A construction method of a double-arc surface three-dimensional intersection horse head gate entry hole in an ultra-deep buried loess stratum

Abstract

The application discloses a kind of super-deep buried loess stratum double-camber surface three-dimensional intersection horse head door entry hole construction method, comprising: the CRD method is used for construction of main line tunnel;When construction is close to the three-dimensional intersection part with horizontal passage, it is carried out to pick high and expand excavation, the number of pilot hole is increased to form the expansion section that satisfies double-camber surface structure, and primary support, temporary mid-partition and temporary inverted arch are constructed;The second lining of expansion section is constructed, and the area of horizontal passage is not poured, and horse head door ring beam is constructed around horizontal passage entrance;Horse head door breaking steel platform is erected;Primary support at horse head door is broken from top to bottom in layer and zone using steel platform, and horizontal passage primary support is promptly constructed;After entry, horizontal passage section is graded and picked high and expanded excavation.This application effectively solves the space timing conflict of CRD method temporary support and horse head door breaking and the double-camber surface three-dimensional intersection section support problem, with the advantages of close process connection, small surrounding rock disturbance, reliable structure stress, safe and efficient construction.

Description

Technical Field

[0001] This invention relates to the field of underground tunnel construction technology for subway stations, specifically to a construction method for safely entering the tunnel through a gate by using the CRD method in conjunction with graded elevation expansion excavation when the main tunnel and the cross passage of an underground subway station form a double-arc three-dimensional intersection under ultra-deep loess strata conditions. Background Technology

[0002] In the construction of urban subway tunnels using the cut-and-cover method, the construction of the "gateway" at the intersection of the construction cross passage and the main tunnel is one of the technical challenges. Excavation of the gateway alters the stress state of the original structure; improper construction can easily cause deformation of the original structure, increase surface settlement, and in severe cases, even lead to structural failure. Under conditions of extremely deep loess strata, due to the loose structure, low strength, poor self-supporting capacity, and sensitivity to water of the loess, the construction of the gateway faces problems such as large deformation of the surrounding rock and complex support stress, significantly increasing the construction difficulty.

[0003] The main technical challenges currently facing the construction of the Matoumen tunnel entrance are as follows: (1) The stability of the surrounding rock in the ultra-deep loess strata is poor. The excavation of the main tunnel causes great disturbance to the surrounding rock. The stress transformation of the surrounding rock-support structure at the Matoumen opening is complex.

[0004] (2) When the main tunnel is constructed using the CRD method, there is a spatial conflict and a construction sequence conflict between the layout of the temporary central partition wall and the temporary invert arch and the subsequent breaking of the cross passage horse head gate to enter the tunnel.

[0005] (3) The cross-section of the transverse passage and the main tunnel changes greatly at the intersection, requiring high-level excavation. The structural cross-section often presents a double-arc shape, which is difficult to adapt to the traditional single-arc support method of the horse-head gate.

[0006] (4) The strength of ultra-deep buried loess drops sharply after encountering water, which puts forward higher requirements for the stability of the surrounding rock and the timeliness of support during the demolition operation at Matoumen.

[0007] Based on the above problems, this invention provides a safe tunneling construction method for the main tunnel using the CRD method and the cross passage forming a double-arc three-dimensional intersection with the main tunnel under ultra-deep loess strata conditions, where the tunnel entrance is excavated in stages with heightening and widening. Summary of the Invention

[0008] The purpose of this invention is to provide a construction method for tunneling in ultra-deep loess strata using a double-arc three-dimensional intersection gate, which aims to solve the technical problems of spatial and temporal conflicts between temporary support and the breaking of the cross passage gate in the CRD method construction of the main tunnel, as well as the poor adaptability of the double-arc three-dimensional intersection section support.

[0009] To achieve the above objectives, the present invention adopts the following technical solution.

[0010] A method for constructing an ultra-deep loess stratum double-arc three-dimensional cross-shaped tunnel entrance includes the following steps.

[0011] Step 1: Construction of the main line tunnel. The CRD method is used for excavation during the construction of the main line tunnel.

[0012] Step 2: When the main tunnel is being constructed to the point where it is close to the predetermined grade-separated intersection with the cross passage, the tunnel is widened by raising the excavation. The number of pilot tunnels is increased according to the cross-sectional shape of the widened section of the main tunnel, and the excavation is carried out in sections to form a widened section that meets the requirements of the double-arc structure cross-section. Then, the initial support, temporary central partition wall and temporary invert arch of the widened section are constructed.

[0013] Step 3: After the excavation and initial support of the widened section of the main tunnel are completed, the secondary lining of the widened section is constructed, and the temporary central partition wall and temporary invert arch in the widened section are removed in sequence. When constructing the secondary lining of the widened section, an area for the design of the transverse passage on the side wall of the main tunnel is reserved, and the secondary lining is not poured in this area. The horse-head gate ring beam is constructed around the entrance of the transverse passage as a structural reinforcement.

[0014] Step 4: Construct a steel platform for breaking up the girder in the widened section, near the girder of the cross passage.

[0015] Step 5: Using the steel platform as the operating platform, the initial support of the main tunnel at the cross passage's head gate is broken up layer by layer and section by section from top to bottom, and the initial support of the cross passage is promptly implemented.

[0016] Step Six: After entering the tunnel through the cross passage's head gate, the cross passage's cross section is excavated in stages to gradually expand the excavation outline to the designed cross section, completing the construction of the double-arc three-dimensional intersection section between the cross passage and the main tunnel.

[0017] During the graded elevation expansion excavation, the cross-section of the transverse passage is excavated in four layers with eight small pilot tunnels. The eight small pilot tunnels are: upper left small pilot tunnel, upper right small pilot tunnel, left middle first small pilot tunnel, right middle first small pilot tunnel, left middle second small pilot tunnel, right middle second small pilot tunnel, lower left small pilot tunnel, and lower right small pilot tunnel. The two small pilot tunnels in each layer are staggered by 5m to 6m.

[0018] Preferably, the main tunnel construction process in step one is as follows.

[0019] Step 1.1: The first section of the main tunnel is excavated using the CRD method, with initial support, temporary central partition wall, and temporary invert arch constructed simultaneously during excavation; the standard section of the main tunnel is divided into four pilot tunnels for excavation.

[0020] Step 1.2: The temporary central partition wall and temporary invert arch in the first section of the main tunnel are dismantled in sections, and the secondary lining is poured in sections immediately after the dismantling to form a permanent structure.

[0021] Step 1.3, repeat steps 1.1 to 1.2 until construction reaches a position close to the designed cross passage.

[0022] Preferably, in step one, when excavating each pilot tunnel using the CRD method, the excavation outline is expanded outward by 80~120mm compared to the design outline as a reserved deformation amount; the longitudinal misalignment between adjacent pilot tunnels on the left and right sides of the same layer is not less than twice the tunnel diameter.

[0023] Preferably, in step four, the steel platform includes embedded steel plates, columns, main beams, secondary beams, and surface steel plates; the embedded steel plates are pre-embedded at intervals on the surface of the secondary lining structure during the pouring of the secondary lining concrete of the main tunnel; the columns are made of I-beams, with their bottoms connected to the embedded steel plates to support the entire platform; there are at least four main beams, each made of I-beams; multiple main beams are horizontally erected on top of the columns; the main beams are welded to the columns; the secondary beams are arranged parallel and spaced on top of the main beams, and are welded to the main beams; the surface steel plates are fully laid on the top surface of the secondary beams and welded to them; side supports are provided between adjacent columns, using diagonal braces or scissor braces to reinforce the columns and prevent them from tilting; safety guardrails are provided around the top of the steel platform, with a height of not less than 1.2m.

[0024] Preferably, in step five, before the initial support of the main tunnel at the cross passage's head gate is removed, advance support is implemented at the top of the head gate arch; the advance support includes advanced pipe roofs installed along the outer contour of the cross passage's head gate and deep-hole grouting of the surrounding rock around the head gate.

[0025] Preferably, after the removal of the upper step gate in step five, the initial support for the upper step of the cross passage is promptly constructed; the ends of the grid steel frame of the initial support for the cross passage are connected and fixed to the ring beam of the gate around the entrance of the main tunnel.

[0026] Preferably, in step five, the removal of the tunnel head is carried out in three layers: upper, middle, and lower steps. First, the initial support of the main tunnel corresponding to the upper step is removed, and then the initial support of the upper step is immediately constructed. Next, the initial support of the main tunnel corresponding to the middle step is removed, and then the initial support of the middle step is constructed. Finally, the initial support of the main tunnel corresponding to the lower step is removed, and then the initial support of the lower step is constructed. The length of each cycle of removal ranges from 2 to 3 grid panels.

[0027] Preferably, during the graded elevation expansion excavation process in step six, the settlement of the arch crown, horizontal convergence, stress of the support structure, and pressure of the surrounding rock in the Matoumen area are monitored in real time, and the excavation sequence and support parameters are adjusted in real time based on the monitoring data; among them, graded early warning values ​​and alarm values ​​are set for the settlement of the arch crown and horizontal convergence.

[0028] Preferably, the order of dismantling the temporary support using the CRD method in step three is as follows: first dismantle the temporary invert arch, then dismantle the temporary intermediate partition wall. The dismantling operation is carried out after the secondary lining concrete has reached the design strength.

[0029] Preferably, the construction of the eight small pilot tunnels in step six specifically includes the following steps.

[0030] Step 6.1: Before excavating the small pilot tunnel on the upper left, first install small guide pipes for advance support along the arch of the transverse passage and grout to reinforce the stratum.

[0031] Step 6.2: Excavate the soil of the upper left small pilot tunnel and reserve core soil, with an advance depth of one grid spacing; construct the initial support of the arch of the upper left small pilot tunnel, temporary invert arch and side wall anchor pipe.

[0032] Step 6.3: Excavate the core soil reserved in the upper left small pilot tunnel and the soil in the middle left small pilot tunnel, and construct the initial sidewall support, temporary invert arch and sidewall anchor bolts for the middle left small pilot tunnel.

[0033] Step 6.4: Excavate the core soil of the left-middle small pilot tunnel and the soil of the upper right small pilot tunnel, and construct the initial sidewall support, temporary invert arch and sidewall anchor bolts for the upper right small pilot tunnel.

[0034] Step 6.5: Excavate the core soil of the upper right small pilot tunnel and the soil of the middle right small pilot tunnel, and construct the initial sidewall support, temporary invert arch and sidewall anchor bolts for the middle right small pilot tunnel.

[0035] Step 6.6: Excavate the core soil reserved for the right center small pilot tunnel and the soil of the left center second small pilot tunnel, and construct the initial sidewall support, temporary invert arch and sidewall anchor bolts for the left center second small pilot tunnel.

[0036] Step 6.7: Excavate the reserved core soil for the left and right middle secondary pilot tunnels and the soil for the right middle secondary pilot tunnel, and construct the initial sidewall support, temporary invert arch, and sidewall anchor bolts for the right middle secondary pilot tunnel.

[0037] Step 6.8: Excavate the core soil reserved for the right middle small pilot tunnel and the soil of the left lower small pilot tunnel, and construct the initial sidewall support and invert arch of the left lower small pilot tunnel.

[0038] Step 6.9: Excavate the core soil reserved for the lower left small pilot tunnel and the soil of the lower right small pilot tunnel, construct the initial support and invert arch of the side wall of the lower right small pilot tunnel, and connect the invert arch of the lower left small pilot tunnel and the invert arch of the lower right small pilot tunnel into a whole by pre-reserved steel bars and shotcrete, so that the initial support of the cross section of this transverse passage is closed into a ring.

[0039] Step 6.10: Remove the temporary inverted arch and temporary central partition wall of the cross passage from top to bottom.

[0040] Repeat steps 6.1 to 6.10 above until the entire cross passage to be excavated is completed.

[0041] Compared with the prior art, the present invention has the following features and beneficial effects.

[0042] 1. This invention involves raising and widening the tunnel during the construction of the main tunnel to the cross passage, forming a double-arc cross-sectional profile that matches the cross passage's gate, and setting a gradual transition section with a span of not less than 1.5 times. This effectively solves the problem that traditional single-arc support for gates cannot adapt to complex cross-sectional changes at three-dimensional intersections, reduces stress concentration caused by abrupt changes in cross-section, and makes the structural stress more reasonable.

[0043] 2. This invention utilizes pre-embedded components in the secondary lining structure of the main tunnel to construct a steel platform for the excavation of the tunnel entrance. The platform serves as an operation and muck removal channel, cleverly avoiding spatial conflicts between the temporary intermediate partition wall and temporary invert arch of the CRD method and the tunnel entrance construction. This allows for orderly connection between the secondary lining of the main tunnel and the excavation of the transverse passage, minimizing process interference and facilitating smoother construction organization. The steel platform, relying on the completed secondary lining structure, provides stable and reliable load-bearing capacity, ensuring safety for high-altitude demolition operations.

[0044] 3. In this invention, after the transverse passage enters the tunnel, a graded expansion excavation is adopted, with each stage of expansion not exceeding 30% of the designed cross-section. Combined with real-time monitoring and measurement of arch settlement, horizontal convergence, support stress, etc., and graded early warning, information-based construction is realized. The expansion excavation sequence and support parameters can be dynamically adjusted according to the monitoring data, and the deformation of the ultra-deep buried loess three-dimensional intersection section is strictly controlled within the allowable range, avoiding structural instability problems that may be caused by a one-time large-section expansion excavation.

[0045] 4. This invention organically integrates the CRD method for mainline tunnel construction, preliminary high-level excavation, steel platform erection, layered demolition of the tunnel entrance, and graded high-level excavation, forming a complete set of construction methods suitable for ultra-deep loess strata with double-arc three-dimensional cross tunnel entrances. It solves the construction problems of existing technologies under such complex conditions, and has the advantages of close process connection, small disturbance to the surrounding rock, reliable structural stress conversion, high construction efficiency and safety, and has good promotion and application value. Attached Figure Description

[0046] The present invention will now be described in further detail with reference to the accompanying drawings.

[0047] Figure 1 This is a construction step diagram of the construction method for the ultra-deep buried loess strata double arc surface three-dimensional cross horse head gate tunnel of the present invention.

[0048] Figure 2 This is a schematic diagram of the structure of the main channel after it has been expanded at the horse-head gate in this invention.

[0049] Figure 3 This is a schematic diagram of the structure of the transverse passage with horse-head gate for subsequent excavation in this invention.

[0050] Figure 4 This is a schematic diagram of the structure of the steel platform for breaking down the horse-head gate in this invention, which is built at the intersection of the transverse passage and the main tunnel.

[0051] Figure 5 This is a layout diagram of the transverse passage and the left and right main tunnels in this invention.

[0052] Attached reference numerals: 1 - Embedded steel plate, 2 - Column, 3 - Main beam, 4 - Secondary beam, 5 - Surface steel plate, 6 - Side support, 7 - Safety guardrail. Detailed Implementation

[0053] This embodiment uses a subway station as an example. This station is a platform-concourse separated station. The concourse level is constructed using the open-cut method, while the platform level is constructed using the cut-and-cover method. The main line tunnel is divided into left and right main line tunnels, connected by a cross passage. The main line tunnels are excavated in sections via shafts #1, #2, and #3, and the cross passage. The cross passage excavation begins after the main line tunnel construction is completed. The total length of the cut-and-cover platform tunnel is 167.2m, with standard dimensions of 10.2m x 10.87m (width x height). The shield tunnel end section has dimensions of 12.05m x 11.37m (width x height); the overburden depth is approximately 39.0m. The strata traversed are mainly old loess and paleosol, with the groundwater level above the tunnel arch. The excavation height of cross passage #5 is 9.3m to 12.23m, the excavation width is 8.9m, and the length is 30.5m. It is constructed using the CRD method.

[0054] The specific construction steps are as follows: Step 1: Both the left and right mainline tunnels are excavated using the CRD method, and initial support, temporary intermediate diaphragm wall and temporary invert arch are constructed. The specific construction process of the main line tunnel is as follows: Step 1.1: The first section of the main tunnel is excavated using the CRD method. Before the main tunnel construction, advance exploratory boreholes are drilled, and advance support is installed. Initial support, temporary intermediate diaphragm walls, and temporary invert arches are installed simultaneously with excavation. The advance support consists of a large pipe roof installed within a 180° range at the top of the main tunnel entrance, and small advance guide pipes installed within a 180° range at the tunnel arch. The circumferential spacing of the small advance guide pipes is 0.3m, and the longitudinal spacing is 1m, arranged in an alternating pattern. The upper and lower benches are excavated using the CRD method, with a longitudinal step distance of 5-6m. The face of the right pilot tunnel should lag behind the face of the left pilot tunnel by 10-20m. When excavating each pilot tunnel using the CRD method, the excavation outline is expanded outward by 80-120mm compared to the design outline as a allowance for deformation. The longitudinal offset between adjacent pilot tunnels at the same level should not be less than twice the tunnel diameter.

[0055] Step 1.2: The temporary central partition wall and temporary invert arch in the first section of the main tunnel are dismantled in sections, and the secondary lining is poured in sections immediately after the dismantling to form a permanent structure; Step 1.3: Repeat steps 1.1 to 1.2 to construct the second section of the main line tunnel, the third section of the main line tunnel, ..., the Nth section of the main line tunnel, until the construction reaches a position close to the designed cross passage.

[0056] In this embodiment, when each main tunnel is excavated in opposite directions, the standard sections of the main tunnels at both ends are constructed simultaneously; when the distance between the two opposing tunnel faces is about 20m, one tunnel face is closed and excavation is carried out from one side in a single direction.

[0057] Step two: When the main tunnel construction reaches the point near the planned grade separation with the cross passage, a raised widening excavation is carried out. Based on the cross-sectional shape of the widened section of the main tunnel, the number of pilot tunnels is increased, and excavation is carried out in sections. The standard single arch excavation is widened into a multi-arc composite cross-section that can connect with the cross passage in a grade separation manner, forming an widened section that meets the requirements of a multi-arc composite cross-section. It is ensured that this cross-section has sufficient structural bearing capacity and spatial clearance to safely allow the cross passage to enter from the side. A gradual transition section is provided between the widened section and the standard section, with a length not less than 1.5 times the tunnel span.

[0058] The composite cross-sectional profile of the multi-arc combination includes a crown arc, side arch arcs, and an inverted arch arc. The crown arc uses a large radius to create an open top space. The side arch arcs are located below the arching line, especially on the side where an opening is needed to connect to the cross passage. To make room for the cross passage, the sidewalls are no longer vertical but instead use another arc with a smaller radius that bulges outward. The inverted arch arc is located at the bottom of the tunnel and is also a reverse arc.

[0059] The multi-arc composite cross-section transforms the original standard tunnel cross-section, which is simple in stress and has vertical sidewalls, into a spatial transition structure with variable cross-section and multiple curvatures. This structure can smoothly transfer the huge eccentric stress generated by the transverse passage entrance to the stable strata by utilizing the "arch effect," and also provides the necessary three-dimensional space for reserving core soil for the upper step of the transverse passage, constructing the gate ring beam, and erecting the steel platform, thereby achieving a safe and stable transition between the main tunnel and the transverse passage structure.

[0060] In this embodiment, the main tunnel is constructed from the standard section to the boundary of the widened section, where the widened section is excavated. Section 3 is excavated using 6 small pilot tunnels. Core soil is reserved in the upper left bench, and the first temporary invert arch is gradually raised, with the height of the upper left bench changing from 5.4m to 4m. A total of 6 temporary invert arches are set up for the first temporary invert arches (the temporary horizontal support structure located between the upper left bench and the lower bench). The 6 temporary invert arches are gradually raised, with each arch raised by about 0.24m. After the left upper small pilot tunnel has advanced about 5m, construction of the left middle small pilot tunnel begins; after the left middle small pilot tunnel has advanced about 3m, construction of the right upper small pilot tunnel begins; after the right upper small pilot tunnel excavates the reserved core soil and advances about 5m, construction of the right middle small pilot tunnel begins. After the right-middle pilot tunnel advanced about 3m, construction began on the left-lower pilot tunnel. After the left-lower pilot tunnel advanced 3m, construction began on the right-lower pilot tunnel. The right-lower pilot tunnel was initially closed into a ring, and section three was completed.

[0061] Step 3: After the excavation and initial support of the widened section of the main tunnel are completed, the secondary lining of the widened section is constructed, and the temporary central diaphragm and temporary invert arch in the widened section are removed in sequence. When constructing the secondary lining of the widened section, an area for the design of the transverse passage on the side wall of the main tunnel is reserved. The secondary lining is not poured in this area, only the initial support structure is retained for subsequent step 5. A horse-head gate ring beam is constructed around the entrance of the transverse passage as a structural reinforcement. The horse-head gate ring beam is cast integrally with the secondary lining structure of the main tunnel. Small steel molds are used for the construction of the secondary lining of the arched section. Secondary ribs are closely attached to the back of the small steel molds along the longitudinal direction of the arch. The secondary ribs are made of 100mm×100mm square timber. The main ribs are made of cold-formed channel steel according to the arc of the arch, with a spacing of 900mm. φ48 steel pipes are used for bracing at the arch foot, with the same spacing as the main ribs. For the secondary lining construction of the elevated section sidewalls, wooden formwork was used. Secondary beams were 100mm x 100mm square timber, spaced 300mm vertically along the sidewalls. Main beams were cold-formed channel steel arranged horizontally at 900mm intervals. Steel pipes were used to brace the backs of the main beams on both sidewalls, with spacing matching the main beam spacing. The bracing steel pipes were connected and fixed to the full-span steel pipe scaffolding system erected inside the tunnel using cross-couplers. The scaffolding was erected with a horizontal spacing of 900mm, a vertical spacing of 900mm, and a step distance of 1500mm; the top step distance was 1000mm, and the side spans had a horizontal spacing of 600mm and a vertical spacing of 900mm. The end formwork of the secondary lining sidewalls was fixed using bracing steel pipes. The formwork for the ring beams at the gable ends used a combination of standardized steel and wooden formwork.

[0062] Step 4: Construct a steel platform for breaking down the tunnel at the intersection of the transverse passage and the main tunnel. The steel platform includes embedded steel plates 1, columns 2, main beams 3, secondary beams 4, and surface steel plates 5. The embedded steel plates 1 are pre-embedded at intervals on the surface of the secondary lining structure during the pouring of the secondary lining concrete of the main tunnel. The columns 2 are made of I-beams and are connected to the embedded steel plates at their bottoms to support the entire platform. There are at least four main beams 3, each made of I-beams. Multiple main beams 3 are horizontally erected on top of the columns 2. The main beams 3 are welded to the columns 2. The secondary beams 4 are... The beam 4 is made of I-beams and is arranged in parallel at a center-to-center spacing of 500mm or 1000mm on the top of the main beam 3. The secondary beam 4 is welded to the main beam 3. The surface steel plate 5 is fully covered on the top surface of the secondary beam and welded to the secondary beam. The surface steel plate 5 is made of 10mm thick anti-slip steel plate. Side supports 6 are set between adjacent columns 2. The side supports 6 are diagonal braces or scissor braces to reinforce the columns 2 and prevent the columns 2 from tilting. Safety guardrails 7 are set around the top of the steel platform. The height of the safety guardrails 7 is not less than 1.2m.

[0063] In this embodiment, a horizontal connecting steel plate is installed at the bottom of the column 2. This horizontal connecting steel plate and the column 2 are pre-processed and connected by welding. The connection part is fully welded, and the weld seam must be full and dense. During the installation of the column, its verticality must be strictly guaranteed, and the hook is slowly lowered into place. After initial calibration to ensure that the vertical deviation is within 10mm and the verticality is verified to be error-free, the horizontal connecting steel plate is firmly connected to the pre-embedded steel plate 1 with bolts. Shims are added at the bolt connection part to prevent stress concentration and damage. After the column 2 is installed, the side support 6 is installed to reinforce the column 2.

[0064] Main beam 3 is made of I-beams and is firmly welded to the columns. Secondary beams are also made of I-beams with a center-to-center spacing of 500 / 1000mm. Secondary beam 4 is tightly abutted against the sidewall of the transverse passage. Space is reserved in the sidewall of the main tunnel corresponding to the construction position of the transverse passage. Temporary transverse supports are erected on the side of the main tunnel entrance (along the tunnel longitudinal direction, adjacent to the edge of the entrance). The two ends of the temporary transverse supports are respectively supported by the left and right sidewalls of the main tunnel. The support top position near the transverse passage opening is located on the solid wall at the side edge of the entrance. After the temporary transverse supports are completed, they are firmly connected with I-beams and welded to the main beams. A 10mm thick steel plate is fully laid on top of the secondary beams and welded to them. Spot welds are also made to the secondary beams, and welds are made to both ends and the center of the secondary beams. The effective weld length at each point is 15cm.

[0065] Welding requirements stipulate that double-sided fillet welds should be used for the connection between the columns and the main beams, and between the main beams and the secondary beams. The connection between the secondary beams and the surface steel plate can be made by single-sided fillet welds or spot welding to ensure a firm weld.

[0066] Step 5: Using a steel platform as the operating platform, the initial support of the main tunnel at the transverse passage's gate is broken layer by layer and section by section from top to bottom, and the initial support of the transverse passage is constructed in a timely manner. The gate is located on the side wall of the main tunnel, which is the area where the initial support shaft wall is broken to open the entrance of the transverse passage. Before breaking the initial support of the main tunnel at the transverse passage's gate, advance support is implemented at the top of the gate arch. The advance support includes advanced pipe roofs installed along the outer contour of the transverse passage's gate and deep-hole grouting of the surrounding rock around the gate. The diameter of the advanced pipe roofs is 108mm, the circumferential spacing between adjacent advanced pipe roofs is 300mm, and double-liquid grout is injected into the pipe roofs.

[0067] During the construction of the tunnel entrance, the work proceeds from top to bottom, proceeding in stages. After the demolition of the stepped tunnel entrances, the initial support for the upper steps of the transverse passage is promptly constructed: In the area of ​​the tunnel entrance, the conventional single-frame arrangement of the lattice steel frame in the initial support of the main tunnel is changed to a continuous arrangement of three frames erected side by side. The three frames are welded together with longitudinal connecting bars to form a whole, thereby enhancing the support rigidity of the entrance area and providing a connecting base for the transverse passage lattice steel frame. The transverse passage lattice steel frame is then firmly welded to the main tunnel lattice steel frame, with the longitudinal connecting bars arranged in a double layer at a circumferential spacing of 0.5m to form a ring beam. This ring beam facilitates the connection and transfer of stress between the initial support structure of the transverse passage and the secondary lining structure of the main tunnel. The two ends of the transverse passage connect to the left and right main tunnels respectively, requiring the demolition of the tunnel entrances at both ends. The demolition of the two end gates should be carried out in stages, and simultaneous operations are strictly prohibited. The demolition of the other end gate can only be carried out after one end gate has been demolished and the excavation and support length of the main tunnel in that direction has reached at least 5m.

[0068] Step Six: After entering the tunnel through the cross passage's main entrance, the cross passage's cross-section is excavated in stages, gradually expanding the excavation outline to the designed cross-section, completing the construction of the double-arc three-dimensional intersection section between the cross passage and the main tunnel. The cross passage excavation adopts the CRD method, with four levels of steps and eight small pilot tunnels excavated, with two small pilot tunnels in each level staggered by 5m~6m. The process flow is as follows: construction of the upper left small pilot tunnel → construction of the first left middle small pilot tunnel → construction of the upper right small pilot tunnel → construction of the first right middle small pilot tunnel → construction of the second left middle small pilot tunnel → construction of the second right middle small pilot tunnel → construction of the lower left small pilot tunnel → construction of the lower right small pilot tunnel.

[0069] Step 6.1: Before excavating the small pilot tunnel on the upper left, first construct the small guide pipe for advance support of the arch of the transverse passage, and grout to reinforce the stratum.

[0070] Step 6.2: Excavate the soil of the upper left small pilot tunnel. When excavating the upper left small pilot tunnel, reserve core soil with a length of 1.5m and a height of 1m. The depth of the excavation is the spacing of one grid. Then, construct the initial support of the arch of the upper left small pilot tunnel, the temporary invert arch and the side wall anchor pipe.

[0071] Step 6.3: Excavate the core soil reserved in the upper left small pilot tunnel and the soil in the middle left small pilot tunnel, and construct the initial sidewall support, temporary invert arch and sidewall anchor bolts for the middle left small pilot tunnel.

[0072] Step 6.4: Excavate the core soil of the left-middle small pilot tunnel and the soil of the upper right small pilot tunnel, and construct the initial sidewall support, temporary invert arch and sidewall anchor bolts for the upper right small pilot tunnel.

[0073] Step 6.5: Excavate the core soil of the upper right small pilot tunnel and the soil of the middle right small pilot tunnel, and construct the initial sidewall support, temporary invert arch and sidewall anchor bolts for the middle right small pilot tunnel.

[0074] Step 6.6: Excavate the core soil reserved for the right center small pilot tunnel and the soil of the left center second small pilot tunnel, and construct the initial sidewall support, temporary invert arch and sidewall anchor bolts for the left center second small pilot tunnel.

[0075] Step 6.7: Excavate the reserved core soil for the left and right middle secondary pilot tunnels and the soil for the right middle secondary pilot tunnel, and construct the initial sidewall support, temporary invert arch, and sidewall anchor bolts for the right middle secondary pilot tunnel.

[0076] Step 6.8: Excavate the core soil reserved for the right middle small pilot tunnel and the soil of the left lower small pilot tunnel, and construct the initial sidewall support and invert arch of the left lower small pilot tunnel.

[0077] Step 6.9: Excavate the core soil reserved for the lower left pilot tunnel and the earthwork for the lower right pilot tunnel. Construct the initial sidewall support and invert arch for the lower left pilot tunnel. The invert arch of the lower left pilot tunnel and the invert arch of the lower right pilot tunnel are connected into a whole by reserved steel bars and shotcrete.

[0078] Step 6.10: Remove the temporary inverted arch and the central partition wall from top to bottom. At this point, the construction of the transverse passage section is complete.

[0079] Step 6.11: Repeat steps 6.1 to 11 until the entire transverse passage to be excavated is completed.

[0080] In this embodiment, the initial support construction process for the tunnel mainline sidewalls and cross passages includes initial shotcrete, erection of grid steel frames, laying of steel mesh, and re-shotcrete. The initial shotcrete is about 3-5 cm thick and seals the surrounding rock in time. The grid steel frames are installed one by one according to the design spacing. The steel mesh is hung on the inside or outside of the steel frame. The re-shotcrete completely wraps the grid steel frame to achieve the design thickness.

[0081] In this embodiment, the removal of the tunnel head in step five is carried out in three layers: upper, middle, and lower steps. First, the initial support of the main tunnel corresponding to the upper step is removed, and then the initial support of the upper step is immediately constructed. Next, the initial support of the main tunnel corresponding to the middle step is removed, and then the initial support of the middle step is constructed. Finally, the initial support of the main tunnel corresponding to the lower step is removed, and then the initial support of the lower step is constructed. The length of removal in each cycle is 2 to 3 grid panels. That is, the length of removal for the upper step is 2 to 3 grid panels each time. After the middle and lower steps are completed, the upper step is removed again, with a length of 2 to 3 grid panels removed. This cycle is repeated.

[0082] The steel platform built in step five also serves as a transportation channel for the subsequent slag removal in the transverse passage. The top surface of the platform is covered with anti-slip steel plates and equipped with safety guardrails no less than 1.2m high.

[0083] The above embodiments are not exhaustive examples of specific implementation methods, and other embodiments are also possible. The purpose of the above embodiments is to illustrate the present invention, rather than to limit the scope of protection of the present invention. All applications derived from simple variations of the present invention fall within the scope of protection of the present invention.

Claims

1. A construction method for a double-arc, three-dimensional, intersecting, horse-head gate entrance in ultra-deep loess strata, characterized in that, Includes the following steps: Step 1: Construction of the main line tunnel. The CRD method is used for excavation during the construction of the main line tunnel. Step 2: When the main tunnel is being constructed to the point where it is close to the predetermined grade-separated intersection with the cross passage, the tunnel is widened by raising the excavation. The number of pilot tunnels is increased according to the cross-sectional shape of the widened section of the main tunnel, and the excavation is carried out in sections to form a widened section that meets the requirements of the double-arc structure cross-section. Then, the initial support, temporary central diaphragm wall and temporary invert arch of the widened section are constructed. Step 3: After the excavation and initial support of the widened section of the main tunnel are completed, the secondary lining of the widened section is constructed, and the temporary central partition wall and temporary invert arch in the widened section are removed in sequence. When constructing the secondary lining of the widened section, a cross passage area is reserved on the side wall of the main tunnel. The secondary lining is not poured in this area. A horse-head gate ring beam is constructed around the entrance of the cross passage as a structural reinforcement. Step 4: Construct a steel platform for breaking up the girder in the widened section, near the girder of the cross passage; Step 5: Using the steel platform as the operating platform, the initial support of the main tunnel at the cross passage gate is broken layer by layer and section by section from top to bottom, and the initial support of the cross passage is promptly implemented. Step 6: After entering the tunnel through the cross passage's horse-head gate, the cross passage's cross section is excavated in stages to gradually expand the excavation outline to the designed cross section, completing the construction of the double-arc three-dimensional intersection section between the cross passage and the main tunnel. During the graded elevation expansion excavation, the cross-section of the transverse passage is excavated in four layers with eight small pilot tunnels. The eight small pilot tunnels are: upper left small pilot tunnel, upper right small pilot tunnel, left middle first small pilot tunnel, right middle first small pilot tunnel, left middle second small pilot tunnel, right middle second small pilot tunnel, lower left small pilot tunnel, and lower right small pilot tunnel. The two small pilot tunnels in each layer are staggered by 5m to 6m.

2. The construction method for ultra-deep buried loess strata with double-arc surface three-dimensional cross-shaped entrance as described in claim 1, characterized in that, The specific construction process of the main tunnel in Step One is as follows: Step 1.1: The first section of the main tunnel is excavated using the CRD method, with initial support, temporary central diaphragm wall, and temporary invert arch constructed simultaneously during excavation; the standard section of the main tunnel is divided into four pilot tunnels for excavation. Step 1.2: The temporary central partition wall and temporary invert arch in the first section of the main tunnel are dismantled in sections, and the secondary lining is poured in sections immediately after the dismantling to form a permanent structure; Step 1.3, repeat steps 1.1 to 1.2 until construction reaches a position close to the designed cross passage.

3. The construction method for ultra-deep buried loess strata with double-arc three-dimensional intersecting horse-head gate as described in claim 1, characterized in that: In step one, when excavating each pilot tunnel using the CRD method, the excavation outline is expanded outward by 80~120mm compared to the design outline as a reserve for deformation; the longitudinal offset between adjacent pilot tunnels on the left and right sides of the same layer is not less than twice the tunnel diameter.

4. The construction method for ultra-deep buried loess strata with double-arc three-dimensional intersecting horse-head gate as described in claim 1, characterized in that: In step four, the steel platform includes embedded steel plates (1), columns (2), main beams (3), secondary beams (4), and surface steel plates (5); the embedded steel plates (1) are embedded at intervals on the surface of the secondary lining structure during the pouring of the secondary lining concrete of the main tunnel; the columns (2) are made of I-beams and are connected at the bottom to the embedded steel plates (1) to support the entire platform; there are at least four main beams (3), each of which is made of I-beams; multiple main beams (3) are horizontally erected on top of the columns (2); the main beams (3) and the columns (2) The secondary beams (4) are arranged parallel to each other on the top of the main beam (3), and the secondary beams (4) are welded to the main beam (3); the surface steel plate (5) is fully laid on the top surface of the secondary beams (4) and welded to the secondary beams (4); side supports (6) are provided between adjacent columns (2), and the side supports (6) are diagonal braces or scissor braces to reinforce the columns (2) and prevent the columns (2) from tilting; safety guardrails (7) are provided around the top of the steel platform, and the height of the safety guardrails (7) is not less than 1.2m.

5. The construction method for ultra-deep buried loess strata with double-arc three-dimensional intersecting horse-head gate as described in claim 1, characterized in that: Before the initial support of the main tunnel at the cross passage's head gate is removed in step five, advance support is implemented at the top of the head gate arch. The advance support includes the installation of advance pipe roofs along the outer contour of the cross passage's head gate and deep-hole grouting of the surrounding rock around the head gate.

6. The construction method for ultra-deep buried loess strata with double-arc three-dimensional intersecting horse-head gate as described in claim 1, characterized in that: After the removal of the upper step gate in step five is completed, the initial support of the upper step of the cross passage should be constructed in a timely manner; the ends of the grid steel frame of the initial support of the cross passage should be connected and fixed to the ring beam of the gate around the entrance of the main tunnel.

7. The construction method for ultra-deep buried loess strata with double-arc three-dimensional intersecting horse-head gate as described in claim 1, characterized in that: In step five, the demolition of the tunnel head is carried out in three layers: upper, middle, and lower steps. First, the initial support of the main tunnel corresponding to the upper step is demolished, and then the initial support of the upper step is immediately constructed. Next, the initial support of the main tunnel corresponding to the middle step is demolished, and then the initial support of the middle step is constructed. Finally, the initial support of the main tunnel corresponding to the lower step is demolished, and then the initial support of the lower step is constructed. The demolition length of each cycle ranges from 2 to 3 grid panels.

8. The construction method for ultra-deep buried loess strata with double-arc three-dimensional intersecting horse-head gate as described in claim 1, characterized in that: In step six, during the graded elevation expansion excavation process, the settlement of the arch crown, horizontal convergence, stress of the support structure, and pressure of the surrounding rock in the Matoumen area are monitored in real time. The excavation sequence and support parameters are adjusted in real time based on the monitoring data. Graded early warning values ​​and alarm values ​​are set for the settlement of the arch crown and horizontal convergence.

9. The construction method for ultra-deep buried loess strata with double-arc three-dimensional intersecting horse-head gate as described in claim 1, characterized in that: The order of dismantling the temporary support using the CRD method in step three is as follows: first dismantle the temporary invert arch, then dismantle the temporary intermediate partition wall. The dismantling work shall be carried out after the secondary lining concrete has reached the design strength.

10. The construction method for ultra-deep buried loess strata with double-arc three-dimensional intersecting horse-head gate as described in claim 1, characterized in that: The construction of the eight pilot tunnels in step six specifically includes the following steps: Step 6.1: Before excavating the small pilot tunnel on the upper left, first install small guide pipes for advance support along the arch of the transverse passage and grout to reinforce the stratum; Step 6.2: Excavate the soil of the upper left small pilot tunnel and reserve core soil, with an advance depth of one grid spacing; construct the initial support of the arch of the upper left small pilot tunnel, temporary invert arch and side wall anchor pipes. Step 6.3: Excavate the core soil reserved in the upper left small pilot tunnel and the soil of the middle left small pilot tunnel, and construct the initial sidewall support, temporary invert arch and sidewall anchor bolts for the middle left small pilot tunnel. Step 6.4: Excavate the core soil of the left middle small pilot tunnel and the soil of the right upper small pilot tunnel, and construct the initial sidewall support, temporary invert arch and sidewall anchor bolts for the right upper small pilot tunnel. Step 6.5: Excavate the core soil of the upper right small pilot tunnel and the soil of the middle right small pilot tunnel, and construct the initial sidewall support, temporary invert arch and sidewall anchor bolts for the middle right small pilot tunnel. Step 6.6: Excavate the core soil reserved for the right center small pilot tunnel and the soil of the left center small pilot tunnel, and construct the initial sidewall support, temporary invert arch and sidewall anchor bolts for the left center small pilot tunnel. Step 6.7: Excavate the reserved core soil for the left and right middle small pilot tunnels and the soil for the right middle small pilot tunnels, and construct the initial sidewall support, temporary invert arch and sidewall anchor bolts for the right middle small pilot tunnels. Step 6.8: Excavate the core soil reserved for the right middle small pilot tunnel and the soil of the left lower small pilot tunnel, and construct the initial sidewall support and invert arch of the left lower small pilot tunnel; Step 6.9: Excavate the core soil reserved for the lower left small pilot tunnel and the soil of the lower right small pilot tunnel, construct the initial support and invert arch of the side wall of the lower right small pilot tunnel, and connect the invert arch of the lower left small pilot tunnel and the invert arch of the lower right small pilot tunnel into a whole by pre-reserved steel bars and shotcrete, so that the initial support of the cross section of this section is closed into a ring. Step 6.10: Remove the temporary inverted arch and temporary central partition wall of this section of the transverse passage from top to bottom; Repeat steps 6.1 to 6.10 above until the entire cross passage to be excavated is completed.

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