Construction method for deep foundation pit excavation of shallow buried tunnel under full coverage

Abstract

The application relates to the technical field of foundation pit construction, and discloses a full-coverage underlying shallow-buried tunnel deep foundation pit excavation construction method, which comprises the following steps: step one, support pile construction: support piles are constructed on both sides of an existing tunnel along the axial direction of the tunnel, the bottom of the support piles is deep into a stable bearing layer below a tunnel bottom plate horizontal plane by not less than 5 m, and the support piles are arranged at intervals along the length direction of the tunnel; and step two, uplift pile construction: uplift piles are constructed at a reserved space above the existing tunnel, the full-coverage underlying shallow-buried tunnel deep foundation pit excavation construction method is provided, a multilevel full-coverage rigid protection system is constructed through the cooperation of support piles, uplift piles, continuous walls, reinforcing seats and cover plates before foundation pit excavation, the unloading effect of the upper foundation pit excavation and the existing tunnel below are effectively isolated, and the occurrence of tunnel uplift deformation is controlled from the root.

Description

Technical Field

[0001] This invention relates to the field of foundation pit construction technology, specifically to a method for excavating deep foundation pits for fully covered shallow-buried tunnels. Background Technology

[0002] With the continuous development and utilization of urban underground space, the demand for deep foundation pit excavation above existing shallow tunnels is increasing. In existing technologies, the excavation of foundation pits above underground tunnels usually adopts a segmented excavation and support method. That is, during the excavation process, earthwork is excavated in layers and sections. After each layer is excavated, steel pipe internal supports or reinforced concrete supports are erected in a timely manner to support the sidewalls of the foundation pit. At the same time, a certain thickness of soil is retained above the tunnel as a counterweight layer to control tunnel heave.

[0003] In actual construction, the above-mentioned existing construction methods have the following shortcomings: the support method of excavating and supporting at the same time involves repeated unloading and loading of the foundation pit retaining structure and the soil above the tunnel during the excavation process. There is a lack of effective means to form a full-coverage protection system before excavation. The continuous unloading of the soil above the tunnel can easily cause the underlying tunnel to bulge and deform. Therefore, a construction method is needed to solve the above problems. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a method for excavating deep foundation pits for shallow-buried tunnels with full coverage, which solves the problem of difficult tunnel heave control in existing construction methods.

[0005] To achieve the above objectives, the present invention provides the following technical solution: a method for excavating deep foundation pits of fully covered shallow-buried tunnels, comprising the following steps; Step 1, Support pile construction: Support piles are constructed along the tunnel axis on both sides of the existing tunnel. The bottom of the support piles extends at least 5m below the horizontal plane of the tunnel floor to enter the stable bearing layer. The support piles are spaced apart along the length of the tunnel. Step 2, Construction of tension piles: Tension piles are constructed in the reserved space above the existing tunnel. The bottom of the tension piles is embedded in the stable bearing layer, and anchor bars are reserved at the top of the tension piles. Step 3, Construction of diaphragm wall: Construct the diaphragm wall along the direction of the support piles constructed in Step 1. The bottom of the diaphragm wall is embedded in the stable bearing layer below the foundation pit bottom slab, and a reinforcing seat is reserved in the middle of the diaphragm wall. Step 4: Construction of reinforced concrete cover plate: Excavate a receiving cavity between the reinforcing seats and above the tunnel, and pour a concrete cover plate in the receiving cavity. The cover plate spans the entire foundation pit area. The two sides of the cover plate are connected to the reinforcing seats, and the bottom of the cover plate is poured and connected to the top of the anti-tension pile. Subsequent construction can only be carried out after the concrete strength of the cover plate reaches 100% of the design value. Step 5, layered and segmented excavation: Earthwork excavation is carried out under the reinforced concrete cover plate by adopting a layered excavation method, with the excavation thickness of each layer controlled at 2m to 3m; in the longitudinal direction, a segmented excavation method is adopted, with the length of each section controlled at 15m to 25m; the excavation sequence follows the principle of excavating the area far from the tunnel axis first and then excavating the area directly above the tunnel, so as to maintain symmetrical and balanced excavation; Step 6, erecting supports along with layers: After the earthwork excavation of each layer is completed, the transverse inner support of this layer is erected. The transverse inner support is a Chinese character "feng"-shaped keel inner support structure, which includes main keels and auxiliary keels. The main keels are arranged along the extension direction of the tunnel, and both ends of the main keels are respectively connected to the underground continuous walls on both sides of the foundation pit. There are a plurality of auxiliary keels, all of which are arranged perpendicular to the main keels and form a Chinese character "feng"-shaped force-bearing framework together with the main keels. Both sides of the Chinese character "feng"-shaped keel inner support structure support the side wall of the foundation pit at the same time, and apply a pre-applied axial force.

[0006] Optionally, the support piles in Step 1 are in the form of bored cast-in-place piles, and a safety distance of 1m to 2m is maintained between the support piles and the outer wall of the existing tunnel.

[0007] Optionally, the connection nodes between the uplift piles in Step 2 and the cover plate in Step 4, as well as between the reinforcing seats and the cover plate, all adopt rigid connection structures, and the length of the reserved anchorage rebars of the uplift piles and the reinforcing seats anchored into the cover plate is not less than 35 times the diameter of the rebars.

[0008] Optionally, in Step 6, when excavating the last 2m to 3m of soil directly above the tunnel, the excavation thickness of each layer is reduced to 1m to 1.5m, and a stepped excavation or strip-drawing excavation method is adopted; the stepped excavation means that the excavation surface is set in a stepped shape and pushed downward step by step, and the strip-drawing excavation means that the soil is divided into a plurality of strip-shaped areas for interval excavation.

[0009] Optionally, after the construction of the reinforced concrete cover plate is completed and before the layered and segmented excavation starts, a foundation pit bottom soil reinforcement step is further included: within the range of 2m to 3m above the designed elevation of the foundation pit bottom, the soil above the tunnel is reinforced by jet grouting piles or sleeve valve pipe grouting, and the unconfined compressive strength of the reinforced soil is not less than 0.5MPa.

[0010] Optionally, in Step 6, the erection of the transverse inner support is completed within 24 hours after the earthwork excavation of each layer is completed.

[0011] Optionally, a plurality of mounting seats are fixedly connected to the side wall of the main keel, the end of the auxiliary keel is fixedly mounted at the end of the mounting seat through a flange, and the end of the auxiliary keel far away from the main keel is fixedly connected with a top support plate through a telescopic mechanism, and the side wall of the top support plate is in tight abutting connection with the inner wall of the continuous wall.

[0012] Optionally, the telescopic mechanism includes an avoidance groove, a telescopic rod and a stop mechanism, the avoidance groove is opened along the length direction of the main keel, the telescopic rod is installed inside the avoidance groove in a guiding and sliding manner, the other end of the telescopic rod is fixedly connected with the top support plate, and the stop mechanism is arranged at the end of the auxiliary keel to limit the telescopic rod from sliding into the avoidance groove.

[0013] Optionally, the stop mechanism includes a limit pin and a stop nut, the outer surface of the telescopic rod is provided with a guide groove, the inner wall of the avoidance groove is provided with a non-return groove, the cross section of the non-return groove is tapered, the limit pin is snap-fitted and installed in the accommodating cavity formed by the guide groove and the non-return groove, a plurality of stop teeth are fixedly connected to the side of the limit pin facing the guide groove, the side of the limit pin facing the non-return groove is inclined and the shape is adapted to the shape of the non-return groove, the stop nut is threadedly connected to the outer surface of the telescopic rod, and one side of the stop nut is in stop fit with the end of the limit pin.

[0014] Optionally, a plurality of connecting lugs are fixedly connected to the surfaces of both the main keel and the auxiliary keel, and a diagonal brace is installed between the main keel and the lower auxiliary keel through the connecting lugs, so as to form a triangular support between the main keel and the auxiliary keel.

[0015] The present invention provides a construction method for excavating a deep foundation pit overlying a fully covered shallow-buried tunnel, which has the following beneficial effects: The present invention provides a construction method for excavating a deep foundation pit overlying a fully covered shallow-buried tunnel. Through the cooperative cooperation of support piles, uplift piles, diaphragm walls, reinforcing seats and cover plates, a multi-level fully covered rigid protection system is constructed before the excavation of the foundation pit, which effectively isolates the unloading effect of the upper foundation pit excavation from the existing tunnel below, and fundamentally controls the occurrence of tunnel uplift deformation; through the rigid connection structure in which the reserved anchor rebars of the uplift piles and the reinforcing seats are anchored into the cover plate, it is ensured that the connection nodes have sufficient anchoring strength and uplift bearing capacity, so that the upper load borne by the cover plate can be reliably transmitted to the diaphragm walls and the support piles through the reinforcing seats, and transmitted to the deep bearing stratum through the uplift piles, forming a clear load transmission path, avoiding the cracking of the cover plate and the deformation of the pile top caused by stress concentration at the nodes; through the Feng-shaped keel inner support structure, the main keel and a plurality of auxiliary keels form a Feng-shaped force-bearing skeleton, so that the auxiliary keels can always maintain reliable support for the diaphragm walls during the whole process of foundation pit excavation, and the stability during the excavation process is improved. Brief Description of the Drawings

[0016] Figure 1 is a schematic structural view after the construction of step 2 of the present invention; Figure 2 is a schematic top-view structural view after the construction of step 2 of the present invention; Figure 3 is a schematic structural view after the accommodating cavity is excavated according to the present invention; Figure 4This is a schematic diagram of the structure of step four of the present invention; Figure 5 This is a schematic diagram of the structure during the excavation of the upper foundation pit according to the present invention; Figure 6 This is a schematic diagram of the first layer of support structure of the present invention; Figure 7 This is a schematic diagram of the second layer support structure of the present invention; Figure 8 This is a schematic diagram of the structure after the excavation of this invention is completed; Figure 9 This is a schematic diagram of the main keel and secondary keel of the present invention; Figure 10 For the present invention Figure 9 Enlarged structural diagram at point A; Figure 11 This is a schematic diagram of the structure of the secondary keel of the present invention.

[0017] In the diagram: 1. Support pile; 2. Existing tunnel; 3. Pull-out pile; 4. Continuous wall; 5. Reinforcing seat; 6. Receiving cavity; 7. Cover plate; 8. Main keel; 9. Secondary keel; 10. Top support plate; 11. Clearance groove; 12. Telescopic rod; 13. Limit pin; 14. Stop nut; 15. Guide groove; 16. Anti-reverse groove; 17. Stop tooth; 18. Connecting lug; 19. Diagonal brace; 20. Mounting seat; 21. Surface layer; 22. Intermediate layer; 23. Bearing layer. Detailed Implementation

[0018] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0019] Please see Figures 1 to 11 The present invention provides a technical solution: a method for excavating deep foundation pits of shallow buried tunnels with full coverage, comprising the following steps; Step 1, Construction of support pile 1: Construct support pile 1 along the tunnel axis on both sides of the existing tunnel 2. The bottom of support pile 1 should penetrate no less than 5m below the horizontal plane of the tunnel floor and enter the stable bearing layer. Support pile 1 should be set at intervals along the length of tunnel 2. Step 2, Construction of tension piles: Construct tension piles 3 in the reserved space above the existing tunnel 2. The bottom of tension piles 3 is embedded in the stable bearing layer, and anchor bars are reserved at the top of tension piles 3. Step 3, Construction of diaphragm wall: Construct diaphragm wall 4 along the direction of the support pile 1 constructed in Step 1. The bottom of diaphragm wall 4 is embedded in the stable bearing layer below the foundation pit bottom slab. A reinforcing seat 5 is reserved in the middle of diaphragm wall 4. Step 4. Construction of reinforced concrete cover plate: Excavate the accommodating cavity 6 between the reinforcing seats 5 and above the tunnel 2, and pour the concrete cover plate 7 in the accommodating cavity 6. The cover plate 7 spans the entire foundation pit range, two sides of the cover plate 7 are connected with the reinforcing seats 5, and the bottom of the cover plate 7 is cast and connected with the top of the uplift pile 3. Subsequent construction can be carried out only after the concrete strength of the cover plate reaches 100% of the design value; Step 5. Layered and segmented excavation: Carry out earthwork excavation under the reinforced concrete cover plate, adopt a layered excavation method, and control the excavation thickness of each layer at 2m to 3m; adopt a segmented excavation method in the longitudinal direction, and control the length of each section at 15m to 25m; the excavation sequence follows the principle of excavating the area far from the tunnel axis first and then excavating the area directly above the tunnel, so as to maintain symmetrical and balanced excavation; Step 6. Erection of supports along with layers: After the excavation of earthwork of each layer is completed, erect the transverse inner support of this layer. The transverse inner support is a Chinese character "feng"-shaped keel inner support structure, which includes main keels 8 and secondary keels 9. The main keels are arranged along the extension direction of the tunnel 2, two ends of the main keel 8 are respectively connected with the underground continuous walls on both sides of the foundation pit. There are a plurality of secondary keels 9, all of which are arranged perpendicular to the main keels and form a "feng"-shaped force-bearing framework with the main keels. Both sides of the "feng"-shaped keel inner support structure support the side wall of the foundation pit at the same time, and apply a pre-applied axial force.

[0020] The support pile 1 provides a vertical bearing foundation for the subsequent continuous wall and cover plate, and is constructed at intervals along the axial direction on both sides of the existing tunnel 2 in the form of a bored cast-in-place pile. The bottom of the support pile 1 penetrates at least 5m below the tunnel floor to make the pile end enter the stable bearing stratum, so as to ensure that the single pile has sufficient vertical bearing capacity and anti-settlement capacity. A safety distance of 1m to 2m is maintained between the support pile 1 and the outer wall of the existing tunnel 2, so as to avoid disturbance or damage to the lining structure of the existing tunnel caused by drilling and concrete pouring during pile foundation construction. The uplift pile 3 is constructed in the reserved space above the tunnel 2, the pile bottom is embedded in the deep stable bearing stratum to provide anti-floating anchoring force for the cover plate, and the anchoring bars reserved at the pile top are anchored into the cover plate during the subsequent pouring of the cover plate to form a rigid connection, which connects the cover plate and the deep bearing stratum into a whole to resist the upward jacking force of the uplift of the underlying soil caused by soil unloading during the excavation of the foundation pit on the cover plate.

[0021] The diaphragm wall 4 is constructed along the arrangement direction of the support piles 1, with its bottom embedded into the stable bearing stratum below the bottom plate of the foundation pit, so that the diaphragm wall 4 not only serves as a lateral retaining structure for foundation pit excavation to bear lateral earth pressure, but also transfers loads to the deep soil mass through the embedded fixed end at the bottom. The reinforcing seat 5 reserved in the middle of the diaphragm wall 4 provides a horizontal rigid connection node for the cover plate, so that both sides of the cover plate form a reliable structural connection with the diaphragm wall 4. After the accommodating cavity 6 is excavated above the tunnel 2, the reinforced concrete cover plate 7 is poured. The cover plate 7 spans the entire range of the foundation pit to form a fully covered horizontal protective layer. Both sides of the cover plate 7 are rigidly connected to the reinforcing seats 5, and the bottom is poured and connected to the top of the uplift pile 3. The length of the reserved anchorage rebars of the uplift pile 3 and the reinforcing seat 5 anchored into the cover plate 7 is not less than 35 times the diameter of the rebar, ensuring that the connection node has sufficient anchorage strength and uplift bearing capacity. The cover plate 7 isolates the unloading effect of the upper foundation pit excavation from the existing tunnel 2 below. Meanwhile, the upper soil load and construction load borne by the cover plate 7 are transmitted to the diaphragm wall 4 and the support piles 1 through the reinforcing seats 5, and transmitted to the deep bearing stratum through the uplift piles 3. Subsequent construction can be carried out only after the concrete strength of the cover plate reaches 100% of the design value, ensuring that the protective structure has sufficient bearing capacity before the foundation pit excavation.

[0022] The lower earthwork excavation is carried out under the protection of the cover plate 7. The thickness of each layer in layered excavation is controlled at 2m to 3m to avoid excessive single unloading amount causing tunnel heave. In the longitudinal direction, segmented excavation is adopted with each segment length of 15m to 25m. The excavation sequence is to excavate the area far away from the tunnel axis first and then excavate the area directly above the tunnel, maintaining symmetry and balance, so that the soil unloading is released gradually and uniformly, and the adverse impact of local concentrated unloading on the tunnel is avoided. When excavating to the last 2m to 3m of soil directly above the tunnel, the excavation thickness of each layer is reduced to 1m to 1.5m, and step-type excavation or strip-drawing excavation is adopted. In the step-type excavation, the excavation surface is set in a step shape and pushed downward step by step. In the strip-drawing excavation, the soil is divided into several strip-shaped areas for interval excavation, so as to further precisely control the unloading process of the last section of soil above the tunnel. The erection of the transverse inner support is completed within 24 hours after the completion of each layer of earthwork excavation, so as to provide support constraints for the side wall of the foundation pit in time. In the inner support structure of the Chinese character "feng" shaped keel, the main keels 8 are arranged along the extension direction of the tunnel 2, and both ends are connected to the diaphragm walls 4 on both sides of the foundation pit. A plurality of secondary keels 9 are arranged perpendicular to the main keels 8 to form a "feng" shaped force-bearing framework, which supports the side wall of the foundation pit from both sides and applies pre-tensioned axial force at the same time, so that the foundation pit retaining structure always maintains a stable force state during the excavation process.

[0023] In this embodiment, as a preferred solution, the support piles 1 in step 1 are in the form of bored cast-in-place piles, and a safety distance of 1m to 2m is maintained between the support piles 1 and the outer wall of the existing tunnel 2.

[0024] The construction technology of bored piles is mature and the quality of pile formation is reliable. The safety distance of 1m to 2m ensures that the support pile 1 and the existing tunnel 2 do not interfere with each other, so that the support pile 1 is arranged as close as possible to both sides of the tunnel, reducing the span of the subsequent continuous wall 4 and optimizing the stress performance of the protection system.

[0025] In this embodiment, as a preferred option, the connection nodes of the tension pile 3 in step two, the cover plate 7 in step four, and the reinforcing seat 5 and the cover plate 7 are all rigidly connected. The length of the reserved anchor bars of the tension pile 3 and the reinforcing seat 5 anchored into the cover plate 7 is not less than 35 times the diameter of the reinforcing bars.

[0026] The anchorage length of the reserved anchorage bar is not less than 35 times the diameter of the steel bar, which meets the requirements of the specification for the anchorage length of the tension steel bar. This ensures that the connection nodes between the tension pile 3 and the cover plate 7, and between the reinforcing seat 5 and the cover plate 7 are rigid connections, and that the load transfer path is clear and reliable, thus avoiding node failure due to insufficient anchorage.

[0027] In this embodiment, as a preferred option, in step six, when the excavation reaches the last 2m to 3m of soil directly above the tunnel, the thickness of each excavation layer is reduced to 1m to 1.5m, and a stepped excavation or strip excavation method is adopted; stepped excavation is to set the excavation surface as a step and advance it downward step by step, and strip excavation is to divide the soil into several strip-shaped areas and excavate them at intervals.

[0028] The soil near the top of the tunnel is the key area for controlling tunnel uplift deformation. Reducing the excavation thickness and using stepped or strip excavation can significantly reduce the unloading amount of each excavation, allowing the tunnel and surrounding soil sufficient time to adjust stress and effectively control tunnel uplift deformation.

[0029] In this embodiment, as a preferred option, after the reinforced concrete cover plate 7 is constructed and before the layered and segmented excavation begins, a soil reinforcement step is also included: within a range of 2m to 3m above the design elevation of the bottom of the foundation pit, the soil above the tunnel is reinforced by jet grouting piles or sleeve valve pipe grouting, and the unconfined compressive strength of the soil after reinforcement is not less than 0.5MPa.

[0030] The soil reinforcement at the bottom of the pit is carried out after the construction of the cover plate 7 and before the excavation begins. The soil within a range of 2m to 3m above the tunnel is improved and reinforced by jet grouting piles or sleeve valve pipe grouting. After reinforcement, the unconfined compressive strength of the soil is not less than 0.5MPa, which improves the heave resistance and overall stability of the soil at the bottom of the pit. It forms a coordinated heave resistance system with the cover plate 7 above, providing additional safety redundancy for the tunnel below.

[0031] In this embodiment, as a preferred option, in step six, the horizontal internal support is erected within 24 hours after the excavation of each layer of earthwork is completed. Multiple mounting seats 20 are fixedly connected to the side wall of the main keel 8. The end of the secondary keel 9 is fixedly installed at the end of the mounting seat 20 through a flange. The end of the secondary keel 9 away from the main keel 8 is fixedly connected to the top support plate 10 through a telescopic mechanism. The side wall of the top support plate 10 is tightly connected to the inner wall of the continuous wall 4. The telescopic mechanism includes a clearance groove 11, a telescopic rod 12 and a stopping mechanism. The clearance groove 11 is opened along the length direction of the main keel 8. The telescopic rod 12 is guided and slidably installed inside the clearance groove 11. The other end of the telescopic rod 12 is fixedly connected to the top support plate 10. The stopping mechanism is located at the end of the secondary keel 9 to restrict the telescopic rod 12 from sliding into the clearance groove 11.

[0032] The successful completion of the support erection within 24 hours effectively controlled the spatiotemporal effects after the excavation of the foundation pit. One end of the secondary keel 9 is fixed to the mounting seat 20 on the main keel 8 via a flange, and the other end pushes the top support plate 10 against the inner wall of the diaphragm wall 4 via a telescopic mechanism. The telescopic rod 12 extends outward from the clearance groove 11 to push the top support plate 10 until it is tightly fitted with the diaphragm wall 4. The blocking mechanism locks the telescopic rod 12 after it extends into place to prevent it from retracting, ensuring that the support system provides continuous and stable support to the diaphragm wall 4.

[0033] In this embodiment, as a preferred solution, the blocking mechanism includes a limiting pin 13 and a blocking nut 14. A guide groove 15 is provided on the outer surface of the telescopic rod 12, and a backstop groove 16 is provided on the inner wall of the clearance groove 11. The cross-section of the backstop groove 16 is tapered. The limiting pin 13 is snapped into the receiving cavity formed by the guide groove 15 and the backstop groove 16. A plurality of stop teeth 17 are fixedly connected to the side of the limiting pin 13 facing the guide groove 15. The side of the limiting pin 13 facing the backstop groove 16 is inclined and its shape matches the shape of the backstop groove 16. The blocking nut 14 is threaded to the outer surface of the telescopic rod 12, and one side of the blocking nut 14 is engaged with the end of the limiting pin 13 for blocking.

[0034] When the telescopic rod 12 is subjected to an inward retraction force, the inclined surface on the limiting pin 13 engages with the inner wall of the anti-reverse groove 16, causing the limiting pin 13 to be stuck between the guide groove 15 and the anti-reverse groove 16 and unable to retract, forming a one-way self-locking. At the same time, the inclined surface causes the limiting pin 13 to clamp the guide groove 15 in the middle. The greater the lateral thrust, the greater the clamping force of the limiting pin 13 on the guide groove 15, thereby preventing the telescopic rod 12 from retracting. After the stop nut 14 is tightened, it engages with the end of the limiting pin 13 to provide double locking protection, effectively avoiding the axial force loss caused by the loosening of the steel wedge block in traditional steel supports.

[0035] In this embodiment, as a preferred option, multiple connecting ears 18 are fixedly connected to the surfaces of the main keel 8 and the secondary keel 9. A diagonal brace 19 is installed between the main keel 8 and the lower secondary keel 9 through the connecting ears 18 to form a triangular support between the main keel 8 and the secondary keel 9.

[0036] The two ends of the diagonal brace 19 are connected to the main keel 8 and the lower secondary keel 9 respectively through the connecting ears 18, forming a stable triangular support structure between the main and secondary keels. The triangular support utilizes the geometric stability of the triangle to significantly improve the overall rigidity and deformation resistance of the Feng-shaped keel structure, enabling the support system to maintain shape stability when subjected to lateral earth pressure in the foundation pit.

[0037] The specific embodiments provided by the present invention have been described in detail above. Specific examples have been used to illustrate the principles and implementation methods of the present invention. The description of the above embodiments is only for the purpose of helping to understand the method and core ideas of the present invention. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of the present invention. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A method for excavating deep foundation pits of shallow-buried tunnels with full coverage, characterized by: Includes the following steps; Step 1, Construction of support piles (1): Support piles (1) are constructed on both sides of the existing tunnel (2) along the tunnel axis. The bottom of the support piles (1) extends at least 5m below the horizontal plane of the tunnel floor and enters the stable bearing layer. The support piles (1) are set at intervals along the length of the tunnel (2). Step 2, Construction of tension piles: Construct tension piles (3) in the reserved space above the existing tunnel (2). The bottom of the tension piles (3) is embedded in the stable bearing layer, and the top of the tension piles (3) is reserved with anchor bars. Step 3, Construction of underground continuous wall: Construct continuous wall (4) along the direction of the support pile (1) constructed in step 1. The bottom of the continuous wall (4) is embedded in the stable bearing layer below the foundation pit bottom plate. A reinforcing seat (5) is reserved in the middle of the continuous wall (4). Step 4, Reinforced concrete cover plate construction: Excavate the receiving cavity (6) between the reinforcing seats (5) and above the tunnel (2), and pour concrete cover plate (7) in the receiving cavity (6). The cover plate (7) spans the entire foundation pit area. The two sides of the cover plate (7) are connected to the reinforcing seats (5). The bottom of the cover plate (7) is poured and connected to the top of the pull-out pile (3). Subsequent construction can only be carried out after the concrete strength of the cover plate reaches 100% of the design value. Step 5: Layered and segmented excavation: Excavation is carried out under the reinforced concrete cover slab using a layered excavation method, with each layer controlled to a thickness of 2m to 3m; longitudinally, segmented excavation is used, with each segment controlled to a length of 15m to 25m; the excavation sequence follows the principle of excavating the area away from the tunnel axis first, and then excavating the area directly above the tunnel, maintaining symmetrical and balanced excavation. Step 6: Layer-by-layer support erection: After the excavation of each layer of earthwork is completed, the transverse internal support of that layer is erected. The transverse internal support is a feng-shaped keel internal support structure, including a main keel (8) and a secondary keel (9). The main keel is set along the extension direction of the tunnel (2). The two ends of the main keel (8) are connected to the underground continuous walls on both sides of the foundation pit respectively. There are multiple secondary keels (9), all of which are set perpendicular to the main keel and form a feng-shaped force-bearing skeleton with the main keel. The two sides of the feng-shaped keel internal support structure simultaneously support the sidewalls of the foundation pit and apply pre-loaded axial force.

2. The method for excavating deep foundation pits of fully covered shallow-buried tunnels according to claim 1, characterized in that: The support pile (1) in step one is a bored pile, and a safe distance of 1m to 2m is maintained between the support pile (1) and the outer wall of the existing tunnel (2).

3. The method for excavating deep foundation pits of fully covered shallow-buried tunnels according to claim 1, characterized in that: The connection nodes of the tension pile (3) in step two and the cover plate (7) in step four, as well as the connection nodes of the reinforcing seat (5) and the cover plate (7), are all rigid connection structures. The length of the reserved anchor bars of the tension pile (3) and the reinforcing seat (5) anchored into the cover plate (7) is not less than 35 times the diameter of the reinforcing bar.

4. The method for excavating deep foundation pits of fully covered shallow-buried tunnels according to any one of claims 1-3, characterized in that: In step six, when the excavation reaches the last 2m to 3m of soil directly above the tunnel, the thickness of each excavation layer is reduced to 1m to 1.5m, and a stepped excavation or strip excavation method is adopted. The stepped excavation method is to set the excavation surface as a step and advance it downward step by step. The strip excavation method is to divide the soil into several strip-shaped areas and excavate them at intervals.

5. The method for excavating deep foundation pits of fully covered shallow-buried tunnels according to any one of claims 1-3, characterized in that: After the construction of the reinforced concrete cover plate (7) is completed and before the excavation begins in layers and sections, the soil reinforcement steps at the bottom of the pit are also included: within 2m to 3m above the design elevation of the bottom of the pit, the soil above the tunnel is reinforced by jet grouting or sleeve valve grouting, and the unconfined compressive strength of the soil after reinforcement is not less than 0.5MPa.

6. The method for excavating deep foundation pits of fully covered shallow-buried tunnels according to any one of claims 1-3, characterized in that: In step six, the horizontal internal supports shall be erected within 24 hours after each layer of earthwork is excavated.

7. The method for excavating deep foundation pits of fully covered shallow-buried tunnels according to claim 6, characterized in that: The side wall of the main keel (8) is fixedly connected with multiple mounting seats (20). The end of the secondary keel (9) is fixedly installed at the end of the mounting seat (20) through a flange. The end of the secondary keel (9) away from the main keel (8) is fixedly connected to a top support plate (10) through a telescopic mechanism. The side wall of the top support plate (10) is tightly connected to the inner wall of the continuous wall (4).

8. The method for excavating deep foundation pits of fully covered shallow-buried tunnels according to claim 7, characterized in that: The telescopic mechanism includes a clearance groove (11), a telescopic rod (12), and a stopping mechanism. The clearance groove (11) is opened along the length direction of the main keel (8). The telescopic rod (12) is guided and slidably installed inside the clearance groove (11). The other end of the telescopic rod (12) is fixedly connected to the top support plate (10). The stopping mechanism is located at the end of the secondary keel (9) to restrict the telescopic rod (12) from sliding into the clearance groove (11).

9. The method for excavating deep foundation pits of fully covered shallow-buried tunnels according to claim 8, characterized in that: The stopping mechanism includes a limiting pin (13) and a stopping nut (14). The outer surface of the telescopic rod (12) is provided with a guide groove (15). The inner wall of the clearance groove (11) is provided with a check groove (16). The cross section of the check groove (16) is tapered. The limiting pin (13) is snapped into the cavity formed by the guide groove (15) and the check groove (16). The limiting pin (13) is fixedly connected to a plurality of stop teeth (17) on the side facing the guide groove (15). The side of the limiting pin (13) facing the check groove (16) is inclined and its shape is adapted to the shape of the check groove (16). The stopping nut (14) is threaded to the outer surface of the telescopic rod (12), and one side of the stopping nut (14) is engaged with the end of the limiting pin (13).

10. The method for excavating deep foundation pits of fully covered shallow-buried tunnels according to any one of claims 1-3, characterized in that: Multiple connecting ears (18) are fixedly connected to the surfaces of the main keel (8) and the secondary keel (9). A diagonal brace (19) is installed between the main keel (8) and the lower secondary keel (9) through the connecting ears (18) to form a triangular support between the main keel (8) and the secondary keel (9).

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