Combined construction method of ultra-shallow-buried miscellaneous fill subsurface tunnel for rail transit

Abstract

The invention relates to the technical field of tunnel excavation, in particular to an ultra-shallow-buried miscellaneous fill subsurface excavation tunnel combined construction method for rail transit, which is characterized in that the range and scale of a shallow-buried miscellaneous fill area are preliminarily locked through early-stage work such as geological survey, historical data analysis and surrounding environment visit investigation before construction; and when excavation is performed to a range close to the shallow-buried miscellaneous fill area, the scale of the shallow-buried miscellaneous fill area is detected in detail by adopting multiple advanced geological forecast means such as transient electromagnetism and advanced drilling, the engineering geological condition of the shallow-buried miscellaneous fill area is mastered comprehensively, and a detailed and reliable basis is provided for formulating excavation measures of the shallow-buried miscellaneous fill area.

Description

Technical Field

[0001] The present invention relates to the technical field of tunnel excavation, in particular to a combined construction method for an ultra-shallow buried mixed fill dark-excavated tunnel for rail transportation. Background Art

[0002] Chongqing Rail Transit crosses mountains. Due to the long route, varying tunnel depths, and complex geological conditions, some sections of tunnel construction generally utilize the mining method. In this region, mining-based tunnels traverse mountains with a wide variety of surrounding rock types and grades, and also contain areas of ultra-shallow miscellaneous fill. Ultra-shallow miscellaneous fill areas are characterized by loose soil, complex composition, and poor self-stabilization. Mining-based tunnels traversing these areas are prone to excessive surface subsidence, collapse, roof falls, or severe water leaks, posing a high safety risk. Therefore, exploring the construction technology for mining-based tunnels traversing ultra-shallow miscellaneous fill areas is crucial to ensuring both a safe and reliable construction process and meeting high construction quality standards and stability requirements to ensure safe line operation. Through continuous exploration, deduction and a large number of field tests, the "Combined Construction Method of Ultra-Shallow Buried Mixed Fill Tunneling for Rail Transit" was summarized and formed, which effectively solved the construction safety and quality problems of the mining method crossing the ultra-shallow buried mixed fill area, thereby ensuring the quality of the project, accelerating the progress of the project, saving construction investment to a large extent, and significantly improving economic benefits. It has high promotion value and application prospects, and has guiding significance for similar projects.

[0003] The advanced geological forecast of this invention is based on preliminary work such as geological surveys, historical data analysis, and visits to the surrounding environment to initially determine the scope and scale of the shallow miscellaneous fill area before construction. When excavation approaches the shallow miscellaneous fill area, various advanced geological forecasting methods such as transient electromagnetic and advanced drilling are used to conduct detailed exploration of the shallow miscellaneous fill area's scale. This allows for a comprehensive understanding of the engineering geology of the shallow miscellaneous fill area, providing a detailed and reliable basis for formulating excavation measures for the shallow miscellaneous fill area. Before tunnel excavation in the shallow miscellaneous fill area, the backfill layer above the tunnel is sloped and excavated to the tunnel top elevation with a slope ratio of 1:1.5. The slope is supported with 100mm thick early-strength C25 shotcrete, a single layer of φ8@200x200 mesh, and φ18 anchors. The tunnel top is constructed with an 800mm-thick C30 steel-concrete arched cover slab, constructed with I20 I-beams spaced 0.5 meters apart. The concrete strength is C30, and the arch guard is 800mm thick. Expanded foundations are used at both ends of the arch guard, with a thickness of 1310mm and a length of 700mm. Once the cover slab reaches the design strength, it is backfilled with soil, and then excavated from within the tunnel using a step-by-step method. Each excavation advance is controlled at 1.0m. After each section is excavated, initial support is immediately applied, including steel frames, steel mesh, and shotcrete, to complete the loop. The principle of this invention can be summarized as a systematic construction method of "advanced geological prediction + open-cut unloading + arch guard installation on the arch top + modified tunnel excavation methods." This method offers simple construction, strong operability, minimal safety risks, and reasonable economics. Summary of the Invention

[0004] Technical problems to be solved: Through preliminary work such as geological surveys, historical data analysis, and visits to the surrounding environment, the scope and scale of the shallow buried miscellaneous fill area were preliminarily determined. When excavation was close to the shallow buried miscellaneous fill area, various advanced geological prediction methods such as transient electromagnetic and advanced drilling were used to conduct detailed detection of the scale of the shallow buried miscellaneous fill area, and to fully understand the engineering geological conditions of the shallow buried miscellaneous fill area, providing a detailed and reliable basis for formulating excavation measures in the shallow buried miscellaneous fill area.

[0005] In view of the shortcomings of the existing technology, the present invention provides a combined construction method for an ultra-shallow buried mixed fill dark-excavated tunnel for rail transit, thereby solving the technical problems mentioned in the background technology.

[0006] To achieve the above objectives, the present invention is implemented through the following technical solutions: The combined construction method for an ultra-shallow mixed fill tunnel excavation for rail transit includes the following steps: a) Before tunnel excavation, a combination of transient electromagnetic (TEM) and advanced horizontal drilling is used to conduct advanced geological prediction to identify the spatial location and engineering characteristics of miscellaneous fill sections with a cover depth of ≤2.3 m, and to establish a three-dimensional geological risk model; b) Based on the model, the miscellaneous fill above the tunnel was unloaded by open excavation with a slope ratio of 1:1.5, and supported as it was excavated: a combined slope support system was formed using 100 mm thick early-strength C25 shotcrete, steel mesh, and mortar anchors; c) At the bottom of the open excavation, a steel-concrete arch cover is cast in situ along the tunnel axis. The I-steel arch frames in the arch cover are spaced 0.5 m apart in the longitudinal direction, with expanded foundations at both ends and connected to the steel arch frames with pre-buried high-strength bolts to form a whole; d) After the concrete strength of the cover slab reaches 100% of the design strength, backfill the soil symmetrically and proceed to the tunnel using the step method of "upper step height ≤ 6 m, lower step height ≤ 3 m, and advance per cycle ≤ 1 m"; e) The primary support in the tunnel shall be supported by a combination of I20b steel frame, hollow grouting anchor, steel mesh + C25 shotcrete, and the secondary concrete lining shall be immediately applied after the primary support is closed into a ring; f) During the entire construction process, real-time monitoring of ground settlement, vault subsidence and clearance convergence is carried out. When the daily deformation rate is greater than 1 mm or the cumulative deformation rate is greater than 10 mm, an early warning is automatically triggered and emergency reinforcement measures are implemented.

[0007] Furthermore, the refined identification in step a) includes: transient electromagnetic method prediction depth of 80 m and front and back overlap of 10 m, advanced horizontal drilling drilling depth of 35 m and front and back overlap of 8 m, and the data of the two methods are integrated through the BIM platform to generate a three-dimensional cloud map of the thickness, strength and permeability coefficient of the miscellaneous fill with a resolution of ≤1 m.

[0008] Furthermore, the slope opening and open excavation unloading in step b) adopts a layered reverse method, with the excavation thickness of each fill layer ≤2m, the rock layer ≤3m, and the width of each step ≥3m. A 0.3m bottom is reserved for manual cleaning at the base, and cement soil is used for replacement when the bearing capacity is insufficient.

[0009] Furthermore, the steel-concrete arched cover plate in step c) adopts double-layer bidirectional φ22 main reinforcement @150 mm, φ12 distribution reinforcement @200 mm, a protective layer of 40 mm, and concrete pads arranged in a plum blossom shape @1 m×1 m to ensure thickness.

[0010] Furthermore, the length of the I20b I-beam arch frame segment in step c) is 2 m, and adjacent segments are connected using 10 mm thick Q235 connecting plates + M24 high-strength bolts, with a bolt torque of not less than 300 N·m. A φ25 locking anchor rod is provided at the arch foot, with an incident angle of 30° to 40°.

[0011] Furthermore, the backfilling and covering process in step d) is as follows: low-permeability clay is used for layered compaction in the area 0.8 m above the arch, with a compaction degree ≥ 90%; graded crushed stone is used for layered compaction in the remaining area, with a compaction degree ≥ 93%, and the height difference of the backfill on both sides is ≤ 0.5 m.

[0012] Furthermore, in step e), the core soil is reserved for excavation using the step method in the tunnel. The upper step advances 0.8-1.0 m per cycle, the lower step lags behind the upper step by 3-5 m, and the core soil on the upper step is retained with a width of ≥3 m to control settlement.

[0013] Furthermore, the secondary lining in step e) uses a 12 m self-propelled hydraulic steel formwork trolley, the formwork is 10 mm thick steel plate + 12# channel steel back rib, the hydraulic cylinder positioning accuracy is ±3 mm, the lining concrete anti-permeability grade is P10 and is mixed with 0.9 kg / m³ polypropylene fiber.

[0014] Furthermore, the real-time monitoring in step f) transmits data to the cloud platform in real time through the wireless sensor network, and the mobile phone APP immediately pushes warning information, and the monitoring frequency is increased to 1-2 times / day during the abnormal deformation stage.

[0015] Furthermore, the emergency reinforcement measures in step f) include: immediately sealing the tunnel face with 200 mm thick C25 sprayed concrete + steel mesh, applying a φ42 advance grouting small pipe @300 mm×3.5 m to inject cement-water glass dual-liquid slurry, and if necessary, backfilling with sand bags with a height of ≥2 m and adding temporary diagonal bracing.

[0016] Beneficial effects compared with existing technologies: 1. The advanced geological forecast of the present invention is to preliminarily lock the scope and scale of the shallow buried miscellaneous fill area through geological survey, historical data analysis, surrounding environment investigation and other preliminary work before construction. When excavation is close to the shallow buried miscellaneous fill area, transient electromagnetic, advanced drilling and other advanced geological forecasting methods are used to conduct detailed detection of the scale of the shallow buried miscellaneous fill area, so as to fully grasp the engineering geological conditions of the shallow buried miscellaneous fill area and provide a detailed and reliable basis for formulating excavation measures in the shallow buried miscellaneous fill area.

[0017] 2. The method of the present invention is simple to construct, highly operable, has low safety risks, and is economical and reasonable. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The above description is only an overview of the technical solution of the present invention. In order to more clearly understand the technical means of the present invention and implement it according to the contents of the specification, the following is a detailed description of the preferred embodiments of the present invention with reference to the accompanying drawings.

[0019] Figure 1 This is a flow chart of the tunnel excavation construction process in an ultra-shallow buried mixed fill area according to the present invention; Figure 2 is a regional map of the ultra-shallow buried section and the miscellaneous fill section of the present application; Figure 3 is a cross section map of the layered support excavation of the upper earthwork of the present application; Figure 4 is a large-scale drawing of the steel arch frame of the present application; Figure 5 is a large-scale drawing of the steel arch frame of the present application; Figure 6 is a cross section map of the layered support excavation of the upper earthwork of the present application; Figure 7 is a cross section map of the layered support excavation of the upper earthwork of the present application; Figure 8 is a cross section map of the layered support excavation of the upper earthwork of the present application; Figure 9 is a cross section map of the layered support excavation of the upper earthwork of the present application; DETAILED DESCRIPTION

[0020] The preferred embodiments of the present application will be described in detail by referring to the attached drawings, however the present application can be realized in various different forms, and therefore the present application is not limited to the embodiments described below; The technical solution in the embodiments of the present application is to solve the problems in the above background art, and the general idea is as follows: As Figures 1 to 9As shown, the present invention adopts a systematic construction method of "advanced geological prediction + open excavation unloading + arch protection + changing the excavation method inside the tunnel", which effectively reduces the construction safety risk and provides reliable guarantees for the safety, quality and progress of excavation construction of mining-based tunnels crossing ultra-shallow miscellaneous fill areas. The advanced geological prediction of the present invention is to preliminarily lock the scope and scale of the shallow miscellaneous fill area through preliminary work such as geological survey, historical data analysis, and surrounding environment visits and investigations before construction. When excavating to the ultra-shallow miscellaneous fill area, a variety of advanced geological prediction methods such as transient electromagnetic and advanced drilling are used to conduct detailed detection of the scale of the shallow miscellaneous fill area, comprehensively grasp the engineering geological conditions of the shallow miscellaneous fill area, and provide a detailed and reliable basis for formulating excavation measures for the shallow miscellaneous fill area. In shallow buried miscellaneous fill areas, the present invention excavates the backfill soil layer above the shallow buried section of the dark-dug tunnel by sloping it to the tunnel top elevation with a slope ratio of 1:1.5. The slope is supported by 100mm thick early-strength C25 shotcrete + a single layer of φ8@200x200 mesh + φ18 anchors. The top of the tunnel is cast with an 800mm thick C30 steel concrete arch cover plate. After the cover plate strength reaches the design strength standard, it is backfilled with soil and then excavated from the inside of the tunnel using the step method. This not only makes the excavation construction progress more reasonable and safe, but also makes the pre-reinforcement range more accurate. After the initial support of the tunnel excavation is completed, the secondary lining structure of the tunnel is constructed in a timely manner, which not only effectively improves the safety and durability of the tunnel structure, but also reduces the waste of resources outside the affected area, and significantly reduces the cost compared to traditional construction methods.

[0021] Before tunnel excavation, the present invention increases the self-stability of the soil above the ultra-shallow buried section of the tunnel by slope-excavating the backfill soil layer above the tunnel to the tunnel top elevation and pouring an 800mm thick C30 steel concrete arch cover plate. This can reduce the possibility of groundwater infiltration, ensure tunnel safety, and reduce ground deformation.

[0022] After pouring the C30 steel concrete arch cover plate, the present invention excavates the lower tunnel and constructs the secondary lining structure in time, thereby improving the tunnel structure stability and anti-destruction ability in the ultra-shallow buried section area and ensuring that the deformation rate is less than 1 mm / d.

[0023] Before tunnel construction, an analysis of survey and design data will be conducted to initially determine the approximate scope and size of the miscellaneous fill area in the ultra-shallow buried section of the tunnel, as well as the length of the route span. The tunnel depth and upper soil thickness of the shallow buried section will also be analyzed. A pre-construction environmental survey of the ultra-shallow buried miscellaneous fill area will also be conducted to investigate the impact of the surrounding environment on construction in this area. This will provide a basis for monitoring and planning the crossing of this area.

[0024] according to Figure 1 The specific excavation process is as follows: Advanced geological forecast The advanced geological prediction of the present invention is mainly divided into three categories: geological survey method, geophysical method and advanced drilling method.

[0025] 1 Advanced geological prediction methods 1) The geological survey method mainly includes supplementary geological survey on the surface and geological sketches of the tunnel faces and tunnel bodies at key locations. The geological sketches are carried out every 5 meters.

[0026] 2) The geophysical prospecting method of this invention is transient electromagnetic method. The transient electromagnetic method predicts a distance of 80m each time, and the overlap length of the two times is 10m.

[0027] 3) The advanced drilling methods include advanced horizontal drilling and deepening blastholes. The predicted distance of advanced horizontal drilling is 35m, with an overlap of 8m before and after. Three holes are drilled and one hole is cored. The drilling depth of the deepened blasthole is not less than 6m, and the final hole position is not less than 2.5m outside the excavation contour line.

[0028] 2. Work sequence of advanced geological prediction 1) After tunnel excavation, geological survey and geological sketching should be carried out immediately. In general areas, records should be made every 5 meters. If geological conditions change or unfavorable geology is encountered, additional sketches should be made.

[0029] 2) Use a transient electromagnetic instrument to detect once about 30m in the ultra-shallow buried section of the miscellaneous fill area to further verify and understand the distribution of the soil in the ultra-shallow buried section.

[0030] 3) If the geophysical prospecting method preliminarily determines that the front has reached the ultra-shallow buried miscellaneous fill area, when the tunnel face is about 10m close to the ultra-shallow buried miscellaneous fill area, the advance drilling method should be used to drill holes for verification.

[0031] 4) Based on the geophysical and drilling results, combined with the previous geological exploration results and geological survey data, the scope of the ultra-shallow buried section miscellaneous fill area is determined.

[0032] Advanced geological forecast analysis After the advanced geological forecast on-site work is completed, analysis work must be carried out immediately. Based on the collected data, the ground conditions at the front end of the tunnel face should be studied and analyzed to determine whether the ultra-shallow buried miscellaneous fill section has been entered before excavation and a clear conclusion should be made to provide a reliable basis for the next excavation and support process.

[0033] Analysis of basic characteristics of miscellaneous fill areas in ultra-shallow buried sections Analysis of geological survey data from the detailed survey phase and advanced geological prospecting revealed that the rock formations between YDK24+510 and YDK24+570 on the upper steps of the tunnel have a dip of 95° / 10°, with the alignment striking approximately 170°. This section is located in a pristine geomorphic area, with the overlying soil primarily consisting of silty clay, approximately 0.6 to 7.6 meters thick. The underlying bedrock is sandy mudstone and sandstone of the Shaximiao Formation of the Middle Jurassic, primarily sandy mudstone. No adverse geological conditions exist, but significant groundwater levels are present. The rock mass, affected by groundwater, exhibits a loose, fragmented structure, and poor surrounding rock stability and integrity. Excavation is located in an extremely shallow section, requiring careful attention to the top rock-soil boundary and the influence of surface precipitation.

[0034] according to Figures 2 to 3 As shown, the upper part of the tunnel is excavated with layered support. According to the results of advanced geological forecast exploration and analysis and the requirements of design change negotiation, before excavating the tunnel in the shallow buried miscellaneous fill area, the backfill soil layer above the underground tunnel will be sloped and excavated to the tunnel top elevation.

[0035] The excavation of foundation pit adopts slope excavation with a slope ratio of 1:1.5. The slope is supported by 100mm thick early strength C25 shotcrete + hanging mesh single layer φ8@200x200 + φ18 anchor nails.

[0036] (1) Excavation of foundation pit The excavation of foundation pit should be carried out in layers and sections from top to bottom in reverse method. The maximum excavation depth of each layer should be determined according to the stability of the rock and soil layer itself. It is generally not more than 2.0m for fill and not more than 3m for rock layer. The minimum width of the layer step is 3m.

[0037] During excavation, the temporary side slope must be kept stable at all times, with a slope ratio of 1:1.5. After each layer of excavation is completed, support structures should be installed promptly, with φ8@200×200mm mesh installed and 100mm thick early-strength concrete sprayed. Over-excavation of the lower soil layer is strictly prohibited until the support reaches normal operation. After the foundation pit is excavated to the design elevation, it should be inspected and leveled, and any accumulated water in the pit should be drained. A 200mm thick rock (soil) layer should be retained at the bottom of the pit and manually excavated and leveled. Inspection and acceptance of the foundation pit should be carried out promptly to avoid over-excavation and disturbance of the bottom soil.

[0038] (2) Excavation of the lowest earthwork: The excavation of the base earth and stone is carried out by combining excavators with manual excavation. When the bottom of the foundation pit is earth, the foundation pit is excavated to 0.3m above the design elevation (when the bottom of the foundation pit is rock, the foundation pit is crushed to the design elevation by machinery), and the mechanical excavation is stopped. The remaining part is excavated manually. After completion, the bottom is manually cleaned and leveled. After acceptance, the arch construction should be organized immediately.

[0039] (3) Excavation precautions ① After the foundation pit is excavated to the bottom, it is excavated and leveled manually. After the foundation pit is inspected and qualified by the five responsible parties involved in the construction, it is reported to the supervision engineer for acceptance and the concrete cushion construction is carried out.

[0040] ② When excavating the foundation pit, support should be provided as the excavation progresses to reduce the exposure time of the foundation pit.

[0041] ③ After the excavation of the foundation pit, the mesh spraying support shall be implemented in a timely manner according to the design requirements.

[0042] ④ After completing one piece of excavation in the foundation pit, clean and level one piece.

[0043] ⑤ During the excavation of the foundation pit, strengthen construction monitoring and measurement, keep abreast of soil pressure, support structure stress, etc., and provide information-based guidance for construction.

[0044] ⑥ The foundation pit is excavated to 4m below the ground, and a dedicated person is arranged to patrol the foundation pit 24 hours a day to ensure that any adverse factors in the foundation pit excavation process can be properly handled in the first place. (4) Measures for over-excavation of foundation pit For areas with over-excavation within 100mm and an area of ​​less than 50㎡, C20 concrete can be directly used for filling and pouring. For areas larger than 50㎡, soil mixed with 10% cement can be used for filling and compaction. For over-excavation exceeding 100mm, it shall be treated according to the requirements of the original soil bearing capacity, and different proportions of ash soil and cement soil can be used for compaction. The treatment principle is to keep consistent with the original soil bearing capacity, and the bearing capacity deviation shall not exceed 20%.

[0045] Steel arch concrete pouring Installation of steel arch frame → Tie up the steel bars of arch foot → Concrete pouring of steel arch guard 1. Installation of steel arch frame, the drawing is as follows Figure 4 As shown: (1) The steel arch frame is connected with C22 steel bars. The spacing of the connecting steel bars along the tunnel ring is 0.5m, and they are staggered at the inner and outer edges of the steel frame support. The steel support and the steel bars are connected by welding. The steel frame and the connecting steel plates are also connected by welding. The connecting steel plates are connected by bolts.

[0046] (2) The steel support should be close to the surrounding rock, and the gaps between the surrounding rock should be filled with sprayed concrete.

[0047] (3) To prevent the I-beam from sinking, the erection steel plate must be set on the bedrock, and a concrete base must be poured if necessary. The erection steel plates at the arch foot foundation on both sides must first be built with M16 bolts, and the exposed parts must be protected with steel sleeves and waterproof tarpaulins to facilitate subsequent steel frame connection. (4) A positioning anchor rod must be set at the bottom of the steel frame and welded to it. The incident angle of the anchor rod is 30°~40° 2. Arch reinforcement binding, the drawing is as follows Figure 5 As shown: (1) The steel bars are installed by manual tying on site. The installation position, spacing, protective layer and size of each steel bar should comply with the design drawings. During installation, the placement points are measured to determine the position of the steel bars.

[0048] (2) The installation position, spacing, protective layer and size of each part of the steel bar should comply with the requirements of the construction details and relevant documents. The steel bar protective layer should be arranged and reserved according to the requirements of the construction details.

[0049] (3) The installed steel bars should have sufficient rigidity and stability.

[0050] (4) The connection of steel bars can be made by: lap joint, mechanical joint or welding according to the requirements of the specification. When the diameter of the steel bar is greater than or equal to 16mm, mechanical joint should be used. For other steel bars, single-sided welding should be used, and the welding length should be not less than 10d.

[0051] (5) The net protective layer thickness of the outermost layer of concrete reinforcement is 40 mm. The net protective layer thickness of the reinforcement refers to the dimension from the edge of the reinforcement to the concrete surface, including the main reinforcement, distribution reinforcement and stirrups.

[0052] (6) To ensure the designed thickness of the concrete protective layer, the protective layer pads can be made of fine stone concrete, and its corrosion resistance and strength should be higher than the concrete of the component body.

[0053] (7) The allowable deviation and inspection method for steel bar installation and protective layer thickness shall comply with the provisions of the following table.

[0054] Table 1 Permissible deviation of steel bar installation and cover thickness (mm) and inspection method

[0055] 3. Steel arch concrete pouring: The concrete for the arch structure is all commercial concrete. After the concrete is delivered to the construction site, it is pumped to the pouring work surface using a ground pump for pouring. The construction method is as follows: (1) Concrete should maintain its workability during transportation and avoid segregation. When transported to the pouring site, a slump test should be carried out before pouring, and the slump requirement of (180 ± 20) should be met.

[0056] (2) During the concrete pouring process, observe the conditions of the formwork, supports, steel bars, embedded parts and reserved holes. When deformation or displacement occurs, reinforcement measures should be taken in a timely manner.

[0057] (3) If the concrete slump is found to be insufficient during construction, water shall not be added without authorization. Instead, the problem shall be solved by adding water-reducing agent under the guidance of technical personnel. If the concrete shows any abnormality, the concrete shall be withdrawn from the site and replaced in time, and shall not be poured into the warehouse.

[0058] (4) Concrete pouring should be carried out continuously. If it is interrupted for some reason, the interval time should be less than the initial setting time of the previous layer of concrete. If the allowable interval time is exceeded, the lining concrete joints must be roughened according to the joint treatment.

[0059] (5) The arch concrete is poured and formed in one step.

[0060] (6) When pouring new concrete on a newly poured lower layer of concrete, the upper layer of concrete should be completed before the lower layer of concrete begins to set.

[0061] (7) During the concrete pouring process, test blocks should be retained in a timely manner according to the design requirements. The retention requirements are as follows: compression test blocks: at least one sample is taken for every 100m³ of concrete poured (one sample is taken for less than 100m³); anti-permeability test blocks: one sample is taken for every 30m of secondary lining concrete, and the sample is arranged reasonably according to the length of the secondary lining pouring; uniform curing test blocks: at least three groups of samples of concrete with the same mix ratio are retained and one sample is retained as needed for each pouring. Concrete test blocks should be retained in a timely manner and well protected according to different curing requirements.

[0062] (8) When using an inserted vibrator, the following provisions shall be met: ① The vibration time at each vibration point should be 20 to 30 seconds, until the concrete stops sinking, bubbles appear, and floating slurry appears on the surface, to prevent over-vibration and missed vibration.

[0063] ② When using an inserted vibrator to vibrate concrete, the moving distance of the vibrator should not be greater than 1.5 times the effective radius of the vibrator, and the depth of insertion into the lower layer of concrete should be 50 to 100 mm. A distance of 50 to 100 mm should be maintained from the side formwork, and collision with steel bars, formwork, embedded parts, etc. should be avoided.

[0064] ③Adhere to the principle of “quick plug and slow pull”.

[0065] Backfilling above the tunnel (1) The backfill material of the foundation pit shall not contain pure clay, silt, silt sand, miscellaneous soil, humus soil with an organic matter content greater than 8%, overly wet soil, and stones with a particle size greater than 150 mm. The area within 0.8 m above the arch should be backfilled with clay soil with poor permeability. Before use, the backfill soil should be sampled and its maximum thousand-density density and optimum moisture content should be measured, and a compaction test should be performed to determine the parameters such as the moisture content control range of the filler, the thickness of the soil, and the number of compaction passes.

[0066] (2) The foundation pit must be backfilled after the arch reaches the designed strength. Before backfilling, the accumulated water and debris in the pit should be cleaned up.

[0067] (3) When backfilling the left and right sides of the foundation pit, the filling should be raised synchronously and balanced to avoid uneven soil pressure in the pit and affect the structure. The foundation pit backfill should be layered and compacted horizontally; at the joints of the foundation pit backfill, the filled slope should be stepped, with a width of not less than 1m and a height of not more than 0.5m.

[0068] (4) During backfilling, machinery or equipment must not collide with the arch sleeve. Manual tamping is used using small equipment within 500mm on both sides and the top of the structure.

[0069] Tunnel step excavation and support The tunnel, which passes through a shallow buried area, was excavated using a bench method combined with mechanical excavation, tailored to the tunnel cross-section and engineering geology. I-beams and a system of anchor bolts and mesh shotcrete were used for support. Construction was strictly conducted in accordance with the principles of "pipeline ahead, rigorous grouting, short advances, strong support, early closure, and frequent measurements."

[0070] 1 The step method is divided into two steps, the upper and lower steps. The excavation cross sections of the upper and lower steps are as follows: Figure 6 and Figure 7 As shown, the excavation height of the upper bench is 6m. After the upper bench excavation is completed, the lower bench should be excavated as soon as possible and supported to form a closed structure. The positive bench method allows for early support closure, which helps control structural deformation and the resulting ground subsidence. The excavation and support advance per cycle of the upper bench should not exceed 1m; the excavation and support advance per cycle of the lower bench should not exceed 1m.

[0071] 2 The tunnel passes through the shallow buried mixed fill area and is excavated using I20b I-steel with a frame spacing of 0.5m. The arch and side walls are equipped with Φ22 hollow grouting anchor rods and Φ8@200*200 single-layer steel mesh sprayed with C25 concrete for strong support.

[0072] 3. The initial support must closely follow the excavation process, must not be exposed for a long time, and must be closed into a ring as soon as possible. After the excavation is completed, it must be monitored according to the monitoring plan to realize information-based construction and dynamic adjustments to ensure excavation safety.

[0073] 4 Excavation and support construction methods 1) The excavation and support advance of each cycle of the upper step shall not exceed 1m; the excavation and support advance of each cycle of the lower step shall not exceed 1m. The excavation height of the step on the section shall not exceed 6m, and the excavation height of the lower step shall not exceed 3m. The longitudinal section diagram of the excavation sequence is as follows Figure 8 shown.

[0074] 2) A breaker hammer is used to remove the large area of ​​earth and rock in the middle, and 300 to 500 mm is reserved on each side for rock surface finishing by an excavator. After the excavation contour is measured and re-measured to meet the requirements, initial support construction is carried out.

[0075] 3) When crushing stones, attention should be paid to on-site safety. Scaffolding railings should be set up for parts with large drop heights, and a dedicated person should be assigned to direct the work.

[0076] 4) The design has taken into account the reserved deformation of tunnel convergence. During tunnel excavation, over-excavation is controlled and under-excavation is strictly prohibited.

[0077] 5) When the surrounding rock deformation is abnormal, or the steel frame sinks or moves inward significantly, emergency reinforcement or bracing, as well as corresponding reinforcement support measures should be taken immediately to ensure safety. At the same time, the cause should be analyzed and the construction method and lining structure should be adjusted accordingly to ensure the safety of the tunnel project and its construction.

[0078] 6) If work needs to be stopped during excavation, the tunnel face must be sealed with mesh spraying and concrete. This is done using a 200mm C25 spray mix + A8@200×200 steel mesh. Ensure the quality of the face seal during construction to ensure safety and control of the face during subsequent construction.

[0079] Secondary lining construction like Figure 9 As shown, the lining construction abandoned traditional loose formwork and steel formwork construction techniques and instead employed an advanced "self-propelled, pre-set steel formwork trolley" construction process, ensuring the lining arch wall structure met the high design standards for both form and precision quality. The A-section lining thickness is 600mm, and the secondary lining steel formwork trolley is a motor-driven, hydraulically attached vibrating trolley. The trolley has a designed clear height of 4.467m and a clear width of 3.8m, allowing for the normal passage of construction vehicles and equipment. Each trolley support system is equipped with one set of formwork, and the trolley formwork is 12m long. The trolley is motor-driven and uses 43kg / m standard rails. Upon installation, the trolley is inspected and accepted as a checkpoint.

[0080] Tunnel and surface monitoring According to the design requirements, the secondary lining must be constructed immediately after the initial support of the shallow buried area is completed, and at the same time, the tunnel vault sinking, clearance convergence and surface settlement points must be monitored more intensively.

[0081] Finally, it should be noted that the above embodiments are merely examples for the purpose of illustrating the present invention and are not intended to limit the embodiments. Those skilled in the art will readily appreciate that other variations or modifications based on the above description are possible. It is not necessary and impossible to provide an exhaustive list of all possible embodiments. However, obvious variations or modifications arising therefrom remain within the scope of protection of the present invention.

Claims

1. A combined construction method for ultra-shallow mixed fill tunnel excavation for rail transit, characterized in that: The following steps are involved: a) Before tunnel excavation, a combination of transient electromagnetic (TEM) and advanced horizontal drilling is used to conduct advanced geological prediction to identify the spatial location and engineering characteristics of miscellaneous fill sections with a cover depth of ≤2.3 m, and to establish a three-dimensional geological risk model; b) Based on the model, the miscellaneous fill above the tunnel was unloaded by open excavation with a slope ratio of 1:1.5, and supported as it was excavated: a combined slope support system was formed using 100 mm thick early-strength C25 shotcrete, steel mesh, and mortar anchors; c) At the bottom of the open excavation, a steel-concrete arch cover is cast in situ along the tunnel axis. The I-steel arch frames in the arch cover are spaced 0.5 m apart in the longitudinal direction, with expanded foundations at both ends and connected to the steel arch frames with pre-buried high-strength bolts to form a whole; d) After the concrete strength of the cover slab reaches 100% of the design strength, backfill the soil symmetrically and proceed to the tunnel using the step method with "upper step height ≤ 6 m, lower step height ≤ 3 m, and advance per cycle ≤ 1 m"; e) The primary support in the tunnel shall be supported by a combination of I20b steel frame, hollow grouting anchor, steel mesh + C25 shotcrete, and the secondary concrete lining shall be immediately applied after the primary support is closed into a ring; f) During the entire construction process, real-time monitoring of ground settlement, vault subsidence and clearance convergence is carried out. When the daily deformation rate is greater than 1 mm or the cumulative deformation rate is greater than 10 mm, an early warning is automatically triggered and emergency reinforcement measures are implemented.

2. The combined construction method for ultra-shallow mixed fill tunneling for rail transit according to claim 1 is characterized in that: The refined identification in step a) includes: transient electromagnetic method prediction depth of 80 m, front and back overlap of 10 m, advanced horizontal drilling drilling depth of 35 m, front and back overlap of 8 m, and the data of these two methods are integrated through the BIM platform to generate a three-dimensional cloud map of the thickness, strength, and permeability coefficient of the miscellaneous fill with a resolution of ≤1 m.

3. The combined construction method for ultra-shallow mixed fill tunneling for rail transit according to claim 1 is characterized in that: The slope cutting and open excavation unloading in step b) adopts the layered reverse method, with the excavation thickness of each fill layer ≤2 m, the rock layer ≤3 m, and the width of each step ≥3 m. A 0.3 m bottom is reserved for manual cleaning at the bottom, and cement soil is used for replacement filling when the bearing capacity is insufficient.

4. The combined construction method for ultra-shallow mixed fill tunneling for rail transit according to claim 1 is characterized in that: The steel-concrete arched cover plate in step c) uses double-layer bidirectional φ22 main reinforcement @150 mm, φ12 distribution reinforcement @200 mm, a protective layer of 40 mm, and concrete pads arranged in a plum blossom shape @1 m×1 m to ensure thickness.

5. The combined construction method for ultra-shallow mixed fill tunneling for rail transit according to claim 1 is characterized in that: The I20b I-beam arch segments in step c) are 2 m long. Adjacent segments are connected using 10 mm thick Q235 connecting plates and M24 high-strength bolts. The bolt torque is no less than 300 N·m. φ25 locking anchor rods are installed at the arch feet, with an incident angle of 30° to 40°.

6. The combined construction method for ultra-shallow mixed fill tunneling for rail transit according to claim 1 is characterized in that: The backfilling and covering process in step d) is as follows: low-permeability clay is compacted layer by layer in the area 0.8 m above the arch, with a compaction degree of ≥90%; graded crushed stone is compacted layer by layer in the remaining area, with a compaction degree of ≥93%, and the height difference of the backfill on both sides is ≤0.5 m.

7. The combined construction method for ultra-shallow mixed fill tunneling for rail transit according to claim 1 is characterized in that: In step e), the core soil is reserved by excavating the steps in the tunnel. The upper step advances 0.8-1.0 m per cycle, the lower step lags behind the upper step by 3-5 m, and the core soil on the upper step is retained with a width of ≥3 m to control settlement.

8. The combined construction method for ultra-shallow mixed fill tunneling for rail transit according to claim 1 is characterized in that: The secondary lining in step e) uses a 12 m self-propelled hydraulic steel formwork trolley. The formwork is 10 mm thick steel plate + 12# channel steel back rib. The hydraulic cylinder positioning accuracy is ±3 mm. The lining concrete has an anti-seepage grade of P10 and is mixed with 0.9 kg / m³ polypropylene fiber.

9. The combined construction method for ultra-shallow mixed fill tunneling for rail transit according to claim 1, characterized in that: The real-time monitoring in step f) transmits data to the cloud platform in real time through the wireless sensor network, and the mobile phone APP immediately pushes warning information, and the monitoring frequency is increased to 1-2 times / day during the abnormal deformation stage.

10. The combined construction method for ultra-shallow mixed fill tunneling for rail transit according to claim 1, characterized in that: The emergency reinforcement measures in step f) include: immediately sealing the tunnel face with 200 mm thick C25 sprayed concrete and steel mesh, installing a φ42 small pre-grouting pipe @300 mm×3.5 m and injecting cement-water glass dual-liquid slurry, and, if necessary, backfilling with sand bags to a height of ≥2 m and adding temporary diagonal bracing.

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