Steel arch assembly for assisting tunnel to gradually enter main tunnel and construction method

Abstract

The invention relates to the technical field of tunnel engineering construction, in particular to a steel arch assembly for assisting a tunnel in gradually changing into a main tunnel and a construction method.The steel arch assembly comprises a plurality of first steel arches, and the ends, close to a main tunnel, of the first steel arches are sequentially provided with multiple sets of gradually-changing steel arches in the extending direction of the auxiliary tunnel; the multiple sets of gradually-changed steel arches comprise a plurality of second steel arches, a plurality of third steel arches, a plurality of fourth steel arches, a plurality of fifth steel arches and a plurality of sixth steel arches which are sequentially connected in the extending direction. The top outlines of the multiple sets of gradually-changing steel arches are gradually changed into straight lines from arc shapes, the supporting structure which smoothly transits from the section of the auxiliary tunnel to the rectangular section of the main tunnel is jointly formed, and the construction method comprises the steps of construction preparation of the gradually-changing section, excavation through a bench method and gradually-changing supporting, section gradually-changing and measurement control, intersection reinforcing supporting and main tunnel connection and main tunnel excavation. Safe and smooth transition from the auxiliary tunnel to the main tunnel is achieved, surrounding rock disturbance is reduced, and supporting stability is enhanced.

Description

Technical Field

[0001] This invention relates to the field of tunnel construction technology, specifically to a steel arch frame assembly and construction method for assisting the gradual transition of a tunnel into the main tunnel. Background Technology

[0002] In the construction of long tunnels for railways and highways, a strategy of "short-term construction of long tunnels" is often adopted to accelerate the construction progress. This involves increasing the working face by setting up auxiliary tunnels such as inclined shafts and cross passages. The intersection section where the auxiliary tunnel transitions into the main tunnel is difficult to construct and carries high safety risks, making it a critical control point in tunnel construction. Traditional construction methods, such as "mechanical top lifting" and "arc-shaped pilot tunnel top lifting," typically involve large-scale excavation of the entire cross-section of the auxiliary tunnel at the top lifting location to achieve the installation of "gantry support." This method not only causes severe disturbance to the surrounding rock but also involves a large excavation face, high support difficulty, and prolonged exposure of workers to exposed surrounding rock, posing significant safety risks. Furthermore, traditional support methods rely on experience, making it difficult to guarantee support quality and prone to arch deformation, which seriously affects construction progress and safety. Therefore, there is an urgent need for a construction method and supporting structure for the transition from the auxiliary tunnel to the main tunnel that can achieve a smooth transition, reliable support, and efficient construction. Summary of the Invention

[0003] The problem to be solved by this invention is to provide a steel arch frame assembly and construction method for the gradual transition of the auxiliary tunnel into the main tunnel. By setting up multiple sets of gradual steel arch frames, a smooth transition from the arc-shaped cross section of the auxiliary tunnel to the rectangular cross section of the main tunnel can be achieved. Combined with the step excavation method and dynamic monitoring and measurement technology, it solves the technical problems of large surrounding rock disturbance, high construction safety risk and low operation efficiency in traditional top-lifting construction.

[0004] To address the shortcomings of existing technologies, the technical solution adopted by this invention is as follows: a steel arch frame assembly for the gradual transition of an auxiliary tunnel into the main tunnel, comprising multiple first steel arch frames, wherein multiple sets of gradual transition steel arch frames are sequentially arranged along the extension direction of the auxiliary tunnel at the end of the multiple first steel arch frames near the main tunnel; the multiple sets of gradual transition steel arch frames include multiple second steel arch frames, multiple third steel arch frames, multiple fourth steel arch frames, multiple fifth steel arch frames, and multiple sixth steel arch frames connected sequentially in the extension direction; the top contour of the multiple sets of gradual transition steel arch frames gradually changes from an arc shape to a straight line shape, together forming a support structure that smoothly transitions from the cross-section of the auxiliary tunnel to the rectangular cross-section of the main tunnel.

[0005] Preferably, the second steel arch frame includes a second steel arch frame section 1, a second steel arch frame section 2, a second steel arch frame section 3, a second steel arch frame section 4, and a second steel arch frame section 5 connected in sequence; wherein, the arc length of the second steel arch frame section 1 and the second steel arch frame section 5 is 2.8 meters and the chord height is 18 mm; the arc length of the second steel arch frame section 2 and the second steel arch frame section 4 is 3.5 meters and the chord height is 18 mm; and the arc length of the second steel arch frame section 3 is 5 meters and the chord height is 2 mm.

[0006] Preferably, the third steel arch frame comprises three sections connected in sequence: a first section, a second section, a third section, a fourth section, and a fifth section; wherein the first and fifth sections have an arc length of 2.8 meters and a chord height of 18 mm; the second and fourth sections have an arc length of 3.5 meters and a chord height of 13 mm; and the third section has an arc length of 5 meters and a chord height of 13 mm.

[0007] Preferably, the fourth steel arch frame comprises four sections connected in sequence: a first section, a second section, a third section, a fourth section, and a fifth section; wherein the first and fifth sections have an arc length of 2.8 meters and a chord height of 18 mm; the second and fourth sections have an arc length of 3.5 meters and a chord height of 12 mm; and the third section has an arc length of 5 meters and a chord height of 7 mm.

[0008] Preferably, the fifth steel arch frame comprises five sections connected in sequence: a first section, a second section, a third section, a fourth section, and a fifth section; wherein the first and fifth sections have an arc length of 2.8 meters and a chord height of 18 mm; the second and fourth sections have an arc length of 4 meters and a chord height of 8 mm; and the third section is a straight section with a length of 5.04 meters.

[0009] Preferably, the sixth steel arch frame is a rectangular shed structure, comprising a top sixth steel arch frame section 2 and a sixth steel arch frame section 1 and a sixth steel arch frame section 3 connected to the two ends of the sixth steel arch frame section 2; wherein, the sixth steel arch frame section 1 and the sixth steel arch frame section 3 are both vertical sections with a length of 4 meters, and the sixth steel arch frame section 2 is a horizontal section with a length of 5.04 meters.

[0010] A construction method for gradually transitioning from an auxiliary tunnel to the main tunnel includes the following steps: S1. Construction preparation for the transition section: When the auxiliary tunnel is constructed to a point 20 meters from the intersection with the main tunnel, the arch is raised at a 5° climbing slope, and the support parameters are determined based on the results of advanced geological forecasts. S2. Step-by-step excavation and gradual support: The two-step method is used for excavation. The excavation height of the upper step is controlled at 3.5 to 4 meters, and the excavation advance in each cycle does not exceed the distance between two arch frames. After excavation, the corresponding steel arch frames are installed immediately, and the system anchor bolts and locking foot anchor pipes are constructed to make the initial support of the invert arch form a ring as soon as possible. The length of the upper step is controlled at 5 meters, and the length of the lower step is controlled at 5 to 8 meters. S3. Section Gradual Change and Measurement Control: According to the preset gradual change drawings, install the second to sixth steel arch frames in sequence along the extension direction of the auxiliary tunnel, so that the support section gradually changes from the arc of the auxiliary tunnel to the rectangular opening; the surveyors need to accurately lay out on site and control the opening position and angle. S4. Intersection reinforcement support and connection with the main tunnel: Reinforced support is used on the last arch frame near the main tunnel; then the sixth steel arch frame 6 is installed to form a rectangular tunnel, and this is used as a passage to excavate to the opposite side of the main tunnel, and the main tunnel arch is constructed. S5. Main Tunnel Excavation: After all the roof lifting work is completed, remove the support legs on both sides of the rectangular tunnel and proceed to the normal excavation process of the main tunnel.

[0011] Preferably, in steps S2 and S4, the blasting operation follows the principles of "short advance, weak blasting, frequent measurement, strong support, and early ring formation".

[0012] Preferably, in steps S2 and S4, monitoring and measurement points are densely set up in the cross-construction area; before the next cycle of blasting, the initial shotcrete strength is not less than 10MPa, and the reserved deformation amount is adjusted according to the measurement data.

[0013] Preferably, in step S4, the reinforced support is a reinforced arch frame made of double-jointed 20a I-beams.

[0014] The beneficial effects of this invention are as follows: By employing a gradual steel arch frame assembly and a matching construction method, this invention achieves a smooth transition from the auxiliary tunnel to the main tunnel, effectively solving the problems of large surrounding rock disturbance, high support difficulty, and high safety risks in traditional cantilever construction. Specifically, this invention, through the sequential arrangement of multiple sets of gradual steel arch frames, gradually transitions the support cross-section from an arc shape to a rectangle, avoiding the large-scale vertical excavation required for installing the gantry in traditional methods, significantly reducing construction difficulty and personnel exposure risks. The use of the step-method excavation and the construction principles of "short advance, weak blasting, frequent measurement, strong support, and early ring formation," combined with dense monitoring and measurement data feedback, ensures dynamic control and safety controllability of the construction process. Furthermore, the introduction of the rectangular tunnel structure not only facilitates the layout of ventilation belts and vehicle passage but also improves the structural stability at intersections. Overall, this invention has significant advantages in improving construction efficiency, ensuring operational safety, and enhancing the deformation resistance of the support structure, demonstrating good engineering applicability and promotional value. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of the structure of the present invention; Figure 2 for Figure 1 Enlarged view of point G in the middle; Figure 3 This is a front view of the first to sixth steel arch frames of the present invention; Figure 4 This is a schematic diagram of the first steel arch frame structure of the present invention; Figure 5 This is a schematic diagram of the second steel arch frame structure of the present invention; Figure 6 This is a schematic diagram of the third steel arch frame structure of the present invention; Figure 7 This is a schematic diagram of the fourth steel arch frame structure of the present invention; Figure 8 This is a schematic diagram of the fifth steel arch frame structure of the present invention; Figure 9 This is a schematic diagram of the sixth steel arch frame structure of the present invention; Figure 10 This is a flowchart of the construction process for a single excavation and support cycle in this invention.

[0016] Explanation of reference numerals in the attached drawings: 1. First steel arch frame; 11. First steel arch frame section 1; 12. First steel arch frame section 2; 13. First steel arch frame section 3; 14. First steel arch frame section 4; 15. First steel arch frame section 5; 2. Second steel arch frame; 21. Second steel arch frame section 1; 22. Second steel arch frame section 2; 23. Second steel arch frame section 3; 24. Second steel arch frame section 4; 25. Second steel arch frame section 5; 3. Third steel arch frame; 31. Third steel arch frame section 1; 32. Third steel arch frame section 2; 33. Third steel arch frame section 3; 34. Third steel arch frame section 4; 35. Third steel arch frame section 5; 4. Fourth steel arch frame... Arch frame; 41. Section 1 of the fourth steel arch frame; 42. Section 2 of the fourth steel arch frame; 43. Section 3 of the fourth steel arch frame; 44. Section 4 of the fourth steel arch frame; 45. Section 5 of the fourth steel arch frame; 5. Fifth steel arch frame; 51. Section 1 of the fifth steel arch frame; 52. Section 2 of the fifth steel arch frame; 53. Section 3 of the fifth steel arch frame; 54. Section 4 of the fifth steel arch frame; 55. Section 5 of the fifth steel arch frame; 6. Sixth steel arch frame; 61. Section 1 of the sixth steel arch frame; 62. Section 2 of the sixth steel arch frame; 63. Section 3 of the sixth steel arch frame; 7. Ventilation belt; 8. Pre-reserved connecting plate for arch support; 9. Centerline of tunnel line; 10. System anchor bolt; 20. Locking foot anchor pipe. Detailed Implementation

[0017] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments. The embodiments of the present invention are given for illustrative and descriptive purposes only, and are not intended to be exhaustive or to limit the invention to the forms disclosed. Many modifications and variations will be apparent to those skilled in the art. The embodiments were chosen and described to better illustrate the principles and practical application of the invention, and to enable those skilled in the art to understand the invention and design various embodiments with various modifications suitable for a particular purpose.

[0018] like Figure 1-10 As shown, this invention provides a steel arch frame assembly and construction method for the gradual transition of an auxiliary tunnel into the main tunnel. It is applicable to the construction of the intersection section where auxiliary tunnels (such as inclined shafts, cross tunnels, and pilot tunnels) transition into the main tunnel in long tunnels such as railways and highways. The steel arch frame assembly includes multiple first steel arch frames 1, each a standard arc-shaped support structure used for the initial support of the normal section of the auxiliary tunnel. Area A represents the first steel arch frame 1, as shown... Figure 4 As shown, the first steel arch frame 1 comprises a first steel arch frame section 11, a first steel arch frame section 12, a first steel arch frame section 13, a first steel arch frame section 14, and a first steel arch frame section 15 connected sequentially. The arc length of the first steel arch frame section 11 and the first steel arch frame section 15 is 2.6 meters, and the chord height is 18 mm. The arc length of the first steel arch frame section 12, the first steel arch frame section 13, and the first steel arch frame section 14 is 3.9 meters, and the chord height is 27 mm. This structure is suitable for conventional cross-sections of auxiliary tunnels, providing stable initial support.

[0019] To ensure a smooth transition to the main tunnel section, multiple sets of gradually changing steel arches are sequentially installed along the extension direction of the auxiliary tunnel at the end of the first steel arch 1 closest to the main tunnel. These sets of gradually changing steel arches include multiple second steel arches 2, multiple third steel arches 3, multiple fourth steel arches 4, multiple fifth steel arches 5, and multiple sixth steel arches 6, connected sequentially in the extension direction. The top contours of these sets of gradually changing steel arches gradually change from an arc shape to a straight line, collectively forming a support structure that smoothly transitions from the arc-shaped cross-section of the auxiliary tunnel to the rectangular cross-section of the main tunnel.

[0020] Throughout the transition section, the segmental chord length of each group of steel arch frames is designed in 1-meter increments to ensure construction precision and consistency. Each group of transitional steel arch frames consists of 5 arch frames of the same size, spaced 1 meter apart, with a total of 5 groups progressively advancing to achieve a smooth transition in the cross-section.

[0021] The second steel arch 2 serves as the starting section for the gradual transition; Group B consists of the second steel arch 2, as shown below. Figure 5As shown, the second steel arch frame 2 includes a second steel arch frame section 21, a second steel arch frame section 22, a second steel arch frame section 23, a second steel arch frame section 24, and a second steel arch frame section 25 connected in sequence. The arc length of the second steel arch frame section 21 and the second steel arch frame section 25 is 2.8 meters, and the chord height is 18 mm. The arc length of the second steel arch frame section 22 and the second steel arch frame section 24 is 3.5 meters, and the chord height is 18 mm. The arc length of the second steel arch frame section 23 is 5 meters, and the chord height is 2 mm; at this point, the top arc section becomes relatively flat.

[0022] The third steel arch frame 3 is further leveled out based on the second steel arch frame 2. Group C is the third steel arch frame 3, as shown below. Figure 6 As shown, the third steel arch frame 3 includes three sections connected in sequence: a first section 31, a second section 32, a third section 33, a fourth section 34, and a fifth section 35. The first section 31 and the fifth section 35 each have an arc length of 2.8 meters and a chord height of 18 mm; the second section 32 and the fourth section 34 each have an arc length of 3.5 meters and a chord height of 13 mm; the third section 33 has an arc length of 5 meters and a chord height of 13 mm, with the top arc further lowered, gradually approaching a straight line shape.

[0023] The fourth steel arch frame 4 continues to advance with a gradual change in cross-section; group D is the fourth steel arch frame 4, as shown below. Figure 7 As shown, the fourth steel arch frame 4 comprises four sections connected in sequence: a first section 41, a second section 42, a third section 43, a fourth section 44, and a fifth section 45. The first section 41 and the fifth section 45 each have an arc length of 2.8 meters and a chord height of 18 mm; the second section 42 and the fourth section 44 each have an arc length of 3.5 meters and a chord height of 12 mm; the third section 43 has an arc length of 5 meters, a chord height of 7 mm, and a flattened top, preparing for the transition to a rectangular cross-section.

[0024] The fifth steel arch 5 serves as a crucial transition section from the arc shape to the rectangle; group E consists of the fifth steel arch 5, as shown below. Figure 8As shown, the fifth steel arch frame 5 comprises five sections connected in sequence: section 51, section 52, section 53, section 54, and section 55. Sections 51 and 55 each have an arc length of 2.8 meters and a chord height of 18 mm; sections 52 and 54 each have an arc length of 4 meters and a chord height of 8 mm; section 53 is a straight section of 5.04 meters, marking the transition from an arc to a flat top. The top width of the fifth steel arch frame 5 is controlled at 4.8 meters. This dimension comprehensively considers the space for the ventilation belts 7 on both sides of the excavation section, while also meeting the requirements for supporting 7 arch frames after the main tunnel is topped out, ensuring the continuity and stability of the support at the intersection.

[0025] The sixth steel arch frame 6 is a rectangular tunnel structure, serving as the final support form for connecting the main tunnel. Group F consists of the sixth steel arch frame 6, as shown below. Figure 9 As shown, the sixth steel arch frame 6 includes a top section 62 of the sixth steel arch frame and sections 61 and 63 of the sixth steel arch frame connected to both ends of section 62. Section 61 and 63 are both vertical sections 4 meters long, while section 62 is a horizontal section 5.04 meters long, together forming a stable rectangular passage. A pre-installed connecting plate 8 is pre-installed at the top of this rectangular opening for reliable connection to the main arch, ensuring the continuity and integrity of the support structure.

[0026] Throughout the construction process, the installation of all steel arch frames was strictly aligned with the centerline 9 of the tunnel line, and repeated calibration was performed using precision instruments such as total stations to ensure that the structural axis was consistent with the design line and to avoid support failure caused by misalignment.

[0027] The construction method for the gradual transition from the auxiliary tunnel to the main tunnel according to the present invention includes the following steps: S1. Construction Preparation for the Gradient Section: After constructing the auxiliary tunnel to a point 20 meters from the intersection with the main tunnel, the arch crown will be raised at a 5° incline. Based on the surrounding rock conditions identified through comprehensive advanced geological forecasting, the support parameters and the starting position of the gradient section will be determined. Surveyors will conduct on-site layout according to the pre-set gradient drawings, marking the excavation outline and the arch frame installation position.

[0028] S2. Bench Excavation and Gradual Support: A two-bench excavation method is used, with the upper bench excavation height controlled at 3.5–4 meters to facilitate mechanical hazard removal and personnel operations. Each excavation cycle should not exceed the distance between two arch frames. Immediately after excavation, the corresponding gradual steel arch frame is installed, and system anchor bolts 10 and locking anchor pipes 20 are constructed. The upper bench length is controlled at 5 meters, and the lower bench length at 5–8 meters. The initial support of the invert arch is quickly formed into a ring, and invert arch lining and filling concrete are constructed to enhance overall stability. Blasting operations strictly adhere to the principles of "short advance, weak blasting, frequent measurements, strong support, and early ring formation" to minimize disturbance to the surrounding rock. Figure 10 As shown, the specific procedures for each excavation and support cycle are as follows: After construction preparation and surveying, drillers drill holes, followed by charging and blasting. After blasting, muck is removed and hazards are cleared, addressing over- or under-excavation at the excavation face. Simultaneously, raw material inspection and steel component processing are carried out. Next, initial shotcrete is applied to the excavation face, and pre-assembled and inspected steel arch frames are erected according to the measured positions (before erection, loose slag at the arch foot must be removed; those that fail inspection are corrected before use). After the steel frame is in place, steel mesh is laid, longitudinal connecting bars are installed, and after inspecting the installation quality, system anchor bolts and locking anchor pipes are installed. Finally, shotcrete is applied to the designed thickness, completing the cycle.

[0029] S3. Cross-section Gradual Transition and Measurement Control: According to the pre-set gradual transition drawings, install the second to sixth steel arch frames 2 to 6 sequentially along the extension direction of the auxiliary tunnel, so that the support cross-section gradually changes from the arc shape of the auxiliary tunnel to a rectangular opening. Each group of cross-sections consists of 5 arch frames of the same size, with a spacing of 1 meter between the frames, and a total of 5 groups are gradually advanced. Surveyors need to conduct precise layout before each cycle of excavation, marking the opening position and angle on the working face to control over-excavation and under-excavation after blasting.

[0030] S4. Intersection Reinforcement and Connection with the Main Tunnel: Reinforced support is used at the last arch frame near the main tunnel (i.e., the fifth steel arch frame 5). Specifically, a reinforced arch frame made of double-jointed 20a I-beams is used to resist deformation caused by stress concentration in the surrounding rock at the intersection. Subsequently, the sixth steel arch frame 6 is installed to form a rectangular tunnel, serving as a stable transition passage through which construction workers can safely excavate to the design boundary on the opposite side of the main tunnel. Construction of the main tunnel arch support begins on the opposite side of the main tunnel. The arch support and the sixth steel arch frame 6 are reliably connected by pre-installed arch support reserved connecting plates 8, ensuring the continuity and integrity of the auxiliary tunnel support and the main tunnel support structure, forming a reliable portal frame support system.

[0031] S5. Main Tunnel Excavation: After all the roof lifting work is completed, remove the supporting legs on both sides of the rectangular tunnel (i.e., the first section 61 of the sixth steel arch and the third section 63 of the sixth steel arch), and proceed to the normal excavation process of the main tunnel to continue moving forward.

[0032] During the aforementioned construction process, the blasting operations in steps S2 and S4 strictly adhered to the principles of "short advance, weak blasting, frequent measurement, strong support, and early ring formation" to minimize disturbance to the surrounding rock. Monitoring and measurement points were densely installed in overlapping construction areas. Before the next blasting cycle, the initial shotcrete strength was no less than 10 MPa, and data from the surrounding rock monitoring and measurement points were collected before blasting. After blasting, data was collected again from the surrounding rock monitoring and measurement points, and the allowable deformation was adjusted based on the measurement data to ensure construction safety and quality.

[0033] In summary, this invention, through the combination of steel arch frame components and construction methods, forms a complete and reliable technology system for the transition of auxiliary tunnels into the main tunnel via roof cantilever construction. This system, with its core design based on gradual cross-sectional changes, achieves a smooth and controllable transition of the support structure from an arc shape to a rectangle through the precise design and sequential installation of the second to sixth steel arch frames (6). This fundamentally avoids the large-scale vertical excavation required for installing gantry frames in traditional processes, significantly reducing safety risks and surrounding rock disturbance. During construction, the principles of "short advances, weak blasting, frequent measurements, strong support, and early ring formation" are strictly followed. Construction management relies on intensified monitoring and measurements, as well as concrete strength indicators, ensuring safety and quality control throughout the entire process of intersection construction under complex conditions. The resulting rectangular tunnel structure not only provides a stable passage and reliable connection for the main tunnel arch construction but also optimizes the working space and ventilation conditions within the tunnel. This invention effectively solves the technical problems existing in traditional roof cantilever construction, demonstrating significant advantages in improving construction efficiency, ensuring operational safety, enhancing structural stability, and promoting standardized tunnel construction, and has extremely high application value.

Claims

1. A steel arch frame assembly for assisting the gradual transition of a tunnel into the main tunnel, comprising multiple first steel arch frames (1), characterized in that: The first steel arch frame (1) near the main tunnel end is provided with multiple sets of gradually changing steel arch frames along the extension direction of the auxiliary tunnel; the multiple sets of gradually changing steel arch frames include multiple second steel arch frames (2), multiple third steel arch frames (3), multiple fourth steel arch frames (4), multiple fifth steel arch frames (5) and multiple sixth steel arch frames (6) connected in sequence in the extension direction; the top outline of the multiple sets of gradually changing steel arch frames gradually changes from an arc shape to a straight line shape, and together they form a support structure that smoothly transitions from the cross-section of the auxiliary tunnel to the rectangular cross-section of the main tunnel.

2. The steel arch frame assembly for the gradual transition from the auxiliary tunnel to the main tunnel according to claim 1, characterized in that: The second steel arch frame (2) includes a second steel arch frame section 1 (21), a second steel arch frame section 2 (22), a second steel arch frame section 3 (23), a second steel arch frame section 4 (24), and a second steel arch frame section 5 (25) connected in sequence; wherein, the arc length of the second steel arch frame section 1 (21) and the second steel arch frame section 5 (25) is 2.8 meters and the chord height is 18 mm; the arc length of the second steel arch frame section 2 (22) and the second steel arch frame section 4 (24) is 3.5 meters and the chord height is 18 mm; the arc length of the second steel arch frame section 3 (23) is 5 meters and the chord height is 2 mm.

3. The steel arch frame assembly for the gradual transition from the auxiliary tunnel to the main tunnel according to claim 1, characterized in that: The third steel arch frame (3) includes a third steel arch frame section 1 (31), a third steel arch frame section 2 (32), a third steel arch frame section 3 (33), a third steel arch frame section 4 (34), and a third steel arch frame section 5 (35) connected in sequence; wherein, the arc length of the third steel arch frame section 1 (31) and the third steel arch frame section 5 (35) is 2.8 meters and the chord height is 18 mm; the arc length of the third steel arch frame section 2 (32) and the third steel arch frame section 4 (34) is 3.5 meters and the chord height is 13 mm; the arc length of the third steel arch frame section 3 (33) is 5 meters and the chord height is 13 mm.

4. The steel arch frame assembly for the gradual transition from the auxiliary tunnel to the main tunnel according to claim 1, characterized in that: The fourth steel arch frame (4) includes a fourth steel arch frame section 1 (41), a fourth steel arch frame section 2 (42), a fourth steel arch frame section 3 (43), a fourth steel arch frame section 4 (44), and a fourth steel arch frame section 5 (45) connected in sequence; wherein, the arc length of the fourth steel arch frame section 1 (41) and the fourth steel arch frame section 5 (45) is 2.8 meters and the chord height is 18 mm; the arc length of the fourth steel arch frame section 2 (42) and the fourth steel arch frame section 4 (44) is 3.5 meters and the chord height is 12 mm; the arc length of the fourth steel arch frame section 3 (43) is 5 meters and the chord height is 7 mm.

5. The steel arch frame assembly for the gradual transition from the auxiliary tunnel to the main tunnel according to claim 1, characterized in that: The fifth steel arch frame (5) includes a fifth steel arch frame section 1 (51), a fifth steel arch frame section 2 (52), a fifth steel arch frame section 3 (53), a fifth steel arch frame section 4 (54), and a fifth steel arch frame section 5 (55) connected in sequence; wherein, the arc length of the fifth steel arch frame section 1 (51) and the fifth steel arch frame section 5 (55) is 2.8 meters and the chord height is 18 mm; the arc length of the fifth steel arch frame section 2 (52) and the fifth steel arch frame section 4 (54) is 4 meters and the chord height is 8 mm; the fifth steel arch frame section 3 (53) is a straight section with a length of 5.04 meters.

6. The steel arch frame assembly for the gradual transition from the auxiliary tunnel to the main tunnel according to claim 1, characterized in that: The sixth steel arch frame (6) is a rectangular shed structure. The sixth steel arch frame (6) includes the second section (62) of the sixth steel arch frame at the top and the first section (61) and the third section (63) of the sixth steel arch frame connected to both ends of the second section (62). The first section (61) and the third section (63) of the sixth steel arch frame are both vertical sections with a length of 4 meters, and the second section (62) of the sixth steel arch frame is a horizontal section with a length of 5.04 meters.

7. A construction method for gradually transitioning from an auxiliary tunnel to the main tunnel, characterized in that: Includes the following steps: S1. Construction preparation for the transition section: When the auxiliary tunnel is constructed to a point 20 meters from the intersection with the main tunnel, the arch is raised at a 5° climbing slope, and the support parameters are determined based on the results of advanced geological forecasts. S2. Step-by-step excavation and gradual support: The two-step method is used for excavation. The excavation height of the upper step is controlled at 3.5 to 4 meters. The excavation advance in each cycle does not exceed the distance between two arch frames. After excavation, the corresponding steel arch frame is installed immediately, and the system anchor rods (10) and locking foot anchor pipes (20) are constructed to make the initial support of the invert arch form a ring as soon as possible. Among them, the length of the upper step is controlled at 5 meters, and the length of the lower step is controlled at 5 to 8 meters. S3. Sectional Gradual Change and Measurement Control: According to the preset gradual change drawings, install the second steel arch frame (2) to the sixth steel arch frame (6) in sequence along the extension direction of the auxiliary tunnel, so that the support section gradually changes from the arc of the auxiliary tunnel to the rectangular opening; the surveyors need to accurately lay out on site and control the opening position and angle. S4. Intersection reinforcement support and connection with the main tunnel: The last arch frame near the main tunnel is reinforced with a reinforced support; then the sixth steel arch frame (6) is installed to form a rectangular tunnel, and this is used as a passage to excavate to the opposite side of the main tunnel, and the main tunnel arch is constructed. S5. Main Tunnel Excavation: After all the roof lifting work is completed, remove the support legs on both sides of the rectangular tunnel and proceed to the normal excavation process of the main tunnel.

8. The construction method according to claim 7, characterized in that: In steps S2 and S4, the blasting operation follows the principles of "short advance, weak blasting, frequent measurement, strong support, and early ring formation".

9. The construction method according to claim 7, characterized in that: In steps S2 and S4, monitoring and measurement points are densely set up in the cross-construction area; before the next cycle of blasting, the initial shotcrete strength is not less than 10MPa, and the reserved deformation amount is adjusted according to the measurement data.

10. The construction method according to claim 7, characterized in that: In step S4, the reinforced support is a reinforced arch frame made of double-jointed 20a I-beams.

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