An excavation support system and construction method of an existing tunnel with rigid foreign matter left in a small-distance overpass arch part of a open-cut tunnel

Abstract

The application discloses an excavation support system and a construction method for a new tunnel crossing an existing tunnel with small clear distance and a rigid foreign matter left in an arch part of the existing tunnel. With the continuous development of traffic construction in China, more and more new tunnels cross existing tunnels. The water stop curtain is arranged above the existing tunnel; the main structure pile foundation is arranged outside the safety distance on both sides of the existing tunnel; the pit bottom grouting layer is arranged above the water stop curtain; the pit retaining structure comprises artificial hole digging retaining piles and mechanical hole forming retaining piles, the artificial hole digging retaining piles are arranged above the existing tunnel, and the mechanical hole forming retaining piles are arranged at other parts of the pit; the first pit support and the second pit support are arranged in the pit retaining structure. The stratum is reinforced by pit bottom grouting, so that the underground water flow in the bottom of the pit is closed, and the strength and deformation resistance of the soil layer between the bottom of the pit and the top of the existing tunnel are improved.

Description

Technical Field

[0001] This invention belongs to the field of tunnel engineering technology, specifically relating to an excavation support system and construction method for an existing tunnel with rigid foreign objects left in the arch of a small-clearance cut-out tunnel. Background Technology

[0002] Tunnel construction methods include cut-and-cover, mining, and shield tunneling. Shield tunneling has become the preferred method in recent years. However, in the 1950s and 60s, mountain tunnels in my country, including railway and highway tunnels, were primarily constructed using the mining method. Due to the lack of advanced construction technology at the time, mountain tunnels were prone to collapse during construction. After a collapse, materials such as steel rails and logs were often used to seal the tunnel arch. These materials were connected to the tunnel lining at one end, while the other end remained in the soil layer of the tunnel arch. Therefore, after the tunnel was completed, rigid objects such as steel rails and logs often remained in the arch, typically within a range of 4m to 6m above the arch crown. Furthermore, even with current technology, mining tunnels still require advanced support measures during excavation, such as advanced guide pipes and radial anchors. These advanced support measures also primarily consist of steel pipes and reinforcing bars. After the tunnel was completed, these rigid foreign objects also remained within a range of 3m to 5m above the tunnel, all connected to the tunnel lining.

[0003] With the continuous development of transportation construction in my country, there are increasingly more cases of new tunnels crossing existing tunnels. Due to factors such as track gradient and surrounding environment, new tunnels inevitably cross existing tunnels with a small clearance. If the existing tunnel is a mining tunnel and has rigid foreign objects such as rails, logs, pre-construction guide pipes, and radial anchors left in its arch, when the new tunnel is located within the range of these rigid foreign objects, the excavation will disturb these objects, directly causing disturbance and damage to the existing tunnel lining, posing a significant threat to the safety of the existing tunnel. Therefore, for such projects, new tunnels not only need to consider measures to control the uplift and deformation of the existing tunnel during the small clearance excavation process, but also need to avoid disturbing the rigid foreign objects left in the arch of the existing tunnel. Summary of the Invention

[0004] To overcome the shortcomings of existing technologies, this invention provides an excavation support system and construction method for existing open-cut tunnels with rigid foreign objects left in the arch section with small clearance. By adopting methods such as grouting to reinforce the strata at the bottom of the pit, using manually excavated bored piles for the pit retaining structure, segmented excavation of the pit, and manual excavation of the pit section, this invention achieves the goal of controlling the heave deformation of the existing tunnel and avoiding disturbance to the rigid foreign objects left in the arch section when excavating an existing open-cut tunnel with rigid foreign objects left in the arch section with small clearance.

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

[0006] An excavation support system for an existing tunnel with rigid foreign objects left in the arch of a small-clearance cut-out tunnel includes a water-stop curtain, which is set above the existing tunnel; and main structure pile foundations are set outside the safety distance on both sides of the existing tunnel.

[0007] A pit bottom grouting layer is provided above the water-stop curtain;

[0008] The retaining structure of the foundation pit includes manually excavated retaining piles and mechanically drilled retaining piles. The manually excavated retaining piles are set above the existing tunnel, and the mechanically drilled retaining piles are set in other parts of the foundation pit.

[0009] The retaining structure of the foundation pit is equipped with the first layer of support and the second layer of support.

[0010] Furthermore, the depth of the water-stop curtain should be at least 1.5m below the vertical clearance between it and the existing tunnel, and it should be located 0.5m below the bottom of the manually excavated retaining piles.

[0011] Furthermore, the pit bottom grouting layer includes a cement-water glass dual-liquid grout pit bottom grouting layer and a cement grout pit bottom grouting layer;

[0012] The cement-water glass double-liquid grouting layer at the bottom of the pit is shaped like a concave character, with the outer side higher than the inner side. The cement grouting layer at the bottom of the pit is located in the low-lying area of ​​the cement-water glass double-liquid grouting layer.

[0013] Furthermore, the thickness of the cement-water glass double-liquid grouting layer and the cement grouting layer at the bottom of the pit is 1:1. The width of the cement-water glass double-liquid grouting layer at the edge of the grouting layer is not less than its own thickness. The entire plane range of the grouting layer at the bottom of the pit covers the entire foundation pit and extends outward to the main influence area of ​​the existing tunnel.

[0014] Furthermore, the main structural pile foundation is located below the bottom slab of the newly built tunnel.

[0015] Furthermore, the water-stopping curtain adopts either grouting curtain water-stopping or jet grouting pile curtain.

[0016] Furthermore, the newly constructed tunnel is designed to accommodate two-way traffic. The tunnel is divided into two independent and parallel structures for both directions. The two parallel pits adopt a support system that shares a common middle row of retaining piles. The manually excavated retaining piles include the shared portion of the manually excavated retaining piles in the pit and the manually excavated retaining piles on both sides. The mechanically drilled retaining piles include the shared portion of the mechanically drilled retaining piles in the pit and the mechanically drilled retaining piles on both sides.

[0017] A construction method for an excavation support system for an existing open-cut tunnel with a small clearance spanning a rigid foreign object left in the arch, comprising the following steps:

[0018] Step 1: Level the site of the proposed open-cut tunnel and simultaneously construct a water-stop curtain from both ends of the open-cut pit toward the middle, symmetrical to the centerline of the existing tunnel, using the centerline of the existing tunnel as the axis of symmetry.

[0019] Step 2: Using the existing tunnel centerline as the axis of symmetry, simultaneously construct the cement-water glass double-liquid grouting layer and the cement grouting layer at the bottom of the pit from both ends of the open excavation pit toward the middle, symmetrical to the existing tunnel centerline.

[0020] Step 3: Using the centerline of the existing tunnel as the axis of symmetry, construct manually excavated retaining piles simultaneously from the middle of the open-cut pit to both ends, symmetrically to the centerline of the existing tunnel. The vertical clearance between the pile tip and the existing tunnel should be 1.5m to 2.5m.

[0021] Step 4: Using the existing tunnel centerline as the axis of symmetry, mechanically drilled retaining piles are simultaneously constructed from the middle of the open-cut foundation pit to both ends, symmetrical to the existing tunnel centerline, for the common part of the foundation pit and the mechanically drilled retaining piles on both sides.

[0022] Step 5: Using the existing tunnel centerline as the axis of symmetry, construct the main structure pile foundation from the middle of the open-cut pit to both ends, symmetrical to the existing tunnel centerline;

[0023] Step Six: Using mechanical excavation, excavate the upward foundation pit of the new tunnel. When the excavation reaches 1m below the first support of the foundation pit, construct the first support of the foundation pit to ensure effective connection with the manually excavated retaining piles and the mechanically drilled retaining piles.

[0024] Step 7: Using mechanical excavation, continue excavating the upward foundation pit of the new tunnel. When the excavation reaches 1m below the second support of the foundation pit, construct the second support of the foundation pit to ensure effective connection with the manually excavated retaining piles and the mechanically drilled retaining piles.

[0025] Step 8: Using manual excavation, continue to excavate the new tunnel's upward foundation pit. With the existing tunnel's centerline as the axis of symmetry, excavate symmetrically from both ends of the existing tunnel section towards the middle in steps with a width of 3m and a height of 1m. Immediately pour the bottom slab for the sections that have been excavated to the bottom, until the excavation and bottom slab pouring are completed.

[0026] Step 9: Pour the main structure sidewalls and top slab of the newly built tunnel's upward section, and remove the second layer of support in the foundation pit;

[0027] Step 10: Construct the new tunnel's downward foundation pit and main structure in the order of Steps 6 to 9;

[0028] Step 11: Remove the first support of the foundation pit, backfill the top of the new tunnel, and restore road traffic.

[0029] Step two is as follows:

[0030] The cement-water glass double-liquid grouting layer is applied before the cement grouting layer, forming a concave grouting layer with a higher outer surface and a lower inner surface at the bottom of the pit. Then, the cement grouting layer is injected into the low-lying area of ​​the concave grouting layer.

[0031] In step five, the main structure pile foundation adopts full-rotation drilling technology, and in the row adjacent to the existing tunnel, the entire section of steel casing is followed during drilling to prevent the collapse of the hole from disturbing the existing tunnel.

[0032] The beneficial effects of this invention are:

[0033] 1) This invention reinforces the strata by grouting at the bottom of the foundation pit, which not only seals off the flow of groundwater at the bottom of the foundation pit, but also improves the strength and deformation resistance of the soil layer between the bottom of the foundation pit and the top of the existing tunnel, and has a positive effect on the resistance of the existing tunnel to uplift deformation during the excavation of the foundation pit.

[0034] 2) In this invention, the existing tunnel top is equipped with manually excavated retaining piles and the foundation pit is excavated manually when the depth of the foundation pit is close to the expected depth of the remaining rigid foreign objects. If the remaining rigid foreign objects are encountered, they can be removed by manual static cutting, which will not cause great disturbance to the foreign objects and thus damage the existing tunnel lining.

[0035] 3) The present invention adopts a support type of shared row of retaining piles for parallel foundation pits, which can reduce the number of pile foundation constructions above the existing tunnel, reduce the disturbance to the existing tunnel and the probability of contact with left-behind rigid foreign objects; the retaining piles of the foundation pit not only play a role in stabilizing the safety of the open-cut foundation pit, but also, by controlling the vertical clearance between the existing tunnel and the retaining piles above it, the retaining piles can also serve as a measure to resist the heave and uplift of the existing tunnel, which is crucial for protecting the safety of the existing tunnel and suppressing heave deformation within the control range;

[0036] 4) The present invention allows for the excavation and construction of the main structure of the parallel foundation pits in sequence, which can avoid large-scale excavation and unloading on the top of the existing tunnel at one time, and reduce the impact of excavation on the heave and deformation of the existing tunnel.

[0037] 5) The present invention sets pile foundations on both sides of the existing tunnel outside the safety distance of the main body of the open-cut tunnel. The load of the new tunnel is borne entirely by the pile foundations and is not transmitted to the existing tunnel below. This can solve the adverse effects on the new tunnel caused by the existing tunnel having a shorter design service life than the new tunnel, such as defects, abandonment, and collapse within the normal service life of the new tunnel. In addition, the pile foundations are also beneficial to the anti-buoyancy of the new tunnel.

[0038] 6) This invention is applicable to engineering scenarios where existing tunnels with rigid foreign objects left on the arch of newly constructed open-cut tunnels with small clearance are crossed. For this special high-risk crossing project, it provides a reliable, simple and effective construction method with high economic and social benefits. It has broad application prospects in urban rail transit, railway, highway and other engineering projects. Attached Figure Description

[0039] Figure 1 Plan view of an existing tunnel with rigid foreign objects remaining in the arch section of a cut-and-cover tunnel with a small clearance;

[0040] Figure 2 This is a cross-sectional view of AA.

[0041] Figure 3 This is a cross-sectional view of BB.

[0042] In the diagram, 1-water-stop curtain, 2-1-cement-water glass double-liquid grouting layer at the bottom of the pit, 2-2-cement grouting layer at the bottom of the pit, 3-manually excavated retaining piles, 3-1-shared part of the foundation pit for manually excavated retaining piles, 3-2-manually excavated retaining piles on both sides, 4-mechanically drilled retaining piles, 4-1-shared part of the foundation pit for mechanically drilled retaining piles, 4-2-mechanically drilled retaining piles on both sides, 5-main structure pile foundation, 6-first support of the foundation pit, 7-second support of the foundation pit, 8-existing tunnel. Detailed Implementation

[0043] The present invention will now be described in detail with reference to specific embodiments.

[0044] like Figure 1 , 2 As shown in Figure 3, this invention includes a water-stop curtain 1, which is installed above the existing tunnel 8. The vertical clearance between the depth of the water-stop curtain 1 and the existing tunnel 8 is not less than 1.5m, and it is located 0.5m below the bottom of the manually excavated retaining piles. The water-stop curtain 1 is the outermost structure of the foundation pit that isolates groundwater from the inside and outside of the pit. The depth of the water-stop curtain 1 above the existing tunnel 8 needs to be reasonably controlled. The vertical clearance between the depth of the water-stop curtain and the existing tunnel can be determined according to the grouting or jet grouting pressure and the protection requirements of the existing tunnel 8. The water-stop curtain 1 adopts either a grouting curtain or a jet grouting pile curtain.

[0045] The main structure pile foundation 5 is set outside the safety distance on both sides of the existing tunnel 8; the safety distance is determined according to the protection requirements of the existing tunnel 8; the main structure pile foundation 5 is located below the bottom slab of the new tunnel, and all the load of the new tunnel is borne by the pile foundation, without being transmitted to the existing tunnel 8 below it.

[0046] A pit bottom grouting layer is set above the water-stop curtain 1; the pit bottom grouting layer includes a cement-water glass double liquid grout pit bottom grouting layer 2-1 and a cement grout pit bottom grouting layer 2-2. The cement-water glass double liquid grout pit bottom grouting layer 2-1 and the cement grout pit bottom grouting layer 2-2 together form a structure for isolating groundwater at the bottom of the foundation pit, and both are implemented by sleeve valve pipe grouting method; the cement-water glass double liquid grout pit bottom grouting layer 2-1 is a "U" shape with the outside higher and the inside lower, and the cement grout pit bottom grouting layer 2-2 is set in the low-lying part of the cement-water glass double liquid grout pit bottom grouting layer 2-1.

[0047] The cement-water glass double-liquid grouting layer 2-1 is applied before the cement grouting layer 2-2, forming a concave "U"-shaped grouting layer at the bottom of the pit, with a higher outer surface and a lower inner surface. Then, the cement grouting layer 2-2 is injected into the lower part of this concave "U"-shaped grouting layer. This grouting method, due to the rapid-setting grout stop formed by the cement-water glass double-liquid grouting layer 2-1, prevents the cement grouting layer from overflowing in highly permeable strata, thus ensuring the grouting reinforcement effect at the bottom of the pit. The depth of the cement-water glass double-liquid grouting layer 2-1 needs to be determined based on the seepage stability calculation at the bottom of the pit and the protection requirements of the existing tunnel. The thickness of the cement-water glass double-liquid grouting layer 2-1 and the cement grouting layer 2-2 at the bottom of the pit is 1:1. The width of the cement-water glass double-liquid grouting layer 2-1 at the edge of the grouting layer is not less than its own thickness. The entire bottom grouting layer covers the entire foundation pit and extends outward to the main influence area of ​​the existing tunnel 8. The determination of the main influence area can be based on the current relevant specifications.

[0048] The retaining structure of the foundation pit includes manually excavated retaining piles 3 and mechanically drilled retaining piles 4. The manually excavated retaining piles 3 are set above the existing tunnel 8, and the mechanically drilled retaining piles 4 are set in other parts of the foundation pit. The retaining structure of the foundation pit is equipped with the first support 6 and the second support 7.

[0049] The newly constructed tunnel is designed to accommodate two-way traffic. The tunnel is divided into two independent and parallel structures for both directions. The two parallel pits are supported by a common middle row of retaining piles. The manually excavated retaining piles 3 include the common part 3-1 of the manually excavated retaining piles in the pit and the manually excavated retaining piles 3-2 on both sides. The mechanically drilled retaining piles 4 include the common part 4-1 of the mechanically drilled retaining piles in the pit and the mechanically drilled retaining piles 4-2 on both sides.

[0050] Manually excavated retaining piles 3 are used above the existing tunnel. Their purpose is to create holes by manually excavating the soil. If any rigid foreign objects are encountered, they can be removed manually using static cutting, minimizing disturbance and preventing damage to the existing tunnel lining. The vertical clearance between the pile tip and the existing tunnel 8 should be 1.5m to 2.5m. Mechanically drilled retaining piles 4 are used in other parts of the foundation pit to improve construction efficiency. The shared retaining piles consist of the shared portion 3-1 of the manually excavated retaining piles and the shared portion 4-1 of the mechanically drilled retaining piles, reducing the number of piles required above the existing tunnel and minimizing disturbance to the existing tunnel and the probability of encountering any remaining rigid foreign objects.

[0051] A construction method for an excavation support system for a cut-and-cover tunnel with a small clearance crossing an existing tunnel includes the following steps:

[0052] Step 1: Level the site of the proposed open-cut tunnel. Using the centerline of the existing tunnel 8 as the axis of symmetry, construct a water-stop curtain 1 symmetrically from both ends of the open-cut pit towards the middle. This can be achieved using methods such as grouting curtain water-stopping or jet grouting pile curtain water-stopping. The depth of the water-stop curtain above the existing tunnel 8 needs to be reasonably controlled. The vertical clearance between the depth of the water-stop curtain 1 and the existing tunnel 8 can be determined based on the grouting or jet grouting pressure and the protection requirements of the existing tunnel 8. Generally, it should not be less than 1.5m, and it should be located 0.5m below the bottom of the common part 3-1 of the manually excavated retaining pile pit and the bottom of the manually excavated retaining pile 3-2.

[0053] Step 2: Using the centerline of the existing tunnel 8 as the axis of symmetry, simultaneously construct the cement-water glass double liquid grouting layer 2-1 and the cement grouting layer 2-2 from both ends of the open-cut pit towards the middle, symmetrical to the centerline of the existing tunnel 8. Both are achieved using the sleeve valve pipe grouting method.

[0054] Step two is as follows:

[0055] The cement-water glass double-liquid grouting layer 2-1 is implemented before the cement grouting layer 2-2, forming a concave grouting layer with a high outer edge and a low inner edge at the bottom of the pit. Then, the cement grouting layer 2-2 is injected into the low-lying part of the concave grouting layer.

[0056] Step 3: Using the centerline of the existing tunnel 8 as the axis of symmetry, construct the common part 3-1 of the open-cut foundation pit and the artificially excavated retaining piles 3-2 on both sides of the foundation pit simultaneously from the middle of the open-cut foundation pit to both ends, symmetrical to the centerline of the existing tunnel 8. The vertical clearance between the pile tip and the existing tunnel 8 should be 1.5m to 2.5m.

[0057] Step 4: Using the centerline of the existing tunnel 8 as the axis of symmetry, and symmetrically about the centerline of the existing tunnel 8, simultaneously construct mechanically drilled retaining piles from the middle of the open-cut foundation pit to both ends for the common part 4-1 of the foundation pit and mechanically drilled retaining piles 4-2 on both sides.

[0058] Step 5: Using the centerline of the existing tunnel 8 as the axis of symmetry, construct the main structure pile foundation 5 from the middle of the open excavation pit to both ends, symmetrical to the centerline of the existing tunnel 8. The main structure pile foundation 5 adopts the full-rotation drilling process, and in the row adjacent to the existing tunnel 8, the entire section of steel casing is followed during drilling to prevent the collapse of the hole from disturbing the existing tunnel 8.

[0059] Step 6: Using mechanical excavation, excavate the upward foundation pit of the new tunnel. When the excavation reaches 1m below the first support 6 of the foundation pit, construct the first support 6 of the foundation pit to ensure effective connection with the manually excavated retaining piles 3 and the mechanically drilled retaining piles 4.

[0060] Step 7: Using mechanical excavation, continue excavating the upward foundation pit of the new tunnel. When the excavation reaches 1m below the second support 7 of the foundation pit, construct the second support 7 of the foundation pit to ensure effective connection with the manually excavated retaining piles 3 and the mechanically drilled retaining piles 4.

[0061] Step 8: Using manual excavation, continue to excavate the upper foundation pit of the new tunnel. With the center line of the existing tunnel 8 as the axis of symmetry, excavate symmetrically from both ends of the foundation pit across the existing tunnel 8 section towards the middle in steps with a width of 3m and a height of 1m. Immediately pour the bottom slab of the section that has been excavated to the bottom until the excavation and bottom slab pouring are completed.

[0062] Step 9: Pour the main structure sidewalls and top slab of the newly built tunnel's upward section, and remove the second support 7 of the foundation pit;

[0063] Step 10: Construct the new tunnel's downward foundation pit and main structure in the order of Steps 6 to 9;

[0064] Step 11: Remove the first support 6 of the foundation pit, backfill the top of the new tunnel, and restore road traffic.

[0065] 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.

[0066] 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 excavation support system for an existing tunnel with a small clearance spanning a rigid foreign object left in the arch, characterized in that: Includes a water-stop curtain (1), which is set above the existing tunnel (8); and main structure pile foundations (5) are set outside the safety distance on both sides of the existing tunnel (8). A pit bottom grouting layer is provided above the water-stop curtain (1); The retaining structure of the foundation pit includes manually excavated retaining piles (3) and mechanically drilled retaining piles (4). The manually excavated retaining piles (3) are set above the existing tunnel (8), and the mechanically drilled retaining piles (4) are set in other parts of the foundation pit. The retaining structure of the foundation pit is equipped with the first layer of support (6) and the second layer of support (7). The pit bottom grouting layer includes a cement-water glass double liquid grout pit bottom grouting layer (2-1) and a cement grout pit bottom grouting layer (2-2). The cement-water glass double-liquid grouting layer (2-1) is a concave shape with a higher outer surface and a lower inner surface. The cement grouting layer (2-2) is located in the low-lying area of ​​the cement-water glass double-liquid grouting layer (2-1).

2. The excavation support system for an existing tunnel with a small clearance spanning a rigid foreign object left in the arch, as described in claim 1, is characterized in that: The depth of the water-stop curtain (1) is not less than 1.5m from the vertical clearance between it and the existing tunnel (8), and it is located 0.5m below the bottom of the manually excavated retaining pile.

3. The excavation support system for an existing tunnel with a small clearance spanning a rigid foreign object left in the arch, as described in claim 2, is characterized in that: The thickness of the cement-water glass double-liquid grouting layer (2-1) and the cement grouting layer (2-2) at the bottom of the pit is 1:

1. The width of the cement-water glass double-liquid grouting layer (2-1) at the edge of the grouting layer is not less than its own thickness. The entire bottom grouting layer covers the entire foundation pit and extends outward to the main influence area of ​​the existing tunnel (8).

4. The excavation support system for an existing tunnel with a small clearance spanning a rigid foreign object in the arch, as described in claim 1, is characterized in that: The main structure pile foundation (5) is located below the bottom slab of the newly built tunnel.

5. The excavation support system for an existing tunnel with a small clearance spanning a rigid foreign object in the arch, as described in claim 1, is characterized in that: The water-stopping curtain (1) adopts either grouting curtain or jet grouting curtain.

6. The excavation support system for an existing tunnel with a small clearance spanning a rigid foreign object left in the arch, as described in claim 1, is characterized in that: The newly built tunnel is designed to accommodate two-way traffic. The new tunnel is divided into two independent and parallel structures for the up and down directions. The two parallel pits adopt a support type with a shared middle row of retaining piles. The manually excavated retaining piles (3) include the shared part of the manually excavated retaining pile pit (3-1) and the manually excavated retaining piles on both sides (3-2); the mechanically drilled retaining piles (4) include the shared part of the mechanically drilled retaining pile pit (4-1) and the mechanically drilled retaining piles on both sides (4-2).

7. A construction method for an excavation support system for an existing tunnel with a small clearance spanning a rigid foreign object left in the arch, characterized in that: Includes the following steps: Step 1: Level the site of the proposed open-cut tunnel and simultaneously construct a water-stop curtain (1) from both ends of the open-cut pit to the middle, symmetrically with the center line of the existing tunnel (8) as the axis of symmetry. Step 2: Using the centerline of the existing tunnel (8) as the axis of symmetry, simultaneously construct the cement-water glass double liquid grouting layer (2-1) and the cement grouting layer (2-2) from both ends of the open excavation pit towards the middle, symmetrical to the centerline of the existing tunnel (8). Step 3: Using the center line of the existing tunnel (8) as the axis of symmetry, construct the common part (3-1) of the open-cut foundation pit and the artificial excavation retaining piles (3-2) on both sides of the pit simultaneously from the middle of the open-cut foundation pit to both ends, symmetrical to the center line of the existing tunnel (8). The vertical net distance between the pile end and the existing tunnel (8) should be 1.5m to 2.5m. Step 4: Using the centerline of the existing tunnel (8) as the axis of symmetry, mechanically drilled retaining piles are simultaneously constructed from the middle of the open-cut foundation pit to both ends, for the common part of the foundation pit (4-1) and the mechanically drilled retaining piles on both sides (4-2). Step 5: Using the centerline of the existing tunnel (8) as the axis of symmetry, construct the main structure pile foundation (5) from the middle of the open excavation pit to both ends, symmetrical to the centerline of the existing tunnel (8). Step 6: Using mechanical excavation, excavate the upward foundation pit of the new tunnel. When the excavation reaches 1m below the first support (6) of the foundation pit, construct the first support (6) of the foundation pit to ensure effective connection with the manually excavated retaining piles (3) and the mechanically drilled retaining piles (4). Step 7: Using mechanical excavation, continue excavating the new tunnel's upward foundation pit. When the excavation reaches 1m below the second support (7) of the foundation pit, construct the second support (7) of the foundation pit to ensure effective connection with the manually excavated retaining piles (3) and the mechanically drilled retaining piles (4). Step 8: Using manual excavation, continue to excavate the new tunnel's upward foundation pit. With the center line of the existing tunnel (8) as the axis of symmetry, excavate symmetrically from both ends of the foundation pit across the existing tunnel (8) section towards the middle in steps with a width of 3m and a height of 1m. Immediately pour the bottom slab into the section that has been excavated to the bottom until the excavation and bottom slab pouring are completed. Step 9: Pour the main structure sidewalls and top slab of the newly built tunnel up section, and remove the second support of the foundation pit (7). Step 10: Construct the new tunnel's downward foundation pit and main structure in the order of Steps 6 to 9; Step 11: Remove the first support of the foundation pit (6), backfill the top of the new tunnel, and restore road traffic; Step two is as follows: The cement-water glass double-liquid grouting layer (2-1) is implemented before the cement grouting layer (2-2) at the bottom of the pit, forming a "U"-shaped grouting layer with a high outer edge and a low inner edge at the bottom of the pit. Then, the cement grouting layer (2-2) at the bottom of the pit is injected into the low-lying area of ​​the "U"-shaped grouting layer.

8. A construction method for an excavation support system for an existing tunnel with a small clearance spanning a rigid foreign object left in the arch, as described in claim 7, characterized in that: In step five, the main structure pile foundation (5) is constructed using a full-rotation drilling process, and a full-section steel casing is used to follow up the drilling in a row adjacent to the existing tunnel (8) to prevent the collapse of the hole from disturbing the existing tunnel (8).

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