. No other loading is allowed in the foundation pit near the subway. This application involves two subway lines. In order to minimize the disturbance to the soil under the subway tunnel, a high-power rotary drilling rig is used for the drilling of support piles. The high power and the weight of the drill bit are used to drill holes to minimize the disturbance to the soil. The slope piles close to the subway structure are followed up with full sleeves to prevent the collapse of the holes from affecting the subway structure. The support construction content of this application includes support piles, grouting reinforcement, prestressed anchor rods, steel supports, and anchor spraying construction, with an emphasis on support pile construction and anchor cable construction. During the construction of support piles, the width of the working surface is 15m, and the width of the anchor cable working surface is 8m. If the soft stratum cannot be used for normal traffic, house slag or steel plates are used as a passage for equipment to walk; and a steel bar processing site is set up.

[0067] The support piles in this application adopt the underwater concrete pouring construction technology of rotary drilling rig drilling, and the drilling rig model shall not be less than TR200~280D; the support pile construction process in this application includes: (1) Measurement and positioning 1) Pile center coordinate calculation and layout: The method of calculating the pile center coordinate is to use the analytical capture method on the AutoCAD drawing. Before capturing on the drawing, the electronic drawing coordinate system is calibrated to calibrate the relationship between the axis and the abutment and the pile. The pile center coordinate is obtained by geometric calculation based on the relative angle and distance relationship between the design center of the slope protection pile and each axis. Based on the plane control wire network point, the total station coordinate method or polar coordinate method is combined with the centering rod with a prism for on-site calibration.

[0068] 2) After the pile position is qualified, the next process can be carried out. The elevation is measured with a level to measure the elevation of the casing top to control the hole depth and the elevation of the steel cage and the concrete surface. After all the pile positions are inspected and found to be correct, they are handed over to the construction workers together with the casing elevation and the construction workers are supervised to arrange the pile positions.

[0069] (2) Buried casing The hole casing plays the role of guiding the drilling tools, controlling the pile position, protecting the hole mouth, isolating the surface water leakage, preventing the collapse of the surface fill, maintaining the water head height in the hole and fixing the steel cage.

[0070] Before drilling, the casing should be accurately buried at the determined pile position, with a length of 3m, and the bottom of the casing should be ensured to be seated in the original soil layer. The drilling position should be accurately fixed, the ground water should be isolated, the soil at the hole mouth should be stabilized, and the hole wall should be protected from collapse to facilitate the drilling work.

[0071] When constructing the supporting piles within 6 m beyond the structural edge lines on both sides of the existing subway that passes through the foundation pit, a full sleeve follow-up construction process is adopted to prevent the collapse of the holes from affecting the subway structure.

[0072] Steel plate casing is used, the diameter of the casing should be 100mm larger than the designed pile diameter, the top elevation of the casing should be 20 to 30cm higher than the construction surface, and ensure that the casing wall is perpendicular to the horizontal plane.

[0073] When positioning the casing, the pile position should be verified first, and then with the pile position as the center, mutually perpendicular cross control pile lines should be determined for cross bolt point control, the casing hole should be dug, the casing should be hoisted in, and the pores around the casing should be filled with soil and compacted with a drill bit. At the same time, the center of the casing and the center of the pile position should be corrected with a cross line to make them coincide.

[0074] (3) Rotary drilling rig wall protection mud In order to ensure the quality of the wall protection, mud is prepared on site and stored in a mud pool. The amount of mud stored can meet the daily demand for hole construction. During rotary drilling, the mud liquid level is maintained to form sufficient mud column pressure, and mud is added to the hole at any time. When pouring concrete, mud should be recycled in a timely manner for reuse and to prevent environmental pollution.

[0075] The mud components in this application are water, bentonite, alkali, and high-viscosity cellulose, and the weight ratio thereof is 100:15:1.8:0.8. The mud performance indicators and test methods are shown in the table below.

[0076] Mud performance indicators and test methods Mud performance Clay soil sandy soil Inspection methods proportion 1.15～1.2 1.15～1.25 NB-1 Mud Density Meter Viscosity 20～24 25～30 ZNN Rotational Viscometer Sand content (%) <3 <4 LNH type mud sand content tester pH 8～9 8～9 PHS-2 acidity meter (4) Drilling 1) When drilling the slope protection piles, the drilling method is to skip every two holes to avoid drilling in series, and the adjacent holes are constructed 24 hours later.

[0077] 2) Put the drilling rig in place, align the drill bit with the pile position, and adjust the verticality of the drilling rig after verification.

[0078] 3) Before drilling, use a level to measure the top elevation of the casing at the hole mouth in order to control the drilling depth. At the beginning of drilling, pay attention to the drilling speed, and the drilling speed should be adjusted appropriately for different formations.

[0079] 4) During the drilling process, observe, check, adjust and control the verticality of the drill rod at any time. The rotary drilling rig can control the verticality through the equipment's own plumb system. At the same time, a plumb is provided on the drill rod, and a theodolite is used to check the verticality of the drill rod. During the construction process, the quality of the hole is strictly controlled, especially in the pile support system. The pile position deviation and the verticality of the pile body are strictly controlled to prevent the quality problem of broken piles after the piles invade the limit.

[0080] 5) The supporting piles shall be constructed by skipping, with one pile driven every two or three times. The safe distance between the newly concreted piles and the temporary pile holes shall not be less than 4 times the pile diameter, or the interval time shall not be less than 24 hours.

[0081] 6) When drilling the west support piles, the original prestressed anchor cables may be encountered, which may cause entanglement on the drill bit. For the slope protection pile construction in this area, a drilling rig larger than TR220D is used for construction. If steel strands are encountered during drilling, the drilling should be carried out slowly and the steel strands should be cut off gradually. During construction, the viscosity of the mud wall protection should be increased to avoid hole collapse. If a hole collapse occurs during the cutting of the steel strands, high-quality clay soil should be backfilled in time and left for at least 5 days before drilling can be continued.

[0082] (5) Clearing the hole After the drilling depth reaches the designed depth, the first hole cleaning is carried out, and the drilling rig is allowed to drill without advancing. At the same time, the hole is cleaned by water jetting. The mud water level in the hole should be kept at about 0.5m below the casing to prevent the hole from collapsing. The second hole cleaning is carried out after the steel cage and the guide tube are installed. When the sediment thickness does not meet the requirements (greater than 100mm), the slag is discharged by air lift reverse circulation and other methods. After the hole is cleaned, the sediment thickness is detected with a measuring rope. If it meets the requirements, the concrete pouring construction is immediately organized.

[0083] (6) Hole formation inspection After the drilling and cleaning is completed, before the installation of the steel cage, the quality inspector will first check the hole quality, mud index test, hole depth, hole diameter, verticality, thickness of sediment at the bottom of the hole, etc.

[0084] (7) Fabrication and hoisting of steel cage In order to facilitate the processing and production of the steel cage on site, a special steel processing and production platform is set up in the middle of the site. The steel cage is made according to the design drawing. The specific processing steps are as follows: ①. Place the main reinforcement of the steel cage on the processing platform, first fix the main reinforcement of the steel cage with the reinforcing hoop, the reinforcing hoop is arranged on the inside of the steel cage, and a reinforcing hoop is set every 2m along the pile length and spot welded to the longitudinal reinforcement to form a steel cage skeleton. After the main reinforcement of the steel cage is positioned, it is then fixedly connected with the reinforcement ring.

[0085] ② After the steel cage frame is processed, the spiral stirrups are installed.

[0086] ③. After the installation of spiral stirrups is completed, the steel cage is reinforced as a whole.

[0087] ④. Carry out hoisting construction.

[0088] (8) Downpipe Use a conduit with a diameter of 250-300mm, and the maximum diameter of the joint is about 380mm. The length of the bottom pipe is 4-6m, the standard section is 2-3m, and the joint is a double-threaded square buckle quick connector. Before the first use, a closed water pressure test should be carried out, and the test pressure is 0.75-1.0MPa. It is qualified if there is no water leakage. The distance from the bottom of the conduit to the bottom of the hole is 300-500mm. The funnel is installed at the top of the conduit.

[0089] (9) Underwater concrete pouring After the steel cage is lowered and the conduit is installed, concrete pouring should be carried out immediately to prevent the hole from being left dry for too long and the sediment from exceeding the standard.

[0090] 1) Concrete requirements The concrete strength of slope protection pile construction is C25, and no admixture containing calcium chloride is allowed to be used in the concrete mix ratio. The prepared concrete should be dense, have good workability and fluidity, and good expansion, and the slump should be controlled at 200mm±20mm. After the concrete is delivered to the site, the slump and workability indicators should be inspected each time, and it can only be poured after passing the inspection. Before pouring concrete, the quality inspector of the professional branch shall check the slump of the concrete and record it. Concrete should not segregate during transportation and pouring.

[0091] 2) Preparation before pouring The drilling depth and the sediment at the bottom of the hole must be accepted before pouring concrete. The mud density within 500mm of the bottom of the hole should be less than 1.25; the sand content should not be greater than 8%; and the viscosity should not be greater than 28s.

[0092] The size of the water stopper ball used in the conduit should be appropriate, and it should be installed correctly, usually above the water surface. The hole should be tightly covered before pouring concrete to prevent concrete from falling into the hole and contaminating the mud. The upper funnel of the conduit is equipped with a steel bar grate with a mesh size not larger than 100×100mm to filter out large lumps or stones to prevent clogging of the pipe.

[0093] 3) Concrete pouring ① After the installation of the steel cage is completed, the concealed project acceptance should be carried out. After the steel cage and the conduit are fully installed, the second hole measurement should be carried out to calculate the thickness of the sediment in the first comparison. If it meets the requirements, the underwater concrete should be poured immediately, otherwise the second hole cleaning should be carried out before pouring.

[0094] ②. The commercial concrete used should have good workability and the slump should be 18 to 22 cm.

[0095] ③. For underwater concrete pouring, a conduit with a diameter of 250-300mm should be used, and the conduit joint should preferably be a key-pin type quick connector. The conduit should be assembled and pressure tested before use, and the test water pressure should be 0.6-1.0Mpa. Damaged sealing rings should be replaced in time.

[0096] ④. After the second hole cleaning test and all indicators of the mud and the thickness of the sediment at the bottom of the hole meet the requirements, pour concrete within 4 hours. If the time is exceeded, the mud should be re-tested and the hole cleaned.

[0097] ⑤. When commercial concrete is used for pouring, after the concrete tank truck arrives at the construction site, the foreman or professional quality inspection personnel shall check the concrete receipt, opening appraisal and other commercial concrete materials to determine the corresponding pile position, concrete strength grade, slump, departure time, etc. of the concrete in the truck. After all the reviews are completed, check whether the slump and workability of the concrete meet the quality requirements. Only after they are qualified can pouring be carried out.

[0098] ⑥ Before pouring concrete, put a spherical ball into the conduit as a water stopper. In order to allow the water stopper to be discharged smoothly, the distance from the bottom of the conduit to the bottom of the hole should be 300-500mm. When pouring concrete, it is required to be released quickly so that the conduit has enough burial depth. Install a slurry pump at the hole mouth, and the returned slurry is recovered into the return slurry pool.

[0099] ⑦. There should be enough concrete reserve to bury the conduit more than 1.0m below the concrete surface for the first time. The calculation formula for the first pouring volume is as follows:

[0100] V——the first pouring volume of concrete (m3); h——pile hole depth (m); h1——The height required for the concrete column inside the conduit to balance with the mud column outside the conduit ; h2——the height of the outer concrete surface of the conduit after the initial concrete is poured, which is 1.5m (the depth from the bottom of the conduit to the bottom of the trench is 0.5m, and the buried depth of the conduit is 1.0m); d——Inner diameter of the catheter (m), take 0.3m; D——pile hole diameter (m); k——filling coefficient, take 1.1; rw——mud density, take 12kN / m3; rc——concrete density, take 24kN / m3 ⑧. The buried depth of the conduit should be 2 to 6 meters. It is strictly forbidden for the conduit to protrude from the concrete surface. A dedicated person should measure the buried depth of the conduit and the height difference of the concrete inside and outside the pipe, and fill in the concrete pouring record.

[0101] ⑨. Underwater concrete must be constructed continuously, and the pouring time of each pile shall be controlled according to the initial setting time of the initial batch of concrete.

[0102] ⑩. Control the last pouring volume. The pile top must not be too low. There is a floating slurry layer in contact with the concrete on the upper layer of concrete that needs to be chiseled off. For this reason, the concrete height needs to be over-poured by 1.0m so that the strength of the concrete can be ascertained after hardening. The part above the design elevation should be chiseled out manually or with a combination of manual and pneumatic picks.

[0103] ⑪. Concrete strength test blocks should be made for concrete pouring, and one group of test blocks should be made for each pile. The test blocks should be well cured, and after reaching a certain strength, they should be immediately removed from the mold and sent to the curing room for standard curing.

[0104] ⑫. After the concrete construction is completed, the concrete factory certificate and concrete strength report should be collected and the concrete strength assessment should be carried out.

[0105] ⑬. Prepare, collect and organize various original construction records, quality inspection records and other original materials, and keep construction logs.

[0106] ⑭. After pouring, cover the hole with steel plate or steel mesh in time and mark it clearly to prevent people from falling into the hole. The empty hole can be backfilled after 6 hours of pouring.

[0107] ⑮. The time for pouring concrete should be accelerated as much as possible to prevent the fluidity of the concrete from decreasing when it enters the steel cage. When the concrete in the hole is close to the steel cage, the buried pipe should be kept deep and the pouring progress should be slowed down. When the pouring is about to end, the height of the concrete column in the conduit decreases, the pressure difference decreases, and the viscosity and specific gravity of the mud and the debris contained in the conduit increase. If there is difficulty in the rising of concrete, water can be added to dilute the mud in the hole, or some sediment can be removed to ensure smooth pouring.

[0108] ⑯. During the concrete pouring process, the conduit should always be kept in the center. When lifting the conduit, a dedicated person should be in charge to prevent the steel frame from tilting or shifting. If the frame is found to be rising, the conduit should be immediately stopped from being lifted, and the conduit should be lowered and gently shaken to separate it from the frame. ⑰. When the concrete is poured to within 5.0m above the pile hole, the guide tube can be lifted no longer until it is poured to the designed elevation and then pulled out at once. After pouring to the designed elevation of the pile top, the over-pouring amount should be not less than 1.0m to ensure the strength of the concrete on the pile top after the floating slurry is chiseled off.

[0109] ⑱. After the concrete construction is completed, the concrete factory certificate and concrete strength report should be collected and the concrete strength assessment should be carried out.

[0110] ⑲. The safe distance between the newly concreted pile and the temporary pile hole should not be less than 4 times the pile diameter, or the interval time should not be less than 24 hours.

[0111] The construction process of the pile top crown beam in this application includes: (1) Template production and installation The template adopts a combined template, and is supported by diagonal braces, horizontal braces, and U-shaped cards according to the designed position, shape, and size. The mold should be clean and free of debris, the joints should be tight and the support should be stable and firm.

[0112] (2) Steel bar production and binding The top of the slope protection pile is roughened to the designed elevation, and the pile head is blown clean with high-pressure air. The steel bars are then tied, and the stirrups and some main bar hooks are processed at the construction site. The lap length of the steel bars during tying is ≥35d, and the number of joints in the same section is not more than 50% of the number of main bars.

[0113] (3) Casting Commercial concrete is used for crown beam casting, and straight stubble is left at each construction joint. Before the next casting, it should be roughened and cleaned. The cumulative time of concrete transportation, pouring and interval should not exceed the initial setting time of concrete. The crown beam concrete should be poured continuously and completed in one go.

[0114] (4) Concrete maintenance Concrete should be covered and maintained within 12 hours after pouring. The time for concrete watering and curing: for concrete made of ordinary Portland cement or slag Portland cement, it should not be less than 7 days; for concrete made of slow-setting admixture, it should not be less than 14 days. The number of watering should be enough to keep the concrete moist. In winter construction, after pouring, it should be covered with insulation cotton and test blocks with the same conditions should be set.

[0115] (5) Demolding The concrete strength when the formwork is removed should be able to ensure that the surface and edges of the crown beam are not damaged. The exposed surface after formwork removal should be cured as soon as possible and protected during the construction process to avoid damage and impact loads.

[0116] The anchor support construction process in this application includes: (1) Anchor hole positioning 1) The anchor hole is located between the two piles, and the anchoring force is transmitted to the supporting piles through the steel waist beam. After the earthwork is excavated to find the anchor position and the anchor construction working surface is leveled, the anchor hole position is checked after the anchor drilling rig is in place. Before construction, the azimuth and inclination of the drilling tool axis are strictly tested to ensure that the azimuth and inclination of the anchor hole meet the design requirements.

[0117] 2) Marking: Use red paint to mark the center point and number of the anchor hole at the measured hole position.

[0118] (2) Drilling rig in place In order to ensure that the depression angle and azimuth angle of the anchor hole axis during construction and after drilling meet the design requirements, the accuracy and stability of the drilling rig must be guaranteed, and the axis of the drilling tool must be strictly checked to ensure that it is consistent with the axis azimuth of the designed anchor hole.

[0119] (3) Drilling 1) The vertical error of the horizontal hole spacing of the anchor drilling holes shall not exceed 100mm, and the deviation shall not exceed 5%.

[0120] 2) Anchor construction should be carried out after the hole position is adjusted according to the designed inclination angle. The hole spacing error of the anchor bolt in the horizontal direction should not exceed 50mm, and the hole spacing error in the vertical direction should not exceed 100mm.

[0121] 3) The anchor hole depth should not be less than the design length and should not be greater than 1% of the design length.

[0122] 4) If the anchor construction encounters underground obstacles, it cannot be forced to drill in. After the obstacles are found, the anchor hole position and angle can be adjusted to avoid obstacles such as pipelines.

[0123] 5) Anchor construction must avoid the drainage well pipe and its filter layer to prevent the well from being silted up during anchor grouting.

[0124] (4) Anchor cable requirements The anchor cable bundle steel strands in this application use 1860MPa prestressed steel strands. Before installation, ensure that each steel strand is straight, not twisted or forked, evenly arranged, and derusted and degreased, and remove any dead bends, mechanical damage, and rust pits. A centering bracket is set every 1.5m within the anchoring section to ensure that the thickness of the anchor cable protective layer is not less than 20mm. When making the anchor cable, cut the material according to the designed length and cover the free end with a hose. After cutting, the steel strands need to be degreased and derusted to keep the steel strands clean. Then tie the steel strands into bundles according to the design requirements, and install the anchor cable bracket, grouting pipe, etc. according to the construction drawings. The manufactured steel strand bundles should be stacked on the brackets for the pipe rack and must not be placed directly on the ground. Classify and number the manufactured anchor cables, and indicate the anchor hole numbers to avoid confusion.

[0125] After the drilling is completed and accepted, the anchor should be lowered into the anchor hole in time. When anchoring, the anchor cable should be kept straight and sent into the hole at a uniform and slow speed. When encountering resistance, do not force it through. The anchor cable should be properly pulled back and then sent in. If it still cannot be lowered after repeated attempts, the cause should be found, and the anchor cable should be removed if necessary, and the hole should be cleaned again with an air compressor.

[0126] (5) Grouting 1) PO42.5 bulk cement is used for anchor grouting. A 70T bulk cement tank is arranged on site to supply cement for on-site anchor construction.

[0127] 2) The slurry material is pure cement slurry, and the cement and water-cement ratio is controlled at 0.5-0.55. In order to improve the early strength of the grouting body, an appropriate proportion of early strength agent should be added. The cement slurry mixed once should be used up before the initial setting.

[0128] 3) The strength grade of grouting slurry material shall not be lower than the design strength grade.

[0129] 4) The grouting adopts the method of combining bottom hole return grouting and pressure grouting. Since the strata are mostly pebble layers, in order to ensure the density of the grouting body, the first grouting adopts the grouting pipe tied to the steel strand for bottom hole return grouting process, and the second grouting adopts 0.5-1.0MPa pressure grouting, injecting 40-45kg cement per linear meter.

[0130] 5) The grouting method is the bottom grouting method. The grouting pipe and the anchor cable are tied together and put into the hole at the same time. The injected cement slurry gradually returns to the hole mouth from the bottom of the hole. The grouting is stopped after overflowing from the hole mouth. The hole mouth is grouted after initial setting. The grouting pipe should be inserted to 500mm from the bottom of the hole, and the hole is sealed and grouting is performed. When the cement slurry is pressed out from the grouting pipe, it is pressurized to the design pressure value, and the pressure is maintained for 3 minutes. The grouting pipe is sealed and the grouting is completed.

[0131] 6) During the grouting process, strictly measure according to the mix ratio, stir evenly and filter. Try to reduce the grouting pressure, appropriately extend the grouting time, and observe the deformation of the foundation pit wall. If there is any sign of deformation into the foundation pit, the construction should be stopped immediately and reported to the owner, supervisor and designer.

[0132] 7) When grouting starts or stops for more than 30 minutes, use water or dilute cement slurry to lubricate the grouting pump and its pipelines. Stir the cement slurry evenly and use it as soon as it is mixed. The cement slurry mixed once should be used up before initial setting.

[0133] 8) When the grouting operation starts or stops for a long time, the pump and grouting pipeline should be moistened with clean water before grouting. After the grouting is completed or when there is a long time before the next grouting, the grouting pipeline should be cleaned in time. If the cement slurry fails to fill the anchor hole after hardening, a second grouting should be carried out and the hole should be sealed.

[0134] (6) Anchor bolt single cycle acceptance and tensioning: After grouting, when the cement slurry strength reaches 75% of the design strength and the pile body concrete strength reaches 70%, the anchor cable is tensioned. The anchor cable tensioning work is carried out section by section along the support pile profile to ensure that the support pile is evenly stressed.

[0135] (7) Anchor bolt acceptance The construction unit shall entrust a third party to conduct multi-cycle tensioning acceptance test on 5% of the total number of prestressed anchor rods and no less than 3 anchor rods. The remaining prestressed anchor rods shall be subject to single-cycle tensioning acceptance test before tensioning construction, and tensioning and locking shall be carried out after passing the test.

[0136] The process flow of pile wall construction in this application includes: (1) Soil finishing between piles After excavation, the soil surface between the piles should be manually trimmed in time, and the allowable deviation of the flatness is ±20mm.

[0137] (2) Hanging steel mesh The specification of the steel mesh is φ6@200mm×200mm. The outer side of the mesh is pressed with Φ16mm steel bars. Use an electric hammer to drill holes on both sides of the pile close to the soil between the piles and perpendicular to the pile side, drive 8~10cm into the pile body, and insert Φ16mm steel bars as mesh pressing bars.

[0138] (3) Surface sprayed concrete The spraying operation should be carried out in sections, and the spraying sequence of the same section should be from bottom to top. During spraying, the nozzle should be kept vertical to the sprayed surface, the spraying distance should be 0.8 to 1.5m, and the spraying thickness of the wall between the outer support piles on the east, west and north sides of the foundation pit is 50mm; the spraying thickness of the wall between the support piles in the area close to Metro Line 6 on the south side and the support piles in the area close to Metro Line 1 in the middle is 80mm. When constructing each layer of shotcrete, make two groups of concrete test blocks and inspect them after 7 and 28 days of standard curing. The anchor spraying operation of this project adopts commercial concrete wet spraying operation.

[0139] The steel support construction process in this application includes: (1) Fabrication and installation of steel purlins Subsequent steel supports cannot be directly supported on piles, and steel purlins need to be set up. Steel purlins are rented or made by yourself, and are fixed on the retaining piles with corbels. Steel purlins are made in sections and welded and installed in the foundation pit.

[0140] (2) Fabrication and installation of diagonal bracing supports The end well position inclined support uses steel inclined support to transmit axial force. The size of the inclined support is set according to the size of the fixed end and the movable end support surface of the steel support. The steel inclined support is processed and manufactured using 20mm steel plate. After being manufactured, the position is determined according to the measurement and layout, and then welded and installed with the surrounding purlin.

[0141] (3) Steel support construction Each steel support is configured with one fixed end and one movable end according to the total length. The middle section is configured with standard pipe joints, and the pipe joints are connected with flange high-strength bolts. The steel pipe support is assembled in a groove and hoisted and erected with a 25-ton truck crane.

[0142] Support installation construction technology ①. Preliminary preparation and support inspection and acceptance Before installing the steel pipe support, the processed steel pipe support must be inspected and accepted. The steel support specifications must be selected according to the design requirements or the design axial force and the requirements of the "Code for Design of Foundation Pit Engineering".

[0143] ②. Steel pipe support assembly After the steel pipe supports are delivered to the site, they are stacked according to specifications. When assembling, the required pipe sections are neatly placed on the site in the assembly order. The whole section is assembled manually using wrenches, and each pipe section is connected with high-strength flange bolts.

[0144] Each steel support is configured with one fixed end and one movable end according to the total length, and the middle section is configured with standard pipe sections.

[0145] ③. Crane hoisting The steel support is hoisted at two points, and the hoisting point is generally about 0.2L to 0.25L from the end. Pay attention to balance and stability during hoisting, and do not hoist too fast to avoid colliding with the erected steel support and causing the steel support to fall and cause a safety accident. According to the on-site construction conditions, a 25t truck crane is used for hoisting. If the site conditions are limited and the 25-ton truck crane cannot meet the construction requirements, a 50t truck crane can be used for hoisting.

[0146] ④. Steel support hanging and adjustment Before lowering the steel support, it should be stabilized just above the center line of the crown beam or corbel at the installation location, and then slowly lowered to place the steel support on the crown beam or corbel.

[0147] ⑤. Apply prestress in stages During construction, a 200T jack is used to apply prestress. The prestress value of the first support should be 50% of the designed prestress value, and the prestress value of the second and third supports should be 60% of the designed prestress value. The axial force application should be dynamically controlled and adjusted according to monitoring. It is not allowed to construct the axial force to 100% of the designed prestress value at one time. The jack itself must be equipped with a pressure gauge and must be calibrated before use. The pressure is applied symmetrically step by step on both sides of the movable end. The prestressed axial force of the steel support is applied step by step twice. The first time it is applied to 70% of the prestressed axial force of the support, it is stopped for 5 minutes, and the second time it is applied to 100% of the prestressed axial force of the support. The jack stops pressurizing. After the pressure gauge reading is stable for 5 minutes, and the prestressed axial force is consistent with the axial force monitoring data of the steel support, the movable end is locked with a steel wedge. The steel support will have axial force reduction when locked, and the axial force of the steel support is basically consistent with the designed prestressed axial force after locking. The pre-stress is applied as per the design requirements and when there is no obvious attenuation of the pressure gauge, the steel support is locked with a specially shaped steel wedge and then the jack is removed.

[0148] ⑥. Wedge locking After the pre-axial force is completed, the gap between the sliding long groove at the rear of the movable end and the end face of the steel pipe is tightly filled with a steel wedge, and then the jack is removed.

[0149] 7. Monitoring After the support is erected, strengthen monitoring and measurement during the construction process, observe and monitor the deformation and displacement of the enclosure structure and steel support according to the monitoring frequency of the monitoring plan, so as to take measures to ensure the safety of the structure and personnel. For specific monitoring methods, see the monitoring plan.

[0150] The soil nail wall construction process includes: (1) Slope repair: artificial slope repair is adopted, and the slope is strictly controlled by surveyors; (2) Hole drilling for soil nailing: Use a casing drill to drill to the designed depth; (3) Steel bar processing: Made of 1DN48 steel pipe; (4) Grouting material: The grouting material is M20 cement slurry for gravity grouting, with a water-cement ratio of 0.5 to 0.55. In order to improve the early strength of the flower tube solid, a certain amount of additives needs to be added if necessary; (5) Grouting liquid stirring: Use a mixer for stirring for no less than 2 minutes; (6) Grouting method: Gravity flow is used for grouting; (7) Surface steel mesh: The surface steel mesh is woven directly on the slope surface by hand. The overlap of the steel mesh is made by combining binding and spot welding. The intersection of horizontal and vertical bars is tied by fire wire. (8) Mixing of shotcrete materials: using commercial concrete wet spraying; (9) Sprayed concrete: The distance between the sprayer and the slope surface should be kept between 1.5 and 2.0 m, and the spray gun should spray vertically on the slope surface. The spray thickness of the soil nail wall layer should not be less than 100 mm.

[0151] The present application realizes a high and low span support structure for a deep foundation pit near an existing subway line. During the excavation of the deep foundation pit, the present application adopts comprehensive support forms such as pile rows, anchor cables, steel supports and full-section grouting according to local conditions, which significantly improves the safety and reliability of the deep foundation pit construction, while ensuring the safety and stability of the deep foundation pit construction near the subway. While achieving the dual goals of construction and protection, it improves work efficiency, reduces the risk factor of the entire project, and reduces unnecessary economic losses and social negative impacts.

[0152] The embodiments of this specific implementation are all preferred embodiments of the present application, and are not intended to limit the protection scope of the present application. The same components are represented by the same figure marks. Therefore, any equivalent changes made based on the structure, shape, and principle of the present application should be included in the protection scope of the present application.

Claims

1. A high-low span support structure for a deep foundation pit near an existing subway line, characterized in that: It includes slope protection piles located around the deep foundation pit and on both sides of the existing subway line running through the deep foundation pit; the slope protection piles are formed by a full sleeve construction process; An upper pressure earthwork is reserved on the upper side of the existing subway line running through the deep foundation pit, and soil nail wall slope protection is arranged on both sides of the upper pressure earthwork; The support composed of the slope protection piles includes: First pile-anchor supports are arranged at both sides of the deep foundation pit and are located above the existing subway line that passes through the pit; the first pile-anchor supports include a plurality of first support piles arranged at intervals and at least one first anchor rod, and a first cap beam is arranged at the top of the first support pile; The second pile-anchor support, the first single-row cantilever pile support and the first steel support cofferdam support are arranged in the deep foundation pit on both sides of the existing subway line passing through; the second pile-anchor support includes a plurality of second support piles arranged at intervals and at least three second anchor rods; the second support piles are provided with a second crown beam on their tops; the first single-row cantilever pile support includes a plurality of third support piles; the third support piles are provided with a third crown beam on their tops; the first steel support cofferdam support includes a steel perimeter purlin, a plurality of fourth support piles and at least two inner support steel pipes; the fourth crown beam is arranged on the fourth support piles, and the inner support steel pipes and the steel perimeter purlin are made of Q235B material; A third pile-anchor support, a second single-row cantilever pile support, a first double-row cantilever pile support and a second steel support cofferdam support are arranged on one side of the deep foundation pit near the existing subway line; the third pile-anchor support includes a plurality of fifth support piles arranged at intervals and at least four third anchor rods, and a fifth crown beam is arranged on the top of the fifth support pile; the second single-row cantilever pile support includes a plurality of sixth support piles, and a sixth crown beam is arranged on the top of the sixth support pile; the first double-row cantilever pile support includes a rigid frame beam and a plurality of seventh support piles, and the plurality of seventh support piles are divided into two rows, front and rear, and a seventh crown beam is arranged on the top of the seventh support pile; the second steel support cofferdam support includes a plurality of eighth support piles and at least six inner support steel pipes; an eighth crown beam is arranged on the top of the eighth support pile; A fourth pile-anchor support is arranged on one side of the deep foundation pit near the high-rise building; a second double-row cantilever pile support is arranged on the other side of the deep foundation pit near the high-rise building; the fourth pile-anchor support comprises a plurality of ninth support piles arranged at intervals and at least two fourth anchor rods, and a ninth crown beam is arranged on the top of the ninth support pile; the second double-row cantilever pile support comprises a rigid frame beam and a plurality of tenth support piles located in the front row, and the rear row support of the second double-row cantilever pile support is reinforced and formed by using a shaft structure and grouting between piles; The side walls around the deep foundation pit and the side walls on both sides of the existing subway line running through the deep foundation pit are formed into protective slopes by full-section grouting.

2. The high-low span support structure for a deep foundation pit near an existing subway line according to claim 1 is characterized in that: A masonry retaining wall is arranged on the surface around the deep foundation pit, and the surface of the masonry retaining wall is plastered with mortar; Longitudinal and transverse blind ditches are set in the deep foundation pit to drain the seepage and rainwater in the deep foundation pit. A collection well is also set in the deep foundation pit.

3. A construction method for a high-low span support structure of a deep foundation pit adjacent to an existing subway line as described in claim 1 or 2, characterized in that: The construction method comprises the following steps: S1: Construction preparation: including site leveling, site inspection, surveying and stake delivery, and neighbor building; S2: Zoning construction: The entire foundation pit is divided into the East Zone located on the east side of the existing subway line, the West Zone located on the west side of the existing subway line, and the Central Zone located above the existing subway line, with the existing subway line running through the foundation pit as the dividing line; Carry out support construction on the support piles and crown beams on the north and south sides of the east and west areas; excavate the east side of the east area, and carry out soil nail wall construction on the upper part of the east side; excavate the west side of the west area, and carry out anchor spraying and backfill construction on the upper part of the west side; Then, the earthwork outside the safety protection zone of the existing subway line running through the foundation pit in the east and west areas was excavated, and the anchor spraying and anchor cable construction between piles was carried out; Afterwards, the upper earthwork of the existing subway line that runs through the foundation pit in the central area was excavated in layers and sections, and support construction was carried out on both sides; S3: Construction of remaining earthwork: The remaining earthwork is constructed in layers and sections, and anchor spraying and anchor cable construction are carried out between piles; S4: Finishing and acceptance: finishing of the bridleway and earthwork abandonment construction, final acceptance and data handover.

4. The construction method of the high-low span support structure of a deep foundation pit near an existing subway line according to claim 3 is characterized in that: In the above step S2, after the earthwork on the upper side of the existing subway line passing through the foundation pit is excavated to the explosion-proof layer area, the explosion-proof layer is broken; then the earthwork is excavated to the support pile elevation adjacent to the subway area; then support piles are constructed on both sides of the existing subway line passing through the foundation pit, and the crown beams are manufactured after the pile heads of the support piles are broken.

5. The construction method of the high-low span support structure of a deep foundation pit near an existing subway line according to claim 3 is characterized in that: In the above step S2, at the intersection of the existing subway line running through the foundation pit and the existing subway line adjacent to one side of the foundation pit, the soil on the upper side of the existing subway line running through the foundation pit is excavated to the explosion-proof layer area, and then the explosion-proof layer is broken; and the excavation is carried out to the elevation of the supporting piles adjacent to the subway area, and then the anchor spraying, anchor rod and soil nailing are carried out between the piles; then the low-elevation supporting piles on the side adjacent to the subway area close to the foundation pit are constructed; and the crown beam is manufactured after the pile heads of the supporting piles are broken.

6. The construction method of the high-low span support structure of a deep foundation pit near an existing subway line according to claim 3 is characterized in that: During the support pile construction process, skip driving is adopted, with every two or three piles being driven. The safe distance between the newly concreted piles and the temporary pile holes shall not be less than 4 times the pile diameter, the interval time shall not be less than 24 hours, and the strength of adjacent bored piles shall not be less than 50% of the design strength; and the mud density during the support pile construction process shall be 1.15-1.

25.

7. The construction method of the high-low span support structure of a deep foundation pit near an existing subway line according to claim 3 is characterized in that: The anchor cable construction spacing is one pile and one anchor, and the full-track anchor hydraulic drilling rig is used for construction. During the construction process, the "jump-drilling" method is used to drill holes to avoid mutual influence between adjacent anchors; When adjacent anchor cables are crossed at the positive corners of the foundation pit, the normal use function of the anchor cables is met by adjusting the anchor cable positions or the anchor cable construction angles; the angle between adjacent anchor rods is 15 to 20 degrees.

8. The construction method of the high-low span support structure of a deep foundation pit near an existing subway line according to claim 3 is characterized in that: The earth excavation adopts the strip excavation method and is carried out in sequence from top to bottom in layers and sections. The span of the section excavation is not more than 15m, the layer height is not more than 2.5m, and the time interval between the excavation of two adjacent warehouses and the upper and lower layers is not less than 24 hours. During the earth excavation on both sides of the existing subway line that runs through the foundation pit, a symmetrical excavation method is adopted. When the excavation is 2m or 3m away from the existing subway line tunnel, manual cleaning is adopted, and 2m or 3m high soil slopes are left on both sides of the existing subway line tunnel structure to avoid damage to the subway structure tunnel.

9. The construction method of the high-low span support structure of a deep foundation pit near an existing subway line according to claim 3 is characterized in that: The soil nail wall construction working surface is excavated in sections and layers, with each section being 15 to 20 meters long. The excavation depth of each layer matches the vertical spacing of the soil nails, and the over-excavation of each layer does not exceed 500 mm.

10. The construction method of the high-low span support structure of a deep foundation pit near an existing subway line according to claim 3, characterized in that: The soil nail wall construction and earth excavation are carried out alternately. The next earth excavation will be carried out after each step of soil nail wall slope protection construction is completed and reaches 70% of the design strength. During the next step of earth excavation, the upper concrete surface layer and soil nails must be protected to avoid touching the concrete surface layer and soil nails of the slope protection surface that have been constructed.

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