A method for quickly expanding and excavating a core soil reserved by a CRD method towards a ring-shaped pit

Abstract

The application relates to a CRD method towards annular guide pit reserved core soil rapid expansion and excavation conversion method, belonging to the technical field of tunnel engineering. In the construction of the underground excavation tunnel which needs to be expanded and excavated and the construction method is converted, in combination with the advanced support, the locking foot anchor rod, the temporary spray mixing, the temporary vertical support and other means, the left one, the left two, the right one and the right two guide holes are formed by the gradual expansion and excavation of the upper left and right guide holes through the CRD method, and the temporary vertical support is added by reserving the core soil, and the conversion of the CRD method towards annular guide pit reserved core soil construction method is completed. Under the premise of ensuring the uninterrupted excavation of the tunnel, the CRD method towards annular guide pit reserved core soil method transition is solved, the small gradual large short-distance excavation construction method is realized, the construction safety is ensured, the tunnel excavation progress is accelerated, the work is reduced, and the application has guiding significance for similar engineering construction.

Description

Technical Field

[0001] This invention belongs to the field of tunnel engineering technology, specifically relating to a method for rapid excavation and conversion of reserved core soil in a CRD normal annular pilot tunnel. Background Technology

[0002] With the rapid development of my country's economy, underground engineering has become increasingly complex and diverse, involving more and more changes in cross-section construction methods. Tunnels with gradually increasing and ultra-large cross-sections are widely used, making method conversion increasingly common. Currently, domestic tunnel construction lacks effective methods for transitioning from CRD (Construction Reduction Drilling) to the pre-reserved core earth method with a circular pilot tunnel, and research on construction methods for converting small cross-sections to ultra-large cross-sections is limited. Therefore, a rapid excavation and reasonable conversion construction scheme is needed, which can not only ensure the rapid expansion and gradual transformation of small cross-sections into ultra-large cross-sections, but also complete the method conversion over a short distance, while controlling the risks of conversion construction and saving project investment. Summary of the Invention

[0003] To overcome the shortcomings of traditional methods for converting small-to-large cross-section tunnels, this invention provides a rapid expansion and excavation method for converting from the CRD method to the ring-shaped pilot tunnel with reserved core soil. In the construction of mined tunnels where the surrounding rock improves, expansion is required, and method conversion is necessary, this method combines advanced support, anchor bolts, temporary shotcrete, and central vertical bracing. Through the gradual expansion of the upper left and right pilot tunnels using the CRD method, left-first, left-second, right-first, and right-second pilot tunnels are formed, and core soil is reserved with temporary central vertical bracing, completing the conversion from the CRD method to the ring-shaped pilot tunnel with reserved core soil method. The lower tunnel is excavated at a safe step distance to the method conversion interface. A step method is used to first excavate the central rock mass and then laterally expand the sidewall rock mass to complete the method conversion. This method solves the transition from the CRD method to the ring-shaped pilot tunnel with reserved core soil method while ensuring uninterrupted tunnel construction, achieving a smooth transition between small-to-large cross-section tunnels with short excavation distances, ensuring construction safety, accelerating tunnel excavation progress, and reducing downtime.

[0004] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for rapid excavation and conversion of CRD-style annular pilot tunnel with reserved core soil includes: Step (1) Upper pilot tunnel I conversion construction: Through the CRD pilot tunnel I, gradually expand upward to form pilot tunnel I-A, and gradually expand horizontally and vertically to form pilot tunnel I-B, so that the pilot tunnel I is converted into fully enclosed pilot tunnels I-A and I-B and core soil is reserved as support. Temporary vertical supports are arranged along the center line of the gradual section. Step (2) Construction of the upper rear tunnel II: The rear tunnel II section is gradually expanded upward to form the II-C tunnel, and gradually expanded horizontally and vertically to form the II-D tunnel, so that the rear tunnel II is converted into the fully enclosed II-C and II-D tunnels; after the surrounding rock has fully deformed and converged and all monitoring data have stabilized, the temporary vertical supports are removed, the core soil is excavated, and the conversion of the I and II sections of the CRD method is completed; The pilot tunnels I-A, I-B, II-C, and II-D are combined to form a fully enclosed initial support arch structure. During the excavation of a single pilot tunnel, the bottom plate and side walls are sealed with concrete, so that the initial support frame of the pilot tunnel and the shotcrete at the bottom form a complete ring of load-bearing body. Among them, Pilot Tunnel I is the upper pre-existing tunnel / upper left pilot tunnel of the CRD method section; Pilot Tunnel II is the upper rear tunnel / upper right pilot tunnel of the CRD method section; Pilot Tunnel I-A is the left first pilot tunnel of the reserved core soil method section of the annular pilot tunnel; Pilot Tunnel I-B is the left second pilot tunnel of the reserved core soil method section of the annular pilot tunnel; Pilot Tunnel II-C is the right first pilot tunnel of the reserved core soil method section of the annular pilot tunnel; and Pilot Tunnel II-D is the right second pilot tunnel of the reserved core soil method section of the annular pilot tunnel.

[0005] Furthermore, in step (1), when converting the I-A pilot tunnel to the I-A pilot tunnel, anchor rods are installed at the arch foot and vertical supports are erected to ensure that the initial support structure of the I-A pilot tunnel arch has support points at both ends.

[0006] Furthermore, in step (2), when converting the II-C guide tunnel to the II-C guide tunnel, anchor rods are installed at the arch foot to ensure that the initial support structure of the II-C guide tunnel arch has support points at both ends.

[0007] Furthermore, advanced support needs to be installed for the excavation of pilot tunnels I-A, I-B, II-C, and II-D. At the junction of construction methods and where the construction method conversion is completed, 3 to 5 grid steel frames should be erected. During the conversion process, the grid steel frames need to be gradually changed according to the actual excavation size.

[0008] Furthermore, it also includes the construction of the conversion between the lower III and IV pilot tunnels: the CRD method III and IV tunnels are excavated successively to the boundary between the construction methods, and the lower section of the gradually changing section tunnel is excavated using the step-by-step method. The lower excavation section needs to be gradually expanded downward to coincide with the design section of the reserved core soil method of the annular pilot tunnel; then, the remaining sidewall rock mass is gradually expanded laterally in a certain order until it coincides with the design section of the reserved core soil method of the annular pilot tunnel, thus completing the conversion between the CRD method III and IV sections; among them, the III pilot tunnel is the lower preliminary tunnel of the CRD method section; the IV pilot tunnel is the lower subsequent tunnel of the CRD method section.

[0009] Furthermore, the spacing between the arch section reserved for the core soil in the excavation of the ring pilot tunnel and the middle section of the ring pilot tunnel reserved for the core soil method is controlled at 6-10m, while the spacing for the remaining sections is 3-5m.

[0010] Furthermore, it includes the following steps: Step (1) Upper part first, I pilot tunnel conversion construction: the face of I pilot tunnel is excavated to the construction method boundary, the arch crown is constructed with advanced support, several grid steel frames are erected, and the excavation is carried out forward frame by frame, with the excavation angle maintained within 30°. During the gradual excavation of I pilot tunnel, it is converted into fully enclosed I-A and I-B pilot tunnels, and core soil is reserved as support conditions; after the excavation of I-A pilot tunnel, the arch foot is installed with locking anchor rods, and the initial support of the arch crown, the erection of vertical bracing, and the spraying of concrete on the bottom plate and side walls are completed in a timely manner.

[0011] After the I-A pilot tunnel has been excavated for 3-5m, the I-B pilot tunnel will be excavated and the initial support of the arch, the bottom slab, and the sidewalls will be shotcreted in a timely manner. Each pilot tunnel will continue to be constructed according to the safe step distance to create conditions for the excavation of the II pilot tunnel.

[0012] Step (2) Upper rear II pilot tunnel conversion construction: The II pilot tunnel face is excavated to the construction method boundary, the arch crown is constructed with advanced support, several grid steel frames are erected, and the excavation is carried out frame by frame. The excavation angle is maintained within 30°. During the gradual excavation of the II pilot tunnel, it is converted into the fully enclosed II-C and II-D pilot tunnels, and the core soil E is reserved as a support condition. After the II-C pilot tunnel is excavated, the arch foot is installed with locking anchor rods, and the initial support of the arch crown, the bottom plate and the side wall shotcrete are completed in a timely manner.

[0013] After the II-C pilot tunnel has been excavated 3-5m forward, the II-D pilot tunnel should be excavated and the initial support of the arch crown and the shotcrete of the bottom slab and side walls should be completed in a timely manner.

[0014] Step (3) Remove temporary vertical supports and excavate the core soil of the tunnel arch: After the II-D pilot tunnel is excavated forward 3-5m, remove the temporary vertical supports after the surrounding rock has fully deformed and converged and all monitoring data have stabilized. Excavate the core soil E and promptly construct the bottom slab shotcrete to make the tunnel arch fully enclosed and form a circular initial support arch structure with good stress, thus completing the conversion of the tunnel arch construction method of the annular pilot tunnel reserved core soil method.

[0015] Furthermore, it also includes: Step (4) Construction of the conversion between the lower III and IV pilot tunnels: After the core soil of the tunnel arch is excavated forward for 6-10m and the deformation is stable, the lower III and IV tunnels of the CRD are excavated to the junction of the construction methods. The middle and lower sections of the tunnel of the transition section are excavated according to the step method, with a spacing of 3-5m. After the excavation of the lower section is completed, the initial support of the invert arch is constructed in time. Then, the remaining sidewall rock mass is excavated laterally in a certain order. Each section is excavated with a safe step distance. After the excavation is completed, the corresponding initial support is constructed in time.

[0016] Step (5) Complete the construction method conversion: After the lower cavern is fully excavated to be basically consistent with the design cross section of the reserved core soil method of the ring pilot tunnel, 3 to 5 grid steel frames are erected here to complete the construction method conversion from the CRD method to the reserved core soil method cross section of the ring pilot tunnel. Each pilot tunnel continues to advance according to the safe step distance.

[0017] In this application, pilot tunnel I is converted into fully enclosed pilot tunnels I-A and I-B, and then pilot tunnel II is converted into fully enclosed pilot tunnels II-C and II-D. Then, I-A, I-B, II-C, and II-D are combined into a fully enclosed initial support arch structure. During the excavation of a single pilot tunnel, the bottom plate and sidewalls are sealed with early-strength shotcrete, so that the initial support frame of the pilot tunnel and the shotcrete at the bottom form a complete ring of load-bearing body, realizing the transition of the surrounding rock from poor to good.

[0018] This invention addresses the challenges of constructing mined tunnels in areas where surrounding rock conditions improve, requiring widening and method conversion. By combining advanced support, anchor bolts, temporary shotcrete, and central vertical bracing, the method utilizes the CRD (Continuous Rock Reduction) method to gradually widen the upper left and right pilot tunnels, creating left-first, left-second, right-first, and right-second pilot tunnels. Temporary central vertical bracing is added to reserve core soil, thus completing the transition from the CRD method to the ring-shaped pilot tunnel with reserved core soil. The lower tunnel is excavated at a safe step distance to the method conversion interface. A step-by-step method is used to first excavate the central rock mass and then laterally widen the sidewall rock mass to complete the method conversion. Furthermore, this invention allows for a gradual transition from small-section tunnels excavated using the CRD method to ultra-large-section tunnels excavated using the ring-shaped pilot tunnel with reserved core soil. This seamless transition from small to ultra-large tunnel construction with a short transition distance avoids time intervals between method conversions, reduces disturbance to the surrounding rock caused by CRD widening, ensures construction safety, accelerates progress, and reduces downtime. This method offers significant guidance for similar engineering projects. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the process conversion steps of the present invention; Figure 2 This is a construction sequence diagram of the upper pre-construction tunnel conversion of the present invention; Figure 3 This is a construction sequence diagram of the upper rear tunnel conversion of the present invention; Figure 4 This is a diagram illustrating the process of removing temporary vertical supports and excavating the core soil of the tunnel arch according to the present invention. Figure 5 This is a schematic diagram of the lower guide tunnel conversion process in this invention; Figure 6 This is a plan view of the lower guide tunnel conversion process in this invention; Figure 7 This is a schematic diagram illustrating the conversion of the arch construction method in this invention; Figure 8 This is a schematic diagram of the longitudinal section of the construction method conversion of the present invention.

[0020] Among them: I-CRD method cross-section upper advance chamber / upper left pilot tunnel; II-CRD method cross-section upper rear advance chamber / upper right pilot tunnel; III-CRD method cross-section lower advance chamber; IV- The lower section of the CRD method cross-section features a rear tunnel; Ⅰ-A - the left first pilot tunnel of the reserved core soil method cross-section in the annular pilot tunnel; Ⅰ-B - the left second pilot tunnel of the reserved core soil method cross-section in the annular pilot tunnel; Ⅱ-C - the right first pilot tunnel of the reserved core soil method cross-section in the annular pilot tunnel; Ⅱ-D - the right second pilot tunnel of the reserved core soil method cross-section in the annular pilot tunnel; E - the reserved core soil at the arch of the reserved core soil method cross-section in the annular pilot tunnel; F - the middle excavation section of the reserved core soil method cross-section in the annular pilot tunnel; G - the lower excavation section of the reserved core soil method cross-section in the annular pilot tunnel; H-1 - the upper excavation section of the left sidewall of the reserved core soil method cross-section in the annular pilot tunnel; H-2 - the upper excavation section of the right sidewall of the reserved core soil method cross-section in the annular pilot tunnel; J-1 - the bottom excavation section of the left sidewall of the reserved core soil method cross-section in the annular pilot tunnel; J-2 - the bottom excavation section of the right sidewall of the reserved core soil method cross-section in the annular pilot tunnel. 1-Left pilot tunnel arch foot anchor bolt; 2-Left pilot tunnel arch crown initial support; 3-Temporary central vertical brace; 4-Left pilot tunnel floor slab and sidewall shotcrete; 5-Left second pilot tunnel arch crown initial support; 6-Left second pilot tunnel floor slab and sidewall shotcrete; 7-Right pilot tunnel arch foot anchor bolt; 8-Right pilot tunnel arch crown initial support; 9-Right pilot tunnel floor slab and sidewall shotcrete; 10-Right second pilot tunnel arch crown initial support; 11-Right second pilot tunnel floor slab and sidewall shotcrete; 12-Arch reserved core soil floor slab shotcrete; 13-Central invert arch initial support; 14-Upper part of tunnel sidewall initial support; 15-Lower part of tunnel sidewall initial support; 16-Tunnel arch crown advance support. Detailed Implementation

[0021] The technical solutions in the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0022] Unless otherwise defined, the technical or scientific terms used in the embodiments of this application shall have the ordinary meaning understood by one of ordinary skill in the art. The terms "first," "second," and similar terms used in this embodiment do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Terms such as "comprising" or "including" mean that the element or object preceding the word encompasses the elements or objects listed after the word and their equivalents, without excluding other elements or objects. "Installed," "connected," and "linked" should be interpreted broadly; for example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal connection of two elements. Terms such as "upper," "lower," "left," "right," "horizontal," and "vertical" are used only relative to the orientation of the components in the accompanying drawings. These directional terms are relative concepts used for relative description and clarification, and they may change accordingly depending on the orientation of the components in the accompanying drawings.

[0023] To better understand the present invention, the invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0024] like Figures 1-8 As shown in this embodiment, the rapid expansion and conversion method of the CRD method for the reserved core soil in the circular pilot tunnel is used in the construction of the cut-and-cover tunnel when the surrounding rock improves, expansion and conversion of construction methods are required. It combines advanced support, dynamic design, information-based construction and enhanced monitoring. The method gradually expands the upper left and right pilot tunnels of the CRD method to form left 1, left 2, right 1 and right 2 pilot tunnels and reserves core soil and adds temporary central vertical supports to complete the conversion of the reserved core soil construction method of the circular pilot tunnel. The lower cavern maintains a safe step distance and adopts the step method to first excavate the middle rock mass and then expand the sidewall rock mass laterally to complete the conversion of construction methods.

[0025] The CRD method first constructs the left pilot tunnel I at the top of the cross-section, and then gradually expands the excavation upward to form the left first pilot tunnel I-A. Gradually expand the excavation laterally and vertically to form the left second pilot tunnel I-B, and reserve core soil E as support conditions. Temporary central vertical supports 3 are arranged along the center line of the gradual expansion section.

[0026] The upper section of the CRD method cross-section is then excavated with the right pilot tunnel II, gradually widening upwards to form the right first pilot tunnel II-C, and then gradually widening laterally to form the right second pilot tunnel II-D. After the surrounding rock has fully deformed and converged and all monitoring data have stabilized, the temporary central vertical support 3 is removed, the core soil E is excavated, and the conversion of the upper pilot tunnel of the CRD method is completed.

[0027] As another implementation method, the method also includes excavating the lower pilot tunnels III and IV of the CRD method to the boundary between construction methods, excavating the lower section of the transition section in the step method, and then expanding the remaining sidewall rock mass laterally in the order of sections H-1, H-2, J-1, J-2 or sections H-1, J-1, H-2, J-2 until it coincides with the design section of the reserved core soil method of the annular pilot tunnel, thus completing the conversion of pilot tunnels III and IV of the CRD method.

[0028] Specifically, the rapid excavation and conversion method for the reserved core soil in the CRD normal annular pilot tunnel in this embodiment includes the following steps: Step (1) Upper pilot tunnel I conversion construction: The face of pilot tunnel I is excavated to the construction method boundary, and the tunnel arch is constructed with 16mm of advance support, such as... Figure 8 As shown, 3 to 5 grid steel frames are erected one after another, and the excavation is carried out forward in stages. The excavation angle is maintained within 30°. During the gradual excavation of pilot tunnel I, it is converted into fully enclosed pilot tunnel I-A and pilot tunnel I-B, and core soil E is reserved as a support condition.

[0029] After the excavation of pilot tunnel I-A, anchor bolts 1 were installed at the arch foot of the left pilot tunnel arch foot, and the initial support 2 of the left pilot tunnel arch crown, the erection of temporary central vertical supports 3, and the spraying of concrete 4 on the bottom slab and sidewalls of the left pilot tunnel were completed in a timely manner. After the excavation of pilot tunnel I-A was 3-5m ahead, pilot tunnel I-B was excavated and the initial support 5 of the left second pilot tunnel arch crown and the spraying of concrete 6 on the bottom slab and sidewalls of the left second pilot tunnel were completed in a timely manner. Each pilot tunnel continued to advance according to the safe step distance to create conditions for the excavation of pilot tunnel II.

[0030] When converting pilot tunnel I to pilot tunnel I-A, temporary vertical supports must be erected and anchor bolts must be installed at the arch foot to ensure full closure of pilot tunnels I-A and I-B.

[0031] Step (2) Upper rear pilot tunnel II conversion construction: The face of pilot tunnel II is excavated to the junction of construction methods. The arch of the tunnel is constructed with 16 advanced supports, and 3 to 5 grid steel frames are erected. The excavation is carried out frame by frame, and the excavation angle is maintained within 30°. During the gradual excavation of pilot tunnel II, it is converted into fully enclosed pilot tunnels II-C and II-D, and the core soil E is reserved as support conditions.

[0032] After the excavation of pilot tunnel II-C, anchor bolts 7 for locking the arch foot of the right pilot tunnel are installed at the arch foot, and the initial support 8 for the arch top of the right pilot tunnel and the shotcrete 9 for the bottom slab and sidewalls of the right pilot tunnel are completed in a timely manner; after the pilot tunnel II-C is excavated forward by 3 to 5 meters, pilot tunnel II-D is excavated and the initial support 10 for the arch top of the right second pilot tunnel and the shotcrete 11 for the bottom slab and sidewalls of the right second pilot tunnel are completed in a timely manner.

[0033] When the pilot tunnel II is converted to pilot tunnel II-C, anchor bolts must be installed at the arch foot to ensure a complete closure of the tunnel when converting to pilot tunnels II-C and II-D.

[0034] Step (3) Remove temporary vertical supports and excavate the core soil of the tunnel arch: After the pilot tunnel II-D has been excavated forward for 3-5m, after the surrounding rock has fully deformed and converged and all monitoring data have stabilized, remove the temporary middle vertical support 3, excavate the core soil E and promptly construct the arch reserved core soil bottom plate shotcrete 12, so that the tunnel arch is fully enclosed to form a circular initial support arch cover structure with better stress, and complete the construction method conversion of the tunnel arch using the annular pilot tunnel reserved core soil method.

[0035] In step (3), after the core soil E is excavated, the bottom slab is sprayed with concrete in a timely manner to ensure that the tunnel arch is fully enclosed.

[0036] As another implementation, the method further includes: Step (4) Construction of the conversion between the lower caverns III and IV: After the core soil E of the tunnel arch is excavated forward for 6-10m and the deformation is stable, the lower caverns III and IV of the CRD are excavated to the junction of the construction methods. The middle excavation section F of the reserved core soil section of the annular pilot tunnel and the lower excavation section G of the reserved core soil section of the annular pilot tunnel are excavated according to the step method, with a spacing of 3-5m. After the excavation of the lower excavation section G of the reserved core soil section of the annular pilot tunnel is completed, the initial support 13 of the bottom invert arch is constructed in time. Then, the remaining sidewall rock mass is horizontally expanded in the order of sections H-1, H-2, J-1, J-2 or sections H-1, J-1, H-2, J-2. Each section is excavated with a safe step distance. After the excavation is completed, the corresponding upper initial support 14 of the tunnel sidewall and the lower initial support 15 of the tunnel sidewall are constructed in time.

[0037] Step (5) Complete the construction method conversion: After the lower cavern is fully excavated to be basically consistent with the design cross section of the reserved core soil method of the ring pilot tunnel, 3 to 5 grid steel frames are erected here to complete the construction method conversion from the CRD method to the reserved core soil method cross section of the ring pilot tunnel. Each pilot tunnel continues to advance according to the safe step distance.

[0038] Steps 3) to 4) are controlled with a step distance of 6 to 10m, and the remaining step distances are 3 to 5m.

[0039] As another implementation method, the grid steel frame needs to be gradually changed according to the actual excavation size during the conversion process of steps (1) to (5); the arch top needs to be set with advance support when excavating the pilot tunnels I-A and I-B in step (1) and the pilot tunnels II-C and II-D in step (2); 3 to 5 grid steel frames need to be erected at the junction of construction methods and at the place where the construction method conversion is completed.

[0040] The specific embodiments described above are only used to explain the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for rapid excavation and conversion of pre-reserved core soil in a CRD-style annular pilot tunnel, characterized in that, include: Step (1) Upper pilot tunnel I conversion construction: Through the CRD pilot tunnel I, gradually expand upward to form pilot tunnel I-A, and gradually expand horizontally and vertically to form pilot tunnel I-B, so that the pilot tunnel I is converted into fully enclosed pilot tunnels I-A and I-B with reserved core soil as support. Temporary vertical supports are arranged along the center line of the transition section; specifically including the following steps: the tunnel face of pilot tunnel I is excavated to the construction method boundary, the arch is constructed with advance support, several grid steel frames are erected, and the excavation is carried out forward frame by frame, with the excavation angle maintained within 30°. During the gradual excavation of pilot tunnel I, it is converted into fully enclosed pilot tunnels I-A and I-B with reserved core soil E as support conditions; After the excavation of the I-A pilot tunnel, anchor bolts were installed at the arch foot, and the initial support of the arch crown, the erection of vertical bracing, and the spraying of concrete on the bottom slab and side walls were completed in a timely manner. After the I-A pilot tunnel has been excavated 3-5m forward, the I-B pilot tunnel will be excavated and the initial support of the arch crown, the bottom slab and the sidewall shotcrete will be completed in a timely manner. Each pilot tunnel will continue to be constructed forward according to the safe step distance to create conditions for the excavation of the II pilot tunnel. Step (2) Construction of the upper rear tunnel II: The rear tunnel II section is gradually expanded upward to form the II-C tunnel, and gradually expanded horizontally and vertically to form the II-D tunnel, so that the rear tunnel II is converted into the fully enclosed II-C and II-D tunnels; after the surrounding rock has fully deformed and converged and all monitoring data have stabilized, the temporary vertical supports are removed, the core soil is excavated, and the conversion of the I and II sections of the CRD method is completed; The pilot tunnels I-A, I-B, II-C, and II-D are combined to form a fully enclosed initial support arch structure. During the excavation of a single pilot tunnel, the bottom plate and side walls are sealed with concrete, so that the initial support frame of the pilot tunnel and the shotcrete at the bottom form a complete ring of load-bearing body. Among them, Pilot Tunnel I is the upper preliminary chamber / upper left pilot tunnel of the CRD method section; Pilot Tunnel II is the upper rear chamber / upper right pilot tunnel of the CRD method section; Pilot Tunnel I-A is the left first pilot tunnel of the reserved core soil method section of the annular pilot tunnel; Pilot Tunnel I-B is the left second pilot tunnel of the reserved core soil method section of the annular pilot tunnel; Pilot Tunnel II-C is the right first pilot tunnel of the reserved core soil method section of the annular pilot tunnel; Pilot Tunnel II-D is the right second pilot tunnel of the reserved core soil method section of the annular pilot tunnel; with The process includes the following steps: the face of the II pilot tunnel is excavated to the boundary of the construction method, the arch crown is constructed with advanced support, several grid steel frames are erected, and the excavation is carried out frame by frame, with the excavation angle maintained within 30°. During the gradual excavation of the II pilot tunnel, it is converted into the fully enclosed II-C and II-D pilot tunnels, and the core soil E is reserved as a support condition. After the excavation of the II-C pilot tunnel, the arch foot is installed with anchor bolts, and the initial support of the arch crown, the bottom slab and the side walls are shotcreted in a timely manner. After the II-C pilot tunnel has been excavated 3-5m forward, the II-D pilot tunnel should be excavated and the initial support of the arch crown and the shotcrete of the bottom slab and side walls should be completed in a timely manner. Step (3) Remove temporary vertical supports and excavate the core soil of the tunnel arch: After the II-D pilot tunnel is excavated forward 3-5m, remove the temporary vertical supports after the surrounding rock has fully deformed and converged and all monitoring data have stabilized, excavate the core soil E and promptly construct the bottom slab shotcrete to make the tunnel arch fully enclosed and form a circular initial support arch structure with good stress, thus completing the conversion of the tunnel arch construction method of the ring pilot tunnel reserved core soil method. It also includes the construction of the conversion between the lower III and IV pilot tunnels: the CRD method III and IV tunnels are excavated successively to the boundary between the construction methods. The lower section of the gradually changing section tunnel is excavated using the step-by-step method. The lower excavation section needs to be gradually expanded downward to coincide with the design section of the reserved core soil method of the annular pilot tunnel. Then, the remaining sidewall rock mass is gradually expanded laterally in a certain order until it coincides with the design section of the reserved core soil method of the annular pilot tunnel, thus completing the conversion between the CRD method III and IV sections. Among them, the III pilot tunnel is the lower section of the CRD method tunnel that is the first tunnel; the IV pilot tunnel is the lower section of the CRD method tunnel that is the last tunnel.

2. The method according to claim 1, characterized in that: In step (1), when converting the pilot tunnel I to the pilot tunnel I-A, anchor rods are installed at the arch foot and vertical supports are erected to ensure that the initial support structure of the arch of the pilot tunnel I-A has support points at both ends.

3. The method according to claim 1, characterized in that: In step (2), when converting the II-C guide tunnel to the II-C guide tunnel, anchor bolts are installed at the arch foot to ensure that the initial support structure of the II-C guide tunnel arch has support points at both ends.

4. The method according to claim 1, characterized in that: For the excavation of pilot tunnels I-A, I-B, II-C, and II-D, advanced support should be installed. At the junction of construction methods and where the construction method conversion is completed, 3 to 5 grid steel frames should be erected. During the conversion process, the grid steel frames should be gradually changed according to the actual excavation size.

5. The method according to claim 1, characterized in that: The spacing between the arch section reserved for the core soil in the excavation of the circular pilot tunnel and the middle section of the circular pilot tunnel reserved for the core soil is controlled at 6-10m, while the spacing for the rest is 3-5m.

6. The method according to claim 1, characterized in that... Also includes: Step (4) Construction of the conversion between the lower III and IV pilot tunnels: After the core soil of the tunnel arch has been excavated forward for 6-10m and the deformation has stabilized, the lower III and IV tunnels of the CRD are excavated to the junction of the construction methods. The lower section of the transition section of the tunnel is excavated using the step-by-step method, with a spacing of 3-5m. After the excavation of the lower section is completed, the initial support of the invert arch is constructed in a timely manner. Then, the remaining sidewall rock mass is excavated laterally in a certain order, with a safe step distance maintained for each section. After the excavation is completed, the corresponding initial support is constructed in a timely manner. Step (5) Complete the construction method conversion: After the lower cavern is fully excavated to be basically consistent with the design cross section of the reserved core soil method of the ring pilot tunnel, 3 to 5 grid steel frames are erected here to complete the construction method conversion from the CRD method to the reserved core soil method cross section of the ring pilot tunnel. Each pilot tunnel continues to advance according to the safe step distance.

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