A tunnel restraining anchor cable and a method for improving the stability of the initial support structure

Abstract

The present invention discloses a tunnel restraining anchor cable and a method for improving the stability of an initial support structure. The tunnel restraining anchor cable includes a steel strand anchored in the surrounding rock, a first anchor plate and a first anchor installed on the surface of the surrounding rock, a longitudinal connector installed between two adjacent steel frames, a second anchor plate and a second anchor installed on the longitudinal connector; the longitudinal connector is arranged between two adjacent steel frames of the initial support, and a third reserved hole is provided in the middle of the longitudinal connector to facilitate the passage of the steel strand; one end of the steel strand is anchored deep in the surrounding rock, and the other end passes through the first and second reserved holes in sequence, and the steel strand applying pre-tightening force is locked by the first and second anchors respectively. The tunnel restraining anchor cable of the present invention simultaneously realizes active reinforcement of the surrounding rock and restraint fixation of the steel frame, has the characteristics of simplicity, speed, and reliability, and avoids interference with subsequent construction processes.

Description

Technical Field

[0001] The present invention relates to the field of tunnel engineering, and in particular to a tunnel restraining anchor cable and a method for improving the stability of an initial support structure. Background Art

[0002] The primary support structure for soft rock tunnels primarily consists of a combination of steel frames and shotcrete, typically with a thickness of 26-28 cm. Compared to the relatively thicker and more rigid secondary tunnel lining, the primary support structure is relatively thin and flexible, possessing a thin-walled arch structure. Its bearing capacity is closely related not only to the material strength and structural stiffness, but also to its structural stability. Especially during the construction of long-span soft rock tunnels, the stability of the primary support structure is significantly reduced, making it prone to instability and failure, leading to premature loss of bearing capacity, thereby exacerbating the occurrence of large deformation disasters in the tunnel. To prevent instability and failure of the primary support structure in soft rock tunnels, existing technical measures include restraint rods, stabilizer rods, or restraint anchor rods. These are essentially different terms for the same technical measure, collectively referred to as restraint anchor rods. These so-called "restraint anchor rods" are essentially steel pipes with one end grouted and anchored in a relatively stable rock layer deep within the surrounding rock, while the other end is connected to the tunnel steel frame. This restraint prevents deformation of the primary support steel frame, thereby improving the stability of the primary support structure.

[0003] For the restraining anchor rods to truly play an effective role in restraining the tunnel steel frame, three conditions must be met simultaneously: first, the restraining anchor rods must be long enough to penetrate deep into the relatively stable rock layer of the surrounding rock; second, the restraining anchor rods must be effectively anchored to the surrounding rock, such as by adopting appropriate anchoring methods and increasing the restraining anchor rod diameter to ensure that the restraining anchor rods have sufficient anchoring force; and third, the restraining anchor rods must be reliably connected to the tunnel steel frame. However, judging from the results of field practice, the existing technical measures still have the following obvious defects and shortcomings:

[0004] (1) Soft rock tunnels are often excavated in sections using the three-step method. The upper step working space is often very limited, and there is a large height difference with the middle step. When drilling holes for constrained anchor rods, the drilling depth and diameter of the YT-28 handheld air drill commonly used in tunnels are limited. To ensure the drilling depth and diameter, if a down-the-hole drill is used, a small down-the-hole drill must be selected. However, field tests have shown that it is still extremely inconvenient to position and drill small down-the-hole drills in tunnels, and the drilling efficiency is very low, which is not conducive to on-site construction. The above reasons lead to the existing constrained anchor rods located on the upper step of the tunnel being limited in length and diameter, making it difficult for the constrained anchor rods to pass through the loose area of ​​the surrounding rock to play an effective anchoring role.

[0005] (2) Since the restraint anchor rods on the upper steps of soft rock tunnels are drilled obliquely upward, the problems of grouting and leakage have been difficult to solve effectively during grouting and anchoring, making it difficult to ensure the grouting and anchoring effect of the restraint anchor rods deep in the surrounding rock. The only option is to drill obliquely downward. Although this is beneficial to grouting and to a certain extent brings into play the bending and shearing properties of the rods, it is not conducive to bringing into play the axial bearing advantages of the restraint anchor rods and their restraining effect on the initial support of the tunnel.

[0006] (3) The restraint anchor rod is connected to the steel frame by welding the contact part with the longitudinal connector. The welding operation takes a long time and is easily affected by the human factors of the operators, making it difficult to ensure the reliability of the connection between the restraint anchor rod and the steel frame.

[0007] In summary, the current constrained anchor rods used to improve the stability of the initial support structure of the upper step of soft rock tunnels still have a series of difficult-to-solve technical problems in the drilling, anchoring, connection and other processes, making it difficult to achieve the expected technical effect of the constrained anchor rods. Therefore, it is necessary to propose a stability control measure and construction method for the initial support structure that is simple to construct, high in construction efficiency, and can guarantee the technical effect. Summary of the Invention

[0008] In response to the above technical problems, the present invention provides a tunnel restraining anchor cable and a method for improving the stability of the initial support structure, which effectively improves the stability of the initial support structure of the soft rock tunnel.

[0009] The technical solution adopted in the present invention is as follows:

[0010] A tunnel restraining anchor cable comprises a steel strand anchored in surrounding rock, a first anchor plate and a first anchor installed on the surface of the surrounding rock, a longitudinal connector installed between two adjacent steel frames, a second anchor plate and a second anchor installed on the longitudinal connector;

[0011] The longitudinal connector is arranged between two adjacent steel frames of the initial support, and a third reserved hole is provided in the middle of the longitudinal connector to facilitate the passage of the steel strand;

[0012] A first reserved hole and a second reserved hole matching the diameter of the steel strand are respectively provided in the middle of the first anchor plate and the second anchor plate; one end of the steel strand is anchored deep in the surrounding rock, and the other end passes through the first reserved hole and the second reserved hole in sequence, and the steel strand that is pre-tightened is locked by the first anchor and the second anchor respectively.

[0013] In the above-mentioned tunnel restraint anchor cable, both ends of the longitudinal connector are tightly attached to the inner side of the steel frame flange close to the surrounding rock and are welded and fixed.

[0014] In the above-mentioned tunnel restraint anchor cable, the length of the steel strand is 5-12m, the diameter is 17-25mm, and 4-8 strands are arranged between every two steel frames.

[0015] In the above-mentioned tunnel restraining anchor cable, the longitudinal connector is made of I14 or I16 I-steel, and its length is 6-8 cm smaller than the distance between adjacent steel frames.

[0016] In the above-mentioned tunnel restraint anchor cable, the length, width and thickness of the first anchor plate are 28-32 cm, 28-32 cm and 14-16 mm respectively, and the first reserved hole is a circular hole with a hole diameter larger than the diameter of the steel strand by 4-6 mm.

[0017] In the above-mentioned tunnel restraint anchor cable, the length, width and thickness of the second anchor plate are 14-16 cm, 10-12 cm and 14-16 mm respectively, and the second reserved hole is a circular hole with a hole diameter 4-6 mm larger than the diameter of the steel strand.

[0018] In the above-mentioned tunnel restraining anchor cable, the third reserved hole is a long hole arranged along the length direction of the longitudinal connecting member.

[0019] In the above-mentioned tunnel restraint anchor cable, the first anchor and the second anchor are both matched with the diameter of the steel strand.

[0020] The method for improving the stability of the initial support structure by using a tunnel restraining anchor cable comprises the following steps:

[0021] 【1】Drilling:

[0022] 【1.1】Preliminary spraying of concrete on the surrounding rock surface after tunnel excavation;

[0023] 【1.2】Drill holes in the middle of two adjacent steel frames at a distance of 0.8-1.2m from the arch foot according to the set number of holes, with a diameter of 28-32mm and a depth of 5-12m;

[0024] 【2】Resin anchor installation:

[0025] After drilling and cleaning the hole, use a plastic tube to send the resin anchor drug roll into the designated position of the drill hole, and then tamp the resin anchor in the drill hole. The cumulative length of the resin anchor drug roll is 1.2-1.8m;

[0026] 【3】Steel strand entry and anchoring:

[0027] After manually feeding the steel strand into the borehole to the location of the resin anchor, a stirring driver is installed at the lower end of the steel strand. The drilling rig drives the stirring driver and the steel strand to rotate, stirring the resin anchor roll. At the same time, the drilling rig is kept advancing. The stirring time is controlled within 20-30 seconds, and the length of the exposed initial support surface of the steel strand is controlled to 25-30 cm.

[0028] 【4】Preload the steel strands on the surrounding rock surface:

[0029] 【4.1】10-15 minutes after the resin anchoring agent is mixed, install the first anchor plate and the first anchor on the surrounding rock surface in sequence;

[0030] [4.2] Use tensioning equipment to apply pre-tightening force to the steel strands, and then use the first anchor to lock the steel strands for the first time, realizing the active reinforcement of the surrounding rock by the restrained anchor cable support system;

[0031] 【5】Connection between steel strand and steel frame:

[0032] 【5.1】Erect the steel frame according to the set spacing;

[0033] [5.2] Install the longitudinal connector, pass the steel strand through the third reserved hole, and then place both ends of the longitudinal connector tightly against the flange of the steel frame close to the surrounding rock, and weld the contact parts between the two.

[0034] [5.3] Install the second anchor plate and the second anchor in sequence, and use the tensioning machine to apply preload force to the steel strands again. The second anchor locks the steel strands a second time to secure the tunnel restraining anchor cables to the steel frame. The preload force is 6-10kN.

[0035] 【5.4】Use an angle grinder to cut off the excess exposed part of the steel strand;

[0036] 【6】Re-spray concrete to the designed thickness.

[0037] In the above method for improving the stability of the initial support structure: Step [1] drilling is carried out using a single-body pneumatic anchor drilling rig equipped with a hollow hexagonal extended drill rod and a matching drill bit.

[0038] The beneficial technical effects of the present invention are as follows:

[0039] (1) The present invention adopts small-diameter steel strands to replace large-diameter steel pipes used in constrained anchor rods, and proposes a technical measure of tunnel constrained anchor rope to improve the stability of the initial support structure of the tunnel. The small-diameter steel strands have certain flexibility, and their installation length is not limited by the tunnel construction work space. The drilling required for the installation of the steel strands is carried out by a single-type pneumatic anchor drill rig, which is equipped with a long drill rod. The drilling depth can reach more than 12m, ensuring that the constrained anchor rope can penetrate into the relatively stable rock layer deep in the surrounding rock. In addition, the drilling efficiency is high, avoiding the problems of being limited by the construction work space of the tunnel upper steps, the limited drilling depth of the YT-28 handheld air drill commonly used in tunnels, and the inconvenience and low efficiency of the down-the-hole drill.

[0040] (2) The steel strands in the tunnel restraint anchor cable of the present invention are anchored to the surrounding rock through a resin anchoring agent, which can quickly obtain anchoring force and completely avoid the problems of grouting and leakage caused by the oblique upward grouting of the restraint anchor rod, thereby ensuring the anchoring effect. At the same time, the restraint anchor cable can be installed on the vertical surrounding rock surface (obliquely upward) of the tunnel arch, which can give full play to the tensile properties of the steel strands, and the tensile strength of the steel strands can reach more than 1860MPa.

[0041] (3) The tunnel restraint anchor cable of the present invention applies a pre-tightening force to the steel strand on the surface of the surrounding rock with the help of a tensioning machine, and the steel strand is locked for the first time by the anchor installed on the surface of the surrounding rock, thereby realizing active reinforcement of the surrounding rock by the restraint anchor cable support system, while the restraint anchor rod does not have this technical effect; in addition, a pre-tightening force is applied to the steel strand again on the surface of the longitudinal connector with the help of a tensioning machine, and the steel strand is locked for the second time by the anchor installed on the longitudinal connector, thereby realizing restraint and fixation of the steel frame by the restraint anchor cable support system. This connection method of the restraint anchor cable and the steel frame is simpler, faster and more reliable.

[0042] (4) One end of the tunnel restraining anchor cable of the present invention is anchored in a relatively stable rock layer deep in the surrounding rock, and the other end is connected to the initial support steel frame, thereby strengthening the restraining effect on the initial support structure. While improving the ability of the initial support structure to resist its own instability and destruction, it can also inhibit the overall settlement and convergence deformation of the initial support structure without setting up temporary arches or temporary cross braces, achieving "cables instead of braces" and avoiding interference with the subsequent construction process. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 Schematic diagram of the arrangement of the tunnel restraint anchor cable support system in an embodiment of the present invention;

[0044] Figure 2 3D schematic diagram of a tunnel restrained anchor cable support system according to an embodiment of the present invention.

[0045] Figure 3 Schematic diagram of a longitudinal connector in an embodiment of the present invention.

[0046] Figure 4 Schematic diagram of the first anchor plate in an embodiment of the present invention.

[0047] Figure 5 Schematic diagram of the second anchor plate in an embodiment of the present invention.

[0048] Figure numerals: 1-steel strand; 2-first anchor plate; 3-first anchor; 4-steel frame; 5-longitudinal connector; 6-second anchor plate; 7-second anchor; 8-third reserved hole; 9-first reserved hole; 10-second reserved hole. DETAILED DESCRIPTION

[0049] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0050] On the contrary, the present invention covers any alternatives, modifications, equivalents, and solutions that fall within the spirit and scope of the present invention as defined by the claims. Furthermore, to facilitate a better understanding of the present invention, certain specific details are described in detail below in the detailed description of the present invention. Those skilled in the art will be able to fully understand the present invention without these details.

[0051] like Figure 1-2 As shown, the present invention takes the construction of a restrained anchor cable set on a step in a tunnel as an example to introduce the specific technical solution of the present invention:

[0052] A tunnel constraint anchor cable consists of a steel strand 1 anchored in the surrounding rock, a first anchor plate 2, a first anchor 3, a longitudinal connector 5, a second anchor plate 6, and a second anchor 7, wherein the first anchor plate 2 and the first anchor 3 are installed on the surrounding rock surface, the longitudinal connector 5 is installed between two adjacent steel frames 4, and the second anchor plate 6 and the second anchor 7 are installed on the longitudinal connector 5.

[0053] The longitudinal connector 5 is set between two adjacent steel frames 4 of the initial support, and its two ends are tightly attached to the inner side of the steel frame flange close to the surrounding rock and welded. Each longitudinal connector 5 is provided with a third reserved hole 8 for the steel strand to pass through;

[0054] like Figure 3-Figure 5 As shown, the first anchor plate 2 and the second anchor plate 6 are each provided with a first reserved hole 9 and a second reserved hole 10 in the middle thereof, each of which matches the diameter of the steel strand 1. The first reserved hole 9 and the second reserved hole 10 are both round holes. The third reserved hole 8 is an elongated hole arranged along the length of the longitudinal connector 5, with a length of 15-20 cm to facilitate the installation of the longitudinal connector 5.

[0055] One end of the steel strand 1 is anchored deep in the surrounding rock, and the other end (exposed end) first passes through the first reserved hole 9, and the first anchor 3 locks the steel strand 1 after the pre-tightening force is applied for the first time; then it passes through the second reserved hole 10, and the second anchor 7 installed on the longitudinal connector locks the steel strand 1 after the pre-tightening force is applied for the second time.

[0056] The steel strands 1 are 10m long and 21.8mm in diameter, with four strands installed between each pair of steel frames on the upper steps of the tunnel. The longitudinal connectors 5 are I16 I-beams, with a length 6cm less than the designed spacing between the steel frames.

[0057] The length, width, and thickness of the first anchor plate 2 are 30 cm, 30 cm, and 16 mm, respectively. The center diameter of the first anchor plate 2 is 5 mm larger than the diameter of the steel strand 1. The length, width, and thickness of the second anchor plate 6 are 16 cm, 12 cm, and 16 mm, respectively. The center diameter of the second anchor plate 6 is 5 mm larger than the diameter of the steel strand 1. The first anchor 3 and the second anchor 7 both match the diameter of the steel strand.

[0058] The method of the present invention for improving the stability of an initial support structure by using a tunnel restraining anchor cable comprises the following steps:

[0059] 【1】Drilling:

[0060] [1.1] After the tunnel upper steps are excavated, the rock surface is immediately sprayed with initial concrete to reduce the impact of falling vault blocks on the drilling construction of the restraining anchor cables.

[0061] [1.2] Between the two steel frames 4 to be installed on the upper step of the tunnel, mark the positions of the restraint anchor cable drilling holes on the surrounding rock surface along the circumference of the tunnel to ensure that the holes are located in the middle of the two adjacent steel frames to facilitate the installation of the longitudinal connector 5; the specific positions of the holes along the circumference of the tunnel should be determined comprehensively considering the number of restraint anchor cables, surrounding rock conditions, and the position of the steel frame connecting plates; among them, the restraint anchor cable drilling holes near the arch feet on both sides of the upper step of the tunnel are set 1.2m above the arch feet.

[0062] 【1.3】Use a single-body pneumatic anchor drilling rig equipped with a hollow hexagonal extended drill rod and a Φ32mm drill bit to drill holes perpendicular to the surrounding rock surface at the marked drilling position.

[0063] [1.4] The drilling depth is determined based on the tunnel over-excavation situation and the exposed length requirement of the steel strand 1 to ensure that the steel strand 1 and the longitudinal connector 5 can be effectively connected.

[0064] 【2】Resin anchor installation:

[0065] [2.1] The steel strand 1 in the restrained anchor cable support system is anchored with a resin anchor end. Before the resin anchor roll is sent into the borehole, the borehole is first cleaned.

[0066] 【2.2】After using a plastic tube to deliver the resin anchor roll into the designated position of the drill hole, tamp the resin anchor in the drill hole.

[0067] 【2.3】The cumulative length of the resin anchor rolls should not be less than the design length of the anchor section of the constrained anchor cable, which is 1.2-1.8m.

[0068] 【3】Steel strand 1 enters the hole and anchors with anchoring agent:

[0069] [3.1] After the steel strand 1 is manually fed into the borehole to the location of the resin anchor, a stirring driver is installed at the lower end of the steel strand 1. The drilling rig drives the stirring driver and the steel strand to rotate and stir the resin anchor roll.

[0070] 【3.2】Keep the drill moving forward while stirring the resin anchor, and control the stirring time to 20-30s to ensure that the anchor is stirred evenly.

[0071] [3.3] When the drill rig is pushing the steel strand 1, it should be noted that the final exposed length of the steel strand 1 should not be too short or too long, and should exceed the initial support surface by 25 cm.

[0072] 【4】Preload the steel strand 1 on the surrounding rock surface:

[0073] [4.1] 10-15 minutes after the resin anchoring agent roll is stirred, the first anchor plate 2 and the first anchor 3 are installed on the surrounding rock surface in sequence.

[0074] [4.2] A tensioning machine is used to apply pre-tightening force to the steel strand 1, which is applied in place at one time. Then, the first anchor 3 installed on the surface of the surrounding rock is used to lock the steel strand 1 for the first time, thereby realizing active reinforcement of the surrounding rock by the constrained anchor cable support system.

[0075] 【5】Connection between steel strand 1 and steel frame:

[0076] 【5.1】After the steel strand 1 is pre-tightened on the surrounding rock surface, the steel frame should be erected in time. The spacing between the steel frames should strictly meet the design requirements.

[0077] [5.2] Install the longitudinal connector 5: Pass the steel strand 1 through the third reserved hole 8 on the longitudinal connector 5, then place both ends of the longitudinal connector 5 tightly against the flange of the steel frame close to the surrounding rock, and weld the contact parts between the two.

[0078] [5.3] Install the second anchor plate 6 and the second anchor 7 on the longitudinal connector 5 in sequence, then use the tensioning equipment to apply pre-tightening force to the steel strand 1 again, and use the second anchor 7 installed on the longitudinal connector to lock the steel strand 1 for the second time to realize the restraint and fixation of the steel frame by the restraint anchor cable support system. The pre-tightening force this time should be controlled within 6-10kN to prevent the steel frame from being pulled and displaced.

[0079] [5.4] Use an angle grinder to cut off the excess exposed part of the steel strand, so that the steel strand 1 is exposed 5 cm from the second anchor installed on the longitudinal connector 5.

[0080] 【6】Spray concrete on the rock surface to the designed thickness to complete the initial support construction work.

[0081] The present invention adopts small-diameter steel strands to replace large-diameter steel pipes used in constrained anchor rods, and proposes the technical measure of tunnel constrained anchor cables to improve the stability of the initial support structure of the tunnel. The drilling required for the installation of the steel strands is carried out by a single-type pneumatic anchor drilling rig, which is equipped with a long drill rod. The drilling depth can reach more than 12m, ensuring that the constrained anchor cables can penetrate into the relatively stable rock layer deep in the surrounding rock. In addition, the drilling efficiency is high, avoiding the problems of being restricted by the working space of the upper steps of the tunnel construction, the limited drilling depth of the YT-28 handheld pneumatic drill commonly used in tunnels, and the inconvenience and low efficiency of down-the-hole drilling rigs.

[0082] The steel strands in the tunnel constraint anchor cable of the present invention are anchored to the surrounding rock through a resin anchor, which can quickly obtain anchoring force and completely avoid the problems of grouting and leakage caused by the oblique upward grouting anchoring of the constraint anchor rod, thereby ensuring the anchoring effect. At the same time, the constraint anchor cable can be installed on the vertical surrounding rock surface (obliquely upward) of the tunnel arch, which can give full play to the tensile properties of the steel strands, and the tensile strength of the steel strands can reach more than 1860MPa.

[0083] The tunnel constraint anchor cable of the present invention applies a pre-tightening force to the steel strand on the surface of the surrounding rock with the help of a tensioning machine, and the first anchor 3 installed on the surface of the surrounding rock locks the steel strand for the first time, thereby realizing the active reinforcement of the surrounding rock by the constraint anchor cable support system, while the constraint anchor rod does not have this technical effect; in addition, a pre-tightening force is applied to the steel strand again on the surface of the longitudinal connector with the help of a tensioning machine, and the second anchor 7 installed on the longitudinal connector locks the steel strand for the second time, thereby realizing the constraint and fixation of the steel frame by the constraint anchor cable support system. This connection method of the constraint anchor cable and the steel frame is simpler, faster and more reliable.

[0084] One end of the tunnel constraint anchor cable of the present invention is anchored in a relatively stable rock layer deep in the surrounding rock, and the other end is connected to the initial support steel frame, thereby strengthening the restraining effect on the initial support structure. While improving the ability of the initial support structure to resist its own instability and destruction, it can also inhibit the overall settlement and convergence deformation of the initial support structure without setting up temporary invert arches or temporary cross braces, achieving "cable instead of brace" and avoiding interference with subsequent construction processes.

[0085] In summary, the present invention's method for confining tunnel anchor cables and improving the stability of initial support structures can simultaneously meet the three conditions required for effective confinement of tunnel steel frames: first, the confining anchor cables are sufficiently long to penetrate deep into the relatively stable rock formations deep within the surrounding rock; second, the confining anchor cables and the surrounding rock can be quickly and effectively anchored, ensuring sufficient anchoring force; and third, the confining anchor cables and the tunnel steel frame are reliably connected. Furthermore, the technical effectiveness of the confining anchor cable support system is ensured, effectively improving the stability of the initial support structure of soft rock tunnels. Simultaneously, it can also inhibit the overall settlement and convergence deformation of the initial support structure.

[0086] The above content is only for explaining the technical idea of ​​the present invention and cannot be used to limit the protection scope of the present invention. Any changes made on the basis of the technical solution in accordance with the technical idea proposed by the present invention shall fall within the protection scope of the claims of the present invention.

Claims

1. A method for improving the stability of an initial support structure, characterized in that: Tunnel restraint anchor cables are used to reinforce the initial support; The tunnel restraining anchor cable comprises a steel strand (1) anchored in the surrounding rock, a first anchor plate (2) and a first anchor (3) installed on the surface of the surrounding rock, a longitudinal connector (5) installed between two adjacent steel frames (4), a second anchor plate (6) and a second anchor (7) installed on the longitudinal connector (5); The longitudinal connector (5) is arranged between two adjacent steel frames (4) of the initial support, and a third reserved hole (8) is provided in the middle of the longitudinal connector (5) for facilitating the steel strand (1) to pass through; A first reserved hole (9) and a second reserved hole (10) matching the diameter of the steel strand (1) are respectively provided in the middle of the first anchor plate (2) and the second anchor plate (6); one end of the steel strand (1) is anchored deep in the surrounding rock, and the other end passes through the first reserved hole (9) and the second reserved hole (10) in sequence, and the steel strand (1) applied with pre-tightening force is locked by the first anchor (3) and the second anchor (7) respectively; The length of the steel strand is 5-12m, the diameter is 17-25mm, and 4-8 strands are set between every two steel frames; The method for improving the stability of the initial support structure by using a tunnel restraining anchor cable comprises the following steps: 【1】Drilling: 【1.1】Preliminary spraying of concrete on the surrounding rock surface after tunnel excavation; 【1.2】Drill holes in the middle of two adjacent steel frames at a distance of 0.8-1.2m from the arch foot according to the set number of holes, with a diameter of 28-32mm and a depth of 5-12m; 【2】Resin anchor installation: After drilling and cleaning the hole, use a plastic tube to send the resin anchor drug roll into the designated position of the drill hole, and then tamp the resin anchor in the drill hole. The cumulative length of the resin anchor drug roll is 1.2-1.8m; 【3】Steel strand entry and anchoring: After manually feeding the steel strand into the borehole to the location of the resin anchor, a stirring driver is installed at the lower end of the steel strand. The drilling rig drives the stirring driver and the steel strand to rotate, stirring the resin anchor roll. At the same time, the drilling rig is kept advancing. The stirring time is controlled within 20-30 seconds, and the length of the exposed initial support surface of the steel strand is controlled to 25-30 cm. 【4】Preload the steel strands on the surrounding rock surface: 【4.1】10-15 minutes after the resin anchoring agent is mixed, install the first anchor plate and the first anchor on the surrounding rock surface in sequence; [4.2] Use tensioning equipment to apply pre-tightening force to the steel strands, and then use the first anchor to lock the steel strands for the first time, realizing the active reinforcement of the surrounding rock by the restrained anchor cable support system; 【5】Connection between steel strand and steel frame: 【5.1】Erect the steel frame according to the set spacing; [5.2] Install the longitudinal connector, pass the steel strand through the third reserved hole, and then place both ends of the longitudinal connector tightly against the flange of the steel frame close to the surrounding rock, and weld the contact parts between the two. [5.3] Install the second anchor plate and the second anchor in sequence, and use the tensioning machine to apply preload force to the steel strands again. The second anchor locks the steel strands a second time to secure the tunnel restraining anchor cables to the steel frame. The preload force is 6-10kN. 【5.4】Use an angle grinder to cut off the excess exposed part of the steel strand; 【6】Re-spray concrete to the designed thickness.

2. The method for improving the stability of the initial support structure according to claim 1, characterized in that: Step [1] Drilling is carried out using a single-body pneumatic anchor drilling rig equipped with a hollow hexagonal extended drill rod and a matching drill bit.

3. The method for improving the stability of the initial support structure according to claim 1, characterized in that: The two ends of the longitudinal connecting piece (5) are closely attached to the inner side of the flange of the steel frame (4) close to the surrounding rock and are fixed by welding.

4. The method for improving the stability of the initial support structure according to claim 1, characterized in that: The longitudinal connector is made of I14 or I16 I-steel, and its length is 6-8 cm smaller than the distance between adjacent steel frames.

5. The method for improving the stability of the initial support structure according to claim 1, characterized in that: The length, width and thickness of the first anchor plate (2) are 28-32 cm, 28-32 cm and 14-16 mm respectively. The first reserved hole (9) is a circular hole with a hole diameter 4-6 mm larger than the diameter of the steel strand.

6. The method for improving the stability of the initial support structure according to claim 1, characterized in that: The length, width and thickness of the second anchor plate (6) are 14-16 cm, 10-12 cm and 14-16 mm respectively. The second reserved hole (10) is a circular hole with a hole diameter 4-6 mm larger than the diameter of the steel strand.

7. The method for improving the stability of an initial support structure according to claim 1, characterized in that: The third reserved hole (8) is a long hole arranged along the length direction of the longitudinal connecting member (5).

8. The method for improving the stability of an initial support structure according to claim 1, characterized in that: The first anchor (3) and the second anchor (7) both match the diameter of the steel strand (1).

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