A tunnel entrance protection structure for V-shaped terrain and its construction method

Abstract

The present invention discloses a tunnel entrance protection structure for a V-shaped terrain and its construction method. The method includes the following steps: S1, construction preparation; S2, construction of the catchment ditch on the tunnel top; S3, excavation and support of the side slope and the front slope; S4, construction of the ring beam and the pipe shed; S5, construction of the special-shaped ring beam for the special-shaped section; S6, construction of the advanced large pipe shed; S7, construction of the hidden tunnel. Through the construction method for entering the tunnel under the "V" - shaped terrain, the present invention adopts the special-shaped ring beam support, enabling the tunnel to enter the hidden excavation construction in advance, reducing the excavation and protection of the high side slope at the tunnel entrance, reducing the potential safety hazards at the tunnel entrance and environmental damage, saving time and labor. The special-shaped ring beam is constructed by shotcrete, saving a certain amount of support formwork and reducing the formwork modulus. Moreover, the setting time of the shotcrete is fast, enabling the structure to enter the stress state in advance, greatly saving the construction time. It can not only reduce the environmental damage and unnecessary excavation construction before entering the tunnel, but also reduce the construction cost while achieving safe and rapid tunnel entrance construction.

Description

Technical Field

[0001] The present invention relates to a tunnel entrance protection structure and a construction method thereof, specifically to a construction method of a tunnel entrance protection structure for a V-shaped terrain, mainly applicable to the entrance construction of highways, railway tunnels, etc. under the "V" shaped terrain, belonging to the technical field of tunnel construction. Background Art

[0002] Generally speaking, the tunnel construction portal project is crucial. Whether the construction procedures of each part at the portal are smooth lays the foundation for safe entrance into the tunnel. For the special situation where the tunnel portal is in a "V" shaped terrain, in the conventional construction methods, generally large-scale excavation and open-cutting of the hidden tunnel are adopted, shortening the hidden tunnel and increasing the open-cut tunnel and backfill. This construction method not only destroys the original landform, damages the ecological environment, and increases the construction cost, but also is prone to cracking and instability of the side slope and the front slope during the excavation process, causing accidents such as landslides and collapses of the mountain body, with serious potential safety hazards.

[0003] At present, with the rapid development of the expressway construction in China, its construction scope has expanded accordingly, and the expressways are gradually extending into mountainous areas. In order to overcome the complexity and particularity of the mountainous regions, tunnels have become one of the necessary options. Due to the limitations of terrain and route selection, tunnel portals often locate in special terrain positions. For a tunnel, the portal project is not only the external part of the whole project but also the first key point and difficulty faced. Now the country highly emphasizes the protection of the ecological environment during construction and the safety guarantee of construction workers, advocating zero-excavation for tunnel entrance construction to reduce the damage to the original portal landform, protect the ecological environment, and adopt appropriate entrance construction techniques to ensure the safety of construction workers.

[0004] To sum up, considering the special situation of the "V" shaped terrain, in order to protect the natural balance of the original landform and quickly enter the tunnel with less large-scale excavation, zero-excavation for tunnel entrance is adopted, and the support effect of a special-shaped ring beam is considered to enter the hidden tunnel construction in advance.

[0005] Therefore, it is the key to solve the above technical problems to develop a construction method of a tunnel entrance protection structure for a V-shaped terrain with strong practicability and high working reliability. Summary of the Invention

[0006] Aiming at the many defects and deficiencies in the above background art, the present invention has made improvements and innovations, aiming to provide a method that can not only reduce the environmental damage and unnecessary excavation construction before tunnel entrance, but also achieve safe and rapid tunnel entrance construction while reducing the construction cost.

[0007] Another object of the present invention is to adopt the "V" - shaped terrain tunneling construction method with special - shaped arch sheathing support, enabling the tunnel to enter the mined - out construction in advance, reducing the excavation and protection of the high - side slope at the tunnel entrance, reducing the potential safety hazards at the tunnel entrance and environmental damage, and saving time and labor.

[0008] Another object of the present invention is that the special - shaped arch sheathing is constructed by shotcrete, saving a certain amount of support formwork and reducing the formwork modulus. Moreover, the setting time of the shotcrete is fast, enabling the structure to enter the stress state in advance and saving a great deal of construction time.

[0009] Another object of the present invention is to achieve rapid tunneling construction by using the support method of special - shaped arch sheathing and large - diameter pipe - shed. Compared with the traditional process, the "zero - excavation" tunneling avoids the damage of the vegetation at the tunnel entrance, not only saving the construction period, but also reducing the construction cost and safety risks, and also generating good social benefits.

[0010] To solve the above problems and achieve the above - mentioned objects of the invention, a construction method of a tunnel entrance protection structure for a "V" - shaped terrain of the present invention is realized by adopting the following design structure and the following technical solutions:

[0011] As an improvement of the construction method of a tunnel entrance protection structure for a "V" - shaped terrain of the present invention, as the above - mentioned improvement of the present invention, it includes a plurality of arched steel arch frames (1) arranged side by side in sequence from the inside to the outside on the outer side of the tunnel entrance and a plurality of special - shaped steel arch frames (2) arranged side by side in sequence from the outside to the inside on the inner side of the tunnel entrance. Among them, the plurality of steel arch frames (1) are connected into a whole by a plurality of longitudinally arranged connecting bars and then concreted to form an arched sheathing. Inside the arched sheathing, a number of arched steel arch frame large - diameter pipe - shed guiding pipes (11) are arranged side by side according to the design requirements, and the arched steel arch frame large - diameter pipe - shed guiding pipes (11) and the connecting bars are arranged in parallel with each other; the plurality of special - shaped steel arch frames (2) are connected into a whole by a plurality of transversely arranged connecting tie rods and then filled with concrete to form a special - shaped sheathing. One end of the special - shaped sheathing is connected to the outer sheathing, and the other end is connected to the mountain rock formation (3); inside the special - shaped sheathing, a number of special - shaped steel arch frame sleeves (21) are arranged side by side in sequence, and the special - shaped steel arch frame sleeves (21) are connected and integrated with the large - diameter pipe - shed guiding pipes (11) to form a tunnel entrance protection structure.

[0012] As a further improvement of the above - mentioned present invention, the arched steel arch frame (1) is a semi - circular or arched I - beam steel arch frame with the same corresponding model size; the special - shaped steel arch frame (2) is a semi - arc I - beam steel arch frame with the same model but different lengths, and the length of the special - shaped steel arch frame (2) decreases in sequence from the outside of the tunnel to the inside of the tunnel.

[0013] As a further improvement of the present invention described above, a plurality of back plates (22) with the same model but different sizes are vertically connected to the free end of the special-shaped steel arch (2). A locking foot bolt (23) is connected to the back plate (22). The locking foot bolt (23) is a grouting locking foot bolt with a hollow interior, and the grouting locking foot bolt is connected to the mountain rock formation (3).

[0014] As a further improvement of the present invention described above, both the large pipe shed guiding pipe (11) and the special-shaped steel arch sleeve (21) are tubular members with a hollow interior; both the connecting bars and the connecting tie rods are tubular or rod-shaped members with a hollow interior or a solid interior;

[0015] On the outer surfaces of the arched steel arch (1), the special-shaped steel arch (2), the connecting bars, and the connecting tie rods, an injection layer, an anti-rust layer, and a waterproof layer are sequentially sprayed from the inside to the outside.

[0016] As a further improvement of the present invention described above, a construction method for a tunnel entrance protection structure for a V-shaped terrain, the method comprising the following steps:

[0017] S1, Construction preparation. First, determine the tunnel entrance construction plan, then conduct surveying and lofting, and finally determine the tunnel entrance construction;

[0018] S2, Top ditch. Before the side slope and overhanging slope construction, a catch ditch should be set 5 meters outside the excavation line of the side slope and overhanging slope;

[0019] S3, Side slope and overhanging slope excavation and support. Mechanical excavation is used for the side slope and overhanging slope excavation. During excavation, the slope should be strictly controlled according to the design. When excavating the side slope and overhanging slope, try to retain the stable structural layer of the slope surface, and adopt a design plan of less or no brushing;

[0020] S4, Invert arch construction. The construction process flow of the invert arch and the pipe shed is: surveying and lofting → fabrication and trial assembly of the steel arch → installation of the steel arch → installation of longitudinal connecting bars → determination and welding of the guiding pipe direction → formwork erection and concrete pouring.

[0021] S5, Special-shaped section invert arch construction. Set an I18 arch as the formwork support skeleton at a spacing of 50 cm below the top triangular void area. Then use bamboo plywood as the bottom formwork, lay a Φ8mm@20cm steel mesh, and then install an I18 I-beam arch at a spacing of 50 cm. Fix and connect the arch with longitudinal connecting bars at a circumferential spacing of 1 m; install the guiding pipe. The model of the guiding pipe is exactly the same as that of the external invert arch of the tunnel, and the length is cut according to different terrains and extends to the inner rock surface of the rock mass. After the guiding pipe is installed, spray 1 m thick C25 shotcrete to quickly form a virtual tunnel wall.

[0022] S6, advanced large pipe shed construction, the construction process flow of the advanced pipe shed is: construction preparation → drilling rig placement → drilling → installation of large pipe shed steel pipes → pipe shed grouting → inspection and acceptance.

[0023] S7, blind tunnel construction, after the pipe roof grouting is completed, according to the advanced geological forecast and monitoring measurement results, the construction adopts the three-step partial excavation method of reserved core soil. After successfully entering the tunnel, the excavation method is adjusted according to the surrounding rock conditions.

[0024] As a further improvement to the above-mentioned invention, in S1, before construction, a surveying team is organized to re-survey the cross-section of the tunnel entrance and release the elevation position of the structure, the entrance mileage, and the blind tunnel mileage, and compare them with the design to see if they are consistent. If there are any deviations, they are reported to the design team for adjustment, and construction can only be carried out after confirmation.

[0025] In said S2, the top intercepting ditch (4) is cast with C20 plain concrete, a retaining wall is set at the top gully position, and the ditch size is increased to prevent the top water from directly scouring the back slope. The top intercepting ditch (4) is adjusted and arranged according to the actual terrain on site and must be completed before the side back slope is constructed. The water in the top intercepting ditch (4) is discharged to the natural ditch on both sides of the tunnel.

[0026] As a further improvement of the above-mentioned present invention, in S3, the side slope includes the entrance side slope and the back slope. The side slope is excavated by open-cut method, excavated from top to bottom, and a step into the cave is reserved. After the side slope is excavated and formed, anchor spraying and hanging mesh are promptly used for protection. In order to reduce disturbance to the rock mass and ensure the stability of the excavated mountain, anchor spraying protection and small pipe grouting reinforcement are performed on the side slope within a certain range during construction. The reinforcement process is anchor rod construction → laying steel mesh → spraying concrete.

[0027] The anchor spraying protection includes mortar anchor rods, hanging steel mesh and C20 sprayed concrete, and the thickness of C20 sprayed concrete is 10 cm; among them, the anchor rods adopt Φ22 ordinary mortar anchor rods, which are 4.5 meters long and arranged in a plum blossom shape with a spacing of 1 meter. They are staggered and φ42mm×4mm grouting small tubes are added on three sides of the side slope within 8 meters above the cave top. The small tubes are 4.5 meters long and arranged in a plum blossom shape with a spacing of 1m×1m.

[0028] As a further improvement of the above-mentioned present invention, in S4, after the excavation of the edge of the cave entrance and the support are completed, the core soil is reserved, and the surveying personnel determine the centerline arch height, the arch position line, and the horizontal cross line on the slope, and then excavate the soil at the arch position on both sides. After excavation and support to the bottom height of the side wall, four I18 I-steel arches are erected with a spacing of 50 cm. Each arch is tightened with a pull rod to prevent tipping. After the I-steel arch is erected, a C20 concrete arch is cast to expand the foundation to cover the arch foot; then, a Φ140mm, 2m long pipe shed guide steel pipe is installed on the steel support at the designed large pipe shed spacing. The centerline must be carefully and accurately checked with an instrument to ensure good guidance and consistent with the direction of the pipe shed position. After the pipe shed guide steel pipe is installed, C20 cast concrete is poured.

[0029] As a further improvement of the above-mentioned present invention, in said S5, the formwork support skeleton also adopts I18 steel arch frame, and the spacing is consistent with the spacing of the sleeve arch frame, which is 50 cm; when the bottom formwork is installed, the arch foot is reserved for the placement of the contraction foot anchor rod, and the formwork is sealed again after the construction of the locking foot anchor rod is completed; the I18 steel arch frame is made and trial-assembled, and after the positions of the two arch feet are accurately measured according to the slope terrain, the cutting length of the steel arch frame is accurately determined by the position of the arch feet, so that the arch frame can be accurately positioned on the rock masses on both sides, and the arch feet are supported by channel steel pads; the guide tube model of the special-shaped sleeve arch is exactly the same as the model of the outer arch of the tunnel, and is connected to the outer arch wire tube of the tunnel, and the length is cut according to the terrain and extended to the inner rock mass and rock surface; after the guide tube is installed, 1m thick C25 sprayed concrete is sprayed to quickly form a virtual tunnel wall.

[0030] As a further improvement of the above-mentioned present invention, in S6, after the construction of the special-shaped sleeve arch is completed, a pipe-roof drilling platform is built, and a drilling rig is installed. Different drilling tool combinations are used according to different strata, and drilling is carried out from the guide pipe inward. Odd-numbered holes are drilled first, and then even-numbered holes are drilled after grouting. Even-numbered holes are not grouted, and consecutive numbering drilling is not allowed to avoid hole collapse and drill sticking. Grouting must be done from both sides toward the arch top.

[0031] In S7, after the pipe roof grouting is completed, according to the advanced geological forecast and monitoring measurement results, the construction adopts the three-step partial excavation method of reserved core soil. After successfully entering the cave, the excavation method is adjusted according to the surrounding rock conditions.

[0032] The beneficial effects of the present invention compared with the prior art are:

[0033] 1. The present invention can not only reduce environmental damage and unnecessary excavation construction before entering the cave, but also reduce construction costs while achieving safe and rapid cave entry construction;

[0034] 2. Through the "underground entry construction method under the 'V'-shaped terrain", the present invention adopts a special-shaped arched shed support, enabling the tunnel to enter the mined tunneling construction in advance, reducing the excavation and protection of the high side slope at the tunnel entrance, reducing the safety hazards and environmental damage at the tunnel entrance, and saving time and labor.

[0035] 3. The special-shaped arched shed of the present invention is constructed by shotcrete, saving a certain amount of support formwork and reducing the formwork modulus. Moreover, the shotcrete has a fast setting time, enabling the structure to enter the stress state in advance and saving a great deal of construction time.

[0036] 4. The construction method of the present invention is convenient to operate, has a low cost, and is safe in construction. It can reduce the amount of earth and rock excavation at the tunnel entrance, reduce unnecessary side slope support, effectively guarantee the construction risks of workers, reduce the damage rate of the construction environment at the tunnel entrance, save the construction cost, shorten the construction period, and has significant economic and social benefits.

[0037] 5. The present invention implements the concepts of "early entry into the tunnel and late exit from the tunnel" and "zero excavation", adapts to local conditions, makes full use of the mountain bodies on both sides of the 'V'-shaped terrain and the support method of "special-shaped arched shed + pipe shed", reduces the excavation of the side slopes at the tunnel entrance, not only avoids the safety risks brought by the high side slopes, but also reduces the impact on the vegetation and soil stability at the tunnel entrance, enables the tunnel to enter the tunnel in advance and start the mined tunneling operation.

[0038] 6. The present invention uses C25 shotcrete and steel mesh to construct the special-shaped section arched shed, without installing the formwork on both sides and the concrete conveying pipeline. Moreover, the shotcrete can obtain a high strength in a short time, is convenient and fast to operate, safe and reliable, and can quickly carry out the construction of the subsequent processes.

[0039] 7. The present invention adopts the method of small pipe grouting for reinforcement treatment at the side slope position, which can greatly improve the stability of the mountain body at the tunnel entrance and ensure the construction safety.

[0040] 8. According to the concepts of "early entry into the tunnel and late exit from the tunnel" and "zero excavation", the present invention adapts to local conditions, optimizes the design of the tunnel entrance, solves the problem of difficult entry into the tunnel when the tunnel entrance is in the 'V'-shaped terrain, makes full use of the support effect of the mountain bodies on both sides of the 'V'-shaped terrain and the "special-shaped arched shed + pipe shed", reduces the damage to the stability of the mountain body in the tunnel entrance section, enables the tunnel to start the mined tunneling operation in advance, so as to ensure the construction and operation safety, and at the same time shorten the construction period and reduce the environmental impact.

[0041] 9. According to the actual terrain conditions at the construction site, the present invention realizes rapid tunnel entry construction by adopting the method of special-shaped socket arch and large pipe shed support. Compared with the traditional process, the "zero-excavation" tunnel entry avoids the destruction of the vegetation at the tunnel entrance, not only saves the construction period, but also reduces the construction cost and safety risks, and also produces good social benefits. In view of the above advantages, it has received praise from the construction unit and the supervision unit. Practice has proved that this construction method is safe and reliable, simple to operate, the quality of the construction products is excellent, the method is operable, has broad application prospects, and has prominent social, economic and environmental protection benefits;

[0042] 10. The exterior of the present invention is coated with anti-rust paint, so it can prevent rust and at the same time extend the service life of the entire protective structure, realizing environmental protection while saving resources. At the same time, a fluorescent material that can emit light by itself is coated on the exterior of the protective structure, which can clearly indicate the position of the protective structure at night, in a dark room or in an underground construction environment, can effectively play a role in safety reminder, improve the visibility, is easy for people to distinguish, and increases the safety in construction and life. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] The following further elaborates on the specific embodiments of the present invention in conjunction with the drawings, where:

[0044] Figure 1 is the overall structural schematic diagram of the present invention;

[0045] Figure 2 is the overall structural schematic diagram of the arched steel arch frame (1) component of the present invention;

[0046] Figure 3 is one of the overall usage state schematic diagrams of the present invention;

[0047] Figure 4 is the second overall structural schematic diagram of the present invention;

[0048] Figure 5 is the third overall structural schematic diagram of the present invention;

[0049] Figure 6 is the fourth overall structural schematic diagram of the present invention;

[0050] Figure 7 is the fifth overall structural schematic diagram of the present invention;

[0051] Figure 8 is the sixth overall structural schematic diagram of the present invention;

[0052] Figure 9 is the seventh overall structural schematic diagram of the present invention;

[0053] Figure 10 is the eighth overall structural schematic diagram of the present invention;

[0054] Figure 11 is the construction process flow chart of the present invention;

[0055] Table 1 is the material and equipment table of the present invention;

[0056] Table 2 is the material table of the present invention;

[0057] Table 3 is the mechanical equipment table of the present invention;

[0058] Among them, the reference numerals in the figure: 1 - arch steel arch frame, 11 - arch steel arch frame sleeve;

[0059] 2 - special-shaped steel arch frame, 21 - special-shaped steel arch frame sleeve, 22 - back plate, 23 - foot-locking bolt;

[0060] 3 - mountain rock stratum;

[0061] S1 - construction preparation;

[0062] S2 - top-of-hole intercepting ditch;

[0063] S3 - side slope and cutting slope excavation and support;

[0064] S4 - construction of arch ring and pipe shed;

[0065] S5 - construction of arch ring in special-shaped section;

[0066] S6 - construction of advanced large pipe shed;

[0067] S7 - construction of blind tunnel;

[0068] A1 - measurement and layout;

[0069] A2 - fabrication and trial assembly of steel arch frame;

[0070] A3 - installation of steel arch frame;

[0071] A4 - installation of guide pipe;

[0072] A5 - formwork erection and concrete pouring;

[0073] B1 - construction preparation;

[0074] B2 - drilling rig positioning;

[0075] B3 - drilling;

[0076] B4 - installation of steel pipe;

[0077] B5 - pipe shed grouting;

[0078] B6 - inspection and acceptance. Specific embodiments

[0079] In order to make the technical means, creative features, achieved objectives and effects of the present invention easily understood, the technical solution of the present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be noted that, without conflict, the embodiments in the present application and the features in the embodiments can be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and embodiments.

[0080] As shown in the accompanying drawings of the specification, a tunnel entrance protection structure for a V-shaped terrain includes a plurality of arched steel arch frames 1 arranged side by side in sequence from inside to outside on the outer side of the tunnel entrance, and a plurality of special-shaped steel arch frames 2 arranged side by side in sequence from outside to inside on the inner side of the tunnel entrance. Among them, the plurality of steel arch frames 1 are connected into a whole by a number of longitudinally arranged connecting bars and concrete is poured to form an arched sleeve arch. A number of large pipe shed guiding pipes 11 of the arched steel arch frames are sequentially arranged side by side in the arched sleeve arch according to the design requirements. The large pipe shed guiding pipes 11 of the arched steel arch frames and the connecting bars are arranged parallel to each other; the plurality of special-shaped steel arch frames 2 are connected into a whole by a number of horizontally arranged connecting tie rods and concrete is filled to form a special-shaped sleeve arch. One end of the special-shaped sleeve arch is connected to the outer sleeve arch, and the other end is connected to the mountain rock formation 3; a number of special-shaped steel arch frame sleeves 21 are sequentially arranged side by side in the special-shaped sleeve arch. The special-shaped steel arch frame sleeves 21 are connected and integrated with the large pipe shed guiding pipes 11 to form a tunnel entrance protection structure.

[0081] Further, the arched steel arch frame 1 is a semi-circular or arched I-beam arch frame with the same corresponding model size; the special-shaped steel arch frame 2 is a semi-circular I-beam arch frame with the same model and different lengths, and the length of the special-shaped steel arch frame 2 decreases sequentially from outside the hole along the hole inside.

[0082] Specifically, a plurality of back plates 22 with the same model and different sizes are vertically connected to the free end of the special-shaped steel arch frame 2. A locking foot bolt 23 is connected to the back plate 22. The locking foot bolt 23 is a grouting locking foot bolt with a hollow interior, and the grouting locking foot bolt is connected to the mountain rock formation 3.

[0083] Further, both the large pipe shed guiding pipe 11 and the special-shaped steel arch frame sleeve 21 are tubular members with a hollow interior; the connecting bars and the connecting tie rods are tubular or rod-shaped members with a hollow interior or a solid interior;

[0084] On the outer surfaces of the arched steel arch frame 1, the special-shaped steel arch frame 2, the connecting bars and the connecting tie rods, an injection layer, an anti-rust layer and a waterproof layer are sequentially sprayed from inside to outside.

[0085] Specifically, a high molecular wear-resistant material is injected on the injection layer; the anti-rust layer includes an epoxy zinc-rich primer, a chlorinated rubber topcoat and an epoxy mica iron intermediate coat located between the epoxy zinc-rich primer and the chlorinated rubber topcoat; the waterproof layer is a polyurethane waterproof coating.

[0086] In the present invention, the arched steel arch frame sleeves 11 and the special-shaped steel arch frame sleeves 21 are both provided in plurality, and the plurality of arched steel arch frame sleeves 11 and the plurality of special-shaped steel arch frame sleeves 21 are arranged at equal or unequal distances.

[0087] The connecting rods and the connecting pull rods are both rod-shaped components with solid interiors, and the rod-shaped components are steel bars. The connecting rods and the connecting pull rods are both provided in multiple numbers, and the multiple connecting rods and the multiple connecting pull rods are arranged at equal or unequal distances.

[0088] A construction method for a tunnel entrance protection structure in a V-shaped terrain, the method comprising the following steps:

[0089] S1, construction preparation, first determine the tunnel entry construction plan, then carry out surveying and setting out, and finally determine the tunnel entrance construction;

[0090] S2, intercepting ditch on the top of the cave. Before the side slope construction, a intercepting ditch should be set up 5 meters away from the side slope excavation line.

[0091] S3, side slope excavation and support, side slope excavation should be carried out by mechanical excavation, the slope should be strictly controlled during excavation, the stable structural layer of the slope surface should be preserved as much as possible during side slope excavation, and a design scheme with little or no brushing should be adopted;

[0092] S4, arch construction, the construction process of arch and pipe shed is: measurement and layout → production and trial assembly of steel arch frame → installation of steel arch frame → installation of longitudinal connecting reinforcement → determination of guide pipe direction and welding → erection of formwork and pouring of concrete.

[0093] S5, special-shaped arch construction, set I18 arch frame as the formwork support skeleton below the top triangular hollow area, with a spacing of 50cm, then use bamboo plywood as the bottom formwork, lay Φ8mm@20cm steel mesh, and then install I18 I-beam arch frame with a spacing of 50cm. Use longitudinal connecting bars to fix the arch frame with a circumferential spacing of 1m; install guide pipes. The model of the guide pipes is exactly the same as that of the outer arch of the tunnel. The length is cut according to the terrain and extended to the inner rock mass and rock surface. After the guide pipes are installed, spray 1m thick C25 shotcrete to quickly form a virtual tunnel wall.

[0094] S6, advanced large pipe shed construction, the construction process flow of the advanced pipe shed is: construction preparation → drilling rig placement → drilling → installation of large pipe shed steel pipes → pipe shed grouting → inspection and acceptance.

[0095] S7, blind tunnel construction, after the pipe roof grouting is completed, according to the advanced geological forecast and monitoring measurement results, the construction adopts the three-step partial excavation method of reserved core soil. After successfully entering the tunnel, the excavation method is adjusted according to the surrounding rock conditions.

[0096] Furthermore, in S1, before construction, a surveying team was organized to re-survey the section of the tunnel entrance, and to release the elevation position of the structure, the mileage of the tunnel entrance, and the mileage of the blind tunnel, and to compare them with the design to see if they were consistent. If there were any deviations, they would be reported to the design team for adjustment, and construction could only be carried out after confirmation.

[0097] In S2, the top intercepting ditch 4 is cast with C20 plain concrete, a retaining wall is set at the top gully position, and the ditch size is increased to prevent the top water from directly eroding the back slope. The top intercepting ditch 4 is adjusted and arranged according to the actual terrain on site and must be completed before the construction of the side back slope. The water in the top intercepting ditch 4 is discharged to the natural ditches on both sides of the tunnel.

[0098] Furthermore, in S3, the side slope includes the entrance side slope and the back slope. The side slope is excavated using the open-cut method, with excavation from top to bottom and a step reserved for entering the tunnel. After the side slope is excavated and formed, it is promptly protected by anchor spraying and mesh hanging. In order to reduce disturbance to the rock mass and ensure the stability of the excavated mountain, anchor spraying and small-duct grouting reinforcement are carried out on the side slope within a certain range during construction. The reinforcement process is anchor construction → laying steel mesh → shotcrete.

[0099] The anchor spraying protection includes mortar anchor rods, hanging steel mesh and C20 sprayed concrete, and the thickness of C20 sprayed concrete is 10 cm; among them, the anchor rods adopt Φ22 ordinary mortar anchor rods, which are 4.5 meters long and arranged in a plum blossom shape with a spacing of 1 meter. They are staggered and φ42mm×4mm grouting small tubes are added on three sides of the side slope within 8 meters above the cave top. The small tubes are 4.5 meters long and arranged in a plum blossom shape with a spacing of 1m×1m.

[0100] Furthermore, in S4, after the excavation of the tunnel entrance and the support are completed, the core soil is reserved, and the surveying personnel determine the centerline arch height, the arch position line, and the horizontal cross line on the slope, and then excavate the soil at the arch position on both sides. After excavation and support to the bottom height of the side wall, four I18 I-steel arches are erected with a spacing of 50cm. Each arch is tightened with a tie rod to prevent tipping. After the I-steel arch is erected, C20 concrete arches are cast to expand the foundation to cover the arch foot; then, Φ140mm, 2m long pipe shed guide steel pipes are installed on the steel support at the designed large pipe shed spacing. The centerline must be carefully and accurately checked with instruments to ensure good guidance and consistent with the direction of the pipe shed position. After the pipe shed guide steel pipes are installed, C20 cast concrete is poured.

[0101] Furthermore, in S5, the template support skeleton also adopts I18 steel arch frames with the same spacing as that of the arch frames of the lagging arch, which is 50 cm. When installing the bottom formwork, positions for installing the shrinkage-foot bolts are reserved at the arch feet, and the formwork is sealed separately after the construction of the shrinkage-foot bolts is completed. For the fabrication and trial assembly of the I18 steel arch frames, after accurately measuring the positions of the two arch feet according to the slope topography, the cutting length of the steel arch frames is accurately determined through the positions of the arch feet, so that the arch frames can be accurately positioned on both sides of the rock mass, and the arch feet are supported by channel steel. The types and positions of the guide pipes of the special-shaped lagging arch are exactly the same as those of the external-hole lagging arch, and they are connected to the guide pipes of the external-hole lagging arch. The length is cut according to different topographies and extends to the rock surface of the inner rock mass. After the installation of the guide pipes is completed, 1 m thick C25 shotcrete is sprayed to quickly form a virtual tunnel wall.

[0102] Furthermore, in S6, after the construction of the special-shaped lagging arch is completed, a pipe shed drilling platform is built and a drill is installed. According to different strata, different drill tool combinations are used to drill inward from the guide pipes. First, the odd-numbered holes are drilled, and after grouting, the even-numbered holes are drilled. The even-numbered holes are not grouted, and continuous serial-numbered hole drilling construction is not allowed to avoid hole collapse and drill sticking. When grouting, it must be grouted from both sides to the crown direction.

[0103] In S7, after the pipe shed grouting is completed, according to the results of the advanced geological prediction and monitoring measurement, the reserved core soil three-bench partial excavation method is adopted for construction. After smoothly entering the tunnel, the excavation method is adjusted according to the surrounding rock conditions.

[0104] Meanwhile, in the present invention, the connections referred to are all fixed connections or movable connections or detachable connections. Among them, the fixed connection is a welded connection or is directly processed into an integral formed structure; the movable connection or detachable connection is a hinged connection, a threaded connection, a bayonet connection, or a plug-in connection.

[0105] In the present invention, the large pipe shed guide pipe 11 and the special-shaped steel arch frame sleeve 21 are both φ140*8 mm guide steel pipes with a hollow interior; the connecting bars are all φ20 ribbed steel bars with a circumferential spacing of 100 cm.

[0106] In the present invention, the construction process flow of the lagging arch is: measurement and layout → fabrication and trial assembly of the steel arch frames → installation of the steel arch frames → determination and welding of the direction of the guide pipes → formwork erection and concrete pouring. Among them,

[0107] Measurement and layout

[0108] After the construction layout of the tunnel entrance position is carried out, the lagging arch is constructed with the reserved core soil in the upper bench. The length of the lagging arch is not less than 2 m, and the pipe shed guide pipes are embedded.

[0109] Fabrication and trial assembly of the steel arch frames

[0110] The steel arch frames are fabricated centrally and subjected to trial assembly after processing. After the assembly is completed, check whether the I-beams are distorted or deformed and whether they are on the same plane.

[0111] Installation of the steel arch frames

[0112] The installation of the steel arch frames is carried out after the side slope and the front slope are excavated to the bottom position of the designed advanced pipe shed and the protection of the front slope surface is well constructed. After ensuring the accuracy in the plane and elevation, the steel arch frames can be installed. When installing, the bottom feet of the steel arch frames should rest on solid ground. If the base is loose, measures should be taken to increase the bearing capacity of the base. First, install the arch frame closest to the rock surface, and install the arch frames one by one from the inside to the outside. Use longitudinal connecting steel bars to connect the arch frames into a whole and fix the arch frames.

[0113] Installation of the guide pipes

[0114] According to the angles and positions in the design drawings, fix the guide pipes on the steel arch frames. The external insertion angle is 1° to 3°. After the guide pipes are welded, the formwork erection of the sleeve arch and the concrete pouring construction can be carried out.

[0115] Formwork erection and concrete pouring

[0116] The sleeve arch is erected with formwork outside the excavation contour line of the tunnel entrance. The internal formwork is supported by the steel arch frames and assembled with steel formwork. The external formwork can be assembled with wooden formwork, fixed and reinforced with stirrups, and movable formwork should be left for easy vibration. The gaps between the formworks should be tightly sealed to ensure no leakage of slurry. After the formworks are installed, concrete is poured. When pouring, the concrete strength required by the design should be met, and the pouring should be symmetric on both the left and right sides and vibrated in time. After pouring, regular maintenance should be carried out.

[0117] In the present invention, the construction process flow of the advanced pipe shed is: construction preparation → drilling rig positioning → drilling → steel pipe installation → pipe shed grouting → inspection and acceptance.

[0118] Among them, in the present invention, the concrete used is all high-strength lightweight polystyrene particle concrete; a cement-based thermal insulation and sound-absorbing layer, a metal plate layer, a rock wool board layer, a shock-absorbing layer, a waterproof layer and a stress dispersion layer are provided in the high-strength lightweight polystyrene particle concrete, and the cement-based thermal insulation and sound-absorbing layer, the metal plate layer, the rock wool board layer, the waterproof layer and the stress dispersion layer are stacked or mixed together in sequence.

[0119] Construction preparation

[0120] Before the construction of the advanced pipe shed support, select 2 holes at the arch feet as test holes for grouting tests to determine or adjust the grouting radius, grouting pressure and single-pipe grouting volume, and select appropriate conduits for grouting. Process the conduits according to the requirements in the processing factory. The main mechanical equipment includes pipe shed drilling rigs, high-pressure grouting machines, high-pressure air equipment and cement mixers, etc. The length of the steel pipes depends on the thickness of the soft and fractured surrounding rock, generally 20m to 30m.

[0121] Drilling rig in place

[0122] For drilling construction, it is advisable to use a crawler-mounted down-the-hole drill or a spiral drill, and drill using the method of casing following. The hole depth shall meet the design requirements, and the hole diameter shall be 20 - 30 mm larger than the diameter of the pipe shed steel pipe. The drilling sequence shall be from the high hole position to the low hole position for hole formation.

[0123] Drilling

[0124] After the drilling rig is in place, drilling can be carried out according to the preset points. The boom of the drilling rig must be tightly pressed against the face or close to the casing arch to prevent excessive vibration. During the drilling process, check the angle of the drilling rig at any time and correct the deviation in a timely manner. When the drilling rig starts drilling, the drilling speed should be low. After drilling 20 cm deep, switch to the normal drilling speed. During the drilling process, pay attention to the collection and analysis of stone powder, take samples of stone powder according to the drilling depth, and make records. After the drilling is completed, use high-pressure air to clean the remaining debris in the hole to prevent pipe jamming during pipe jacking.

[0125] Installation of steel pipe

[0126] When the drilling reaches the design depth and passes the inspection, push the steel pipe into the hole. The steel pipe shall be selected as hot-rolled seamless steel pipe with a diameter of φ108 mm, a wall thickness of 6 mm, and joint lengths of 4 m and 6 m. The pipe mouth section of 2.5 m is not perforated. The longitudinal spacing of the steel pipes is 15 - 20 cm, and the circumferential spacing is 9 - 12 cm. The grouting holes of φ10 mm are arranged in a plum blossom shape. The steel pipes are connected by screw threads, and the screw thread length shall not be less than 150 mm. The steel pipes are pre-arranged, with long and short pipes matched to ensure that the number of joints in the same cross-section does not exceed 50%. The steel pipes can be manually pushed into the hole or assisted by machinery. The pushing-in shall be slow and uniform. According to the design requirements, place a steel reinforcement cage inside the pipe shed steel pipe to increase the stiffness of the pipe shed steel pipe. After the steel pipe with holes is installed, plug the gap between the pipe mouth and the hole wall with a grout stopper and embed an exhaust hole.

[0127] Pipe shed grouting

[0128] After the steel pipe is jacked in place, use high-pressure air to clean the pipe to ensure the smoothness of the grouting pipeline. Seal the pipe orifice of the steel pipe with a steel plate and weld the grouting conduit. For pipe shed grouting, single-component or two-component grout can be selected. During grouting construction, start from both ends and push towards the tunnel crown, or use interval grouting, that is, grout the odd-numbered holes first and then the even-numbered holes. The grout concentration at the beginning of grouting should be lower and gradually thicken to the designed concentration. The grout is prepared according to the mix ratio, and the mixing is uniform, with good fluidity and viscosity. The initial pressure is 0.5 - 1.0 MPa. Use a grouting pump to press the grout into the holes, and then keep the pressure for more than 10 minutes to ensure that the grout can quickly penetrate into the surrounding rock fissures. When slurry discharges from the exhaust port, conduct final pressure grouting, and the final pressure is 2.0 MPa. Stop grouting when the grout volume reaches the design requirements. After grouting, sweep and drain the gelled grout in the pipe, and tightly fill it with cement mortar to enhance the stiffness and strength of the pipe shed. After grouting, the inspection personnel should detect the grouting effect. After passing the inspection, when the strength of the grout reaches 70%, the next process can be carried out.

[0129] The construction process flow of the special-shaped casing arch is as follows: construction preparation → installation of formwork support framework → installation of bottom formwork → installation of steel mesh → fabrication and trial assembly of I18 steel arch frames → installation of arch frames → welding and fixing of longitudinal connecting bars → driving and connection of locking foot bolts → grouting of shrinking foot bolts → determination and welding of the direction of guide pipes → formwork erection and concrete pouring.

[0130] Quality control:

[0131] 7.1 Quality assurance system

[0132] (1) Organizational guarantee

[0133] The project department forms a quality management leading group with the project manager as the team leader, the project deputy manager, the chief engineer, and the heads of each department as members, responsible for project quality decision-making. A quality inspection department is set up below, responsible for the daily affairs of quality management.

[0134] (2) System guarantee

[0135] Establish and improve quality management rules and regulations such as quality responsibility system, inspection system, and reward and punishment system, and strictly implement them. In particular, seriously carry out the mid-month and end-of-month project quality inspections and the irregular inspections of the quality inspection department. Establish a project quality plan, quality system documents, and operation documents, and strictly manage the project quality process according to the quality system procedures; establish a quality tracking system, with the responsibility implemented to specific operators, and traceability.

[0136] (3) Ideological guarantee

[0137] Strengthen quality awareness education to let employees establish the quality awareness of putting quality first and the quality awareness of lifelong responsibility.

[0138] 2. Process control

[0139] (1) Set out the lines precisely in strict accordance with the design drawings to ensure accurate alignment, and regularly recheck the guiding lines and calibrate the surveying instruments.

[0140] (2) Attach importance to the construction technical disclosure work and the quality disclosure before work, clarify the respective responsibilities, ensure that every construction operator understands the design intention, and construct strictly according to the requirements of the design drawings. In view of the large mobility of the migrant workers in this project, timely conduct technical disclosure for the newly recruited migrant workers.

[0141] (3) Before the construction of each process, it is necessary to obtain the construction instructions from the supervising engineer before starting work.

[0142] (4) During the construction process, strictly implement the construction technical specifications and the special specifications of the project in the contract documents to ensure that the construction of each part meets the technical specifications and design requirements.

[0143] (5) Strictly control the quality inspection of the materials entering the site. Implement the method of special person inspection and random sampling inspection according to the regulations. For the materials that require certificates according to the specifications, the certificates must be obtained, and unqualified materials shall not be used. Ensure that all raw materials used in the structure meet the design and specification requirements. At the same time, properly store the materials entering the site to prevent pollution and rust.

[0144] (6) Conduct mix proportion tests and selections for concrete construction, and construct strictly according to the selected mix proportion.

[0145] (7) Inspect and accept the molds, machinery and other equipment entering the site to ensure that their technical indicators meet the standards, operate normally, and meet the construction requirements.

[0146] (8) The excavation of the arch foundation must be in place, and it must be treated strictly according to the design requirements. The next process can be carried out only after the bearing capacity of the foundation reaches the design requirements.

[0147] (9) For the pipe shed drilling and grouting, it must be constructed by skipping holes, and construct from both sides to the middle.

[0148] (10) When excavating, try to use machinery in cooperation with manual excavation to reduce the disturbance to the surrounding rock and the damage to the surrounding vegetation.

[0149] (11) During the processing of the arch frame, strictly process the arch frame radian according to the design to ensure the smoothness of the arch frame. The welding of the connecting section must be full and meet the design and specification requirements.

[0150] (12) During the construction process of shotcrete, strictly control the thickness and density of the shotcrete, and it is strictly prohibited to use fillers for filling.

[0151] (13) Summarize and improve. According to the previous construction situation, timely hold technical and construction summaries to guide the construction of the next stage.

[0152] Safety measures:

[0153] 1. Safety assurance system

[0154] (1) Organizational assurance. Establish a safety leading group with the project manager as the team leader, responsible for making decisions on project safety work; set up a security department in the project management department, responsible for the daily affairs of project safety work; the security department has full-time safety officers who follow the operations to be responsible for on-site safety management work.

[0155] (2) System assurance. Establish and improve safety responsibility systems, safety inspection systems, safety reward and punishment systems and other rules and regulations and strictly enforce them. In particular, earnestly carry out the safety inspection systems in the middle and at the end of each month and the irregular inspections by the security department, earnestly carry out the pre-shift safety briefing system, compile various safety operation procedures for the project and strictly organize the construction according to the operation procedures.

[0156] (3) Ideological assurance. Strengthen the safety awareness education and safety skill training of employees, organize employees to conduct safety technical disclosure, so that employees have safety skills suitable for their corresponding occupations and avoid hurting others and being hurt by others during the construction process.

[0157] (4) Technical assurance. Optimize the construction organization design and strictly construct according to the construction organization design to ensure the safety of project construction.

[0158] (5) Economic assurance. Strengthen communication with all parties such as design, supervision, and the owner, increase the capital investment in safety facilities, and promptly honor the rewards and punishments according to the reward and punishment system.

[0159] 2. On-site safety management

[0160] (1) The construction electricity strictly follows the three-phase five-wire system, and all electrical equipment implements one machine, one switch, and one protection to prevent electric shock accidents.

[0161] (2) For high-altitude operations, correctly wear safety protection supplies.

[0162] (3) Insist that special operation personnel hold certificates to work, and there should be special personnel to command and operate the equipment, and the command signals should be clear and accurate;

[0163] (4) Construction machinery and equipment, etc., are provided with edge protection and warning signs as required to ensure construction safety.

[0164] (5) During the construction process, strengthen the monitoring and measurement of surrounding rocks and the advanced geological prediction, and timely adjust the construction plan and support parameters according to the monitoring and measurement of surrounding rocks and various geological data to ensure construction safety.

[0165] Environmental protection measures:

[0166] (1) Implement the requirements of the Environmental Management Manual and procedural documents, establish an environmental management system, and strictly abide by the laws, regulations and rules on environmental protection issued by the national and local governments;

[0167] (2) Strengthen employee education on civilized construction and environmental protection awareness through the Environmental Protection Law, relevant regulations and typical cases.

[0168] (3) The construction and work sites should be reasonably arranged, with complete and clear signage, various signs that are eye-catching, and the construction sites clean and tidy;

[0169] (4) Construction wastewater and domestic sewage must not pollute the surrounding environment. Wastewater must be precipitated or filtered to meet environmental protection requirements. Solid waste must be collected, stored, and disposed of in a centralized manner.

[0170] (5) Based on the principle of occupying less arable land and reducing pollution, reasonably select and arrange living camps and construction access roads;

[0171] (6) Take measures such as planting trees to restore vegetation on slopes damaged by clearing, building of access roads, etc., so as to minimize ecological damage.

[0172] Benefit Analysis

[0173] 1. Economic benefits

[0174] This method still uses conventional equipment and materials, and through improving the construction technology and optimizing the construction procedures, the production process is simplified, standardized, and humanized, with less equipment and material consumption.

[0175] This method leverages the support provided by the special-shaped sleeve arches to enable tunnel construction to begin earlier, reducing the excavation and protection of the high-side slope at the tunnel entrance, minimizing safety hazards and environmental damage at the entrance, and saving time and manpower. The special-shaped sleeve arches are constructed using shotcrete, which saves a certain amount of support formwork and reduces the amount of formwork required. Furthermore, shotcrete sets quickly, allowing the structure to enter a stress-bearing state earlier, significantly reducing construction time.

[0176] Due to the fast construction speed, close connection of work processes and early completion of construction period, a certain amount of machinery and equipment costs and labor costs were saved.

[0177] Compared with the traditional method of brushing the side and slope, this method can save one month of construction time. The corresponding labor and equipment cost savings are as follows:

[0178] Personnel costs: 38 on-site construction personnel and 20 project management personnel, so the labor cost savings are (38 + 20) people * 6,000 yuan / month = 348,000 yuan;

[0179] Cost of large machinery and equipment:

[0180] Excavator cost: One Hitachi 220 excavator is invested, calculated at 26,000 yuan per month, that is, the loader saves 26,000 yuan;

[0181] Loader cost: Two 50-type loaders are invested to assist in construction such as support erection. The 50-type loader is calculated at 17,000 yuan per month, that is, the loader saves 17,000 * 2 = 34,000 yuan;

[0182] Generator cost: One 350kw generator is invested, calculated at 13,000 yuan per month, that is, the generator saves 13,000 yuan;

[0183] Dump truck cost: Three 20m 3 dump trucks are invested, calculated at 22,000 yuan per month, that is, the dump truck saves 22,000 * 3 = 66,000 yuan;

[0184] Mixing plant and tanker cost: One 120 mixing plant and three tankers are invested, calculated at 100,000 yuan per month, that is, the mixing plant and tankers save 100,000 yuan;

[0185] In summary, through this method, it can save

[0186] 348,000 + 26,000 + 34,000 + 13,000 + 66,000 + 100,000 = 587,000 yuan

[0187] Table 1 is the material and equipment table of the present invention;

[0188] Serial Number Item Number of People Remarks 1 Foreman 1 2 Driller 12 Including Blaster 3 Install Bolts and Steel Frames 6 4 Operate Grouting Pump 4 5 Shotcrete Worker 4 Use Shotcreting Machine 6 Muck Loader Driver 2 7 Truck Driver 4 8 Carpenter 2 9 Steelworker 4 Total 38

[0189] Table 2 is the material table of the present invention;

[0190]

[0191]

[0192] Table 3 is the mechanical equipment table of the present invention;

[0193]

[0194] 2. Social benefits

[0195] (1) This method basically adopts the "zero excavation" method for constructing the side and slope, reducing the damage to the original entrance topography, protecting the ecological environment, and this construction method is safe and reliable, avoiding high slopes and high embankments, ensuring the operation safety during construction and the safety during operation, reflecting good social benefits.

[0196] (2) The special-shaped segment socket arch construction of this method uses shotcrete, enabling the structure to enter the stress state in advance, ensuring the safety of construction operations, and reducing the erection of supports, saving resources.

[0197] (3) The use of this construction method can advance the process time, shorten the total construction period, and produce better social and economic benefits.

[0198] Project Examples

[0199] The Huimin long tunnel of the 9th Civil Engineering Division of Yuanlu Expressway has a single tunnel length of 8475m. The left tunnel entrance of the Huimin tunnel exit is a "V"-shaped tunnel entry construction. This method was used during construction and the tunnel was successfully excavated on December 1, 2017.

[0200] The first civil engineering section of Yuanlu Expressway was responsible for the construction of Mamuzhai No. 1 Tunnel. The single tunnel is 2727m long. The right entrance of Mamuzhai No. 1 Tunnel is constructed in a "V"-shaped terrain. This method was used during the construction, and the tunnel was successfully excavated on April 21, 2018.

[0201] The single tunnel of Qixingzhai Tunnel of Yuanlu Expressway, which was constructed by the third civil engineering division of Yuanlu Expressway, is 2934.7 meters long. The right tunnel entrance of Qixingzhai Tunnel is constructed in a "V"-shaped terrain. This method was used during the construction, and the tunnel was successfully excavated on May 16, 2019.

[0202] In summary, the Yuanlu Expressway Project Management Department has developed a method for rapid tunnel entry construction under "V"-shaped special terrain through engineering practice, technical research and summary. After engineering entity testing and application effect evaluation, this method has the characteristics of easy operation, low cost and safe construction. It can also effectively reduce the environmental damage caused by tunnel entrance construction and effectively shorten the construction period, with significant economic and social benefits.

[0203] After comprehensive consideration and in combination with the actual situation of the project, it was decided to adopt the zero excavation method to enter the tunnel. After appropriately trimming the side slope and carrying out reinforcement support, the tunnel blind hole construction was carried out in advance through the support effect of the special-shaped arch.

[0204] Finally, it should be noted that the above has clearly and completely described the concept, specific structure and technical effects of the present invention in combination with the embodiments and drawings, so as to fully understand the purpose, characteristics and effects of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, other embodiments obtained by those skilled in the art without creative work are within the scope of protection of the present invention. In addition, all the connections and connection relationships mentioned in the text do not refer to the direct connection of components, but refer to the fact that a better connection structure can be formed by adding or reducing connection accessories according to the specific implementation situation. The various technical features in the invention can be combined interactively without conflicting with each other.

Claims

1. A construction method for a tunnel entrance protection structure for V-shaped terrain, characterized in that: The method comprises the following steps: S1, construction preparation, first determine the tunnel entry construction plan, then carry out surveying and setting out, and finally determine the tunnel entrance construction; S2, intercepting ditch on the top of the cave. Before the side slope construction, a intercepting ditch should be set up 5 meters away from the side slope excavation line. S3, side slope excavation and support, side slope excavation should be carried out by mechanical excavation, the slope should be strictly controlled during excavation, the stable structural layer of the slope surface should be preserved as much as possible during side slope excavation, and a design scheme with little or no brushing should be adopted; S4, arch construction. The construction process of arch and pipe shed is as follows: measurement and layout → production and trial assembly of steel arch frame → installation of steel arch frame → installation of longitudinal connecting reinforcement → determination of guide pipe direction and welding → formwork erection and concrete pouring; S5, special-shaped arch construction: I18 arch frames are set below the top triangular hollow area as the formwork support frame, with a spacing of 50cm. Then, bamboo plywood is used as the bottom formwork, and Φ8mm*20cm steel mesh is laid. Then, I18 I-beam arch frames are installed with a spacing of 50cm. The arch frames are fixed with longitudinal connecting bars, with a circumferential spacing of 1m. Guide pipes are installed. The guide pipe model is exactly the same as the outer arch model of the tunnel. The length is cut according to different terrain and extended to the inner rock mass and rock surface. After the guide pipe is installed, 1m thick C25 shotcrete is sprayed to quickly form a virtual tunnel wall. S6, advanced large pipe shed construction. The construction process of the advanced pipe shed is as follows: construction preparation → drilling rig positioning → drilling → installation of large pipe shed steel pipes → grouting of pipe shed → inspection and acceptance; S7, blind tunnel construction, after the pipe roof grouting is completed, according to the advanced geological forecast and monitoring measurement results, the construction adopts the three-step partial excavation method of reserved core soil. After successfully entering the tunnel, the excavation method is adjusted according to the surrounding rock conditions.

2. The construction method of a tunnel entrance protection structure for a V-shaped terrain according to claim 1, characterized in that: The construction method also includes a tunnel entrance protection structure, wherein the tunnel entrance protection structure includes a plurality of arched steel arch frames (1) arranged in parallel from the inside to the outside on the outside of the tunnel entrance, and a plurality of special-shaped steel arch frames (2) arranged in parallel from the outside to the inside on the inside of the tunnel entrance, wherein the plurality of steel arch frames (1) are connected as a whole by a plurality of longitudinally arranged connecting bars and poured with concrete to form an arched sleeve, and a plurality of arched steel arch frame large pipe shed guide pipes (11) are sequentially installed in parallel in the arched sleeve according to design requirements. The arched steel arch frame large pipe shed guide pipe (11) and the connecting reinforcement are arranged in parallel with each other; a plurality of special-shaped steel arch frames (2) are connected as a whole through a plurality of transversely arranged connecting rods and filled with concrete to form a special-shaped sleeve arch, one end of the special-shaped sleeve arch is connected to the outer sleeve arch, and the other end is connected to the mountain rock layer (3); a plurality of special-shaped steel arch frame sleeves (21) are sequentially installed in parallel in the special-shaped sleeve arch, and the special-shaped steel arch frame sleeves (21) are connected to the large pipe shed guide pipe (11) to form a whole, forming a tunnel entrance protection structure.

3. The construction method of a tunnel entrance protection structure for V-shaped terrain according to claim 2, characterized in that: The arched steel arch frame (1) is a semicircular or arched I-beam arch frame of the same size and model; the special-shaped steel arch frame (2) is a semi-arc I-beam arch frame of the same size and model but different lengths, and the length of the special-shaped steel arch frame (2) decreases from the outside of the hole to the inside of the hole.

4. The construction method of a tunnel entrance protection structure for V-shaped terrain according to claim 3, characterized in that: The free end of the special-shaped steel arch frame (2) is vertically connected to a plurality of back plates (22) of the same model but different sizes, and the back plates (22) are connected to locking foot anchor rods (23), which are grouting locking foot anchor rods with a hollow interior, and the grouting locking foot anchor rods are connected to the mountain rock layer (3).

5. The construction method of a tunnel entrance protection structure for a V-shaped terrain according to claim 2, characterized in that: The large pipe rack guide pipe (11) and the special-shaped steel arch frame sleeve (21) are both hollow tubular components; the connecting ribs and the connecting rods are both hollow or solid tubular or rod-shaped components; An injection molding layer, an anti-rust layer, and a waterproof layer are sprayed in sequence from the inside to the outside on the outer surfaces of the arched steel arch frame (1), the special-shaped steel arch frame (2), the connecting ribs, and the connecting pull rods.

6. The construction method of a tunnel entrance protection structure for a V-shaped terrain according to claim 1, characterized in that: In S1, before construction, a surveying team is organized to re-survey the section of the tunnel entrance and release the elevation position of the structure, the tunnel entrance mileage and the blind tunnel mileage, and compare them with the design to see if they are consistent; if there are any deviations, they are reported to the design for adjustment in a timely manner, and construction can only be carried out after confirmation; In the above-mentioned S2, the top intercepting ditch (4) is cast with C20 plain concrete, a retaining wall is set at the top gully position, and the ditch size is increased to prevent the top water from directly scouring the back slope. The top intercepting ditch (4) is adjusted and arranged according to the actual terrain on site and must be completed before the side back slope is constructed. The water in the top intercepting ditch (4) is discharged to the natural ditch on both sides of the tunnel.

7. The construction method of a tunnel entrance protection structure for V-shaped terrain according to claim 1, characterized in that: In S3, the side slope includes the entrance side slope and the back slope. The side slope is excavated using the open-cut method, excavated step by step from top to bottom, and a step into the tunnel is reserved. After the side slope is excavated and formed, it is promptly protected by anchor spraying and mesh hanging. In order to reduce disturbance to the rock mass and ensure the stability of the excavated mountain, anchor spraying protection and small pipe grouting reinforcement are carried out on the side slope within a certain range during construction. The reinforcement process is anchor rod construction → laying steel mesh → shotcrete. The anchor spraying protection includes mortar anchor rods, hanging steel mesh and C20 sprayed concrete, and the thickness of C20 sprayed concrete is 10 cm; among them, the anchor rods adopt Φ22 ordinary mortar anchor rods, which are 4.5 meters long and arranged in a plum blossom shape with a spacing of 1 meter. They are staggered and φ42mm×4mm grouting small tubes are added on three sides of the side slope within 8 meters above the cave top. The small tubes are 4.5 meters long and arranged in a plum blossom shape with a spacing of 1m×1m.

8. The construction method of a tunnel entrance protection structure for V-shaped terrain according to claim 1, characterized in that: In the above S4, after the excavation of the opening side and the support are completed, the core soil is reserved, and the surveying personnel determine the centerline arch height, the arch position line, and the horizontal cross line on the slope, and then excavate the soil at the arch position on both sides. After excavation and support to the height of the bottom of the side wall, four I18 I-steel arches are erected with a spacing of 50cm. Each arch is tightened with a tie rod to prevent tipping. After the I-steel arch is erected, a C20 concrete arch is cast to expand the foundation to cover the arch foot; then, a Φ140mm, 2m long pipe shed guide steel pipe is installed on the steel support at the designed large pipe shed spacing. The centerline must be carefully and accurately checked with an instrument to ensure good guidance and consistent with the position direction of the pipe shed. After the pipe shed guide steel pipe is installed, C20 cast concrete is poured.

9. The construction method of a tunnel entrance protection structure for V-shaped terrain according to claim 1, characterized in that: In S5, the template support skeleton also uses I18 steel arch frames with the same spacing as that of the arch frames of the sleeve arch, which is 50 cm. When installing the bottom formwork, a position for driving the shrinkage-foot anchor bolts is reserved at the arch feet, and the formwork is sealed separately after the construction of the locking-foot anchor bolts is completed. For the fabrication and trial assembly of the I18 steel arch frames, after accurately measuring the positions of the two arch feet according to the slope topography, the cutting length of the steel arch frames is accurately determined through the arch-foot positions, so that the arch frames can be accurately positioned on both sides of the rock mass, and the arch feet are supported with channel steel. The guide pipe types and positions of the special-shaped sleeve arch are exactly the same as those of the external sleeve arch of the tunnel, and are connected to the guide pipe of the external sleeve arch of the tunnel. The length is cut according to different topographies and extends to the rock surface of the inner rock mass. After the installation of the guide pipes is completed, 1 m thick C25 shotcrete is sprayed to quickly form a virtual tunnel wall.

10. The construction method of a tunnel entrance protection structure for V-shaped terrain according to claim 1, characterized in that: In S6, after the construction of the special-shaped sleeve arch is completed, a pipe-shed drilling platform is built and the drilling rig is installed. According to different strata, different drill tool combinations are used to drill inward from the guide pipes. First, the odd-numbered holes are drilled, and then the even-numbered holes are drilled after grouting. The even-numbered holes are not grouted, and continuous sequential drilling construction is not allowed to avoid hole collapse and drill sticking. When grouting, grouting must be carried out from both sides towards the arch crown direction. In S7, after the pipe-shed grouting is completed, according to the results of the advanced geological forecast and monitoring measurement, the construction is carried out by using the reserved core soil three-step partial excavation method, and the excavation method is adjusted according to the surrounding rock conditions after successfully entering the tunnel.

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