Underpass construction method for small-diameter shield of shallow layer at bottom of expressway

Abstract

The invention discloses an expressway bottom shallow layer small-diameter shield under-crossing construction method, and relates to the field of shield under-crossing construction.The expressway bottom shallow layer small-diameter shield under-crossing construction method comprises the following steps that cement paste is injected into an outer ring grouting hole in a reinforcing area of an expressway soil layer through the retreating type grouting technology, and the reinforcing area of the soil layer of the expressway is reinforced; a soil layer is reinforced, pipe shed steel pipes are inserted into the drill holes below the outer ring grouting holes, the pipe shed steel pipes are filled with cement paste, the shape of the soil layer is reinforced, small guide pipes are inserted into the drill holes in the soil layer below the pipe shed steel pipes, the cement paste is injected into the small guide pipes, the soil layer is reinforced, a reinforced area is divided into a plurality of areas through the small guide pipes and the pipe shed steel pipes, and inner ring grouting holes are drilled in the areas; and filling cement paste into the soil layer by adopting a retreating type grouting process to reinforce a deeper soil layer, and reinforcing the soil layer below the aquilaria sinensis high-speed soil layer by adopting a mode of long pipe shed supporting, small guide pipe grouting and retreating type grouting.

Description

Technical Field

[0001] This invention relates to the field of shield tunneling construction technology, specifically a method for shallow, small-diameter shield tunneling construction at the bottom of a highway. Background Technology

[0002] With the continuous densification of urban rail transit networks, shield tunnels frequently face engineering challenges during construction, particularly when they pass under existing highways. Especially when the tunnel depth is shallow (usually less than twice the tunnel diameter) and traverses a complex stratum of soft overlying and hard subsurface soil, shield excavation easily disturbs the overlying soil, leading to an imbalance in the redistribution of ground stress. This can result in significant surface subsidence, and in extreme cases, may even induce sudden ground collapse, seriously endangering the structural safety and normal traffic flow of the highway.

[0003] Currently, commonly used ground reinforcement methods are relatively limited. One method is grouting reinforcement, which involves injecting cement grout or cement-water glass grout into the pores or fissures of the soil through drilling, promoting soil particle cementation and thus improving the overall strength and impermeability of the ground. Another method is pre-support, such as driving steel pipes or self-drilling anchors in front of the tunnel excavation face to form a "scaffolding"-like load-bearing structure to share the overlying load. This is often combined with grouting to enhance the stability of the surrounding rock. However, facing complex and variable shallow-buried composite ground conditions, these traditional reinforcement methods are still insufficient in terms of adaptability, control precision, and settlement suppression effect, making it difficult to fully guarantee the safety and controllability of underpass construction. In response to the shortcomings of existing technologies, this invention provides a method for shallow, small-diameter shield tunneling under highways to solve the above problems. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a method for shallow, small-diameter shield tunneling under highways, solving the problem of reinforcing existing highways.

[0005] To achieve the above objectives, the present invention provides the following technical solution: a method for shallow, small-diameter shield tunneling under highway foundations. S1. Grouting holes on the outer ring of the reinforced area of ​​the highway soil layer are injected with cement grout using a backward grouting process to reinforce the soil layer and form a top shell in the soil layer. S2. Insert the steel pipe of the pipe roof into the drill hole below the grouting hole of the outer ring, fill the steel pipe with cement grout, reinforce the soil layer to form a protective shell, and prevent the soil from sinking. S3. Insert the small guide pipe into the borehole in the soil layer below the steel pipe of the pipe shed, and inject cement grout into the small guide pipe to reinforce the soil layer. S4. The reinforcement area is divided into multiple zones by small guide pipes and pipe roof steel pipes. Inner ring grouting holes are drilled in the zones, and cement grout is filled into them using a backward grouting process to reinforce deeper soil layers. S5. Start the tunnel boring machine to excavate the soil layer under the highway; The above-mentioned shield tunneling construction method also involves a highway ground reinforcement structure: a pipe roof steel pipe. The pipe roof steel pipe has an outer ring of grouting holes drilled above it and a small guide pipe installed below it, so that the small guide pipe and the pipe roof steel pipe divide the reinforcement area into multiple areas, and an inner ring of grouting holes are drilled in each area.

[0006] Preferably, in steps S1 and S4, the steps of the backward grouting process are as follows: A1. Lay out the grouting hole positions according to the hole position layout diagram, and mark the hole numbers with red paint; A4. Move the drilling machine to the designated position, perform horizontal correction, align the drill bit with the center of the marked hole number, and drill the hole to the designed rock surface depth. Use high-pressure air to clean the hole to ensure that the hole is unobstructed. A3. Connect the segmented sealing device to the grouting pipe and lower it to the bottom of the hole; A4. The grouting machine injects cement grout from the bottom of the hole through the grouting pipe. After injecting 80% of the designed grout volume, the pressure of the cement grout is checked and it rises to the final pressure of 0.5MPa and is kept stable for 10 minutes. Grouting is then stopped, the grouting pipe is moved back 1.0 meter, the grouting pump is restarted, and the above process is repeated until the hole opening is reached. The hole opening must be sealed during the last grouting section to prevent backflow of grout. The grouting of the grouting hole is then completed.

[0007] Preferably, the cement slurry mix ratio is cement:water:admixture = 1:0.8:0.02, and the cement used is 42.5 grade ordinary Portland cement.

[0008] Preferably, in step S2, the pipe roof grouting construction steps are as follows: B1. Move the drilling machine to the designated hole position, ensuring that the center of the drill rod is aligned with the center of the hole position; B2. During the drilling process, the φ200mm pipe roof casing is pushed to the bottom of the hole along with the drill bit; B3. After drilling, pull out the drill rod and drill bit. The grouting machine continuously injects the casing material into the pipe roof casing from the bottom of the hole through a φ25mm rubber hose. Then, insert a φ108 pipe roof steel pipe into the pipe roof casing. B4. After the grouting is completed, the casing is slowly pulled out, and then the cement grout is injected into the pipe roof steel pipe in one go through the grouting machine. The cement grout flows through the pipe roof grouting hole on the pipe roof steel pipe and spreads to the surrounding soil layer, filling the soil layer voids, forming a reinforcement ring or water-stop curtain, and improving the stability of the stratum. B5. When the cement grout injection volume reaches more than 80% of the design value, the pressure of the cement grout reaches 0.5MPa, and the pressure stabilization time reaches 10 minutes, and the grouting volume reaches more than 80% of the standard, the grouting of the hole is deemed qualified and the grouting of the steel pipe of the pipe roof is completed. If any of the above conditions are not met, secondary grouting is required.

[0009] Preferably, the ratio of the shell material is water:bentonite:cement = 100:8:6, the ratio of the cement slurry is water:cement = 1:1, and 5% by volume of 35° Baume water glass is added to the cement slurry.

[0010] Preferably, in step S3, the grouting construction steps for the small-diameter pipe are as follows: C1. According to the design drawings, use a total station or theodolite to locate the hole positions; C2. The drilling machine drills into the highway strata at an inclination angle of 15° to 25° with a drilling diameter of 133mm, and the casing is pushed to the bottom of the hole along with the drill bit during the drilling process; C3. After drilling, insert a small guide tube with a diameter of φ42~φ48mm into the borehole; C4. Grouting material is applied around the small guide pipe at a flow rate of 0.3m / s. 3 After injecting at a rate of / min and stabilizing the pressure for 2 minutes after the slurry returns from the orifice, the casing material is initially stabilized before slowly, uniformly, and steadily pulling out the casing. C5. By adopting a backward segmented grouting method, the grouting machine starts grouting into the small guide pipe through the grouting pipe. The cement grout diffuses into the surrounding soil layer through the grouting holes of the small guide pipe, filling the voids in the soil layer and forming a reinforcement ring or water-stop curtain to improve the stability of the stratum. C6. After injecting 80% of the designed grout volume, and after the first grouting is completed, check that the grouting pressure meets the requirements of a final pressure of 0.5MPa, a pressure stabilization time of ≥2min, and a single grouting volume of ≥80% of the design value before proceeding to the second grouting. If the requirements are not met, grouting should be repeated, and the above process should be repeated, grouting backwards segment by segment, until the grouting of the entire small guide tube is completed.

[0011] Preferably, the drilling depth in step C2 extends above the shield axis, and the final hole is 1.5m from the top of the tunnel.

[0012] Preferably, the pipe roof steel pipe has a plurality of pipe roof grouting holes, with a spacing of 500mm between every two pipe roof grouting holes, and the pipe roof grouting holes are arranged in a quincunx pattern. The tail end of the pipe roof steel pipe is provided with a drilled grout-stopping section, and the front end of the pipe roof steel pipe is formed into a cone shape by cutting.

[0013] Preferably, the small guide tube has a plurality of small guide tube grouting holes, the distance between any two small guide tube grouting holes is 500mm, and the small guide tube grouting holes are arranged in a quincunx pattern. The tail end of the small guide tube is provided with a drilling grout-stopping section, and the front end of the small guide tube is formed into a cone shape by cutting.

[0014] The technical effects and advantages of this invention are as follows: The shallow, small-diameter shield tunneling method used for the highway's underpass reinforces the soil beneath the highway using a combination of long pipe roof support, small-diameter pipe grouting, and retreating grouting. First, the site was leveled and a reinforcement platform was constructed next to the toe of the highway slope on the starting side. Then, pipe roof construction was carried out, forming a preliminary support system under the highway. This system provides stable external support for subsequent small-diameter pipe grouting and retreating grouting, effectively controlling ground deformation. Following this, oblique grouting operations using small-diameter pipes and retreating grouting were performed, filling the inner and outer grouting holes. Drill holes according to the marked locations into the soil layer below the pipe shed. Using a grouting machine, cement grout is forcefully injected deep into the soil layer along small guide pipes, filling the pores, cracks, and weak interlayers in the soil layer. After the grout solidifies and binds, it forms a crisscrossing network or continuous reinforcement plate in the soil layer, thereby reinforcing the soil under the highway, improving the density, integrity, and deformation resistance of the original foundation soil, effectively controlling the risk of collapse caused by uneven settlement of the roadbed, and ensuring the safe operation of the highway during the shield tunneling under the Dongguan-Foshan Expressway. Attached Figure Description

[0015] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0016] Figure 1 This is a schematic cross-sectional view of the highway structure of the present invention; Figure 2 This is a top view of the structure of the present invention; Figure 3 This is a schematic diagram of the steel pipe structure for the pipe shed of the present invention; Figure 4 This is a schematic diagram of the small catheter structure of the present invention; Figure 5 This is a schematic diagram of the small guide tube of the present invention inside a borehole; Figure 6 This is a schematic diagram of the highway reinforcement process structure of the present invention; Figure 7 This is a schematic diagram of the construction process of the steel pipe for the pipe shed according to the present invention; Figure 8 This is a schematic diagram of the small catheter construction process of the present invention; Figure 9 This is a schematic diagram of the backward grouting process structure of the present invention.

[0017] In the diagram: 1. Expressway; 2. Reinforcement platform; 3. Guide wall; 4. Pipe roof steel pipe; 41. Pipe roof grouting hole; 5. Small guide pipe; 51. Small guide pipe grouting hole; 6. Outer ring grouting hole; 7. Inner ring grouting hole. Detailed Implementation

[0018] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] This embodiment discloses a method for shallow, small-diameter shield tunneling under highway foundations, according to the appendix... Figure 1 To be continued Figure 9 As shown, it includes the following steps: After clearing and leveling the site at the toe of the Dongguan-Foshan Expressway, the grouting reinforcement hole positions were measured and marked. HPRB3008@150*150 double-layer steel mesh was laid in the reinforcement area, with the steel bars staggered from the reinforcement hole positions. After being hardened with C20 concrete, it served as reinforcement platform 2. Then, guide wall 3 was laid on reinforcement platform 2 according to the drawings and laid out. φ200mm seamless steel pipes with a wall thickness of 6mm and a single length of 80cm were embedded inside the guide wall 3, and a total of 30 pipes were arranged and fixed according to the designed hole positions. Then, on reinforcement platform 2, the positions of the outer ring grouting holes 6 are laid out according to the hole layout diagram above the guide pipe, and the hole numbers are marked with red paint. A drilling machine is used to drill holes in the soil layer below the highway from the guide pipe, using a top-down sequential drilling method. Each row of grouting holes skips holes at intervals. The holes drilled by the drilling machine reach the designed rock surface depth, and high-pressure air is used to clean the holes to ensure unobstructed flow, thus reinforcing the soil layer below the highway. Finally, the segmented sealing device is connected to the grouting pipe on the grouting machine. The grouting pipe is lowered to the bottom of the hole, and the cement grout is injected using a backward grouting process. After injecting 80% of the designed grout volume, the pressure of the cement grout is checked and it rises to the final pressure of 0.5 MPa and is kept stable for 10 minutes. The grouting is then stopped, the grouting pipe is moved back 1.0 meter, the grouting pump is restarted, and the above process is repeated until the hole is reached. The hole is sealed during the last grouting section to prevent backflow of grout. The grouting of the outer ring grouting hole 6 is completed, and the area above the steel pipe of the pipe shed is reinforced to provide a top shell for the subsequent reinforcement of the pipe shed. The aforementioned cement slurry is prepared by a pulping machine with a cement:water:admixture ratio of 1:0.8:0.02 and using 42.5 grade ordinary Portland cement. By carrying out pre-grouting reinforcement work on the upper area of ​​the arch of the pipe roof design, the soil in front of the excavation face is effectively reinforced and the stratum deformation is controlled; Guided by the guide pipe on guide wall 3, the drilling position for the pipe roof is determined. The drilling machine is then moved to the position, the drill bit aligned with the center, and drilling is performed. During drilling, the φ200mm pipe roof casing is advanced to the bottom of the hole along with the drill bit. After drilling, the drill rod and drill bit are removed. Then, the grouting machine continuously injects casing material into the pipe roof casing from the bottom of the hole through a φ25mm hose. Next, the φ108 pipe roof steel pipe 4 is inserted into the casing. After grouting, the casing material is allowed to stabilize initially before the pipe roof casing is slowly, evenly, and smoothly pulled out. Care must be taken not to pull out the casing too quickly, otherwise the casing material may be sucked out or voids may form. Finally, the water is injected through the grouting machine in one go. The slurry is injected into the steel pipe 4 of the pipe roof. The cement slurry flows through the grouting hole 41 on the steel pipe 4 and diffuses into the surrounding soil layer, filling the soil layer voids and forming a reinforcement ring or water-stop curtain to improve the stability of the stratum. When the cement slurry injection volume reaches more than 80% of the design value, the pressure of the cement slurry reaches 0.5MPa and the pressure stabilization time reaches 10 minutes, and the grouting volume reaches more than 80% of the standard, the grouting of the hole is deemed qualified and the grouting of the steel pipe 4 of the pipe roof is completed. If any of the above conditions are not met, secondary grouting is required until the grouting of the steel pipe 4 of the pipe roof is completed, so that the steel pipe 4 of the pipe roof forms a rigid protective shell in the reinforcement zone of the highway soil layer to prevent the top soil from sinking. The ratio of the shell material is water: bentonite: cement = 100:8:6, the ratio of cement slurry is water: cement = 1:1, and 5% by volume of 35° Baume water glass is added to the cement slurry. After grouting of several sections of pipe roof steel pipe 4, the pipe roof steel pipe 4 is used as a support to form a support form with greater rigidity, which can prevent and limit the deformation of the surrounding rock and can withstand the early pressure of the surrounding rock in advance. The roadbed section is pre-reinforced before the shield tunnel passes through, reducing the impact on the highway pavement during the crossing process. In the soil layer below the pipe roof, according to the design drawings, a total station or theodolite is used to locate the boreholes. The drilling rig drills into the highway stratum at an inclination angle of 15° to 25°, with a borehole diameter of 133mm. The drilling depth is extended to above the shield tunnel axis, with the final hole being 1.5m from the tunnel top. During the drilling process, the casing is advanced to the bottom of the hole along with the drill bit. After the hole is formed, a small guide pipe 5 with a diameter of φ42 to φ48mm is inserted into the borehole. Grouting material is then injected around the small guide pipe 5 at a flow rate of 0.3m / s. 3After injection at a rate of / min and grout return at the orifice, the pressure is stabilized for 2 minutes. Then, after the casing material has initially stabilized, the casing is slowly, uniformly, and steadily pulled out. Then, a backward segmented grouting process is adopted, so that the grouting machine starts to inject cement grout into the small guide pipe 5 under high pressure through the grouting pipe. The cement grout flows through the small guide pipe grouting hole 51 on the small guide pipe 5 and diffuses to the surrounding soil layer, filling the soil layer voids and forming a reinforcement ring or water-stop curtain to improve the stability of the stratum. When 80% of the designed grout volume is injected, the first segment of grouting is completed. Check that the grouting pressure meets the requirements of cement grout pressure reaching 0.5MPa, pressure stabilization time ≥2min, and single segment grouting volume ≥80% of the design value. Then, the second segment of grouting is carried out. If the requirements are not met, the grouting is repeated. The above process is repeated segment by segment of backward grouting until the entire small guide pipe 5 is grouted. The aforementioned shell material ratio is water: bentonite: cement = 100:8:6; After the grouting of several small guide pipes 5 is completed, small guide pipes 5 are drilled and buried in the weak soil layer of the highway. Cement grout is injected into the weak layer under high pressure through the small guide pipes 5, which allows the grout to penetrate, compact, cut, and ultimately cement the weak layer, forming a gel-like cement solidified body. This achieves the purpose of reinforcing the weak soil layer and reducing settlement. The small guide pipes 5 form a physical isolation zone to control the grout diffusion range and ensure the grouting effect in the area. At the same time, the small guide pipes 5, together with the pipe roof steel pipes 4, form an outer ring seal on the reinforced area of ​​the highway soil layer, dividing the reinforced area of ​​the highway soil layer into multiple zones. According to the design drawings, the positions of the inner ring grouting holes 7 are marked in each area, and the hole numbers are marked with red paint. A drilling machine is used to drill holes in the soil layer below the highway from the guide pipe. The grouting holes are drilled sequentially from top to bottom, with each row of grouting holes skipping holes at intervals. The holes drilled by the drilling machine reach the rock surface depth, and high-pressure air is used to clean the holes to ensure that the channels are unobstructed. Then, the segmented sealing device is connected to the grouting pipe on the grouting machine, and the grouting pipe is lowered to the bottom of the hole. The cement grout is injected using a backward grouting process. After injecting 80% of the designed grout volume, the pressure of the cement grout is checked and it rises to the final pressure of 0.5MPa and is maintained for 10 minutes. Grouting is stopped, the grouting pipe is backed up 1.0 meter, the grouting pump is restarted, and the above process is repeated until the hole is reached. The hole opening of the last section of grouting needs to be sealed to prevent backflow of grout. The grouting of the inner ring grouting holes 7 is completed to achieve deep reinforcement of the soil layer in the reinforcement area of ​​the highway and to achieve the effect of strengthening and consolidating the soil in the underpass area. The aforementioned cement slurry is prepared by a pulping machine with a cement:water:admixture ratio of 1:0.8:0.02 and using 42.5 grade ordinary Portland cement. After the grouting of the above-mentioned grouting holes is completed, segmented retreat grouting skip-hole construction is carried out in the middle area after the construction of small guide pipe 5. This realizes the segmented densification of the stratum, avoids long-distance flow of grout, improves the uniformity of grouting, and reduces the risk of surface heave. The soil beneath the Dongguan-Foshan Expressway was reinforced using a combination of pipe roof support, small-diameter pipe 5 grouting, and retreating grouting. First, the site was leveled and a reinforcement platform 2 was constructed beside the toe of the expressway slope on the starting side. Then, pipe roof construction was carried out, forming a preliminary support system beneath the highway. This provided stable external support for subsequent small-diameter pipe 5 grouting and retreating grouting, effectively controlling ground deformation. Following this, oblique grouting operations were performed using small-diameter pipe 5 and retreating grouting, with the inner grouting holes 7 and outer grouting holes 6 positioned according to the marked locations. Drilling is carried out to the soil layer below the pipe shed. Cement grout is forcefully injected into the soil layer through a grouting machine along a small guide pipe 5, filling the pores, cracks and weak interlayers in the soil layer. After the grout solidifies and binds, it forms a crisscrossing network or continuous reinforcement plate in the soil layer, thereby reinforcing the soil under the existing expressway, improving the density, integrity and deformation resistance of the original foundation soil, effectively controlling the risk of collapse caused by uneven settlement of the roadbed, and ensuring the safe operation of the highway during the shield tunneling under the Dongguan-Foshan Expressway. Then, the tunnel boring machine is started for excavation, and cement-based grout of 3.4–4.7 m³ is injected simultaneously during the excavation process. 3 / ring, the initial setting time is 4 to 6 hours; after each ring is excavated, cement-water glass double liquid grout is injected through the segment hoisting hole for secondary reinforcement, and the setting time is controlled at 30 to 60 seconds to quickly fill potential voids; The above-mentioned shield tunneling construction method also involves a highway ground reinforcement structure: including a reinforcement platform 2 laid beside the highway, a guide wall 3 laid on the reinforcement platform 2, and several guide pipes buried in the guide wall 3. The pipe roof steel pipe 4 is inserted into the highway ground through the guide pipes. Several outer ring grouting holes 6 are distributed above the pipe roof steel pipe 4, and cement grout is injected into the outer ring grouting holes 6 to form a top shell above the pipe roof steel pipe 4, which reinforces the soil layer above the pipe roof steel pipe 4. Several small guide pipes 5 are inserted into the soil layer below the pipe roof steel pipe 4, and the arrangement of the small guide pipes 5 is coordinated with the arrangement of the pipe roof steel pipe 4 to form an outer ring seal in the reinforcement area of ​​the highway soil layer, dividing the reinforcement area of ​​the highway soil layer into multiple areas. Each area is equipped with an inner ring grouting hole 7, and cement grout is injected into the inner ring grouting hole 7 to deeply reinforce the soil in the reinforcement area of ​​the highway soil layer, so as to achieve the effect of strengthening and consolidating the soil in the tunneling area. Several grouting holes 41 are opened on the pipe roof steel pipe. The distance between every two grouting holes 41 is 500mm, and the grouting holes 41 are arranged in a quincunx pattern. A manually installed drilled grout-stopping section is set at the tail end of the pipe roof steel pipe 4, and the front end of the pipe roof steel pipe 4 is formed into a cone shape by cutting. The small guide tube 5 has several small guide tube grouting holes 51. The distance between every two small guide tube grouting holes 51 is 500mm, and the small guide tube grouting holes 51 are arranged in a quincunx pattern. The tail end of the small guide tube 5 is provided with a manually installed drilled grouting stop section, and the front end of the small guide tube 5 is formed into a cone shape by cutting.

[0020] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for shallow, small-diameter shield tunneling under the bottom of a highway, characterized in that, Includes the following steps: S1. Grouting holes (6) on the outer ring of the reinforced area of ​​the highway soil layer are injected with cement grout through a backward grouting process to reinforce the soil layer and form a top shell in the soil layer. S2. Insert the steel pipe (4) of the pipe roof into the drill hole below the grouting hole (6) of the outer ring, fill the steel pipe (4) with cement grout, reinforce the soil layer to form a protective shell, and prevent the soil from sinking. S3. Insert the small guide pipe (5) into the borehole in the soil layer below the steel pipe (4) of the pipe shed, and inject cement grout into the small guide pipe (5) to reinforce the soil layer; S4. The reinforcement area is divided into multiple areas by small guide pipe (5) and pipe roof steel pipe (4). Inner ring grouting holes (7) are drilled in the area. Cement grout is filled into the area by the backward grouting process to reinforce the deeper soil layer. S5. Start the tunnel boring machine to excavate the soil layer under the highway; The above-mentioned shield tunneling construction method also involves a highway ground reinforcement structure: a pipe roof steel pipe (4), with an outer ring grouting hole (6) drilled above the pipe roof steel pipe (4) and a small guide pipe (5) set below it, so that the small guide pipe (5) and the pipe roof steel pipe (4) divide the reinforcement area into multiple areas, with an inner ring grouting hole (7) drilled in the area.

2. The method for shallow, small-diameter shield tunneling under a highway as described in claim 1, characterized in that, In steps S1 and S4, the steps of the backward grouting process are as follows: A1. Lay out the grouting hole positions according to the hole position layout diagram, and mark the hole numbers with red paint; A4. Move the drilling machine to the designated position, perform horizontal correction, align the drill bit with the center of the marked hole number, and drill the hole to the designed rock surface depth. Use high-pressure air to clean the hole to ensure that the hole is unobstructed. A3. Connect the segmented sealing device to the grouting pipe and lower it to the bottom of the hole; A4. The grouting machine injects cement grout from the bottom of the hole through the grouting pipe. After injecting 80% of the designed grout volume, the pressure of the cement grout is checked and it rises to the final pressure of 0.5MPa and is kept stable for 10 minutes. Grouting is then stopped, the grouting pipe is moved back 1.0 meter, the grouting pump is restarted, and the above process is repeated until the hole opening is reached. The hole opening must be sealed during the last grouting section to prevent backflow of grout. The grouting of the grouting hole is then completed.

3. The method for shallow, small-diameter shield tunneling under a highway as described in claim 2, characterized in that, The cement slurry mix ratio is cement:water:admixture = 1:0.8:0.02, and the cement used is 42.5 grade ordinary Portland cement.

4. The method for shallow, small-diameter shield tunneling under a highway as described in claim 1, characterized in that, In S2, the grouting construction steps for the pipe roof are as follows: B1. Move the drilling machine to the designated hole position, ensuring that the center of the drill rod is aligned with the center of the hole position; B2. During the drilling process, the φ200mm pipe roof casing is pushed to the bottom of the hole along with the drill bit; B3. After drilling, pull out the drill rod and drill bit. The grouting machine continuously injects the casing material into the pipe roof casing from the bottom of the hole through the φ25mm rubber hose. Then, insert the φ108 pipe roof steel pipe into the pipe roof casing (4). B4. After the grouting is completed, the casing is slowly pulled out, and then the cement grout is injected into the pipe roof steel pipe (4) in one go through the grouting machine. The cement grout flows through the pipe roof grouting hole (41) on the pipe roof steel pipe (4) and spreads to the surrounding soil layer, filling the soil layer voids, forming a reinforcement ring or water-stop curtain, and improving the stability of the stratum. B5. When the amount of cement grout injected reaches more than 80% of the design value, the pressure of the cement grout is tested to reach 0.5MPa, and the pressure stabilization time reaches 10 minutes, and the amount of grouting reaches more than 80%, the grouting of the hole is deemed qualified and the grouting of the steel pipe (4) of the pipe shed is completed. If any of the above conditions are not met, secondary grouting is required.

5. The method for shallow, small-diameter shield tunneling under a highway as described in claim 1, characterized in that, The shell material is mixed in the ratio of water:bentonite:cement = 100:8:6, and the cement slurry is mixed in the ratio of water:cement = 1:1, with 5% by volume of 35° Baume water glass added to the cement slurry.

6. The method for shallow, small-diameter shield tunneling under a highway as described in claim 1, characterized in that, In step S3, the grouting construction steps of the small guide pipe (5) are as follows: C1. According to the design drawings, use a total station or theodolite to locate the hole positions; C2. The drilling machine drills into the highway strata at an inclination angle of 15° to 25° with a drilling diameter of 133mm, and the casing is pushed to the bottom of the hole along with the drill bit during the drilling process; C3. After drilling, insert the φ42~φ48mm small guide tube (5) into the borehole; C4. Grouting material is injected around the small guide pipe (5) at a flow rate of 0.3 m / s. 3 After injecting at a rate of / min and stabilizing the pressure for 2 minutes after the slurry returns from the orifice, the casing material is initially stabilized before slowly, uniformly, and steadily pulling out the casing. C5. By adopting a backward segmented grouting method, the grouting machine starts grouting into the small guide pipe (5) through the grouting pipe. The cement grout diffuses into the surrounding soil layer through the small guide pipe grouting hole (51) on the small guide pipe (5), filling the soil layer voids, forming a reinforcement ring or water-stop curtain, and improving the stability of the stratum. C6. After injecting 80% of the designed grout volume, after the first grouting is completed, check that the grouting pressure meets the requirements of final pressure reaching 0.5MPa, pressure stabilization time ≥2min, and single-section grouting volume ≥80% of the design value. Then proceed with the second grouting. If the requirements are not met, grouting is carried out again. Repeat the above process, grouting backwards section by section until the entire small guide tube (5) is grouted.

7. The method for shallow, small-diameter shield tunneling under a highway as described in claim 6, characterized in that, The shell material is prepared in the following ratio: water: bentonite: cement = 100:8:

6.

8. The method for shallow, small-diameter shield tunneling under a highway as described in claim 6, characterized in that, In step C2, the drilling depth extends above the shield axis, and the final hole is 1.5m from the top of the tunnel.

9. The method for shallow, small-diameter shield tunneling under a highway as described in claim 1, characterized in that, The steel pipe (4) of the pipe roof is provided with a number of grouting holes (41), and the interval between each two grouting holes (41) is 500mm. The grouting holes (41) are arranged in a plum blossom shape. The tail end of the steel pipe (4) is provided with a drilling grout stop section. The front end of the steel pipe (4) is formed into a cone shape by cutting.

10. The method for shallow, small-diameter shield tunneling under a highway as described in claim 1, characterized in that, The small guide tube (5) has several small guide tube grouting holes (51), the distance between each two small guide tube grouting holes (51) is 500mm, and the small guide tube grouting holes (51) are arranged in a plum blossom shape. The tail end of the small guide tube (5) is provided with a drilling grouting section, and the front end of the small guide tube (5) is formed into a cone shape by cutting.

Images