Underground excavation station primary support arch cover method large arch foot structure and construction method thereof

Abstract

The invention discloses an underground excavation station primary support arch cover method large arch springing structure and a construction method thereof, and aims at solving the technical problems that arch springing stress is concentrated in an upper-soft lower-hard stratum and traditional support settlement is large, and technical breakthrough is achieved through innovative combination of a dynamic adjustable support system and an intelligent grouting process. In the aspect of the structure, the self-adjusting steel pipe pile comprises inward inclination bottom feet with the inward inclination angle of 1: 6-8, a hinged angle adjusting base, a self-adjusting steel pipe pile with the dynamic adjustment of + / -5 degrees, a 600 * 600 * 25 mm partial pressure steel plate, a 45-degree inclined grouting hole channel, a check valve and a bottom longitudinal beam, a pile-plate-beam three-in-one structure is formed, and an optical fiber sensor is arranged in the steel pipe pile to monitor surrounding rock deformation in real time; according to the construction method, three-stage intelligent grouting, 0.1-0.3 MPa low-pressure filling, 0.5-0.8 MPa high-pressure reinforcing and compensation grouting based on monitoring are adopted, concrete is sprayed in cooperation with the telescopic formwork, and installation of the water stop type feet-lock anchor pipe and the water swelling rubber strip are completed synchronously.

Description

Technical Field

[0001] The present invention relates to the technical field of building construction, and in particular to a large arch foot structure of a concealed excavation station using an initial support arch covering method and a construction method thereof. Background Art

[0002] There are many types of construction methods for urban subway underground stations, and many safety accidents are caused by geological disasters, design and construction, etc. In construction in soft upper and hard lower strata, the arch cover method is often used. The arch cover method is simpler and requires less investment than traditional complex construction methods such as the pile method. However, how to ensure the safety of the arch cover method construction requires further research, especially in key nodes and core technologies. For example, the construction technology of the large arch foot is the key to ensuring the stability of the initial support of the upper part of the primary support arch cover method, or the key to ensuring that the secondary lining arch does not fall off. It is necessary to improve the stability of the large arch foot and ensure sufficient bearing capacity.

[0003] For example, Chinese invention patent application CN201910787403.8 discloses a construction method for enlarging the arch foot and initially supporting the arch capping method. This method includes: excavating a construction channel into the station; constructing the upper portion of the station using the arch capping method; constructing the middle and lower portions using a top-down construction method, excavated in three layers; excavating the side walls in sections, supporting them as they are excavated, and installing steel supports; casting the inverted arch and fill layer; and sequentially setting up full-height scaffolding for a continuous, segmented process to cast the platform slab, running slab, concourse slab, track-top air duct, and structural columns. Excavating the construction channel into the station further includes: excavating the station basement level from branch channel B, using controlled blasting and mechanical excavation at the intersection to protect the surrounding rock, excavating in sections, and promptly implementing initial support after slag removal during construction. Monitoring and measurement are also strengthened during construction. This method creates a streamlined construction process, which strengthens construction organization, ensures construction safety, and ensures timely and high-quality completion.

[0004] For another example, Chinese invention patent application number CN202411736808.6 discloses a construction technology for large-section subway underground excavation stations using the initial support arch method. This technology addresses the problems of high construction difficulty, long construction period, and high cost in traditional underground excavation methods when faced with large-span shallow-buried stations. By introducing innovative methods such as a double-layer initial support arch structure, gradual support and expanded arch foot design, automated monitoring, and three-dimensional modeling analysis, it effectively solves the problem of high construction difficulty in weathered rock strata. This technology not only significantly improves construction efficiency, shortens construction period, and reduces costs, but also significantly reduces the disturbance of construction to the surrounding environment and enhances the safety and stability of the structure. Its flexible adaptability and broad application prospects provide an efficient, economical, and environmentally friendly construction solution for the field of subway construction, promoting the rapid development of subway construction.

[0005] For another example, Chinese invention patent application number CN201510045392.8 discloses a method for the underground excavation of a large-span subway station in hard rock formations, which effectively reduces the risk of excavation construction and improves the efficiency of excavation construction. The method includes the following steps: excavating the middle of the station arch in two steps, first excavating the upper step in the middle of the tunnel arch and constructing the primary arch cover of this step; excavating the lower step in the middle of the tunnel arch; excavating the left and right sides of the tunnel arch, constructing locking anchor rods at the arch foot position, extending the primary arch cover of the arch top, and completing the construction of the primary arch cover; under the protection of the primary arch cover, trenching and excavating the lower rock mass of the station, dividing the excavation into 9 parts (left, middle, and right) and three steps (upper, middle, and lower), and constructing the initial support of the side walls of the corresponding sections until the excavation reaches the bottom; and constructing the main structure of the station from bottom to top to complete the station construction.

[0006] The above-mentioned prior art discloses a construction method and process for the primary support arch covering method for expanding the arch foot. It studies the entire construction process of the primary support arch covering method, but lacks research on the specific structure of the large arch foot and its construction method. Based on the above-mentioned technical problems existing in the prior art, the present invention provides a large arch foot structure and a construction method of the primary support arch covering method for a concealed excavation station. Summary of the Invention

[0007] In order to solve the above-mentioned technical problems existing in the prior art, the present invention provides a large arch foot structure of a concealed excavation station with an initial support arch cover method and a construction method thereof.

[0008] The present invention adopts the following technical solutions: The present invention provides a large arch foot structure of a concealed excavation station using an initial support arch cover method, comprising: Inward-inclined footing: The inward-inclined footing is a structure that tilts inward, with an inclination angle of 1:6~8; Self-adjusting steel pipe piles are installed at the large arch feet in the guide pits on both sides. The self-adjusting steel pipe piles are connected to the bottom base of the bottom longitudinal beam through an articulated angle adjustment base. The articulated angle adjustment base includes a fixed plate with a slide groove and a ball joint. The outer wall of the self-adjusting steel pipe pile is embedded with a fiber optic sensor along the axial direction. The pressure dividing steel plate is set at the pile-beam node between the top of the self-adjusting steel pipe pile and the bottom longitudinal beam. The pressure dividing steel plate is a 600×600×25mm steel plate. Two symmetrical grouting channels are opened on the pressure dividing steel plate, and the grouting pipe joint is installed, and the outer wall is fixed; Bottom longitudinal beam, set along the longitudinal direction of the tunnel, its steel bars are welded to the pre-buried joint bars of the first support, and the concrete strength grade is C30; The primary support includes steel grilles, system anchor rods, steel mesh, and shotcrete. The longitudinal spacing of the steel grilles is 0.5-1m, and joint bars are embedded on both sides of the foot of each steel grille. The joint bars are welded to the stirrups of the bottom longitudinal beam. The system anchor rods include water-stopping locking anchor pipes or anchor rods. The rod body of the water-stopping locking anchor pipes or anchor rods is coated with two slow-expansion water-swelling water-stop rubber strips. The locking anchor pipes are φ42mm seamless steel pipes, and the anchor rods are φ25mm hollow anchor rods. The second support includes a grid steel frame, steel mesh, shotcrete, and reserved beard bars or joint bars connecting the first and second supports. The circumferential spacing of the beard bars is 1 to 1.2 meters. The bottom grid of the second support is located inside the bottom longitudinal beam. Furthermore, the included angle θ between the inward-inclined footing and the axis of the self-adjusting steel pipe pile satisfies: θ=arctan(1 / 8)+Δα, where Δα is the real-time adjustment angle of the articulated base, -5°≤Δα≤+5°; Self-adjusting steel pipe piles, pressure-dividing steel plates, and bottom longitudinal beams form a pile-plate-beam trinity structure. The inward-inclined footing, self-adjusting steel pipe piles, pressure-dividing steel plates, and bottom longitudinal beams together constitute a joint rigid support system that resists surrounding rock deformation.

[0009] Furthermore, in the articulated angle adjustment base of the self-adjusting steel pipe pile, the rotation angle range of the ball joint is ±5°, and the slide fixing plate is fixed to the bottom base of the bottom longitudinal beam by bolts; when the rock layer inclination changes by more than 3°, the hydraulic jacking system automatically compensates for the angle deviation to ensure that the pile-rock contact surface pressure is ≥0.5Mpa; the optical fiber sensors on the outer wall of the self-adjusting steel pipe pile are arranged axially with a spacing of 1.5m. The sensors are covered with a polyurethane protective cover with a thickness of 2mm and a compressive strength of ≥50Mpa, which is used to monitor the inclination of the self-adjusting steel pipe pile and the stress changes of the surrounding rock in real time.

[0010] Furthermore, the slow-expanding water-swelling water-stop strip of the water-stop locking anchor pipe is a composite strip of butyl rubber and water-swelling rubber, with a thickness of 5~10mm, a 24h expansion rate of 80%, and a shear strength of ≥0.15MPa after expansion in water. The durability requirement is that the expansion rate is retained at ≥85% after 50 dry-wet cycles; the exposed end of the anchor pipe is fixed by squeezing the tray and the base rock. The tray is a circular steel plate with a diameter of φ60mm and a thickness of 10mm.

[0011] Furthermore, the grouting channels of the pressure-dividing steel plate are two symmetrically distributed φ15mm circular holes. One of the channels has a quick-connect grouting pipe joint that is detachably connected to the secondary grouting pipe, and the other channel serves as an exhaust hole. The pressure-dividing steel plate is fully welded to the top of the self-adjusting steel pipe pile. The weld thickness is ≥8mm, the weld metal yield strength is ≥355MPa, and the ultrasonic flaw detection complies with GB / T11345-2013 Class B. The embedded joint reinforcement at the first support footing is HRB400 steel bar with a diameter of φ22mm and a length of 300mm. It is double-sided welded to the bottom longitudinal beam stirrups with a diameter of φ12mm and a spacing of 200mm. The welding length is ≥5d, where d is the diameter of the steel bar.

[0012] The present invention also provides a method for constructing a large arch foot using the initial support arch cover method for a concealed excavation station, comprising: Step 1: Advance support. Before the excavation of the horse head gate, advance support is carried out. The advance support includes an advance large pipe shed and an advance small pipe shed. The large pipe shed is constructed by a pipe shed drilling rig, and the small pipe shed is drilled by an air leg type air drill. The large pipe shed is constructed 5 to 10 cm away from the contour line of the outer arch. The diameter of the large pipe shed is φ108mm, the spacing is 40cm, the outward angle is not more than 5°, and the length is 15 to 20m. Small pipes are set between the large pipe sheds. The diameter of the small pipe is φ42mm and the length is 3 to 5m. After the advance support is completed, a plunger pump is used to grout and reinforce the surrounding rock. The grouting slurry is pure cement slurry with a water-cement ratio of 0.8 to 1:1. The skip hole grouting method is adopted during grouting. Step 2: Double-side pilot pit construction, using mechanical crushing horse head gate, staggered excavation of the two side wall pilot pits; when the distance between the two side wall pilot pits is less than 1 times the tunnel diameter, the staggered distance is ≥15m; when the distance exceeds 1 times the tunnel diameter, the staggered distance is 3~5m, or parallel excavation is adopted to reduce the flow step time and save excavation period; the pilot pit is excavated by weak blasting or mechanical method, and the short step method is used in the same pilot pit, with the upper and lower steps 3~5m apart; the footing is mechanically excavated to form an inward inclined surface to protect the integrity of the footing rock, with an inward inclination angle of 1:6~8; after the pilot pit is excavated, the first support is promptly applied, including the first support steel The reinforcement grid is installed, the anchor rods of the first support system are installed, the first support reinforcement mesh is hung, and the first support concrete is sprayed, and beard reinforcement is reserved; a water-stopping locking anchor pipe or anchor rod is installed at the foot of each step, and the gap between the water-stopping locking anchor pipe / anchor rod body and the water-stop rubber strip is filled and the gap between the rod body and the rock wall is coated with a slow-expansion water-stop rubber strip. The water-stopping locking anchor pipe or anchor rod is grout-reinforced to the surrounding rock of the footing, and after grouting and reinforcement, the first support concrete is sprayed; the joint reinforcement is embedded on both sides of the foot of the first support reinforcement grid, and the joint reinforcement is firmly welded on both sides. When the bottom longitudinal beam reinforcement is tied, it is welded to the bottom longitudinal beam stirrups; Step 3: Construction of the middle pilot tunnel. The middle pilot tunnel is excavated in two layers. The first layer is 2.5m high. After weak blasting or mechanical excavation, the primary support is installed. This includes connecting the top steel grids of the left and right primary supports, driving the primary support arch system anchors, hanging the primary support steel mesh, and spraying the primary support concrete. The second layer of the middle pilot tunnel is excavated after the secondary support construction is completed. Step 4: Self-adjusting steel pipe pile construction. PD45 down-the-hole drills were used to drill holes at the large arch feet in the pilot pits on both sides. The BIM model was used to preview the layout of the self-adjusting steel pipe piles. Laser-guided down-the-hole drilling was used to achieve a hole position error of ≤2cm, a hole diameter of 400mm, and a hole angle of 60°~70°. The self-adjusting steel pipe piles had a diameter of φ219mm and a wall thickness of 10mm. 235B hot-rolled steel pipes are 7-9m long and spaced 1-1.2m apart longitudinally along the arch foot. The self-adjusting steel pipe piles are bored at an angle of 60-70° to the horizontal to facilitate down-the-hole drilling. The self-adjusting steel pipe piles are threaded in sections, with internal and external threads of 8-10cm. Each section of steel pipe is ≤1.5m long, and fiber optic sensors are installed on the side walls and protected with polyurethane sleeves. Plum blossom-shaped grouting holes are drilled within 3m of the bottom of the self-adjusting steel pipe piles. The grouting holes are φ6-10mm in diameter and are four symmetrical holes circumferentially spaced 40-50cm apart longitudinally. The bottom of the self-adjusting steel pipe piles features a wedge-shaped tip. The inclination angle is dynamically adjusted within ±5° using an articulated angle adjustment base, and precise control is achieved using a bottom inclination sensor. A skip-hole grouting and three-stage intelligent grouting process is used. The first grouting process involves inserting a sleeve valve tube into the middle of the self-adjusting steel pipe pile, sealing the grouting holes with tape, and using pure cement slurry with a water-cement ratio of 0.5-0.75:1 and a pressure of 0.1-0.3 MPa to fill the gaps inside the steel pipe and between the steel pipe and the surrounding rock. The second grouting process involves sealing the pile top with a divided steel plate. The grouting material is pure cement slurry and a micro-expansive agent or water-glass double-liquid slurry, at a pressure of 0.5-0.8 MPa, to fill the surrounding rock cracks. The third stage grouting process involves real-time monitoring of surrounding rock deformation data using fiber optic sensors, intelligent compensatory grouting at a pressure of 0.8-1 MPa, and micro-expansive cement slurry is injected to form a pile-slab-beam trinity structure. The expansive agent is magnesium oxide, with an activity of 60-80 seconds and a dosage of 8-10%. In rock fracture zones, three φ28 main bars are added to the self-adjusting steel pipe pile, connected by sleeves, with the segment length matching that of the steel pipe pile. The grouting slurry contains: sulphoaluminate cement 70-80% by volume, silica fume 10-15% by volume, and magnesium oxide expansion agent 8-10% by volume; Step 5: Second support construction: Mechanically break the sprayed concrete within 1m of the top of the middle partition wall, install the second support top grid steel frame between the two grids of the middle partition wall, bend and anchor the second support top grid steel frame and the first support beard bar, hang the second support top steel mesh, and connect the bottom grids on both sides of the second support in the side wall guide pit to the bottom longitudinal beam; after the bottom longitudinal beam construction is completed, install the remaining second support steel mesh, install a retractable formwork system consisting of a hydraulic folding steel frame and a silicone resin panel on the grid steel frame, expand it to fit the shape of the second support grid, shrink it after spraying, and quickly demould it, realizing the concrete spraying machine hanging formwork spraying operation; Step 6, bottom longitudinal beam construction. After the second support steel bar grid is installed, anchor steel bars are welded around the self-adjusting steel pipe piles, and the bottom longitudinal beam steel bars are tied. The main bars of the bottom longitudinal beam on both sides of the second support bottom grid are abutted against the second support grid steel frame. The bottom longitudinal beam stirrups are welded to the first support joint bars. After the steel bars are tied, the formwork is erected and C30 concrete is poured. After the excavation of the upper section is completed, the middle partition wall is removed to start the excavation of the lower section. The excavation of the large arch foot of the lower section adopts mechanical excavation or smooth blasting to control the stability of the rock mass of the large arch foot.

[0013] Furthermore, step 2 also includes: Step 21: The offset length depends on the distance between the two side wall pilot pits. When the hole diameter D is less than or equal to 8m, the offset L is greater than or equal to 15m. When D is greater than or equal to 8m, the offset L is equal to min(3-5m, D / 2), where D is the excavation hole diameter and the surface settlement rate is less than or equal to 0.5mm / d. Step 22: The anchor pipe is a seamless steel pipe with a diameter of 42 mm, and the anchor rod is a hollow anchor rod with a diameter of 25 mm. The water-stopping locking anchor pipe or anchor rod is used to grout the surrounding rock of the footing to improve the bearing capacity of the surrounding rock at the footing. After the grouting is completed, the first support shotcrete operation is carried out; Step 23: Embed beard bars at a circumferential spacing of 1 to 1.2 m in the first support steel grid to strengthen the connection between the first support and the second support. Embed joint bars on both sides of the bottom foot of each steel grid of the first support. The joint bars are welded firmly on both sides. When the bottom longitudinal beam reinforcement is tied, weld them to the bottom longitudinal beam stirrups to enhance the integrity of the connection between the support and the bottom longitudinal beam.

[0014] Furthermore, step 4 also includes: Step 41: Fix the steel pipe at the hole with a clamp and connect it section by section from bottom to top with a pipe clamp. Adjacent steel pipe piles are connected in a staggered manner. The length of the steel pipe section is adjusted to 50 cm. Step 42: An articulated angle-adjustable base is provided at the top of the self-adjusting steel pipe pile. The base consists of a fixed plate with a slide groove and a ball joint. This allows the self-adjusting steel pipe pile to dynamically adjust its inclination angle within a ±5° range. The bottom end of the steel pipe pile is automatically adjusted by an articulated hydraulic support system. A fiber optic sensor at the bottom is used for precise measurement to control the angle accuracy of the steel pipe pile installation, significantly improving its adaptability to the inclination angle of the rock layer. Step 43: After the down-the-hole is drilled, high-pressure air is used to clean the mud and sand in the hole, and then the self-adjusting steel pipe piles are lowered section by section. After the self-adjusting steel pipe piles are installed, grouting and filling are performed on the self-adjusting steel pipe piles; Step 44: Weld a pressure-dividing steel plate with dimensions of 600×600×25 mm at the orifice of the pile-beam node at the top of the pile. The pressure-dividing steel plate at the pile-beam node is provided to eliminate stress concentration. Two symmetrical grouting channels are provided on the plate, one of which is installed with a grouting pipe joint, and the other serves as a grouting exhaust hole. The grouting channel forms a 45° angle with the plane of the steel plate. A check valve with a pressure resistance of 1.2 MPa is provided at the orifice. The grouting slurry uses pure cement slurry and a micro-expansion agent or water glass double liquid slurry with a water-cement ratio of 1:1 and a double liquid slurry ratio of cement slurry: water glass = 1:1. The grouting pressure is controlled at 0.5-0.8 MPa. When the rock integrity coefficient is less than 0.35, the secondary grouting pressure is increased to 0.8-1.0 MPa. The functional relationship between grouting pressure and rock integrity is P = 0.5 + 0.3 × (0.45-Kv) MPa, where Kv is the rock integrity coefficient. In step 45, the pressure data collected in real time by the fiber optic sensor is transmitted to the grouting pump through the PLC controller. When the monitored pressure is lower than the set threshold, the primary grouting threshold is 0.2 MPa and the secondary grouting threshold is 0.6 MPa, and the grouting pump is automatically started to supplement the grouting. After the grouting is completed, the fiber optic sensor continuously monitors the changes in the surrounding rock pressure. When the pressure fluctuation exceeds 10% within 72 hours, compensatory grouting is triggered.

[0015] Furthermore, step 5 also includes: Step 51: Mechanically break up the shotcrete within 1m of the top of the temporary middle partition wall in the double-side wall pilot pit, install the second support top grid steel frame between the two grids of the middle partition wall, bend and anchor the reserved beard reinforcement of the first support to the second support grid, hang the second support top steel mesh, install the second support bottom grid steel frame in the two side wall pilot pits, and install longitudinal connecting reinforcement to connect the second support grid steel frame; Step 52: After the bottom longitudinal beam is constructed, the remaining second support steel mesh is installed. A retractable sprayed concrete formwork system is installed on the grid steel frame. The formwork is composed of a hydraulic folding steel frame and a silicone resin panel. The formwork is unfolded to fit the shape of the second support grid and shrunk and demoulded after spraying. The deflection of the hydraulic steel frame after unfolding is ≤ L / 500, where L is the span and the elastic modulus of the silicone resin panel is ≥ 1.5 GPa. A concrete sprayer is used to extend the nozzle into the formwork for hanging formwork spraying to reduce concrete rebound and speed up the construction progress of the second support. Step 53: The hydraulic folding steel frame is unfolded at an angle of 0° to 90°. The thickness of the silicone resin panel is 5 mm. The surface is coated with a silicone release agent with a coating amount of 0.2 kg / m². When spraying concrete, the distance between the nozzle and the inner surface of the formwork is controlled at 0.3 to 0.5 m, and the spraying pressure is 0.1 to 0.3 MPa.

[0016] Furthermore, step 6 also includes: Step 61: After the second support steel bar grid is installed, anchor steel bars are welded around the steel pipe piles, and the bottom longitudinal beam steel bars are tied. The main bars of the bottom longitudinal beam on both sides of the second support bottom grid are abutted against the second support grid steel frame. The first support pre-buried joint bars are anchored into the bottom longitudinal beam and firmly welded to the bottom longitudinal beam stirrups. The bottom longitudinal beam concrete formwork is erected, and the bottom longitudinal beam concrete is poured. The concrete strength grade is C30. Step 62: After the excavation of the upper section is completed, the middle partition wall is removed and the lower section is excavated. When excavating the lower section, mechanical excavation or smooth blasting should be used under the arch foot to control the rock mass stability of the arch foot. Step 63: Use a rock drilling rig in conjunction with a hydraulic breaker, with a blasthole depth of 1.5-2.0 m, a peripheral hole spacing of 0.4-0.5 m, a peripheral hole charge concentration of 0.18±0.02 kg / m, and a detonating cord relay detonation network. After smooth blasting, the surrounding rock disturbance range at the large arch foot is controlled within 0.5 m, and the over-excavation is ≤100 mm.

[0017] The present invention also provides a monitoring device for the large arch foot construction method of the initial support arch cover method for a concealed excavation station, comprising a fiber optic sensor buried in a self-adjusting steel pipe pile, a grouting pump PLC controller, and a cloud analysis platform. The fiber optic sensor monitors the axial strain ε and the inclination angle α in real time, and when Δε / Δt>0.1% / h or Δα>0.5° / h, the grouting pump is triggered to compensate for grouting.

[0018] Compared with the prior art, the present invention has the following advantages: 1. The large arch foot structure of the primary support arch capping method for underground tunnel stations described in this invention, based on rigidity theory, constructs a combined rigid support system of "double-layer primary support + water-stop anchor pipe + large arch foot bottom longitudinal beam + self-adjusting steel pipe piles." This significantly improves the ability to resist surrounding rock deformation, especially for shallow tunnels or tunnels in thin overburden strata, and has a good effect on controlling surface settlement and surrounding rock convergence. 2. The large arch foot structure of the underground station primary support arch capping method described in this invention fully utilizes self-adjusting steel pipe piles, pre-embeds fiber optic sensors, and employs a fiber optic sensor grouting linkage algorithm. Through data acquisition, data processing, linkage control, and feedback adjustment, an intelligent three-stage grouting process is implemented. This implements graded pressure control, significantly improving the cohesion of the broken rock formation by more than 25% compared to traditional grouting methods, significantly increasing the bearing capacity of the surrounding rock of the large arch foot. 3. The large arch foot structure of the underground excavation station with primary support arch capping method described in this invention uses composite pressure-bearing steel plates at the pile-beam nodes to eliminate stress concentration. This is then reinforced through intelligent grouting to form a pile-plate-beam trinity structure. This changes the traditional load transfer model that relies solely on steel frames, dispersing local compressive stress by 30-40%. 4. The large arch foot structure of the underground station primary support arch cover method described in the present invention significantly improves the anti-seepage performance of the arch foot and the stress conditions of the surrounding rock by designing a water-stopping locking foot anchor pipe or anchor rod; 5. The large arch foot structure of the initial support arch cover method of the underground excavation station described in the present invention adopts an automatic retractable formwork system. The formwork is composed of a hydraulic folding steel frame and a silicone resin panel. It is unfolded to fit the shape of the second support grid and shrinks after spraying for rapid demoulding, which significantly saves the time of installation and removal of the formwork and reduces the amount of concrete rebound. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is a front view of the large arch foot structure of the primary support arch cover method for a concealed excavation station according to the present invention; Figure 2 yes Figure 1 A partial enlarged view of area A in the middle; Figure Number: 1-Large pipe shed; 2-Small pipe; 3-Large arch foot; 31-Inward inclined surface; 4-Steel pipe pile; 41-Fiber optic sensor; 42-Pressure dividing steel plate; 43-Hinged rod; 44-Base; 45-Grouting hole; 5-Locked anchor pipe and anchor rod; 51-Slow-expansion water-stop strip; 6-First support; 61-First support steel grid; 7-Second support; 71-Second support steel grid; 8-Beard bar. DETAILED DESCRIPTION

[0020] In order to more clearly understand the above-mentioned objects, features and advantages of the present invention, the present invention is further described in detail below with reference to the accompanying drawings and specific implementation methods. It should be noted that the embodiments of the present application and the features in the embodiments can be combined with each other unless there is a conflict. Example

[0021] like Figures 1 to 2 As shown, the large arch foot structure of the initial support arch cover method of the underground excavation station includes: Inward-inclined footing: The inward-inclined footing is a structure that tilts inward, with an inclination angle of 1:6~8; Self-adjusting steel pipe piles 4 are arranged at the large arch feet 3 in the guide pits on both sides. The self-adjusting steel pipe piles 4 are connected to the bottom base surface of the bottom longitudinal beam via an articulated angle adjustment base 44. The articulated angle adjustment base 44 includes a fixed plate with a slide groove and a ball joint. The outer wall of the self-adjusting steel pipe pile 4 is axially embedded with an optical fiber sensor 41; The pressure dividing steel plate 42 is set at the pile-beam node between the top of the self-adjusting steel pipe pile 4 and the bottom longitudinal beam. The pressure dividing steel plate 42 is a 600×600×25mm steel plate. Two symmetrical grouting holes 45 are opened on the pressure dividing steel plate 42, and a grouting pipe joint and a check valve are installed. Bottom longitudinal beam, set along the longitudinal direction of the tunnel, its steel bars are welded to the pre-buried joint bars of the first support 6, and the concrete strength grade is C30; The primary support 6 includes a steel grille, system anchor rods, steel mesh, and shotcrete. The longitudinal spacing of the steel grille is 0.5 to 1 meter, and pre-embedded joint bars are provided on both sides of the foot of each steel grille. The joint bars are welded to the stirrups of the bottom longitudinal beam. The system anchor rods include water-stopping locking foot anchor pipe anchor rods 5. The rod body of the water-stopping locking foot anchor pipe anchor rods 5 is coated with two slow-expansion water-swelling water-stop rubber strips 51. The locking foot anchor pipe is a φ42 mm seamless steel pipe, and the anchor rod is a φ25 mm hollow anchor rod. The second support 7 includes a grid steel frame, a steel mesh, shotcrete, and reserved beard bars 8 or joint bars connecting the first support 6 and the second support 7. The circumferential spacing of the beard bars 8 is 1 to 1.2 meters. The bottom grid of the second support 7 is located inside the bottom longitudinal beam. Furthermore, the included angle θ between the inward-inclined footing of the structure and the 4-axis of the self-adjusting steel pipe pile satisfies: θ=arctan(1 / 8)+Δα, where Δα is the real-time adjustment angle of the articulated base, -5°≤Δα≤+5°; The self-adjusting steel pipe piles 4, the pressure-dividing steel plates 42, and the bottom longitudinal beam form a pile-plate-beam trinity structure. The inward-inclined footing, the self-adjusting steel pipe piles 4, the pressure-dividing steel plates 42, and the bottom longitudinal beam together constitute a joint rigid support system to resist deformation of the surrounding rock. The hinged rod 43 is a key component connecting the self-adjusting steel pipe pile 4 and the bottom longitudinal beam. Through flexible angle adjustment and stress transmission, it solves problems such as the complex inclination of the rock strata at the large arch foot 3, the high installation accuracy requirements of the self-adjusting steel pipe pile 4, and the poor adaptability to surrounding rock deformation.

[0022] Furthermore, in the articulated angle adjustment base 44 of the self-adjusting steel pipe pile 4, the rotation angle range of the ball joint is ±5°, and the slide fixing plate is fixedly connected to the bottom base surface of the bottom longitudinal beam by bolts; when the rock layer inclination changes by more than 3°, the hydraulic jacking system automatically compensates for the angle deviation to ensure that the pile-rock contact surface pressure is ≥0.5MPa; the optical fiber sensor 41 on the outer wall of the self-adjusting steel pipe pile 4 is arranged axially with a spacing of 1.5m. The sensor is covered with a polyurethane protective sleeve with a thickness of 2mm and a compressive strength of ≥50Mpa, which is used to monitor the inclination of the self-adjusting steel pipe pile 4 and the stress changes of the surrounding rock in real time.

[0023] Furthermore, the slow-expansion water-swelling water-stopping strip 51 of the water-stopping locking anchor pipe anchor rod 5 is a composite strip of butyl rubber and water-swelling rubber, with a thickness of 5~10mm, a 24h expansion rate of 80%, a shear strength of ≥0.15MPa after swelling in water, and a durability requirement that the expansion rate is retained at ≥85% after 50 dry-wet cycles; the exposed end of the anchor pipe is fixed by squeezing it with the base rock through a tray, and the tray is a circular steel plate with a diameter of φ60mm and a thickness of 10mm.

[0024] Furthermore, the grouting channels of the pressure dividing steel plate 42 are two symmetrically distributed φ15mm circular holes. One of the channels has a quick connector for detachable connection to the secondary grouting pipe, and the other channel serves as an exhaust hole. The pressure dividing steel plate 42 is fully welded to the top of the self-adjusting steel pipe pile 4. The weld thickness is ≥8mm, the weld metal yield strength is ≥355MPa, and the ultrasonic flaw detection complies with GB / T 11345-2013 Class B. The embedded joint reinforcement at the foot of the first support 6 is HRB400 steel bar with a diameter of φ22mm and a length of 300mm. It is welded on both sides to the bottom longitudinal beam stirrups with a diameter of φ12mm and a spacing of 200mm. The welding length is ≥5d, where d is the diameter of the steel bar.

[0025] The present invention also provides a method for constructing a large arch foot using the initial support arch cover method for a concealed excavation station, comprising: Step 1: Advance support. Before excavating the horse head gate, advance support is implemented. The advance support includes an advance large pipe shed 1 and an advance small pipe 2. The large pipe shed 1 is bored by a pipe shed drilling rig, and the small pipe 2 is bored by an air-leg air drill. The large pipe shed 1 is constructed 5 to 10 cm away from the contour line of the outer arch. The diameter of the large pipe shed 1 is φ108 mm, the spacing is 40 cm, the outward angle is not more than 5°, and the length is 15 to 20 m. Small pipes 2 are set between the large pipe sheds 1. The diameter of the small pipe 2 is φ42 mm and the length is 3 to 5 m. After the advance support is completed, a plunger pump is used to grout and reinforce the surrounding rock. The grouting slurry is pure cement slurry with a water-cement ratio of 0.8 to 1:1. The skip hole grouting method is adopted during grouting. Step 2: Double-side pilot tunnel construction, using mechanical crushing horse head gate, staggered excavation of the two side wall pilot tunnels; when the distance between the two side wall pilot tunnels is less than 1 times the tunnel diameter, the staggered distance is ≥15m; when the distance exceeds 1 times the tunnel diameter, the staggered distance is 3~5m, or parallel excavation is adopted to reduce the flow step time and save excavation period; the pilot tunnel is excavated by weak blasting or mechanical method, and the short step method is used in the same pilot tunnel, with the upper and lower step distances of 3~5m; the footing is mechanically excavated to form an inward inclined surface 31 to protect the integrity of the footing rock, with an inward inclination angle of 1:6~8; after the pilot tunnel is excavated, the first support 6 is promptly applied, which includes erecting the first support steel grid 61, driving the first The support system anchors, hangs the first support steel mesh, and sprays the first support concrete, and reserves beard bars 8, with a circumferential spacing of 1 to 1.2 meters. A water-stopping locking anchor pipe anchor rod 5 is driven at the foot of each step. The gap between the rod body and the rock wall is filled with a slow-expansion water-stopping rubber strip 51 on the surrounding wall of the water-stopping locking anchor pipe anchor rod 5. The water-stopping locking anchor pipe anchor rod 5 is grout-reinforced to the surrounding rock of the footing, and after grouting and reinforcement, the first support concrete is sprayed. Pre-embedded joint bars are placed on both sides of the foot of the first support steel grid 61. The joint bars are firmly welded on both sides and are welded to the stirrups of the bottom longitudinal beam when the bottom longitudinal beam reinforcement is tied. Step 3: Construction of the middle pilot tunnel. The middle pilot tunnel is excavated in two layers. The first layer is 2.5m high. After weak blasting or mechanical excavation, the first support 6 is installed. This includes connecting the top steel grids of the first support on both sides, driving the first support arch system anchor rods, hanging the first support steel mesh, and spraying the first support concrete. The second layer of the middle pilot tunnel is excavated after the construction of the second support 7 is completed. Step 4: Construction of self-adjusting steel pipe piles 4. 3 large arch feet in the pilot pit on both sides are drilled with Epiroc PD45 down-the-hole drills. The BIM model is used to preview the layout of the self-adjusting steel pipe piles 4. Laser-guided down-the-hole drilling is used to achieve a hole position error of ≤2cm, a hole diameter of 400mm, and a hole angle of 60°~70°. The diameter of the self-adjusting steel pipe piles 4 is φ219, the wall thickness is 10mm, and the Q 235B hot-rolled steel pipes are 7-9m long and spaced 1-1.2m apart longitudinally along the arch foot. The self-adjusting steel pipe piles 4 are bored at an angle of 60-70° to the horizontal, facilitating down-the-hole drilling. The self-adjusting steel pipe piles 4 are threaded in sections, with internal and external threads of 8-10cm. Each section of steel pipe is ≤1.5m long, and fiber optic sensors 41 are installed on their side walls and protected with polyurethane sleeves. Plum blossom-shaped grouting holes 45 are drilled within 3m of the bottom of the self-adjusting steel pipe piles 4. The grouting holes 45 have a diameter of φ6-10mm and are four symmetrical holes circumferentially spaced 40-50cm apart longitudinally. The bottom of the self-adjusting steel pipe piles 4 feature a wedge-shaped tip. The inclination angle is dynamically adjusted within a ±5° range using an articulated angle adjustment base 44, and precise control is achieved using a bottom inclination sensor. A skip-hole grouting and three-stage intelligent grouting process is used. The first grouting process involves inserting a sleeve valve tube into the middle of the self-adjusting steel pipe pile 4, sealing the grouting hole 45 with tape, and using pure cement slurry with a water-cement ratio of 0.5-0.75:1 and a pressure of 0.1-0.3 MPa to fill the gaps inside the steel pipe and between the steel pipe and the surrounding rock. The second grouting process involves sealing the pile top with a pressure-dividing steel plate 42. The grouting material is pure cement slurry and a micro-expansive agent or water-glass dual-liquid slurry, at a pressure of 0.5-0.8 MPa, to fill the gaps in the surrounding rock. The third stage grouting process involves real-time monitoring of surrounding rock deformation data using an optical fiber sensor 41, and intelligent compensatory grouting at a pressure of 0.8-1 MPa. Micro-expansive cement slurry is injected to form a pile-plate-beam trinity structure. The expansive agent used is magnesium oxide, with an activity of 60-80 seconds and a dosage of 8-10%. In the rock fracture zone, three φ28 main bars are added to the self-adjusting steel pipe pile 4, connected by sleeves, with the segment length consistent with that of the self-adjusting steel pipe pile 4. The grouting slurry contains 70-80% by volume of sulphoaluminate cement, 10-15% by volume of silica fume, and 8-10% by volume of magnesium oxide expansion agent; Step 5, construction of the second support 7, manually breaking the 1m range of the top of the middle partition wall for spraying concrete, installing the second support top grid steel frame between the two grids of the middle partition wall, bending and anchoring the second support top grid steel frame and the first support beard bar, hanging the second support top steel mesh, and connecting the bottom grids on both sides of the second support in the side wall guide pit to the bottom longitudinal beam; after the bottom longitudinal beam construction is completed, install the remaining second support steel mesh, install a retractable formwork system consisting of a hydraulic folding steel frame and a silicone resin panel on the grid steel frame, expand it to fit the shape of the second support grid, shrink it after spraying, and quickly demould it, realizing the concrete spraying machine hanging formwork spraying operation; Step 6, bottom longitudinal beam construction, after the second support steel bar grid 71 is installed, anchor steel bars are welded around the self-adjusting steel pipe piles 4, and the bottom longitudinal beam steel bars are tied. The main bars of the bottom longitudinal beam on both sides of the second support 7 bottom grid are abutted against the second support 7 grid steel frame, and the bottom longitudinal beam stirrups are welded to the first support 6 joint bars 8. After the steel bars are tied, the formwork is erected and C30 concrete is poured; after the excavation of the upper section is completed, the middle partition wall is removed to excavate the lower section. The excavation of the 3 large arch feet of the lower section adopts mechanical excavation or smooth blasting to control the rock stability of the large arch foot 3.

[0026] Furthermore, step 2 also includes: Step 21: The offset length depends on the distance between the two side wall pilot pits. When the hole diameter D is less than or equal to 8m, the offset L is greater than or equal to 15m. When D is greater than or equal to 8m, the offset L is equal to min(3-5m, D / 2), where D is the excavation hole diameter and the surface settlement rate is less than or equal to 0.5mm / d. Step 22: The anchor pipe is made of seamless steel pipe with a diameter of 42 mm, and the anchor rod is made of hollow anchor rod with a diameter of 25 mm. The water-stop type locking anchor pipe 5 or anchor rod is used to grout the surrounding rock of the footing to improve the bearing capacity of the surrounding rock at the footing. After the grouting is completed, the first support 6 sprayed concrete operation is carried out; Step 23: Pre-embed beard bars at a circumferential spacing of 1 to 1.2 m in the first support steel bar grid 61 to strengthen the connection between the first support 6 and the second support 7. Pre-embed joint bars on both sides of the bottom foot of each steel bar grid of the first support 6. The joint bars are firmly welded on both sides. When the bottom longitudinal beam reinforcement is tied, they are welded to the bottom longitudinal beam stirrups to enhance the integrity of the connection between the support and the bottom longitudinal beam.

[0027] Furthermore, step 4 also includes: Step 41: Fix the steel pipe at the hole with a clamp and connect it section by section from bottom to top with a pipe clamp. Adjacent steel pipe piles 4 are connected in a staggered manner. The length of the steel pipe section is adjusted to 50 cm. Step 42: An articulated angle-adjusting base 44 is provided at the top of the self-adjusting steel pipe pile 4. The base 44 consists of a fixed plate with a slide groove and a ball joint, which allows the inclination angle of the self-adjusting steel pipe pile 4 to be dynamically adjusted within a range of ±5°. The bottom end of the self-adjusting steel pipe pile 4 is moved by an articulated hydraulic jacking system, and the bottom optical fiber sensor is used for precise measurement to control the angle accuracy of the installation of the self-adjusting steel pipe pile 4, greatly improving the adaptability to the inclination angle of the rock layer. Step 43, after the down-the-hole is drilled, the mud and sand in the hole are blown clean by high-pressure air, and then the self-adjusting steel pipe piles 4 are lowered section by section. After the self-adjusting steel pipe piles 4 are installed, the self-adjusting steel pipe piles 4 are grouting treated; Step 44: Weld a pressure-dividing steel plate 42 at the orifice of the pile-beam node at the top of the pile. The plate 42 is sized at 600×600×25 mm. The plate 42 is provided to eliminate stress concentration. Two symmetrical grouting channels are provided on the plate. One of the channels is equipped with a grouting pipe joint and a check valve, and the other channel serves as a grouting exhaust hole. The grouting channel forms a 45° angle with the plane of the steel plate. The orifice is equipped with a check valve and has a pressure resistance of 1.2 MPa. The grouting slurry uses pure cement slurry and a micro-expansion agent or water glass double liquid slurry with a water-cement ratio of 1:1 and a double liquid slurry ratio of cement slurry: water glass = 1:1. The grouting pressure is controlled at 0.5-0.8 MPa. When the rock integrity coefficient is less than 0.35, the secondary grouting pressure is increased to 0.8-1.0 MPa. The functional relationship between grouting pressure and rock integrity is P = 0.5 + 0.3 × (0.45-Kv) MPa, where Kv is the rock integrity coefficient. In step 45, the pressure data collected in real time by the optical fiber sensor 41 is transmitted to the grouting pump through the PLC controller. When the monitored pressure is lower than the set threshold, the primary grouting threshold is 0.2 MPa and the secondary grouting threshold is 0.6 MPa, and the grouting pump is automatically started to supplement the grouting. After the grouting is completed, the optical fiber sensor 41 continuously monitors the changes in the surrounding rock pressure. When the pressure fluctuation exceeds 10% within 72 hours, compensatory grouting is triggered.

[0028] Furthermore, step 5 also includes: Step 51: Mechanically break up the shotcrete within 1m of the top of the temporary middle partition wall in the double-side wall pilot pit, install the top grid steel frame of the second support 7 between the two grids of the middle partition wall, bend and anchor the reserved beard reinforcement 8 of the first support 6 into the grid of the second support 7, hang the top steel mesh of the second support 7, install the grid steel frame of the second support 7 in the guide pits on both sides, and install the longitudinal connecting reinforcement to connect the grid steel frame of the second support 7; Step 52: After the construction of the bottom longitudinal beam is completed, the remaining steel mesh of the second support 7 is installed, and a retractable formwork for sprayed concrete is installed on the grid steel frame. The formwork is composed of a hydraulic folding steel frame and a silicone resin panel. It is unfolded to fit the shape of the grid of the second support 7 and shrunk and demoulded after spraying. The deflection of the hydraulic steel frame after unfolding is ≤ L / 500, where L is the span, and the elastic modulus of the silicone resin panel is ≥1.5 GPa. A concrete spraying machine is used to extend the nozzle into the formwork for hanging formwork spraying operation to reduce the amount of concrete rebound and speed up the construction progress of the second support 7. Step 53: The hydraulic folding steel frame is unfolded at an angle of 0° to 90°. The thickness of the silicone resin panel is 5 mm. The surface is coated with a silicone release agent with a coating amount of 0.2 kg / m². When spraying concrete, the distance between the nozzle and the inner surface of the formwork is controlled at 0.3 to 0.5 m, and the spraying pressure is 0.1 to 0.3 MPa.

[0029] Furthermore, step 6 also includes: Step 61: After the second support steel bar grid 71 is installed, anchor steel bars are welded around the self-adjusting steel pipe piles 4, and the bottom longitudinal beam steel bars are tied. The main bars of the bottom longitudinal beam on both sides of the second support 7 bottom grid are in contact with the second support 7 grid steel frame. The embedded joint bars of the first support 6 are anchored into the bottom longitudinal beam and firmly welded to the bottom longitudinal beam stirrups. The bottom longitudinal beam concrete formwork is erected and the bottom longitudinal beam concrete is poured. The concrete strength grade is C30. Step 62: After the excavation of the upper section is completed, the middle partition wall is removed and the lower section is excavated. When excavating the lower section, mechanical excavation or smooth blasting should be used under the large arch foot 3 to control the rock mass stability of the large arch foot 3. Step 63: Use a rock drilling rig in conjunction with a hydraulic breaker, with a blasthole depth of 1.5-2.0 m, a peripheral hole spacing of 0.4-0.5 m, a peripheral hole charge concentration of 0.18±0.02 kg / m, and a detonating cord relay detonation network. After smooth blasting, the surrounding rock disturbance range at the large arch foot 3 is controlled within 0.5 m, and the over-excavation is ≤100 mm.

[0030] The present invention also provides a monitoring device for the large arch foot construction method of the initial support arch cover method for a concealed excavation station, comprising a fiber optic sensor 41 buried in a self-adjusting steel pipe pile 4, a grouting pump PLC controller and a cloud analysis platform, wherein the fiber optic sensor 41 monitors the axial strain ε and the inclination angle α in real time, and when Δε / Δt>0.1% / h or Δα>0.5° / h, the grouting pump is triggered to compensate for grouting.

[0031] The present invention is not limited to the above-described embodiments. The above-described embodiments and descriptions are merely illustrative of the principles of the present invention. Various changes and modifications may be made to the present invention without departing from the spirit and scope of the present invention. Such changes and modifications are intended to fall within the scope of the present invention. The scope of protection claimed in the present invention is defined by the appended claims.

Claims

1. A large arch foot structure of a concealed excavation station with an initial arch cover method, characterized in that: include: Inward-inclined footing: The inward-inclined footing is a structure that tilts inward, with an inclination angle of 1:6~8; Self-adjusting steel pipe piles are installed at the large arch feet in the guide pits on both sides. The self-adjusting steel pipe piles are connected to the bottom base of the bottom longitudinal beam through an articulated angle adjustment base. The articulated angle adjustment base includes a fixed plate with a slide groove and a ball joint. The outer wall of the self-adjusting steel pipe pile is embedded with a fiber optic sensor along the axial direction. The pressure dividing steel plate is set at the pile-beam node between the top of the self-adjusting steel pipe pile and the bottom longitudinal beam. The pressure dividing steel plate is a 600×600×25mm steel plate. Symmetrical grouting channels are opened on the pressure dividing steel plate, and grouting pipe joints and check valves are installed. Bottom longitudinal beam, set along the longitudinal direction of the tunnel, its steel bars are welded to the pre-buried joint bars of the first support, and the concrete strength grade is C30; The primary support includes steel grilles, system anchor rods, steel mesh, and shotcrete. The longitudinal spacing of the steel grilles is 0.5-1m, and joint bars are embedded on both sides of the foot of each steel grille. The joint bars are welded to the stirrups of the bottom longitudinal beam. The system anchor rods include water-stopping locking anchor pipes or anchor rods. The rod body of the water-stopping locking anchor pipes or anchor rods is coated with two slow-expansion water-swelling water-stop rubber strips. The locking anchor pipes are φ42mm seamless steel pipes, and the anchor rods are φ25mm hollow anchor rods. The second support includes a grid steel frame, steel mesh, shotcrete, and reserved beard bars or joint bars connecting the first and second supports. The circumferential spacing of the beard bars is 1 to 1.2 meters. The bottom grid of the second support is located inside the bottom longitudinal beam. The angle θ between the inward-inclined footing and the axis of the self-adjusting steel pipe pile satisfies: θ=arctan(1 / 8)+Δα, where Δα is the real-time adjustment angle of the articulated base, -5°≤Δα≤+5°; Self-adjusting steel pipe piles, pressure-dividing steel plates, and bottom longitudinal beams form a pile-plate-beam trinity structure. The inward-inclined footing, self-adjusting steel pipe piles, pressure-dividing steel plates, and bottom longitudinal beams together constitute a joint rigid support system that resists surrounding rock deformation.

2. The large arch foot structure of the initial support arch cover method for a dark excavation station according to claim 1 is characterized in that: In the articulated angle adjustment base of the self-adjusting steel pipe pile, the rotation angle range of the ball joint is ±5°, and the slide fixing plate is fixed to the bottom base of the bottom longitudinal beam by bolts; when the rock layer inclination changes by more than 3°, the hydraulic jacking system automatically compensates for the angle deviation to ensure that the pile-rock contact surface pressure is ≥0.5MPa; the optical fiber sensors on the outer wall of the self-adjusting steel pipe pile are arranged axially with a spacing of 1.5m. The sensors are covered with a polyurethane protective sleeve with a thickness of 2mm and a compressive strength of ≥50Mpa, which is used to monitor the inclination of the self-adjusting steel pipe pile and the stress changes of the surrounding rock in real time.

3. The large arch foot structure of the primary support arch cover method for a concealed excavation station according to claim 1 is characterized in that: The slow-expansion water-swelling water-stop strip of the water-stop locking anchor pipe is a composite strip of butyl rubber and water-swelling rubber, with a thickness of 5~10mm, a 24h expansion rate of 80%, and a shear strength of ≥0.15MPa after expansion in water. The durability requirement is that the expansion rate is retained at ≥85% after 50 dry-wet cycles; the exposed end of the anchor pipe is fixed by squeezing the tray and the base rock. The tray is a circular steel plate with a diameter of φ60mm and a thickness of 10mm.

4. The large arch foot structure of the primary support arch cover method for a concealed excavation station according to claim 1 is characterized in that: The grouting channels of the pressure-dividing steel plate consist of two symmetrically distributed φ15mm circular holes. One of the grouting pipe joints is a quick connector, which is detachably connected to the secondary grouting pipe. The other hole serves as an exhaust hole. The pressure-dividing steel plate is connected to the top of the self-adjusting steel pipe pile by full welding. The weld thickness is ≥8mm, the weld metal yield strength is ≥355MPa, and the ultrasonic flaw detection complies with GB / T 11345-2013 Class B. The embedded joint reinforcement at the first support footing is HRB400 steel bar with a diameter of φ22mm and a length of 300mm. It is double-sided welded to the bottom longitudinal beam stirrups with a diameter of φ12mm and a spacing of 200mm. The welding length is ≥5d, where d is the diameter of the steel bar.

5. A method for constructing a large arch foot of a dark-excavated station using the primary arch cover method, applied to the large arch foot structure of a dark-excavated station using the primary arch cover method according to any one of claims 1 to 4, characterized in that: include: Step 1: Advance support. Before the excavation of the horse head gate, advance support is carried out. The advance support includes an advance large pipe shed and an advance small pipe shed. The large pipe shed is constructed by a pipe shed drilling rig, and the small pipe shed is drilled by an air leg type air drill. The large pipe shed is constructed 5 to 10 cm away from the contour line of the outer arch. The diameter of the large pipe shed is φ108mm, the spacing is 40cm, the outward angle is not more than 5°, and the length is 15 to 20m. Small pipes are set between the large pipe sheds. The diameter of the small pipe is φ42mm and the length is 3 to 5m. After the advance support is completed, a plunger pump is used to grout and reinforce the surrounding rock. The grouting slurry is pure cement slurry with a water-cement ratio of 0.8 to 1:

1. The skip hole grouting method is adopted during grouting. Step 2: Double-side pilot pit construction, using mechanical crushing horse head gate, staggered excavation of the two side wall pilot pits; when the distance between the two side wall pilot pits is less than 1 times the tunnel diameter, the staggered distance is ≥15m; when the distance exceeds 1 times the tunnel diameter, the staggered distance is 3~5m, or parallel excavation is adopted to reduce the flow step time and save excavation period; the pilot pit is excavated by weak blasting or mechanical method, and the short step method is used in the same pilot pit, with the upper and lower steps 3~5m apart; the footing is mechanically excavated to form an inward inclined surface to protect the integrity of the footing rock, with an inward inclination angle of 1:6~8; after the pilot pit is excavated, the first support is promptly applied, including the first support steel The reinforcement grid is installed, the anchor rods of the first support system are installed, the first support reinforcement mesh is hung, and the first support concrete is sprayed, and beard reinforcement is reserved; a water-stopping locking anchor pipe or anchor rod is installed at the foot of each step, and the gap between the water-stopping locking anchor pipe / anchor rod body and the water-stop rubber strip is filled and the gap between the rod body and the rock wall is coated with a slow-expansion water-stop rubber strip. The water-stopping locking anchor pipe or anchor rod is grout-reinforced to the surrounding rock of the footing, and after grouting and reinforcement, the first support concrete is sprayed; the joint reinforcement is embedded on both sides of the foot of the first support reinforcement grid, and the joint reinforcement is firmly welded on both sides. When the bottom longitudinal beam reinforcement is tied, it is welded to the bottom longitudinal beam stirrups; Step 3: Construction of the middle pilot tunnel. The middle pilot tunnel is excavated in two layers. The first layer is 2.5m high. After weak blasting or mechanical excavation, the primary support is installed. This includes connecting the top steel grids of the left and right primary supports, driving the primary support arch system anchors, hanging the primary support steel mesh, and spraying the primary support concrete. The second layer of the middle pilot tunnel is excavated after the secondary support construction is completed. Step 4: Self-adjusting steel pipe pile construction. Epiroc PD45 down-the-hole drills were used to drill holes at the large arch feet in the pilot pits on both sides. The self-adjusting steel pipe pile layout was previewed using the BIM model. Laser-guided down-the-hole drilling was used to achieve a hole position error of ≤2cm, a hole diameter of 400mm, and a hole angle of 60°~70°. The self-adjusting steel pipe pile had a diameter of φ219 and a wall thickness of 10mm. 235B hot-rolled steel pipes are 7-9m long and spaced 1-1.2m apart longitudinally along the arch foot. The self-adjusting steel pipe piles are bored at an angle of 60-70° to the horizontal to facilitate down-the-hole drilling. The self-adjusting steel pipe piles are threaded in sections, with internal and external threads of 8-10cm. Each section of steel pipe is ≤1.5m long, and fiber optic sensors are installed on the side walls and protected with polyurethane sleeves. Plum blossom-shaped grouting holes are drilled within 3m of the bottom of the self-adjusting steel pipe piles. The grouting holes are φ6-10mm in diameter and are four symmetrical holes circumferentially spaced 40-50cm apart longitudinally. The bottom of the self-adjusting steel pipe piles features a wedge-shaped tip. The inclination angle is dynamically adjusted within ±5° using an articulated angle adjustment base, and precise control is achieved using a bottom inclination sensor. A skip-hole grouting and three-stage intelligent grouting process is used. The first grouting process involves inserting a sleeve valve tube into the middle of the self-adjusting steel pipe pile, sealing the grouting holes with tape, and using pure cement slurry with a water-cement ratio of 0.5-0.75:1 and a pressure of 0.1-0.3 MPa to fill the gaps inside the steel pipe and between the steel pipe and the surrounding rock. The second grouting process involves sealing the pile top with a divided steel plate. The grouting material is pure cement slurry and a micro-expansive agent or water-glass double-liquid slurry, at a pressure of 0.5-0.8 MPa, to fill the surrounding rock cracks. The third stage grouting process involves real-time monitoring of surrounding rock deformation data using fiber optic sensors, intelligent compensatory grouting at a pressure of 0.8-1 MPa, and micro-expansive cement slurry is injected to form a pile-slab-beam trinity structure. The expansive agent is magnesium oxide, with an activity of 60-80 seconds and a dosage of 8-10%. In rock fracture zones, three φ28 main bars are added to the self-adjusting steel pipe pile, connected by sleeves, with the segment length matching that of the steel pipe pile. The grouting slurry contains: sulphoaluminate cement 70-80% by volume, silica fume 10-15% by volume, and magnesium oxide expansion agent 8-10% by volume; Step 5: Second support construction: manually break the 1m range on the top of the middle partition wall for spraying concrete, install the second support top grid steel frame between the two grids of the middle partition wall, bend and anchor the second support top grid steel frame and the first support beard bar, hang the second support top steel mesh, and connect the bottom grids on both sides of the second support in the side wall guide pit to the bottom longitudinal beam; after the bottom longitudinal beam construction is completed, install the remaining second support steel mesh, install a retractable formwork system consisting of a hydraulic folding steel frame and a silicone resin panel on the grid steel frame, expand it to fit the shape of the second support grid, shrink it after spraying, and quickly demould, realizing the concrete spraying machine hanging formwork spraying operation; Step 6: Construction of the bottom longitudinal beam. After the installation of the second support steel grid is completed, anchor steel bars are welded around the steel pipe piles, and the bottom longitudinal beam steel bars are tied. The main bars of the bottom longitudinal beam on both sides of the second support bottom grid are abutted against the second support grid steel frame, and the bottom longitudinal beam stirrups are welded to the first support joint bars. After the steel bars are tied, the formwork is erected and C30 concrete is poured. After the excavation of the upper section is completed, the middle partition wall is removed to start the excavation of the lower section. The excavation of the large arch foot of the lower section adopts mechanical excavation or smooth blasting to control the stability of the rock mass of the large arch foot.

6. The method for constructing a large arch foot of a dark-excavated station using the primary arch cover method according to claim 5 is characterized in that: Step 2 also includes: Step 21: The offset length depends on the distance between the two side wall pilot pits. When the hole diameter D is less than or equal to 8m, the offset L is greater than or equal to 15m. When D is greater than or equal to 8m, the offset L is equal to min(3-5m, D / 2), where D is the excavation hole diameter and the surface settlement rate is less than or equal to 0.5mm / d. Step 22: The anchor pipe is a seamless steel pipe with a diameter of 42 mm, and the anchor rod is a hollow anchor rod with a diameter of 25 mm. The water-stopping locking anchor pipe or anchor rod is used to grout the surrounding rock of the footing to improve the bearing capacity of the surrounding rock at the footing. After the grouting is completed, the first support shotcrete operation is carried out; Step 23: Embed beard bars at a circumferential spacing of 1 to 1.2 m in the first support steel grid to strengthen the connection between the first support and the second support. Embed joint bars on both sides of the bottom foot of each steel grid of the first support. The joint bars are welded firmly on both sides. When the bottom longitudinal beam reinforcement is tied, weld them to the bottom longitudinal beam stirrups to enhance the integrity of the connection between the support and the bottom longitudinal beam.

7. The method for constructing a large arch foot of a concealed excavation station using the primary arch cover method according to claim 5 is characterized in that: Step 4 also includes: Step 41: Fix the steel pipe at the hole with a clamp and connect it section by section from bottom to top with a pipe clamp. Adjacent self-adjusting steel pipe piles are connected in a staggered manner. The length of the steel pipe section is adjusted to 50 cm. Step 42: An articulated angle-adjustable base is provided at the top of the self-adjusting steel pipe pile. The base consists of a fixed plate with a slide groove and a ball joint. This allows the inclination angle of the self-adjusting steel pipe pile to be dynamically adjusted within a ±5° range. The bottom end of the steel pipe pile is automatically adjusted for movement through an articulated hydraulic jacking system. A fiber optic sensor at the bottom is used for precise measurement to control the angle accuracy of the self-adjusting steel pipe pile installation, significantly improving its adaptability to the inclination angle of the rock layer. Step 43: After the down-the-hole is drilled, high-pressure air is used to clean the mud and sand in the hole, and then the steel pipe piles are lowered section by section. After the self-adjusting steel pipe piles are installed, grouting and filling are performed on the self-adjusting steel pipe piles; Step 44: Weld a pressure-dividing steel plate with dimensions of 600×600×25 mm at the orifice of the pile-beam node at the top of the pile. The pressure-dividing steel plate at the pile-beam node is provided to eliminate stress concentration. Two symmetrical grouting channels are provided on the plate. One channel is equipped with a grouting pipe joint and a check valve, and the other channel serves as a grouting exhaust hole. The grouting channel forms a 45° angle with the plane of the steel plate. A check valve is provided at the orifice, with a pressure resistance of 1.2 MPa. The grouting slurry uses pure cement slurry and a micro-expansion agent or water glass double liquid slurry with a water-cement ratio of 1:1 and a double liquid slurry ratio of cement slurry: water glass = 1:

1. The grouting pressure is controlled between 0.5 and 0.8 MPa. When the rock integrity coefficient is less than 0.35, the secondary grouting pressure is increased to 0.8-1.0 MPa. The functional relationship between grouting pressure and rock integrity is P = 0.5 + 0.3 × (0.45 - Kv) MPa, where Kv is the rock integrity coefficient. In step 45, the pressure data collected in real time by the fiber optic sensor is transmitted to the grouting pump through the PLC controller. When the monitored pressure is lower than the set threshold, the primary grouting threshold is 0.2 MPa and the secondary grouting threshold is 0.6 MPa, and the grouting pump is automatically started to supplement the grouting. After the grouting is completed, the fiber optic sensor continuously monitors the changes in the surrounding rock pressure. When the pressure fluctuation exceeds 10% within 72 hours, compensatory grouting is triggered.

8. The method for constructing a large arch foot of a concealed excavation station using the primary arch cover method according to claim 5, characterized in that: Step 5 also includes: Step 51: Mechanically break up the shotcrete within 1m of the top of the temporary middle partition wall in the double-side wall pilot pit, install the second support top grid steel frame between the two grids of the middle partition wall, bend and anchor the reserved beard reinforcement of the first support into the second support grid, hang the second support top steel mesh, install the second support bottom grid steel frame in the two side wall pilot pits, and install the longitudinal connecting reinforcement to connect the second support grid steel frame; Step 52: After the bottom longitudinal beam is constructed, the remaining secondary support steel mesh is installed. A retractable sprayed concrete formwork is installed on the grid steel frame. The formwork is composed of a hydraulic folding steel frame and a silicone resin panel. The formwork is unfolded to fit the shape of the secondary support grid and shrunk and demolded after spraying. The deflection of the hydraulic steel frame after unfolding is ≤ L / 500, where L is the span and the elastic modulus of the silicone resin panel is ≥ 1.5 GPa. A concrete sprayer is used to extend the nozzle into the formwork for hanging formwork spraying to reduce concrete rebound and speed up the construction progress of the secondary support. Step 53: The hydraulic folding steel frame is unfolded at an angle of 0° to 90°. The thickness of the silicone resin panel is 5 mm. The surface is coated with a silicone release agent with a coating amount of 0.2 kg / m². When spraying concrete, the distance between the nozzle and the inner surface of the formwork is controlled at 0.3 to 0.5 m, and the spraying pressure is 0.1 to 0.3 MPa.

9. The method for constructing a large arch foot of a concealed excavation station using the primary arch cover method according to claim 5, characterized in that: Step 6 also includes: Step 61: After the second support steel bar grid is installed, anchor steel bars are welded around the steel pipe piles, and the bottom longitudinal beam steel bars are tied. The main bars of the bottom longitudinal beam on both sides of the second support bottom grid are abutted against the second support grid steel frame. The first support pre-buried joint bars are anchored into the bottom longitudinal beam and firmly welded to the bottom longitudinal beam stirrups. The bottom longitudinal beam concrete formwork is erected, and the bottom longitudinal beam concrete is poured. The concrete strength grade is C30. Step 62: After the excavation of the upper section is completed, the middle partition wall is removed and the lower section is excavated. When excavating the lower section, mechanical excavation or smooth blasting should be used under the arch foot to control the rock mass stability of the arch foot. Step 63: Use a rock drilling rig in conjunction with a hydraulic breaker, with a blasthole depth of 1.5-2.0 m, a peripheral hole spacing of 0.4-0.5 m, a peripheral hole charge concentration of 0.18±0.02 kg / m, and a detonating cord relay detonation network. After smooth blasting, the surrounding rock disturbance range at the large arch foot is controlled within 0.5 m, and the over-excavation is ≤100 mm.

10. The method for constructing a large arch foot of a concealed excavation station using the primary support arch covering method according to claim 5, characterized in that: Step 6 also includes: Step 61: After the second support steel bar grid is installed, anchor steel bars are welded around the steel pipe piles, and the bottom longitudinal beam steel bars are tied. The main bars of the bottom longitudinal beam on both sides of the second support bottom grid are abutted against the second support grid steel frame. The first support pre-buried joint bars are anchored into the bottom longitudinal beam and firmly welded to the bottom longitudinal beam stirrups. The bottom longitudinal beam concrete formwork is erected, and the bottom longitudinal beam concrete is poured. The concrete strength grade is C30. Step 62: After the excavation of the upper section is completed, the middle partition wall is removed and the lower section is excavated. When excavating the lower section, mechanical excavation or smooth blasting should be used under the arch foot to control the rock mass stability of the arch foot. Step 63: Use a rock drilling rig in conjunction with a hydraulic breaker, with a blasthole depth of 1.5-2.0 m, a peripheral hole spacing of 0.4-0.5 m, a peripheral hole charge concentration of 0.18±0.02 kg / m, and a detonating cord relay detonation network. After smooth blasting, the surrounding rock disturbance range at the large arch foot is controlled within 0.5 m, and the over-excavation is ≤100 mm.

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