Design and construction method of upper foundation pit of tunnel upheaval with axial force servo control

Abstract

The application discloses a kind of axial force servo control tunnel upheaval upper foundation pit design construction method, to provide a kind of little pit bottom reinforcement, construction is convenient, can effectively control the axial force servo control tunnel upheaval upper foundation pit design construction method of existing tunnel's upheaval deformation problem.It successively includes the following steps: one, division construction area, foundation pit is divided into first construction block and after construction block, wherein first construction block is located in the direct upper of tunnel and extends along the length direction of tunnel;Construction foundation pit enclosure and support column pile, foundation pit enclosure includes foundation pit enclosure wall and horizontal support beam, and support column pile supports horizontal support beam;Two, first construction block foundation pit construction;Specific steps are as follows, first construction block is divided into several excavation blocks along the length direction of tunnel;Excavation is carried out to each excavation block.Three, after construction block foundation pit construction.

Description

Technical Field

[0001] This invention relates to the field of design and construction of foundation pits above tunnels, and specifically to a method for designing and constructing foundation pits above tunnels using axial force servo control for tunnel uplift. Background Technology

[0002] With the acceleration of urbanization, the number of urban populations and building density are increasing daily, posing a severe challenge to urban surface transportation. Developing urban underground rail transit is an important way to solve surface traffic congestion. In addition, the construction of urban infrastructure and the development of underground space are also developing rapidly, with an increasing number of construction projects above existing subway tunnels. Construction projects above tunnels will cause changes in the confining pressure of existing tunnels, which will lead to uneven longitudinal or circumferential deformation of the tunnel structure or the opening of joints. In severe cases, it will cause concrete to crush and fall off, bolts to break, and the tunnel structure to reach its load-bearing capacity limit and fail, endangering tunnel safety.

[0003] To control tunnel uplift deformation, the above-ground foundation pit of existing tunnels is often reinforced using a combination of full-span foundation reinforcement and segmented excavation. However, full-span foundation reinforcement is time-consuming, costly, and has significant construction impacts. For example, in a residential area in soft soil, the maximum vertical deformation during soil reinforcement of a subway tunnel using triaxial cement mixing piles was -50mm, and the maximum convergence deformation was +37mm. In a river-crossing tunnel, the maximum vertical deformation during soil reinforcement of a subway tunnel using MJS (all-around high-pressure jet grouting) was -5mm / +12mm, and the maximum convergence deformation was +9mm. This also caused structural defects such as cracks, damage, water leakage, misalignment, joint opening, and track bed separation, which are detrimental to the normal operation and structural safety of rail transit.

[0004] The segmented approach utilizes the spatial effect of the foundation pit. By setting up isolation piles or reinforcing the soil within the excavation area and strictly limiting excavation, it aims to "reduce the size of the pit," control the amount of unloading at one time, and limit the time for excavation, support, and bottom slab construction. This is supplemented by the use of steel ingots for weight-bearing, thereby reducing tunnel uplift deformation. However, this construction method is a passive control technology, making it difficult to control the development of tunnel deformation during the construction process.

[0005] In summary, the conventional design and construction methods for foundation pits above existing tunnels have the following drawbacks: First, the foundation pit design relies on passive deformation control technology, which cannot control and adjust deformation during the actual construction process. Second, during the construction of large-area soil reinforcement, the construction disturbance is large, which often results in significant deformation. Moreover, the reinforcement construction period is long, often exceeding the time required for subsequent foundation pit excavation and main structure construction. The extended construction period makes tunnel deformation more uncontrollable. Summary of the Invention

[0006] The purpose of this invention is to provide an axial force servo control method for the design and construction of an upper foundation pit for tunnel heave that requires less reinforcement at the bottom of the pit, is easy to construct, and can effectively control the heave deformation problem of existing tunnels. This method effectively overcomes the problems of large reinforcement requirements, large construction disturbances, long reinforcement construction periods, and inability to control and adjust the tunnel heave deformation during actual construction in existing tunnel upper foundation pit design and construction methods.

[0007] The technical solution of this invention is: A method for designing and constructing an upper foundation pit for tunnel heave controlled by axial force servo, comprising the following steps: First, the construction area is divided into a first-construction block and a second-construction block. The first-construction block is located directly above the tunnel and extends along the length of the tunnel. Construction pit retaining wall and supporting column piles; the pit retaining wall includes the pit retaining wall and horizontal support beams, and the supporting column piles support the horizontal support beams. Second, construct the foundation pit for the first construction block; Third, the construction of the foundation pit in the subsequent construction area.

[0008] The specific steps for constructing the foundation pit in the first construction block of the second step are as follows: The initial construction area is divided into several excavation blocks along the tunnel length; Excavation was carried out on each excavation block, and the excavation steps for each excavation block are as follows. a. Lay counterweight units on the horizontal support beams of the excavation block; b. Excavate the excavation block to the bottom of the pit and lay a precast subbase slab at the bottom of the pit in the excavation block; c. A vertical axial force servo control device is arranged between the precast subbase slab and the horizontal support beam in the excavation block. The vertical axial force servo control device includes a vertical support and a jack. The jack applies vertical axial force between the precast subbase slab and the horizontal support beam through the vertical support. d. The construction begins with the construction of the structural base slab at the bottom of the pit. The structural base slab is a reinforced concrete base slab. Then, the base slab counterweight is laid on the structural base slab. e. Remove the vertical axial force servo control device of the previously constructed block and remove the counterweight unit.

[0009] This scheme proposes a design and construction method for the upper foundation pit of a tunnel under axial force servo control. It employs segmented excavation combined with a vertical axial force servo control device to actively control tunnel uplift deformation. Specifically, during the construction of the excavation blocks above the tunnel, prestress is applied through a combination of counterweight units, precast foundation slabs, and the vertical axial force servo control device. (The reaction force of the jacks is provided by the pull-out resistance of the supporting column piles, the self-weight of the horizontal support beams, the counterweight units, and the self-weight of the vertical supports, solving the problem of prestressing the upper foundation pit.) This achieves the goal of actively controlling deformation, significantly reducing the uplift deformation of the pit bottom and the underlying tunnel caused by unloading above. Compared to the traditional full-slab foundation reinforcement method, this scheme eliminates construction disturbance during the foundation reinforcement stage, requires less reinforcement at the pit bottom, is easier to construct, and achieves cost savings, time savings, and environmental friendliness.

[0010] In addition, this scheme involves constructing precast foundation slabs after excavating the foundation pit to the bottom, achieving the purpose of rapid bottom sealing. At the same time, combined with a vertical axial force servo control device, prestress can be applied quickly, which can effectively reduce the problem of creep and bulging deformation caused by long-term exposure after foundation pit excavation due to multiple construction procedures and long pouring time of the foundation slab.

[0011] Preferably, the vertical axial force applied by the jacks between the precast subbase slab and the horizontal support beam is adjusted based on tunnel deformation, which is monitored by a tunnel deformation monitoring system. When the system detects that the tunnel uplift deformation exceeds a set value, the jacks gradually increase the vertical axial force to control the deformation within the set range. This allows for real-time adjustment of the prestress applied by the jacks based on data from the tunnel deformation monitoring system, achieving the requirements for micro-deformation control of the tunnel. Simultaneously, each jack in the vertical axial force servo control device can independently control the prestress value, enabling not only control of tunnel uplift deformation but also correction of horizontal displacement to a certain extent through different prestress values.

[0012] Preferably, the counterweight unit includes: Steel trestle decks are laid on horizontal support beams; Several counterweights are laid on the steel trestle deck. This facilitates the installation and disassembly of the counterweight units, while also enabling recycling, thus achieving the goals of green infrastructure and low-carbon environmental protection.

[0013] Preferably, the weight of the counterweight unit is determined by the following formula. The weight G4 of the counterweight unit is: G4 = k × G1 - G2 - G3 - G5 - G6 - n1 × N; In the formula, G1 is the unloading amount of the soil (kN); G2 — Self-weight of the horizontal support beam corresponding to the excavation block (kN); G3 – Self-weight of the steel trestle deck (kN); G5 – The sum of the vertical supports and jacks' self-weight (kN) of each vertical axial force servo control device within the excavation block; G6 — Self-weight of precast subfloor (kN); N——Design value of pull-out bearing capacity of a single support column pile (kN); n1 — The number of support piles within the excavation block; k — reaction force coefficient (kN); The unloading amount G1 of the soil is: G1=γ×(B2×L2+B1×L1)×H / 2; In the formula, γ is the average unit weight of the soil in the excavated block (kN / m³). 2 ); L1—Length of the pit bottom along the tunnel side of the excavation block (m); B1—Length of the vertical tunnel sidewall at the bottom of the excavation block (m); L2—Length of the excavation block along the tunnel side (m); B2—Length of the vertical tunnel sidewall at the top of the excavation block (m); H—Excavation depth of the foundation pit (m); The design value of the pull-out bearing capacity of a single supported column pile is: N = T u / 2+G7; In the formula, T u —Standard value of ultimate uplift bearing capacity of a single supported column pile (kN); G7 – Self-weight of the supporting column pile (kN); The standard value of the ultimate uplift bearing capacity of the supporting column pile is: T u =∑λ i q sik u i l i ; In the formula, λ i — Pull-out coefficient (kN); q sik —Standard value of the ultimate compressive lateral resistance of the i-th soil layer on the pile side surface (kPa); u i —Pile circumference (m); l i —Thickness (m) of the i-th layer of soil around the pile.

[0014] The reaction force of the jacks is provided by the pull-out resistance of the supporting piles, the self-weight of the horizontal support beam, and the self-weight of the counterweight unit and vertical support. The weight setting of the counterweight unit is crucial for addressing the prestressing application challenge in the excavation above the tunnel. If the counterweight unit is too small, it cannot reliably support the jack reaction force, affecting the control of tunnel uplift deformation. If the counterweight unit is too large, it reduces construction efficiency, affecting the efficiency of prestressing application by the jacks, and may also adversely affect the excavation retaining wall and supporting piles. In actual construction, the weight of the counterweight unit is often estimated by construction personnel based on experience, making accurate setting impossible. To solve this problem, the weight of the counterweight unit in this scheme is calculated using the formula mentioned above. This ensures the accuracy and reliability of the counterweight unit weight, providing a basis for engineering design and facilitating accurate and reliable counterweight unit weight determination by construction personnel, thus ensuring safe and reliable construction.

[0015] Preferably, the number of vertical axial force servo control devices within the excavation block can be determined by the following calculation formula. The number of vertical axial force servo control devices within the excavation block is n² ≥ 1.2 × F. 合 / P; In the formula, F 合 —Design value of the resultant prestressing force acting on the precast subbase slab (kN); P—Design value of bearing capacity (kN) of a single set of vertical axial force servo control device, determined by the minimum bearing capacity of the vertical support and jack; Design value of prestressed resultant force F acting on precast subbase slab 合 For: F 合 =G1-G6.

[0016] In this way, the number of vertical axial force servo control devices in the excavation block can be accurately calculated according to the formula, providing a basis for the design of the number of vertical axial force servo control devices in the engineering, facilitating construction personnel and ensuring construction safety and reliability.

[0017] Preferably, the steel trestle platform is constructed by welding an upper steel plate, a lower steel plate, and sandwiched steel sections together to form a single unit, with a suspension rod installed on one side of the upper steel plate. This facilitates the movement of the steel trestle platform and makes construction easier.

[0018] Preferably, the precast subbase slab comprises several subbase slabs distributed sequentially along the tunnel width direction. Adjacent subbase slabs are constrained by a tenon structure, which includes two tenons positioned vertically, one tenon resting on one subbase slab and the other on the other. During the laying of the precast subbase slabs, the tenon of the first laid subbase slab is positioned above the tenon of the subsequently laid subbase slab. This facilitates the laying of the precast subbase slabs and, more importantly, the tenon structure between the subbase slabs allows for better control of tunnel uplift deformation. Furthermore, it allows for better control of tunnel uplift deformation after the subbase slabs are laid, but before the vertical axial force servo control device above the subbase slab is installed and prestressed.

[0019] Preferably, the supporting pile includes a pile and a steel lattice column, with the steel lattice column connecting the top of the pile to the horizontal supporting beam. This facilitates the actual construction operation of the supporting pile.

[0020] As a preferred option, the specific steps for the construction of the foundation pit in the post-construction block in the three-step process are as follows: excavate the post-construction block to the bottom of the pit; then, construct the structural base plate of the pit bottom in the post-construction block. The design and construction method for the upper foundation pit of a tunnel under axial force servo control also includes the following steps. Fourth, after the structural base slabs of the first and second construction blocks are completed, remove the horizontal support beams and cut off the support column piles above the structural base slabs. Fifth, construct the structural side walls, structural partition walls, and structural top slab of the foundation pit upwards.

[0021] The beneficial effects of this invention are: First, the method employs segmented excavation combined with a vertical axial force servo control device to actively control tunnel heave deformation, significantly reducing heave deformation at the bottom of the pit and beneath the tunnel caused by unloading from above. Compared to the traditional full-span foundation reinforcement method, this scheme's axial force servo control method for tunnel heave design and construction eliminates construction disturbance during the foundation reinforcement stage. It requires less reinforcement at the bottom of the pit, is easier to construct, and achieves cost savings, time savings, and environmental friendliness.

[0022] Second, the use of precast subfloor slabs achieves the purpose of rapid bottom sealing. At the same time, combined with the vertical axial force servo control device, prestress can be applied quickly, which can effectively reduce the problem of creep and bulging deformation caused by long-term exposure after foundation pit excavation due to the many construction procedures and long pouring time of the bottom slab.

[0023] Third, the reaction force of the jacks is provided by the pull-out resistance of the supporting column piles, the self-weight of the horizontal support beam, the counterweight unit, and the self-weight of the vertical support, thus solving the problem of applying prestress to the foundation pit above the tunnel. A calculation formula for the weight of the counterweight unit is also provided, ensuring its accuracy and reliability. This provides a basis for the design of counterweight unit weights in engineering projects, facilitating construction personnel to obtain accurate and reliable counterweight unit weights and ensuring safe and reliable construction.

[0024] Fourth, the prestress value applied by the jacks is adjusted in real time based on the data monitored by the tunnel deformation monitoring system to meet the requirements for tunnel micro-deformation control. Attached Figure Description

[0025] Figure 1 This is a three-dimensional partial structural diagram of the design and construction method of the upper foundation pit for axial force servo control of tunnel heave according to the present invention during the construction process.

[0026] Figure 2 This is a structural schematic diagram of a certain stage in the construction process of the foundation pit of the pre-construction block of the present invention.

[0027] Figure 3 yes Figure 2 Top view.

[0028] Figure 4 This is a structural schematic diagram of another stage in the construction process of the foundation pit of the first construction block of the present invention.

[0029] Figure 5 This is a schematic diagram of a structure after the foundation pit of the first construction block of the present invention has been completed.

[0030] Figure 6 This is a schematic diagram of a structure after the completion of the post-construction block foundation pit construction of the present invention.

[0031] Figure 7 This is a structural schematic diagram of the completed construction of the structural side walls, structural partition walls and structural top slab of the present invention.

[0032] Figure 8 This is a partial structural diagram of the connection node between the supporting column pile and the horizontal supporting beam of the present invention.

[0033] Figure 9 This is a schematic diagram of a prefabricated subfloor plate according to the present invention.

[0034] In the picture: Horizontal support beam 1.1, water-stop curtain 1.2, bored cast-in-place pile 1.3, capping beam 1.4, bent-up steel bar 1.5; Supporting column pile 2, column pile 2.1, steel lattice column 2.2; Counterweight unit 3, steel trestle plate 3.1, counterweight block 3.2; Precast subfloor 4, subfloor 4.1, tenon 4.2; Vertical axial force servo control device 5, jack 5.0, upper support column 5.1, lower prestress transfer support 5.2; Tunnel 6; Structural base plate 7; 8 structural side walls; 9 partition walls in the structure; Structural top plate 10. Detailed Implementation

[0035] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments: Specific Implementation Example 1, such as Figures 1-7 As shown, a method for designing and constructing an upper foundation pit for tunnel heave controlled by axial force servo includes the following steps: First, the construction area is divided into a pre-construction zone A and a post-construction zone B. Pre-construction zone A is located directly above tunnel 6 and extends along the tunnel's length. There can be one, two, or more tunnels below the excavation pit; in this embodiment, there are two tunnels 6 below the excavation pit. Pre-construction zone A corresponds one-to-one with the tunnels below the excavation pit. Each pre-construction zone is located directly above its corresponding tunnel and extends along the tunnel's length.

[0036] The construction includes the foundation pit retaining wall and several supporting piles 2. The foundation pit retaining wall consists of a retaining wall, a capping beam 1.4, and a horizontal support beam 1.1. The retaining wall is composed of bored piles 1.3 and a cutoff wall 1.2. The main reinforcement bars at the top of the bored piles are anchored into the capping beam 1.4. The supporting piles are fixed within the foundation pit soil. The supporting piles support the horizontal support beam.

[0037] Second, the construction of the foundation pit of the first construction block can be carried out simultaneously during actual construction. Alternatively, the first construction block above one tunnel can be constructed first, and then the first construction block above the other tunnel can be constructed.

[0038] like Figures 2-5 As shown, the specific steps for the foundation pit construction in the same pre-construction block are as follows: The first construction block A is divided into several excavation blocks A1 along the tunnel length direction.

[0039] Excavation was carried out on each excavation block, and the excavation steps for each excavation block are as follows. a. Lay counterweight units 3 on the horizontal support beam 1.1 of the excavation block.

[0040] b. Excavate the excavation block to the bottom of the pit and lay a precast subfloor 4 at the bottom of the pit in the excavation block.

[0041] c. Several vertical axial force servo control devices 5 are arranged between the precast subbase slab and the horizontal support beam in the excavation block. The vertical axial force servo control device includes a vertical support and a jack 5.0. The jack applies vertical axial force between the precast subbase slab and the horizontal support beam through the vertical support.

[0042] d. First, construct the structural base slab 7 at the bottom of the pit in the construction area. The structural base slab is a reinforced concrete base slab (with a precast pad slab embedded in the structural base slab); then lay the base slab counterweight on the structural base slab.

[0043] e. Remove the vertical axial force servo control device of the previously constructed block and remove the counterweight unit.

[0044] Third, such as Figure 6 As shown, the construction of the foundation pit in the subsequent construction block is carried out in the following steps: excavation to the bottom of the pit in the subsequent construction block B; then, construction of the structural base plate 7 at the bottom of the pit in the subsequent construction block.

[0045] Fourth, after the structural base slabs of the first and subsequent construction blocks are completed, remove the horizontal support beams and cut off the support column piles above the structural base slabs.

[0046] Fifth, such as Figure 7 As shown, the structural side walls 8, structural partition walls 9, and structural top slab 10 of the excavation pit are constructed upwards. Next, the excavation pit is backfilled with soil, and after backfilling, the counterweight on the bottom slab of the structure is removed.

[0047] This embodiment describes a method for designing and constructing the foundation pit above a tunnel under axial force servo control. It employs segmented excavation combined with a vertical axial force servo control device to actively control tunnel uplift deformation. Specifically, during the construction of the excavation blocks above the tunnel, prestress is applied through a combination of counterweight units, precast foundation slabs, and the vertical axial force servo control device. (The reaction force of the jacks is provided by the pull-out resistance of the supporting column piles, the self-weight of the horizontal support beams, the counterweight units, and the self-weight of the vertical supports, solving the problem of prestressing the foundation pit above the tunnel). This achieves the goal of actively controlling deformation and significantly reduces the uplift deformation of the pit bottom and the underlying tunnel caused by unloading above. Compared to the traditional full-slab foundation reinforcement method, this method eliminates construction disturbance during the foundation reinforcement stage, requires less reinforcement at the pit bottom, and is easier to construct, achieving cost savings, time savings, and environmental friendliness.

[0048] In addition, this scheme involves constructing precast foundation slabs after excavating the foundation pit to the bottom, achieving the purpose of rapid bottom sealing. At the same time, combined with a vertical axial force servo control device, prestress can be applied quickly, which can effectively reduce the problem of creep and bulging deformation caused by long-term exposure after foundation pit excavation due to multiple construction procedures and long pouring time of the foundation slab.

[0049] Furthermore, during the construction of the foundation pit in the initial construction block, the vertical axial force applied by the jacks between the precast subbase slab and the horizontal support beams is adjusted based on tunnel deformation, which is monitored by a tunnel deformation monitoring system. When the system detects that the tunnel uplift deformation exceeds a set value, the jacks gradually increase the vertical axial force to control the deformation within the set range. In this way, the prestress value applied by the jacks can be adjusted in real time based on the data monitored by the tunnel deformation monitoring system, achieving the requirements for micro-deformation control of the tunnel.

[0050] Each vertical axial force servo control device's jacks are independently controlled. In this embodiment, the jacks are hydraulic jacks. Each vertical axial force servo control device's jacks are controlled by a hydraulic servo control system. In this way, not only can the tunnel's uplift deformation be controlled, but also a certain degree of horizontal displacement correction can be achieved by applying different prestress values.

[0051] Furthermore, such as Figure 1 As shown, the vertical support consists of an upper support column 5.1 and a lower prestress transfer support 5.2, with the jack located between the upper support column and the lower prestress transfer support. The upper support column is welded from H-beams and upper and lower end plates located above and below the H-beams. Of course, H-beams can also be replaced by steel pipes or multi-section steel components.

[0052] The top of the lower prestress transfer support is located above the upper surface of the structural base slab. In this embodiment, the lower prestress transfer support is formed by welding an upper bearing plate, a lower bearing plate, and a vertical bearing plate located between the upper and lower bearing plates. The gap between the upper and lower bearing plates facilitates the passage of the reinforcing bars in the structural base slab and subsequent concrete pouring.

[0053] The jack 5.0 is positioned close to the bottom of the foundation pit. This ensures better prestressing and reduces prestress loss.

[0054] Furthermore, such as Figure 1 , Figure 8 As shown, the supporting pile 2 includes a pile 2.1 and a steel lattice column 2.2. The upper end of the pile is flush with the bottom of the foundation pit. The steel lattice column connects the top of the pile to the horizontal support beam. This facilitates the actual construction operation of the supporting pile.

[0055] In the four steps, after the construction of the structural base plate of the first and second construction blocks is completed, the specific operation of cutting off the supporting column piles above the structural base plate is to cut off the steel lattice column.

[0056] The horizontal support beams 1.1 are arranged in a grid pattern. The lattice columns are located at the intersections of the horizontal support beams, with the top of the lattice column anchored into the horizontal inner support beam. Bent-up reinforcing bars 1.5 are arranged at the intersections of the horizontal support beams, and these bent-up reinforcing bars are welded to the top of the lattice column to form a single unit. After the vertical axial force servo control device applies a load, it bears the shear force at the lattice column joints.

[0057] Furthermore, such as Figure 1 As shown, the counterweight unit 3 includes a steel trestle plate 3.1 and several counterweight blocks 3.2. The steel trestle plate is laid on the horizontal support beam. The counterweight blocks are laid on the steel trestle plate. This facilitates the installation and disassembly of the counterweight unit, while also enabling recycling and achieving the goals of green infrastructure and low-carbon environmental protection.

[0058] The steel trestle platform is formed by welding an upper steel plate, a lower steel plate, and sandwiched steel sections together. A suspension rod is installed on one side of the upper steel plate. This facilitates the movement of the steel trestle platform and makes construction easier.

[0059] In this embodiment, the counterweight is composed of concrete blocks and can be arranged in layers, thus facilitating the installation and removal of the counterweight. Of course, the counterweight can also be replaced by the soil excavated during the earthwork excavation process, thereby reducing the number of counterweights brought to the site and reducing the pressure on on-site earthwork transportation.

[0060] Furthermore, such as Figure 9 As shown, the precast subbase slab 4 comprises several subbase slabs 4.1 distributed sequentially along the tunnel width direction, with adjacent subbase slabs constrained by a tenon structure. The tenon structure includes two tenons 4.2 distributed vertically, one tenon positioned on one subbase slab and the other on the other. During the laying of the precast subbase slabs, the tenon of the first laid subbase slab is positioned above the tenon of the subsequently laid subbase slab. This facilitates the laying of the precast subbase slabs; furthermore, the tenon structure between the subbase slabs allows for better control of tunnel uplift deformation. More importantly, it allows for better control of tunnel uplift deformation after the subbase slabs are laid, but before the vertical axial force servo control device above the subbase slab is installed and prestressed.

[0061] In this second specific embodiment, the remaining construction methods are the same as in the first specific embodiment, except that the weight of the counterweight unit is calculated and determined using the following formula. The weight G4 of the counterweight unit is: G4 = k × G1 - G2 - G3 - G5 - G6 - n1 × N; In the formula, G1 is the unloading amount of the soil (kN); G2 — Self-weight of the horizontal support beam corresponding to the excavation block (kN); G3 – Self-weight of the steel trestle deck (kN); G5 – The sum of the vertical supports and jacks' self-weight (kN) of each vertical axial force servo control device within the excavation block; G6 — Self-weight of precast subfloor (kN); N——Design value of pull-out bearing capacity of a single support column pile (kN); n1 — The number of support piles within the excavation block; k — reaction force coefficient (kN); The unloading amount G1 of the soil is: G1=γ×(B2×L2+B1×L1)×H / 2; In the formula, γ is the average unit weight of the soil in the excavated block (kN / m³). 2 ); L1—Length of the pit bottom along the tunnel side of the excavation block (m); B1—Length of the vertical tunnel sidewall at the bottom of the excavation block (m); L2—Length of the excavation block along the tunnel side (m); B2—Length of the vertical tunnel sidewall at the top of the excavation block (m); H—Excavation depth of the foundation pit (m); The design value of the pull-out bearing capacity of a single supported column pile is: N = T u / 2+G7; In the formula, T u —Standard value of ultimate uplift bearing capacity of a single supported column pile (kN); G7 – Self-weight of the support column pile (kN). Specifically, the buoyancy weight of the support column pile below the groundwater level is taken. The standard value of the ultimate uplift bearing capacity of the supporting column pile is: T u =∑λ i q sik u i l i ; In the formula, λ i — Pull-out coefficient (kN); q sik —Standard value of the ultimate compressive lateral resistance of the i-th soil layer on the pile side surface (kPa); u i —Pile circumference (m); l i —Thickness (m) of the i-th layer of soil around the pile.

[0062] The reaction force of the jacks is provided by the pull-out resistance of the supporting piles, the self-weight of the horizontal support beam, and the self-weight of the counterweight unit and vertical support. The weight setting of the counterweight unit is crucial for addressing the prestressing application challenge in the excavation above the tunnel. If the counterweight unit is too small, it cannot reliably support the jack reaction force, affecting the control of tunnel uplift deformation. If the counterweight unit is too large, it reduces construction efficiency, affecting the efficiency of prestressing application by the jacks, and may also adversely affect the excavation retaining wall and supporting piles. In actual construction, the weight of the counterweight unit is often estimated by construction personnel based on experience, making accurate setting impossible. To solve this problem, the weight of the counterweight unit in this scheme is calculated using the formula mentioned above. This ensures the accuracy and reliability of the counterweight unit weight, providing a basis for engineering design and facilitating accurate and reliable counterweight unit weight determination by construction personnel, thus ensuring safe and reliable construction.

[0063] The number of vertical axial force servo control devices within the excavation block can be determined by the following calculation formula. The number of vertical axial force servo control devices within the excavation block is n² ≥ 1.2 × F. 合 / P; In the formula, F 合 —Design value of the resultant prestressing force acting on the precast subbase slab (kN); P—Design value of bearing capacity (kN) of a single set of vertical axial force servo control device, determined by the minimum bearing capacity of the vertical support and jack; Design value of prestressed resultant force F acting on precast subbase slab 合 For: F 合 =G1-G6.

[0064] In this way, the number of vertical axial force servo control devices in the excavation block can be accurately calculated according to the formula, providing a basis for the design of the number of vertical axial force servo control devices in the engineering, facilitating construction personnel and ensuring construction safety and reliability.

[0065] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any simple modifications, alterations, and equivalent transformations made to the above embodiments based on the technical essence of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A method for designing and constructing an upper foundation pit for tunnel heave controlled by axial force servo, characterized in that, The steps are as follows: First, divide the construction area into a first-construction block and a second-construction block. The first-construction block is located directly above the tunnel and extends along the length of the tunnel. Construction pit retaining wall and supporting column piles; the pit retaining wall includes the pit retaining wall and horizontal support beams, and the supporting column piles support the horizontal support beams. Second, construct the foundation pit for the first construction block. The initial construction area is divided into several excavation blocks along the tunnel length; Excavation was carried out on each excavation block, and the excavation steps for each excavation block are as follows. a. Lay counterweight units on the horizontal support beams of the excavation block; b. Excavate the excavation block to the bottom of the pit, and lay a precast subbase slab at the bottom of the pit in the excavation block; c. A vertical axial force servo control device is arranged between the precast subbase slab and the horizontal support beam in the excavation block. The vertical axial force servo control device includes a vertical support and a jack. The jack applies vertical axial force between the precast subbase slab and the horizontal support beam through the vertical support. d. The construction begins with the construction of the structural base slab at the bottom of the pit, followed by the laying of the base slab counterweight on the structural base slab. e. Remove the vertical axial force servo control device of the previously constructed block and remove the counterweight unit; Third, the construction of the foundation pit in the subsequent construction area. Excavate the subsequent construction block to the bottom of the pit; then construct the structural base slab at the bottom of the pit in the subsequent construction block. Fourth, after the structural base slabs of the first and second construction blocks are completed, remove the horizontal support beams and cut off the support column piles above the structural base slabs. Fifth, construct the structural side walls, structural partition walls, and structural top slab of the foundation pit upwards.

2. The method for designing and constructing the upper foundation pit for axial force servo control of tunnel heave according to claim 1, characterized in that, The vertical axial force applied by the jacks between the precast subbase slab and the horizontal support beam is adjusted by the tunnel deformation, which is monitored by the tunnel deformation monitoring system. When the tunnel deformation monitoring system detects that the tunnel uplift deformation exceeds the set value, the jacks gradually increase the vertical axial force value to control the tunnel uplift deformation within the set range.

3. The method for designing and constructing the upper foundation pit of a tunnel under axial force servo control according to claim 1 or 2, characterized in that, The counterweight unit includes: Steel trestle decks are laid on horizontal support beams; Several counterweights are laid on the steel trestle deck.

4. The method for designing and constructing the upper foundation pit of a tunnel under axial force servo control according to claim 3, characterized in that, The weight of the counterweight unit is calculated using the following formula. Weight of the counterweight unit G 4 is: G 4= k × G 1- G 2- G 3- G 5- G 6- n1 ×N; In the formula, G 1 —— Soil unloading (kN); G 2 —— Self-weight of the horizontal support beam corresponding to the excavation block (kN); G 3 —— Self-weight of steel trestle deck (kN); G 5 —— The sum of the vertical supports and jacks of each vertical axial force servo control device within the excavation block (kN). G 6 —— Self-weight of precast subbase slab (kN); N—— Design value of pull-out bearing capacity of a single support column pile (kN); n1—— The number of supporting piles within the excavation block; k—— Reaction coefficient; Soil unloading G 1 is: G 1= γ ×( B 2× L 2+ B 1× L 1)× H / 2; In the formula, γ—— Average unit weight of soil in the excavation block (kN / m) 3 ); L 1 —— The length of the pit bottom along the tunnel side of the excavation block (m); B 1 —— The vertical tunnel side length at the bottom of the excavation block (m). L 2 —— The length of the excavation block's pit top along the tunnel side (m); B 2 —— The vertical tunnel side length at the top of the excavation block (m). H — Excavation depth of the foundation pit (m); The design value of the pull-out bearing capacity of a single support column pile is: N = T u / 2+ G 7; In the formula, T u —— Standard value of ultimate uplift bearing capacity of a single supported column pile (kN); G 7 —— Self-weight of the supporting column pile (kN); The standard value of the ultimate uplift bearing capacity of the supporting column pile is: T u =∑ λ i q sik u i l i ; In the formula, λ i —— Pull-out coefficient; q sik —— The first pile side surface i Standard value of compressive ultimate lateral resistance of soil layer (kPa); u i —— Pile circumference (m); l i —— Piling Perimeter i The thickness of the soil layer (m).

5. The method for designing and constructing the upper foundation pit of a tunnel under axial force servo control according to claim 4, characterized in that, The number of vertical axial force servo control devices within the excavation block can be determined by the following calculation formula. The number of vertical axial force servo control devices within the excavation block is: n2 ≥1.2× F 合 / P ; In the formula, F 合 —— Design value of the resultant prestressing force (kN) acting on the precast subbase slab; P—— The design value (kN) of the bearing capacity of a single set of vertical axial force servo control device is determined by the minimum bearing capacity of the vertical support and jack; Design value of prestressed resultant force acting on precast subbase slab F 合 for: F 合 = G 1- G 6.

6. The method for designing and constructing the upper foundation pit for axial force servo control of tunnel heave according to claim 3, characterized in that, The steel trestle deck is formed by welding an upper steel plate, a lower steel plate, and sandwich steel sections together, with a suspension rod installed on one side of the upper steel plate.

7. The method for designing and constructing the upper foundation pit of a tunnel under axial force servo control according to claim 1 or 2, characterized in that, The precast subbase slab includes several subbase slabs distributed sequentially along the tunnel width direction. Adjacent subbase slabs are constrained by a tenon structure. The tenon structure includes two tenons distributed vertically, one tenon being set on a subbase slab and the other tenon being set on a subbase slab. When laying precast subbase slabs, the tenon of the first subbase slab is positioned above the tenon of the second subbase slab.

8. The method for designing and constructing the upper foundation pit of a tunnel under axial force servo control according to claim 1 or 2, characterized in that, The supporting column piles include column piles and steel lattice columns.

9. The method for designing and constructing the upper foundation pit for axial force servo control of tunnel heave according to claim 8, characterized in that, The steel lattice column connects the top of the column pile to the horizontal support beam.

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