Large-section tunnel underpassing existing municipal road tunnel construction method

Abstract

This application relates to the field of tunnel construction technology, specifically disclosing a construction method for a large-section tunnel passing under an existing municipal road, including the following steps: Step 1, monitoring of the existing tunnel; Step 2, construction of an ultra-long pipe roof in the tunnel body; Step 3, construction of advanced small guide pipes; Step 4, advanced reinforcement construction of the tunnel face; Step 5, excavation and support construction of the tunnel face; Step 6, grouting after initial support. Through the construction of ultra-long pipe roofs and advanced small guide pipes, all the loads released outward by the rock mass during excavation can be effectively diffused and transferred. During the specific excavation process, the load borne by the steel pipes in the tunnel rock mass can be diffused in all directions, effectively preventing the rate of arch settlement, greatly improving the strength of the surrounding rock in the tunnel project, and effectively avoiding the probability of tunnel collapse and other safety accidents. The CRD method is used for mechanical excavation, and each step of the excavation is closed into a ring, which strictly controls the settlement and deformation of the surrounding rock, which is conducive to the stability of the surrounding rock, reduces the construction difficulty, and ensures construction safety.

Description

Technical Field

[0001] This invention relates to the field of tunnel construction technology, specifically a method for constructing large-section tunnels that pass under existing municipal roads. Background Technology

[0002] The Caijiaguan No. 1 Tunnel Project on the Guiyang Taijin Line is a municipal engineering mountain tunnel. The entrance of the Caijiaguan No. 1 Tunnel adopts a two-way six-lane, low-clearance tunnel design, transitioning to a separated tunnel design, and finally reaching the exit as a low-clearance tunnel. The left tunnel's mileage markers are MLK1+780~MLK3+056, with a length of 1276 meters. The right tunnel's mileage markers are MRK1+772.603~MRK3+015.559, with a length of 1243 meters. From the entrance, the tunnel runs westward, passing under the Sichuan-Guizhou Expressway (both directions), Yan'an South Road (the section affected by the Wetland Park), and the Miaopu No. 1 Tunnel. After exiting the tunnel, it intersects with Jinzhu and connects to the Yunyan section of the Taijin Line. Figure 7 and Figure 8 As shown, the left and right tunnels are designed with straight sections at the entrance and exit, and a circular curve in the middle section. The longitudinal design features V-shaped slopes of -2.95% and 4.5%. For the left tunnel (MLK2+440-MLK2+500), the clearance between this tunnel and the Miaopu No. 1 Tunnel is 6.3-6.6m. For the right tunnel (MRK2+430-MRK2+490), the clearance is 5.2-5.6m. The small clearance poses a high construction risk. Both tunnels pass under the Miaopu No. 1 Tunnel, and the surrounding rock is classified as Class V, with significant variations in strata attitude, highly developed joints and fissures, and extreme instability. The existing Miaopu No. 1 Tunnel is a major urban road with heavy traffic. Due to the small clearance and unstable geological features, direct excavation under the existing municipal road tunnel could lead to over-excavation, increasing risk, reducing safety, and posing significant construction challenges. Summary of the Invention

[0003] In view of the shortcomings of existing technologies, the technical problem solved by the present invention is to provide a construction method for large-section tunnels passing under existing municipal roads, thereby solving the problem of high construction difficulty of existing large-section tunnels passing under existing municipal roads.

[0004] To solve the above problems, the technical solution adopted by the present invention is: a construction method for large-section tunnels passing under existing municipal roads, comprising the following steps: Step 1: Monitoring existing tunnels. Evidence is collected on the current quality defects of existing tunnels, such as structural cracks, water seepage, and road surface damage. Settlement observation points are set up, including arch subsidence monitoring points and road settlement monitoring points. Step 2: Construction of the extra-long pipe roof in the tunnel. Laser-guided positioning is used for pipe roof construction. The pipe roof adopts hot-rolled seamless steel pipes and steel flower pipes with an outer diameter of 89mm and a wall thickness of 5mm. The length of the pipe roof is 9m per ring, with an overlap length of 3m between two rings. The circumferential spacing of the steel pipes is 40cm, and the external insertion angle is no more than 12°. Grouting holes are drilled on the steel flower pipes with a diameter of 10-16mm, arranged in a quincunx pattern. A 150cm section without drilling is left at the end to stop the grouting. After the pipe roof construction is completed, grout is injected into the steel flower pipes. After the grouting is completed, it is filled with M20 cement mortar. Step 3: Construction of advanced small guide pipes. φ42×4mm hot-rolled seamless steel pipes are used, with a length of 4.5m. The diameter of the grouting holes of the small guide pipes is 6-8mm, the spacing is 10-20cm, and they are arranged in a quincunx pattern. The front end of the small guide pipe is processed into a cone shape, and the tail length is not less than 30cm for the grouting section. The circumferential spacing is 40cm, the longitudinal spacing is 3m, the external insertion angle is 10°-15°, and the longitudinal overlap length is greater than 1m. The tail of the advanced small guide pipe is welded to an I-beam. Step four: Advanced reinforcement of the tunnel face. The upper step tunnel face is reinforced using 89mm outer diameter, 6mm wall thickness steel pipes. A single-cycle reinforcement length is 18m, with the steel pipes spaced 1.5m apart. The steel pipes are arranged in a quincunx pattern with a spacing of 1.5m. The overlap length between the two circulations is 3m. Cement grout is used for grouting the steel pipes with a water-cement ratio of 1:1 and a grouting pressure of 0.5-1MPa. After the steel pipes are grouted, M30 cement mortar is poured into the pipes for sealing. Step 5: Excavation and support construction at the tunnel face. The CRD method is used for mechanical excavation, dividing the tunnel cross-section into four parts: the upper section of the pilot tunnel, the lower section of the pilot tunnel, the upper section of the subsequent pilot tunnel, and the lower section of the subsequent pilot tunnel. Initial support (including temporary support) is installed after excavation of each part. I18 I-beams are used for support, and Φ22mm tack anchors with a length of 3000mm are used for the anchor bolts. A single layer of steel mesh is laid, and C25 concrete is sprayed with a thickness of 200mm. After the initial support is installed, the temporary support is removed, and the invert arch is filled with C35 waterproof concrete. After the waterproof layer is laid, the secondary lining is integrally cast, and the sidewalls are also filled with C35 waterproof concrete. Step 6: Initial support back grouting. C35 concrete is poured behind the initial support. The initial support back grouting is carried out in a cycle of 5m. During the construction of the initial support in each cycle, the circumferential spacing is 500mm and the longitudinal spacing is 1000mm. A grouting pipe with a length of 1000mm and a diameter of 30mm is reserved.

[0005] Furthermore, in step one, there is a settlement observation point every 10m, with 5 settlement observation points set up on each monitoring section, including three arch settlement monitoring points and two road settlement monitoring points.

[0006] Furthermore, in step two, laser-guided positioning is achieved using a laser transmitter and receiver. The laser transmitter and receiver are mounted on a support, with the laser transmitter positioned at the beginning of the pipe roof construction section to emit a laser as a baseline. This laser baseline coincides with the axis of the pipe roof design hole. The receiver is mounted on the drilling rig and kept parallel to the drill rod axis, ensuring that the laser baseline emitted by the laser transmitter coincides with the axis of the pipe roof design hole.

[0007] Furthermore, in step two, C30 concrete is injected into the steel pipe through the orifice in one go. The grouting adopts a dual control measure of final grouting pressure and grouting volume. The initial pressure is 0.5-1.0MPa, the final pressure is controlled at 2MPa, and grouting is stopped after holding the pressure for 3-5mm. After the grouting is completed, M20 cement mortar is used to fill the pipe roof to enhance its rigidity and strength.

[0008] Furthermore, the soil after the face reinforcement in step four is tested. Excavation can only proceed after the test is passed. The test requirements are: 28-day unconfined compressive strength qu≥0.8Mpa and permeability coefficient≤10-7cm / sec.

[0009] Furthermore, in step five, after the excavation, initial support, and temporary initial support of the upper section of the left pilot tunnel are completed to a length not exceeding 8 meters, the excavation, initial support of the sidewalls, and initial support of the invert arch of the lower section of the left pilot tunnel are carried out. After the excavation and initial support of the lower section of the left pilot tunnel are completed to a length of 21-24 meters, the construction on the right side is carried out. After the excavation and initial support of the upper section of the right rear pilot tunnel are completed to a length not exceeding 8 meters, the excavation, initial support of the sidewalls, and initial support of the invert arch of the lower section of the rear pilot tunnel are carried out. The secondary lining follows the excavation face by no more than 6 meters.

[0010] Furthermore, immediately after excavation in step five, engineering geological and hydrogeological conditions should be observed and recorded, and a geological description should be made. After excavation, the surrounding rock and initial support displacement and arch settlement should be measured in a timely manner. After the initial support is completed, the surface of the sprayed layer should be observed and recorded, and cracks should be described. Monitoring and measurement locations and points should be installed as soon as possible within 2 meters of the excavation surface.

[0011] The beneficial effects of this scheme are as follows: By injecting cement grout and other materials into the surrounding rock through openings in the steel pipes, the stability of the surrounding rock structure can be improved, providing excellent load-bearing capacity. Furthermore, the steel pipes, penetrating deep into the rock mass, can effectively withstand stress loads generated by changes in the rock mass structure, significantly enhancing the stability and safety of the tunnel structure. The application of pipe roof pre-support technology can effectively diffuse and transfer all loads released outwards from the rock mass during excavation. During the actual excavation process, the load borne by the steel pipes in the tunnel rock mass can be diffused in all directions. In terms of construction effectiveness, tunnel pipe roof pre-support technology can effectively prevent the rate of arch settlement, greatly increasing the strength of the surrounding rock in the tunnel project and effectively avoiding the probability of tunnel collapses and other safety accidents. The use of the CRD method for mechanical excavation, where each step of the excavation forms a closed loop, combines the advantages of the bench method and the double-sidewall pilot tunnel method, providing strict control over surrounding rock settlement and deformation, which is beneficial for surrounding rock stability, reduces construction difficulty, and ensures construction safety. Attached Figure Description

[0012] Figure 1 This is a flowchart of the construction method of the present invention; Figure 2 This is a schematic diagram of the frontal arrangement of the tunnel long pipe shed and the advanced small pipe in this invention; Figure 3 This is a schematic diagram of the longitudinal arrangement of the tunnel long pipe shed and the advanced small pipe in this invention; Figure 4 This is a frontal schematic diagram of the advanced reinforcement of the working face according to the present invention; Figure 5 This is a cross-sectional view of the excavation and support construction process at the working face of the present invention. Figure 6 This is a schematic diagram of the CRD method construction support design of the present invention; Figure 7 This is a schematic diagram of the existing tunnel monitoring and measurement point layout according to the present invention. Figure 1 ; Figure 8 This is a schematic diagram of the existing tunnel monitoring and measurement point layout according to the present invention. Figure 2 . Detailed Implementation

[0013] 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.

[0014] Implementation, for example, attached Figures 1 to 8 The construction method for a large-section tunnel passing under an existing municipal road includes the following steps: Step 1: Monitoring existing tunnels. Evidence is collected on the existing tunnels for structural cracks, water seepage, and road surface damage. Settlement observation points are set up, including arch settlement monitoring points and road settlement monitoring points. There is a monitoring section every 10m, and 5 settlement observation points are set up in each monitoring section, including three arch settlement monitoring points and two road settlement monitoring points. The monitoring equipment can be high-precision total station, level, spirit level, steel ruler, and measuring rod. Step two, construction of the extra-long pipe roof in the tunnel, such as Figure 2 and Figure 3 As shown, laser-guided positioning is used for pipe roof construction. A laser transmitter and receiver are used for guidance, mounted on supports. The laser transmitter is positioned at the beginning of the pipe roof construction section, emitting a laser beam as a baseline. This baseline coincides with the axis of the designed pipe roof hole, with a deviation ≤2mm. The receiver is mounted on the drilling rig and kept parallel to the drill rod axis, ensuring that the laser baseline emitted by the transmitter coincides with the axis of the designed pipe roof hole. During installation, a total station can be used for auxiliary positioning to adjust the support angle and height. According to the pipe roof design parameters, a total station was used to accurately lay out the center points of each borehole in the pipe roof. The borehole positions were marked with paint on the guide wall surface, along with the borehole number, drilling depth, and inclination angle. The drilling rig was moved to the designated position according to the borehole number, and its position was adjusted so that the center line of the drill rod coincided with the laser reference line. A level and angle gauge were used to adjust the drilling rig's inclination and azimuth angles. After the drilling rig was in place, steel supports and anchor bolts were used to fix it to the ground or tunnel sidewall. A suitable drilling speed was selected based on the surrounding rock geological conditions for drilling. During the drilling process, the data processor displays the drill rod's deflection angle, direction, and amount. Adjustments are made based on the displayed data to ensure accurate drilling. After drilling, the hole is cleaned. Hot-rolled seamless steel pipes and steel perforated pipes are used, with an outer diameter of 89mm and a wall thickness of 5mm. The pipe roof is 9m long per ring, with a 3m overlap between two rings. The circumferential spacing of the steel pipes is 40cm, and the external insertion angle is no greater than 12°. Grouting holes with a diameter of 10-16mm are drilled on the steel perforated pipes in a quincunx pattern, with a 150cm un-drilled grout stop section at the end. After the pipe roof construction is completed... Grouting is performed inside the steel pipe. C30 concrete is injected into the steel pipe in one go from the borehole opening. The grouting adopts dual control measures for final grouting pressure and grouting volume. The initial pressure is 0.5-1.0MPa, and the final pressure is controlled at 2MPa. Grouting is stopped after holding the pressure for 3-5mm. If the grouting volume exceeds the limit and the pressure requirement is not met, the grout concentration should be adjusted and grouting should continue until the grouting quality standard is met. Grouting can only be terminated when it is ensured that the rock mass around the borehole and the pores around the steel pipe are filled with grout. After grouting is completed, M20 cement mortar is used to fill the hole to enhance the rigidity and strength of the pipe roof. Step 3, advanced small-diameter conduit construction, such as... Figure 2 and Figure 3As shown, φ42×4mm hot-rolled seamless steel pipes are used, with a length of 4.5m. The grouting holes of the small guide pipes have a diameter of 6-8mm and a spacing of 10-20cm, arranged in a quincunx pattern. The front end of the small guide pipes is processed into a cone shape, and the tail end is not less than 30cm long as a grouting section. The circumferential spacing is 40cm, the longitudinal spacing is 3m, the external insertion angle is 10°-15°, the longitudinal overlap length is greater than 1m, and the tail end of the advanced small guide pipes is welded to an I-beam. The grouting material is cement grout with a water-cement ratio of 1:1, and the grouting pressure is 0.5-1MPa. Step four, advance reinforcement construction of the tunnel face, such as... Figure 4 As shown, the working face of the upper step is reinforced using steel perforated pipes with an outer diameter of 89mm and a wall thickness of 6mm. The single-cycle reinforcement length is 18m, and the steel perforated pipes are installed at 1.5m intervals. The steel pipes are arranged in a quincunx pattern with a spacing of 1.5m. The overlap length between two cycles is 3m. Cement grout is used for grouting the steel pipes with a water-cement ratio of 1:1 and a grouting pressure of 0.5-1MPa. After the steel pipes are grouted, M30 cement mortar is injected into the pipes for sealing. After the working face is reinforced, the soil is tested. Excavation can only proceed after the test is passed. The test requirements are: 28-day unconfined compressive strength qu≥0.8MPa and permeability coefficient≤10-7cm / sec. Step 5: Excavation and support construction of the tunnel face, using the CRD method for mechanical excavation, such as... Figure 5 and Figure 6As shown, the construction process is from ① to ⑩, specifically: ① Excavation of the upper section of the pilot tunnel, ② Initial support of the upper section of the pilot tunnel (including temporary support), ③ Excavation of the lower section of the pilot tunnel, ④ Initial support of the lower section of the pilot tunnel (including temporary support), ⑤ Excavation of the upper section of the subsequent tunnel, ⑥ Initial support of the upper section of the subsequent tunnel, ⑦ Excavation of the lower section of the subsequent tunnel, ⑧ Initial support of the lower section of the subsequent tunnel, ⑨ Removal of temporary support and casting of inverted arch, ⑩ Overall lining casting after laying waterproof layer. The tunnel section is divided into four parts: the upper section of the pilot tunnel, the lower section of the pilot tunnel, the upper section of the subsequent tunnel, and the lower section of the subsequent tunnel. After excavating each part in sequence, initial support (including temporary support) is installed, supported by I18 steel beams. The locking foot bolts use Φ22mm cartridge bolts, L = 3000mm, a single-layer steel mesh is laid, and C25 concrete is sprayed with a thickness of 200mm. After installing the initial support, the temporary support is removed and the inverted arch is cast and filled. The temporary support must be removed after the initial support monitoring measurement values are stable, and oxygen and acetylene cutting can be used. During the removal process, the secondary lining closely follows the construction to ensure the overall closure of the tunnel into a ring. The inverted arch is cast with C35 waterproof concrete, and the overall lining is cast after laying the waterproof layer. The side wall is cast with C35 waterproof concrete. After the excavation, initial support and temporary initial support of the upper section of the left pilot tunnel are completed within no more than 8 meters, the excavation of the lower section of the left pilot tunnel, side wall initial support and inverted arch initial support are carried out. After the excavation and initial support of the lower section of the left pilot tunnel are completed for 21 - 24 meters, the right side construction is carried out. After the excavation and initial support of the upper section of the right subsequent tunnel are completed within no more than 8 meters, the excavation of the lower section of the subsequent tunnel, side wall initial support and inverted arch initial support are carried out. The secondary lining closely follows the excavation face within no more than 6 meters. After excavation, the engineering geology and hydrogeology conditions are immediately observed and recorded, and geological descriptions are made. After excavation, the surrounding displacement measurement of the surrounding rock and initial support and the crown settlement measurement are carried out in a timely manner; after the initial support is completed, the surface of the sprayed layer is observed and recorded, and crack descriptions are made; the monitoring measurement parts and measuring point layouts are installed as soon as possible within 2 meters from the excavation face. The designed value of the surrounding rock displacement is Ud, and the measured value of the surrounding rock displacement is U. When U < Ud / 3, normal construction can be carried out. When Ud / 3 < U < 2Ud / 3, the support is strengthened. When the surrounding rock displacement rate < 0.2mm / d, it indicates basic stability. When the surrounding rock displacement rate is 0.2 - 1.0mm / d, enhanced observation is required; Step 6, grouting behind the initial support. C35 concrete is cast behind the initial support. Grouting behind the initial support is carried out in a cycle of every 5m. During the construction of the initial support in each cycle, the circumferential spacing is 500mm, the longitudinal spacing is 1000mm, and a grouting pipe with a diameter of 30mm and a reserved length of 1000mm is left to ensure that there is no void between the tunnel and the rock-soil surface after the initial support is completed, and to reduce the impact on the settlement of the existing tunnel.

[0015] Repeat steps two through six until tunnel construction is completed. Before and during construction, existing municipal road tunnels need to be monitored. Taking the Caijiaguan No. 1 Tunnel project as an example, the Caijiaguan No. 1 Tunnel includes a left tunnel and a right tunnel. After the secondary lining construction of the right tunnel passes under the Miaopu Tunnel and reaches 80% of its design strength, the left tunnel will be constructed using the same process. Before the construction of the Caijiaguan No. 1 Tunnel enters the area affected by the Miaopu No. 1 Tunnel, the current status of the existing left and right tunnels will be investigated. Before construction, evidence of existing quality defects such as structural cracks, water seepage, and road surface damage in the Miaopu No. 1 Tunnel will be collected and submitted to the property owner for confirmation. During construction, inspections will be conducted three times a day to monitor and record any development trends of existing cracks and other new problems. Throughout the construction process, monitoring and measurement points will be arranged at the arch tops of the left and right lines of the Miaopu No. 1 Tunnel and the sidewalks of the road, with five settlement observation points set up at each monitoring section every 10 meters. The specific layout is as follows: Figure 7 and Figure 8 As shown, this includes three arch settlement monitoring points and two road settlement monitoring points, with the monitoring range extending 30m beyond the project area. This effectively ensures construction safety. High-precision total stations and other equipment can be used for monitoring to understand the deformation state of the end face, determine the stability of the tunnel arch, and assess the impact of construction on the existing tunnel based on the ground settlement displacement. The monitoring frequency is as follows: 1-2 times / day for 1-15 days of construction, 1 time / 2 days for 16-30 days, 1-2 times / week for 30-90 days, and 1-3 times / month for more than 90 days, reducing construction risks and ensuring construction safety.

[0016] The above descriptions are merely embodiments of the present invention, and common knowledge regarding specific structures and characteristics is not elaborated upon here. It should be noted that those skilled in the art can make various modifications and improvements without departing from the structure of the present invention, and these should also be considered within the scope of protection of the present invention. These modifications and improvements will not affect the effectiveness of the present invention or the practicality of the patent. The scope of protection claimed in this application should be determined by the content of its claims, and the specific embodiments described in the specification can be used to interpret the content of the claims.

Claims

1. Construction method for large-section tunnels passing under existing municipal roads, including the following steps: Step 1: Monitoring existing tunnels. Evidence is collected on the current quality defects of existing tunnels, such as structural cracks, water seepage, and road surface damage. Settlement observation points are set up, including arch subsidence monitoring points and road settlement monitoring points. Step 2: Construction of the extra-long pipe roof in the tunnel. Laser-guided positioning is used for pipe roof construction. The pipe roof adopts hot-rolled seamless steel pipes and steel flower pipes with an outer diameter of 89mm and a wall thickness of 5mm. The length of the pipe roof is 9m per ring, with an overlap length of 3m between two rings. The circumferential spacing of the steel pipes is 40cm, and the external insertion angle is no more than 12°. Grouting holes are drilled on the steel flower pipes with a diameter of 10-16mm, arranged in a quincunx pattern. A 150cm section without drilling is left at the end to stop the grouting. After the pipe roof construction is completed, grout is injected into the steel flower pipes. After the grouting is completed, it is filled with M20 cement mortar. Step 3: Construction of advanced small guide pipes. φ42×4mm hot-rolled seamless steel pipes are used, with a length of 4.5m. The diameter of the grouting holes of the small guide pipes is 6-8mm, the spacing is 10-20cm, and they are arranged in a quincunx pattern. The front end of the small guide pipe is processed into a cone shape, and the tail length is not less than 30cm for the grouting section. The circumferential spacing is 40cm, the longitudinal spacing is 3m, the external insertion angle is 10°-15°, and the longitudinal overlap length is greater than 1m. The tail of the advanced small guide pipe is welded to an I-beam. Step four: Advanced reinforcement of the tunnel face. The upper step tunnel face is reinforced using 89mm outer diameter, 6mm wall thickness steel pipes. A single-cycle reinforcement length is 18m, with the steel pipes spaced 1.5m apart. The steel pipes are arranged in a quincunx pattern with a spacing of 1.5m. The overlap length between the two circulations is 3m. Cement grout is used for grouting the steel pipes with a water-cement ratio of 1:1 and a grouting pressure of 0.5-1MPa. After the steel pipes are grouted, M30 cement mortar is poured into the pipes for sealing. Step 5: Excavation and support construction at the tunnel face. The CRD method is used for mechanical excavation, dividing the tunnel cross-section into four parts: the upper section of the pilot tunnel, the lower section of the pilot tunnel, the upper section of the subsequent pilot tunnel, and the lower section of the subsequent pilot tunnel. Initial support (including temporary support) is installed after excavation of each part. I18 I-beams are used for support, and Φ22mm tack anchors with a length of 3000mm are used for the anchor bolts. A single layer of steel mesh is laid, and C25 concrete is sprayed with a thickness of 200mm. After the initial support is installed, the temporary support is removed, and the invert arch is filled with C35 waterproof concrete. After the waterproof layer is laid, the secondary lining is integrally cast, and the sidewalls are also filled with C35 waterproof concrete. Step 6: Initial support back grouting. C35 concrete is poured behind the initial support. The initial support back grouting is carried out in a cycle of 5m. During the construction of the initial support in each cycle, the circumferential spacing is 500mm and the longitudinal spacing is 1000mm. A grouting pipe with a length of 1000mm and a diameter of 30mm is reserved.

2. The construction method for a large-section tunnel passing under an existing municipal road according to claim 1, characterized in that: In step one, settlement monitoring points are set up at 10m intervals, with 5 settlement monitoring points set up at each monitoring section, including three arch settlement monitoring points and two road settlement monitoring points.

3. The construction method for a large-section tunnel passing under an existing municipal road according to claim 1, characterized in that: In step two, laser-guided positioning is achieved using a laser transmitter and receiver. The laser transmitter and receiver are mounted on a support. The laser transmitter is installed at the beginning of the pipe roof construction section to emit a laser as a baseline. The laser baseline coincides with the axis of the pipe roof design hole. The receiver is installed on the drilling rig and kept parallel to the drill rod axis to ensure that the laser baseline emitted by the laser transmitter coincides with the axis of the pipe roof design hole.

4. The construction method for a large-section tunnel passing under an existing municipal road according to claim 1, characterized in that: In step two, C30 concrete is injected into the steel pipe through the orifice in one go. The grouting adopts dual control measures for final grouting pressure and grouting volume. The initial pressure is 0.5-1.0MPa, the final pressure is controlled at 2MPa, and the grouting is stopped after holding the pressure for 3~5mm. After the grouting is completed, it is filled with M20 cement mortar.

5. The construction method for a large-section tunnel passing under an existing municipal road according to claim 1, characterized in that: After the soil at the working face is reinforced in step four, it is tested. Excavation can proceed after the test is passed. The test requirements are: 28-day unconfined compressive strength qu≥0.8Mpa and permeability coefficient≤10-7cm / sec.

6. The construction method for a large-section tunnel passing under an existing municipal road according to claim 1, characterized in that: In step five, after the excavation, initial support, and temporary initial support of the upper section of the left pilot tunnel are completed to a length not exceeding 8 meters, the excavation, initial support of the sidewalls, and initial support of the invert arch of the lower section of the left pilot tunnel are carried out. After the excavation and initial support of the lower section of the left pilot tunnel are completed to a length of 21-24 meters, the construction on the right side is carried out. After the excavation and initial support of the upper section of the right rear pilot tunnel are completed to a length not exceeding 8 meters, the excavation, initial support of the sidewalls, and initial support of the invert arch of the lower section of the rear pilot tunnel are carried out. The secondary lining follows the excavation face by no more than 6 meters.

7. The construction method for a large-section tunnel passing under an existing municipal road according to claim 1, characterized in that: Step 5: Immediately after excavation, observe and record the engineering geological and hydrogeological conditions, and make a geological description. After excavation, promptly measure the displacement of the surrounding rock and initial support, as well as the settlement of the arch. After the initial support is completed, observe and record the surface of the sprayed layer, and describe the cracks. Install the monitoring and measurement points within 2 meters of the excavation surface as soon as possible.

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