Advanced support method for large-section soft rock tunnel to pass through existing old tunnel in close range

Abstract

The invention relates to the field of construction, and provides a forepoling method for a large-section soft rock tunnel to underpass an existing old tunnel in a close range. The forepoling method for the large-section soft rock tunnel to pass through the existing old tunnel in the close-range underneath mode comprises the steps of pipe shed working chamber construction, concrete guide wall construction, super-long and super-large pipe shed construction, pipe shed working chamber section second-layer primary supporting, lower step and inverted arch excavation and primary supporting. According to the forepoling method for the large-section soft rock tunnel to underpass the existing old tunnel in the close range, the stability and the bearing capacity of the excavation face and the surrounding soil body are enhanced, and the influence of excavation operation on the existing line is reduced. Disturbance to a soil body below the existing railway line is reduced, the risks of land subsidence and line deformation caused by construction are reduced, and normal operation of the existing railway line is guaranteed. The structure stability can be enhanced, the construction efficiency is improved, the supporting structure performance is optimized, the influence on the existing line is reduced, and the structure safety in the construction period and the subsequent operation stage is ensured.

Description

Technical Field

[0001] The invention relates to the field of construction and provides an advance support method for a large-section soft rock tunnel passing under an existing old tunnel at a close distance. Background Art

[0002] Existing tunnel construction technology often faces huge challenges in the construction of long pipe sheds for advanced support through other tunnels. Especially under weak geological conditions, for example, the strata include sericite quartz schist, chlorite sodium schist and tuff weak weathering layer: scaly metamorphic structure, flaky structure, occasionally phylloxetine structure, mainly composed of clay minerals, containing a small amount of quartz and sericite, which are easy to soften and disintegrate when exposed to water, and crack in areas where water loss occurs. It is difficult to drill holes in one go during the long pipe shed drilling process. Traditional construction methods are difficult to ensure the construction quality of the long pipe shed, and both tunnel operation safety and construction efficiency are improved.

[0003] Under such geological conditions, the construction of super-long and super-large pipe-roof advance support mainly faces the following technical difficulties, including: 1. High safety risk: The excavation section of the new tunnel is large, the tunnel body passes through soft surrounding rock and is close to the existing tunnel. The clay minerals are easily softened and disintegrated when exposed to water. If the support measures are not safe and timely, it is easy to cause the surrounding rock bearing capacity to decrease, instability and collapse, threatening the safety of construction workers and the stability of the existing tunnel structure.

[0004] 2. Difficulty in quality control: Drilling in soft surrounding rock formations is difficult due to the well-developed joints and fissures in the surrounding rock. It is difficult to drill a 45-meter-long hole with a diameter of Φ180 at a time. During the drilling process, it is difficult to control the hole direction, external insertion angle and grouting quality, which can easily cause the hole direction to be non-parallel, the external insertion angle to be too large or to invade the excavation surface, making it difficult to form an effective shed-like structure and achieving the applied support effect.

[0005] 3. Low construction efficiency: The initial support work of the pipe-roof studio is large, the construction period is long, the construction process of the formwork support system of the cast-in-place concrete guide wall is complicated, the flexibility is poor, and the construction efficiency is low.

[0006] Therefore, there is an urgent need for a new advanced support construction technology that can effectively solve the above problems to ensure the quality and safety of tunnel construction and the stable operation of existing tunnels. Summary of the invention

[0007] The embodiment of the present invention provides a method for advanced support of a large-section soft rock tunnel passing under an existing old tunnel at a close distance, so as to solve the defect that the traditional construction method in the related technology is difficult to ensure the safety, quality and progress of the tunnel construction and the stable operation of the existing tunnel.

[0008] The present invention provides an advance support method for a large-section soft rock tunnel passing under an existing old tunnel at a close distance, comprising: The construction steps of the pipe shed studio are as follows: on the basis of the previous cycle of advance support, the upper and middle steps are excavated and initial concrete is sprayed, steel mesh is laid, steel frame is erected, locking foot anchor pipes and pipe sheds are installed, and radial anchors are drilled to form a pipe shed studio; The construction steps of the concrete guide wall are as follows: when the excavation reaches the preset length of the pipe shed studio, a guide arch frame and a guide steel pipe are set, and at the same time, sprayed concrete is filled and compacted between the guide arch frame and the expansion arch frame to quickly form a sprayed concrete guide wall; The construction steps of super-long and super-large pipe sheds are as follows: while processing pipe sections on site, special machinery, equipment and instruments are used on site to drill, clean, inspect and detect inside the holes. The pipe sections processed on site are connected as a whole and installed into the holes. A water pressure test is carried out before grouting. The test pressure is equal to the final grouting pressure. The grouting adopts double-tube grouting to form an advanced support shed-like structure. The second-layer initial support steps of the pipe-roof studio section include erecting steel frames again on the middle and upper steps, setting up locking foot anchor pipes and pipe-roofs, laying steel mesh, and spraying concrete to fill the space between the two layers of arch frames to form a double-layer initial support structure; The steps of excavation and initial support of the lower step and invert arch are as follows: after the second layer of initial support reaches the design strength, excavation and initial support of the lower step and invert arch are carried out to form a closed-loop structure in time.

[0009] According to one embodiment of the present invention, the construction steps of the pipe-roof studio include: The upper and middle steps are excavated, initial concrete is sprayed, steel mesh is laid, steel frames are erected, locking-foot anchor pipes are installed, and radial anchors are drilled to enhance the stability of the support structure and provide a safe working platform for the construction of extra-long and extra-large pipe sheds.

[0010] According to one embodiment of the present invention, the concrete guide wall construction steps include: A guide arch frame and a guide steel pipe are arranged in the concrete guide wall. The guide arch frame enhances the rigidity of the concrete guide wall. The guide steel pipe ensures the hole direction, hole position and hole spacing. Sprayed concrete is filled and compacted between the guide arch frame and the expanded excavation arch frame to ensure the overall stability and bearing capacity of the concrete guide wall, speed up the construction progress, improve the construction efficiency, and ensure the construction accuracy and quality.

[0011] According to one embodiment of the present invention, while processing the extra-long and extra-large pipe rack, plain shotcrete or anchors, nets, and spraying are used to seal the excavation surface to form an air-stopping curtain and a slurry-stopping wall.

[0012] According to one embodiment of the present invention, the extra-long and extra-large pipe rack is processed into a plurality of segments using hot-pressed seamless steel pipes, and the segments are connected by threaded connections, which facilitates the installation of the pipe rack in a limited space. A steel cage is provided inside the extra-long and extra-large pipe rack to increase the rigidity and bearing capacity of the pipe body, and grouting holes are provided on the segments to ensure the effect of slurry consolidating the surrounding rock.

[0013] According to one embodiment of the present invention, endoscopic in-hole photography is used to conduct lithology analysis to understand the development of joints and fissures in the hole and groundwater, which serves as reference material for geological prediction during tunnel excavation.

[0014] According to one embodiment of the present invention, after drilling, a pipe is lowered and a water pressure test is performed, the test pressure is equal to the final grouting pressure, and then double-tube grouting is used to overcome the negative pressure generated in the pipe, thereby ensuring the grouting pressure, penetration effect and coupling between the pipe wall and the hole wall.

[0015] According to one embodiment of the present invention, a second layer of initial support is added to the pipe-roof studio section, including erecting steel frames on the middle and upper steps again, installing locking feet and anchoring pipes on the pipe-roof, laying steel mesh, and spraying concrete to fill the space between the two layers of arch frames to form a double-layer initial support structure. The design of the double-layer support structure improves the bearing capacity and deformation resistance of the support structure.

[0016] According to one embodiment of the present invention, a closed-loop structure is formed in time through lowering steps, invert excavation and initial support to ensure the overall stability of the initial support structure and control the deformation of the surrounding rock.

[0017] According to one embodiment of the present invention, the super-long and super-large pipe rack construction steps further include: Grouting process; Among them, the grouting pressure meets: p'<p<(3~5)p'; Where, p, final pressure value, MPa; p', hydrostatic pressure, MPa; or, Grouting pressure meets the following requirements: p=KH Where, p, final pressure value, MPa; H, depth of grouting, meter; K, pressure coefficient; The grouting volume meets the following requirements: Q = Anα(1+β); Where, Q, total grouting volume, cubic meters; A, rock volume within the grouting range, cubic meters; n, surrounding rock porosity, %; α, slurry filling coefficient, 0.7-0.9; β, grouting material loss coefficient, 0.1; By controlling the grouting pressure and grouting volume, the grouting effect is ensured and the construction safety and quality are guaranteed.

[0018] The advance support method for a large-section soft rock tunnel under an existing old tunnel at a close distance provided by the embodiment of the present invention greatly enhances the stability and bearing capacity of the excavation surface and the surrounding soil through multi-step and multi-level initial support measures (including initial shotcrete, laying steel mesh, erecting steel frame, setting lock-foot anchor pipe shed and drilling radial anchor rods, etc.), effectively reduces the impact of excavation operations on the existing tunnel, and ensures the structural safety during the construction period and the subsequent operation stage. The setting of the pipe shed studio and the construction of the super-long and super-large pipe shed not only provide effective advance support for subsequent excavation, but also ensure the accuracy and efficiency of the pipe shed construction through the precise control of the guide arch frame and the guide steel pipe. The shotcrete is filled and compacted between the guide arch frame and the expansion arch frame to quickly form a concrete guide wall, which speeds up the overall construction progress. By implementing the second layer of initial support, the overall strength and stability of the support structure are further enhanced, the stress concentration is effectively dispersed, the deformation of the support structure is reduced, and the support effect is improved. At the same time, the design of the two-layer support structure also provides a better working platform and safety guarantee for subsequent construction. The use of a phased and step-by-step excavation and support method can strictly control the exposure time and range of the excavation surface, reduce the disturbance to the surrounding rock below the existing line, reduce the risk of tunnel structure settlement and line deformation caused by construction, and ensure the stable operation of the existing line. Therefore, the advance support method for a large-section soft rock tunnel passing under an existing old tunnel at a close distance provided by the embodiment of the present invention shows significant technical effects in terms of enhancing structural stability, improving construction efficiency and accuracy, optimizing support structure performance, reducing the impact of construction on existing tunnels, achieving controllability of construction quality, and reducing construction costs and environmental impacts. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the technical solutions in the present invention or the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0020] Figure 1 It is a schematic flow chart of the advance support method for a large-section soft rock tunnel passing under an existing old tunnel at a close distance provided by the present invention. DETAILED DESCRIPTION

[0021] The following embodiments of the present invention are described in further detail in conjunction with the accompanying drawings and examples. The following examples are used to illustrate the present invention, but are not intended to limit the scope of the present invention.

[0022] like Figure 1 As shown, the present invention provides an advance support method for a large-section soft rock tunnel passing under an existing old tunnel at a close distance, comprising: Step 10, the construction steps of the pipe shed studio, on the basis of the previous cycle of advance support, excavate the upper step and the middle step, spray concrete, lay steel mesh, erect steel frame, set up the locking foot anchor pipe shed and drill radial anchor rods to form the pipe shed studio; Step 20, concrete guide wall construction step, when excavation reaches the preset length of the pipe shed studio, a guide arch frame and a guide steel pipe are set, and at the same time, spray concrete is filled and compacted between the guide arch frame and the expansion arch frame to quickly form a spray concrete guide wall; Step 30, the construction steps of the super-long and super-large pipe shed, while processing the pipe segments on site, use special machinery, equipment and instruments on site to drill, clean, inspect and detect the holes, connect the pipe segments processed on site into a whole and install them into the hole, and conduct a water pressure test before grouting. The test pressure is equal to the final grouting pressure. The grouting adopts double-tube grouting to form an advanced support shed-like structure; Step 40, the second layer initial support step of the pipe shed studio section, including erecting steel frames again on the middle and upper steps, setting up locking foot anchor pipes and pipe sheds, laying steel mesh, and spraying concrete to fill the space between the two layers of arch frames to form a double-layer initial support structure; Step 50, the step of excavating and initially supporting the lower step and the invert, after the second layer of initial support reaches the designed strength, excavate and apply the initial support of the lower step and the invert to form a closed-loop structure in time.

[0023] The advance support method for a large-section soft rock tunnel under an existing old tunnel at a close distance provided by the embodiment of the present invention greatly enhances the stability and bearing capacity of the excavation surface and the surrounding soil through multi-step and multi-level initial support measures (including initial shotcrete, laying steel mesh, erecting steel frame, setting lock-foot anchor pipe shed and drilling radial anchor rods, etc.), effectively reduces the impact of excavation operations on existing lines, and ensures the structural safety during construction and subsequent operation stages. The setting of the pipe shed studio and the construction of super-long and super-large pipe sheds not only provide effective advance support for subsequent excavation, but also ensure the accuracy and efficiency of pipe shed construction through precise control of the guide arch frame and the guide steel pipe. The shotcrete is filled and compacted between the guide arch frame and the expansion arch frame to quickly form a concrete guide wall, which speeds up the overall construction progress. By implementing the second layer of initial support, the overall strength, rigidity and stability of the support structure are further enhanced, the stress concentration is effectively dispersed, the deformation of the support structure is reduced, and the support effect is improved. At the same time, the design of the two-layer support structure also provides a better working platform and safety guarantee for subsequent construction. The excavation and support method that is carried out in stages and steps can strictly control the exposure time and range of the excavation surface, reduce the disturbance to the surrounding rock below the existing line, reduce the risk of settlement of the existing tunnel structure and deformation of the line caused by construction, and ensure the normal operation of the existing line. Therefore, the advance support method for a large-section soft rock tunnel passing under an existing old tunnel at a close distance provided by an embodiment of the present invention shows significant technical effects in terms of enhancing structural stability, improving construction efficiency and accuracy, optimizing support structure performance, reducing the impact of construction on existing lines, achieving controllability of construction quality, and reducing construction costs and environmental impacts.

[0024] Continue to see Figure 1 The method for advanced support of a large-section soft rock tunnel under an existing old tunnel at a close distance proposed in an embodiment of the present invention is a tunnel construction technology for complex geological conditions and high requirements for existing line protection.

[0025] Construction steps of the pipe shed studio: Under the protection of the advanced support structure of the previous cycle, the upper step (including the arch expansion part) is first excavated. After the excavation is completed, the initial support operation is immediately carried out, including the initial spraying of concrete to quickly close the excavation surface to prevent the surrounding rock from collapsing due to long exposure time; laying steel mesh to enhance the integrity and crack resistance of the support structure; erecting a steel frame as the main bearing structure; setting up a locking foot anchor pipe shed to fix the steel frame to prevent its displacement; drilling radial anchor rods to further reinforce the surrounding rock and improve the support effect.

[0026] According to one embodiment of the present invention, the construction of the pipe-roof studio includes: The upper step (including the expanded arch part) and the middle step were excavated and initially sprayed with concrete, steel mesh was laid, steel frames were erected, locking anchor pipes and pipe sheds were installed, and radial anchors were drilled to enhance the stability of the support structure and create a safe working platform for the pipe shed construction.

[0027] Specifically, in this step, under the advanced support of the previous cycle, the upper step (including the arch excavation part) is excavated, and the initial support around the upper step (including the arch excavation part) is applied, that is, the initial concrete is sprayed, the steel mesh is laid, the steel frame of the excavation part is erected, and the locking foot anchor pipe shed is set up, the radial anchor rod is drilled, and the arch excavation part is backfilled with concrete; After lagging behind the upper step (including the expanded arch part) by a preset distance, the middle step is excavated at a staggered angle to the left and right, and initial support is applied, i.e. initial spraying of concrete, laying of steel mesh, erection of steel frames, installation of locking anchor pipes and sheds, drilling of radial anchors and then spraying of concrete to the designed thickness, and installation of temporary cross braces to close the ring and ensure structural stability.

[0028] Preparation: First, determine the excavation location and size of the pipe-roof studio based on the hydrogeological conditions and the requirements for the configuration of construction machinery and operators. Then, check and prepare the machinery, equipment, personnel and safety protection measures required for excavation.

[0029] Excavation: Under the protection of the previous cycle advance support structure, use appropriate excavation methods (such as step method, CD method, etc.) to excavate the pipe-roof studio. During the excavation process, the shape and size of the excavation surface should be strictly controlled to avoid over-excavation or under-excavation.

[0030] Excavation progress control: The excavation progress should be reasonably arranged according to the actual situation on site and the construction capacity of the support structure to ensure the coordination and timeliness of excavation and support.

[0031] Initial spraying of concrete: After the excavation is completed, the initial spraying of concrete should be carried out immediately. The concrete should be sprayed evenly on the excavation surface to form a concrete layer of a certain thickness to quickly seal the excavation surface and prevent the surrounding rock from being exposed for too long, resulting in weathering, instability and other adverse phenomena.

[0032] Laying steel mesh: Lay steel mesh on the initial shotcrete layer. The steel mesh should fit closely to the excavation surface and be firmly connected to anchor rods, steel frames, etc. by welding or tying to enhance the integrity and crack resistance of the support structure.

[0033] Erecting steel frame: According to the design requirements, erect steel frame under the steel mesh. The steel frame should have good bearing capacity and rigidity, and can effectively support the pressure of the excavation surface and the surrounding rock above it. The steel frames should be connected by connectors to form a stable overall support system.

[0034] Locking anchor pipes are installed at the bottom of the steel frame. The locking anchor pipes should be inserted into the surrounding rock to a certain depth and be tightly combined with the surrounding rock through grouting and other methods. The main function of the locking anchor pipe is to fix the steel frame to prevent it from displacement or overturning due to the pressure of the surrounding rock.

[0035] Drilling radial anchors: Drill a certain number of radial anchors on the excavation surface and its periphery. The anchors should penetrate the surrounding rock and penetrate a certain depth inside it, and be tightly combined with the surrounding rock through grouting and other methods. The main function of the radial anchors is to reinforce the surrounding rock, fix the key blocks of the surrounding rock, prevent the expansion of the loose circle, and improve the bearing capacity and stability of the support structure.

[0036] Through the implementation of the above steps, the pipe-roof studio construction steps can form an effective support barrier, protect the stability and safety of the tunnel excavation surface, and provide a safe working platform for subsequent construction.

[0037] Construction steps of shotcrete guide wall: When excavation reaches the preset length of the pipe shed studio, first set up the guide arch and guide steel pipe to ensure the accurate construction of the extra-long and extra-large pipe shed. Then, use the guide steel pipe to construct the extra-long and extra-large pipe shed to provide strong advance support for subsequent excavation. At the same time, the shotcrete is filled and compacted in the gap between the guide arch and the arch of the expansion part to ensure the overall stability and bearing capacity of the concrete guide wall. The longitudinal length, thickness, circumferential length, bearing capacity of the foundation of the shotcrete guide wall and the depth of the embedded working platform must meet the design requirements.

[0038] Setting of guide arch and guide steel pipe: When excavating to the length of the pipe shed studio, firstly, according to the design requirements, three I18 guide arches (1.0m spacing) and guide steel pipes with a length of 2m / root and a diameter of 200mm are accurately set to ensure the accuracy of the pipe shed construction. The guide steel pipe is set on the guide arch. The guide steel pipe is used as a guide tool for extra-long and extra-large pipe sheds. Its installation accuracy directly affects the construction quality of the pipe shed.

[0039] The gap between the guide arch and the expansion arch is filled with sprayed concrete. The key to this step is to ensure that the concrete can fully fill the gap between the arches to form an overall stable concrete guide wall annular support structure.

[0040] In the concrete guide wall construction step, the embodiment of the present invention particularly emphasizes the importance of filling and compacting the space between the guide arch frame and the expansion arch frame by spraying concrete.

[0041] Before spraying concrete, the spraying surface needs to be cleaned to ensure that there are no loose rocks and loose soil. During the spraying process, the spraying pressure, spraying angle and spraying distance should be controlled to ensure that the concrete can be evenly and densely attached to the arch frame.

[0042] The sprayed concrete filling and compacting between the guide arch and the expansion arch can enhance the overall stiffness and bearing capacity of the support structure, and improve the overall stability and bearing capacity of the concrete guide wall. This helps to resist the surrounding rock pressure and other external loads during the subsequent construction process. Shotcrete can also effectively close the gaps between the arches to prevent groundwater and other fluids from penetrating into the support structure, thereby maintaining the stability of the support structure. This helps to reduce the risk of surrounding rock deformation and support structure failure caused by seepage. Using sprayed concrete for filling and compacting can save construction time and labor costs. At the same time, sprayed concrete has good early strength and can quickly form a stable support structure, providing strong guarantee for subsequent construction. The wet sprayed concrete construction process produces less dust, which is conducive to improving the construction environment and reducing the health impact on construction workers. In addition, sprayed concrete can also cover the exposed rock surface and reduce the occurrence of weathering and erosion.

[0043] In summary, the embodiments of the present invention can significantly improve the overall stability and bearing capacity of the concrete guide wall by spraying concrete to fill the space between the guide arch frame and the expansion arch frame during the concrete guide wall construction step, while improving construction efficiency and working environment.

[0044] Construction steps for super long and super large pipe shed: According to one embodiment of the present invention, the construction of super-long and super-large pipe sheds uses steel pipes as guides and uses pipe shed drilling rigs to drill holes for super-long and super-large pipe sheds. The construction methods of pipe sheds mainly include pipe shed lead-hole top-entry method and pipe shed follow-pipe drilling method. The specific method to be selected must be determined according to the on-site geological conditions and construction conditions.

[0045] The steel pipes of the pipe-roof are driven into the rock surface at a small external insertion angle to form a simply supported beam structure to bear the pressure of the surrounding rock. The diameter and center distance of the steel pipes need to be determined according to the design requirements and geological conditions. Generally, the diameter of the steel pipes is 80-180mm and the center distance is 30-50cm.

[0046] According to one embodiment of the present invention, the extra-long and extra-large pipe shed is processed into several segments using hot-pressed seamless steel pipes, and the segments are connected by threads. A steel cage is penetrated inside the extra-long and extra-large pipe shed, and grouting holes are drilled on the segments.

[0047] In the embodiment of the present invention, the super-long and super-large pipe rack adopts hot-pressed seamless steel pipe with an outer diameter of 159 mm and a wall thickness of 10 mm. The 45-meter-long pipe rack is processed in sections, and the section lengths are 4 meters and 6 meters respectively. The sections are connected by threaded connection, and the joints of adjacent steel pipes are staggered front and back. The number of joints in the same cross section is not more than 50%, and the joints of adjacent steel pipes are staggered by at least 1 meter. The odd-numbered hole steel flower pipe is arranged as (4+6+6+6+6+6+6+6) meters, and the even-numbered hole steel flower pipe is arranged as (6+6+6+6+6+6+6+4) meters. To ensure a firm connection and good bearing capacity, the length of the thread should not be less than 15 cm. The thread is a 2 mm spiral square wire. One end of the steel pipe is an inner thread and the other end is an outer thread. The thread must be processed straight and must not be misaligned. Grouting holes with a diameter of 5 to 8 mm are drilled on the pipe wall. The spacing between the grouting holes is 20 to 30 cm and arranged in a plum blossom shape. No holes are drilled within 4 to 6 meters of the hole mouth as a grouting stop section. The tail thread of the last section of the jacking pipe in each hole is connected to the short section of the jacking tool pipe. The front end of the first section of the pipe in each hole is compressed into a cone with a length of 10 cm.

[0048] In order to increase the rigidity and bearing capacity of the pipe body, a steel cage is added in the pipe shed. The steel cage is composed of four main bars and fixed rings welded together. The main bars are 4 threaded steel bars with a diameter of 18 mm. The fixed rings are made of short pipe sections with a diameter of 42 mm, a wall thickness of 3.5 mm, and a length of 5 cm welded to the main bars. The spacing between the fixed rings is 1 meter.

[0049] According to one embodiment of the present invention, in the processing of super-long and super-large pipe racks, plain shotcrete or anchors, nets, and spraying are used to seal the excavation surface, the drill rod is used in conjunction with the drill bit to repeatedly clean the hole, and an endoscope is used to take in-hole photos for lithology analysis. After the hole is formed, the pipe is lowered and a water pressure test is carried out. The test pressure is equal to the final grouting pressure, and then double-tube grouting is used.

[0050] Specifically, before drilling, in order to ensure the stability of the excavation surface and the safety of construction, plain shotcrete or anchors, nets, and spraying are used to seal the excavation surface, while forming a slurry-stopping wall and an air-stopping curtain to prevent slag discharge difficulties during hole cleaning and slurry leakage during grouting.

[0051] In order to facilitate the lowering of the pipe, the hole diameter is 2 cm larger than the pipe body diameter. The drill bit uses a concentric drill bit with a diameter of 180 mm. To prevent the drill bit from falling, the drill rod rigidity and diameter are increased. The drill rod diameter is 168 mm. To prevent the surrounding rock from softening and disintegrating when exposed to water, the drilling process is dry drilling; the external insertion angle is 1~3 degrees (excluding the longitudinal slope), and it is minimized as much as possible without invading the tunnel excavation contour line, ensuring that all holes are located on the same arc surface, forming a shed-like structure together. It is set within a range of 150 degrees along the arch, with a circumferential spacing of 40 cm, a single length of 45 meters, and a total of 52 per ring.

[0052] During the drilling process, a guide tube is used to control the direction of the hole, and a laser guide instrument is used to check the direction of the hole. A 1-meter-long digital slope ruler is used to control the external insertion angle of the drill rod. If the hole direction and straightness are found to be deviated, they will be corrected in time. Holes that cannot be directly corrected are defined as waste holes, and waste holes are drilled again after grouting backfill.

[0053] The drilling sequence is from the arch to the two sides and from the high hole position to the bottom hole position. Due to the small distance between the pipe racks, pressure relief and air leakage during the drilling process and grouting during the grouting process are prevented. The construction is carried out in a "skip hole" manner, with "drilling one hole at a time" being the first step.

[0054] When the drilling rig starts drilling, the drilling speed and air pressure are low. After the hole is 10 meters deep, the drilling speed and air pressure are gradually adjusted according to the geological conditions.

[0055] Use the drill rod and drill bit to clean the hole repeatedly, and use high-pressure air to remove scum from the bottom of the hole to the hole mouth to ensure that the hole diameter and hole depth meet the requirements and prevent hole blockage and drill jam.

[0056] An endoscope is used to take in-hole video of the drilled hole, and the images are used to determine whether the hole wall has collapsed or is straight, providing a basis for lowering the pipe. The images are used for lithology analysis to determine the development of joints and fissures in the hole and groundwater, which are used as reference data for geological prediction and forecasting during tunnel excavation, thereby guiding the tunnel excavation.

[0057] For in-hole video recording, a PVC tube is used to slowly push the high-definition camera to the bottom of the hole. To ensure that the camera is pushed to the bottom of the hole smoothly, a pulley trolley is installed at the front end of the camera during the pushing process, and the camera connecting wires are tied to the PVC tube with tape. To prevent the camera from being contaminated by dust, mud and water in the hole during the pushing process, which will affect the shooting effect, a plastic bottle is used as a temporary protective cover, and then the camera is pulled out evenly and slowly from the bottom of the hole to the hole mouth, and photos and videos are taken during the pulling process to ensure a clear image.

[0058] After the hole is formed, the pipe is put down immediately. The threaded pipe is tightened with a matching special pipe clamp, and then pushed in with an excavator or a drill through the tool pipe. Before grouting, a steel cage, an exhaust pipe and a grouting pipe are put into the pipe. The grouting pipe and the exhaust pipe are both PE pipes with a diameter of 20 mm. In order to overcome the negative pressure generated during the grouting process, an exhaust pipe is set on the upper part of the pipe, and the exhaust pipe is tied to the steel cage. A grouting pipe is set at the lower part of the pipe, and both the grouting pipe and the exhaust pipe extend to the bottom of the hole. The pipe mouth is welded with a flange and a fast high-pressure grouting joint, a grouting stop valve and an exhaust hole are set. The exhaust pipe and the exhaust hole are sealed with hot melt sealant.

[0059] Before grouting, use anchoring agent to seal around the hole, and spray concrete to seal the excavation surface for the second time. If necessary, use anchor, net, and spray to seal, especially the weak initial support between the guide arch and the excavation surface. To ensure the grouting pressure, penetration effect and coupling degree between the pipe wall and the hole wall, the grouting pipeline should be moistened before grouting to check whether there is leakage or air leakage, and timely block it. A water pressure test should be carried out before grouting, and water should be injected into the pipe using a high-pressure grouting pump; The grouting machine uses a piston-type variable-flow high-pressure grouting pump with a maximum working pressure of no less than 15 MPa, a main motor power of no less than 22 kW, and a high-pressure grouting pipe with a pressure resistance of no less than 25 MPa; The grouting liquid in general areas adopts cement single liquid slurry, and the cement grade is PO42.5 or above; the water-cement ratio of the slurry is 0.5:1~1:1, and the grouting pressure is generally 1.0~2.0 MPa. The specific slurry mix ratio and grouting pressure are determined by the on-site test according to the hydrogeological conditions. The water-cement ratio can be slightly larger at the beginning, and then gradually decrease. The initial concentration of grouting is determined according to the grouting amount; in areas with broken surrounding rocks and developed groundwater, cement-water glass double liquid grouting is adopted. The water glass is 40°Be, and the water-cement ratio is 0.8:1~1:1; the cement: water glass slurry is 1:0.8.

[0060] The end standard of single-hole grouting is that the grouting pressure gradually increases, and when the designed final pressure is reached and grouting continues for more than 10 minutes; the grouting volume is not less than 80% of the designed volume, the grouting speed is 1 / 4 of the initial grouting speed, and the grouting volume at the end of grouting is generally below 20 to 30 liters per minute.

[0061] According to one embodiment of the present invention, the construction steps of the super-long and super-large pipe shed for the advance support method of a large-section soft rock tunnel under an existing old tunnel at a close distance also include: Extra-long and extra-large pipe shed grouting technology; Among them, the grouting pressure meets: p'<p<(3~5)p'; Where, p, final pressure value, MPa; p', hydrostatic pressure, MPa; or, Grouting pressure meets the following requirements: p = KH; Where, p is the final pressure value, MPa; H is the depth of grouting, meter; K is the pressure coefficient.

[0062] In the process of tunnel excavation and initial support, drilling, hole cleaning, pipe laying and grouting are key steps to ensure tunnel stability and safety. Among these steps, the control of grouting volume and grouting pressure is particularly important because it directly affects the grouting effect and the performance of the tunnel support structure.

[0063] Grouting pressure control solution 1: First, according to the given grouting pressure range formula: p′ <p<(3∼5)p′; Where p is the final grouting pressure (in MPa) and p′ is the hydrostatic pressure (in MPa).

[0064] Hydrostatic pressure p′: This is the pressure generated by groundwater at the tunnel excavation face or grouting location, which usually needs to be determined through on-site measurements or based on geological survey reports.

[0065] Grouting final pressure p: The pressure value that needs to be reached and maintained during the grouting process to ensure that the slurry can fully fill the cracks and pores and achieve the expected reinforcement effect.

[0066] According to this formula, the final grouting pressure p should be greater than the hydrostatic pressure p', but should not exceed 3 to 5 times the hydrostatic pressure. This range is selected to avoid excessive grouting pressure leading to formation damage or slurry loss, while ensuring the grouting effect.

[0067] Grouting pressure control scheme 2: Another grouting pressure control scheme is based on the formula of grouting depth and pressure coefficient: p = KH; Among them, p is the final pressure value of grouting (unit: MPa), H is the depth of grouting (unit: meter), and K is the pressure coefficient, which can be selected by referring to the table below.

[0068] Grouting depth H: This is the depth of the grouting hole or grouting pipe below the tunnel excavation surface, which needs to be determined based on specific design requirements and geological conditions.

[0069] Pressure coefficient K: This is an empirical value or design parameter used to adjust the grouting pressure to suit different geological conditions and engineering requirements. The size of the pressure coefficient usually needs to be determined based on experiments or experience from similar projects. Please refer to the table below for selection.

[0070] When using this formula, it is necessary to first determine the grouting depth H and the pressure coefficient K, and then calculate the final grouting pressure p. This method more directly considers the effect of the grouting position on the grouting pressure, which helps to achieve more accurate grouting control.

[0071] According to one embodiment of the present invention, the grouting volume of the super-long and super-large pipe rack meets the following requirements: Q = Anα(1+β); In the formula, Q is the total grouting volume, cubic meter; A is the rock volume within the grouting range, cubic meter; n is the porosity of the surrounding rock, %; α is the slurry filling coefficient, 0.7~0.9; β is the grouting material loss coefficient, 0.1.

[0072] In tunnel engineering, accurate calculation of grouting volume is crucial to ensure grouting effect and tunnel stability. According to one embodiment of the present invention, the calculation formula of grouting volume is: Q = Anα(1+β); The meaning of each parameter is as follows: Q: total grouting volume, in cubic meters; A: rock volume within the grouting range, in cubic meters; n: surrounding rock porosity, in percentage (%); α: slurry filling coefficient, ranging from 0.7 to 0.9; β: grouting material loss coefficient, with a value of 0.1, which indicates the proportion of material loss caused by factors such as pipeline friction and slurry overflow during the grouting process.

[0073] Calculation steps: Determine the rock volume A within the grouting range: This usually needs to be determined based on the tunnel design drawings, geological survey reports and actual excavation conditions.

[0074] Determine the surrounding rock porosity n: Porosity can be estimated through laboratory measurements, geological surveys, or empirical data from similar projects.

[0075] Determine the slurry filling coefficient α: According to the properties of the slurry, grouting process and engineering requirements, select an appropriate value within the range of 0.7 to 0.9.

[0076] Determine the grouting material loss coefficient β: In this embodiment, β=0.1 is given, which means that 10% of the grouting material will be lost during the grouting process.

[0077] Substitute the formula to calculate the total grouting volume Q: Q=A⋅n⋅α⋅(1+β).

[0078] Second layer initial support steps of the pipe shed studio section: In order to further enhance the support effect, the second layer initial support is carried out. This step includes erecting the steel frame again, setting the locking foot anchor pipe shed, laying the steel mesh, and spraying concrete to fill the space between the two layers of arch frames. The design of the double-layer support structure improves the bearing capacity and deformation resistance of the support structure.

[0079] Steps for excavation and initial support of lower steps and inverts: After the second layer of initial support arch is completed and reaches the preset strength, the lower steps and inverts are excavated with staggered positions on the left and right. Initial support is performed by spraying concrete, laying steel mesh, erecting side walls and invert steel frames, and installing locking anchor pipes and sheds. By quickly closing the ring, the overall stability of the initial support structure is ensured and deformation of the surrounding rock is controlled.

[0080] The embodiment of the present invention provides a method for advanced support of a large-section soft rock tunnel under an existing old tunnel at a close distance. The method effectively enhances the stability of the excavation surface, prevents excessive deformation or instability of the surrounding rock, and provides safety guarantee for subsequent construction operations by timely applying initial support in each construction cycle, especially focusing on the upper step (including the arch expansion part) and the middle step of the pipe-roof studio. The middle and lower steps adopt a left-right staggered excavation method, which is not only conducive to controlling the stress release during the excavation process and reducing the disturbance to the surrounding rock mass, but also can quickly support after excavation to ensure that the excavation surface is always in a controlled state. This step-by-step implementation strategy improves the safety and efficiency of construction. When the excavation reaches the specified length of the pipe-roof studio, by setting a guide arch frame and a guide steel pipe and spraying concrete to fill it densely, and then applying an extra-long and extra-large pipe shed, this step not only ensures the accurate position and direction of the pipe shed, but also further enhances the stability in front of the excavation surface through the advance pre-support effect of the pipe shed, creating favorable conditions for subsequent excavation operations. By constructing the second layer of initial support for the pipe-roof studio, and continuing to excavate and construct initial support for the lower steps and inverts after reaching the preset strength, a multi-level, all-round support system was constructed. This system can effectively disperse and bear the surrounding rock pressure, and improve the bearing capacity and stability of the overall structure. The entire construction process followed the cyclic operation mode of "excavating one section, supporting one section, and then excavating the next section", which not only ensured the construction progress, but also ensured the construction quality. Through refined construction organization and strict quality control measures, efficient and high-quality underpass tunnel excavation and support operations were achieved.

[0081] The embodiment of the present invention provides an advance support method for a large-section soft rock tunnel passing under an existing old tunnel at a close distance. Through the application of a multi-layer support structure and advance support measures, the disturbance to the surrounding rock during tunnel excavation and the risk of deformation of the existing tunnel structure are effectively reduced, thereby improving the safety during the construction process.

[0082] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for advanced support of a large-section soft rock tunnel under an existing old tunnel at a close distance, characterized in that: include: The construction steps of the pipe shed studio are as follows: on the basis of the previous cycle of advance support, the upper and middle steps are excavated and initial concrete is sprayed, steel mesh is laid, steel frame is erected, locking foot anchor pipes and pipe sheds are installed, and radial anchors are drilled to form a pipe shed studio; The construction steps of the concrete guide wall are as follows: when the excavation reaches the preset length of the pipe shed studio, a guide arch frame and a guide steel pipe are set, and at the same time, sprayed concrete is filled and compacted between the guide arch frame and the expansion arch frame to quickly form a sprayed concrete guide wall; The construction steps of super-long and super-large pipe sheds are as follows: while processing pipe sections on site, special machinery, equipment and instruments are used on site to drill, clean, inspect and detect inside the holes. The pipe sections processed on site are connected as a whole and installed into the holes. A water pressure test is carried out before grouting. The test pressure is equal to the final grouting pressure. The grouting adopts double-tube grouting to form an advanced support shed-like structure. The second-layer initial support steps of the pipe-roof studio section include erecting steel frames again on the middle and upper steps, setting up locking foot anchor pipes and pipe-roofs, laying steel mesh, and spraying concrete to fill the space between the two layers of arch frames to form a double-layer initial support structure; The steps of excavation and initial support of the lower step and invert arch are as follows: after the second layer of initial support reaches the design strength, excavation and initial support of the lower step and invert arch are carried out to form a closed-loop structure in time.

2. The method for advanced support of a large-section soft rock tunnel under an existing old tunnel at a close distance according to claim 1 is characterized in that: The construction steps of the pipe-roof studio include: The upper and middle steps are excavated, initial concrete is sprayed, steel mesh is laid, steel frames are erected, locking-foot anchor pipes are installed, and radial anchors are drilled to enhance the stability of the support structure and provide a safe working platform for the construction of extra-long and extra-large pipe sheds.

3. The method for advanced support of a large-section soft rock tunnel under an existing old tunnel at a close distance according to claim 1 is characterized in that: The concrete guide wall construction steps include: A guide arch frame and a guide steel pipe are arranged in the concrete guide wall. The guide arch frame enhances the rigidity of the concrete guide wall. The guide steel pipe ensures the hole direction, hole position and hole spacing. Sprayed concrete is filled and compacted between the guide arch frame and the expanded excavation arch frame to ensure the overall stability and bearing capacity of the concrete guide wall, speed up the construction progress, improve the construction efficiency, and ensure the construction accuracy and quality.

4. The method for advanced support of a large-section soft rock tunnel under an existing old tunnel at a close distance according to claim 1 is characterized in that: While processing the extra-long and extra-large pipe shed, plain shotcrete or anchors, nets and spraying are used to seal the excavation surface to form an air-stopping curtain and a slurry-stopping wall.

5. The method for advanced support of a large-section soft rock tunnel under an existing old tunnel at a close distance according to claim 1 is characterized in that: The extra-long and extra-large pipe rack is made of hot-pressed seamless steel pipes into several sections, and the sections are connected by threads, which facilitates the installation of the pipe rack in a limited space. A steel cage is penetrated inside the extra-long and extra-large pipe rack to increase the rigidity and bearing capacity of the pipe body. Grouting holes are opened on the sections to ensure the effect of slurry consolidating the surrounding rock.

6. The method for advanced support of a large-section soft rock tunnel under an existing old tunnel at a close distance according to claim 1 is characterized in that: Endoscopic in-hole photography is used to conduct lithology analysis in order to understand the development of joints and fissures in the hole and groundwater, which serves as a reference for geological prediction during tunnel excavation.

7. The method for advanced support of a large-section soft rock tunnel under an existing old tunnel at a close distance according to claim 1 is characterized in that: After drilling, pipes are lowered and a water pressure test is carried out. The test pressure is equal to the final grouting pressure. Double-tube grouting is then used to overcome the negative pressure generated in the pipe and ensure the grouting pressure, penetration effect and coupling between the pipe wall and the hole wall.

8. The method for advanced support of a large-section soft rock tunnel under an existing old tunnel at a close distance according to claim 1 is characterized in that: A second layer of initial support is added to the pipe-roof studio section, including erecting steel frames on the middle and upper steps again, installing locking-foot anchor pipes on the pipe-roof, laying steel mesh, and spraying concrete to fill the space between the two arch frames to form a double-layer initial support structure. The design of the double-layer support structure improves the bearing capacity and deformation resistance of the support structure.

9. The method for advanced support of a large-section soft rock tunnel under an existing old tunnel at a close distance according to claim 1 is characterized in that: Through lower steps, invert excavation and initial support, a closed-loop structure is formed in time to ensure the overall stability of the primary support structure and control the deformation of the surrounding rock.

10. The method for advanced support of a large-section soft rock tunnel under an existing old tunnel at a close distance according to any one of claims 1 to 8, characterized in that: The super-long and super-large pipe shed construction steps also include: Grouting process; Among them, the grouting pressure meets: p'<p<(3~5)p'; Where, p, final pressure value, MPa; p', hydrostatic pressure, MPa; or, Grouting pressure meets the following requirements: p=KH Where, p, final pressure value, MPa; H, depth of grouting, meter; K, pressure coefficient; The grouting volume meets the following requirements: Q = Anα(1+β); Where, Q, total grouting volume, cubic meters; A, rock volume within the grouting range, cubic meters; n, surrounding rock porosity, %; α, slurry filling coefficient, 0.7-0.9; β, grouting material loss coefficient, 0.1; By controlling the grouting pressure and grouting volume, the grouting effect is ensured and the construction safety and quality are guaranteed.

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