A single tunnel construction method for a composite middle-wall multi-arch tunnel

Abstract

The invention discloses a single tunnel construction method of a composite middle wall multi-arch tunnel, comprising the following steps: 1) excavating a first tunnel and treating the foundation of a middle partition wall; 2) tying a steel bar skeleton of the middle partition wall, performing initial support, and spraying the tied steel bar skeleton of the middle partition wall as a whole to form a middle partition wall; 3) pouring and filling the invert of the first tunnel; 4) laying a waterproof layer of the first tunnel, and pouring secondary lining; 5) excavating the upper part of the rear tunnel, installing a joint with the middle partition wall, and initially supporting the upper part of the excavated rear tunnel; 6) reserving rock pillars, excavating the lower part of the rear tunnel, and initially supporting it after excavation; 7) excavating the reserved rock pillars, completing all initial support and closing them into a ring; 8) pouring and filling the invert; 9) laying a waterproof layer of the rear tunnel, and pouring secondary lining. The construction method of the invention can reduce procedures, simplify processes, improve construction efficiency, and save engineering costs, and is suitable for wide promotion and application.

Description

Technical Field

[0001] The invention relates to the field of tunnel construction methods, and in particular to a single tunnel construction method for a composite middle-wall multi-arch tunnel. Background Art

[0002] The structure of the arch tunnel is complex, with many construction steps and a long construction period. The stress state of the structure changes frequently, and the quality control is difficult. Any negligence during construction will cause lining cracks, leakage of the middle partition wall and other defects. Therefore, the quality control of the arch tunnel construction is extremely important. The composite middle wall arch tunnel is the most commonly used arch tunnel. The current construction method of the composite middle wall arch tunnel mainly adopts the middle guide tunnel method. For the composite middle wall arch tunnel, the role of the middle guide tunnel is mainly to form the middle wall core. The height and width of the middle guide tunnel are determined to facilitate the construction of the middle wall core, and are generally around 5m. Since it is an auxiliary project for tunnel construction, the excavation method and support method of the middle guide tunnel are flexibly determined by specific engineering conditions.

[0003] During the construction of the composite middle wall and arch tunnel using the middle guide tunnel method, it is necessary to excavate the middle guide tunnel and set up and dismantle its support; the middle partition wall is cast on site with concrete, which requires a special formwork trolley, and after it reaches the designed strength, it is necessary to backfill and grout the top; the main tunnel can only be excavated after the middle guide tunnel is through and the middle partition wall is cast and reaches the designed strength; the initial support of the main tunnels on both sides needs to be connected to the middle partition wall, which has many processes and great construction interference. In addition, in the construction process of the middle guide tunnel method, there are many processes with high construction requirements, such as the process requirements for dense backfill grouting on the top of the middle partition wall, the process requirements for blasting control of the subsequent tunnel, and the process requirements for the connection between the initial support of the main tunnel and the middle partition wall, etc., and the quality control of the connection process between the initial support of the main tunnel and the middle partition wall is very difficult. Summary of the invention

[0004] In view of the deficiencies in the prior art, the object of the present invention is to provide a single tunnel construction method for a composite middle-wall multi-arch tunnel with reduced procedures, simplified technology, shortened construction period and improved work efficiency.

[0005] In order to achieve the above object, the present invention is implemented by the following technical scheme: a single tunnel construction method of a composite middle wall multi-arch tunnel, comprising the following steps:

[0006] (1) Carry out preliminary tunnel excavation and treat the intermediate wall foundation according to the construction drawings;

[0007] (2) Tie the steel frame of the middle partition wall according to the foundation of the middle partition wall, then provide initial support for the good advance tunnel, and spray-cast the tied steel frame of the middle partition wall as a whole to form the middle partition wall;

[0008] (3) Casting and filling of the invert of the first tunnel;

[0009] (4) Then lay the waterproof layer of the first tunnel and pour the secondary lining of the first tunnel;

[0010] (5) Excavate the upper part of the rear tunnel, install the joint with the middle partition wall in the excavated rear tunnel, and then provide initial support for the upper part of the excavated rear tunnel;

[0011] (6) Reserve rock pillars at the middle partition wall of the rear tunnel, then excavate the lower part of the rear tunnel, and after excavation, provide initial support for the lower part of the rear tunnel;

[0012] (7) Excavate the reserved rock pillar at the middle partition wall of the rear tunnel, provide initial support for the excavated part of the rear tunnel, close it with the initial support of the previous rear tunnel to form a ring, and complete the pouring and filling of the rear tunnel invert;

[0013] (8) Finally, lay the waterproof layer of the rear tunnel and pour the secondary lining of the rear tunnel.

[0014] The principle of this technical solution is that there is no excavation and support process for the auxiliary guide tunnel, and the construction of the leading tunnel is carried out directly; the middle partition wall and the initial support of the leading tunnel are integrated into one, and the connection process of the initial support of the leading tunnel and the middle partition wall is cancelled; the middle partition wall is constructed by shotcrete, and no special construction equipment is required; the initial support of the rear tunnel needs to be connected to the middle partition wall; and rock pillars need to be reserved at the middle partition wall of the rear tunnel. The excavation and support of the middle guide tunnel are saved, the process is relatively small, and the construction interference is small.

[0015] In order to better implement the method of the present invention, further, the excavation span of the leading tunnel is 0.8-1.2 m larger than the excavation span of the succeeding tunnel.

[0016] In order to better implement the method of the present invention, further, if the preceding tunnel or the succeeding tunnel is of grade V or IV surrounding rock, its initial support consists of an I-beam arch frame, radial anchor rods, a steel mesh and shotcrete, and an I-beam arch frame with a steel mesh and shotcrete anchor combined support is adopted; if the preceding tunnel or the succeeding tunnel is of grade III surrounding rock, the initial support consists of a radial anchor rod, a steel mesh and shotcrete, the steel arch frames are connected with longitudinal steel bars, and are welded together with the radial anchor rods and the steel mesh, and are closely attached to the surrounding rock to form a load-bearing reinforced concrete structure; the shotcrete adopts a wet spraying process.

[0017] In order to better implement the method of the present invention, further, if the preceding tunnel or the succeeding tunnel has grade V or IV surrounding rock, a shallow buried section of the tunnel entrance and a fault fracture zone section with grade V or IV surrounding rock, its secondary lining adopts a reinforced concrete structure, and the reasonable time for the construction of the secondary lining should be finally determined based on the construction monitoring and measurement data.

[0018] In order to better implement the method of the present invention, further, it also includes tunnel auxiliary construction measures, including an advanced long pipe shed at the tunnel entrance, an advanced small guide tube in the tunnel, an advanced anchor rod, and peripheral curtain grouting.

[0019] In order to better implement the method of the present invention, further, in the V-level surrounding rock section, for both the leading tunnel and the subsequent tunnel, the three-step annular reserved core soil method is adopted for excavation; in the IV-level surrounding rock section, the leading tunnel is excavated by the two-step core soil method, and the subsequent tunnel is excavated by the three-step method; the subsequent tunnel should be excavated after the secondary lining of the leading tunnel is completed for not less than 40m and reaches the design strength, and the secondary lining of the leading tunnel should always be not less than 40m ahead of the secondary lining of the preceding and following tunnels; in the leading tunnel and the subsequent tunnel, the excavation on the left and right sides of the lower step is staggered by 5 to 10m, and the arch adopts smooth blasting technology.

[0020] In order to better implement the method of the present invention, further, the initial support steel frame of the rear tunnel is connected to the initial support steel frame of the leading tunnel as a whole by welding, and an anti-slip block is welded at the lower end of the joint. In hard rock, after the excavation of the reserved rock column of the rear tunnel, the mesh should be sprayed with concrete in time, and the joints of the middle partition wall should be rounded.

[0021] In order to better implement the method of the present invention, further, the leading tunnel should promptly implement foundation treatment and end anchoring measures for the base of the middle partition wall, and the initial support of the subsequent tunnel should be connected to the upper end of the middle partition wall as soon as possible.

[0022] In order to better implement the method of the present invention, further, a shock-absorbing layer is laid between the initial support and the secondary lining of the advance tunnel, rock pillars are reserved in the partition wall of the subsequent tunnel, and more peripheral holes, less explosives, and more cycles of excavation are adopted, and on-site blasting test plans and blasting vibration monitoring work of the subsequent tunnel are carried out in a timely manner.

[0023] In order to better implement the method of the present invention, further, the step distance between the working face of the preceding tunnel and the working face of the succeeding tunnel is ≥120 linear meters, and the step distance between the secondary lining of the preceding tunnel and the secondary lining of the succeeding tunnel is ≥90 linear meters.

[0024] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0025] (1) Compared with the traditional middle-guide tunnel construction method, the composite middle-wall multi-arch tunnel single tunnel construction method of the present invention has a smoother transition of the middle partition wall structure stiffness, a more reasonable structure, more reliable end constraints and connections, and is more conducive to resisting bias loads and improving stress concentration states;

[0026] (2) In the construction method of the present invention, the intermediate partition wall is set in the pilot tunnel to achieve a combination of permanent and temporary, and the material utilization rate is high. From the construction point of view, there is no excavation and support process of the auxiliary pilot tunnel, and there is no connection process between the initial support of the pilot tunnel and the intermediate partition wall. In addition, the intermediate partition wall is constructed by spraying concrete and does not require special construction equipment. The process is reduced and the process is simplified, which can shorten the construction period and improve work efficiency.

[0027] (3) The construction method provided by the present invention can realize the smooth connection and combination of permanent and temporary support structures. Under the premise of ensuring the quality of the project, it can reduce the number of procedures, simplify the process, and improve the construction efficiency. Since there is no need to set up an advance auxiliary guide tunnel (middle guide tunnel, side guide tunnel), the setting of temporary support can be reduced or even cancelled, which greatly improves the construction progress, reduces the investment in construction personnel and equipment, and saves project costs. It can reduce the disturbance to the surrounding rock, is beneficial to the stress of the structure, and has a positive impact on guiding the construction of the single tunnel method of the composite middle wall and arch tunnel. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Other features, objects and advantages of the present invention will become more apparent from the detailed description of non-limiting embodiments with reference to the following drawings:

[0029] Figure 1 It is a schematic diagram of the construction process of the present invention;

[0030] Figure 2 It is a schematic diagram of the middle partition wall in the construction method of the present invention;

[0031] Figure 3 It is an excavation sequence diagram of the construction method of the present invention;

[0032] Figure 4 This is the construction step control diagram of the preceding tunnel and the succeeding tunnel in the present invention.

[0033] Among them: 1—advance tunnel, 2—middle partition wall, 3—initial support layer of advance tunnel, 4—waterproof layer of advance tunnel, 5—secondary lining layer of advance tunnel, 6—rear tunnel, 7—initial support layer of rear tunnel, 8—waterproof layer of rear tunnel, 9—secondary lining layer of rear tunnel, 10—heading face of advance tunnel, 11—heading face of rear tunnel. DETAILED DESCRIPTION

[0034] In order to make the purpose, process conditions and advantages of the present invention more clearly understood, the present invention is further described in detail in conjunction with the following embodiments, but the embodiments of the present invention are not limited thereto. Without departing from the above-mentioned technical ideas of the present invention, various substitutions and changes are made according to the common technical knowledge and customary means in the field, which should all be included in the scope of the present invention. The specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0035] Embodiment 1:

[0036] This embodiment provides a single tunnel construction method for a composite middle wall multi-arch tunnel. The specific process is as follows: Figure 1 As shown, the following steps are included:

[0037] (1) Carry out preliminary tunnel excavation and treat the intermediate wall foundation according to the construction drawings;

[0038] (2) Tie the steel frame of the middle partition wall according to the foundation of the middle partition wall, then provide initial support for the good advance tunnel, and spray-cast the tied steel frame of the middle partition wall as a whole to form the middle partition wall;

[0039] (3) Casting and filling of the invert of the first tunnel;

[0040] (4) Then lay the waterproof layer of the first tunnel and pour the secondary lining of the first tunnel;

[0041] (5) Excavate the upper part of the rear tunnel, install the joint with the middle partition wall in the excavated rear tunnel, and then provide initial support for the upper part of the excavated rear tunnel;

[0042] (6) Reserve rock pillars at the middle partition wall of the rear tunnel, then excavate the lower part of the rear tunnel, and after excavation, provide initial support for the lower part of the rear tunnel;

[0043] (7) Excavate the reserved rock pillar at the middle partition wall of the rear tunnel, provide initial support for the excavated part of the rear tunnel, close it with the initial support of the previous rear tunnel to form a ring, and complete the pouring and filling of the rear tunnel invert;

[0044] (8) Finally, lay the waterproof layer of the rear tunnel and pour the secondary lining of the rear tunnel.

[0045] The partition wall obtained through the above steps is as follows: Figure 2As shown, the middle partition wall is arranged in the leading tunnel, and after the lower part of the leading tunnel is excavated and the initial support is constructed, it is then connected to the initial support of the corresponding part by on-site steel skeleton tying or steel frame, and sprayed with shotcrete (it can also be made of on-site concrete pouring using formwork). Its cross-sectional dimensions are small at the top and large at the bottom, approximately trapezoidal. The construction of the rear tunnel of the arch tunnel is close to the construction condition of the leading tunnel, and is the closest to the working condition. For the single tunnel construction method, the middle partition wall is not only the initial support of the leading tunnel, but also the enclosure structure of the secondary lining and waterproof layer of the leading tunnel. Therefore, the middle partition wall should be structurally designed as a rigid support that bears all loads to prevent it from causing harmful deformation and affecting the safety, durability and waterproofing effect of the leading tunnel structure. The minimum cross-section of the middle partition wall of the single tunnel construction method is the joint between the initial support of the rear tunnel and the middle partition wall. Its thickness should be designed and constructed according to the compression short column component, and its slenderness ratio should not be greater than 8 to prevent it from being unstable and damaged during the excavation of the rear tunnel. The middle partition wall is usually about 4m high, so its minimum thickness should not be less than 50cm. At the same time, compared with the mid-guide tunnel method, the base width of the middle partition wall of the single tunnel construction method is 1 / 3 to 1 / 4 smaller, and the foundation bearing capacity requirements are higher. For this reason, measures such as steel pipe piles are usually set at the bottom of the middle partition wall in the design to meet the foundation bearing capacity requirements and improve the constraint state of the lower end of the middle partition wall.

[0046] Embodiment 2:

[0047] Based on the above embodiment, this embodiment further limits the excavation span of the leading tunnel to be 0.8~1.2m larger than the excavation span of the following tunnel. In the single tunnel construction method, due to the need to set up a middle partition wall, the excavation span of the leading tunnel is usually larger than the excavation span of the following tunnel, about 1m. Therefore, the geometric dimensions of the leading tunnel and the corresponding single tunnel are not much different, and the general single tunnel excavation method can be adopted. It is worth pointing out that the foundation treatment measures of the middle partition wall must be completed in the leading tunnel in a timely manner according to the design.

[0048] Embodiment 3:

[0049] On the basis of the above embodiments, this embodiment further defines that if the preceding tunnel or the following tunnel is of Grade V or IV surrounding rock, its initial support is composed of I-beam arch frame, radial anchor rod, steel mesh and shotcrete, and the I-beam arch frame with steel mesh and shotcrete joint support is adopted; if the preceding tunnel or the following tunnel is of Grade III surrounding rock, the initial support is composed of radial anchor rod, steel mesh and shotcrete, the steel arch frames are connected with longitudinal steel bars, and are welded with radial anchor rod and steel mesh as one, closely attached to the surrounding rock, forming a bearing structure reinforced concrete structure; the shotcrete adopts wet spraying technology. For Grade V and IV surrounding rock, it is composed of I-beam arch frame, radial anchor rod, steel mesh and shotcrete, while for Grade III surrounding rock, it is composed of radial anchor rod, steel mesh and shotcrete (partly I-beam). The steel arch frames are connected with longitudinal steel bars, and are welded with radial anchor rod and steel mesh as one, closely attached to the surrounding rock, forming a bearing structure.

[0050] Initial support: The V and IV surrounding rock sections are supported by I-beam arch frame with steel mesh and sprayed anchor. The sprayed concrete is sprayed with wet technology, drilling with pneumatic drill, manual installation of anchor rods, and small pipe grouting to reinforce the surrounding rock.

[0051] Embodiment 4:

[0052] On the basis of the above embodiments, this embodiment further defines that if the preceding tunnel or the succeeding tunnel is of Grade V or IV surrounding rock, or a shallow buried section of the tunnel entrance of Grade V or Grade IV surrounding rock, or a section of a fault crushing zone, then its secondary lining shall adopt a reinforced concrete structure, and the reasonable time for the construction of the secondary lining shall be finally determined based on the construction monitoring and measurement data. Secondary lining: A reinforced concrete structure shall be adopted for the shallow buried section of the tunnel entrance of Grade V or Grade IV surrounding rock, and a section of a fault crushing zone, to ensure the safety of the tunnel support structure. The reasonable time for the construction of the secondary lining shall be finally determined (verified) based on the construction monitoring and measurement data, so as to maximize the bearing capacity of the initial support, but not exceed its bearing capacity.

[0053] Embodiment 5:

[0054] On the basis of the above-mentioned embodiments, this embodiment further adds tunnel auxiliary construction measures, including an advanced long pipe shed at the tunnel entrance, an advanced small guide tube in the tunnel, an advanced anchor rod, and peripheral curtain grouting;

[0055] The tunnel entrance advance long pipe shed is set at the tunnel entrances at both ends. Grouting is used to improve the bearing capacity of the surrounding rock itself, improve the elastic resistance of the rock mass to the structure, and improve the stress conditions of the structure. The steel pipes of the pipe shed are all Φ108×6mm hot-rolled seamless steel pipes with a circumferential spacing of 40cm. The steel pipes are set in the lining arch, and the distance between the pipe core and the outer contour line of the lining design is greater than 20cm, and the angle with the center line of the road surface is 1°.

[0056] The small advance pipe in the tunnel is suitable for tunnels with poor surrounding rock of grade V and IV. The bearing capacity of the surrounding rock itself is improved by grouting the small pipe, the elastic resistance of the rock mass to the structure is improved, and the stress condition of the structure is improved. The small pipe adopts a hot-rolled seamless steel pipe with an outer diameter of 42mm and a length of 400cm, with an annular spacing (SL5a\SL5jq) of 30cm\SL4a40cm, an outward inclination angle of about 10°~15°, and a longitudinal horizontal overlap length of not less than SL5a1.5m\SL5jq2.0m\SL4a1.3m;

[0057] The advance anchor is set in the tunnel advance pilot pit IV surrounding rock section, using a 350cm long Φ22 drug coil anchor, with a circumferential spacing of about 40cm, an outward angle of about 10° to 15°, and a longitudinal overlap length of not less than 100cm. During construction, the best direction of the anchor should be determined according to the occurrence of the rock joint surface;

[0058] The peripheral curtain grouting is used in areas with large water gushing volume. It is used to grout and seal the gaps in the rock mass around the tunnel, limit the large-scale discharge of groundwater, and protect the original water system from being damaged. The grouting pipe adopts a hot-rolled seamless steel pipe with an outer diameter of 42mm and a wall thickness of 4.0mm. A grouting plug needs to be set at the tail of the steel pipe according to the size of the drill hole. 8mm grouting holes are drilled around the pipe wall, but there are 15cm at the tail without grouting holes. The spacing between the steel pipes is 200*200cm.

[0059] Embodiment 6:

[0060] Based on the above embodiment, this embodiment is further limited to the V-level surrounding rock section. For both the leading tunnel and the following tunnel, the three-step circular reserved core soil method is used for excavation; in the IV-level surrounding rock section, the leading tunnel is excavated by the two-step core soil method, and the following tunnel is excavated by the three-step method; the following tunnel should be excavated after the secondary lining of the leading tunnel is completed for not less than 40m and reaches the design strength, and the secondary lining of the leading tunnel should always be not less than 40m ahead of the secondary lining of the leading and following tunnels; in the leading tunnel and the following tunnel, the excavation on the left and right sides of the lower step is staggered by 5 to 10m, and the arch adopts smooth blasting technology. In order to effectively change the biased stress state of the leading tunnel and the stress concentration state at the joint, engineering practice shows that if Figure 3As shown in the figure, the construction process is Z1→Z2→Z2→Y1→Y2→Y3→Y4. In the V-level surrounding rock section, the three-step ring reserved core soil method is used for both the leading tunnel and the trailing tunnel; in the IV-level surrounding rock section, the leading tunnel is excavated by the two-step core soil method, and the trailing tunnel is excavated by the three-step method. The trailing tunnel should be excavated after the secondary lining of the leading tunnel is completed for no less than 40m and reaches the design strength, and the secondary lining of the leading tunnel should always be no less than 40m ahead of the secondary lining of the leading and trailing tunnels. In the leading tunnel and the trailing tunnel, the excavation on the left and right sides of the lower step is staggered by 5 to 10m. The arch adopts smooth blasting technology to maximize the protection of the surrounding rock stability, reduce over-excavation, and improve the bearing capacity of the initial support.

[0061] Embodiment 7:

[0062] Based on the above embodiments, this embodiment further stipulates that the initial support steel frame of the rear tunnel should be connected to the initial support steel frame of the leading tunnel as a whole by welding, and an anti-slip block should be welded at the lower end of the joint. In hard rock, after the reserved rock column of the rear tunnel is excavated, the mesh should be sprayed with concrete in time, and the joint of the middle partition wall should be rounded. In order to ensure the construction quality of the joint and improve the biased stress condition of the middle partition wall, under normal circumstances, the initial support steel frame of the rear tunnel should be connected to the initial support steel frame of the leading tunnel by welding, and an anti-slip block should be welded at the lower end of the joint to prevent the rear tunnel from slipping.

[0063] The steel frame at the joint of the preceding tunnel is displaced. Moreover, in hard rock, after the excavation of the reserved rock column of the subsequent tunnel, the mesh should be hung and concrete sprayed in time, and the joints of the middle partition wall should be rounded to avoid excessive stress concentration caused by the excavation of the previous step, which may cause cracks in the lining of the preceding tunnel.

[0064] Embodiment 8:

[0065] On the basis of the above embodiments, this embodiment further stipulates that the first tunnel should promptly implement the foundation treatment and end anchoring measures of the base of the middle partition wall, and the initial support of the subsequent tunnel should be connected to the upper end of the middle partition wall as soon as possible. During construction, the first tunnel should promptly implement the foundation treatment and end anchoring measures of the base of the middle partition wall, and the initial support of the subsequent tunnel should be connected to the upper end of the middle partition wall as soon as possible to be jointly stressed, so as to improve the biased stress state of the middle partition wall.

[0066] Embodiment 9:

[0067] On the basis of the above-mentioned embodiments, this embodiment further defines that a shock-absorbing layer is laid between the initial support and the secondary lining of the leading tunnel, a rock pillar is reserved in the partition wall of the trailing tunnel, and a plan of drilling more peripheral holes, less explosives, and more cycles of excavation is adopted, and the on-site blasting test plan and blasting vibration monitoring of the trailing tunnel are carried out in a timely manner. In order to ensure the quality of the secondary lining of the leading tunnel, shock-absorbing measures should be taken, such as laying a shock-absorbing layer between the initial support and the secondary lining of the leading tunnel, a rock pillar is reserved in the partition wall of the trailing tunnel, and a plan of drilling more peripheral holes, less explosives, and more cycles of excavation is adopted, and the on-site blasting test plan and blasting vibration monitoring of the trailing tunnel are carried out in a timely manner, and the blasting vibration speed is strictly controlled.

[0068] Embodiment 10:

[0069] Controlling the construction step distance of the leading tunnel and the following tunnel is the key to reducing the surrounding rock disturbance. Therefore, based on the above embodiment, this embodiment further limits the construction step distance of the leading tunnel and the following tunnel, such as Figure 4 As shown, the step distance between the working face of the preceding tunnel and the working face of the succeeding tunnel is ≥120 linear meters, and the step distance between the secondary lining of the preceding tunnel and the secondary lining of the succeeding tunnel is ≥90 linear meters.

[0070] Although the embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the claims and their equivalents.

Claims

1. A composite middle wall multi-arch tunnel single tunnel construction method, characterized in that: The following steps are involved: (1) Carry out preliminary tunnel excavation and treat the intermediate wall foundation according to the construction drawings; (2) The steel frame of the middle partition wall is tied according to the foundation of the middle partition wall, and then the initial support is carried out on the good advance tunnel, and the tied steel frame of the middle partition wall is sprayed as a whole to form the middle partition wall; the middle partition wall is arranged in the advance tunnel, and after the lower part of the advance tunnel is excavated and the initial support is applied, the steel frame is tied on site or the steel frame is connected with the initial support of the corresponding part as a whole, and it is sprayed with shotcrete or cast in site with formwork. Its cross-sectional size is small at the top and large at the bottom, and is approximately trapezoidal. The partition wall is not only the initial support of the leading tunnel, but also the enclosure structure of the secondary lining and waterproof layer of the leading tunnel. The middle partition wall should be structurally designed as a rigid support that bears all loads. The minimum section of the middle partition wall is the joint between the initial support of the rear tunnel and the middle partition wall. Its thickness should be designed and constructed as a compressed short column component, and its slenderness ratio should not be greater than 8. The middle partition wall is usually 4m high and the minimum thickness is not less than 50cm. Steel pipe piles are set at the bottom of the middle partition wall to meet the bearing capacity requirements of the foundation and improve the constraint state of the lower end of the middle partition wall. (3) Casting and filling of the invert of the first tunnel; (4) Then lay the waterproof layer of the first tunnel and pour the secondary lining of the first tunnel; (5) Excavate the upper part of the rear tunnel, install the joint with the middle partition wall in the excavated rear tunnel, and then provide initial support for the upper part of the excavated rear tunnel; (6) Reserve rock pillars at the middle partition wall of the rear tunnel, then excavate the lower part of the rear tunnel, and after excavation, provide initial support for the lower part of the rear tunnel; (7) Excavate the reserved rock pillar at the middle partition wall of the rear tunnel, provide initial support for the excavated part of the rear tunnel, close it with the initial support of the previous rear tunnel to form a ring, and complete the pouring and filling of the rear tunnel invert; (8) Finally, lay the waterproof layer of the rear tunnel and pour the secondary lining of the rear tunnel.

2. A composite middle wall multi-arch tunnel single tunnel construction method according to claim 1, characterized in that: The excavation span of the leading tunnel is 0.8-1.2 m larger than the excavation span of the following tunnel.

3. A composite middle wall multi-arch tunnel single tunnel construction method according to claim 1 or 2, characterized in that: If the preceding tunnel or the succeeding tunnel has Grade V or IV surrounding rock, its initial support is composed of I-beam arch frames, radial anchors, steel mesh and shotcrete, and the combined support of I-beam arch frames hanging steel mesh and shotcrete anchors is adopted; if the preceding tunnel or the succeeding tunnel has Grade III surrounding rock, its initial support is composed of radial anchors, steel mesh and shotcrete, the steel arch frames are connected with longitudinal steel bars, and are welded to the radial anchors and steel mesh as a whole, closely attached to the surrounding rock, forming a load-bearing reinforced concrete structure; the shotcrete adopts the wet spraying process.

4. A composite middle wall multi-arch tunnel single tunnel construction method according to claim 1 or 2, characterized in that: If the preceding tunnel or the succeeding tunnel has Grade V or Grade IV surrounding rock, or a shallow buried section of the tunnel entrance and a fault fracture zone section with Grade V or Grade IV surrounding rock, its secondary lining shall adopt a reinforced concrete structure, and the reasonable time for the construction of the secondary lining shall be finally determined based on the construction monitoring and measurement data.

5. A composite middle wall multi-arch tunnel single tunnel construction method according to claim 1 or 2, characterized in that: It also includes tunnel auxiliary construction measures, including advanced long pipe sheds at the tunnel entrance, advanced small guide tubes in the tunnel, advanced anchor rods, and peripheral curtain grouting.

6. A composite middle wall multi-arch tunnel single tunnel construction method according to claim 3, characterized in that: In the Grade V surrounding rock section, the three-step circular core reserved soil method is adopted for both the leading tunnel and the trailing tunnel in excavation; in the Grade IV surrounding rock section, the two-step core reserved soil method is adopted for the leading tunnel, and the three-step method is adopted for the trailing tunnel; the trailing tunnel is excavated after the secondary lining of the leading tunnel is completed for not less than 40m and reaches the design strength, and the secondary lining of the leading tunnel is always not less than 40m ahead of the secondary lining of the leading and trailing tunnels; in the leading tunnel and the trailing tunnel, the excavation on the left and right sides of the lower step is staggered by 5 to 10m, and the smooth blasting technology is adopted for the arch.

7. A composite middle wall multi-arch tunnel single tunnel construction method according to claim 1 or 2, characterized in that: The initial support steel frame of the rear tunnel is connected to the initial support steel frame of the advance tunnel by welding as a whole, and an anti-slip block is welded at the lower end of the joint. In the hard rock, the rock pillars reserved for the rear tunnel are excavated and mesh sprayed with concrete in time, and the joints of the middle partition walls are rounded.

8. A composite middle wall multi-arch tunnel single tunnel construction method according to claim 1 or 2, characterized in that: The advance tunnel should promptly implement foundation treatment and end anchoring measures for the base of the middle partition wall, and the initial support of the follow-up tunnel should be connected to the upper end of the middle partition wall as soon as possible.

9. A composite middle wall multi-arch tunnel single tunnel construction method according to claim 1 or 2, characterized in that: A shock-absorbing layer is laid between the initial support and the secondary lining of the advance tunnel. Rock pillars are reserved in the partition wall of the follow-up tunnel, and more peripheral holes, less explosives, and more cycles of excavation are adopted. In addition, on-site blasting test plans and blasting vibration monitoring work are carried out in a timely manner for the follow-up tunnel.

10. A composite middle wall multi-arch tunnel single tunnel construction method according to claim 1 or 2, characterized in that: The step distance between the working face of the preceding tunnel and that of the succeeding tunnel shall be ≥120 linear meters, and the step distance between the secondary lining of the preceding tunnel and that of the succeeding tunnel shall be ≥90 linear meters.

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