A structure and construction method of a shallow-buried tunnel excavated from a open-cut working well

Abstract

The present application relates to the technical field of tunnel and underground engineering construction, and specifically provides a structure and a construction method for a shallow-buried tunnel to be excavated from an open-cut working well, which comprises an open-cut support system, a concealed support system, an interface reinforcement system, a temporary plugging and backfill system and a linking section. The construction method comprises the following steps: firstly, constructing the open-cut working well enclosure structure; secondly, excavating the upper step to the enclosure pile, installing the steel waist beam and constructing the anchor cable; thirdly, excavating the lower step to the vicinity of the pile, closing and setting the fiber anchor rod; fourthly, constructing the second lining and the temporary plugging wall; fifthly, backfilling the upper step to wrap the steel waist beam; sixthly, excavating the open-cut working well; and finally, breaking the interface structure and the temporary plugging wall, and pouring the linking section to realize the through connection. The present application realizes the safe advance and stop of the concealed tunnel under the condition of the lag of the open-cut working well, effectively solves the problem of time limit and saves the cost and reduces the risk.

Description

Technical Field

[0001] This invention belongs to the field of tunnel and underground engineering construction technology, specifically relating to the structure and construction method of a shallow buried tunnel that exits through an open-cut working shaft. Background Technology

[0002] In urban tunnel construction, a combination of open-cut and shallow-buried cut-and-cover methods is commonly used. Cut-and-cover tunnels typically begin or end with an open-cut working shaft. Traditionally, the open-cut working shaft must be completed before cut-and-cover tunnel construction can commence. However, open-cut sections in urban areas are often constrained by factors such as land acquisition and demolition, pipeline relocation, and traffic management, resulting in significant uncertainty in the construction period. This often leads to prolonged work stoppages in the cut-and-cover section, as the open-cut working shaft remains incomplete when construction reaches the cut-and-cover boundary. Shallow-buried cut-and-cover tunnels are frequently located in soft, water-rich strata, posing significant safety risks such as collapses and water leakage during work stoppages, further delaying the project and increasing costs. Therefore, there is an urgent need for an exit structure and construction method that can overcome the constraints of the open-cut process and ensure the safe and continuous construction of cut-and-cover tunnels.

[0003] Currently, there is limited research on exit construction methods for shallow-buried cut-and-cover tunnels in urban areas. With the accelerated development of urban tunnel construction in my country, more and more tunnel projects are facing the challenge of exit construction. Therefore, it is imperative to develop a safe, fast, economical, and reasonable construction method for the exit construction of shallow-buried cut-and-cover tunnels. Summary of the Invention

[0004] To address the problems existing in the prior art, the present invention provides a structure and construction method for a shallow buried tunnel that exits through an open-cut working shaft. This allows the tunnel to be constructed to the boundary of the working shaft and safely stopped when the open-cut working shaft is not ready for excavation. The tunnel can then be safely connected after the working shaft is excavated, thereby reducing construction delays and lowering safety risks and project costs.

[0005] In a first aspect, the present invention proposes a structure for a shallow-buried mined tunnel that exits through an open-cut working shaft, located at the junction of the open-cut working shaft and the shallow-buried mined tunnel, characterized in that it includes:

[0006] The open-cut support system, including retaining piles, is set around the open-cut working shaft;

[0007] The support system for mined tunnels includes initial tunnel support and advance support;

[0008] The interface reinforcement system includes anchor cables and fiber anchors; the anchor cables are installed in the arch of the upper bench of the tunnel, with one end anchored to the retaining pile or the soil behind the pile, and the other end tensioned and locked to the steel waist beam; the steel waist beam is supported by the initial support of the tunnel and is located in the excavated space of the upper bench of the tunnel; the fiber anchors are installed on the excavation face of the lower bench of the tunnel and the surrounding soil;

[0009] The temporary sealing and backfilling system includes a temporary sealing wall and plain concrete backfill; the temporary sealing wall is set on the lower step of the tunnel and connected to the already constructed secondary lining; the plain concrete backfill fills the excavated space of the upper step of the tunnel and the gap between it and the retaining piles; the plain concrete encloses the steel waist beam.

[0010] The connecting section is the remaining secondary lining structure of the tunnel that connects with the secondary lining that has already been constructed. The connecting section is constructed after the open-cut foundation pit has been excavated to the design elevation and the retaining piles, plain concrete backfill, temporary sealing walls and initial support of the lower bench of the tunnel within the tunnel area have been removed.

[0011] Furthermore, the reinforcement of the temporary sealing wall is consistent with that of the secondary lining, and they are connected by a pre-reserved connector or a pre-reserved steel bar.

[0012] The reinforcement of the temporary sealing wall is consistent with that of the secondary lining and is reliably connected via pre-installed connectors or steel bars. This makes the temporary sealing wall structurally an extension of the already constructed secondary lining, forming a unified whole of equal strength. This allows the temporary sealing wall to share the water and soil pressure from the constructed section behind the tunnel with the secondary lining, ensuring the airtightness and structural strength of the temporary working face during the long waiting period, thus forming a robust temporary barrier.

[0013] Furthermore, the retaining piles are bored cast-in-place piles, with their bottoms penetrating stable strata and below the design elevation of the tunnel floor.

[0014] By placing the bottom of the retaining piles in stable strata and below the tunnel floor, the stability of the retaining piles themselves and sufficient embedment depth are ensured, and the path of heave or seepage that may occur from the bottom of the piles during excavation is blocked. This provides an immovable rear support boundary for the initial construction of the tunnel up to the piles, which is the basis for all subsequent safety measures and improves the overall interface stability and project safety.

[0015] Furthermore, the open-cut support system also includes a ring beam and supports or anchor cables; the ring beam is connected to the retaining piles, and the supports or anchor cables are laid out to avoid the design outline of the cut-and-cover tunnel.

[0016] When laying supports or anchor cables, avoid the design outline of the cut-and-cover tunnel. This avoids the need for complex support removal or replacement work during the later stage of breakthrough, eliminates the safety risks and construction delays caused by this, ensures the efficiency of open-cut construction, and facilitates the final breakthrough operation.

[0017] Furthermore, both ends of the steel waist beam are connected to the initial support of the tunnel; multiple anchor cables are spaced apart along the length of the steel waist beam.

[0018] Furthermore, concrete is sprayed onto the excavation face of the lower step of the tunnel to form a concrete sealing layer; the fiber anchor is a glass fiber anchor.

[0019] By utilizing the full-length bonded anchoring characteristics of fiberglass anchors, the soil in front of and around the anchor is reinforced, increasing the soil's cohesion and internal friction angle, thereby enhancing its self-stabilizing ability. Simultaneously, while fiberglass possesses high tensile strength, it is easily broken by machinery; therefore, while providing reliable temporary reinforcement, it does not affect the efficiency of mechanical demolition operations during the final breakthrough stage.

[0020] Secondly, the present invention proposes a construction method for a shallow-buried, mined tunnel that exits through an open-cut working shaft, wherein the structure is the one proposed in the first aspect above, and the construction method includes the following steps:

[0021] S1: Before the cut-and-cover tunnel is constructed to the cut-and-cover interface, construct the cut-and-cover support system for the cut-and-cover working shaft;

[0022] S2: The upper step of the tunnel is excavated to the retaining piles of the open-cut support system, and a steel waist beam is installed to support it on the initial support of the tunnel. Anchor cables are constructed and tensioned and locked onto the steel waist beam.

[0023] S3: The lower step of the tunnel is excavated to a predetermined distance from the retaining piles, and shotcrete is sprayed to seal the excavation surface of the lower step, forming a concrete sealing layer, and fiber anchor bolts are installed.

[0024] S4: The secondary lining of the tunnel is constructed to the predetermined position, followed by the construction of a temporary reinforced concrete sealing wall;

[0025] S5: Plain concrete is used to backfill the excavated space of the tunnel upper step and the gap between it and the retaining piles, to wrap the steel waist beam, and to fill the arch crown with grout.

[0026] S6: Carry out the excavation of the foundation pit for the open-cut working well;

[0027] S7: Remove the retaining piles, plain concrete backfill, temporary sealing walls, and initial support of the lower bench within the scope of the mined tunnel, and construct the connection section to achieve the connection between the mined tunnel and the open-cut section.

[0028] By completing the cut-and-cover interface treatment before carrying out open-cut excavation, the passive waiting process was transformed into proactively creating conditions, and the traditional irreversible process was reorganized. This enabled partial parallel construction on two work faces, maximizing the degree of parallel operation and shortening the overall project duration along the critical path.

[0029] Furthermore, the open-cut support system also includes a ring beam and supports or anchor cables; step S6 also includes the construction of the ring beam and supports or anchor cables, connecting the ring beam to the retaining piles, and the supports or anchor cables of the open-cut support system avoid the tunnel penetration area.

[0030] Furthermore, in step S7, the demolition work and the pouring of the connecting section need to be carried out in stages.

[0031] The construction and pouring of the connecting sections are carried out in stages, controlling the extent of soil exposure and the duration of free face exposure each time, and following the principle of soil mechanics spatiotemporal effects of compartmentalized and timely closure. Small-scale, rapid support and closure methods are used to maintain the stability of the surrounding rock at the foundation pit and tunnel entrance, preventing large-area exposure from causing excessively rapid stress release and instability in the soil, thus ensuring construction safety in this final high-risk stage of breakthrough.

[0032] Furthermore, in step S1, the bottom of the retaining piles needs to enter a stable stratum and be lower than the design elevation of the tunnel floor.

[0033] The beneficial effects of this invention are as follows: By setting up an interface reinforcement system and a temporary sealing and backfilling system, the mined tunnel is physically isolated and reinforced from the unexcavated open-cut area. A self-stabilizing temporary force-bearing terminal that can be permanently stationary is created, transferring the soil and water pressure of the tunnel to the retaining piles and the initial tunnel support through anchor cables and steel girders, and stabilizing the tunnel entrance through temporary sealing walls and backfill. This eliminates the need for the mined tunnel construction to rely on the completed state of the open-cut working shaft, fundamentally solving the problem of inter-process constraints, shortening the overall construction period, and reducing overall risk. Anchor cables are used to tension and connect the retaining piles to the steel girders supported on the initial tunnel support, forming a powerful active support system. This system effectively transfers and distributes the soil pressure and load above the tunnel arch to the initial tunnel support structure and the more stable retaining piles and the soil behind them through the steel girders. The force transmission path is clear and reliable, significantly enhancing the integrity and rigidity of the tunnel entrance transition area, actively controlling settlement, and preventing the collapse of the soil at the tunnel entrance top. Attached Figure Description

[0034] Figure 1 This is a schematic diagram of the longitudinal section of the shallow-buried tunnel structure of the present invention, which exits through an open-cut working shaft.

[0035] Figure 2 This is a schematic cross-sectional view of the shallow-buried tunnel structure of the present invention, which exits through an open-cut working shaft.

[0036] In the diagram: 1-Retaining pile; 2-Ground line; 3-Open-cut section; 4-Foundation pit bottom line; 5-Surface reinforcement layer; 6-Interface; 7-Initial support; 8-Steel waist beam; 9-Anchor cable; 10-Fiber anchor rod; 11-Secondary lining; 12-Temporary sealing wall; 13-Plain concrete backfill; 14-Tunnel lower bench; 15-Concrete sealing layer. Detailed Implementation

[0037] The present application will be further described in detail below with reference to the accompanying drawings and specific embodiments:

[0038] Definitions:

[0039] Shallow-buried tunneling method: a construction method for excavating tunnels in shallow soil beneath the surface without prior excavation, emphasizing pre-support and timely support.

[0040] Open-cut method: A construction method in which the ground is excavated to form a foundation pit, the structure is constructed inside the foundation pit, and then backfilled.

[0041] Working shaft: An open-cut foundation pit used for purposes such as starting and receiving tunnel construction, equipment entry and exit, and earthwork transportation.

[0042] Pre-support small guide pipe: A small-diameter steel pipe driven into the soil ahead of the excavation face and grouted for pre-support.

[0043] Upper step / lower step: In the step excavation method, the upper part that is excavated first is called the upper step, and the lower part that is excavated later is called the lower step.

[0044] Anchor cable: A component consisting of a cable body, anchorage, etc., which provides anchoring force by tensioning the cable body.

[0045] Steel wainscoting: A horizontal steel member installed inside the retaining piles to bear the tension of the anchor cables and transfer it to the supporting structure (in this case, the initial support of the tunnel).

[0046] Fiber anchor: A type of soil nail or anchor that provides full-length anchoring. The material of the anchor can be glass fiber, steel fiber, etc. Here, it specifically refers to anchors that are easily broken by machinery later.

[0047] Secondary lining (secondary lining): A permanent reinforced concrete lining structure poured inside the initial support of the tunnel.

[0048] Temporary sealing wall: A reinforced concrete wall used to temporarily close the tunnel working face, awaiting later demolition.

[0049] Connection section: refers to the remaining permanent secondary lining section poured after the temporary structure is removed in order to complete the permanent outline of the tunnel.

[0050] like Figure 1 , Figure 2 The shallow-buried tunnel shown is constructed by exiting through an open-cut working shaft. It is located at the junction of the open-cut working shaft and the shallow-buried tunnel and includes: an open-cut support system, a tunnel support system, an interface reinforcement system, a temporary sealing and backfilling system, and a connecting section.

[0051] The open-cut support system includes retaining piles 1, a ring beam, and supports or anchor cables 9. The retaining piles 1 are located around the open-cut working shaft. The retaining piles 1 are bored cast-in-place piles, with their bottoms penetrating stable strata and below the design elevation of the tunnel floor. The ring beam is connected to the retaining piles 1, and the supports or anchor cables 9 are laid out to avoid the design outline of the cut-and-cover tunnel.

[0052] The support system for mined tunnels includes initial tunnel support and advance support.

[0053] The interface reinforcement system includes anchor cables 9 and fiber anchors 10. The anchor cables 9 are installed in the arch of the upper tunnel bench, with one end anchored to the retaining pile 1 or the soil behind the pile, and the other end tensioned and locked to the steel waist beam 8. The steel waist beam 8 is supported by the initial tunnel support 7 and is located within the excavated space of the upper tunnel bench; both ends of the steel waist beam 8 are connected to the initial tunnel support 7. Multiple anchor cables 9 are spaced along the length of the steel waist beam 8. The fiber anchors 10 are installed on the excavation face of the lower tunnel bench 14 and the surrounding soil; the fiber anchors 10 are glass fiber anchors. Concrete is sprayed onto the excavation face of the lower tunnel bench 14 to form a concrete sealing layer 15.

[0054] The temporary sealing and backfilling system includes a temporary sealing wall 12 and plain concrete backfill 13. The temporary sealing wall 12 is located on the lower bench 14 of the tunnel and connected to the already constructed secondary lining 11. The reinforcement of the temporary sealing wall 12 is consistent with that of the secondary lining 11, and they are connected by pre-reserved connectors or pre-reserved steel bars. The plain concrete backfill 13 fills the excavated space of the upper bench of the tunnel and the gap between it and the retaining piles 1. The plain concrete encloses the steel waist beam 8.

[0055] The connecting section is the remaining secondary lining 11 structure of the tunnel that connects to the already constructed secondary lining 11; the connecting section is constructed after the open-cut foundation pit is excavated to the design elevation and the retaining piles 1, plain concrete backfill 13, temporary sealing wall 12 and initial support 7 of the tunnel lower step 14 within the tunnel area are removed.

[0056] A shallow-buried tunnel project in a certain city requires an exit through an open-cut shaft. Due to land acquisition issues, construction of the open-cut shaft cannot commence on schedule. The construction method according to this invention includes the following steps:

[0057] S0: Construction preparation: First, conduct surveying and positioning to accurately mark the boundary between the open-cut working shaft and the cut-and-cover tunnel.

[0058] S1: Open-cut support construction: Drilled cast-in-place piles will be constructed within the planned area of ​​the working shaft as retaining piles. The piles will have a diameter of 1.2m, a spacing of 1.5m, and a length extending 5m into the stable silty clay layer below the tunnel floor. A ring beam will be constructed at the top of the piles. Grouting will be performed to reinforce the ground surface around the working shaft, forming a surface reinforcement layer to prevent settlement.

[0059] Construction of the normal section of the mined tunnel: The mined tunnel is excavated normally from another working shaft, using the bench method, combined with Φ42mm double-layer advanced small pipe support.

[0060] S2: Upper Bench Construction and Interface Treatment: When the upper bench of the mined tunnel reaches the designed position of the open-cut retaining piles, excavation is stopped. I-beam steel girders are installed, with both ends reliably connected to the initial tunnel support via steel plates or steel columns. Four bundles of Φ15.2mm prestressed anchor cables are installed through holes drilled in the retaining piles, with a design tension of 300kN. The other end is tensioned and locked to the steel girders. The concrete supports above the open-cut excavation pit that affect the tunnel outline are removed.

[0061] S3: Construction and Sealing of the Lower Bench: Excavate the lower bench to approximately 2 meters from the retaining piles. Spray a 20cm thick layer of C25 concrete to form a concrete sealing layer to seal the working face of the lower bench. Install Φ25mm fiberglass anchors, 6m in length, spaced 1m x 1m in a quincunx pattern, on the working face and in the surrounding soil.

[0062] S4: Construction of Secondary Lining and Temporary Sealing Wall: The secondary lining concrete pouring immediately follows the lower step, continuing until the temporary sealing wall is positioned as designed. The reinforcement bars for the temporary sealing wall are tied, with the same reinforcement ratio as the secondary lining, and reliably connected to the secondary lining reinforcement bars through pre-embedded connectors. C35 concrete is then poured to form a robust temporary sealing wall.

[0063] S5: Upper Bench Backfill: C15 plain concrete will be used to backfill the entire excavated space of the tunnel upper bench, including the gap between the tunnel arch and the retaining piles, ensuring that the steel girders are completely and tightly encased. Grouting will be used to supplement any areas that are not properly backfilled.

[0064] Work on the mined tunnel is suspended pending further notice: At this point, the working face of the mined tunnel has been safely and stably sealed off, and work can be suspended pending the construction of the open-cut working shaft.

[0065] S6: Open-cut excavation: After the land acquisition is resolved, open-cut excavation of the working shaft will commence, followed by the layered installation of the open-cut support system. The support locations will be pre-designed to avoid the area where the future tunnel will be constructed.

[0066] S7: Construction of the Through Section and Permanent Structure: After the foundation pit is excavated to the design elevation of the base, within the through section of the cut-and-cover tunnel, a combination of hydraulic hammer and manual labor is used to carefully remove the retaining piles, plain concrete backfill, temporary sealing walls, and initial support of the tunnel lower bench in sections (each section not exceeding 6m in length). Subsequently, the remaining secondary lining reinforcement is tied, formwork is erected, and concrete is poured to complete the permanent lining of the entire tunnel section (i.e., the connecting section), ensuring a smooth connection between the cut-and-cover tunnel and the bottom slab and sidewalls of the open-cut section, achieving safe breakthrough.

[0067] The above are merely preferred embodiments of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principle of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. A structure for a shallow-buried mined tunnel that exits through an open-cut working shaft, located at the junction of the open-cut working shaft and the shallow-buried mined tunnel, characterized in that... include: The open-cut support system, including retaining piles, is set around the open-cut working shaft; The support system for mined tunnels includes initial tunnel support and advance support; The interface reinforcement system includes anchor cables and fiber anchors; the anchor cables are installed in the arch of the upper bench of the tunnel, with one end anchored to the retaining pile or the soil behind the pile, and the other end tensioned and locked to the steel waist beam; the steel waist beam is supported by the initial support of the tunnel and is located in the excavated space of the upper bench of the tunnel; the fiber anchors are installed on the excavation face of the lower bench of the tunnel and the surrounding soil; The temporary sealing and backfilling system includes a temporary sealing wall and plain concrete backfill; the temporary sealing wall is set on the lower step of the tunnel and connected to the already constructed secondary lining; the plain concrete backfill fills the excavated space of the upper step of the tunnel and the gap between it and the retaining piles; the plain concrete encloses the steel waist beam. The connecting section is the remaining secondary lining structure of the tunnel that connects with the secondary lining that has already been constructed. The connecting section is constructed after the open-cut foundation pit has been excavated to the design elevation and the retaining piles, plain concrete backfill, temporary sealing walls and initial support of the lower bench of the tunnel within the tunnel area have been removed.

2. The structure of a shallow-buried, mined tunnel exiting through an open-cut working shaft according to claim 1, characterized in that, The reinforcement of the temporary sealing wall is the same as that of the secondary lining, and they are connected by a pre-reserved connector or a pre-reserved steel bar.

3. The structure of a shallow-buried, mined tunnel exiting through an open-cut working shaft according to claim 1, characterized in that, The retaining piles are bored cast-in-place piles, with their bottoms penetrating stable strata and below the design elevation of the tunnel floor.

4. The structure of a shallow-buried, mined tunnel exiting through an open-cut working shaft according to claim 3, characterized in that, The open-cut support system also includes a ring beam and supports or anchor cables; the ring beam is connected to the retaining piles, and the supports or anchor cables are laid out to avoid the design outline of the cut-and-cover tunnel.

5. The structure of a shallow-buried, mined tunnel exiting through an open-cut working shaft according to claim 1, characterized in that, The two ends of the steel waist beam are connected to the initial support of the tunnel; multiple anchor cables are spaced apart along the length of the steel waist beam.

6. The structure of a shallow-buried, mined tunnel exiting through an open-cut working shaft according to claim 1, characterized in that, The excavation face of the lower step of the tunnel is sprayed with concrete to form a concrete sealing layer; the fiber anchor is a glass fiber anchor.

7. A construction method for a shallow-buried, cut-and-cover tunnel exiting through an open-cut working shaft, wherein the structure is as described in any one of claims 1-6, characterized in that... The construction method includes the following steps: S1: Before the cut-and-cover tunnel is constructed to the cut-and-cover interface, construct the cut-and-cover support system for the cut-and-cover working shaft; S2: The upper step of the tunnel is excavated to the retaining piles of the open-cut support system, and a steel waist beam is installed to support it on the initial support of the tunnel. Anchor cables are constructed and tensioned and locked onto the steel waist beam. S3: The lower step of the tunnel is excavated to a predetermined distance from the retaining piles, and shotcrete is sprayed to seal the excavation surface of the lower step, forming a concrete sealing layer, and fiber anchor bolts are installed. S4: The secondary lining of the tunnel is constructed to the predetermined position, followed by the construction of a temporary reinforced concrete sealing wall; S5: Plain concrete is used to backfill the excavated space of the tunnel upper step and the gap between it and the retaining piles, to wrap the steel waist beam, and to fill the arch crown with grout. S6: Carry out the excavation of the foundation pit for the open-cut working well; S7: Remove the retaining piles, plain concrete backfill, temporary sealing walls, and initial support of the lower bench within the scope of the mined tunnel, and construct the connection section to achieve the connection between the mined tunnel and the open-cut section.

8. The construction method for a shallow-buried, mined tunnel with an exit shaft as described in claim 7, characterized in that, The open-cut support system also includes a ring beam and supports or anchor cables; step S6 also includes the construction of the ring beam and supports or anchor cables, connecting the ring beam to the retaining piles, and the supports or anchor cables of the open-cut support system avoid the tunnel penetration area.

9. A construction method for a shallow-buried, mined tunnel with an exit shaft as described in claim 7, characterized in that, In step S7, the demolition work and the pouring of the connecting section need to be carried out in sections.

10. The construction method of a shallow-buried, mined tunnel with an exit shaft as described in claim 7, characterized in that, In step S1, the bottom of the retaining piles must enter a stable stratum and be lower than the design elevation of the tunnel floor.

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