An excavation face sealing and reinforcing construction method for underpassing existing underground structures by excavation

Abstract

The application discloses a kind of excavation face water stop and reinforcement construction methods of undermining underpass existing underground structure, comprising the following steps: step (one), excavation face advanced detection;Step (two), existing underground structure side wall outside seepage passage investigation;Step (three), stop grout wall construction;Step (four), existing underground structure side wall outside seepage passage plugging;Step (five), layout drain hole;Step (six), full-face grouting reinforcement;Step (seven), acceptance.Compared with the traditional grouting water blocking as the main water stop measure, the excavation face is comprehensively treated by adopting the combination of blocking and dredging, the water is drained through the drain hole, the seepage passage is backfilled and plugged, the grout wall is waterproofed, and the full-face grouting reinforcement is combined to ensure the water stop effect of the excavation face, effectively ensuring the stability of the excavation face and the operation safety of the existing underground structure.

Description

Technical Field

[0001] This invention relates to the field of underground tunnel construction technology, specifically to a method for water-stopping and reinforcement of the excavation face when tunneling under an existing underground structure. Background Technology

[0002] With the continuous development of underground space, the planning and design of tunnels and subways inevitably involve the construction of new underground structures that tunnel under existing underground structures. Since existing underground structures are already in use and operation, they are highly sensitive to construction disturbances caused by tunneling. Maintaining the stability of the excavation face during tunneling can effectively control the settlement and deformation of existing underground structures, thereby ensuring their operational safety. After backfilling of existing underground structures, surface water infiltration and leakage from surrounding pipelines can easily lead to the formation of partially stagnant water on the outer side of the waterproof layer. Furthermore, tunneling inevitably disturbs the existing underground structure's retaining structure and the existing soil. During construction, due to pipeline leakage and rainfall, surface water continuously infiltrates, increasing the soil moisture content and causing softening and loss of the soil around the underground structure. This leads to seepage channels forming between the retaining structure and sidewalls, resulting in seepage water at the excavation face and potentially significant settlement and deformation, seriously threatening the operational safety of existing underground structures and the safety of tunneling construction. Therefore, effective water-stopping and reinforcement of the excavation face is crucial for maintaining its stability and ensuring the safe use of existing structures. Conventional water-stopping and reinforcement techniques typically only involve grouting reinforcement of the excavation face, failing to address water-stopping throughout the entire excavation process. Furthermore, they do not adequately consider the disturbance to existing underground structures with high safety sensitivity caused by grouting, thus failing to achieve the desired water-stopping and reinforcement effect and making it difficult to guarantee water-free operation at the excavation face.

[0003] Therefore, there is an urgent need to provide a construction method for water-stopping and reinforcement of the excavation face when tunneling under existing underground structures, in order to solve the problem of water leakage at the excavation face that is very likely to occur during the construction of tunneling under existing underground structures. Summary of the Invention

[0004] The technical problem to be solved by this invention is to overcome the defects of the above-mentioned technologies and provide a construction method for water-stopping and reinforcement of the excavation face of a tunnel under an existing underground structure. The method involves determining the water content of the soil at the excavation face through advance boreholes, using ground-penetrating radar to scan the sidewalls of the existing underground structure to determine the location of seepage channels between the retaining structure and the sidewalls, setting a grout-stopping wall below the bottom slab of the existing underground structure to prevent water seepage, grouting and backfilling to seal the seepage channels on the outer sidewalls of the existing underground structure near the tunnel excavation face, setting drainage holes on the outer sidewalls of the existing underground structure to drain water, and performing full-section double-liquid grouting reinforcement at the tunnel excavation face. This method effectively seals the seepage of surface water and reinforces the excavation face, ensuring the stability of the excavation face during construction and the operational safety of the existing underground structure.

[0005] To solve the above-mentioned technical problems, the technical solution provided by this invention is a method for water-stopping and reinforcement of the excavation face when tunneling under an existing underground structure, comprising the following steps:

[0006] Step (1) Advanced exploration of the excavation face: Use a geological drilling rig to conduct advanced geological exploration of the excavation face. Arrange a row of advanced exploration holes 1m away from the upper and lower edges of the excavation face. The horizontal spacing of the advanced exploration holes is 3-5m, the hole diameter is not less than 5cm, and the hole depth is not less than 8m. Core samples are taken through the windows of the advanced exploration holes to observe the water content of the excavation face.

[0007] Step (II) Investigation of seepage channels on the outside of the existing underground structure sidewall: Use ground-penetrating radar to scan the existing underground structure sidewall at least 3 times, generate a scanning report, and mark the voids and loose locations identified in the report on the existing underground structure sidewall retaining structure to determine the location of seepage channels on the outside of the existing underground structure sidewall.

[0008] Step (3) Construction of the grout-stopping wall: A grout-stopping wall is constructed below the existing underground structure slab to restrict the seepage of water from the excavation surface. The grout used in the grout-stopping wall is a two-component grout. The cement grout ratio is 1:1 (water-cement ratio) and 1:1 (volume ratio of cement grout to water glass). The solidification time of the two-component grout is 2-3 minutes. The grouting pipes of the grout-stopping wall are arranged in a quincunx pattern with a spacing of 1*1m. Each grouting pipe is 4 meters long and uses φ42mm steel pipes. The grouting pressure during grouting is not greater than 0.3Mpa.

[0009] Step (IV) Sealing of seepage channels on the outer side of the existing underground structure sidewall: After the construction of the grout-stopping wall at the excavation face is completed, grouting is used to backfill the seepage channels on the outer side of the existing underground structure sidewall. The backfilling grouting pipes are arranged in a quincunx pattern with a spacing of 2m*2m. The backfilling grouting pipes are made of φ42mm steel pipes. The grout is a two-component grout. The cement grout ratio is 1:1 (water-cement ratio) and 1:1 (volume ratio of cement grout to water glass). The grouting is injected slowly, and the grouting pressure is controlled at 0.1~0.2Mpa.

[0010] Step (5) Layout of drainage holes: Layout one row of drainage holes 1m away from each side of the backfill grouting area of ​​the existing underground structure. Each row of drainage holes consists of 5 rows with a spacing of 1m. The depth of the drainage holes extends to 10cm from the outer edge of the sidewall of the existing underground structure. The drainage holes are made of φ42mm steel pipes. The pipe wall of the drainage holes is drilled every 10cm with a hole diameter of 8mm. The outside is wrapped with geotextile and gravel.

[0011] Step (VI) Full-section grouting reinforcement: After the water stop is completed at the excavation face, full-section grouting pre-reinforcement is carried out on the excavation face outside the grouting wall range. The full-section grouting pipes are arranged in a quincunx pattern with a spacing of 1m*1m. The full-section grouting pipes are 8m long and are made of φ42mm steel pipe. The grout is a two-component grout and the grouting pressure is 0.3~0.5Mpa.

[0012] Step (7) Acceptance: After the water-stopping and reinforcement construction of the excavation face is completed, core drilling is carried out on the excavation face according to the design drawings and relevant specifications to test the soil grouting reinforcement. After ensuring that the construction quality meets the requirements, the acceptance of the underground excavation conditions is organized, and subsequent construction can begin.

[0013] As an improvement, after core sampling at the advance borehole window in step (I), the mechanical properties and moisture content of the soil in the advance borehole core sample are tested, and the mix design and optimization of the two-liquid grout are carried out based on the mechanical performance index of the soil sample at the excavation face to ensure that the grout stop wall and the full-section grouting reinforcement strength meet the design requirements and are not less than 0.8 MPa.

[0014] As an improvement, in step (ii), when using ground-penetrating radar to scan the area of ​​the existing underground structure sidewall, the scan is performed from the outside of the existing underground structure sidewall enclosure and the inside of the existing underground structure sidewall exposed surface, respectively. The location of the seepage channel on the outside of the existing underground structure sidewall is compared and verified to ensure that the seepage channel is accurately identified without omission.

[0015] As an improvement, in step (iii), the grout-stopping walls are arranged in a rectangular pattern, consisting of transverse and longitudinal grout-stopping walls. The transverse grout-stopping walls have two rows of grouting pipes, with a width equal to the width of the excavation face. The vertical grout-stopping walls have three rows of grouting pipes, with a height equal to the height of the excavation face. During construction, the transverse grout-stopping walls are constructed first, followed by the vertical grout-stopping walls, ensuring that the grout-stopping walls reach their design strength before the underground excavation begins.

[0016] As an improvement, in step (iv), the backfill grouting pipes are arranged outside the existing underground structure side wall retaining structure. The horizontal boundary of the backfill grouting pipes is 2m on both sides of the excavation face, and three rows are arranged vertically. The bottom row of backfill grouting pipes is arranged 1m on the upper edge of the existing underground structure bottom plate.

[0017] As an improvement, in step (iv), the backfill grouting pipe is drilled manually, and the drilling is carried out in a row of skip-hole operations. The hole depth is controlled at 10cm away from the outer edge of the existing underground structure sidewall to prevent damage to the waterproof layer on the outer sidewall of the existing underground structure. After the drilling is completed, grouting and backfilling are carried out in a timely manner. The grouting method of the backfill grouting pipe is upward grouting. Each grouting hole is grouted twice. The first grouting uses a two-liquid grout with an initial setting time of 2 minutes, and the second grouting uses a two-liquid grout with an initial setting time of 1 minute.

[0018] As an improvement, in step (v), each row of drainage holes is equipped with a vertical water inlet pipe. The vertical water inlet pipe is a φ100mm PVC pipe, which concentrates the water flow from the drainage holes into a water collection tank outside the excavation face for unified discharge.

[0019] As an improvement, in step (VI), the full-section grouting adopts an upward grouting method for zoned construction. The excavation surface outside the grout stop wall is divided into four zones: Zone 1, Zone 2, Zone 3, and Zone 4. Drilling and grouting operations are carried out from bottom to top in the order of Zone 1 to Zone 4. The grouting of Zone 1 uses a two-component grout with an initial setting time of 2 minutes and the grouting pressure is controlled at 0.5 MPa. The grouting of Zone 2 uses a two-component grout with an initial setting time of 2 minutes and the grouting pressure is controlled at 0.4 MPa. The grouting of Zone 3 uses a two-component grout with an initial setting time of 1 minute and the grouting pressure is controlled at 0.4 MPa. The grouting of Zone 4 uses a two-component grout with an initial setting time of 1 minute and the grouting pressure is controlled at 0.3 MPa. Grouting of adjacent zones cannot be carried out at the same time.

[0020] The advantages of this invention compared to the prior art are:

[0021] 1. Compared with traditional water-stopping measures that mainly rely on grouting, this method combines blocking and drainage to comprehensively treat the excavation face. It ensures the water-stopping effect of the excavation face by combining methods such as drainage holes, backfilling to seal seepage channels, waterproofing with grout-stopping walls, and full-section grouting reinforcement. This effectively guarantees the stability of the underground excavation face and the operational safety of existing underground structures.

[0022] 2. Advanced boreholes are used to understand the geological conditions of the excavation face, to detect seepage points and water conditions, and to test and analyze the mechanical properties of core samples taken from the boreholes. The test results are used to design the mix ratio of the grout for the grout stop wall and the full-section grouting, so as to improve the grout performance and ensure the reinforcement effect.

[0023] 3. By using ground-penetrating radar to scan the open surfaces on both sides of the existing underground structure sidewalls, the seepage channels and cavities were identified and accurately located. Targeted drilling and grouting were then carried out to ensure the sealing effect of the seepage channels, reduce the impact of surface water infiltration on the excavation face construction, and ensure construction safety.

[0024] 4. Drainage holes are installed on the outer sidewalls of the existing underground structure to divert and collect water, which greatly improves the ability of the water-stopping structure to cope with the enhanced surface water infiltration during heavy rainfall. Similarly, the drainage holes can also serve as drainage and reserve measures in extreme weather conditions, effectively improving the risk resistance of the excavation face and the existing underground structure as a whole.

[0025] 5. The full-section grouting reinforcement of the excavation face adopts a bottom-up phased construction approach, and the grouting control pressure and initial setting time of the grouting holes in the later construction areas are gradually reduced according to the construction sequence. This can effectively reduce the soil squeezing effect of grouting construction, ensure the stability of the soil at the excavation face during grouting construction, avoid soil softening and deformation caused by grouting construction, and ensure that the settlement of the existing underground structure is controlled. Attached Figure Description

[0026] Figure 1 This is a cross-sectional schematic diagram of the positional relationship between the existing underground structure and the excavation face in the construction method for water-stopping and reinforcement of the excavation face of the underground tunnel under the present invention.

[0027] Figure 2 This is a longitudinal cross-sectional schematic diagram of the positional relationship between the existing underground structure and the excavation face in the construction method for water-stopping and reinforcement of the excavation face of the underground tunnel under the present invention.

[0028] Figure 3 This is a schematic cross-sectional view of the layout of advance detection boreholes at the excavation face in the construction method for water-stopping and reinforcement of an existing underground structure through a tunnel, according to the present invention.

[0029] Figure 4 This is a schematic cross-sectional view of the arrangement of the grout-stopping wall at the excavation face in the construction method for water-stopping and reinforcement of the excavation face of the underground tunnel under the existing underground structure according to the present invention.

[0030] Figure 5 This is a schematic cross-sectional view of the grouting pipe arrangement for sealing and backfilling seepage channels in the sidewall of the existing underground structure in the construction method for water-stopping and reinforcement of the excavation face of the tunnel under the existing underground structure according to the present invention.

[0031] Figure 6 This is a longitudinal cross-sectional diagram of the grouting pipe arrangement for sealing and backfilling seepage channels in the sidewall of the existing underground structure in the construction method for water-stopping and reinforcement of the excavation face of the tunnel under the existing underground structure according to the present invention.

[0032] Figure 7 This is a schematic cross-sectional view of the arrangement of drainage holes on the side wall of the existing underground structure in the construction method for water-stopping and reinforcement of the excavation face of the tunnel under the existing underground structure according to the present invention.

[0033] Figure 8 This is a cross-sectional view of the grouting pipe layout across the entire excavation face outside the grouting wall area in the construction method for water-stopping and reinforcement of the excavation face of a tunnel under an existing underground structure, according to the present invention.

[0034] As shown in the figure: 1. Excavation face; 2. Existing underground structure; 201. Existing underground structure base slab; 202. Existing underground structure sidewall; 203. Inner side of existing underground structure sidewall; 204. Outer side of existing underground structure sidewall; 205. Existing underground structure sidewall retaining structure; 3. Pre-exploration borehole; 4. Grout stop wall; 401. Vertical grout stop wall; 402. Horizontal grout stop wall; 403. Grouting pipe for grout stop wall; 5. Backfill grouting pipe; 6. Drainage hole; 601. Vertical water pipe; 602. Water collection tank; 7. Excavation face outside the grout stop wall area; 701. Area 1; 702. Area 2; 703. Area 3; 704. Area 4; 705. Full-section grouting pipe. Detailed Implementation

[0035] To make the objectives, technical solutions, and advantages of the embodiments of the invention clearer, the technical solutions of the embodiments of the invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the invention, not all embodiments. The components of the embodiments of the invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0036] In the description of the embodiments of the invention, it should be noted that if terms such as "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," or "outer" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship commonly used when the product of the invention is in use, they are only for the convenience of describing the invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation on the invention. Furthermore, terms such as "first," "second," and "third" are only used to distinguish descriptions and should not be construed as indicating or implying relative importance.

[0037] Furthermore, the use of terms such as "horizontal," "vertical," and "sag" does not imply that the component must be absolutely horizontal or suspended, but rather that it can be slightly tilted. For example, "horizontal" simply means that its direction is more horizontal relative to "vertical," and does not mean that the structure must be completely horizontal, but can be slightly tilted.

[0038] In the description of the embodiments of the invention, "a plurality of" means at least two.

[0039] In the description of the embodiments of the invention, it should also be noted that, unless otherwise explicitly specified and limited, the terms "set," "install," "connect," and "link" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; 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; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in the invention according to the specific circumstances.

[0040] Referring to the attached diagram, a method for water-stopping and reinforcement of the excavation face when tunneling under an existing underground structure includes the following steps:

[0041] Step (1) Advanced exploration of excavation face 1: Use a geological drilling rig to conduct advanced geological exploration of excavation face 1. Arrange a row of advanced exploration holes 3 1m away from the upper and lower edges of excavation face 1. The horizontal spacing of the advanced exploration holes 3 is 3-5m, the hole diameter is not less than 5cm, and the hole depth is not less than 8m. Core samples are taken through the window of the advanced exploration holes 3 and the water content of the excavation face is observed.

[0042] In practice, after core sampling at window 3 of the advance test borehole, the mechanical properties and moisture content of the soil sample from core sample 3 of the advance test borehole are tested. Based on the mechanical performance indicators of soil sample from excavation face 1, the mix design and optimization of the two-liquid grout ratio are carried out to ensure that the grout stop wall 4 and the grouting reinforcement strength of the entire section meet the design requirements and are not less than 0.8 MPa.

[0043] Step (II) Investigation of seepage channels on the outer side of the existing underground structure sidewall 204: Use ground-penetrating radar to scan the existing underground structure sidewall 202, scan it no less than 3 times, generate a scanning report, and mark the voids and loose locations identified in the report on the existing underground structure sidewall retaining structure 205 to determine the location of the seepage channels on the outer side of the existing underground structure sidewall 204.

[0044] In the specific implementation, when using ground-penetrating radar to scan the area of ​​the existing underground structure sidewall 202, scanning is carried out from the outside of the existing underground structure sidewall retaining structure 205 and the open surface of the existing underground structure sidewall 203, respectively. The location of the seepage channel on the outside of the existing underground structure sidewall 204 is compared and verified to ensure that the seepage channel is accurately identified without omission.

[0045] Step (3), Construction of Grout Stop Wall 4: Grout stop wall 4 is constructed below the existing underground structure slab 201 to restrict the seepage of water from the excavation face 1. The grout for grout stop wall 4 is a two-component grout. The cement grout ratio is 1:1 (water-cement ratio) and 1:1 (volume ratio of cement grout to water glass). Specifically, the mix ratio of the two-component grout is adjusted according to the soil mechanical properties revealed by the core sample of the advanced exploratory borehole 3. The solidification time of the two-component grout is 2-3 minutes. The grouting pipes 403 of the grout stop wall are arranged in a quincunx pattern with a spacing of 1*1m. Each grouting pipe 403 is 4 meters long. The grouting pipes 403 of the grout stop wall are made of φ42mm steel pipes. The grouting pressure during grouting is not greater than 0.3Mpa.

[0046] In practice, the grout-stopping walls 4 are arranged in a rectangular shape, consisting of a transverse grout-stopping wall 402 and a longitudinal grout-stopping wall 401. The transverse grout-stopping wall 402 is equipped with two rows of grouting pipes 403, with a width equal to that of the excavation face 1. The longitudinal grout-stopping wall 401 is equipped with three rows of grouting pipes 403, with a height equal to that of the excavation face 1. During construction, the transverse grout-stopping wall 402 is constructed first, followed by the longitudinal grout-stopping wall 401, ensuring that the grout-stopping walls 4 reach their design strength before the underground excavation of the excavation face 1.

[0047] Step (IV) Sealing of the 204 seepage channel on the outer side of the existing underground structure sidewall: After the construction of the grout stop wall 4 at the excavation face 1 is completed, the 204 seepage channel on the outer side of the existing underground structure sidewall is backfilled with grout. The backfill grouting pipes 5 are arranged in a quincunx pattern with a spacing of 2m*2m. The backfill grouting pipes 5 are made of φ42mm steel pipes. The grout is a two-component grout. The cement grout ratio is 1:1 (water-cement ratio) and 1:1 (volume ratio of cement grout to water glass). The grouting is injected slowly. Specifically, the two-component grout mix ratio is adjusted according to the soil mechanical properties revealed by the core sample of the advanced exploratory borehole 3. The grouting pressure is controlled at 0.1~0.2Mpa.

[0048] In specific implementation, the backfill grouting pipe 5 is arranged outside the existing underground structure side wall retaining structure 205. The horizontal boundary of the backfill grouting pipe 5 is 2m on both sides of the excavation face 1, and 3 rows are arranged vertically. The bottom row of backfill grouting pipe 5 is arranged 1m above the upper edge of the existing underground structure bottom plate 201.

[0049] Meanwhile, the backfill grouting pipe 5 is drilled manually, and the drilling is carried out in a row of skip-hole operations. The hole depth is controlled at 10cm away from the edge of the 204 outer side wall of the existing underground structure to prevent damage to the 204 waterproof layer on the outer side wall of the existing underground structure. After the drilling is completed, grouting and backfilling are carried out in a timely manner. The grouting method of the backfill grouting pipe 5 is upward grouting. Each grouting hole is grouted twice. The first grouting uses a two-liquid grout with an initial setting time of 2 minutes, and the second grouting uses a two-liquid grout with an initial setting time of 1 minute.

[0050] Step (5) Layout of drainage holes 6: Layout one row of drainage holes 6 at 1m on each side of the backfill grouting area of ​​the existing underground structure 2. Each row of drainage holes 6 has 5 rows with a spacing of 1m. The depth of the drainage holes 6 extends to 10cm from the outer edge of the sidewall 204 of the existing underground structure. The drainage holes 6 are made of φ42mm steel pipes. The pipe wall of the drainage holes 6 is provided with drill holes every 10cm with a diameter of 8mm. The outside is wrapped with geotextile and gravel.

[0051] In practice, each row of drainage holes 6 is equipped with a vertical water inlet pipe 601. The vertical water inlet pipe 601 is a φ100mm PVC pipe, which concentrates the water flow from the drainage holes 6 into the water collection tank 602 outside the excavation face 1 for unified discharge.

[0052] Step (VI) Full-section grouting reinforcement: After the water stop is completed at the excavation face 1, full-section grouting pre-reinforcement is carried out on the excavation face 7 outside the grouting wall range. The full-section grouting pipes 705 are arranged in a quincunx pattern with a spacing of 1m*1m. The full-section grouting pipes 705 are 8m long and are made of φ42mm steel pipes. The grout is a two-component grout and the grouting pressure is 0.3~0.5Mpa.

[0053] In practice, the full-section grouting adopts an upward grouting method for zoned construction. The excavation surface outside the grout stop wall is divided into four zones: Zone 1 (701), Zone 2 (702), Zone 3 (703), and Zone 4 (704). Drilling and grouting operations are carried out from bottom to top, skipping zones from Zone 1 (701) to Zone 4 (704). The grouting in Zone 1 (701) uses a two-component grout with an initial setting time of 2 minutes and the grouting pressure is controlled at 0.5 MPa. The grouting in Zone 2 (702) uses a two-component grout with an initial setting time of 2 minutes and the grouting pressure is controlled at 0.4 MPa. The grouting in Zone 3 (703) uses a two-component grout with an initial setting time of 1 minute and the grouting pressure is controlled at 0.4 MPa. The grouting in Zone 4 (704) uses a two-component grout with an initial setting time of 1 minute and the grouting pressure is controlled at 0.3 MPa. Grouting in adjacent zones cannot be carried out simultaneously.

[0054] Step (7) Acceptance: After the water-stopping and reinforcement construction of excavation face 1 is completed, core drilling is carried out on excavation face 1 according to the design drawings and relevant specifications to test the soil grouting reinforcement. After ensuring that the construction quality meets the requirements, the acceptance of the underground excavation conditions is organized, and subsequent construction begins.

[0055] The present invention and its embodiments have been described above. This description is not restrictive, and the accompanying drawings are only one embodiment of the present invention; the actual structure is not limited thereto. In conclusion, if those skilled in the art are inspired by this description and design similar structures and embodiments without departing from the spirit of the invention, such designs should fall within the protection scope of the present invention.

Claims

1. A method for waterproofing and reinforcement of the excavation face when tunneling under an existing underground structure, characterized in that, Includes the following steps: Step (1) Advanced exploration of the excavation face (1): Use a geological drilling rig to conduct advanced geological exploration of the excavation face (1). Arrange a row of advanced exploration holes (3) 1m away from the upper and lower edges of the excavation face (1). The horizontal spacing of the advanced exploration holes (3) is 3~5m, the hole diameter is not less than 5cm, and the hole depth is not less than 8m. Core samples are taken through the windows of the advanced exploration holes (3) and the water content of the excavation face is observed. Step (II) Investigation of seepage channels on the outer side wall (204) of the existing underground structure: Use ground-penetrating radar to scan the existing underground structure side wall (202) at least 3 times, form a scanning report, and mark the voids and loose locations found in the report on the retaining structure (205) of the existing underground structure side wall to determine the location of seepage channels on the outer side wall (204) of the existing underground structure. Step (3), Construction of the grout-stopping wall (4): A grout-stopping wall (4) is constructed below the existing underground structure base slab (201) to restrict the seepage of water from the excavation face (1). The grout of the grout-stopping wall (4) is a two-component grout. The cement grout ratio is 1:1 (water-cement ratio of cement grout, volume ratio of cement grout to water glass of 1:1). The solidification time of the two-component grout is 2-3 minutes. The grout-stopping wall grouting pipes (403) are arranged in a quincunx pattern with a spacing of 1*1m. Each grout-stopping wall grouting pipe (403) is 4 meters long. The grout-stopping wall grouting pipes (403) are made of φ42mm steel pipe. The grouting pressure during grouting is not greater than 0.3Mpa. Step (IV) Sealing of seepage channels on the outer side of the existing underground structure sidewall (204): After the construction of the grout-stopping wall (4) on the excavation face (1) is completed, the seepage channels on the outer side of the existing underground structure sidewall (204) are backfilled with grout. The backfilling grouting pipes (5) are arranged in a quincunx pattern with a spacing of 2m*2m. The backfilling grouting pipes (5) are made of φ42mm steel pipes. The grout is a two-component grout. The cement grout ratio is 1:1 (water-cement ratio) and 1:1 (volume ratio of cement grout to water glass). The grouting is injected slowly, and the grouting pressure is controlled at 0.1~0.2Mpa. Step (5), Drainage holes (6): Drainage holes (6) are arranged 1m away from both sides of the backfill grouting area of ​​the existing underground structure (2). Each row of drainage holes (6) consists of 5 rows with a spacing of 1m. The depth of the drainage holes (6) extends to 10cm from the edge of the outer sidewall (204) of the existing underground structure. The drainage holes (6) are made of φ42mm steel pipes. The pipe wall of the drainage holes (6) is provided with drill holes every 10cm with a diameter of 8mm. The outer side is wrapped with geotextile and gravel. Step (VI) Full-section grouting reinforcement: After the water is stopped at the excavation surface (1), the excavation surface (7) outside the grouting wall is pre-reinforced with full-section grouting. The full-section grouting pipes (705) are arranged in a quincunx pattern with a spacing of 1m*1m. The full-section grouting pipes (705) are 8m long and are made of φ42mm steel pipe. The grout is a two-component grout and the grouting pressure is 0.3~0.5Mpa. Step (7) Acceptance: After the water-stopping and reinforcement construction of the excavation surface (1) is completed, core drilling is carried out on the excavation surface (1) according to the design drawings and relevant specifications to test the soil grouting reinforcement. After ensuring that the construction quality meets the requirements, the acceptance of the underground excavation conditions is organized and the subsequent construction begins.

2. The method for water-stopping and reinforcement of the excavation face when tunneling under an existing underground structure according to claim 1, characterized in that: After core sampling at the window of the advanced probe hole (3) in step (1), the mechanical properties and water content of the soil sample from the advanced probe hole (3) are tested. Based on the mechanical performance index of the soil sample from the excavation face (1), the mix design and optimization of the two-liquid grout ratio are carried out to ensure that the grout stop wall (4) and the grouting reinforcement strength of the whole section meet the design requirements and are not less than 0.8 MPa.

3. The method for water-stopping and reinforcement of the excavation face when tunneling under an existing underground structure according to claim 1, characterized in that: In step (ii), when using ground-penetrating radar to scan the area of ​​the existing underground structure sidewall (202), the scan is performed from the outside of the existing underground structure sidewall retaining structure (205) and the open surface of the existing underground structure sidewall (203). The location of the seepage channel on the outside of the existing underground structure sidewall (204) is compared and verified to ensure that the seepage channel is accurately identified without omission.

4. The method for water-stopping and reinforcement of the excavation face when tunneling under an existing underground structure according to claim 1, characterized in that: Step (3) The grout-stopping wall (4) is arranged in a rectangular shape, divided into a transverse grout-stopping wall (402) and a longitudinal grout-stopping wall (401). The transverse grout-stopping wall (402) is equipped with two rows of grouting pipes (403), and its arrangement width is the same as that of the excavation face (1). The longitudinal grout-stopping wall (401) is equipped with three rows of grouting pipes (403), and its arrangement height is the same as that of the excavation face (1). During the construction process, the transverse grout-stopping wall (402) is constructed first, and then the longitudinal grout-stopping wall (401) is constructed to ensure that the grout-stopping wall (4) reaches the design strength before the excavation face (1) is excavated.

5. The method for water-stopping and reinforcement of the excavation face when tunneling under an existing underground structure according to claim 1, characterized in that: In step (four), the backfill grouting pipe (5) is arranged outside the existing underground structure side wall retaining structure (205). The horizontal boundary of the backfill grouting pipe (5) is 2m on both sides of the excavation surface (1), and 3 rows are arranged vertically. The bottom row of backfill grouting pipe (5) is arranged 1m above the upper edge of the existing underground structure bottom plate (201).

6. The method for water-stopping and reinforcement of the excavation face of a tunnel under an existing underground structure according to claim 1, characterized in that: In step (four), the backfill grouting pipe (5) is drilled manually. The drilling is done by skipping holes in the same row. The hole depth is controlled at 10cm from the edge of the outer side wall (204) of the existing underground structure to prevent damage to the waterproof layer of the outer side wall (204) of the existing underground structure. After the drilling is completed, grouting is carried out in time. The grouting method of the backfill grouting pipe (5) is upward grouting. Each grouting hole is grouted twice. The first grouting uses a two-liquid grout with an initial setting time of 2 minutes, and the second grouting uses a two-liquid grout with an initial setting time of 1 minute.

7. The method for water-stopping and reinforcement of the excavation face when tunneling under an existing underground structure according to claim 1, characterized in that: In step (5), each row of drainage holes (6) is equipped with a vertical water inlet pipe (601). The vertical water inlet pipe (601) is made of φ100mm PVC pipe, which concentrates the water flow from the drainage holes (6) into the water collection tank (602) outside the excavation face (1) for unified discharge.

8. The method for water-stopping and reinforcement of the excavation face of a tunnel under an existing underground structure according to claim 1, characterized in that: In step (six), the full-section grouting adopts the upward grouting method for zoned construction. The excavation surface (7) outside the grout stop wall is divided into four areas: area one (701), area two (702), area three (703), and area four (704). The drilling and grouting operations are carried out from bottom to top in the order from area one (701) to area four (704). The grouting of area one (701) uses a two-liquid grout with an initial setting time of 2 minutes and the grouting pressure is controlled at 0.5 MPa. The grouting of area two (702) uses a two-liquid grout with an initial setting time of 2 minutes and the grouting pressure is controlled at 0.4 MPa. The grouting of area three (703) uses a two-liquid grout with an initial setting time of 1 minute and the grouting pressure is controlled at 0.4 MPa. The grouting of area four (704) uses a two-liquid grout with an initial setting time of 1 minute and the grouting pressure is controlled at 0.3 MPa. The grouting of adjacent areas cannot be carried out at the same time.

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