Structure of newly-added station of underground line and construction method of structure

Abstract

The invention relates to a structure of an underground line newly-added station and a construction method of the structure. The construction method of the structure comprises the following steps that S1, reinforcement is conducted in an existing tunnel; s2, building envelope construction; s3, a station foundation pit is excavated; s4, reinforcement and freezing in the existing tunnel hole; s5, connecting the tunnel and the station; and S6, plugging recovery. The reverse circulation hole forming technology and the CSM waterproof curtain are combined, so that the adverse effect on the existing tunnel in the enclosure construction period can be effectively controlled, deformation of the enclosure can be reduced to the maximum extent through the cover and excavation top-down technology in the foundation pit excavation process, upward floating of the existing tunnel is controlled, and finally reinforcement and radial freezing are conducted in the existing tunnel. The personnel operation safety and the surrounding environment safety in the lower excavation process of the newly-added station are guaranteed, and it is guaranteed that external water and soil do not permeate into the foundation pit.

Description

Technical Field

[0001] The present invention belongs to the technical field of rail transit, and particularly relates to the structure of a newly added station on an underground line and its construction method. Background Art

[0002] With the rapid development of rail transit projects and the development and construction of surrounding plots in major cities, the passenger volume of some line stations has greatly exceeded the original designed passenger volume, resulting in a poor passenger experience at some stations; or in areas originally designed as underground tunnels, with the development and construction of surrounding plots, in order to meet the passenger flow needs of the surrounding area, adding a station on the existing underground line has become the preferred solution; currently, there is no relevant technology for adding a station to an existing operating underground line.

[0003] Currently, during the construction of underground rail transit, the conventional construction sequence is to build the station first and then the tunnel. This process can minimize the risks of tunnel construction to the greatest extent, such as the risks of shield tunneling in and out of the tunnel, and provide a working surface for shield tunneling in and out of the tunnel. The biggest difficulty in realizing this construction sequence lies in the implementation of the station. The construction areas of some stations are densely populated with various demolished buildings and structures, and it often happens that the construction of the station is delayed due to demolition, thus affecting the opening and operation of the entire line.

[0004] To accelerate the progress of the project construction, the method often adopted is to "build the tunnel first and then the station", that is, to first penetrate the tunnel and then build the station. Some technologies are to implement the retaining structure at the end of the station before tunnel excavation to ensure the stability of the side wall of the foundation pit during the excavation of the station foundation pit. For example, in the Chinese invention patent with the publication number CN115929323A, this technology proposes to arrange glass fiber bars in advance in the tunneling range of the end diaphragm wall for pouring, and the underground floors 1-2 of the station are constructed in the conventional order, while the underground floors 3-4 are constructed in the reverse order. This technology is applicable to strata that are non-aqueous and have good stability around the tunnel.

[0005] In addition, there are also cases where the retaining structure of the station is constructed after tunnel excavation. For example, in the Chinese invention patent with the publication number CN109208637A, diaphragm walls are set on both sides of the tunnel to retain soil and water, a filling wall and an overlapping pile are set in the tunnel, and the freezing of vertical and inclined holes on the ground is used to strengthen the water stop in the area around the tunnel. This technology is applicable to water-rich sand layers and requires an open ground site to arrange the inclined hole freezing on the ground; in the Chinese invention patent with the publication number CN110424969A, two plugging walls are set in the shield tunnel to facilitate the formation of the retaining pile holes, and the secondary grouting is used for water stop outside the tunnel. This technology is prone to water leakage in the gap between the tunnel and the station and is applicable to non-aqueous strata.

[0006] All of the above "tunnel first, station later" technologies require the pre - setting of retaining structures on the ground, which has high requirements for the ground site; or the construction of filling walls in the tunnel to facilitate the construction of retaining structures. The construction of filling walls has a great impact on the existing segment structure and takes a long construction time. Therefore, it is necessary to study a technology with little impact on the existing segments and short construction time during the construction period, as well as related technologies for new stations with little interference to the operating line during the operation period. Summary of the Invention

[0007] The present invention is proposed to solve the above - mentioned deficiencies, and its purpose is to provide a structure and construction method for adding a new station to an underground line. This construction method is applicable to adding an operating station on an underground tunnel with a large passenger volume, can protect the operating underground tunnel, shorten the operation - influencing time as much as possible, and can also protect the existing underground shield tunnel, overcome the problem of narrow site, and solve problems such as side - wall leakage during deep foundation pit excavation.

[0008] To achieve the above - mentioned purpose, the present invention adopts the following solutions:

[0009] A construction method for adding a new station to an underground line of the present invention includes the following steps:

[0010] S1: Reinforcement inside the existing tunnel

[0011] Inside the existing shield tunnel, longitudinally tie the shield segments within the scope of the proposed new station.

[0012] S2: Construction of retaining structure

[0013] According to the measured ring numbers and mileage of the shield segments of the existing shield tunnel on - site, determine the plane position of the end retaining structure of the new station; then construct the retaining structure. First, construct reverse - circulation bored piles, and after the reverse - circulation bored piles reach the design strength, construct a CSM water - stop curtain outside the reverse - circulation bored piles.

[0014] S3: Excavation of the station foundation pit

[0015] After the CSM water - stop curtain reaches the design strength, use the top - down and reverse - construction method to excavate the station foundation pit layer by layer from top to bottom; after excavating each layer of soil, pour the corresponding layer of the station floor structure and station side - wall structure until excavating to the upper part of the existing shield tunnel.

[0016] S4: Reinforcement and freezing inside the existing tunnel

[0017] Arrange radial freezing pipes and / or inclined freezing pipes in the existing shield tunnel to form a frozen body, and freeze and reinforce the gap between the existing shield tunnel, reverse - circulation bored piles, and CSM water - stop curtain.

[0018] S5: Connection between the tunnel and the station

[0019] Continue to excavate downward in sequence, pour the side wall structure of the station and the tunnel ring beam. When the foundation pit of the main body of the station is excavated to the bottom, pour the structural floor slab to complete the connection between the station and the tunnel portal.

[0020] S6: Plugging and restoration

[0021] Plug the orifices of the radial freezing pipes or inclined freezing pipes in the tunnel, remove the longitudinal ties, construct the internal structure of the station, and restore the normal operation of the existing shield tunnel.

[0022] As a preferred implementation method, in step S2, during the hole formation process of the reverse circulation bored pile, the hole should always be filled with slurry; after the excavation of the hole section is completed, the hole position, hole depth, hole width and hole wall verticality should be checked, and the hole cleaning and slurry replacement work can be carried out only after passing the inspection; the control of the final hole depth of the hole section should meet the following requirements: a. The final hole depth of the hole section must ensure the design depth. Within the same hole section, the bottom excavation depth of the hole bottom is consistent and flat; b. The bottom of the extended bored pile within the same hole section needs to be consistent with the bottom excavation depth of the previous hole section; after the excavation of the dug hole is completed, the sediment and sundries at the bottom of the hole should be cleaned up. 1 hour after the hole bottom cleaning and slurry replacement are completed, the slurry specific gravity within the height of 500 mm from the hole bottom is not greater than 1.15, and the sediment thickness shall not be greater than 100 mm.

[0023] As a preferred implementation method, in step S2, the CSM cut-off wall forms a continuous cement-soil wall of equal thickness by injecting cement slurry while rotating and sinking the milling wheel to forcefully cut the original soil; the width of the CSM cut-off wall is 0.7 - 1.0 m.

[0024] As a preferred implementation method, in step S2, the clear distance in the height direction between the bottom of the retaining structure and the top of the existing shield tunnel is not less than 0.5 m.

[0025] As a preferred implementation method, in step S4, the reinforcement requirements for in-tunnel radial freezing: the uniaxial compressive strength of the reinforced soil is not less than 3.6 MPa, the flexural tensile strength is not less than 2.0 MPa, and the shear strength is not less than 1.5 MPa (-10 °C).

[0026] As a preferred implementation method, in step S4, a heat preservation layer is laid on the inner side of the tunnel structure around the freezing wall of the frozen body, and the laying range extends 3 m outside the designed freezing wall boundary; the heat preservation layer uses a flame-retardant soft plastic foam board with a thickness of not less than 40 mm and a thermal conductivity of not greater than 0.04 w / mk; the designed active freezing time is 40 - 50 days; the single-hole flow rate of the freezing hole is not less than 5 m 3 / h.

[0027] As a preferred embodiment, in step S4, the following rules are adopted to adjust the brine temperature during active freezing: during active freezing for 7 days, the brine temperature drops below -18°C; during active freezing for 15 days, the brine temperature drops below -24°C; during excavation, the temperature difference between the supply and return brine is not greater than 2°C, and the brine temperature drops below -28°C.

[0028] As a preferred embodiment, in step S4, during the excavation of the main foundation pit of the station, for every 2 - 4 m of excavation, a heat preservation layer is laid on the exposed surface of the frozen body.

[0029] The present invention also provides a structure of a newly added station on an underground line, which is constructed by the above construction method.

[0030] As a preferred embodiment, the structure of the newly added station on the underground line includes

[0031] An existing shield tunnel, and longitudinal tie rods for improving the longitudinal stiffness can be arranged on the tunnel segments of the existing shield tunnel;

[0032] A newly added station, located above the existing shield tunnel, the newly added station at least includes a station floor structure and a station side wall structure, and is communicated with the existing shield tunnel through a connecting portal structure between the station and the tunnel;

[0033] A retaining structure, arranged around the newly added station and maintaining a set safety clearance from the existing shield tunnel, the retaining structure is composed of reverse circulation bored piles and a CSM water stop curtain on its outer side, and is used for supporting the side wall of the foundation pit and preventing water seepage;

[0034] A frozen body, which is formed between the existing shield tunnel and the retaining structure through radial freezing pipes and / or inclined freezing pipes, and is used for strengthening the surrounding soil layers.

[0035] Compared with the prior art, the present invention has the following beneficial effects:

[0036] First, the present invention adopts the reverse circulation hole forming process + CSM water stop curtain to effectively control the adverse effects on the existing tunnel during the construction of the retaining structure. The top - down reverse construction process during the foundation pit excavation can minimize the deformation of the retaining structure and control the floating of the existing tunnel. Finally, reinforcement is carried out inside the existing tunnel and radial freezing is carried out to ensure the safety of personnel during the lower part excavation of the newly added station and the safety of the surrounding environment, and ensure that external water and soil do not seep into the foundation pit.

[0037] Second, the present invention combines the advantages of various processes, can effectively solve the problems encountered in the construction of new stations for underground lines in various complex strata, is particularly suitable for complex strata such as water-rich sand strata, and can also be applied to the construction of stations where the construction of the station lags behind the tunnel construction due to ground demolition problems, thereby ensuring that the goal of line opening to traffic is controllable. In addition, this construction method can be applied to the construction of stations with relatively good formation conditions and adjacent to the mined tunnel.

[0038] Third, within the scope of the proposed new station, the present invention longitudinally ties the segments of the existing shield tunnel to protect the integrity of the existing tunnel structure: by longitudinal tying, the circumferential and longitudinal overall stiffness of the shield segments is enhanced, so that the tunnel remains stable during the processes such as the excavation of the foundation pit above the station or subsequent freezing construction, and the deformation is reduced; the construction is simple and the interference is small: longitudinal tying only requires local reinforcement operations in the tunnel, and the construction can be organized during the period when the night operation stops, and the influence time on the normal operation line is relatively short; compared with setting large-scale infill walls or reconstructing the structure in a large area in the tunnel, local tying construction can minimize the interference to the tunnel structure and greatly reduce the risk of damage or deformation of the shield segments.

[0039] Fourth, the construction of the enclosure structure of the present invention is carried out in two processes: reverse circulation bored piles, and a CSM water stop curtain is constructed outside the reverse circulation bored piles; both support and water stop are considered: the reverse circulation bored piles can play a supporting role in the side wall of the foundation pit, and the CSM (cement soil mixing and milling method) water stop curtain outside forms a continuous cement soil wall with equal thickness to provide a reliable water stop barrier. The side wall of the foundation pit realizes double protection of "stiffness + water stop" as a whole, overcoming the high requirements for both support and water stop of the enclosure structure in the "tunnel first and then station" situation; the requirements for the ground site are relatively low: the combination of reverse circulation drilling and CSM is adopted, and the equipment scale and construction area are relatively smaller than those of the traditional diaphragm wall, which is suitable for narrow urban environments; the risk of leakage is reduced: the CSM water stop curtain and the reverse circulation bored piles form a composite enclosure structure, and a safety clearance of not less than 0.5 m can be maintained at the top of the shield tunnel, and there is no need to set large-scale enclosure structures or demolish a large area on the ground in advance, avoiding direct interference with the existing segments. At the same time, the cooperation of the two can better cope with water-rich strata and reduce the hidden danger of leakage.

[0040] Fifthly, the station foundation pit excavation of the present invention adopts the top-down inverse construction method, with excavation carried out layer by layer from top to bottom. After excavating one layer, the floor slab and side wall structures of the corresponding layer are poured until reaching above the existing tunnel. The inverse construction method reduces the impact on the tunnel and its surroundings: during inverse construction, after the excavation of each layer of soil, the floor slab and side wall structures can be quickly formed, playing a role in enclosing and supporting the foundation pit, and reducing the disturbance of the deep foundation pit to the surrounding soil layers. Construction safety: the layered synchronous support can reduce the risk of large-scale collapse or large-scale ground displacement. When the foundation pit excavation reaches the top of the tunnel, the tunnel has been reinforced by longitudinal tie rods and subsequent freezing measures, further ensuring the safety of the operation. It can be in three-dimensional intersection with the ground construction: during inverse construction, the upper structure of the station can be constructed first, and the lower space can be excavated synchronously, not only making use of the tight construction period in the city but also reducing the impact on the surrounding traffic and municipal pipelines.

[0041] Sixthly, in the shield tunnel of the present invention, radial or inclined freezing pipes are arranged to form an annular frozen body for freezing and reinforcing the gap between the shield tunnel, the reverse circulation bored cast-in-place pile, and the CSM water stop curtain. Further water stop and ground stability: through the circulation of low-temperature brine or refrigerant, a frozen body is formed in the soil layer around the gap between the tunnel and the retaining structure, realizing temporary airtight water stop and reinforcement and eliminating the risk of leakage. It does not rely on a large area of ground site: traditional inclined holes or deep grouting often require a large number of equipment to be arranged on the ground and occupy a large area. In the present invention, the freezing work can be completed by arranging freezing pipes in the tunnel, which is more suitable for construction in narrow plots above existing lines. It has less interference with the shield segments: in cooperation with the previous longitudinal tie rods and in-tunnel reinforcement measures, during freezing, through the circumferential or radial pipeline arrangement, the overall stability of the shield segments is improved, effectively reducing the potential hazards of shield segment cracking or dislocation.

[0042] Seventhly, the present invention conducts safe operations within the enclosed frozen body and retaining system: the double protection of the frozen body and the retaining structure ensures that the foundation pit soil layer at the top of the tunnel will not become unstable or have large-scale water leakage, and the portal excavation and structure construction can be safely completed. The portal structure is reliable: by setting a ring beam during the excavation process, it can not only ensure the stability of the retaining structure but also closely connect the new station main body and the existing tunnel at the portal, reducing the risk of uneven settlement or leakage during the later operation. Efficient connection and shortening the impact time on operation: the in-tunnel structure construction and the station foundation pit construction cooperate with each other and are carried out crosswise, achieving a shorter construction period. The station and the tunnel can be quickly connected after freezing.

[0043] In summary, the present invention effectively overcomes many limitations in the existing construction modes of "tunnel first and then station" and "station first and then tunnel" in terms of site, demolition, retaining structure, and tunnel safety, etc., and successfully realizes an efficient, safe, and feasible construction method for adding a station above an existing operating shield tunnel and the corresponding structural system, which has significant application value and promotion prospects for the demand of adding stations under the background of the increasing passenger volume of urban rail transit. Brief Description of the Drawings

[0044] Figure 1 It is a structural plan view of a newly added station on an underground line;

[0045] Figure 2 It is a structural sectional view of a newly added station on an underground line;

[0046] Figure 3 It is the front view of the first construction step in the embodiment of the present invention;

[0047] Figure 4 It is the side view of the first construction step in the embodiment of the present invention;

[0048] Figure 5 It is the front view of the second construction step in the embodiment of the present invention;

[0049] Figure 6 It is the side view of the second construction step in the embodiment of the present invention;

[0050] Figure 7 It is the front view of the third construction step in the embodiment of the present invention;

[0051] Figure 8 It is the side view of the third construction step in the embodiment of the present invention;

[0052] Figure 9 It is the front view of the fourth construction step in the embodiment of the present invention;

[0053] Figure 10 It is the side view of the fourth construction step in the embodiment of the present invention;

[0054] Figure 11 It is the front view of the fifth construction step in the embodiment of the present invention;

[0055] Figure 12 It is the side view of the fifth construction step in the embodiment of the present invention;

[0056] Figure 13 It is the front view of the sixth construction step in the embodiment of the present invention;

[0057] Figure 14 It is the side view of the sixth construction step in the embodiment of the present invention;

[0058] In the figure: existing shield tunnel 1, reserved section 1-1 of the existing shield tunnel, abandoned section 1-2 of the existing shield tunnel, longitudinal tie 1-3 of the existing shield tunnel, newly added station 2, station floor structure 2-1, station side wall structure 2-2, connecting portal structure 2-3 between the station and the tunnel, retaining structure 3, reverse circulation bored pile 3-1, CSM water stop curtain 3-2, frozen body 4, radial freezing pipe 4-1, inclined freezing pipe 4-2. Detailed implementation mode

[0059] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0060] It should be understood that the accompanying drawings are only for illustrative purposes and cannot be construed as a limitation of this patent; for better illustration of this embodiment, some components in the accompanying drawings will be omitted, enlarged or reduced, which do not represent the dimensions of the actual product; for those skilled in the art, it is understandable that some well-known structures and their descriptions in the accompanying drawings may be omitted. The positional relationships described in the accompanying drawings are only for illustrative purposes and cannot be construed as a limitation of this patent.

[0061] As Figure 1 and Figure 2 shown, a structure of a newly added station for an underground line of the present invention includes an existing shield tunnel 1, a newly added station 2, a retaining structure 3, and a frozen body 4. The existing shield tunnel 1 includes an existing shield tunnel reserved section 1-1 and an existing shield tunnel abandoned section 1-2. An existing shield tunnel longitudinal tie 1-3 for improving the longitudinal stiffness can be arranged on the tunnel segment of the existing shield tunnel 1; the newly added station 2 is located above the existing shield tunnel 1, and the newly added station 2 at least includes a station floor structure 2-1 and a station side wall structure 2-2, and is communicated with the existing shield tunnel 1 through a station-tunnel connection portal structure 2-3; the retaining structure 3 is arranged around the newly added station 2 and maintains a set safety clearance from the existing shield tunnel 1. The retaining structure 3 is composed of a reverse circulation bored pile 3-1 and a CSM water stop curtain 3-2 located outside it, and is used for supporting the side wall of the foundation pit and preventing water seepage; the frozen body 4 is formed between the existing shield tunnel 1 and the retaining structure 3 through a radial freezing pipe 4-1 and / or an inclined freezing pipe 4-2, and is used for strengthening the surrounding soil layer.

[0062] A construction method for a newly added station of an underground line of the present invention includes the following steps:

[0063] S1: Reinforcement inside the existing tunnel

[0064] As Figures 3 to 4 shown, inside the operating existing shield tunnel 1, during the skylight time, the longitudinal tie of the shield segments within the range of the proposed newly added station is strengthened to form the existing shield tunnel longitudinal tie 1-3, improve the longitudinal stiffness of the shield segments, and reduce the uneven settlement and floating caused by the excavation of the later station foundation pit;

[0065] S2: Construction of the retaining structure

[0066] like Figures 5 to 6 As shown in the figure, according to the shield segment ring number and mileage of the existing shield tunnel 1 in underground operation measured on site, the plane position of the new station end retaining structure is determined to ensure that the center of the retaining structure overlaps with the center of a certain ring segment, reducing the impact range of the retaining structure construction on the segment. Then the retaining structure 3 is constructed, and the reverse circulation bored pile 3-1 is constructed first. After the reverse circulation bored pile reaches the design strength, the CSM water-stop curtain 3-2 is constructed outside the reverse circulation bored pile to ensure that the side wall of the proposed station foundation pit does not leak water or sand.

[0067] In order to minimize the scale of the new station, the clear distance between the retaining structure and the tunnel along the line direction of the new station is arranged to be no less than 1m. In addition, when adding a retaining structure above the existing tunnel, taking into account the equipment construction error and the safety of the existing tunnel, the clear distance between the bottom of the retaining structure and the height direction of the existing tunnel is no less than 0.5m. When constructing retaining structures around existing tunnels, it is necessary to ensure the safety of the existing tunnel while ensuring the safety of the main foundation pit excavation of the station in the later stage. Because during the construction of the retaining structure, the disturbance of the surrounding soil by the retaining structure will inevitably affect the existing tunnel, so it is important to select a structure that meets the safety of the main foundation pit excavation and ensures the safety of the existing tunnel structure.

[0068] In foundation pit projects with high groundwater levels, poor ground conditions, and deep excavation, the station main foundation pit retaining structure mostly adopts support structures such as bored cast-in-place piles and underground continuous walls. Limited by the mechanical construction equipment of underground continuous walls, the minimum plane length of each underground continuous wall is 2.8m, and the plane width is generally 0.8-1.0m. The potential risk of collapse during the formation of underground continuous walls is greater than that of bored cast-in-place piles (generally 1.0-1.2m in diameter). Therefore, underground continuous walls are not suitable for being arranged near existing tunnels.

[0069] In order to reduce the impact of bored pile construction on existing tunnels, mud-walled bored piles are selected in non-squeezing drilling. Common bored pile drilling processes include positive circulation, reverse circulation, impact drilling, rotary drilling, full casing and full rotation. Although the rotary drilling drilling process has a faster construction speed, there is a hidden danger of hole collapse during the drilling process. The collapse of bored piles near existing tunnels will cause soil disturbance around the existing tunnel, which may affect the safety of the tunnel structure in severe cases. In order to reduce the risk of bored pile collapse in the stratum, the reverse circulation drilling process is selected. The gaps between bored piles are sealed with cement reinforced soil such as mixing piles and high-pressure rotary jet piles; in addition, in order to improve the water-stopping effect of the foundation pit, underground continuous walls are often used for underground three-story stations with deeper excavation depths. I-beams and other materials are used for water-stopping between underground continuous walls, which has a good water-stopping effect.

[0070] Therefore, it is necessary to select a retaining structure with high stiffness, good water-stop effect and little impact on existing tunnels, that is, to adopt bored cast-in-place piles with reverse circulation hole formation + CSM water-stop curtain. When there is no groundwater leakage problem, the CSM water-stop curtain can be cancelled.

[0071] 1) Reverse circulation bored cast-in-place piles with reverse circulation hole formation

[0072] During the hole formation process of the reverse circulation bored cast-in-place pile 3-1, the hole should always be filled with mud to maintain the stability of the hole wall. During the hole formation period, the observation of stability should be strengthened, and the specific gravity and viscosity of the mud should be increased to prevent the hole from collapsing. During the construction, if the mud leaks, it should be replenished in time to always maintain the necessary liquid level height. Regularly check the quality of the mud and adjust the mud index in time. After the excavation of the hole section is completed, the hole position, hole depth, hole width and hole wall perpendicularity should be checked. Only after passing the inspection can the hole cleaning and slurry replacement work be carried out. The control of the final hole depth of the hole section should meet the following requirements: a. The final hole depth of the hole section must ensure the design depth. Within the same hole section, the bottom excavation depth of the hole bottom is consistent and flat; b. The bottom of the extended bored cast-in-place pile within the same hole section needs to be the same as the bottom excavation depth of the first-stage hole section; The length, thickness, inclination, etc. of the bored cast-in-place pile should meet the following requirements: a. The allowable deviation of the bored cast-in-place pile length is ±2.0%; b. The allowable deviation of the bored cast-in-place pile thickness is ±10mm; c. The allowable deviation of the bored cast-in-place pile perpendicularity is ±1 / 300; d. The local protrusion on the surface of the retaining pile should not be greater than 100mm; e. There should be no mud inclusion or water leakage at the joint of the bored cast-in-place pile. After the excavation of the hole is completed, the sediment and other sundries at the bottom of the hole should be cleaned up. One hour after the hole bottom cleaning and slurry replacement are completed, the specific gravity of the mud within 500mm height from the hole bottom is not greater than 1.15, and the sediment thickness shall not be greater than 100mm.

[0073] 2) CSM water-stop curtain

[0074] The CSM method is the abbreviation of Cutter Soil Mixing (milling deep mixing technology). This method combines the existing hydraulic milling trencher and deep mixing technology. By rotating and sinking the milling wheel, while forcibly cutting the original soil, cement slurry is injected to form a continuous cement-soil wall with equal thickness, which is often used in anti-seepage and other projects. This process has a wide range of formation adaptability and can be used for cutting and mixing from soft soil to soft rock formations. It has strong adaptability to complex geological conditions, high strength, good uniformity and continuity, and reliable water-stop effect. The effective reinforcement depth can reach 60m. Since this process is to stir the original soil in-situ and the slurry injection pressure is small, this process has little impact on existing tunnels.

[0075] According to the needs of the project construction, the depth of the CSM water-stop curtain 3-2 needs to be set in combination with the specific geological conditions, hydrological conditions and dewatering requirements. The width of the CSM water-stop curtain 3-2 is generally 0.7-1.0m.

[0076] S3: Excavation of the station foundation pit

[0077] As Figures 7 to 8 shown, after the CSM cut-off curtain 3-2 reaches the designed strength, the top-down cut-and-cover construction method is adopted to excavate the station foundation pit layer by layer from top to bottom; after each layer of soil is excavated, the corresponding layer of the station floor structure 2-1 and the station side wall structure 2-2 are poured until the excavation reaches above the existing shield tunnel 1.

[0078] Common methods for excavating the station foundation pit include the open cut and cover method, the top-down cut-and-cover method, the bottom-up cut-and-cover method, etc. When it is necessary to focus on protecting the existing tunnel below during the excavation stage of the station foundation pit, a construction technology with little impact on the safety of the existing tunnel structure and short impact time should be selected.

[0079] To protect the existing tunnel below and reduce the deformation of the retaining structure, the top-down cut-and-cover method with the largest support stiffness is selected for the excavation of the station foundation pit. At the same time, to shorten the impact time on the existing tunnel, the existing tunnel is interrupted only when the excavation reaches the bottommost layer of soil.

[0080] S4: Reinforcement and freezing inside the existing tunnel

[0081] As Figures 9 to 10 shown, radial freezing pipes 4-1 and / or inclined freezing pipes 4-2 are arranged in the existing shield tunnel to form a frozen body 4, and the gaps between the existing shield tunnel, the reverse circulation bored pile and the CSM cut-off curtain are frozen and reinforced; the operation of this section of the line is interrupted or operated in sections, and radial freezing pipes are arranged in 2-3 rings of segments in the tunnel to provide a safe working surface for the excavation of the foundation pit.

[0082] To reduce the impact on the existing tunnel during the construction of the retaining structure, longitudinal channel steel reinforcement is set in advance in the existing tunnel to improve the longitudinal stiffness of the existing tunnel.

[0083] There are gaps of 0.5m - 1m between the newly added retaining structure and the existing tunnel in both the plane and height directions. If there is no groundwater leakage and the formation stability is good, excavation can be directly carried out without freezing; when encountering groundwater leakage problems and poor formation stability, radial freezing holes can be considered to be arranged in the tunnel (such as during the skylight time on the operating tunnel). This construction method is not restricted by the site and can ensure the stability of the formation around the existing tunnel during the excavation of the foundation pit.

[0084] Technical requirements for the in-tunnel radial freezing method for reinforcement: The uniaxial compressive strength of the reinforced soil should not be less than 3.6MPa, the flexural tensile strength should not be less than 2.0MPa, and the shear strength should not be less than 1.5MPa (-10°C).

[0085] The designed longitudinal thickness of the frozen curtain is two to three ring segment widths. Among them, in the area near the station, a semi-circle of inclined freezing holes is set to ensure the safety of the later excavation below the tunnel. The average temperature of the frozen curtain is -10°C.

[0086] To ensure that the average temperature of the frozen soil reaches the calculated value during design, the average temperature during the acceptance of the frozen soil should not be higher than -10°C. An insulation layer is laid on the inner side of the tunnel structure around the frozen wall, and the laying range extends 3m outside the designed frozen wall boundary. The insulation layer uses a flame-retardant (or difficult-to-combust) soft plastic foam board with a thickness of not less than 40mm and a thermal conductivity of not more than 0.04w / mk. The designed active freezing time is 40 - 50 days. It is required that the single-hole flow rate of the freezing holes is not less than 5m 3 / h;

[0087] During active freezing, the brine temperature should be adjusted according to the following rules: the brine temperature drops below -18°C after 7 days of active freezing; the brine temperature drops below -24°C after 15 days of active freezing. When excavating, the temperature difference between the supply and return brine is not more than 2°C, and the brine temperature drops below -28°C.

[0088] During the excavation of the main foundation pit of the station, for every 2m of excavation, an insulation layer needs to be laid on the exposed surface of the frozen body.

[0089] S5: Connection between the tunnel and the station

[0090] As Figures 11 to 12 shown, continue to excavate downward in sequence, pour the side wall structure of the station and the tunnel ring beam. When the main foundation pit of the station is excavated to the bottom, pour the structural floor slab to complete the connection between the station and the tunnel portal; for the loss of the frozen body thickness caused by dewatering during the excavation process, it is solved by accelerating the construction progress and increasing the thickness of the frozen wall.

[0091] S6: Plugging and restoration

[0092] As Figures 13 to 14 shown, plug the orifices of the radial freezing pipes 4-1 or the inclined freezing pipes 4-2 in the tunnel, remove the longitudinal ties, construct the internal structure of the station, and restore the normal operation of the existing shield tunnel.

[0093] The above embodiments are only illustrative examples of the technical solutions of the present invention. What the present invention relates to is not limited to what is described in the above embodiments, but is subject to the scope defined by the claims. Any modification, supplement, or equivalent replacement made by those skilled in the art in the field of the present invention based on this embodiment is within the scope protected by the claims of the present invention.

Claims

1. A construction method for adding a new station to an underground line, characterized in that: It includes the following steps: S1: Reinforcement inside the existing tunnel Inside the existing shield tunnel, longitudinally tie the shield segments within the scope of the proposed new station; S2: Construction of the retaining structure According to the measured shield segment ring numbers and mileage of the existing shield tunnel on-site, determine the plane position of the retaining structure at the end of the new station; then construct the retaining structure. First, construct the reverse circulation bored piles. After the reverse circulation bored piles reach the design strength, construct the CSM water-stop curtain outside the reverse circulation bored piles; S3: Excavation of the station foundation pit After the CSM water-stop curtain reaches the design strength, adopt the top-down and reverse construction method to excavate the station foundation pit layer by layer from top to bottom; after excavating each layer of soil, pour the corresponding layer of the station floor structure and the station side wall structure until excavating to above the existing shield tunnel; S4: Reinforcement and freezing inside the existing tunnel Arrange radial freezing pipes and / or inclined freezing pipes inside the existing shield tunnel to form a frozen body, and freeze and reinforce the gap between the existing shield tunnel, the reverse circulation bored piles, and the CSM water-stop curtain; S5: Connection between the tunnel and the station Continue to excavate downward in sequence, pour the station side wall structure and the tunnel ring beam. When the main foundation pit of the station is excavated to the bottom, pour the structural floor slab to complete the connection between the station and the tunnel portal; S6: Plugging and restoration Plug the orifices of the radial freezing pipes or inclined freezing pipes inside the tunnel, remove the longitudinal ties, construct the internal structure of the station, and restore the normal operation of the existing shield tunnel.

2. The construction method of the newly added station for the underground line according to claim 1, characterized in that: In step S2, during the hole-forming process of the reverse circulation bored piles, the hole should always be filled with mud; After the excavation of the hole section is completed, check the hole position, hole depth, hole width, and hole wall verticality. Only after passing the inspection can the hole cleaning and slurry replacement work be carried out; The control of the final hole depth of the hole section should meet the following requirements: a. The final hole depth of the hole section must ensure the design depth. Within the same hole section, the excavation depth at the bottom of the hole is consistent and flat; b. The bottom of the extended bored pile within the same hole section needs to be consistent with the excavation depth at the bottom of the previous hole section; after the excavation of the dug hole is completed, clean the sediment and sundries at the bottom of the hole. 1 hour after the hole cleaning and slurry replacement are completed, the mud specific gravity within a height of 500 mm from the bottom of the hole is not greater than 1.15, and the sediment thickness shall not be greater than 100 mm.

3. The construction method of adding a new station to an underground line according to claim 2, characterized in that: In step S2, the CSM water-stop curtain forms a continuous cement-soil wall with equal thickness by rotating and sinking the milling wheel and injecting cement slurry while forcibly cutting the original soil; the width of the CSM water-stop curtain is 0.7 - 1.0 m.

4. The construction method of the newly added station for the underground line according to claim 1 or 2 or 3, characterized in that: In step S2, the net distance in the height direction between the bottom of the retaining structure and the top of the existing shield tunnel is not less than 0.5 m.

5. The construction method of the newly added station for the underground line according to claim 1 or 2 or 3, characterized in that: In step S4, the reinforcement requirements for in-tunnel radial freezing: The uniaxial compressive strength of the reinforced soil is not less than 3.6 MPa, the flexural tensile strength is not less than 2.0 MPa, and the shear strength is not less than 1.5 MPa (-10 °C).

6. The construction method of adding a new station to an underground line according to claim 5, characterized in that: In step S4, a heat-insulating layer is laid on the inner side of the tunnel structure around the freezing wall of the frozen body, and the laying range extends 3 m outside the designed freezing wall boundary; the heat-insulating layer is made of a flame-retardant soft plastic foam board with a thickness of not less than 40 mm and a thermal conductivity of not more than 0.04 w / mk; the designed active freezing time is 40 to 50 days; the single-hole flow rate of the freezing holes is not less than 5 m 3 / h.

7. The construction method of the newly added station for the underground line according to claim 6, characterized in that: In step S4, the following rules are adopted to adjust the brine temperature during active freezing: During active freezing for 7 days, the brine temperature drops below -18 °C; during active freezing for 15 days, the brine temperature drops below -24 °C; when excavating, the temperature difference between the incoming and outgoing brine is not greater than 2 °C, and the brine temperature drops below -28 °C.

8. The construction method of adding a new station to an underground line according to claim 7, characterized in that: In the step S4, during the excavation of the foundation pit of the station main body, every 2 - 4 m of excavation is carried out, and a heat preservation layer is laid on the exposed surface of the frozen body.

9. A structure of a newly added station for an underground line, characterized in that: Constructed by the construction method according to any one of claims 1 - 8.

10. The structure of the newly added station for the underground line according to claim 9, characterized in that: Comprising An existing shield tunnel (1), and an existing shield tunnel longitudinal tie (1 - 3) for improving the longitudinal stiffness can be arranged on the tunnel segments of the existing shield tunnel (1); A new station (2), located above the existing shield tunnel (1), the new station (2) at least includes a station floor structure (2 - 1) and a station side wall structure (2 - 2), and is communicated with the existing shield tunnel (1) through a station - tunnel connection portal structure (2 - 3); A retaining structure (3), arranged around the new station (2) and maintaining a set safety clearance from the existing shield tunnel (1), the retaining structure (3) is composed of a reverse - circulation bored pile (3 - 1) and a CSM water - stop curtain (3 - 2) located on its outer side, and is used for supporting the side wall of the foundation pit and preventing water seepage; A frozen body (4), the frozen body (4) is formed between the existing shield tunnel (1) and the retaining structure (3) through radial freezing pipes (4 - 1) and / or inclined freezing pipes (4 - 2), and is used for strengthening the surrounding soil layers.

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