Deep and large foundation pit support system and construction method spanning over existing subway interval structure

Abstract

This application relates to a deep and large foundation pit support system and construction method for straddling an existing subway section structure, belonging to the technical field of civil engineering construction. The deep and large foundation pit support system in this application includes a number of full casing support piles located on both sides of the existing subway line and a beam-slab covering the top of the existing subway line; the beam-slab includes a rigid frame beam and a reinforced concrete slab cast integrally; the rigid frame beam in the beam-slab straddles above the existing subway line and is correspondingly arranged with the full casing support piles, and both ends of the rigid frame beam are fixedly connected to the tops of the corresponding full casing support piles on both sides of the existing subway line; a number of rotary drilling support piles are arranged on the outside of the full casing support piles on each side, the rotary drilling support piles are located on the side and obliquely below the existing subway line, and there are a number of cross-laid anchor rods between the rotary drilling support piles on both sides. In this application, the effective protection of the subway structure is realized, and the adverse impact of construction on the existing subway is reduced.

Description

Technical Field

[0001] This application relates to the technical field of civil engineering construction, and particularly to a deep and large foundation pit support system and construction method for spanning an existing subway interval structure. Background Art

[0002] With the continuous improvement of the urban modernization level and motorization level in China, the rail transit system and planning layout have been gradually improved, and the situation where new urban projects are located within the subway protection area has also increased. In engineering cases of large deep foundation pits adjacent to existing subways, in most cases, single-sided support excavation is carried out with the foundation pit adjacent to the existing subway on one side, or excavation and unloading and protection measures are adopted when the foundation pit is located above the existing subway to ensure operation safety. However, in some project works, the foundation pit straddles the subway line, the buried depth of the line interval is more than ten meters, and it is in the state of "closing the station without stopping operation". During the construction process, it is necessary to ensure the protection of the finished products of the existing subway line and operation safety. The excavation process requires excavation in three directions: left, up, and right. The working conditions are complex and the safety risks are high. There is no mature construction technology yet. Traditional subway protection technologies, such as soil layer reinforcement, setting anti-pulling piles, continuous wall portal rigid frames, etc., are not applicable to the working conditions of excavation in three directions: left, up, and right, adjacent to the subway. Summary of the Invention

[0003] In order to ensure the construction of a deep and large foundation pit spanning an existing subway interval structure without disturbing the normal operation of the subway and reduce the construction risks, this application provides a deep and large foundation pit support system and construction method for spanning an existing subway interval structure.

[0004] In the first aspect, a deep and large foundation pit support system for spanning an existing subway interval structure provided by this application adopts the following technical solutions:

[0005] A deep and large foundation pit support system for spanning an existing subway interval structure includes a number of full casing support piles located on both sides of the existing subway line and a beam-slab covering the top of the existing subway line;

[0006] The full casing support piles on each side are arranged along the length direction of the existing subway line, and the full casing support piles on both sides are symmetrically arranged;

[0007] The beam-slab includes a rigid frame beam and a reinforced concrete slab cast integrally; the rigid frame beam in the beam-slab spans above the existing subway line and is arranged corresponding to the full casing support piles, and both ends of the rigid frame beam are fixedly connected to the tops of the corresponding full casing support piles on both sides of the existing subway line;

[0008] On the outer side of the full casing support piles on each side, a number of rotary drilling support piles are arranged. The rotary drilling support piles are located on the side and obliquely below the existing subway line. A number of anchor rods are arranged between the rotary drilling support piles on both sides. One end of the anchor rod is fixedly connected to the rotary drilling support pile, and the other end of the anchor rod extends obliquely downward under the bottom plate of the existing subway line, and the corresponding anchor rods on both sides are arranged in a cross pattern.

[0009] By adopting the above technical solution, full casing support piles and beam-slab structures are arranged on both sides and the top of the existing subway section, effectively protecting the safety of the existing subway. The full casing support piles have high pile-forming quality. The reinforced concrete slab in the beam-slab structure is located 1 m above the existing subway section, covering and shielding the existing subway, solving the problem of water seepage and leakage, and effectively protecting the existing subway structure and waterproof layer. The rigid frame beam in the beam-slab structure acts as a tie beam to restrain the top of the full casing support piles. The full casing support piles on both sides and the beam-slab structure on the top form an integral whole, ensuring its stability and reliability and ensuring the finished product protection effect of the existing subway. In addition, in this application, rotary drilling support piles and cross-set anchor rods are used as a foundation pit support combination system. The rotary drilling support piles and the cross-set anchor rods cooperate with each other. The rotary drilling support piles provide a stable anchoring foundation for the anchor rods, and the cross-set anchor rods can apply tensile forces to the rotary drilling support piles in different directions, jointly improving the anti-tilting deformation ability of the support structure and enhancing the protection of the existing subway line. In this application, the characteristics of non-unidirectional tension of the cross anchor rods are fully utilized, and they are reasonably arranged, improving the anti-tilting deformation ability of the support structure, also improving the bearing capacity of the foundation under the subway to prevent the subway from sinking and deforming, with clear force, convenient operation, time-saving and efficient.

[0010] Optionally, in some sections of the rotary drilling support piles on each side, they are single-row rotary drilling support piles, and in some sections, single-row rotary drilling support piles and double-row rotary drilling support piles are arranged alternately in sequence. The soil between the rotary drilling support piles on the same side is reinforced by grouting to improve the anti-tilting deformation ability of the support structure.

[0011] By adopting the above technical solution, the rotary drilling support piles are reasonably arranged, and they form a symmetric support with the cross-set anchor cables on both sides of the existing subway, solving the construction problems of the deep foundation pits on both sides of the existing subway section structure, and being stable and reliable.

[0012] Optionally, each side of the anchor rods is divided into three layers: upper, middle and lower. The upper anchor rod and the lower anchor rod are inclined towards one end direction of the existing subway line, and the middle anchor rod is inclined towards the other end direction of the existing subway line.

[0013] By adopting the above technical solution, the need for long-distance high-precision of the tie-back method for the anchor rods in the pile-anchor system is avoided, and at the same time, the anchoring effect is ensured. By calculating the self-weight of the subway structure and the axle weight of the fully loaded vehicle load, considering the dynamic coefficient load of 1.3, and calculating according to the static earth pressure with relatively strict deformation control, the spacing of the anchor rods under the side of the subway structure is 3000 mm, and a total of 3 anchor rod supports need to be set. This solution can meet the usage requirements.

[0014] In a second aspect, a construction method of a deep and large foundation pit support system spanning an existing subway interval structure provided by the present application adopts the following technical solution:

[0015] A construction method of a deep and large foundation pit support system spanning an existing subway interval structure, the construction method comprising the following steps:

[0016] S1: Construction preparation: fabricating the surcharge counterweight blocks and setting up the enclosure;

[0017] S2: Demolishing the explosion-proof layer: Excavating the overburden soil above the existing subway explosion-proof layer, and then demolishing the explosion-proof layer of the existing subway;

[0018] S3: Constructing the full casing support piles: Constructing the full casing support piles on both sides of the existing subway;

[0019] S4: Constructing the beam and slab: Excavating the overburden soil above the existing subway structure, constructing the beam and slab above the existing subway structure, and connecting the rigid frame beam in the beam and slab with the top of the full casing support pile;

[0020] S5: Constructing the rotary drilling support piles: Excavating the soil on both sides of the existing subway, and then constructing the rotary drilling support piles on both sides of the bottom slab of the existing subway structure;

[0021] S6: Constructing the anchor rods: Reinforcing and grouting the soil between the piles on both sides of the existing subway, and monitoring and controlling the deformation of the existing subway structure. After the monitoring parameters are qualified, excavating the soil under the side of the existing subway, and then setting the anchor rods;

[0022] S7: Constructing the foundation base: Excavating the foundation base soil, and then conducting the acceptance.

[0023] By adopting the above technical solutions, the construction of a deep and large foundation pit with all three sides of the existing subway, namely the left, upper, and right sides, being excavated and exposed was realized. The finished products of the existing subway were protected by the full casing support piles on both sides and the beam-slab structure at the top, solving the problem of seepage and leakage, preventing the top of the existing subway from being exposed to oxidation or its waterproof layer from being damaged; the uplift deformation was controlled by calculation and finite element simulation for the surcharge; the earth excavation method of skip charging and layered excavation with time-limited support was adopted; through the automated monitoring technology, the actual impact of on-site construction was effectively reflected to control the on-site construction; automated monitoring data acquisition devices were arranged on both sides of the structural joint to monitor the deformation data on both sides of the structural joint in real time, and at the same time, the on-site inspection of the structural joint was strengthened during the earth unloading period, thereby minimizing the impact on the subway tunnel structure.

[0024] Optionally, in the above step S2, a water drill is used to cut and statically break the explosion-proof layer, and the reinforced concrete blocks after cutting are hoisted out of the construction area in sequence by a crane; after the explosion-proof layer is broken, the soil covering the upper part of the subway structure needs to be covered.

[0025] By adopting the above technical solutions, using a water drill to cut and statically break the explosion-proof layer ensures that the subway is not disturbed during the breaking process of the explosion-proof layer. Covering the soil protects the exposed subway structure as a finished product, preventing various external factors from damaging the waterproof layer on the upper part of the subway structure and avoiding potential hazards.

[0026] Optionally, in the above step S3, the construction of the full casing support piles is carried out in sequence by the construction method of "driving one every two". The full casing support piles are located within 10 m of the existing subway section; the upper end of the full casing support pile is at least 500 mm higher than the top plate of the existing subway; the lower end of the casing support pile is obliquely below the bottom plate of the existing subway.

[0027] By adopting the above technical solutions, not only the pile-forming quality is improved, but also the disturbance to the existing subway structure is effectively reduced. The full casing support piles are located within 10 m of the existing subway section, the upper end is at least 500 mm higher than the subway top plate, and the lower end is obliquely below the subway bottom plate, forming a strong lateral support and ensuring the safety and stability of the subway structure. This construction method effectively prevents the displacement and deformation of the subway structure under complex geological conditions and ensures the normal operation of the subway during construction.

[0028] Optionally, in the above step S4, the beam-slab is poured with C25 commercial concrete. The cumulative time of concrete transportation, pouring, and intermittent time should not exceed the initial setting time of the concrete. The concrete is continuously poured and completed at one time. The space between the rigid frame beams of the beam-slab is hardened with 200-mm-thick and C25-strength concrete to form a reinforced concrete slab; the concrete is covered and water-cured within 12 hours after pouring, and the number of water-sprinkling times should keep the concrete in a moist state.

[0029] By adopting the above technical solutions, the construction quality and safety of the beams and slabs above the subway structure are ensured. First, the selection of C25 commercial concrete ensures that the beams and slabs have sufficient strength and durability to effectively support the loads generated during the excavation of the foundation pit. Secondly, the concrete is poured continuously and completed in one go, which avoids the generation of cold joints and enhances the integrity and stability of the beams and slabs. Thirdly, the timely covering and watering maintenance measures after the concrete is poured ensure that the concrete maintains appropriate humidity during the early hardening process, promotes the full curing of the concrete, and reduces the generation of cracks. These measures work together to not only improve the construction quality of the beams and slabs, but also effectively control the impact on the existing subway structure during the construction process, ensuring the normal operation of the subway.

[0030] Optionally, in the above step S5, the earthwork needs to be excavated to the middle or upper part of the full casing support piles on both sides of the existing subway structure bottom plate, and the earthwork plane is excavated in 5 steps with a span of 10m to 20m, and the excavation is carried out by using a bilaterally symmetrical, layered, skipped, and time-limited excavation process, and the excavation depth of each layer is not more than 2.5m; during the excavation process, the excavation rate of the two adjacent bins and the upper and lower layers is controlled, and the time interval shall not be less than 24 hours to prevent excessive stress release due to excessive earthwork excavation rate;

[0031] After the construction of rotary drilling support piles is completed, grouting pipes are installed between the rotary drilling support piles, and the soil between the rotary drilling support piles is reinforced with grouting by the backward grouting method; the grouting body in each cubic meter of soil is not less than 500kg, and each step is retreated by 0.25m to 0.35m; the grouting pressure is 0.3 to 0.5MPa, and the pressure is maintained for 3 to 5s before stopping.

[0032] By adopting the above technical solution, the stress release caused by the excessive excavation rate is avoided, thereby reducing the disturbance to the subway structure. At the same time, the stability and shear resistance of the soil are enhanced through grouting and solidification, further ensuring the safety of the subway structure.

[0033] Optionally, the deformation of the existing subway structure is monitored and controlled during construction: if the structure floats up, information-based loading is performed on the beams and slabs of the existing subway using counterweights; if the structure sinks, the counterweights are unloaded or compensatory grouting is performed on the soil on both sides of the existing subway;

[0034] The counterweight blocks are made of prefabricated reinforced concrete blocks; the load is loaded from the middle of the subway structure to both ends, and is symmetrical and layered. When unloading, the two sides are unloaded symmetrically and in steps. If the sinking exceeds the warning level after all the counterweight blocks are unloaded, the soil under the existing subway floor is compensated for by grouting to slightly reinforce the jacking.

[0035] It is considered that the maximum upward floating of the subway section will occur when the foundation pit is excavated to about half of its position. An upward floating of 5 mm has been predicted in the design and numerical simulation. According to the requirement of controlling the upward floating by 2 mm in the first stage, information-based surcharge measures are considered, and reinforced concrete blocks are used for surcharge. It is considered that the maximum subsidence of the subway section will occur when the foundation pit is excavated to the bottom of the foundation pit. A subsidence of 5 mm has been predicted in the design and numerical simulation. According to the requirement of controlling the subsidence by 3 mm in the first stage, information-based unloading or construction compensation grouting methods are considered for slightly strengthening and jacking measures. By adopting the above technical solutions, the real-time monitoring of the deformation of the subway structure is carried out, possible upward floating or subsidence problems are discovered and processed in time, the dynamic control during the construction process is ensured, and the construction safety and reliability are improved. Information-based construction is carried out according to the monitoring deformation results of the existing subway line. Such dynamic adjustment measures not only improve the construction safety, but also reduce the impact on the operation of the existing subway, realizing precise control and efficient construction; it is more scientific, reasonable and efficient, shortening the construction period, reducing the consumption of materials and machinery, reducing the overall construction cost, and enhancing the economic benefits of the project.

[0036] Optionally, in the above step S6, the soil under the side of the subway structure is excavated by a symmetric layered excavation method. The excavation depth of each layer is not greater than 2.5 m, and the width is not greater than 15 m. The high and low spans of the excavated area and the unexcavated area are protected by a temporary slope with a slope ratio of 1:1 to avoid the disturbance of the soil excavation to the tunnel and the surrounding soil.

[0037] Before the formal construction of the anchor rod, two anchor rods are taken for experimental operations of drilling, grouting, tensioning and locking to assess the adaptability of the construction technology and construction equipment; after confirmation, the formal construction is carried out.

[0038] The spacing of the anchor rods is one anchor per pile, and the anchor rods are arranged in three layers, upper, middle and lower; a full-track anchor rod hydraulic drill is used for construction, and the hole forming is carried out in the way of drilling one and skipping two during the construction process to avoid the mutual influence between adjacent anchor rods.

[0039] By adopting the above technical solutions, the disturbance of the soil excavation to the tunnel and the surrounding soil is effectively avoided, and the stability and safety of the foundation pit support are ensured. Before the formal construction of the anchor rod, two anchor rods are taken for experimental operations of drilling, grouting, tensioning and locking to assess the adaptability of the construction technology and construction equipment, ensuring the quality and reliability of the anchor rod construction. The spacing of the anchor rods is one anchor per pile, and the anchor rods are arranged in three layers, upper, middle and lower, enhancing the overall stiffness and stability of the support structure, effectively resisting the lateral pressure of the foundation pit, and preventing the inclination and deformation of the subway structure. A full-track anchor rod hydraulic drill is used for construction and the hole forming is carried out in the way of drilling one and skipping two during the construction process, avoiding the mutual influence between adjacent anchor rods, and improving the construction efficiency and project quality.

[0040] In summary, the present application includes at least one of the following beneficial technical effects:

[0041] 1. In this application, full casing support piles and beam-slab structures are arranged on both sides and the top of the existing subway section, effectively protecting the safety of the existing subway. The pile-forming quality of the full casing support piles is high. The reinforced concrete slab in the beam-slab structure is located 1 m above the existing subway section, shielding and covering the existing subway, solving the problem of water leakage, and effectively protecting the existing subway structure and waterproof layer.

[0042] 2. In this application, the rigid frame beam in the beam-slab structure acts as a tie beam to restrain the top of the full casing support piles. The full casing support piles on both sides are combined with the beam-slab structure at the top to form a whole, ensuring its stability and reliability and the protection effect of the existing subway finished product.

[0043] 3. In this application, a rotary drilling support pile and cross-set anchor rods are used as the foundation pit support combined system. By making full use of the characteristic that the cross anchor rods are subjected to tensile forces in non-parallel directions and arranging them reasonably, the anti-tilting deformation ability of the support structure is improved, and the construction difficulty is reduced.

[0044] 4. In this application, the design of the three-layer anchor rods improves the bearing capacity of the foundation under the subway to prevent the subway from sinking and deforming. The force is clear, the operation is convenient, and it is time-saving and efficient. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 is a schematic structural diagram of the support system in this application.

[0046] Figure 2 is a schematic partial structural diagram of the support system in this application.

[0047] Figure 3 is a construction flow chart of the support system in this application.

[0048] Figure 4 is a schematic diagram of the step-by-step earth excavation in this application.

[0049] Figure 5 is a schematic diagram of the sequential construction of the full casing support piles in this application.

[0050] Figure 6 is a schematic diagram of the anchor rod distribution in this application.

[0051] In the figure:

[0052] 10. Foundation pit; 20. Full casing support pile; 30. Beam-slab structure; 31. Rigid frame beam; 40. Rotary drilling support pile; 50. Anchor rod; 60. Retaining pile; 70. Explosion-proof layer; 80. Counterweight. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0053] The following will be combined with the attached Figure 1 - attached Figure 6, 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 fully combine the embodiments of the present invention and obtain other embodiments without creative labor, and these embodiments are also within the protection scope of the present invention. Embodiment 1

[0054] Referring to Figure 1 and Figure 2 As shown, the deep and large foundation pit support system straddling the existing subway interval structure in this embodiment includes a number of full casing support piles 20 located on both sides of the existing subway line and a beam-slab 30 covering the top of the existing subway line; the full casing support piles 20 on each side are arranged along the length direction of the existing subway line and the full casing support piles 20 on both sides are symmetrically arranged; the beam-slab 30 includes a rigid frame beam 31 and a reinforced concrete slab cast integrally; the rigid frame beam 31 in the beam-slab 30 straddles above the existing subway line and is arranged corresponding to the full casing support piles 20, and both ends of the rigid frame beam 31 are fixedly connected to the tops of the corresponding full casing support piles 20 on both sides of the existing subway line; a number of rotary drilling support piles 40 are arranged outside each side of the full casing support piles 20, the rotary drilling support piles 40 are located at the side and obliquely below the existing subway line, and part of the rotary drilling support piles 40 on each side are single-row rotary drilling support piles 40 in some sections, and part of the sections are arranged alternately with single-row rotary drilling support piles 40 and double-row rotary drilling support piles 40 in turn; the rotary drilling support piles 40 on the same side are used to improve the anti-tilt deformation ability of the support structure by grouting and consolidating the soil body. A number of anchor rods 50 are arranged between the rotary drilling support piles 40 on both sides; one end of the anchor rod 50 is fixedly connected to the rotary drilling support pile 40, the other end of the anchor rod 50 extends obliquely downward under the bottom plate of the existing subway line and the corresponding anchor rods 50 on both sides are cross-arranged; the anchor rods 50 on each side are divided into three layers: upper, middle and lower. The upper anchor rod 50 and the lower anchor rod 50 are inclined towards one end of the existing subway line, and the middle anchor rod 50 is inclined towards the other end of the existing subway line. The retaining pile 60 is arranged on the side wall of the foundation pit 10 far from the existing subway structure, and three layers of inclined downward anchor rods 50 are also arranged on the retaining pile 60.

[0055] The implementation principle is as follows: Full casing support piles 20 and beam-slab structures 30 are arranged on both sides and the top of the existing subway section, effectively protecting the safety of the existing subway. The full casing support piles 20 have high pile-forming quality. The reinforced concrete slab in the beam-slab structure 30 is located 1 m above the existing subway section, covering and shielding the existing subway, solving the problem of water seepage and leakage, and effectively protecting the existing subway structure and waterproof layer. The rigid frame beams 31 in the beam-slab structure 30 act as tie beams to restrain the tops of the full casing support piles 20. The full casing support piles 20 on both sides and the beam-slab structure 30 at the top are combined to form a whole, ensuring its stability and reliability and guaranteeing the finished product protection effect of the existing subway. In addition, in this application, a rotary drilling support pile 40 and cross-set anchor rods 50 are used as the foundation pit 10 support combination system. The rotary drilling support pile 40 and the cross-set anchor rods 50 cooperate with each other. The rotary drilling support pile 40 provides a stable anchoring foundation for the anchor rods 50, and the cross-set anchor rods 50 can apply tensile forces to the rotary drilling support pile 40 in different directions, jointly improving the anti-tilting and deformation resistance ability of the support structure and enhancing the protection of the existing subway line. In this application, the characteristics of the cross anchor rods 50 having non-unidirectional tension are fully utilized, and they are reasonably arranged, improving the anti-tilting and deformation resistance ability of the support structure, increasing the bearing capacity of the foundation under the subway to prevent the subway from sinking and deforming, with clear stress, convenient operation, time-saving and high efficiency. Embodiment 2

[0056] In this embodiment, a construction method for a deep foundation pit support system for a structure straddling an existing subway section is provided. Referring to Figure 3 and Figure 4 as shown, the construction method includes the following steps:

[0057] I. Construction preparation is carried out, specifically including but not limited to the following:

[0058] (1). Before construction, a construction operation schedule is formulated, and the labor resources are configured professionally. According to different types of work and different construction parts, several professional teams are divided, so that each professional team is engaged in the same or similar work, improving their operation proficiency and labor productivity, and ensuring that the construction quality, construction progress, etc. meet the requirements of the construction deployment.

[0059] (2). Before construction, the construction geological conditions are surveyed and an investigation report is formed. Based on the investigation report, the groundwater level of the site is judged. If the groundwater level of the site is buried deep and the foundation is above the groundwater level, the influence of groundwater does not need to be considered during construction, but the influence of rainwater and large-scale perched water on construction needs to be considered during the rainy season. If perched water is encountered during construction, it is necessary to consider installing drain holes for drainage. Based on the investigation report, it is judged whether there will be geological disaster problems such as rock mass collapse, cracking, landslide and soil slope instability in the whole field area, and corresponding pre-treatment plans are made.

[0060] (3)Design monitoring points before construction, install monitoring equipment to monitor the construction process and the surrounding environment for guiding information-based construction. The monitoring starts before the self-supporting construction and continues during the implementation stage of the supporting construction and the completion and maintenance stage. Through continuous monitoring, the adverse impacts on the existing subway section during construction are reduced and controlled to ensure the smooth progress of the supporting project.

[0061] (4)Plan or prepare various required materials and tools before construction. According to the material analysis in the construction progress plan and construction budget, do a good job in material preparation, supply, and determine the warehouse and storage yard. Organize materials to enter the site according to the plan before construction and do a good job in storage. According to the construction deployment and combined with the construction sequence and quantity of each sub-item project, the site needs to be equipped with tools and equipment required for the construction of retaining piles, anchor rods, and earthwork. Before construction, carefully check the model, variety, specification of the raw materials and the quality of each component of the anchor rod, and check whether the raw materials and main technical performance meet the design requirements. To ensure the quality of the retaining pile wall protection, prepare slurry on-site and store it in a slurry pit. The stored slurry volume can meet the daily demand for hole-forming construction. The anchor cable uses 1860-grade steel strands with a diameter of 15.2mm. Before installation, ensure that each steel strand is straight, not twisted or crossed, arranged evenly, rust-removed, and oil-removed. Those with dead bends, mechanical damage, and rust pits are removed. The cement for grouting the 50mm anchor rod uses PO42.5 bulk cement. Arrange a 70t bulk cement silo on-site to supply the cement for the 50mm anchor rod construction on-site. The construction content includes shotcrete, prestressed anchor rod 50 grouting, retaining pile construction, and grouting reinforcement construction, as well as the construction water for on-site safety and civilized construction. Using a DN200mm steel pipe as the upper water main pipe can meet the water demand.

[0062] (5)Precast counterweight blocks 80 for surcharge before construction. The counterweight blocks 80 are made of concrete and are used to enclose the foundation pit 10 in the construction area.

[0063] II. Earthwork excavation above the explosion-proof layer and explosion-proof layer demolition:

[0064] Refer to Figure 4As shown in the figure, in this embodiment, the entire earthwork excavation process adopts a layered excavation method. By controlling the excavation rate, it is possible to prevent the stress from being released too quickly due to the overly fast earthwork excavation rate, which may cause disturbances to the subway section. The earthwork from the upper part of the subway structure to the base range can be divided into several steps for excavation. For example, in this embodiment, the earthwork from the upper part of the subway structure to the base is excavated in eight steps. In other actual construction scenarios, the number of excavation steps can be adjusted adaptively according to the depth of the base. There is a reinforced concrete explosion-proof layer 70 above the subway structure. Within the construction influence range, this explosion-proof layer 70 needs to be demolished. The excavation of the surface soil covering the explosion-proof layer 70 is the first step of excavation. By excavating the surface soil covering, the existing subway explosion-proof layer 70 is exposed, and then the explosion-proof layer 70 is demolished. In this embodiment, to ensure that the subway is not disturbed during the demolition of the explosion-proof layer 70, a water jet cutting static demolition method is used to demolish the explosion-proof layer 70.

[0065] During the process of demolishing the explosion-proof layer 70 by water jet cutting, according to the layout plan, the position lines for water jet drilling are found. In this embodiment, the diameter of the water jet is 200 mm, and the overlap between hole positions is 10 mm. The cut reinforced concrete blocks are hoisted by a crane. The crane is supported in the two-side foundation pits 10. During hoisting, the concrete blocks are lifted in sequence from east to west and from north to south, and the cut concrete blocks are hoisted out of the construction area.

[0066] After the explosion-proof layer 70 is demolished, attention needs to be paid to the protection of the existing structure. After the explosion-proof layer 70 is demolished, there is still soil covering above the subway structure. The exposed earthwork needs to be covered, and no other protective measures are required. If the subway structure is exposed after the explosion-proof layer 70 is demolished, the exposed subway structure needs to be protected as a finished product to prevent the upper waterproof layer of the subway structure from being damaged by various external factors, and close cooperation with relevant trades is required to protect the waterproof layer well; a layer of flame-retardant and waterproof tarpaulin is covered on the upper part of the subway structure, and bricks are pressed for protection to prevent damage and pollution of the waterproof layer; when operating personnel are constructing on the upper part of the subway structure, they should wear cloth shoes and not wear shoes with nails for operation. During the construction period, if any problems are found, they should be repaired in time to avoid potential hazards.

[0067] III. Construction of full casing support piles on both sides of the subway structure:

[0068] Refer to Figure 5 As shown in the figure, the full casing support piles 20 are arranged along both sides of the subway structure. The construction method of "driving one pile every two piles" is adopted, that is, the full casing support piles 20 are constructed in three batches to ensure the quality of the formed piles and the effect of protecting the subway as a finished product. After the full casing support piles 20 are poured, the pile top elevation needs to reach the top of the rigid frame beam 31. The specific construction process of the full casing support piles 20 is an existing technology, and its construction procedures are carried out in accordance with the current "Technical Specification for Building Foundation Pit Support" (JGJ120 - 2016) and "Technical Code for Building Pile Foundations" (JGJ94 - 2008), which will not be elaborated here.

[0069] IV. Earthwork excavation and beam-slab construction on the top of the subway structure:

[0070] (1) There is a gap between the explosion-proof layer and the top of the subway structure. After the explosion-proof layer is broken, the soil covering the top of the subway structure is excavated, which is the second step of excavation in the earthwork excavation project. In the earthwork excavation project, a symmetric and layered excavation method is adopted. When the excavation reaches 2 m thick above the subway structure, manual excavation is used to prevent over-excavation. A 500-mm-thick original soil layer or pebble layer is reserved above the existing subway structure.

[0071] (2) A rigid frame beam 31 is set on the upper part of the original soil layer or pebble layer reserved in the subway. The two ends of the rigid frame beam 31 are respectively connected to the full casing support piles 20 on both sides of the subway, so that the rigid frame beam 31 and the full casing support piles 20 become an integral body to strengthen the protection of both sides and the upper part of the subway structure.

[0072] The specific construction process of the rigid frame beam 31 is as follows:

[0073] ① Fabrication and installation of the rigid frame beam formwork: The formwork adopts combined formwork and is supported in accordance with the designed position, shape and dimensions with inclined braces, cross braces and U-shaped clamps. The inside of the formwork should be clean without sundries, the joints should be tight without slurry leakage, and the supports should be stable and firm. ② Chiseling of the full casing support piles 20 on both sides of the subway structure: The surveyors level according to the designed elevation of the pile top of the full casing support piles 20 and make marks on each full casing support pile 20. The chiseling personnel chisel out all the main bars above the line around the pile body according to the given elevation line. When chiseling, it is strictly prohibited to bend or damage the main bars. After chiseling, the pile head is blown clean with high-pressure air. ③ Binding of the rigid frame beam steel bars: The stirrups and some main bar bends of the rigid frame beam are processed on the construction site. The main bars chiseled out from the full casing support piles 20 are anchored into the upper and lower layer steel bars of the rigid frame beam 31, and the anchorage length meets the specifications. When binding the rigid frame beam steel bars, the steel bar lap length ≥ 35d, and the number of joints in the same section does not exceed 50% of the number of main bars. ④ Pouring of the rigid frame beam concrete: C25 commercial concrete is used for pouring. The cumulative time of concrete transportation, pouring and intermittent should not exceed the initial setting time of the concrete. The concrete should be continuously poured and completed at one time. A 200-mm-thick, C25-strength concrete is used for hardening between the rigid frame beams 31 (cast integrally with the rigid frame beam 31). ⑤ Concrete curing: The concrete is covered and watered for curing within 12 hours after pouring. The number of watering times should keep the concrete in a moist state. During winter construction, it is covered with heat preservation cotton after pouring, and standard-cured specimens are set. ⑥ Form removal: The concrete strength at the time of form removal should ensure that the surface and edges of the rigid frame beam 31 are not damaged. The exposed surface should be cured as soon as possible after form removal and protected during the construction process to avoid damage and impact loads.

[0074] V. Layered excavation of the soil on both sides of the subway structure:

[0075] After the construction of the full casing support piles 20 on both sides of the subway structure and the upper beam and slab 30 of the subway structure is completed, the next step of earth excavation is carried out, which is the third and fourth steps of the eight-step earth excavation. The earth needs to be excavated to the upper part of the full casing support piles 20 on both sides of the subway structure floor. The plane earth excavation is divided into 5 sequences in total, with an excavation span of 15 m. The excavation is carried out symmetrically on both sides, in layers, in a skip-joint manner, and within a limited time. During the excavation process, the time interval between the excavation rates of the adjacent two bins and the upper and lower layers in the third and fourth steps of earth excavation shall not be less than 24 hours to prevent the stress from being released too quickly due to the too fast earth excavation rate.

[0076] VI. Construction of Rotary Drilling Support Piles:

[0077] The rotary drilling support piles 40 are located on both sides of the subway structure floor and are arranged at intervals along the length direction of the subway structure. The rotary drilling support piles 40 are arranged in a single row in some areas and in a double row in some areas. The rotary drilling construction technology is adopted to ensure the stability of the soil mass under the subway structure. The specific construction process of the rotary drilling support piles 40 is the existing technology, and its construction procedures are carried out in accordance with the current "Technical Specification for Building Foundation Pit Support" (JGJ120-2016) and "Technical Code for Building Pile Foundations" (JGJ94-2008), which will not be elaborated here.

[0078] VII. Reinforcement Grouting of the Soil between Piles:

[0079] After the construction of the rotary drilling support piles 40 is completed, the soil between the rotary drilling support piles 40 on both sides of the subway is reinforced by grouting. According to the grouting test results, the backward grouting method is adopted; grouting pipes are set between the rotary drilling support piles 40, and the grouting body in each cubic meter of soil is not less than 500 kg, and each step retreats 0.3 m. If the soil layer in the grouting area is mainly cobblestone layer with a large permeability coefficient, a quick-setting agent can be added to the grouting slurry, and the dosage of the quick-setting agent is 6% of the cement dosage. The grouting pressure is 0.3 - 0.5 MPa, and the pressure is maintained for 3 - 5 s before stopping the grouting. The grouting starts from the side far away from the subway. After grouting two rows on each side, the construction of the tracking compensation grouting holes is carried out, and then the remaining grouting is constructed. By controlling the grouting pressure, the lifting distance, and the grouting time maintained in each step, the influence on the exposed existing operating subway section is effectively reduced, and the grouting effect meets the design requirements. During the grouting process, the subway structure is monitored in real time. When the deformation of the subway structure exceeds the warning value, deformation control measures are taken.

[0080] VIII. Preparation before Anchor Rod Construction:

[0081] In this embodiment, before the construction of the anchor rod 50, it is necessary to monitor and control the deformation of the existing subway structure. Considering that the stage control index is stricter than the numerical simulation index, it is necessary to issue construction measures to prevent the subway from exceeding the stage deformation during the design stage. For example, according to the monitoring data of the floating and sinking of the existing subway structure, counterweight blocks 80 can be piled and unloaded on the upper part of the rigid frame beam 31 of the existing subway, and information-based construction can be carried out according to the monitoring deformation results of the existing subway structure during construction. Considering that the maximum floating of the subway section will occur when the foundation pit 10 is excavated to about half of its position, and a floating of 5 mm has been predicted in the design and numerical simulation. According to the requirement of controlling the floating by 2 mm in the first stage, it is considered to adopt the information-based piling measure with the counterweight blocks 80. When piling, the piling should be carried out from the middle of the subway structure to both ends, and precast reinforced concrete blocks are used for piling, with symmetrical loading on the left and right and layered loading. Considering that the maximum sinking of the subway section will occur when the foundation pit 10 is excavated to the bottom of the foundation pit 10, and a sinking of 5 mm has been predicted in the design and numerical simulation. According to the requirement of controlling the sinking by 3 mm in the first stage, it is considered to adopt the measure of information-based unloading with the counterweight blocks 80 or the measure of slightly strengthening and jacking by construction compensation grouting. When unloading the counterweight blocks 80, symmetrical unloading and step-by-step unloading should be carried out on both sides of the subway structure. When compensating grouting on both sides of the subway, it is required that the grouting body in each cubic meter of soil is not less than 500 kg, the grouting pressure is 0.3 - 0.5 MPa, the pressure is maintained for 3 - 5 s and then the grouting is stopped, and the setting time is controlled by adding the dosage of the accelerator to achieve the purpose of slightly strengthening and jacking the subway structure. The next step of construction can be carried out only after the deformation detection value of the subway structure reaches the normal value.

[0082] IX. Anchor Rod Construction Test:

[0083] Before the construction of the anchor rod 50, it is advisable to take two anchor rods 50 for experimental operations of drilling, grouting, tensioning and locking to assess the adaptability of the construction technology and construction equipment.

[0084] X. Anchor Rod Construction:

[0085] According to the test of the anchor rod 50, the foundation pit support form of cross anchor rods 50 is adopted under the side of the subway structure. Calculated according to the earth pressure at rest with stricter deformation control, 3 anchor rods 50 are required to be adopted under the side of the subway structure. During the sequential construction of the 3 anchor rods 50, it is necessary to excavate the soil under the side of the subway structure in layers and steps. The symmetrical excavation method is adopted during the soil excavation process, and the excavation depth of each layer is not greater than 2.5 m, and the width is not greater than 15 m. The temporary slope protection is carried out for the high and low spans between the excavated area and the unexcavated area with a slope of 1:1 to avoid disturbing the tunnel and the surrounding soil as much as possible during the soil excavation. In each step of soil excavation, one anchor rod 50 is set. There are three anchor rods 50 in total in this embodiment, so it corresponds to three steps of excavation; each step of soil needs to be excavated 500 mm below the elevation of the next anchor rod 50, and then the construction of the anchor rod 50 is carried out. The next step of excavation can be carried out only after the tensioning construction of the anchor rod 50 is completed.

[0086] Refer to Figure 6 As shown, the spacing of the anchor rods 50 is one anchor rod per pile. The full-track anchor rod hydraulic drill is used for construction. During the construction process, the "drill one skip two" method is adopted for hole formation to avoid mutual influence between adjacent anchor rods 50. For the anchor rods 50 at the corner of the foundation pit 10, when there is cross-construction between adjacent anchor rods 50, measures such as adjusting the position of the anchor rod or the construction angle of the anchor rod should be taken to meet the normal use function of the anchor rod 50. At the same time, the angles of adjacent anchor rods are adjusted, and the angle is controlled between 15° and 20°. If such an angle cannot stagger the position of the anchor rod 50, the position shall be adjusted according to the design drawings. Considering the balance of cross-construction of the three vertical anchor rods 50, the construction angle of the second anchor rod 50 is adjusted in the reverse direction. After the construction of the anchor rod 50 is completed, the soil between the piles is supported by hanging a steel mesh and spraying concrete. The specific construction process of the anchor rod 50 shall be carried out in accordance with the current "Technical Specification for Building Foundation Pit 10 Support" (JGJ120-2016).

[0087] XI. Excavation of foundation pit soil:

[0088] After the construction of the last anchor rod 50 is completed, the foundation pit soil is excavated. As the last step of the step-by-step excavation of the soil, the soil 30 cm away from the bottom of the foundation pit 10 is excavated manually and shall not be over-excavated. After the excavation of the foundation pit soil is completed, the main body of the project shall be inspected and accepted in accordance with the "Unified Standard for Construction Quality Acceptance of Building Engineering" GB50300-2013 and the "Technical Specification for Building Foundation Pit Support" DB11 / 489-2016.

[0089] In this application, during the construction of the support structure and the excavation of the foundation pit 10, the disturbance of the construction to the stratum and the redistribution of the foundation soil stress inside and outside the foundation pit 10 may cause deformation or settlement of the retaining structure and the ground surface, endangering the safety of the subway structure and the surrounding environment. In this application, in order to ensure the safety of the subway section and the foundation pit construction during the construction of the foundation pit support, real-time monitoring and measurement are carried out during the construction. Monitoring points are set in the subway structure section, and monitoring instruments are buried. The deformation of the foundation pit 10 itself and the deformation of the surrounding buildings (structures) are mainly monitored. High-precision static level instruments can be used to complete the automatic data collection and upload of subway monitoring. According to the monitoring data, the construction of the support structure is guided, the construction design is optimized, and information-based construction is achieved, providing strong support for the dynamic control of the design and construction. During the construction process, the inspection of the foundation pit 10 itself and the surrounding environment shall also be strengthened, and the monitoring points shall be protected to ensure the continuity of the monitoring data.

[0090] In this application, field data collection is carried out by connecting electronic devices such as levels and total stations through mobile phone Bluetooth. The data is transmitted to the monitoring platform through the mobile phone signal. The platform automatically processes the monitoring data, generates monitoring analysis reports and deformation curve graphs, screens out the maximum deformation value, and sends information such as text messages and WeChat in the first time after exceeding warning standards, etc., to realize an automatic monitoring data analysis system for fieldwork - indoor work, closely monitor the deformation of the subway structure in real time, and ensure the safety of subway operation. By using an integrated automation monitoring information platform for indoor and outdoor monitoring, from field data collection to indoor data processing, it is integrated, intelligent, improves monitoring efficiency, and reflects monitoring data in real time, quickly and accurately. Thus, it guides and evaluates the construction sequence.

[0091] The implementation principle is as follows: The construction method in this application is applicable to the excavation construction of the foundation pit 10 within the protection area of the existing line, especially applicable to the close support and deep foundation pit construction of the three-way adjacent existing subway structure in the pebble stratum within 10m of the existing subway section.

[0092] In this application, by reasonably setting the full casing support piles 20 and the beam slab 30, the safety of the existing subway is effectively protected, and the full casing support piles 20 and the beam slab 30 at the top are combined to form a whole, ensuring its stability and reliability. In this application, the rotary drilling support piles 40 and the cross-set anchor rods 50 are also used as the foundation pit support combination system, making full use of the characteristics of the cross anchor rods 50 having non-unidirectional tension, arranging them reasonably, improving the anti-tilt deformation ability of the support structure, and also improving the foundation bearing capacity under the subway to prevent the subway from sinking and deforming.

[0093] In the specific construction process, the water drill static cutting method is adopted in this application to break the explosion-proof layer 70, avoiding disturbing the subway structure during the breaking process. For the deformation control of the existing subway structure: information-based guidance for stacking counterweight blocks 80 is achieved through calculation and finite element simulation to control the floating deformation; the combination of strong support and prestressed anchor rods 50 is used to control the sinking deformation; and dynamic design is carried out with the help of monitoring means. When the foundation pit 10 is excavated, the retaining piles are higher than the bottom plate of the subway section for horizontal deformation constraint; the principle of synchronous construction is adopted on both sides of the subway, and layered and segmented excavation is adopted at the symmetric excavation area. At the asymmetric excavation area, synchronous unloading is carried out on both sides of the subway, and the unilateral support is strengthened by supplementary grouting at the lower part to control the lateral inclination deformation. The skip-joint construction is adopted for the longitudinal excavation and support along the existing subway to control the longitudinal deformation curvature. For the subway structure displacement control: through the earthwork skip-joint, layered excavation, and time-limited support earthwork excavation method, reasonable stacking combined with the automatic monitoring technology and the method of strictly controlling the construction angle of the cross anchor rods 50, the influence on the exposed existing operating subway section is effectively reduced, and the displacement data is controlled within the required range. For the influence of subway vibration: the full casing method is adopted for pile forming within 10 m of the subway section; the double-casing construction process is used for the construction of the anchor rods 50; grouting is carried out in the pebble layer to strengthen the stability of the soil between piles and the grouting quality of the anchor rods 50, reducing the influence of construction on the subway structure vibration.

[0094] In this application, automatic monitoring data acquisition equipment is arranged for the existing subway structure, and the monitoring data is transmitted to the computer automatic processing platform. The subway monitoring data is collected, processed, analyzed, and automatically warned through the monitoring data management platform, and the analyzed data is timely fed back to the on-site construction unit to guide the construction. It can timely correct the deviation of the on-site earthwork unloading to avoid the inclination of the existing subway structure caused by asymmetric earthwork unloading. The monitoring data of the subway structure is reported in real time. It can also distinguish the floating or sinking deformation of the subway, and then carry out corresponding treatment, providing strong support for the dynamic management and control of design and construction and ensuring construction safety. Finally, the control of ultra-low allowable deformation of the subway can be realized, and the influence of construction on the existing subway is minimized. The construction problems of the deep foundation pits on both sides of the existing subway section structure are solved; information-based design, refined construction, and deformation control are realized, ensuring the safety of subway operation during construction without station closure and interruption, and the construction is of high quality and efficiency. This construction method is safe, reliable, economical, practical, and has strong applicability, with remarkable economic and social benefits. It can provide reference for similar projects and has the value of popularization and application.

[0095] The embodiments of this specific implementation manner are all preferred embodiments of this application, and do not limit the protection scope of this application accordingly. The same components are represented by the same reference numerals. Therefore, all equivalent changes made according to the structure, shape, and principle of this application should be covered within the protection scope of this application.

Claims

1. A deep and large foundation pit support system that spans over an existing subway section structure, characterized in that: The deep and large foundation pit support system comprises a plurality of full casing support piles (20) located on both sides of the existing subway line and a beam plate (30) covering the top of the existing subway line; The full casing support piles (20) on each side are arranged along the length direction of the existing subway line and the full casing support piles (20) on both sides are arranged symmetrically; The beam slab (30) comprises an integrally cast rigid frame beam (31) and a reinforced concrete slab; the rigid frame beam (31) in the beam slab (30) spans over the existing subway line and is arranged corresponding to the full casing support piles (20), and the two ends of the rigid frame beam (31) are respectively fixedly connected to the top ends of the corresponding full casing support piles (20) on both sides of the existing subway line; A plurality of rotary drilling support piles (40) are arranged outside the full casing support piles (20) on each side, the rotary drilling support piles (40) are located at the side and obliquely below the existing subway line, and a plurality of anchor rods (50) are arranged between the rotary drilling support piles (40) on both sides; one end of the anchor rod (50) is fixedly connected to the rotary drilling support pile (40), and the other end of the anchor rod (50) is obliquely extended downwardly into the bottom of the existing subway line floor, and the corresponding anchor rods (50) on both sides are arranged crosswise; Some sections of the rotary excavation support piles (40) on each side are single-row rotary excavation support piles (40), and some sections are single-row rotary excavation support piles (40) and double-row rotary excavation support piles (40) arranged alternately in sequence; The deep and large foundation pit support system further comprises retaining piles (60) arranged on the side wall of the foundation pit (10) away from the existing subway structure.

2. The deep and large foundation pit support system for crossing an existing subway section structure according to claim 1 is characterized in that: The soil between the rotary drilling support piles (40) on the same side is reinforced by grouting to improve the ability of the support structure to resist tilting and deformation.

3. The deep foundation pit support system for overpassing an existing subway section structure according to claim 1 is characterized in that: The anchor rods (50) on each side are divided into three layers: upper, middle and lower. The upper anchor rods (50) and the lower anchor rods (50) are inclined toward one end of the existing subway line, and the middle anchor rods (50) are inclined toward the other end of the existing subway line.

4. A construction method of a deep and large foundation pit support system that crosses an existing subway section structure, applied to the construction of a deep and large foundation pit support system that crosses an existing subway section structure as described in any one of claims 1 to 3, characterized in that: The construction method comprises the following steps: S1: Construction preparation: making stacking weight blocks (80) and fencing; S2: removing the explosion-proof layer (70): excavating the top of the explosion-proof layer (70) of the existing subway, and then removing the explosion-proof layer (70) of the existing subway; S3: Construction of full casing support piles (20): Construction of full casing support piles (20) on both sides of the existing subway; S4: Construction of beam and slab (30): Excavation is performed on the top of the existing subway structure, and the beam and slab (30) is constructed on the top of the existing subway structure, and the rigid frame beam (31) in the beam and slab (30) is fixedly connected with the top of the full casing support pile (20); S5: Construction of rotary excavation support piles (40): Excavation of earthwork on both sides of the existing subway, and then construction of rotary excavation support piles (40) on both sides of the existing subway structure bottom plate; S6: Anchor rod (50) construction: reinforce the soil between the piles on both sides of the existing subway by grouting, and monitor and control the deformation of the existing subway structure. After the monitoring parameters are qualified, excavate the soil below the side of the existing subway, and then install the anchor rod (50); S7: Foundation pit (10) Base construction: excavation of the base, followed by acceptance inspection.

5. The construction method of a deep and large foundation pit support system that crosses an existing subway section structure according to claim 4 is characterized in that: In the above step S2, the explosion-proof layer (70) is statically broken by water drilling cutting, and the cut reinforced concrete blocks are sequentially hoisted to the outside of the construction area by a crane; after the explosion-proof layer (70) is broken, the soil covering the upper part of the subway structure needs to be covered.

6. The construction method of a deep and large foundation pit support system that crosses an existing subway section structure according to claim 4 is characterized in that: In the above step S3, the full casing support piles (20) are constructed in sequence using a two-for-one construction method, and the full casing support piles (20) are located within 10m of the existing subway section; the upper end of the full casing support pile (20) is at least 500mm higher than the existing subway roof; the lower end of the casing support pile is located obliquely below the existing subway floor.

7. The construction method of a deep and large foundation pit support system that crosses an existing subway section structure according to claim 4 is characterized in that: In the above step S4, the beam and slab (30) are cast with C25 commercial concrete, and the cumulative time of concrete transportation, casting and interval should not exceed the initial setting time of concrete. The concrete is cast continuously and completed in one time. The steel frame beams (31) of the beam and slab (30) are hardened with 200mm thick and strong C25 concrete to form a reinforced concrete slab; after the concrete casting is completed, the concrete is covered and watered within 12 hours for curing, and the number of watering times should be sufficient to keep the concrete in a moist state.

8. The construction method of a deep and large foundation pit support system that crosses an existing subway section structure according to claim 4 is characterized in that: In the above step S5, the earthwork needs to be excavated to the middle or upper part of the full casing support piles (20) on both sides of the existing subway structure bottom plate, and the earthwork plane is excavated in 5 steps with a span of 10m to 20m, and the excavation is carried out by adopting a bilaterally symmetrical, layered, skipped, and time-limited excavation process, and the excavation depth of each layer is not greater than 2.5m; during the excavation process, the excavation rate of the two adjacent bins and the upper and lower layers is controlled, and the time interval shall not be less than 24 hours to prevent excessive stress release due to excessive earthwork excavation rate; After the construction of the rotary excavation support piles (40) is completed, a grouting pipe is arranged between the rotary excavation support piles (40), and the soil between the rotary excavation support piles (40) is reinforced with grouting by a backward grouting method; the grouting body in each cubic meter of soil is not less than 500 kg, and each step is retreated by 0.25 m to 0.35 m; the grouting pressure is 0.3 to 0.5 MPa, and the pressure is maintained for 3 to 5 seconds before stopping.

9. The construction method of a deep and large foundation pit support system that crosses an existing subway section structure according to claim 4 is characterized in that: The earthwork below the side of the subway structure is excavated symmetrically in layers, with each layer excavated no deeper than 2.5m and no wider than 15m. A 1:1 slope is used for temporary slope protection between the excavated area and the unexcavated area to avoid disturbance of the tunnel and surrounding soil by soil excavation. Before the formal construction of the anchor rods (50), two anchor rods (50) are taken to perform experimental operations of drilling, grouting, tensioning and locking to assess the adaptability of the construction process and construction equipment; after confirmation, the formal construction is carried out; The spacing between anchor rods (50) is one pile and one anchor, and the anchor rods (50) are arranged in three layers: upper, middle and lower. A full-track anchor rod (50) hydraulic drilling rig is used for construction, and during the construction process, a one-drill-two-jump method is used to drill holes to avoid mutual influence between adjacent anchor rods (50).

10. The construction method of a deep and large foundation pit support system that crosses an existing subway section structure according to claim 4, characterized in that: During construction, the deformation of the existing subway structure is monitored and controlled: if the structure floats up, information-based loading is performed on the beam and slab (30) of the existing subway using a stacking weight block (80); if the structure sinks, the stacking weight block (80) is unloaded or compensatory grouting is performed on the soil on both sides of the existing subway; The counterweight blocks (80) are prefabricated reinforced concrete blocks; the load is loaded from the middle of the subway structure to both ends, and the loading is symmetrical and layered. When unloading, the two sides are unloaded symmetrically and in steps; if the sinking exceeds the warning level after all the counterweight blocks (80) are unloaded, the soil under the existing subway floor is compensated by grouting to reinforce the jacking.

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