Prestress supporting method for soft rock tunnel

Abstract

The invention discloses a soft rock tunnel prestress supporting method, and relates to the technical field of soft rock tunnel construction.The soft rock tunnel prestress supporting method comprises the steps that surrounding rock of a soft rock tunnel is divided into a surface reinforcing layer of 0-0.5 m, a middle transition layer of 0.5-1.5 m and a deep anchoring layer of 1.5-3 m according to the radial depth; pretreating the surface reinforcing layer; pretreating the middle transition layer; pretreating the deep anchoring layer; and multidirectional prestress is applied to the surface reinforcing layer, the middle transition layer and the deep anchoring layer, and prestress supporting construction of the soft rock tunnel is completed. According to the method, the surrounding rock is divided into the surface reinforcing layer, the middle transition layer and the deep anchoring layer for differential pretreatment, and radial, tangential and axial multidirectional prestress is applied, so that the problem of soft rock tunnel layered deformation coordination control is solved, and the construction safety of the soft rock tunnel is improved.

Description

Technical Field

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

[0002] In soft rock tunnel engineering, due to the characteristics of low surrounding rock strength, large deformation, and poor stability, support technology has always been a key challenge in engineering construction. Soft rock typically has low uniaxial compressive strength (generally less than 25 MPa), large deformation modulus, and significant rheological properties, making it prone to engineering accidents such as large deformation and collapse after tunnel excavation. Summary of the Invention

[0003] The main objective of this invention is to propose a prestressed support method for soft rock tunnels, which aims to improve the construction safety of soft rock tunnels.

[0004] To achieve the above objectives, the present invention proposes a prestressed support method for soft rock tunnels, comprising:

[0005] The surrounding rock of the soft rock tunnel is divided into a surface reinforcement layer of 0-0.5m, an intermediate transition layer of 0.5-1.5m, and a deep anchoring layer of 1.5-3m according to radial depth.

[0006] Pre-treat the surface reinforcement layer;

[0007] Preprocess the intermediate transition layer;

[0008] Pre-treat the deep anchoring layer;

[0009] Multi-directional prestress is applied to the surface reinforcement layer, the intermediate transition layer, and the deep anchoring layer respectively to complete the prestressed support construction of the soft rock tunnel.

[0010] In one embodiment, the step of pretreating the surface reinforcement layer includes:

[0011] Grouting holes are drilled to a depth of 0.3 to 0.5 m at a circumferential spacing of 1.5 m and a longitudinal spacing of 2 m in the surface reinforcement layer to form the first grouting channel;

[0012] Grout A is injected into the surface reinforcement layer using the first grouting channel at a grouting pressure of 0.2 to 0.5 MPa and a penetration time of at least 30 minutes.

[0013] In one embodiment, the A slurry comprises 100 parts by weight of silicate cement, 20 parts by weight of water glass, and 5 parts by weight of micro-expansion agent.

[0014] In one embodiment, the step of injecting grout A into the surface reinforcement layer using the first grouting channel at a grouting pressure of 0.2–0.5 MPa and a penetration time of at least 30 minutes includes:

[0015] The first injection of 60% of the total amount of grout A into the first grouting channel;

[0016] Let it stand for 10 minutes, and tap the wall of the first grouting channel;

[0017] Inject 40% of the total amount of grout A into the first grouting channel again.

[0018] In one embodiment, the step of preprocessing the intermediate transition layer includes:

[0019] Anchor bolt holes are drilled in the intermediate transition layer at a circumferential spacing of 2m and a longitudinal spacing of 3m, and composite anchor bolts are inserted.

[0020] B grout is injected into the intermediate transition layer using the anchor bolt holes at a grouting pressure of 0.8 to 1.2 MPa.

[0021] In one embodiment, the B slurry comprises 100 parts cement, 3 parts quick-setting agent and 1 part water-reducing agent by mass.

[0022] In one embodiment, the composite anchor bolt forms a radial angle of 15° to 25° with the soft rock tunnel.

[0023] In one embodiment, the steps of applying multi-directional prestress to the surface reinforcement layer, the intermediate transition layer, and the deep anchoring layer to complete the prestressed support construction of the soft rock tunnel include:

[0024] Radial prestress, tangential prestress, and axial prestress are simultaneously applied to the surface reinforcement layer, the intermediate transition layer, and the deep anchoring layer to complete the prestressed support construction of the soft rock tunnel.

[0025] Wherein, the tension ratio satisfies the following: the tension of the tangential prestress is the tension of the radial prestress * C, 0.6≤D≤0.8; the tension of the axial prestress is the tension of the radial prestress * E, 0.4≤E≤0.6.

[0026] In one embodiment, the steps of applying multi-directional prestress to the surface reinforcement layer, the intermediate transition layer, and the deep anchoring layer to complete the prestressed support construction of the soft rock tunnel include:

[0027] Circumferential prestressed tendons are arranged every 2 to 3 meters along the axial direction of the soft rock tunnel, and each circumferential prestressed tendon includes 6 steel strands;

[0028] The circumferential prestressed tendon is tensioned in one stage until the prestress value of the circumferential prestressed tendon reaches the preset value of 30%, and the load is held for 5 minutes.

[0029] The circumferential prestressed tendon is subjected to secondary tensioning until the prestress value of the circumferential prestressed tendon reaches 70% of the preset value, and the deformation F of the surrounding rock is measured.

[0030] When F≤2mm, the circumferential prestressed tendon is tensioned in three stages until the prestress value of the circumferential prestressed tendon reaches 100% of the preset value, thus completing the prestressed support construction of the soft rock tunnel.

[0031] In one embodiment, the steps of applying multi-directional prestress to the surface reinforcement layer, the intermediate transition layer, and the deep anchoring layer to complete the prestressed support construction of the soft rock tunnel include:

[0032] Longitudinal prestressing tendons are applied to the surface reinforcement layer, the intermediate transition layer and the deep anchoring layer respectively. The longitudinal prestressing tendons include 4 to 6 steel strands with a circumferential spacing of 4 to 6 m.

[0033] Tension is applied simultaneously at both ends of the longitudinal prestressed tendon, with the tension value being 70% ± 5% of the radial tension value and the tensioning speed being ≤ 50 kN / min.

[0034] The technical solution of this invention solves the problem of coordinated control of layered deformation in soft rock tunnels by dividing the surrounding rock into a surface reinforcement layer, an intermediate transition layer, and a deep anchoring layer for differentiated pretreatment and applying radial, tangential, and axial multi-directional prestress. Surface grouting seals cracks to prevent local collapse, intermediate anchoring enhances shear resistance and suppresses shear failure, and deep prestressed anchor cables balance ground stress to prevent large deformations. The three layers form a progressive support structure, enabling the shallow fractured zone, intermediate plastic zone, and deep elastic zone of the surrounding rock to bear load collaboratively, significantly improving the overall stability of the support system, achieving control over the layered deformation characteristics of soft rock tunnels, and effectively coordinating the three-dimensional stress state of the surrounding rock and preventing stress concentration, thereby improving the construction safety of soft rock tunnels. Attached Figure Description

[0035] 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.

[0036] Figure 1 This is a flowchart illustrating an embodiment of the prestressed support method for soft rock tunnels provided by the present invention.

[0037] 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

[0038] 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.

[0039] It should be noted that if the embodiments of the present invention involve directional indicators (such as up, down, left, right, front, back, etc.), the directional indicators 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 indicators will also change accordingly.

[0040] 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.

[0041] In soft rock tunnel engineering, support technology has always been a key challenge due to the low strength, large deformation, and poor stability of the surrounding rock. Soft rock typically has low uniaxial compressive strength (generally less than 25 MPa), large deformation modulus, and significant rheological properties, making it prone to large deformations and collapses after tunnel excavation. Traditional support methods often employ single grouting or anchor bolt support, which is insufficient to effectively control the layered deformation characteristics of soft rock. Especially in deeply buried soft rock tunnels, the radial stress distribution of the surrounding rock is uneven, and the deformation characteristics of the surface, intermediate, and deep layers differ significantly. Conventional support methods cannot achieve differentiated reinforcement for surrounding rock at different depths. Furthermore, existing prestressing methods are mostly unidirectional or simple combinations, failing to fully coordinate the triaxial stress state of the surrounding rock, leading to incoordination between the support structure and the surrounding rock deformation, and easily causing stress concentration and localized failure. These problems seriously affect the construction safety and long-term stability of soft rock tunnels.

[0042] To address this technical problem, this invention proposes a prestressed support method for soft rock tunnels.

[0043] Please see Figure 1 In one embodiment of the present invention, the prestressed support method for soft rock tunnels includes:

[0044] Step S10: Divide the surrounding rock of the soft rock tunnel into a surface reinforcement layer of 0-0.5m, an intermediate transition layer of 0.5-1.5m, and a deep anchoring layer of 1.5-3m according to radial depth.

[0045] Step S20: Pre-process the surface reinforcement layer;

[0046] Step S30: Preprocess the intermediate transition layer;

[0047] Step S40: Pre-process the deep anchoring layer;

[0048] Step S50: Apply multi-directional prestress to the surface reinforcement layer, the intermediate transition layer and the deep anchoring layer respectively to complete the prestressed support construction of the soft rock tunnel.

[0049] Radial depth division refers to determining the reinforcement range based on the stress distribution gradient of the surrounding rock. For example, the surface reinforcement layer corresponds to the excavation disturbance zone, the intermediate transition layer corresponds to the plastic deformation zone, and the deep anchoring layer corresponds to the elastic deformation zone. Pretreatment includes processes such as grouting and anchoring to improve the structural integrity of each layer of surrounding rock. Multi-directional prestressing includes radial, tangential, and axial components, applying constraint forces in different directions through tensioning equipment.

[0050] Specifically, the surface reinforcement layer seals surface cracks through grouting, the intermediate transition layer enhances shear resistance with anchor bolts, and the deep anchoring layer controls deep deformation with prestressed anchor cables. During construction, the surface layer is first permeated with grout to form a load-bearing shell, then composite anchor bolts are installed in the intermediate layer to form a shear-resistant layer, and finally, prestressed tendons are arranged in the deep layer to form a restraint layer. The three levels of pretreatment, combined with multi-directional prestressing, work synergistically to form a gradient-decreasing support system.

[0051] Compared with existing technologies, traditional methods, which use grouting layers of uniform thickness or single anchor bolt arrangements, cannot adapt to the mechanical properties of surrounding rock at different depths. This solution achieves coordinated control of stress redistribution and deformation by employing layered differentiated treatment: surface grouting to prevent spalling, middle-layer anchoring to suppress shear slip, and deep-layer prestressing to control creep.

[0052] Through the above technical solution, this application solves the problem of coordinated control of layered deformation in soft rock tunnels by dividing the surrounding rock into a surface reinforcement layer, an intermediate transition layer, and a deep anchoring layer for differentiated pretreatment and applying radial, tangential, and axial multi-directional prestress. Surface grouting seals cracks to prevent local collapse, intermediate anchoring enhances shear resistance and suppresses shear failure, and deep prestressed anchor cables balance ground stress to prevent large deformations. The three layers form a progressive support structure, enabling the shallow fractured zone, intermediate plastic zone, and deep elastic zone of the surrounding rock to bear load collaboratively, significantly improving the overall stability of the support system, achieving control over the layered deformation characteristics of soft rock tunnels, effectively coordinating the three-dimensional stress state of the surrounding rock, preventing stress concentration, and improving the construction safety of soft rock tunnels.

[0053] In an embodiment of the present invention, the step of pre-treating the surface reinforcement layer includes:

[0054] Step S21: Drill grouting holes to a depth of 0.3 to 0.5 m at a circumferential spacing of 1.5 m and a longitudinal spacing of 2 m in the surface reinforcement layer to form the first grouting channel;

[0055] Step S22: Inject grout A into the surface reinforcement layer using the first grouting channel at a grouting pressure of 0.2 to 0.5 MPa and a penetration time of at least 30 minutes.

[0056] The circumferential spacing of 1.5m and the longitudinal spacing of 2m refer to the distribution density of grouting holes in the tunnel's circumference and axial direction. This can be achieved using a laser positioning device in conjunction with drilling machinery. This spacing ensures grout coverage while avoiding excessive hole density that could damage the rock mass. The grouting hole depth of 0.3–0.5m refers to the critical value at which the borehole penetrates the loose surface rock layer without disturbing the intermediate transition layer. This can be achieved using a drilling rig equipped with a depth sensor. This depth range ensures effective grout reinforcement of the surface layer while avoiding damage to the deep surrounding rock structure. The grouting pressure of 0.2–0.5MPa refers to the dynamic equilibrium pressure required for grout diffusion in micro-fractures. This can be adjusted using a variable frequency grouting pump. This pressure range drives sufficient grout penetration while preventing excessive pressure that could cause rock fracturing. The penetration time of at least 30 minutes refers to the minimum duration required for the grout to complete filling and initial consolidation in the pores. This can be controlled by a grouting flow monitoring system. This time threshold ensures that the grout fully fills the fractures and forms a continuous reinforcement network.

[0057] Specifically, by rationally setting the spacing and depth of grouting holes, uniformly distributed grouting channels are formed in the surface reinforcement layer. A staged grouting method is adopted. After the initial injection of 60% grout, the hole is allowed to stand for 10 minutes and the hole wall is tapped to promote the grout's diffusion into the depths of the fractures under the action of gravity and vibration. The second injection of the remaining 40% grout fills the voids formed after the initial infiltration, preventing localized unfilled areas due to premature grout solidification. During this process, the micro-expansion agent continuously acts within the A grout, ensuring a tight bond between the consolidated body and the surrounding rock, forming a uniform load-bearing structure.

[0058] Compared to existing technologies, traditional surface grouting often employs a single high-pressure grouting method, which can easily lead to rapid solidification of the grout in the shallow layer, preventing it from penetrating deep into the fractures. This solution, through multi-stage grouting combined with intermittent hammering, utilizes the synergistic effect of the grout's own weight and mechanical vibration to significantly improve the uniformity of grout penetration into micro-fractures. While existing technologies typically use equidistant grouting hole spacing, this solution dynamically adjusts the hole density according to the degree of fracturing in the surface rock mass, making it more suitable for the heterogeneous characteristics of soft rock.

[0059] Through the above technical solution, this application effectively solves the problem of local collapse caused by the development of surface fissures in soft rock, and the resulting uniform load-bearing body significantly improves the overall stability of the surface reinforcement layer. The staged grouting process avoids weak areas caused by uneven grout solidification, and the micro-expansion characteristics ensure a tight bond between the solidified grout and the surrounding rock, providing a reliable foundation for subsequent prestressing application.

[0060] In an embodiment of the present invention, the A slurry comprises 100 parts by mass of silicate cement, 20 parts by mass of water glass and 5 parts by mass of micro-expansion agent.

[0061] Among them, silicate cement refers to a hydraulic cementitious material with calcium silicate as its main component, specifically ordinary silicate cement or slag silicate cement, which serves as the basic bonding material for the grout and provides the main strength support for the surrounding rock. Water glass refers to an aqueous solution of alkali metal silicates, specifically sodium water glass or potassium water glass, which acts as a quick-setting agent to regulate the setting time of the grout and enhance its permeability in fissures. Micro-expansion agents are materials that expand in volume during hydration, specifically calcium sulfoaluminate or magnesium oxide-based expansion agents, used to compensate for the shrinkage after the grout solidifies, ensuring the compactness of the filled fissures.

[0062] Specifically, when silicate cement is mixed with water glass, the alkaline environment of the water glass accelerates the cement hydration reaction, shortens the initial setting time of the grout, and prevents the grout from being lost due to prolonged lack of solidification during the infiltration process. The micro-expansion agent generates moderate volume expansion during the grout solidification stage, offsetting the inherent shrinkage effect of cement-based materials, thereby reducing the voids between the grout and the surrounding rock, and enhancing the integrity of the surface reinforcement layer. Through the synergistic effect of these three components, the grout can effectively fill the micro-cracks on the surface of the surrounding rock and form a uniform load-bearing body, preventing surface spalling of soft rock due to stress release.

[0063] Compared to existing technologies, traditional grouting materials typically use only cement-based grout, which presents a challenge in balancing setting speed and permeability, and is prone to shrinkage and secondary cracking after curing. This solution introduces water glass and a micro-expansion agent, which shortens the setting time while ensuring the fluidity of the grout, and eliminates shrinkage defects through the expansion effect, significantly improving the deformation resistance of the surface reinforcement layer.

[0064] Through the above technical solution, this application can effectively solve the problem of reinforcement failure caused by uneven grout penetration or solidification shrinkage of the surrounding rock of soft rock tunnels, ensure that the grouting layer is tightly bonded to the surrounding rock, form a stable and uniform load-bearing body, and suppress the initial deformation of soft rock tunnels.

[0065] In an embodiment of the present invention, the step of injecting grout A into the surface reinforcement layer using the first grouting channel at a grouting pressure of 0.2 to 0.5 MPa and a penetration time of at least 30 minutes includes:

[0066] Step S221: 60% of the total amount of grout A is injected into the first grouting channel for the first time;

[0067] Step S222: Let stand for 10 minutes and tap the wall of the first grouting channel;

[0068] Step S223: Inject 40% of the total amount of grout A into the first grouting channel again.

[0069] In an embodiment of the present invention, the step of preprocessing the intermediate transition layer includes:

[0070] Step S31: Drill anchor bolt holes in the intermediate transition layer at a circumferential spacing of 2m and a longitudinal spacing of 3m, and insert composite anchor bolts.

[0071] Step S32: Inject grout B into the intermediate transition layer using the anchor bolt hole at a grouting pressure of 0.8 to 1.2 MPa.

[0072] The circumferential spacing of 2 meters refers to the distance between adjacent anchor holes along the tunnel circumference. For example, a laser positioning device can be used for hole placement to ensure uniform stress on the support system. The longitudinal spacing of 3 meters refers to the distance between adjacent anchor holes along the tunnel axis. This can be achieved using a segmented marking method to ensure the anchor group forms a spatial network support structure. The 95:5 mass ratio of the steel core to the outer resin bonding layer of the composite anchor means that the steel core accounts for 95% of the total mass of the composite. For example, using 25 mm diameter threaded steel as the core, with a 1.5 mm thick epoxy resin layer coated on the surface through a hot-melt process, this ratio ensures both the tensile strength of the anchor and improves its bonding performance with the surrounding rock. The grouting pressure of 0.8-1.2 MPa refers to the pressure range applied when injecting grout into the hole. For example, a staged booster pump can be used to achieve precise pressure control. This pressure range ensures that the grout fully fills the rock fissures while avoiding excessive pressure that could cause the surrounding rock to split.

[0073] Specifically, during the construction of the intermediate transition layer, drilling is first carried out at predetermined intervals, for example, using a hydraulic drilling rig at a speed of 15 revolutions per minute to form anchor holes with a diameter of 42 mm. Then, the composite anchor is inserted into the hole. During installation, it is slowly screwed in using a rotary propulsion device at a speed not exceeding 2 revolutions per minute, allowing the resin bonding layer to evenly coat the steel core surface under the action of frictional heat. After the anchor installation is completed, grout B is injected into the hole in two stages through the grouting pipeline. For example, after injecting 60% of the total amount, it is allowed to stand for 10 minutes to allow the grout to initially penetrate before injecting the remaining 40%. This staged grouting method avoids the problem of incomplete filling caused by premature solidification of the grout.

[0074] Compared to existing technologies, traditional anchor bolt support often involves directly inserting a single metal rod and then grouting, which can easily lead to anchoring failure due to insufficient bonding between the rod and the grout. This solution, however, utilizes a composite anchor bolt structure design to form a resin bonding transition layer on the steel core surface. For example, during anchor bolt installation, low-speed rotation allows the resin layer to mechanically interlock with the borehole wall, significantly improving the coordinated deformation capability between the anchor bolt and the surrounding rock. Simultaneously, the staged grouting process overcomes the defects of single-stage grouting, such as porosity or segregation. For instance, after the initial grouting, the grout is allowed to naturally seep into micro-cracks under gravity, while the secondary grouting focuses on filling structurally weak areas.

[0075] Through the above technical solutions, this application effectively improves the integrity and deformation resistance of the intermediate transition layer. The resin bonding layer of the composite anchor bolt can generate elastic deformation buffer when the surrounding rock undergoes minor displacement, avoiding stress concentration caused by direct contact between the steel core and the rock mass. The dense grout network formed by the staged grouting process can uniformly transmit prestress and prevent stress imbalance caused by uneven grout penetration in local areas. The spatial arrangement of the anchor bolt group enables the support system to form a three-dimensional stress frame, significantly suppressing the rheological deformation trend of soft rock.

[0076] In addition, composite anchor bolts refer to rods composed of a steel core and an outer resin bonding layer, with a mass ratio of 95:5. This can be achieved by using a hot-melt extrusion process to uniformly coat the steel core surface with resin, with the resin layer thickness controlled within the range of 1-2 mm. The circumferential spacing of 2 m and the longitudinal spacing of 3 m refer to the arrangement density of the anchor bolts in the circumferential and axial directions of the tunnel. Precise hole placement can be achieved using a laser positioning device in conjunction with drilling machinery. The grouting pressure of 0.8-1.2 MPa refers to the output pressure range of the grout pumping system, which can be dynamically adjusted using a hydraulic servo control system.

[0077] Specifically, during the construction of the intermediate transition layer, anchor holes are first drilled at predetermined intervals, and composite anchors are installed to the designed depth using a rotary advance method. During this process, the synergistic effect of the steel core and the outer resin layer forms a dual anchoring mechanism: the steel core provides the main tensile strength, while the resin layer absorbs energy through viscoelastic deformation when the surrounding rock deforms. Subsequently, pressure grouting is implemented through the anchor holes. Grout B, under specific pressure, penetrates and fills the rock fissures, forming a mesh-like reinforcement structure. The grouting pressure is controlled within the range of 0.8–1.2 MPa to ensure effective grout diffusion while avoiding excessive pressure that could cause rock fracturing.

[0078] Compared to existing technologies, traditional anchor bolt support often uses a single metal rod for direct grouting, which suffers from insufficient interfacial adhesion and stress concentration. This solution utilizes a composite anchor bolt structure design, where the resin layer is rotated and compressed during installation to form a uniform coating, effectively improving the stress transfer path between the steel core and the surrounding rock. The step-by-step grouting process, compared to conventional one-time grouting, enhances the uniformity of grout penetration into fractured rock masses.

[0079] Through the above technical solution, this application achieves a synergistic reinforcement effect of the intermediate transition layer. The resin layer of the composite anchor bolt plays a role in the early stage of surrounding rock deformation, delaying the time when the peak stress of the steel core appears. The reinforcement network formed by pressure grouting and the anchor bolt system form a spatial coupling effect, significantly improving the shear deformation resistance of the transition layer and effectively suppressing the gradual collapse phenomenon common in soft rock tunnels.

[0080] In an embodiment of the present invention, the B slurry comprises 100 parts cement, 3 parts quick-setting agent and 1 part water-reducing agent by mass.

[0081] Cement refers to the hydraulic cementitious material used as the matrix material of the grout, specifically silicate cement, which provides structural strength after the grout has cured. Accelerator is an additive used to shorten the grout setting time, specifically aluminate compounds, used to accelerate the curing process after grouting the intermediate transition layer and prevent grout loss. Water-reducing agent is an admixture used to improve the fluidity of the grout, specifically polycarboxylate polymers, used to reduce the water-cement ratio of the grout and improve the uniformity of grout penetration within the anchor bolt holes.

[0082] Specifically, during the pretreatment of the intermediate transition layer, cement, accelerator, and water-reducing agent are mixed in a specific ratio to form grout B. This ratio balances the grout's fluidity and setting speed. When grout B is injected into the anchor bolt holes at a grouting pressure of 0.8–1.2 MPa, the water-reducing agent ensures that the grout fully fills the surrounding rock fissures, while the accelerator promotes rapid hardening of the grout within a predetermined time, forming a stable reinforcement structure in conjunction with the composite anchor bolt. This ratio design ensures that the grout maintains fluidity during penetration while preventing uncontrolled deformation of the surrounding rock due to slow setting.

[0083] Compared with existing technologies, traditional grouting materials often suffer from insufficient grout penetration depth or premature solidification due to an imbalance in the ratio of accelerator and water-reducing agent. However, this solution ensures effective diffusion of the grout under high-pressure grouting by precisely controlling the addition ratio of accelerator and water-reducing agent, while also shortening the curing waiting time and significantly improving the reinforcement efficiency of the intermediate transition layer.

[0084] Through the above technical solution, this application solves the problem of uneven grout penetration and mismatch between curing speed during the grouting process of the intermediate transition layer in soft rock tunnels. By optimizing the grout composition ratio, it ensures that the surrounding rock fissures are fully filled and a load-bearing structure is quickly formed, thereby effectively suppressing surrounding rock deformation and improving the overall stability of the support system.

[0085] In an embodiment of the present invention, the composite anchor rod forms a radial angle of 15° to 25° with the soft rock tunnel.

[0086] Composite anchor bolts refer to anchor bolts composed of a steel core and an outer resin bonding layer, with a steel core to resin layer mass ratio of 95:5. This can be achieved using a hot-melt resin coating process, where a resin layer is pre-coated onto the steel core surface to form a composite structure. The 15°–25° included angle refers to the range of inclination angles between the anchor bolt axis and the tunnel radial direction. This angle can be adjusted using a drilling guide device, balancing the tangential anchoring force and radial support force of the anchor bolt. Rotary propulsion refers to the process of simultaneously rotating and advancing the anchor bolt axially during installation. This can be achieved using hydraulic rotary propulsion equipment, applying rotational torque and propulsion force simultaneously to ensure uniform distribution of the resin layer. The rotation speed is controlled to no more than 2 revolutions per minute, which refers to the upper limit of the anchor bolt's rotational angular velocity. This can be achieved using a speed control valve or a variable frequency motor. Low-speed rotation prevents uneven distribution of the resin layer due to centrifugal force.

[0087] Specifically, during the construction of the intermediate transition layer, anchor holes are first drilled at predetermined circumferential and longitudinal intervals. Then, composite anchors are inserted into the holes at an angle of 15° to 25°. During installation, the anchors are driven by a rotary propulsion device, rotating at a speed not exceeding 2 revolutions per minute while simultaneously penetrating the borehole. Under the rotational shearing action, the resin layer evenly coats the surface of the steel core and makes full contact with the surrounding rock borehole wall. This angle range allows the anchors to disperse shear stress through the tilt angle when subjected to surrounding rock deformation, while also utilizing the frictional bonding between the resin layer and the rock mass to form a composite load-bearing structure.

[0088] Compared to existing technologies, traditional anchor installation often employs vertical drilling or a single insertion method, which can easily lead to uneven resin distribution or angle mismatch causing stress concentration. In existing technologies, excessively high anchor rotation speeds may damage the integrity of the resin layer, while excessively large angles weaken the radial support effect. This solution, by limiting the tilt angle and rotation speed, ensures a continuous and uniform resin coating layer, while optimizing the anchor's stress direction and enhancing the overall collaborative load-bearing capacity of the intermediate transition layer.

[0089] Through the above technical solution, this application solves the problem of interface peeling in the anchoring structure of the intermediate transition layer in soft rock tunnels, achieves reliable bonding between the resin layer and the steel core and surrounding rock interface, and enhances the adaptability of the anchor system to surrounding rock deformation. By combining angle control and rotation speed, the risk of stress concentration at the anchor ends is effectively reduced, and the long-term stability of the support structure under complex stress conditions is improved.

[0090] In an embodiment of the present invention, the steps of applying multi-directional prestress to the surface reinforcement layer, the intermediate transition layer, and the deep anchoring layer to complete the prestressed support construction of the soft rock tunnel include:

[0091] Step S51: Simultaneously apply radial prestress, tangential prestress, and axial prestress to the surface reinforcement layer, the intermediate transition layer, and the deep anchoring layer to complete the prestressed support construction of the soft rock tunnel.

[0092] Wherein, the tension ratio satisfies the following: the tension of the tangential prestress is the tension of the radial prestress * C, 0.6≤D≤0.8; the tension of the axial prestress is the tension of the radial prestress * E, 0.4≤E≤0.6.

[0093] Radial prestress refers to compressive stress perpendicular to the tunnel wall, which can be applied using hydraulic jacks or prestressed anchors to suppress radial deformation of the surrounding rock. Tangential prestress refers to tensile stress distributed along the circumference of the tunnel, which can be achieved by tensioning circumferential prestressing tendons to balance tangential stress concentration in the surrounding rock. Axial prestress refers to compressive stress distributed along the longitudinal direction of the tunnel, which can be achieved by synchronous tensioning of longitudinal prestressing tendons to constrain axial creep of the surrounding rock. The coefficients C, D, and E in the tension ratio refer to the mechanical equilibrium parameters between prestresses in different directions, which can be determined through numerical simulation or field tests to avoid instability of the support structure caused by excessive stress in one direction.

[0094] Specifically, during construction, radial prestress is transmitted through the contact surface between the anchor bolts or grouting body and the surrounding rock; tangential prestress is formed by tensioning circumferential steel strands to create a continuous constraint ring; and axial prestress is formed by synchronous tensioning of longitudinal steel strands to create a longitudinal constraint chain. By applying multi-directional prestress in stages—for example, first applying 30% of the radial prestress, and then adjusting the ratio of tangential and axial prestress according to the deformation of the surrounding rock—the stresses in each direction can work synergistically. During the load-bearing stage, the tension force is dynamically adjusted by monitoring the deformation of the surrounding rock; for example, when the tangential deformation exceeds a threshold, the tangential prestress is increased by the upper limit of the proportional coefficient D.

[0095] Compared with existing technologies, traditional support methods typically apply prestress in only one direction, resulting in uneven stress distribution in the surrounding rock and easily leading to local cracking or overall instability. This solution, however, achieves three-dimensional equilibrium of the surrounding rock stress field through the simultaneous application and proportional control of multi-directional prestress. For example, in the rheological stage of soft rock, the introduction of axial prestress can effectively suppress longitudinal shrinkage deformation, while the gradient loading of tangential prestress can prevent the propagation of circumferential cracks.

[0096] Through the above technical solution, this application can form a spatial network prestressed system, enabling the surface reinforcement layer, intermediate transition layer, and deep anchoring layer to form a synergistic load-bearing structure. By dynamically adjusting the proportional coefficient, it can adapt to the deformation characteristics of the surrounding rock under different geological conditions. For example, a lower axial prestress ratio can be used in soft rock with high water content to prevent grout seepage damage, while the tangential prestress ratio can be increased in strata with well-developed joints to enhance circumferential restraint. This solution controls large deformations of the surrounding rock while reducing the risk of failure of the support structure due to stress concentration.

[0097] In an embodiment of the present invention, the steps of applying multi-directional prestress to the surface reinforcement layer, the intermediate transition layer, and the deep anchoring layer to complete the prestressed support construction of the soft rock tunnel include:

[0098] Step S501: Arrange circumferential prestressed tendons every 2 to 3 m along the axial direction of the soft rock tunnel, and each circumferential prestressed tendon includes 6 steel strands.

[0099] Step S502: Perform primary tensioning on the circumferential prestressed tendon until the prestress value of the circumferential prestressed tendon reaches the preset value of 30%, and hold the load for 5 minutes.

[0100] Step S503: Perform secondary tensioning on the circumferential prestressed tendon until the prestress value of the circumferential prestressed tendon reaches 70% of the preset value, and measure the deformation F of the surrounding rock;

[0101] Step S504: When F≤2mm, the circumferential prestressed tendon is tensioned in three stages until the prestress value of the circumferential prestressed tendon reaches 100% of the preset value, thus completing the prestressed support construction of the soft rock tunnel.

[0102] Among them, circumferential prestressed strands refer to prestressed steel strand bundles spaced along the tunnel axis. Specifically, multiple high-strength steel strands can be combined to form a circumferential constraint structure, gradually enhancing the circumferential bearing capacity of the surrounding rock through staged tensioning. The selection of steel strand diameter and tensile strength parameters can balance material cost and mechanical performance requirements. For example, 15.2mm diameter steel strands can provide sufficient tensile strength while avoiding excessively increasing construction difficulty. In the multi-stage tensioning process, the first stage tensioning is used to initially establish the prestressed frame; the second stage tensioning, combined with deformation monitoring, can dynamically adjust the tensioning process; and the third stage tensioning applies the final prestress after confirming the stability of the surrounding rock, releasing the creep stress of the surrounding rock during the holding stage. The threshold setting of the deformation amount F is based on the deformation characteristics of soft rock. For example, a critical value of 2mm can effectively identify whether the surrounding rock is in a controllable deformation stage.

[0103] Specifically, circumferential prestressed tendons are installed at regular intervals (e.g., 2-3 m) along the tunnel axis, each tendon consisting of multiple steel strands (e.g., 6 strands). The tensioning process is divided into three stages: first, an initial prestress of 30% of the preset value is applied and the load is maintained to induce initial compression of the surrounding rock; then, the load is increased to 70% of the preset value, and the deformation is monitored in real time. If the deformation does not exceed the threshold, the entire tensioning process continues; if the deformation exceeds the threshold, construction must be suspended and reinforcement measures must be taken. The setting of the load-bearing stage allows the surrounding rock to gradually adjust its internal stress distribution under the action of prestress, avoiding localized damage caused by sudden loading.

[0104] In some specific implementations, low-relaxation materials can be used for the steel strands to reduce prestress loss, and the tensioning equipment can be equipped with a synchronous control system to ensure uniform stress distribution across multiple steel strands. Deformation measurement can be performed using a laser rangefinder or strain gauge, and the data acquisition frequency can be set to once per second for real-time monitoring.

[0105] Compared with existing technologies, traditional prestressed support often uses a one-time tensioning method, which is prone to prestress loss or surrounding rock overload failure due to the rheological properties of soft rock. This solution uses staged tensioning combined with deformation monitoring to dynamically assess the surrounding rock response during the application of prestress, which can effectively control deformation development and avoid structural damage caused by premature application of the full load.

[0106] Through the above technical solution, this application enables precise application of prestress during soft rock tunnel support. By combining staged loading with real-time monitoring, the risk of sudden large deformations in the surrounding rock is significantly reduced, while simultaneously improving the uniformity of prestress distribution. This solution is particularly suitable for weak surrounding rock formations with significant rheological properties, optimizing construction efficiency while ensuring support effectiveness.

[0107] In an embodiment of the present invention, the steps of applying multi-directional prestress to the surface reinforcement layer, the intermediate transition layer, and the deep anchoring layer to complete the prestressed support construction of the soft rock tunnel include:

[0108] Step S510: Apply longitudinal prestressing tendons to the surface reinforcement layer, the intermediate transition layer and the deep anchoring layer respectively. The longitudinal prestressing tendons include 4 to 6 steel strands with a circumferential spacing of 4 to 6 m.

[0109] In step S520, tension is applied simultaneously at both ends of the longitudinal prestressed tendon. The tension value is 70% ± 5% of the radial tension value, and the tensioning speed is ≤ 50 kN / min.

[0110] In this context, longitudinal prestressed tendons refer to prestressed structures arranged along the tunnel axis. These can be achieved by combining multiple steel strands to form a bundle-like structure, applying axial tension to enhance the longitudinal integrity of the surrounding rock. Simultaneous tension application means tensioning the prestressed tendons at both ends simultaneously. This can be achieved using hydraulic jacks in conjunction with a synchronous control system to avoid uneven stress distribution caused by unilateral tensioning. Tensioning speed refers to the increment of tension applied per unit time, which can be controlled by adjusting the flow rate of the hydraulic pump to prevent excessively rapid loading that could cause instantaneous deformation of the surrounding rock.

[0111] Specifically, longitudinal prestressed strands are arranged along the tunnel axis, and the circumferential spacing is adjusted according to the surrounding rock conditions. The number and spacing of steel strands are matched to form continuous support. During tensioning, tension is applied simultaneously at both ends, and the tension value is dynamically adjusted based on the radial tension force to ensure the synergistic effect of axial and radial prestress. The tensioning speed is precisely controlled by a hydraulic system to avoid excessive speed causing impact and localized damage to the surrounding rock.

[0112] Compared with existing technologies, traditional longitudinal prestressing construction often uses unilateral tensioning or fixed-speed loading, which can easily lead to uneven stress distribution or impact deformation of the surrounding rock. This scheme, through synchronous tensioning and speed control, ensures that the axial prestress is evenly transferred to each reinforced layer, reducing the risk of stress concentration.

[0113] Through the above technical solution, this application achieves uniform distribution of longitudinal prestress in soft rock tunnels, effectively suppresses the rheological deformation of the surrounding rock along the tunnel axis, improves the overall stability of the support system, and reduces the risk of local collapse caused by longitudinal stress imbalance.

[0114] Based on the above embodiments, a specific implementation method is shown here for ease of understanding:

[0115] The prestressed support method for soft rock tunnels proposed in this invention comprises three main technical steps: a layered progressive surrounding rock pretreatment step, a multi-directional synergistic prestressing application step, and a dynamic control and maintenance step. These three steps are executed sequentially to form a complete support system. The layered progressive surrounding rock pretreatment step creates conditions for subsequent prestressing application, the multi-directional synergistic prestressing application step establishes the main support structure, and the dynamic control and maintenance step ensures long-term stability.

[0116] Detailed description of the steps for layered and progressive surrounding rock pretreatment:

[0117] The layered, progressive surrounding rock pretreatment step is the foundation of the entire support method. This step lays the groundwork for the effective operation of the subsequent prestressed system by pretreating the soft rock in layers. After the soft rock tunnel excavation is completed, a detailed geological condition assessment of the tunnel cross-section is first conducted to identify the distribution of weak interlayers, joints, fissures, and rock strata with different degrees of weathering in the surrounding rock.

[0118] The pretreatment process begins at the tunnel inner wall, dividing the surrounding rock into three treatment layers according to radial depth: a surface reinforcement layer, an intermediate transition layer, and a deep anchoring layer. The surface reinforcement layer, located within 0.5 meters of the tunnel inner wall, is the critical area directly bearing the prestressing load. A progressive grouting reinforcement method is used for the surface reinforcement layer. First, grouting holes are arranged around the tunnel perimeter in a grid pattern with a circumferential spacing of 1.5 meters and a longitudinal spacing of 2 meters, with the hole depth controlled between 0.3 and 0.5 meters. The grouting process employs a low-pressure, slow-speed injection method, with the grouting pressure controlled between 0.2 and 0.5 MPa to ensure the grout fully penetrates the micro-fractures in the surrounding rock, improving its integrity and bearing capacity.

[0119] The intermediate transition layer is located 0.5-1.5 meters from the inner wall of the tunnel. Its main function is to transfer and disperse stress from the surface layer, preventing stress concentration. A segmented anchor bolt pre-embedding method is used to treat the intermediate transition layer. After surface grouting reinforcement, anchor bolt holes are installed along the tunnel perimeter at 2-meter circumferential and 3-meter longitudinal intervals. The anchor bolts are 1.2-1.8 meters long, and the angle between the anchor bolt and the tunnel radial direction is controlled between 15-25 degrees. During anchor bolt installation, the anchor bolt is first inserted into the pre-drilled hole, followed by secondary grouting to ensure a good bonding interface between the anchor bolt and the surrounding rock.

[0120] The deep anchorage layer is located 1.5-3 meters from the tunnel wall and forms the fundamental support for the entire prestressed support system. The deep anchorage layer is treated using a long anchor group anchoring method, with anchor lengths ranging from 2.5 to 4 meters, arranged at 3-meter circumferential and 4-meter longitudinal intervals. The deep anchors are installed in a multi-directional arrangement, including radial anchors and inclined anchors at 30-45 degrees, forming a three-dimensional anchorage network. After the deep anchors are installed, the anchorage area is reinforced by high-pressure grouting, with the grouting pressure controlled between 1-2 MPa to ensure a stable load-bearing structure in the deep surrounding rock.

[0121] Detailed steps for applying multi-directional synergistic prestress:

[0122] The multi-directional coordinated prestressing application step is the core technical aspect of this invention. Based on the pretreatment of the surrounding rock, this step applies prestress to the tunnel surrounding rock through an innovative multi-directional coordinated method, forming a stable support system. The prestressing application process employs a three-dimensional prestressing system with radial dominance, tangential assistance, and axial coordination.

[0123] Radial prestressing is the main component of the entire prestressing system, achieved by installing circumferential prestressing strands along the tunnel wall. These circumferential prestressing strands are arranged along the tunnel axis at intervals of 2-3 meters. Each strand consists of multiple prestressed steel strands distributed in a ring around the tunnel perimeter. The tensioning process employs a segmented symmetrical tensioning method. The tunnel cross-section is first divided into four quadrants, and tensioning is performed simultaneously within each quadrant to ensure uniform distribution of prestress. The tension force is determined based on the geological conditions of the surrounding rock and design requirements, generally controlled between 100-200 kN per steel strand.

[0124] Tangential prestressing is achieved through longitudinal prestressing tendons, which are arranged along the tunnel's axial direction to constrain longitudinal deformation. The longitudinal prestressing tendons are arranged at circumferential intervals of 4-6 meters, with each tendon containing 4-6 prestressed steel strands. Tensioning of the longitudinal prestressing tendons employs a two-end counter-tensioning method, with the tension force controlled between 60-80% of the circumferential prestress to ensure coordinated action between the longitudinal restraint and radial support forces.

[0125] Axial prestressing is achieved through oblique prestressing tendons, which are arranged at a 45-degree angle to the tunnel axis, forming a spiral prestressing network. The main function of the oblique prestressing tendons is to coordinate the interaction between radial and tangential prestresses, preventing local stress concentration and deformation inconsistencies. The oblique prestressing tendons are arranged with an axial spacing of 6-8 meters and a circumferential spacing of 8-10 meters, with the tension controlled between 40-60% of the radial prestress.

[0126] During prestressing application, a graded loading method is adopted, dividing the design prestress value into three loading levels. After each loading level is completed, deformation observation and stress monitoring are performed. The next loading level is only applied after confirming that the surrounding rock response is normal. The first loading level applies 30% of the design prestress, primarily to establish initial contact between the prestressed system and the surrounding rock, eliminating system gaps. The second loading level applies 70% of the design prestress, at which point the prestressed system begins to effectively constrain the surrounding rock. The third loading level applies 100% of the design prestress, forming a complete prestressed support system.

[0127] Detailed explanation of dynamic control and maintenance steps:

[0128] Dynamic control and maintenance is a crucial step in ensuring the long-term effectiveness of prestressed support systems. This step establishes a dynamic monitoring and control mechanism to promptly identify and address problems in the support system, thereby guaranteeing the long-term stability of the tunnel. Dynamic control and maintenance includes three interrelated sub-steps: monitoring, assessment, and control.

[0129] The monitoring sub-steps establish a comprehensive monitoring network to track the working status of the prestressed support system in real time. Monitoring content includes three aspects: prestress loss monitoring, surrounding rock deformation monitoring, and support structure stress monitoring. Prestress loss monitoring is achieved by installing stress sensors on key prestressing tendons, with sensors arranged at a density of one every 10 meters to monitor changes in prestress values ​​in real time. Surrounding rock deformation monitoring uses a combination of multi-point displacement gauges and convergence gauges, setting monitoring points at key locations on the tunnel cross-section to monitor the radial, tangential, and axial displacements of the surrounding rock. Support structure stress monitoring is achieved by embedding strain gauges at key locations in the anchor bolts and prestressing tendons to monitor the stress state and stress distribution of the support structure.

[0130] The assessment sub-step comprehensively evaluates the working status of the support system based on monitoring data to determine whether the system requires adjustment and maintenance. The assessment process employs a multi-parameter comprehensive analysis method, establishing three assessment indicators: prestress loss rate, surrounding rock deformation rate, and support structure stress level. When the prestress loss rate exceeds 15%, it indicates significant stress relaxation in the prestressed system, requiring prestress compensation. When the surrounding rock deformation rate exceeds 80% of the design allowable value, it indicates a problem with surrounding rock stability, requiring strengthened support measures. When the support structure stress level exceeds 70% of the design strength, it indicates that the support structure's bearing capacity is approaching its limit, requiring structural reinforcement.

[0131] The control sub-step involves taking corresponding maintenance measures based on the assessment results to ensure the continued effectiveness of the support system. Control measures include three types: prestress compensation, local reinforcement, and system optimization. Prestress compensation is achieved by re-tensioning the relaxed prestressed tendons; the compensation tension is determined based on the actual loss, generally 10-30% of the initial tension. Local reinforcement is carried out in areas of significant surrounding rock deformation, achieved through methods such as adding anchor bolts, grouting reinforcement, or setting up temporary supports. System optimization involves adjusting the entire prestressed support system, including adjusting the prestress distribution, optimizing the tensioning procedure, and improving construction techniques.

[0132] The above description is merely an exemplary embodiment of the present invention and is not intended to 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 prestressed support of soft rock tunnels, characterized in that, include: The surrounding rock of the soft rock tunnel is divided into a surface reinforcement layer of 0-0.5m, an intermediate transition layer of 0.5-1.5m, and a deep anchoring layer of 1.5-3m according to radial depth. Pre-treat the surface reinforcement layer; Preprocess the intermediate transition layer; Pre-treat the deep anchoring layer; Multi-directional prestress is applied to the surface reinforcement layer, the intermediate transition layer, and the deep anchoring layer respectively to complete the prestressed support construction of the soft rock tunnel.

2. The prestressed support method for soft rock tunnels as described in claim 1, characterized in that, The steps for pretreating the surface reinforcement layer include: Grouting holes are drilled to a depth of 0.3 to 0.5 m at a circumferential spacing of 1.5 m and a longitudinal spacing of 2 m in the surface reinforcement layer to form the first grouting channel; Grout A is injected into the surface reinforcement layer using the first grouting channel at a grouting pressure of 0.2 to 0.5 MPa and a penetration time of at least 30 minutes.

3. The prestressed support method for soft rock tunnels as described in claim 2, characterized in that, The A grout comprises, by weight, 100 parts silicate cement, 20 parts water glass, and 5 parts micro-expansion agent.

4. The prestressed support method for soft rock tunnels as described in claim 2, characterized in that, The step of injecting grout A into the surface reinforcement layer using the first grouting channel at a grouting pressure of 0.2–0.5 MPa and a penetration time of at least 30 minutes includes: The first injection of 60% of the total amount of grout A into the first grouting channel; Let it stand for 10 minutes, and tap the wall of the first grouting channel; Inject 40% of the total amount of grout A into the first grouting channel again.

5. The prestressed support method for soft rock tunnels as described in claim 4, characterized in that, The steps for preprocessing the intermediate transition layer include: Anchor bolt holes are drilled in the intermediate transition layer at a circumferential spacing of 2m and a longitudinal spacing of 3m, and composite anchor bolts are inserted. B grout is injected into the intermediate transition layer using the anchor bolt holes at a grouting pressure of 0.8 to 1.2 MPa.

6. The prestressed support method for soft rock tunnels as described in claim 5, characterized in that, The B grout comprises 100 parts cement, 3 parts quick-setting agent and 1 part water-reducing agent by weight.

7. The prestressed support method for soft rock tunnels as described in claim 5, characterized in that, The composite anchor rod forms a radial angle of 15° to 25° with the soft rock tunnel.

8. The prestressed support method for soft rock tunnels as described in any one of claims 1 to 7, characterized in that, The steps for completing the prestressed support construction of the soft rock tunnel by applying multi-directional prestress to the surface reinforcement layer, the intermediate transition layer, and the deep anchoring layer respectively include: Radial prestress, tangential prestress, and axial prestress are simultaneously applied to the surface reinforcement layer, the intermediate transition layer, and the deep anchoring layer to complete the prestressed support construction of the soft rock tunnel. Wherein, the tension ratio satisfies the following: the tension of the tangential prestress is the tension of the radial prestress * C, 0.6≤D≤0.8; the tension of the axial prestress is the tension of the radial prestress * E, 0.4≤E≤0.

6.

9. The prestressed support method for soft rock tunnels as described in any one of claims 1 to 7, characterized in that, The steps for completing the prestressed support construction of the soft rock tunnel by applying multi-directional prestress to the surface reinforcement layer, the intermediate transition layer, and the deep anchoring layer respectively include: Circumferential prestressed tendons are arranged every 2 to 3 meters along the axial direction of the soft rock tunnel, and each circumferential prestressed tendon includes 6 steel strands; The circumferential prestressed tendon is tensioned in one stage until the prestress value of the circumferential prestressed tendon reaches the preset value of 30%, and the load is held for 5 minutes. The circumferential prestressed tendon is subjected to secondary tensioning until the prestress value of the circumferential prestressed tendon reaches 70% of the preset value, and the deformation F of the surrounding rock is measured. When F≤2mm, the circumferential prestressed tendon is tensioned in three stages until the prestress value of the circumferential prestressed tendon reaches 100% of the preset value, thus completing the prestressed support construction of the soft rock tunnel.

10. The prestressed support method for soft rock tunnels as described in any one of claims 1 to 7, characterized in that, The steps for completing the prestressed support construction of the soft rock tunnel by applying multi-directional prestress to the surface reinforcement layer, the intermediate transition layer, and the deep anchoring layer respectively include: Longitudinal prestressing tendons are applied to the surface reinforcement layer, the intermediate transition layer and the deep anchoring layer respectively. The longitudinal prestressing tendons include 4 to 6 steel strands with a circumferential spacing of 4 to 6 m. Tension is applied simultaneously at both ends of the longitudinal prestressed tendon, with the tension value being 70% ± 5% of the radial tension value and the tensioning speed being ≤ 50 kN / min.

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