Initial support methods for soft rock tunnels

Abstract

This invention discloses a method for initial support of soft rock tunnels, relating to the field of soft rock tunnel construction technology. The method includes dividing the cross-section of the soft rock tunnel into multiple continuous excavation units according to a spiral line; excavating multiple excavation units to form multiple support faces; simultaneously installing prestressed anchor cables on each support face formed by excavating each excavation unit; after all support faces located in the same spiral segment have been equipped with prestressed anchor cables, installing flexible mesh to form a prestressed mesh; and spraying concrete onto the prestressed mesh to complete the initial support of the soft rock tunnel. This invention improves the construction safety of soft rock tunnels by dividing the excavation units along a spiral line and simultaneously installing prestressed anchor cables and flexible mesh after excavation, combined with a layered sprayed concrete construction method.

Description

Technical Field

[0001] This invention relates to the field of soft rock tunnel construction technology, and in particular to a method for initial support of soft rock tunnels. Background Technology

[0002] Soft rock tunnel support technology is an important branch of underground engineering, and an increasing number of tunnel projects need to traverse soft rock strata. Soft rock generally refers to rocks with a uniaxial compressive strength of less than 30 MPa, including mudstone, shale, weathered rock, and soft sandstone. These rocks are characterized by low strength, easy weathering, significant rheological properties, and softening upon contact with water, posing a significant challenge to tunnel construction.

[0003] Currently, anchor-sprayed support technology is mainly used for soft rock tunnel support. However, in soft rock tunnels, there are problems such as insufficient anchoring force, large stiffness of steel mesh that is difficult to adapt to soft rock deformation, and easy cracking of sprayed concrete. It cannot provide sufficient support resistance under the low strength conditions of soft rock, resulting in poor safety. Summary of the Invention

[0004] The main objective of this invention is to propose an initial support method for soft rock tunnels, which aims to provide sufficient support resistance for soft rock tunnels and improve the construction safety of soft rock tunnels.

[0005] To achieve the above objectives, the present invention proposes a method for initial support of soft rock tunnels, comprising:

[0006] The cross-section of the soft rock tunnel is divided into multiple continuous excavation units according to the spiral line; the spiral line is wrapped around the secondary lining of the soft rock tunnel and forms at least two spiral segments;

[0007] Multiple excavation units are excavated separately to form multiple support surfaces;

[0008] When each excavation unit is excavated to form a support face, prestressed anchor cables are installed simultaneously on the support face;

[0009] After the prestressed anchor cables are installed on multiple support surfaces located in the same spiral segment, a flexible mesh is installed to form a prestressed mesh.

[0010] Concrete is sprayed onto the prestressed grid to complete the initial support for the soft rock tunnel.

[0011] In one embodiment, the helix is ​​wound clockwise around the outer lining of the soft rock tunnel;

[0012] The steps of excavating multiple excavation units to form multiple support faces include:

[0013] Starting from the arch of the soft rock tunnel, multiple excavation units are excavated sequentially in a clockwise direction to form multiple support surfaces.

[0014] In one embodiment, the depth of each excavation unit is h, where 0.5m ≤ h ≤ 0.8m.

[0015] In one embodiment, the width of each excavation unit is d, and the perimeter of the soft rock tunnel is c, where c*1 / 8≤d≤c*1 / 6.

[0016] In one embodiment, before the step of installing a flexible mesh to form a prestressed mesh after all the prestressed anchor cables have been installed on the multiple support surfaces located in the same helical segment, the initial support method for soft rock tunnels further includes:

[0017] Each of the prestressed anchor cables is initially tensioned until the prestress of the prestressed anchor cable reaches 60% of the design prestress value;

[0018] After 24 hours, each of the prestressed anchor cables is tensioned a second time until the prestress of the prestressed anchor cable reaches 85% of the design prestress value;

[0019] After 72 hours, each of the prestressed anchor cables is finally tensioned until the prestress of the prestressed anchor cable reaches 100% of the design prestress value.

[0020] In one embodiment, the flexible mesh includes a plurality of longitudinally and transversely distributed steel wires, and the prestressed anchor cable includes an anchor cable head and a connecting steel plate, wherein the connecting steel plate is connected to the anchor cable head;

[0021] After the prestressed anchor cables are installed on multiple support surfaces located in the same helical segment, the step of installing a flexible mesh to form a prestressed mesh includes:

[0022] After the prestressed anchor cables are installed on multiple support surfaces in the same spiral segment, the flexible mesh is tied to the connecting steel plate with tie wire and spot welded to form the prestressed mesh.

[0023] In one embodiment, the step of spraying concrete onto the prestressed grid to complete the initial support of the soft rock tunnel includes:

[0024] After the first excavation unit has installed the prestressed anchor cable and connected the flexible grid, the first layer of concrete is sprayed onto the spiral segment corresponding to the first excavation unit to form the first concrete layer.

[0025] After the prestressed anchor cables and flexible mesh are installed on all the support surfaces of the same spiral segment, a second layer of concrete is sprayed onto the first concrete layer to obtain the second concrete layer, thus completing the initial support for the soft rock tunnel.

[0026] In one embodiment, before the step of spraying a second layer of concrete onto the first concrete layer to form the prestressed mesh after the prestressed anchor cables and flexible mesh have been installed on all the support surfaces of the same spiral segment, the initial support method for soft rock tunnels further includes:

[0027] The first concrete layer is roughened.

[0028] In one embodiment, the step of spraying concrete onto the prestressed grid to complete the initial support of the soft rock tunnel further includes:

[0029] After the next spiral segment is excavated and the prestressed anchor cables are installed and connected to the flexible grid, a third layer of concrete is sprayed onto the second concrete layer to complete the initial support for the soft rock tunnel.

[0030] In one embodiment, the thickness of the first concrete layer is a, where 30mm ≤ a ≤ 40mm; the first concrete layer comprises 100 parts cement, 230 parts fine aggregate, and 8 parts aluminate quick-setting agent by mass.

[0031] The thickness of the second concrete layer is b, 50mm≤b≤60mm; the second concrete layer comprises 100 parts cement, 240 parts mixed aggregate and 1 part polycarboxylate superplasticizer by mass.

[0032] The thickness of the third concrete layer is e, 40mm≤e≤50mm; the third concrete layer comprises 100 parts cement, 200 parts aggregate and 2 parts steel fiber by mass.

[0033] The technical solution of this invention divides the excavation units into spiral sections and simultaneously installs prestressed anchor cables and flexible grids after the excavation units are excavated. Combined with the construction method of layered shotcrete, it effectively reduces the exposure time of the surrounding rock and establishes a support system in a timely manner. This effectively controls the deformation of the surrounding rock, improves construction efficiency and the support effect on soft rock tunnels, provides sufficient support resistance for soft rock tunnels, and improves the construction safety of soft rock tunnels. Attached Figure Description

[0034] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0035] Figure 1 This is a flowchart illustrating an embodiment of the initial support method for soft rock tunnels provided by the present invention.

[0036] The realization of the objective, functional features and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0037] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0038] It should be noted that if the embodiments of the present invention involve directional indications (such as up, down, left, right, front, back, etc.), the directional indications are only used to explain the relative positional relationship and movement of the components in a specific posture. If the specific posture changes, the directional indications will also change accordingly.

[0039] Furthermore, if the embodiments of this invention involve descriptions such as "first" or "second," these descriptions are for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined with "first" or "second" may explicitly or implicitly include at least one of those features. Additionally, the use of "and / or" or "and / or" throughout the text includes three parallel solutions. For example, "A and / or B" includes solution A, solution B, or a solution where both A and B are satisfied simultaneously. Furthermore, the technical solutions of the various embodiments can be combined with each other, but this must be based on the ability of those skilled in the art to implement them. When the combination of technical solutions is contradictory or impossible to implement, it should be considered that such a combination of technical solutions does not exist and is not within the scope of protection claimed by this invention.

[0040] Soft rock tunnel support technology is an important branch of underground engineering. With the rapid development of infrastructure construction in my country, more and more tunnel projects need to traverse soft rock strata. Soft rock generally refers to rocks with a uniaxial compressive strength of less than 30 MPa, including mudstone, shale, weathered rock, and soft sandstone. These rocks are characterized by low strength, easy weathering, significant rheological properties, and softening upon contact with water, posing a significant challenge to tunnel construction. When excavating tunnels in soft rock strata, the surrounding rock has poor self-stabilizing ability, making it prone to large deformations or even collapses, which seriously threatens construction safety and project quality.

[0041] Currently, the main technical solutions for soft rock tunnel support are as follows: Traditional anchor-sprayed support technology, while widely used, suffers from insufficient anchoring force, high rigidity of the steel mesh making it difficult to adapt to soft rock deformation, and easy cracking of the sprayed concrete in soft rock tunnels; steel arch support technology, although possessing strong load-bearing capacity, has rigid structures that are difficult to adapt to the rheological deformation of soft rock, and also involves large steel consumption and high costs; advanced support technology, while capable of pre-reinforcing the surrounding rock, has limited support effect on already excavated areas and involves complex construction processes. These traditional technologies generally suffer from drawbacks such as long exposure time of the surrounding rock, delayed support, and poor structural adaptability.

[0042] From a construction technology perspective, most existing technologies adopt the traditional sequence of excavation followed by support, which has significant drawbacks under soft rock conditions. Full-face excavation exposes a large area of ​​surrounding rock, which, under the influence of the low strength and easy weathering characteristics of soft rock, is prone to instability. While sectional excavation reduces the exposed area, it still results in a long exposure time for the surrounding rock. Support lags behind excavation, failing to control surrounding rock deformation in a timely manner and affecting the support effect. Furthermore, existing support structures are mostly rigid designs, lacking adaptability to the rheological properties of soft rock, and are unable to provide sufficient support resistance under the low strength conditions of soft rock.

[0043] A comprehensive analysis of existing technologies reveals that the main technical challenges facing soft rock tunnel support include: how to provide sufficient support resistance under low-strength conditions in soft rock; how to adapt to the large deformation and rheological properties of soft rock; how to reduce the exposure time of the surrounding rock and establish effective support in a timely manner; how to ensure the long-term stability and durability of the support structure; and how to improve construction efficiency and economy while ensuring safety. These problems severely restrict the development of soft rock tunnel construction technology, and there is an urgent need for an initial support method for soft rock tunnels that can simultaneously solve the above problems.

[0044] To address this technical problem, this invention proposes a method for initial support of soft rock tunnels.

[0045] Please see Figure 1 In one embodiment of the present invention, the initial support method for soft rock tunnels includes:

[0046] Step S10: Divide the cross-section of the soft rock tunnel into multiple continuous excavation units according to the spiral line; the spiral line is wrapped around the secondary lining of the soft rock tunnel and forms at least two spiral segments;

[0047] Step S20: Excavate multiple excavation units to form multiple support surfaces;

[0048] Step S30: When each excavation unit is excavated to form a support face, prestressed anchor cables are installed simultaneously on the support face;

[0049] Step S40: After the prestressed anchor cables are installed on all the support surfaces of the same spiral segment, a flexible mesh is installed to form a prestressed mesh.

[0050] Step S50: Spray concrete onto the prestressed grid to complete the initial support for the soft rock tunnel.

[0051] It should be noted that the soft rock tunnel secondary lining mentioned in this embodiment is not actually formed; it is only used here to describe the positional relationship.

[0052] It should be understood that a spiral refers to a trajectory line that continuously curves around the tunnel cross-section, with its winding direction forming a specific angle with the tunnel axis, used to guide the division of excavation units. An excavation unit refers to a segmented work area formed by cutting along the spiral. The excavation volume of each unit can be adjusted according to the surrounding rock conditions, for example, the depth can be controlled within the range of 0.5 to 0.8 meters, reducing the area exposed at one time by segmenting the excavation. Prestressed anchor cables are anchoring components that apply prestress, specifically using steel strands in conjunction with expansion-shell anchor ends, installed immediately after excavation to quickly provide support. Flexible mesh refers to a metal mesh structure with extensibility, specifically using galvanized steel wire woven into a diamond mesh, absorbing the deformation energy of the surrounding rock through its deformable characteristics.

[0053] More specifically, the tunnel cross-section is first divided into multiple continuous excavation units based on the design spiral parameters, with appropriate rock pillars retained between adjacent units as temporary supports. Excavation begins at the crown and proceeds along the spiral, with prestressed anchor cables installed immediately upon completion of each unit on the freshly excavated surface. Once all units within the same spiral segment have completed anchor cable installation, the flexible mesh is connected to the anchor cables to form an integrated load-bearing system. Finally, layered shotcrete is used to tightly bond the support structure to the surrounding rock, forming a prestressed composite support layer. Throughout the process, excavation, support, and concrete work alternate spatially, ensuring close coordination between each step.

[0054] Compared to existing technologies, this solution achieves rapid segmented support through spiral division, transforming traditional large-area exposure into localized exposure, for example, reducing the single excavation area to one-eighth to one-sixth of the tunnel perimeter. The synchronous installation of prestressed anchor cables changes the traditional delayed support mode, establishing active support force immediately after the excavation face is formed. The combination of flexible mesh and anchor cables forms a deformation-adaptive support system, reducing stress concentration compared to traditional steel mesh. The spiral propulsion method allows for phased concrete spraying, avoiding shrinkage cracking problems caused by single-layer thick spraying. This application effectively controls the surrounding rock deformation of soft rock tunnels, shortening the exposure time after a single excavation to less than two hours, significantly reducing the risk of surrounding rock relaxation. The flexible support system dynamically coordinates with surrounding rock deformation, avoiding rigid structure failure. The phased construction method enables parallel operations of excavation and support, improving overall construction efficiency by approximately 30% while reducing concrete material usage by approximately 15%.

[0055] The technical solution provided by this invention divides the excavation units into spiral sections and installs prestressed anchor cables and flexible grids simultaneously after the excavation units are excavated. Combined with the construction method of layered shotcrete, the exposure time of the surrounding rock is effectively reduced and the support system is established in a timely manner. This effectively controls the deformation of the surrounding rock, improves construction efficiency and the support effect on soft rock tunnels, provides sufficient support resistance for soft rock tunnels, and improves the construction safety of soft rock tunnels.

[0056] In an embodiment of the present invention, the spiral is wound clockwise around the outer lining of the soft rock tunnel;

[0057] The steps of excavating multiple excavation units to form multiple support faces include:

[0058] Step S21: Starting from the arch of the soft rock tunnel, excavate multiple excavation units in a clockwise direction to form multiple support surfaces.

[0059] It should be understood that a clockwise spiral winding means that the extension direction of the spiral is consistent with the direction of clock rotation. This can be achieved using a laser positioning device in conjunction with a total station for spatial positioning, ensuring that the spiral trajectory conforms to the design parameters. Excavating sequentially in a clockwise direction starting from the arch means starting from the highest point of the tunnel top and excavating in sections in a clockwise sequence. This can be implemented step-by-step using a hydraulic breaker in conjunction with an excavator, with the excavation sequence synchronized with the spiral's direction.

[0060] More specifically, after the tunnel cross-section is divided into several excavation units by a spiral, construction machinery begins work from the arch top and advances unit by unit in a clockwise direction. A support face is immediately formed after each unit is excavated, and the excavation of adjacent units continues along the spiral trajectory. During excavation, the construction direction is consistent with the spiral winding direction, ensuring a correspondence between the spatial position of each excavation unit and the support structure. This coordinated design of the excavation sequence and the spiral direction enables an orderly connection between the excavation face and the support face.

[0061] This embodiment establishes a spatial continuity between excavation units by defining the correspondence between the spiral winding direction and the excavation sequence, reducing the exposure time of the surrounding rock's free face. Simultaneously, the clockwise advancement method aligns better with the operating habits of most construction machinery, avoiding directional conflicts during excavation. This effectively controls the deformation rate of the surrounding rock during soft rock tunnel construction, achieving spatiotemporal coordination between excavation and support processes. By defining the starting position and direction of excavation, it ensures timely follow-up of the support structure, avoiding support delays caused by chaotic excavation sequences in traditional methods. Furthermore, the clearly defined construction process improves mechanical operation efficiency and reduces time losses during process transitions.

[0062] In an embodiment of the present invention, the depth of each excavation unit is h, where 0.5m ≤ h ≤ 0.8m.

[0063] It should be understood that the excavation unit depth refers to the distance advanced along the tunnel axis in each excavation operation. This can be achieved through real-time monitoring using a laser rangefinder in conjunction with a total station. During the excavation process, the excavation depth is controlled by adjusting the travel parameters of the tunneling machinery. This depth range can balance construction efficiency with the requirements of surrounding rock stability, avoiding both excessively small excavation volume leading to frequent process changes and excessively large excavation volume causing stress redistribution in the surrounding rock.

[0064] More specifically, after dividing the tunnel cross-section into excavation units along a spiral path, the excavation of each unit is carried out using a hydraulic breaker or a tunnel boring machine. During excavation, operators control the swing amplitude and excavation speed of the robotic arm according to a preset spiral trajectory, ensuring that the actual depth of each excavation unit remains within a defined range. For example, when encountering a locally fractured zone, the excavation depth can be adjusted to the lower limit to enhance the self-stability of the surrounding rock; in sections with intact rock strata, the upper limit can be used to improve construction efficiency.

[0065] Compared to existing technologies, traditional full-face excavation methods often employ cyclic advances of 1.2–1.5 m, resulting in a large exposed area of ​​surrounding rock and a long support lag time. This embodiment, by limiting the excavation unit depth, reduces the exposed area of ​​each excavation unit by approximately 40%–60%, significantly shortening the interval between surrounding rock exposure and support completion, effectively suppressing the development of rheological deformation in soft rock. This embodiment enables a close integration of excavation and support processes, forming a continuous and stable support system in soft rock strata. By limiting the excavation depth range, the deformation of the surrounding rock in each excavation unit is controlled within the elastic deformation stage, avoiding the accumulation of plastic deformation that could lead to localized collapses. Simultaneously, this depth range allows construction machinery to operate at its optimal efficiency, reducing project delays caused by frequent adjustments to operating parameters.

[0066] In an embodiment of the present invention, the width of each excavation unit is d, the perimeter of the soft rock tunnel is c, and c*1 / 8≤d≤c*1 / 6.

[0067] It should be understood that the width of an excavation unit refers to the arc length of each excavation unit divided along the tunnel's circumference. Specifically, it can be proportionally divided using the tunnel's cross-sectional perimeter as a reference parameter. This width range, by defining the proportional relationship between the excavation unit and the tunnel perimeter, ensures that the construction scale of each excavation unit meets the space requirements for mechanical operations while effectively controlling the exposed area of ​​the surrounding rock. The tunnel perimeter refers to the total length of the outer contour of the tunnel's excavation cross-section, which can be measured using a total station. This parameter serves as the basis for dividing excavation units and can adapt to the engineering needs of tunnels with different cross-sectional dimensions.

[0068] More specifically, the width of the excavation unit is proportional to the tunnel perimeter. During construction, the tunnel cross-sectional perimeter is measured first, and the theoretical width range of each excavation unit is calculated based on this perimeter. Construction workers divide the excavation units according to this width range and excavate sequentially along the tunnel's circumference. Prestressed anchor cables are installed immediately after each excavation unit is completed, forming a continuous support system. By establishing a link between the width of the excavation unit and the tunnel perimeter, the excavation scale can be dynamically adjusted according to actual engineering conditions. This avoids the low construction efficiency caused by traditional fixed-size divisions and prevents the risk to surrounding rock stability caused by excessively large excavation units.

[0069] This embodiment constrains the excavation unit width with the tunnel perimeter by establishing a proportional relationship, automatically adapting the excavation scale to the tunnel dimensions. This effectively controls the exposed area of ​​the surrounding rock while ensuring construction efficiency. It effectively controls the deformation rate of the surrounding rock during soft rock tunnel construction. By optimizing the ratio between the excavation unit size and the tunnel cross-section, it ensures both the construction efficiency of single-cycle excavation operations and shortens the exposure time of the surrounding rock. This technical solution can adapt to the engineering needs of tunnels with different cross-sectional dimensions, forming a progressive and highly adaptable excavation and support system in soft rock strata, significantly improving the collaborative bearing capacity of the initial support structure.

[0070] In an embodiment of the present invention, before the step of installing a flexible mesh to form a prestressed mesh after all the prestressed anchor cables have been installed on the multiple support surfaces located on the same spiral segment, the initial support method for soft rock tunnels further includes:

[0071] Step S401: Initial tensioning is performed on each of the prestressed anchor cables until the prestress of the prestressed anchor cable reaches 60% of the design prestress value;

[0072] Step S402: After 24 hours, each of the prestressed anchor cables is tensioned a second time until the prestress of the prestressed anchor cable reaches 85% of the design prestress value;

[0073] Step S403: After 72 hours, each of the prestressed anchor cables is finally tensioned until the prestress of the prestressed anchor cable reaches 100% of the design prestress value.

[0074] It should be understood that initial tensioning refers to the process of applying prestress for the first time, which can be achieved using hydraulic jacks with staged loading. Applying load in stages avoids excessive stress on the surrounding rock instantaneously. Secondary tensioning refers to supplementary tensioning after the anchoring system has initially stabilized. This can be achieved using intelligent tensioning equipment with pressure sensors, which adjusts the tension force by monitoring prestress loss. Final tensioning refers to the final step of applying all prestress, which can be achieved by linking the locking device with the tensioning equipment to ensure that the prestress accurately reaches the set value.

[0075] More specifically, after the anchor cables are installed on all support surfaces within the same helical segment, initial tensioning is first performed using hydraulic equipment, for example, controlling the prestress at 60% of the design value. After 24 hours of surrounding rock stress adjustment, secondary tensioning is performed using an intelligent tensioning system, for example, increasing the prestress to 85% of the design value. After 72 hours when the surrounding rock rheology tends to stabilize, the final application of 100% of the design prestress is completed using precisely controlled equipment. This staged loading method allows the surrounding rock to gradually undergo adaptive deformation, enabling the anchoring system and the surrounding rock to form a synergistic load-bearing system.

[0076] This embodiment achieves dynamic matching between load application and surrounding rock rheological properties in the time dimension through three-stage interval tensioning, ensuring timely establishment of the support system while avoiding the risk of overload. It effectively solves the problem of prestress loss caused by soft rock rheology, ensuring continuous and stable transmission of anchoring force to the deep rock mass. The staged tensioning process enables a gradual redistribution of the surrounding rock stress field, avoiding structural interface damage caused by sudden loading in traditional methods. This significantly improves the collaborative performance between the support system and the surrounding rock, creating a stable mechanical environment for subsequent flexible mesh installation.

[0077] In an embodiment of the present invention, the flexible mesh includes a plurality of longitudinally and transversely distributed steel wires, and the prestressed anchor cable includes an anchor cable head and a connecting steel plate, wherein the connecting steel plate is connected to the anchor cable head;

[0078] After the prestressed anchor cables are installed on multiple support surfaces located in the same helical segment, the step of installing a flexible mesh to form a prestressed mesh includes:

[0079] Step S410: After the prestressed anchor cables are installed on all the support surfaces of the same spiral segment, the flexible mesh is tied to the connecting steel plate with tie wire and spot welded to form the prestressed mesh.

[0080] It should be understood that flexible mesh refers to a mesh structure woven from high-strength steel wire. Specifically, it can be achieved by weaving cold-drawn steel wire with a diameter of 8 mm into a mesh size of 100 mm × 100 mm. Its ductility can adapt to the deformation characteristics of soft rock. Prestressed anchor cables refer to anchoring components that apply prestress. Specifically, they can be achieved using an anchor cable head structure with a connecting steel plate. The connecting steel plate serves as a mesh fixing base to distribute stress. Wire binding refers to mechanical fixing using metal wire. Specifically, galvanized iron wire can be used for cross-wrapping to temporarily fix the mesh position. Spot welding connection refers to forming a permanent connection through local fusion welding. Specifically, resistance welding can be used to weld at key nodes to ensure the coordinated stress distribution between the mesh and the anchor cable.

[0081] More specifically, after all prestressed anchor cables are installed within the helical section, the flexible mesh is unfolded to cover the support surface. Each intersection of the mesh is initially secured to the connecting steel plate at the anchor cable head using tie wire, followed by spot welding reinforcement at the edges of the connecting steel plate. This combined connection method ensures installation efficiency while, through a composite connection mode combining mechanical fixing and welding, enabling the mesh and anchor cables to form an integral load-bearing structure. As the helical excavation progresses, the meshes of adjacent support surfaces are connected in the same way, ultimately forming a continuous prestressed support layer.

[0082] This embodiment achieves rapid positioning through wire binding and then enhances the strength of key nodes through selective spot welding, ensuring connection reliability and improving construction efficiency. Compared to traditional rigid steel mesh, the connection method of flexible steel wire mesh and anchor cables is more adaptable to soft rock deformation and avoids connection failure caused by stress concentration. It achieves a reliable connection between the flexible support system and prestressed anchor cables, effectively transferring prestress and evenly distributing support stress. The combined connection method of wire binding and spot welding ensures construction speed and improves the shear strength of the connection nodes, preventing the mesh from slipping and falling off during soft rock deformation. This connection structure allows the support system to coordinate with the deformation of the surrounding rock, fully utilizing the extensibility of the flexible mesh and the prestressing effect of the anchor cables to form a dynamically stable support system.

[0083] In an embodiment of the present invention, the step of spraying concrete onto the prestressed grid to complete the initial support of the soft rock tunnel includes:

[0084] Step S51: After the first excavation unit has installed the prestressed anchor cable and connected the flexible grid, the first layer of concrete is sprayed onto the spiral segment corresponding to the first excavation unit to form the first concrete layer.

[0085] Step S52: After the prestressed anchor cables and flexible mesh are installed on all the support surfaces of the same spiral segment, a second layer of concrete is sprayed onto the first concrete layer to obtain the second concrete layer, thus completing the initial support for the soft rock tunnel.

[0086] It should be understood that the first layer of concrete refers to the concrete overlay layer formed by the initial spraying, which can be achieved using a wet spraying process. The spraying location corresponds to the inner surface of the secondary lining of the soft rock tunnel, framed by the spiral section. This layer of concrete is applied immediately after the anchor cables and mesh are installed, serving to initially seal the surrounding rock. The second layer of concrete refers to the concrete structural layer superimposed on the first layer, which can be achieved using a layered spraying process, further increasing the support thickness after the first layer has hardened. Layered spraying controls the thickness of each spray application, preventing cracks caused by concrete shrinkage.

[0087] More specifically, during the spiral segment advancement, once the anchor cable installation and mesh connection of the first excavation unit are completed, the first layer of shotcrete is immediately applied. This layer of concrete covers the spiral segment area corresponding to the excavation unit, forming a preliminary sealing layer. As the spiral excavation continues, once all support surfaces within the same spiral segment have completed anchor cable and mesh installation, a second layer of shotcrete is applied to the area where the first layer has been applied. The coverage area of ​​the second layer of concrete overlaps with the area of ​​the first layer, forming a continuous support structure through layered superposition.

[0088] This embodiment employs a staged, layered spraying method. By controlling the thickness of each layer, the shrinkage stress generated during the hardening process of each concrete layer is effectively released, preventing structural cracks. Simultaneously, the superimposed structure formed by layered spraying improves interlayer bonding and enhances the overall integrity of the support system. This effectively solves the problem of easy cracking of concrete structures in the initial support of soft rock tunnels. The layered spraying process reduces shrinkage stress by controlling the thickness of each layer, and the superimposed structure enhances interlayer bonding. The resulting prestressed mesh works synergistically with the concrete layers to adapt to the deformation characteristics of soft rock and improve the durability of the support structure. This scheme significantly improves the crack resistance and overall stability of the shotcrete structure while ensuring construction efficiency.

[0089] In an embodiment of the present invention, before the step of spraying a second layer of concrete onto the first concrete layer to form a prestressed mesh after all the prestressed anchor cables and flexible meshes have been installed on the multiple support surfaces of the same spiral segment, the initial support method for soft rock tunnels further includes:

[0090] Step S400: Roughen the first concrete layer.

[0091] It should be understood that roughening treatment refers to creating a uniform textured surface on the concrete surface using mechanical or manual methods. Specifically, an electric roughening machine or a handheld steel chisel can be used to scratch the surface of the first concrete layer, creating a rough surface with a depth of 1 to 3 millimeters. This treatment can effectively increase the mechanical interlocking force and bonding area between the old and new concrete layers, preventing interlayer delamination.

[0092] More specifically, after the first layer of concrete is sprayed, while it has reached its initial setting state but is not fully hardened, the concrete surface is uniformly roughened using a specialized tool. The roughening direction is perpendicular to the flow direction of the subsequently sprayed concrete, creating a cross-hatching pattern. The roughening depth should be controlled to remove the surface laitance and expose the aggregate particles, avoiding damage to the structural integrity of the concrete. After roughening, a high-pressure air gun is used to remove surface debris, ensuring a clean interface. This pretreatment provides an ideal bonding base for the second layer of concrete, allowing the two layers to form a continuous, load-bearing whole.

[0093] In an embodiment of the present invention, the step of spraying concrete onto the prestressed grid to complete the initial support of the soft rock tunnel further includes:

[0094] Step S510: After the next spiral segment is excavated and the prestressed anchor cable is installed and connected to the flexible grid, a third layer of concrete is sprayed onto the second concrete layer to complete the initial support for the soft rock tunnel.

[0095] It should be understood that the third layer of concrete refers to the shotcrete layer used to form the final support structure, which can be achieved using cement-based composite materials incorporating steel fibers. The incorporation of steel fibers can improve the crack resistance and toughness of the concrete. The second concrete layer refers to the intermediate transition layer, which can be achieved using concrete incorporating polycarboxylate superplasticizers. The addition of superplasticizers can improve the workability and density of the concrete. The first concrete layer refers to the initial support layer, which can be achieved using concrete with added aluminate accelerators. Accelerators can accelerate the setting and hardening process of the concrete.

[0096] More specifically, during the helical segment excavation, after the installation of anchor cables and the connection of flexible mesh for the first excavation unit are completed, the first layer of shotcrete is immediately applied to form the initial support layer. As the next helical segment is excavated, after the installation of the second layer of anchor cables and mesh is completed, the second layer of shotcrete is applied to the formed initial support layer to form an intermediate transition layer. After the third helical segment is completed, the third layer of concrete is shotcreted on the intermediate transition layer to form the final support structure. The timing of each layer of concrete shotcreting is synchronized with the progress of the helical excavation, forming a layered and progressive support system.

[0097] In some specific implementations, the initial support layer can use fast-setting concrete to quickly seal the surrounding rock, the intermediate transition layer uses high-flowability concrete to fill structural voids, and the final support layer uses fiber-reinforced concrete to improve the overall deformation resistance. The sprayed thickness of each concrete layer can be adjusted according to the surrounding rock conditions, but the interlayer bonding quality must be ensured.

[0098] This embodiment employs three layers of sprayed concrete with different formulations, ensuring timely initial support while achieving synergistic development of strength and toughness in the support structure through a gradient configuration of material properties. It effectively controls the generation of concrete shrinkage cracks, adapts to the rheological deformation characteristics of soft rock, and forms a composite support structure with gradient properties. The layered spraying process is closely coordinated with the helical excavation process, improving the integrity and durability of the support system while ensuring construction efficiency, and solving the technical problems of easy cracking and incompatibility with surrounding rock deformation inherent in traditional sprayed concrete.

[0099] In an embodiment of the present invention, the thickness of the first concrete layer is a, 30mm≤a≤40mm; the first concrete layer comprises 100 parts cement, 230 parts fine aggregate and 8 parts aluminate quick-setting agent by mass.

[0100] The thickness of the second concrete layer is b, 50mm≤b≤60mm; the second concrete layer comprises 100 parts cement, 240 parts mixed aggregate and 1 part polycarboxylate superplasticizer by mass.

[0101] The thickness of the third concrete layer is e, 40mm≤e≤50mm; the third concrete layer comprises 100 parts cement, 200 parts aggregate and 2 parts steel fiber by mass.

[0102] It should be understood that aluminate accelerators are admixtures that accelerate the hydration reaction of cement, specifically using sulfoaluminate compounds, and their function is to shorten the initial setting time of concrete. Polycarboxylate superplasticizers are surfactants with a high molecular weight structure, specifically using ether polymers, and their function is to improve the fluidity of concrete while maintaining its workability. Steel fibers are short fibers made of metallic materials, specifically using cold-drawn low-carbon steel wire, and their function is to enhance the tensile strength and crack resistance of concrete. Mixed aggregates refer to a combination of aggregates with different particle sizes, specifically achieved by mixing fine and coarse sand in a specific ratio, and their function is to optimize the density and structural stability of concrete.

[0103] More specifically, after the anchor cable mesh installation is completed, a first layer of concrete containing a fast-setting agent is sprayed to form a thin support layer, which hardens rapidly to form the initial support surface. After the overall excavation of the spiral section is completed, a second layer of concrete containing a water-reducing agent is sprayed onto the surface of the initial set layer. This layer improves the structural density by optimizing the aggregate gradation. As the construction of subsequent spiral sections progresses, a third layer of concrete containing steel fibers is sprayed onto the formed double-layer structure. The fiber reinforcement effectively inhibits the development of shrinkage cracks. The three spraying operations correspond to different construction stages, and the progressive improvement of support performance is achieved through adjustments to the material ratio.

[0104] This embodiment effectively solves the shrinkage crack problem caused by thick-layer shotcreting. The rapid-setting thin layer shortens the exposure time of the surrounding rock, the intermediate water-reducing layer improves the structural density, and the fiber-reinforced layer enhances the long-term deformation resistance. The three-layer structure forms a complementary and synergistic effect, achieving full-cycle performance optimization of the support system while controlling the construction pace. It is particularly suitable for the special requirements of the support structure on the rheological characteristics of soft rock strata.

[0105] The above description is merely an exemplary embodiment of the present invention and does not limit the scope of protection of the present invention. Any equivalent structural transformations made based on the technical concept of the present invention and the contents of the specification and drawings of the present invention, or direct / indirect applications in other related technical fields, are included within the scope of protection of the present invention.

Claims

1. A method for initial support of soft rock tunnels, characterized in that, include: The cross-section of the soft rock tunnel is divided into multiple continuous excavation units according to the spiral line; The spiral is wound around the outer lining of the soft rock tunnel and forms at least two spiral segments; Multiple excavation units are excavated to form multiple support surfaces; When each excavation unit is excavated to form a support face, prestressed anchor cables are installed simultaneously on the support face; After the prestressed anchor cables are installed on multiple support surfaces in the same spiral segment, a flexible mesh is installed to form a prestressed mesh. The flexible mesh includes multiple longitudinally and transversely distributed steel wires. The prestressed anchor cable includes an anchor cable head and a connecting steel plate, and the connecting steel plate is connected to the anchor cable head. After the prestressed anchor cables are installed on multiple support surfaces in the same spiral segment, the flexible mesh is tied to the connecting steel plate with tie wires and spot-welded to form the prestressed mesh. After the first excavation unit has installed the prestressed anchor cable and connected the flexible grid, the first layer of concrete is sprayed onto the spiral segment corresponding to the first excavation unit to form the first concrete layer. After the prestressed anchor cables and flexible mesh are installed on all the support surfaces of the same spiral segment, a second layer of concrete is sprayed onto the first concrete layer to obtain the second concrete layer; after the next spiral segment is excavated and the prestressed anchor cables are installed and the flexible mesh is connected, a third layer of concrete is sprayed onto the second concrete layer to complete the initial support of the soft rock tunnel. Before the step of installing flexible mesh to form a prestressed mesh after all the prestressed anchor cables have been installed on the multiple support surfaces set in the same spiral segment, the initial support method for soft rock tunnels further includes: Each of the prestressed anchor cables is initially tensioned until the prestress of the prestressed anchor cable reaches 60% of the design prestress value; After 24 hours, each of the prestressed anchor cables is tensioned a second time until the prestress of the prestressed anchor cable reaches 85% of the design prestress value; After 72 hours, each of the prestressed anchor cables is finally tensioned until the prestress of the prestressed anchor cable reaches 100% of the design prestress value.

2. The initial support method for soft rock tunnels as described in claim 1, characterized in that, The spiral is wound clockwise around the outer lining of the soft rock tunnel; The steps of excavating multiple excavation units to form multiple support faces include: Starting from the arch of the soft rock tunnel, multiple excavation units are excavated sequentially in a clockwise direction to form multiple support surfaces.

3. The initial support method for soft rock tunnels as described in claim 2, characterized in that, The depth of each excavation unit is h, where 0.5m ≤ h ≤ 0.8m.

4. The initial support method for soft rock tunnels as described in claim 1, characterized in that, Before the step of spraying a second layer of concrete onto the first concrete layer to form a prestressed mesh after all the prestressed anchor cables and flexible meshes have been installed on the multiple support surfaces of the same spiral segment in the soft rock tunnel, the initial support method for soft rock tunnels further includes: The first concrete layer is roughened.

5. The initial support method for soft rock tunnels as described in claim 1, characterized in that, The thickness of the first concrete layer is a, 30mm≤a≤40mm; the first concrete layer comprises 100 parts cement, 230 parts fine aggregate and 8 parts aluminate quick-setting agent by mass. The thickness of the second concrete layer is b, 50mm≤b≤60mm; the second concrete layer comprises 100 parts cement, 240 parts mixed aggregate and 1 part polycarboxylate superplasticizer by mass. The thickness of the third layer of concrete is e, 40mm≤e≤50mm; the third layer of concrete comprises 100 parts cement, 200 parts aggregate and 2 parts steel fiber by mass.

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