Interface for reconstruction and extension of underground engineering based on permanent and temporary combination and construction method thereof

Abstract

This invention discloses a novel interface and its construction method for underground engineering renovation and expansion based on a combination of permanent and temporary structures. Conventional interface methods at the junction of new and existing underground structures are prone to leakage and joint separation. In this invention, the new underground structure connects to the existing underground structure, with a I-beam and crossbeams wrapping the existing structure's top slab in a C-shape; one side of the I-beam connects to the support within the new structure's foundation pit; vertical and horizontal embedded steel plates are respectively installed on the right side and bottom of the I-beam; the I-beam and crossbeam are connected by horizontal steel plates, and connected to the new structure's top slab by vertical steel plates; the new structure's top slab contains reinforcing bars, which are welded to the vertical steel plates, and the upper and lower layers of reinforcing bars are connected by tie rods; a waterproof layer is applied to the new structure's top slab, and backfilling is carried out on top. This invention forms an effective force transmission system between the new and old structures, avoiding the tendency for subsequent concentrated deformation to cause interface opening and leakage.

Description

Technical Field

[0001] This invention belongs to the field of underground engineering technology, specifically relating to an interface for the renovation and expansion of underground engineering based on a combination of permanent and temporary structures, and its construction method. Background Technology

[0002] Currently, with the increasing intensity of underground space development in major cities across the country, such as urban rail transit transfer stations, TOD development and station-city integration of existing rail transit stations, and various comprehensive transportation hubs, there is a growing number of renovation and expansion projects for existing underground structures. The connection points between new and existing structures face significant implementation risks and safety hazards in terms of overall stress and joint waterproofing. Currently, conventional interface solutions almost always have obvious quality defects. Furthermore, due to the influence of existing underground structures, the requirement that new and existing structures be at the same elevation often leads to difficulties in erecting supports within the foundation pit of new underground structures using conventional methods, which does not conform to the basic design principles of green, low-carbon, and environmentally friendly practices.

[0003] Generally, the interface between a newly constructed underground structure (or underground passage) and an existing structure is treated using pre-installed rebar connectors or rebar anchoring, with a "flat joint" being the primary structural type. While conventional joint methods can temporarily meet the load-bearing requirements of the project and satisfy normal usage requirements for a certain period after construction, they are prone to leakage and joint separation during long-term use, failing to meet the 100-year design life requirement. Furthermore, due to the clearance requirements of the newly constructed underground structure (or underground passage), it is generally required that the structural slabs of each layer at the connection point be smoothly connected, i.e., the bottom of the top slab should be flush with the ground. Based on load-bearing requirements, at the interface, the first internal support in the new foundation pit often has to be directly applied to the top slab of the existing underground structure (effective horizontal force transfer), leading to the need for subsequent support replacement, increasing implementation difficulty and engineering risks, and affecting the one-time integral casting of the new structure's sidewalls and top slab, thus increasing structural joints. Summary of the Invention

[0004] To overcome the shortcomings of existing technologies, this invention provides an interface for underground engineering renovation and expansion based on a combination of permanent and temporary structures and its construction method. This not only meets the overall stress and waterproofing requirements of the interface, but also solves the problem of difficult support erection in the foundation pit of newly built underground structures. At the same time, the support system in the foundation pit can be integrated into the permanent structure as appropriate, avoiding the removal of the existing foundation pit support system in the later stage, and carrying out the whole life cycle scheme design in a green and low-carbon manner.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] The interface for underground engineering renovation and expansion based on the combination of permanent and temporary structures is characterized by:

[0007] The new underground structure is connected to the existing underground structure. The interface at the connection point adopts a combination of I-beams and crossbeams. The I-beams and crossbeams wrap the top slab of the existing structure in a C-shape.

[0008] The joints between the I-beam and the existing structural top slab and side walls are roughened and coated with epoxy resin; the joints between the I-beam and the existing underground structure are connected by pre-reserved steel reinforcement connectors; one side of the I-beam is connected to the support inside the foundation pit of the new underground structure.

[0009] Vertical and horizontal embedded steel plates are installed on the right side and bottom of the I-beam, respectively; the I-beam and the crossbeam are connected by the horizontal embedded steel plate; the I-beam and the top slab of the new structure are connected by the vertical embedded steel plate.

[0010] The newly constructed top slab is equipped with longitudinal and transverse reinforcing bars, which are welded to vertically embedded steel plates. The upper and lower layers of reinforcing bars are mechanically connected by tie rods.

[0011] A waterproof layer is applied to the top slab of the new structure, and then backfilled with soil.

[0012] Furthermore, the top of the support inside the first newly constructed underground foundation pit is flush with the top of the I-shaped beam;

[0013] Furthermore, one side of the crossbeam connects to the lower armpit corner of the existing structural top plate, and the other side is flush with the I-shaped beam;

[0014] Furthermore, the projected length of the top of the I-shaped beam above the existing structural top slab is not less than 1 times the thickness of the existing structural top slab, and not less than 500mm.

[0015] The method for constructing interfaces for underground engineering renovation and expansion based on the combination of permanent and temporary structures includes the following steps:

[0016] Step 1: Excavation of the foundation pit and construction of new underground structures

[0017] 1.1: Based on the design requirements, and the layout of the existing underground structure location and the outer retaining structure outline of the new underground structure, dewatering wells are set up and dewatering construction is carried out in advance. Then, the foundation pit retaining structure is constructed as required, and then the foundation pit excavation construction is carried out according to the requirements of regional, layered and block construction.

[0018] 1.2: The area of ​​the interface between the old and new underground structures is first excavated as a small foundation pit with a slope of 1:0.5 to 1:0.75. The horizontal distance between the bottom of the slope and the bottom of the two sides of the I-beam should not be less than 0.5m until the top slab of the existing structure at the interface is completely exposed and the bottom of the foundation pit is flush with the bottom of the I-beam. Then, the I-beam is cast and the vertical and horizontal steel plates are pre-embedded as required. The joint between the I-beam and the top slab and side wall of the existing structure is roughened and coated with two coats of epoxy resin. The joint between the I-beam and the existing underground structure is connected by the pre-reserved steel bar connector.

[0019] 1.3: After the foundation pit is excavated to the bottom of the existing underground structure top slab, the first foundation pit support is erected. On one side of the existing underground structure, the top of the first foundation pit support is flush with the top of the I-beam. The first foundation pit support is made of reinforced concrete.

[0020] 1.4: In accordance with the design requirements and based on the principle of excavation and support as needed, the excavation of the new foundation pit will continue on the side adjacent to the existing underground structure. Stress gauges and deformation monitoring points will be installed at the corresponding locations of the existing structure's top slab, middle slab, bottom slab, and side walls as needed to ensure the safety and normal use of the existing underground structure.

[0021] 1.5: After the new foundation pit has been excavated to the bottom, the bottom of the pit shall be cleaned and the site leveled. Manual excavation shall be carried out within 500mm of the bottom of the pit. Finally, plain concrete cushion layer shall be constructed and the bottom of the pit shall be sealed in time.

[0022] 1.6: The construction sequence of conventional underground structures is as follows: the waterproof layer of the base slab is laid in sequence as required, followed by the pouring of the new structure base slab, the new structure side walls, the new structure middle slab and the new structure top slab in sequence. The waterproof layer is laid before the side walls are constructed. The new structure side walls at the interface can be considered as reserved openings instead of waterproofing. The clear space of the structures on both sides at the connection between the old and new structures should be consistent.

[0023] Step 2: Construct new underground structure interfaces

[0024] 2.1: Longitudinal and transverse reinforcing bars are installed in the top slab of the new structure. The reinforcing bars are welded to the vertical embedded steel plates, and the upper and lower layers of reinforcing bars are mechanically connected by tie bars.

[0025] 2.2: After the construction of all new underground structures is completed, the existing structural sidewalls at the connection points of the existing underground structures shall be demolished. At the same time, at least 0.5-1.0m of the sidewalls around the interface opening shall be manually removed.

[0026] 2.3: Utilize the pre-embedded steel plate under the 1-shaped beam and the existing structural side wall within the opening area after chiseling away the reserved steel bars, erect the formwork, tie the steel bars, and then pour the horizontal beam. One side of the horizontal beam connects to the underside of the existing structural top slab, and the other side is flush with the 1-shaped beam. The horizontal beam and the 1-shaped beam are connected by the pre-embedded steel plate.

[0027] 2.4: The left and right sides and bottom of the opening are effectively connected to the newly built underground structure by steel bar connectors or rebar installation to meet the connection requirements between the old and new structures. The elevations of the top of the top slab and the top of the bottom slab at the connection points between the old and new structures should be consistent.

[0028] 2.5: Apply a waterproof layer to the top slab of the new structure. The waterproof layer should overlap the existing waterproof layer on the top slab by at least 0.5m. Then carry out backfilling above the top slab. During this period, the structure within 2m below the ground surface needs to be removed in advance. The first foundation pit support outside 2m below the ground surface can be retained and share the load with the main structure.

[0029] 2.6: Carry out road surface restoration work according to the requirements of urban roads or greening.

[0030] The beneficial effects of this invention are:

[0031] 1) This invention utilizes a 1-shaped beam and a crossbeam to wrap the existing underground structure top slab in a C-shape. At the same time, the 1-shaped beam and the crossbeam are effectively connected to the newly built underground structure through pre-embedded steel plates to form a composite component that bears the overall load. An effective force transmission system is formed between the old and new structures. The 1-shaped beam and the crossbeam replace the direct connection of the structural plates at the conventional interface between the old and new structures. Since the 1-shaped beam and the crossbeam have a large overall stiffness, it avoids the relatively weak stiffness of the components at the interface in the past, which led to relatively concentrated deformation and easy opening and leakage of the interface.

[0032] 2) During the excavation stage of the new underground structure foundation pit, in order to ensure the safety of the foundation pit and the effective transfer of horizontal loads, the erection of the first internal support often requires the use of the existing structural top slab. However, since the new structural top slab is basically flush with the existing structural top slab, conventional solutions may require replacement of the support later. The top of the I-beam of this invention is significantly higher than the existing structural top slab. Therefore, the top of the I-beam can provide the first internal support with a function similar to a capping beam. At the same time, it can ensure that the first internal support is spatially staggered from the new structural top slab below. In addition, the first internal support can be retained as part of the permanent structure as appropriate, ensuring the safety and long-term stability of the underground structure.

[0033] 3) The underground structures, foundation pit retaining structures and internal supports, embedded steel plates and reinforcing bars used in the implementation of this invention are all conventional types, which are easy to process and manufacture;

[0034] 4) The invention has a simple process and is easy to construct, with high economic and social benefits. It has broad application prospects in the renovation and expansion of existing structures such as urban rail transit, railways, and municipal tunnels. Attached Figure Description

[0035] Figure 1 This is a diagram showing the connection between the newly constructed underground structure and the existing structure in this invention.

[0036] Figure 2 This is a cross-sectional layout diagram of the existing underground structure;

[0037] Figure 3 Diagram showing the connection between the internal support and the I-beam in the newly constructed foundation pit;

[0038] Figure 4 Detailed drawing of the connection between the I-shaped beam and the roof slab of the newly built station;

[0039] Figure 5 A schematic diagram of a small foundation pit for constructing the I-shaped beam at the interface;

[0040] In the diagram, 1-existing underground structure, 2-new underground structure, 3-existing structure top slab, 4-existing structure middle slab, 5-existing structure bottom slab, 6-existing structure side wall, 7-new structure top slab, 8-new structure middle slab, 9-new structure bottom slab, 10-new structure side wall, 11-I-shaped beam, 12-horizontal beam, 13-vertical embedded steel plate, 14-horizontal embedded steel plate, 15-new underground foundation pit internal support, 16-longitudinal and transverse reinforcing bars, 17-ties, 18-slope line. Detailed Implementation

[0041] The present invention will now be described in detail with reference to specific embodiments.

[0042] This invention addresses the common practice in underground renovation and expansion projects of directly connecting structural slabs using a "flat joint" method, which poses a risk of opening and leakage later on. Therefore, it proposes an interface system based on a 1-shaped beam and a crossbeam to achieve effective force transfer between the old and new structures and ensure the anti-seepage requirements at the interface. At the same time, the top of the 1-shaped beam provides a support similar to a cap beam, thereby solving the problem of erecting the first layer of internal support in a new foundation pit.

[0043] like Figure 1 , Figure 2 As shown, the existing underground structure 1 and the newly built underground structure 2 are generally connected on the first or second underground floor. The interface at the connection point adopts a combination of a 1-shaped beam 11 and a crossbeam 12 to meet the requirement of continuous use of the underground space.

[0044] The existing underground structure 1 includes the existing structure top slab 3, the existing structure middle slab 4, the existing structure bottom slab 5, and the existing structure side walls 6; the newly constructed underground structure 2 includes the newly constructed structure top slab 7, the newly constructed structure middle slab 8, the newly constructed structure bottom slab 9, and the newly constructed structure side walls 10.

[0045] like Figure 3 As shown, the joint between the I-beam 11 and the existing structural top slab 3 and the existing structural side wall 6 is roughened and coated with epoxy resin; the I-beam 11 and the crossbeam 12 wrap the existing structural top slab 3 in a C-shape.

[0046] Vertical embedded steel plates 13 and horizontal embedded steel plates 14 are respectively installed on the right side and bottom of the I-shaped beam 11; the I-shaped beam 11 and the cross beam 12 are connected by the horizontal embedded steel plates 14; the I-shaped beam 11 and the newly built structural top slab 7 are connected by the vertical embedded steel plates 13. The bottom elevation of the newly built structural top slab 7 is basically the same as that of the existing structural top slab 3, so as to meet the requirements of underground space use to the greatest extent.

[0047] The I-beam 11 is connected to the existing underground structure 1 via a pre-reserved steel reinforcement connector; one side of the I-beam 11 is connected to the internal support 15 in the newly built underground foundation pit; the I-beam 11 is equivalent to the foundation pit capping beam, and the top of the I-beam 11 is at least 1.0m higher than the top of the new structure's top slab 7; the sloping design at the top of the I-beam 11 can effectively transfer the horizontal load of the internal support 15 to the direction of the existing structure's top slab 3; the internal support 15 can be retained and not removed in the future, and will share the horizontal ground load with the new structure's top slab 7;

[0048] like Figure 4 As shown, the newly built top slab 7 is equipped with longitudinal and transverse reinforcing bars 16 and tie bars 17 to meet the structural stress requirements. The reinforcing bars 16 are welded to the vertical embedded steel plate 13. The upper and lower reinforcing bars 16 are mechanically connected by tie bars 17 to meet the overall structural stress requirements at the interface.

[0049] like Figure 5 As shown, during construction, a 1-shaped beam 11 is first constructed to meet the needs of the support 15 erection in the new foundation pit. During the construction of the new underground structure 2, the existing structural sidewall 6 is removed according to the interface connection range as required. Generally, static demolition is considered. At the same time, at least 0.5-1.0m of the sidewall 6 is reserved around the interface opening range for manual removal to ensure that a certain length of steel bars is retained around the interface opening range for interface connection. The length of the reserved steel bars is stripped from the sidewall being removed, and the length is generally not less than 20d (d is the diameter of the steel bar). Before the portal is opened when connecting the old and new structures, temporary steel supports should be set inside the existing structure if necessary, based on calculations and monitoring of the existing underground structure 1, and active support measures should be considered.

[0050] The I-beam 11 and the crossbeam 12 are also connected by a pre-embedded steel plate 14. The projection length of the top of the I-beam 11 above the existing structural top plate 3 is not less than 1 times the thickness of the existing structural top plate 3, and generally not less than 500mm. The top of the first newly built underground foundation pit support 15 is flush with the top of the I-beam 11. One side of the crossbeam 12 is connected to the lower armhole of the existing structural top plate 3, and the other side is flush with the I-beam 11.

[0051] A waterproof layer shall be applied to the top slab 7 of the new structure. The old and new waterproof layers must be overlapped. The structure within 2m below the ground surface of the foundation pit shall be removed and compacted with backfill soil that meets the requirements.

[0052] The method for constructing interfaces for underground engineering renovation and expansion based on a combination of permanent and temporary structures is characterized by the following steps:

[0053] Step 1: Excavation of the foundation pit and construction of new underground structures

[0054] 1.1: According to the design requirements, based on the location of the existing underground structure 1 and the outline of the outer retaining structure of the new underground structure 2, dewatering wells are set up and dewatering construction is carried out in advance. Then, the foundation pit retaining structure is constructed as required, and then the foundation pit excavation construction is carried out according to the requirements of regional, layered and block construction.

[0055] 1.2: The area of ​​the interface between the old and new underground structures should be excavated first as a small foundation pit with a slope. The slope ratio can generally be considered as 1:0.5 to 1:0.75. The horizontal distance between the bottom of the slope and the bottom of both sides of the I-beam 11 should not be less than 0.5m until the top plate 3 of the existing structure at the interface is completely exposed. The bottom of the foundation pit is flush with the bottom of the I-beam 11. Then, the I-beam 11 is erected and cast as required, and the vertical steel plate 13 and the horizontal steel plate 14 are pre-embedded. The joint between the I-beam 11 and the top plate 3 of the existing structure and the side wall 6 of the existing structure is roughened and coated with two coats of epoxy resin. The joint between the I-beam 11 and the existing underground structure 1 is connected by the pre-reserved steel bar connector. If there is no pre-reserved steel bar connector, it is treated as rebar installation.

[0056] 1.3: After the foundation pit is excavated to about 1m below the top slab of the existing underground structure 1, and the excavation depth of the foundation pit generally does not exceed 3m and meets safety requirements, the first foundation pit support 15 is erected on one side of the existing underground structure 1. The top of the first foundation pit support 15 is flush with the top of the I-beam 11. At this time, the I-beam 11 can be considered as the capping beam of the foundation pit support 15. The first foundation pit support 15 should be made of reinforced concrete.

[0057] 1.4: According to the design requirements and based on the principle of excavation and support, the excavation of the new foundation pit will continue on the side adjacent to the existing underground structure 1. Stress gauges and deformation monitoring points will be installed at the corresponding parts of the existing structure top slab 3, existing structure middle slab 4, existing structure bottom slab 5 and existing structure side wall 6 as needed to ensure the safety and normal use of the existing underground structure 1.

[0058] 1.5: After the new foundation pit has been excavated to the bottom, the bottom of the pit shall be cleaned and the site leveled. The area within 500mm of the bottom of the pit shall be excavated manually. Finally, a plain concrete cushion layer shall be constructed and the bottom of the pit shall be sealed in a timely manner.

[0059] 1.6: According to the conventional underground structure construction sequence, the waterproof layer of the bottom slab shall be laid in sequence as required. Then, the new structure bottom slab 9, the new structure side wall 10, the new structure middle slab 8 and the new structure top slab 7 shall be poured in sequence. The waterproof layer shall be laid before the side wall is constructed. The new structure side wall 10 at the interface can be considered as a reserved opening. The net clearance of the structure on both sides at the connection between the new and old structures shall be consistent.

[0060] Step 2: Construct new underground structure interfaces

[0061] 2.1: Longitudinal and transverse reinforcing bars 16 are installed in the top slab 7 of the new structure. The reinforcing bars 16 are welded to the vertical embedded steel plate 13 to achieve structural connection. The upper and lower reinforcing bars are mechanically connected by tie bars 17 to meet the structural requirements.

[0062] 2.2: After the construction of all new underground structures 2 is completed, the existing structural sidewalls 6 at the connection points of the existing underground structures 1 are demolished. Generally, static demolition is considered. At the same time, at least 0.5-1.0m of the sidewalls 6 around the interface opening area are reserved for manual chiseling to retain a certain length of steel bars for the purpose of connecting the interface steel bars.

[0063] 2.3: Using the pre-embedded steel plate 14 under the I-beam 11 and the existing structural side wall 6 within the opening range after chiseling out the reserved steel bars, erect the formwork, tie the steel bars, and then pour the horizontal beam 12. The welding of the steel bars of the horizontal beam 12 and the steel bars after chiseling out the opening should avoid all being located in the same position and meet the structural requirements. One side of the horizontal beam 12 is connected to the lower armhole of the existing structural top plate 3, and the other side is flush with the I-beam 11. The horizontal beam 12 and the I-beam 11 are connected through the pre-embedded steel plate 14.

[0064] 2.4: In addition to the connection at the top slab of the existing underground structure 1, the left and right sides and bottom of the opening are effectively connected to the newly built underground structure 2 by means of steel bar connectors or rebar installation to complete the connection between the old and new structures. The elevations of the top slab and the bottom slab on both sides of the connection between the old and new structures should be consistent.

[0065] 2.5: Apply a waterproof layer on the newly constructed top slab 7. The waterproof layer should overlap with the existing top slab 3 by at least 0.5m. Then carry out backfilling above the top slab. During this period, the structure within 2m below the ground surface needs to be removed in advance. The first foundation pit support 15 outside 2m below the ground surface can be retained and share the load with the main structure.

[0066] 2.6: Carry out road surface restoration work according to the requirements of urban roads or greening.

[0067] In the description of this invention, unless otherwise explicitly specified and limited, the terms "set," "install," "connect," "link," and "fix" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0068] The content of this invention is not limited to the embodiments listed. Any equivalent modifications made by those skilled in the art to the technical solutions of this invention by reading this specification are covered by the claims of this invention.

Claims

1. An interface for underground engineering renovation and expansion based on a combination of permanent and temporary structures, characterized in that: The newly built underground structure (2) is connected to the existing underground structure (1). The interface at the connection point adopts a combination of a 1-shaped beam (11) and a crossbeam (12). The 1-shaped beam (11) and the crossbeam (12) wrap the top plate (3) of the existing structure in a C-shape. The joint between the 1-beam (11) and the existing structural top slab (3) and the existing structural side wall (6) is roughened and coated with epoxy resin; the joint between the 1-beam (11) and the existing underground structure (1) is connected by a pre-reserved steel bar connector; one side of the 1-beam (11) is connected to the support (15) in the foundation pit of the newly built underground structure (2); Vertical embedded steel plates (13) and horizontal embedded steel plates (14) are respectively installed on the right side and bottom of the 1-shaped beam (11); the 1-shaped beam (11) and the cross beam (12) are connected by the horizontal embedded steel plates (14); the 1-shaped beam (11) and the newly built structural top slab (7) are connected by the vertical embedded steel plates (13); The newly built top slab (7) is provided with longitudinal and transverse reinforcing bars (16), which are welded to the vertical embedded steel plate (13). The upper and lower layers of reinforcing bars (16) are mechanically connected by tie bars (17). A waterproof layer is applied to the top slab (7) of the new structure, and backfill soil on top.

2. The interface for underground engineering renovation and expansion based on permanent and temporary combination as described in claim 1, characterized in that: The top of the first newly built underground foundation pit support (15) is flush with the top of the I-shaped beam (11).

3. The interface for underground engineering renovation and expansion based on permanent and temporary combination as described in claim 2, characterized in that: One side of the beam (12) is connected to the lower armpit of the existing structural top plate (3), and the other side is flush with the I-shaped beam (11).

4. The interface for underground engineering renovation and expansion based on permanent and temporary combination as described in claim 3, characterized in that: The projected length of the top of the 1-shaped beam (11) above the existing structural top plate (3) is not less than 1 times the thickness of the existing structural top plate (3) and not less than 500mm.

5. A method for constructing interfaces for the renovation and expansion of underground engineering projects based on a combination of permanent and temporary structures, characterized in that: Includes the following steps: Step 1: Excavation of the foundation pit and construction of new underground structures 1.1: According to the design requirements, based on the location of the existing underground structure (1) and the outline of the outer retaining structure of the new underground structure (2), dewatering wells are set up and dewatering construction is carried out in advance. Then, the foundation pit retaining structure is constructed as required, and then the foundation pit excavation construction is carried out according to the requirements of dividing the area, layer and block. 1.2: The projection range of the interface between the old and new underground structures is first excavated by sloping the foundation pit with a slope of 1:0.5 to 1:0.

75. The horizontal distance between the bottom of the slope and the bottom of the two sides of the 1-shaped beam (11) should not be less than 0.5m until the top plate (3) of the existing structure at the interface is completely exposed. The bottom of the foundation pit is flush with the bottom of the 1-shaped beam (11). Then, the 1-shaped beam (11) is erected and cast according to the requirements, and vertical steel plates (13) and horizontal steel plates (14) are pre-embedded. The joint between the 1-shaped beam (11) and the top plate (3) of the existing structure and the side wall (6) of the existing structure is roughened and coated with two coats of epoxy resin. The joint between the 1-shaped beam (11) and the existing underground structure (1) is connected by the pre-reserved steel bar connector. 1.3: After the foundation pit is excavated to the bottom of the existing underground structure (1), the first foundation pit support (15) is erected. On one side of the existing underground structure (1), the top of the first foundation pit support (15) is flush with the top of the 1-shaped beam (11). The first foundation pit support (15) is made of reinforced concrete. 1.4: According to the design requirements, based on the principle of excavation and support, the excavation of the new foundation pit will continue on the side adjacent to the existing underground structure (1). Stress gauges and deformation monitoring points will be installed at the corresponding parts of the existing structure top slab (3), existing structure middle slab (4), existing structure bottom slab (5) and existing structure side wall (6) as needed to ensure the safety and normal use of the existing underground structure (1). 1.5: After the new foundation pit has been excavated to the bottom, the bottom of the pit shall be cleaned and the site leveled. Manual excavation shall be carried out within 500mm of the bottom of the pit. Finally, plain concrete cushion layer shall be constructed and the bottom of the pit shall be sealed in time. 1.6: The construction sequence of conventional underground structures is as follows: the waterproof layer of the bottom slab is laid in sequence as required, and then the construction is carried out in the order of the new structure bottom slab (9), the new structure side wall (10), the new structure middle slab (8) and the new structure top slab (7). The waterproof layer is laid before the side wall is constructed. The new structure side wall (10) at the interface is a reserved opening structure. The net clearance of the structure on both sides of the connection between the old and new structures should be consistent. Step 2: Construct new underground structure interfaces 2.1: Longitudinal and transverse reinforcing bars (16) are installed in the top slab (7) of the new structure. The reinforcing bars (16) are welded to the vertical embedded steel plate (13). The upper and lower reinforcing bars are mechanically connected by tie bars (17). 2.2: After the construction of all new underground structures (2) is completed, the existing structural sidewalls (6) at the connection of the existing underground structures (1) are demolished. At the same time, at least 0.5-1.0m of the sidewalls (6) around the interface opening area are reserved for manual removal. 2.3: Using the horizontally embedded steel plate (14) under the 1-shaped beam (11) and the existing structural side wall (6) within the opening range, the steel bars are left after being chiseled out. The formwork is erected, the steel bars are tied, and then the horizontal beam (12) is poured. One side of the horizontal beam (12) is connected to the lower armpit corner of the existing structural top plate (3), and the other side is flush with the 1-shaped beam (11). The horizontal beam (12) and the 1-shaped beam (11) are connected by the embedded steel plate (14). 2.4: The left and right sides and bottom of the opening are effectively connected to the newly built underground structure (2) by steel bar connectors or rebar installation to complete the connection between the old and new structures. The elevations of the top of the top plate and the top of the bottom plate at the connection between the old and new structures should be consistent. 2.5: Apply a waterproof layer on the newly built top slab (7). The waterproof layer should overlap with the waterproof layer on the existing top slab (3) by at least 0.5m. Then carry out backfilling above the top slab. During this period, the structure within 2m below the ground surface needs to be removed in advance. The first foundation pit support (15) outside 2m below the ground surface can be retained and share the load with the main structure. 2.6: Carry out road surface restoration work according to the requirements of urban roads or greening.

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