Tunnel construction method for soft rock large deformation section

Abstract

This invention discloses a tunnel construction method for soft rock sections with large deformation, relating to the field of tunnel construction technology for soft rock sections with large deformation. The method includes drilling shallow, large-diameter and deep, small-diameter stress relief holes to form a layered stress relief system; differential pressure grouting around the stress relief holes to form a deep reinforcement ring and a shallow coupling ring; erecting a steel frame and installing a three-stage extension locking bottom support assembly to form a closed support ring; cross-connecting the bottoms of adjacent steel frames with prestressed tie rods to form an overall rigid grid; constructing secondary lining after determining the longitudinal attenuation transmission period based on the spatiotemporal characteristics of surrounding rock deformation; and employing a segmented pouring method, first the invert arch and then the arch wall, with the arch wall concrete incorporating a delayed expansion agent. This invention effectively controls the deformation of the surrounding rock during tunnel construction in soft rock sections with large deformation, improves the overall stiffness and stability of the support system, ensures a tight fit between the lining and the surrounding rock, and reduces construction risks.

Description

Technical Field

[0001] This invention relates to the field of tunnel construction technology for soft rock sections with large deformation, and particularly to a tunnel construction method for soft rock sections with large deformation. Background Technology

[0002] As tunnel engineering continues to expand into areas with complex geological conditions, especially in mountainous areas, plateau regions, and the construction of long, deep-buried tunnels, the problem of large deformation of soft rock is increasingly becoming a key bottleneck restricting the safety and progress of the project.

[0003] Soft rock typically refers to rock masses with low surrounding rock strength, poor self-stabilizing ability, and slow stress release. After tunnel excavation, due to the stress redistribution effect of the surrounding rock, it often exhibits continuous and significant deformation characteristics. This deformation not only threatens the safety of construction workers but may also lead to instability or even collapse of the support structure. Although support technology has gradually developed from traditional rigid support to diversified strategies such as flexible support and pressure relief support in recent years, existing methods still have systemic defects in dealing with large deformations in soft rock.

[0004] Currently, soft rock tunnel construction generally adopts the principle of "strong support and early closure," which means that heavy steel frames are erected immediately after excavation, combined with systematic anchor bolts and shotcrete to form initial support, and secondary lining is constructed as soon as possible to achieve structural closure. Some projects have attempted to apply stress relief hole technology, which alleviates stress concentration by drilling holes in the surrounding rock. However, the arrangement of these holes often lacks a scientific hierarchical design, and only a single depth of drilling is used. This cannot coordinate and control the stress release process between shallow and deep surrounding rock, which can easily lead to excessive relaxation of shallow surrounding rock and insufficient stress release in deep rock, resulting in uneven deformation and localized failure. In terms of support structure, I-beams or grid steel frames are used as the main load-bearing components, and each steel frame is connected by longitudinal connecting bars. However, the bottom support often relies on concrete foundations or simple steel structures. Under weak bottom conditions, these supports are difficult to quickly establish a stable spatial closed loop, and the lack of an effective connection mechanism between the bottoms of adjacent steel frames significantly reduces the overall stiffness and deformation resistance of the support system. Furthermore, the timing of secondary lining construction is often determined based on construction experience rather than objective monitoring data, lacking scientific analysis of the spatiotemporal evolution of surrounding rock deformation. Premature construction can easily lead to lining cracking due to continuous deformation of the surrounding rock, while delayed construction prolongs the construction period and increases risks. More seriously, voids often occur between the lining structure and the surrounding rock, preventing a tight fit and weakening the long-term stability and load-bearing capacity of the support system. These combined problems result in high construction risks, delays, and increased costs in soft rock tunnel construction with large deformation, urgently requiring an innovative construction method that can systematically solve these problems. Summary of the Invention

[0005] The main objective of this invention is to propose a tunnel construction method for soft rock sections with large deformation, which aims to control the deformation of the surrounding rock during the tunnel construction process, improve the overall stiffness and stability of the support system, ensure that the lining is closely fitted to the surrounding rock, thereby reducing construction risks and improving project safety and efficiency.

[0006] To achieve the above objectives, the present invention proposes a tunnel construction method for soft rock sections with large deformation, the method comprising: After the tunnel excavation and initial shotcreting are completed, the first set of shallow, large-diameter stress relief holes are drilled in the arch and sidewall areas of the excavation profile, and highly compressible filler is placed in the holes. At the grid node locations formed by the first group of shallow, large-diameter stress relief holes, a second group of deep, small-diameter stress relief holes is drilled. Around each set of stress relief holes, anchor holes are drilled and grouting anchors are installed. Grouting operations are carried out in stages and with differential pressure. First, high-pressure grouting is performed on the anchors around the second set of deep small-diameter stress relief holes to form a deep reinforcement ring. Then, low-pressure grouting is performed on the anchors around the first set of shallow large-diameter stress relief holes to form a shallow coupling ring. An arch wall steel frame is erected within the shallow coupling ring, and a bottom support assembly with a three-level extension and locking function is installed simultaneously. The three-level extension and locking function includes a first-level hinged unfolding, a second-level hydraulic extension, and a third-level mechanical locking, so that a single steel frame can instantly form a spatially closed temporary support ring. As the tunnel is excavated, after the longitudinal connecting bars of the temporary support ring are welded, prestressed tie rods are immediately used to cross-connect the bottom support components of at least three adjacent temporary support rings in the horizontal and vertical directions to form a bottom support grid with overall rigidity. During tunnel construction, based on the spatiotemporal evolution characteristics of surrounding rock deformation at the monitoring section, secondary lining is started when it is determined that the deformation has entered the longitudinal attenuation and transmission period. The criteria for determining the longitudinal attenuation and transmission period are: among at least three continuous monitoring sections arranged along the longitudinal direction of the tunnel, the deformation rate of the section closest to the working face has passed the peak and begun to decrease, while the deformation rate of the more distant section has dropped to below 30% of the peak rate. When constructing the secondary lining, the pouring sequence of first the inverted arch and then the arch wall, with sections skipping sections, is adopted. A delayed expansion agent is added to the arch wall lining concrete to allow the lining concrete to undergo controllable micro-expansion within 7 to 14 days after pouring.

[0007] In one embodiment, after tunnel excavation and initial shotcreting, the step of drilling a first set of shallow, large-diameter stress relief holes in the arch and sidewall areas of the excavation profile, and placing highly compressible filler in the holes, includes: Using a hydraulic rock drill with an 85 mm diameter drill bit, drill holes along the normal direction of the tunnel excavation outline. The drilling depth is strictly controlled at 2.0 meters. During the drilling process, retract the drill every 0.5 meters and clean the hole with high-pressure air to ensure that the hole wall is straight and there is no collapse. After the hole-forming acceptance, a pre-cut cylindrical open-cell polyurethane foam with a length of 2.1 meters and a diameter of 83 millimeters was selected as the high compressibility filler. Use a metal push rod with a guide cone at the front end to push the foam into the hole until the depth mark on the rod is flush with the hole opening, ensuring that the foam completely fills the hole.

[0008] In one embodiment, the step of drilling a second set of deep, small-diameter stress relief holes at the grid node locations formed by the first set of shallow, large-diameter stress relief holes includes: After the first set of holes is completed and the working face is cleaned, the same rock drill is used with a 45 mm diameter drill bit. Drilling is carried out at the center point of the marked diamond grid. The drilling direction is parallel to the tunnel axis or slightly inclined into the tunnel at a degree not greater than 3 degrees. The drilling depth reaches 3.2 meters. After drilling is completed, a PVC plastic pipe with an outer diameter of 42 mm is inserted into the hole. The front end of the PVC plastic pipe is closed, the pipe wall is drilled with overflow holes, and the rear end protrudes 10 cm above the rock surface without grouting, so that the rear end protruding above the rock surface serves as a channel for deep stress release.

[0009] In one embodiment, anchor bolt holes are drilled and grouting anchor bolts are installed around each group of stress relief holes. A staged, differential pressure grouting operation is then performed. First, high-pressure grouting is applied to the anchor bolts surrounding the second group of deep, small-diameter stress relief holes to form a deep reinforcement ring. Then, low-pressure grouting is applied to the anchor bolts surrounding the first group of shallow, large-diameter stress relief holes to form a shallow coupling ring. The steps include: High-pressure grouting is performed using a screw-type grouting pump. Through the grouting joints connected to the anchor rods around the second set of holes, ultrafine cement grout with a water-cement ratio of 0.35:1 is injected into the holes. The initial grouting pressure is 0.5 MPa, and the pressure is gradually increased to 1.2 MPa within 1 minute and maintained at this pressure value. The grouting is stopped when the grouting volume reaches 80% of the design value or the pressure rises to 1.5 MPa. After an interval of at least 4 hours, low-pressure grouting is performed using the same equipment but with ordinary cement grout. Grouting is carried out through the anchor bolts around the first set of holes, and the grouting pressure is always controlled at 0.4 MPa. When the grout seeps steadily from the adjacent anchor bolt holes or rock fissures, the pressure is maintained for another 30 seconds before grouting ends.

[0010] In one embodiment, the bottom support assembly consists of a fixed hinge, a two-stage telescopic sleeve, and a mechanical locking clamp; The steps of erecting an arch wall steel frame within the shallow coupling ring and simultaneously installing a bottom support assembly with a three-stage extension and locking function, wherein the three-stage extension and locking function includes a first-stage hinged deployment, a second-stage hydraulic extension, and a third-stage mechanical locking, to instantly form a spatially closed temporary support ring for a single steel frame include: First, connect the fixed hinge seat to the arch foot base plate of the arch wall steel frame with high-strength bolts; Swing the first-stage sleeve to rotate it around the hinge axis to the designed angle, and insert the limit pin to complete the first-stage hinge deployment; The micro hydraulic cylinder integrated in the secondary sleeve is activated, pushing the secondary sleeve downward until the pressure plate at its bottom is in close contact with the rock surface of the tunnel floor. The contact pressure is monitored by a pressure gauge to reach 8 kN, thus completing the second stage of hydraulic extension. Tighten the handwheel of the mechanical locking clamp set on the secondary sleeve, so that the wedge block inside the clamp bites the sleeve, thus completing the third-stage mechanical locking.

[0011] In one embodiment, the prestressed tie rod is a 20 mm diameter precision-rolled threaded steel bar, equipped with dedicated tensioning end and fixing end anchors; As tunnel excavation progresses, after the longitudinal connecting bars of the temporary support rings are welded, the step of immediately using prestressed tie rods to cross-connect the bottom support assemblies of at least three adjacent temporary support rings in both the transverse and longitudinal directions to form a bottom support grid with overall rigidity includes: During connection, the prestressed tie rod is passed through the pre-reserved transverse connection holes on the bottom support components of the three adjacent steel frames along the transverse direction of the tunnel; Along the longitudinal direction of the tunnel, another set of the prestressed tie rods is arranged in the same manner, and the two sets of prestressed tie rods intersect in an X-shape in the plane; Two hydraulic jacks are used to tension the rods simultaneously at both ends. The tension force is controlled at 80 kN. Once the predetermined force value is reached, the rods are immediately anchored with lock nuts, so that the temporary support rings in this area are connected into an integral load-bearing structure through the bottom grid.

[0012] In one embodiment, the secondary lining is constructed using a pouring sequence of first the invert arch and then the arch wall, with sections poured in stages. A delayed expansion agent is added to the arch wall lining concrete to induce controlled micro-expansion of the lining concrete within 7 to 14 days after pouring. This process includes: The first section of the invert arch, which is 6 meters long, and the invert arch filling concrete were cast in one go using C35 concrete and covered with the bottom support grid. After the first section of the invert arch and the invert arch filling concrete have reached a compressive strength of 25MPa as tested by test blocks cured under the same conditions, skip the adjacent 6-meter section and pour the second section of the invert arch and the invert arch filling concrete in the next 6-meter section after the interval. After the first section of the invert and the invert filling concrete and the second section of the invert and the invert filling concrete have reached the predetermined strength, an arch wall lining trolley is erected in the 6-meter skip section between the first section of the invert and the invert filling concrete and the second section of the invert and the invert filling concrete, steel bars are tied, and secondary arch wall lining concrete is poured in this section.

[0013] In one embodiment, the delayed expansion agent is an ettringite-based expansion agent, which is added during concrete mixing at a ratio of 7% of the total mass of cementitious materials. When constructing the secondary lining, the pouring sequence is to first pour the inverted arch and then the arch wall, using a segmented, skip-construction method. A delayed expansion agent is added to the arch wall lining concrete to induce controlled micro-expansion of the lining concrete within 7 to 14 days after pouring. This process also includes: After the concrete is poured into the formwork, the internal strain development is monitored by strain sensors embedded in the lining, and its expansion process is controlled by heat preservation and moisture retention curing. The expansion rate of the concrete is less than 0.002% / day from the 3rd to the 7th day after pouring, the main expansion occurs from the 7th to the 14th day, and the final restricted expansion rate is stable at 0.020%.

[0014] In one embodiment, prior to the step of immediately using prestressed tie rods to cross-connect the bottom support components of at least three adjacent temporary support rings in both the transverse and longitudinal directions to form a bottom support grid with overall rigidity after the longitudinal connecting bars of the temporary support rings are welded during tunnel excavation, the tunnel construction method for soft rock sections with large deformation further includes: A geological drilling rig was used to drill grouting holes with a depth of 2.5 meters and a diameter of 60 millimeters at the base plate position corresponding to the arch foot. Through the grouting valve installed at the orifice, water glass-cement dual-liquid grout with added quick-setting agent is injected into the hole at a pressure of 0.8-1.0 MPa. Grouting is stopped when the pressure rises to 1.5 MPa or when grout appears to precipitate on the surface of the bottom plate, forming a local reinforcement cushion layer with a thickness of not less than 0.5 meters. The bottom support assembly is then supported on the local reinforcement cushion layer.

[0015] In one embodiment, after tunnel excavation and initial shotcreting, the step of drilling a first set of shallow, large-diameter stress relief holes in the arch and sidewall areas of the excavation profile, and placing highly compressible filler in the holes, further includes: A wet spraying robot is used to spray C30 early-strength shotcrete mixed with nano-silica powder, wherein the amount of nano-silica is 3% of the cement mass. The spraying operation is carried out in two layers. The thickness of the first layer is controlled at 30 mm. Immediately after spraying, a steel mesh with a grid spacing of 150 mm * 150 mm is hung and welded to the end of the anchor bolt. A second layer of concrete is sprayed to increase the total thickness to 55 mm. During the spraying process, the concrete completely covers the steel mesh and anchor plate.

[0016] The technical solution of this invention effectively controls the stress release process of surrounding rock at different depths during tunnel construction in soft rock sections with large deformation through the synergistic effect of layered stress relief holes and high-compression fillers. Simultaneously, the use of a bottom support component with a three-stage extension locking function, and the formation of an integral rigid bottom support grid using prestressed tie rods, solves the problems of insufficient stiffness and connectivity in traditional bottom supports. Furthermore, the timing of secondary lining construction based on the spatiotemporal evolution characteristics of surrounding rock deformation avoids the drawbacks of lining construction too early or too late. The lining concrete incorporating a delayed expansion agent achieves close adhesion to the surrounding rock, improving the long-term stability and load-bearing capacity of the support system, thereby effectively addressing the problem of large deformation in soft rock. Attached Figure Description

[0017] 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.

[0018] Figure 1 This is a schematic flowchart of an embodiment of the tunnel construction method for soft rock sections with large deformation provided by the present invention.

[0019] 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

[0020] 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.

[0021] 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.

[0022] 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.

[0023] Existing construction methods for soft rock tunnels with large deformation have shortcomings in areas such as stress relief hole layout, the stiffness and installation of the steel frame bottom support structure, the timing of secondary lining construction, and the fit between the lining structure and the surrounding rock. Specifically: stress relief holes lack hierarchy and coordination, making it difficult to effectively control stress in the surrounding rock at different depths; traditional bottom supports have insufficient stiffness and are complex to install, making it difficult to quickly form a closed support ring, and lacking overall bottom connection; the timing of secondary lining construction lacks scientific basis; and the lining is prone to separation from the surrounding rock, affecting long-term stability and load-bearing capacity. These problems increase the construction risks, extend the construction period, and increase costs of soft rock tunnels with large deformation.

[0024] To address this technical problem, this invention proposes a tunnel construction method for soft rock sections with large deformation.

[0025] Please see Figure 1 In one embodiment of the present invention, the tunnel construction method for soft rock sections with large deformation includes: S10, after the tunnel excavation and the initial shotcrete are completed, the first set of shallow large-diameter stress relief holes are drilled in the arch and sidewall areas of the excavation outline, and highly compressible filler is placed in the holes. S20, at the grid node positions formed by the first group of shallow large-diameter stress relief holes, drill the second group of deep small-diameter stress relief holes; S30, around each group of stress relief holes, drill anchor holes and install grouting anchors, and carry out grouting operations in stages and with differential pressure. First, high-pressure grouting is carried out on the anchors around the second group of deep small-diameter stress relief holes to form a deep reinforcement ring. Then, low-pressure grouting is carried out on the anchors around the first group of shallow large-diameter stress relief holes to form a shallow coupling ring. S40, erect the arch wall steel frame within the shallow coupling ring, and simultaneously install the bottom support assembly with a three-level extension and locking function. The three-level extension and locking function includes the first level of hinged deployment, the second level of hydraulic extension, and the third level of mechanical locking, so that a single steel frame can instantly form a spatially closed temporary support ring. S50, as the tunnel is excavated, after the longitudinal connecting bars of the temporary support ring are welded, prestressed tie rods are immediately used to cross-connect the bottom support components of at least three adjacent temporary support rings in the horizontal and vertical directions to form a bottom support grid with overall rigidity. S60, during tunnel construction, based on the spatiotemporal evolution characteristics of surrounding rock deformation at the monitoring section, secondary lining is started when it is determined that the deformation has entered the longitudinal attenuation and transmission period; wherein, the criteria for determining the longitudinal attenuation and transmission period are: among at least three continuous monitoring sections arranged along the longitudinal direction of the tunnel, the deformation rate of the section closest to the working face has passed the peak and begun to decrease, while the deformation rate of the section further away has decreased to less than 30% of the peak rate. S70, when constructing the secondary lining, adopt the pouring sequence of first the inverted arch and then the arch wall, and add a delayed expansion agent to the arch wall lining concrete to make the lining concrete undergo controllable micro-expansion within 7 to 14 days after pouring.

[0026] To facilitate understanding of the technical solution in this embodiment, some key terms are explained below: The first group of shallow, large-diameter stress relief holes refers to large-diameter, shallow-depth holes drilled in the arch and sidewall areas of the tunnel excavation profile to release shallow surrounding rock stress. These holes are designed to provide initial stress adjustment space for the surrounding rock.

[0027] High-compression fillers are materials placed inside stress-relief holes that can undergo large deformations under external forces without losing their integrity. These fillers can be compressed during surrounding rock deformation, thereby further promoting stress release.

[0028] The second group of deep, small-diameter stress relief holes refers to the small-diameter and deep holes drilled at the grid node locations formed by the first group of shallow, large-diameter stress relief holes to release stress in the deep surrounding rock. These holes work in synergy with the shallow holes to achieve layered stress release.

[0029] A deep reinforcement ring refers to a reinforced area located deep within the surrounding rock, formed by high-pressure grouting of anchor bolts around a second set of deep, small-diameter stress-relieving holes. This reinforcement ring aims to improve the overall stability and bearing capacity of the deep surrounding rock.

[0030] A shallow coupling ring refers to a reinforced area located in the shallow layer of the surrounding rock, formed by low-pressure grouting of anchor bolts around the first set of shallow, large-diameter stress relief holes. This coupling ring aims to tightly connect the shallow surrounding rock with the initial support structure, forming a synergistic force-bearing system.

[0031] The three-stage extension-locking function refers to the bottom support assembly's ability to extend and ultimately lock in place through three stages. This function ensures that the bottom support assembly can quickly and reliably contact the tunnel floor and provide support.

[0032] The bottom support assembly refers to the structural component used to support the bottom of the arch wall steel frame, forming a closed support ring. This assembly connects the arch foot of the steel frame to the tunnel floor through its extension locking function.

[0033] A temporary support ring is a spatial closed support structure formed immediately after tunnel excavation, consisting of a steel frame for the arch wall and bottom support components. This support ring provides immediate, initial stable support for the tunnel.

[0034] A prestressed tie rod is a member used to connect adjacent support structures after prestressing is applied, thereby improving their overall stiffness and stability. Through tension, the connected structure is placed under compression, thus enhancing its resistance to deformation.

[0035] A bottom support grid refers to a rigid, grid-like structure formed by connecting at least three adjacent temporary support rings in both the transverse and longitudinal directions using prestressed tie rods. This grid aims to improve the overall integrity and deformation resistance of the bottom support.

[0036] The longitudinal attenuation and conduction period refers to the stage during tunnel construction where the deformation of the surrounding rock exhibits a specific pattern along the longitudinal direction of the tunnel. In this stage, the deformation rate of the section closest to the tunnel face has passed its peak and begun to decline, while the deformation rate of the sections farther away has dropped to below 30% of the peak rate.

[0037] Delayed expansion agents are materials added to concrete that cause it to expand only after a certain period of time following pouring. These agents are designed to induce micro-expansion in the lining concrete over a specific time period, thereby compensating for concrete shrinkage and ensuring a tight bond between the concrete and the surrounding rock.

[0038] The tunnel construction method in this embodiment involves first drilling a first set of shallow, large-diameter stress relief holes in the arch and sidewall areas of the excavated profile after tunnel excavation and initial shotcreting. High-compressibility fillers are then placed inside these holes. For example, a conventional impact drill with a general-purpose drill bit can be used to drill holes approximately perpendicular to the excavated profile. The drilling depth can be set to about 2 meters based on experience. After drilling is completed, the next step can be performed directly. When placing the fillers inside the holes, prefabricated wooden blocks or sandbags can be manually inserted, or loose sand can be directly filled into the holes to provide a certain amount of compressible space.

[0039] Furthermore, at the grid node locations formed by the first group of shallow, large-diameter stress relief holes, a second group of deep, small-diameter stress relief holes will be drilled. Specifically, after the first group of holes is constructed, a handheld pneumatic drill can be used to drill at the grid node locations according to the site markings. The drilling direction should be roughly parallel to the tunnel axis, and the drilling depth can be set to approximately 3 meters. After drilling, the holes can be left open without any further treatment, serving as stress relief channels.

[0040] Around each group of stress relief holes, anchor bolt holes are drilled and grouting anchor bolts are installed. Grouting is then performed in stages with differential pressure. First, high-pressure grouting is applied to the anchor bolts surrounding the deep, small-diameter stress relief holes of the second group to form a deep reinforcement ring. Then, low-pressure grouting is applied to the anchor bolts surrounding the shallow, large-diameter stress relief holes of the first group to form a shallow coupling ring. For example, after the anchor bolts are installed, a single grouting method can be used, employing a conventional grouting pump to inject ordinary cement grout into all anchor bolt holes at a constant pressure until the grout overflows from the hole openings. Alternatively, staged grouting can be used, but the grouting pressure is not differentiated; for example, medium pressure can be applied to all anchor bolts to form a reinforced area.

[0041] Based on this, an arch wall steel frame is erected within the shallow coupling ring, and a bottom support assembly with a three-stage extension and locking function is installed simultaneously. This three-stage extension and locking function includes a first-stage hinged deployment, a second-stage hydraulic extension, and a third-stage mechanical locking, enabling a single steel frame to instantly form a spatially closed temporary support ring. Specifically, after the steel frame is erected, the bottom support assembly can use simple fixed steel columns or wooden piles, connected to the arch foot of the steel frame by welding or bolting, and placed directly on the tunnel floor to form a closed ring. Alternatively, the bottom support assembly can be designed as a two-section telescopic structure, with length adjustment achieved by manually adjusting bolts or pins, and then fixed by simple mechanical clips to make it contact the floor.

[0042] As tunnel excavation progresses, after the longitudinal connecting bars of the temporary support ring are welded, prestressed tie rods are immediately used to cross-connect at least three adjacent bottom support components of the temporary support ring in both the transverse and longitudinal directions, forming a bottom support grid with overall rigidity. For example, after the longitudinal connecting bars are welded, ordinary steel bars or wire ropes can be used to connect the bottom support components of adjacent steel frames through simple binding or welding to form a basic bottom connection structure. Alternatively, ordinary tie rods can be used for connection, but without applying prestress, relying solely on the connecting action of the tie rods to enhance the overall integrity of the bottom structure.

[0043] During tunnel construction, based on the spatiotemporal evolution characteristics of surrounding rock deformation at monitoring sections, secondary lining is initiated when the deformation is determined to have entered the longitudinal attenuation and transmission period. The criteria for determining this longitudinal attenuation and transmission period are: among at least three continuous monitoring sections arranged longitudinally along the tunnel, the deformation rate of the section closest to the tunnel face has passed its peak and begun to decline, while the deformation rate of the more distant sections has decreased to below 30% of its peak rate. Specifically, the timing of secondary lining construction can be based on field experience, such as after the initial support deformation has stabilized for a period of time, or directly arranged according to the construction schedule. Alternatively, it can be determined through simple visual observation or periodic measurement of deformation at several points; when the deformation change is not significant, secondary lining construction is considered permissible.

[0044] Finally, when constructing the secondary lining, a pouring sequence of first the invert arch and then the arch wall, using segmented pouring, is adopted. A delayed expansion agent is added to the arch wall lining concrete to allow for controlled micro-expansion within 7 to 14 days after pouring. For example, the secondary lining can be poured using the traditional full-section one-time pouring method, or following the conventional sequence of first the arch wall and then the invert arch, without segmented pouring. Alternatively, no expansion agent can be added to the lining concrete, and ordinary concrete can be used directly, relying on the concrete's own shrinkage characteristics.

[0045] The tunnel construction method in this embodiment effectively controls the stress release process of surrounding rock at different depths through the synergistic effect of layered stress relief holes and high-compression fillers. Simultaneously, the use of a bottom support component with a three-stage extension locking function, and the formation of an integral rigid bottom support grid using prestressed tie rods, solves the problems of insufficient stiffness and connectivity in traditional bottom supports. Furthermore, the timing of secondary lining construction based on the spatiotemporal evolution characteristics of surrounding rock deformation avoids the drawbacks of lining construction too early or too late. The lining concrete incorporating a delayed expansion agent achieves close adhesion to the surrounding rock, improving the long-term stability and load-bearing capacity of the support system, thereby effectively addressing the problem of large deformation in soft rock.

[0046] In an embodiment of the present invention, after tunnel excavation and completion of the initial shotcrete, the step of drilling a first set of shallow, large-diameter stress relief holes in the arch and sidewall areas of the excavation profile, and placing highly compressible filler in the holes, includes: S11 uses a hydraulic rock drill with an 85 mm diameter drill bit to drill along the normal direction of the tunnel excavation outline. The drilling depth is strictly controlled at 2.0 meters. During the drilling process, the drill is withdrawn every 0.5 meters and the hole is cleaned with high-pressure air to ensure that the hole wall is straight and there is no collapse. S12, After the hole-forming acceptance, a pre-cut cylindrical open-cell polyurethane foam with a length of 2.1 meters and a diameter of 83 millimeters is selected as the high compressibility filler; S13, use a metal push rod with a guide cone at the front end to push the foam into the hole until the depth mark on the rod is flush with the hole opening, ensuring that the foam completely fills the hole.

[0047] Specifically, a hydraulic rock drill was used for drilling, a device characterized by high precision and efficiency, ensuring the accuracy of the drilling position and direction. An 85mm diameter drill bit was used, its dimensions optimized to provide suitable space for the subsequent installation of highly compressible filler, while ensuring the effective range of the stress relief holes. Drilling along the normal direction of the tunnel excavation outline maximizes the interruption of stress transmission paths in the surrounding rock, thereby more effectively inducing stress release. The drilling depth was strictly controlled at 2.0 meters, the optimal depth determined based on the characteristics of the surrounding rock and stress distribution patterns in the soft rock section with large deformation. This depth effectively releases shallow stress while avoiding excessive disturbance to the deep surrounding rock. During drilling, the drill was retracted and a high-pressure air cleaning was performed every 0.5 meters to promptly remove rock cuttings and dust from the hole, prevent drill bit jamming, and check the hole wall condition to ensure it is straight and free of collapse, creating favorable conditions for the smooth installation of the filler.

[0048] After borehole acceptance, a pre-cut cylindrical open-cell polyurethane foam with a length of 2.1 meters and a diameter of 83 millimeters was selected as the highly compressible filler. This foam, with its open-cell structure, possesses excellent compressibility and elastic recovery, allowing it to be effectively compressed during surrounding rock deformation, thereby absorbing and releasing surrounding rock stress. Its precise pre-cut dimensions (2.1 meters in length and 83 millimeters in diameter) closely match the borehole depth and diameter, ensuring that the filler completely covers the stress-relief area and maintains close contact with the borehole wall, avoiding any voids.

[0049] During filler installation, a metal pusher rod with a guide cone at the front end is used to push the foam into the hole. The guide cone design effectively guides the foam into the hole, preventing it from getting stuck or damaged during insertion. The metal pusher rod provides sufficient rigidity and thrust to ensure the foam smoothly reaches the predetermined position. The depth mark on the rod is flush with the hole opening; this design allows the operator to accurately control the filling depth, ensuring the foam completely fills the hole, thereby guaranteeing the continuity and uniformity of stress release and avoiding localized stress concentration.

[0050] By employing the aforementioned technical solution, using a hydraulic rock drill with a specific diameter drill bit, and strictly controlling the drilling direction, depth, and hole cleaning process, the construction quality and geometric accuracy of the stress relief holes were ensured, laying the foundation for the effective installation of subsequent high-compressibility fillers. Accurately sized, high-compressibility open-cell polyurethane foam was selected and precisely installed using a push rod with a guide cone, ensuring that the filler completely and uniformly filled the holes, thereby maximizing its high compressibility and effectively absorbing and releasing surrounding rock stress. This refined drilling and filler installation method significantly improved the stress release efficiency and uniformity of the first group of shallow, large-diameter stress relief holes, avoiding stress concentration or insufficient release due to improper construction, thus more effectively controlling large deformations in soft rock and ensuring the stability of the initial support and the safety of tunnel construction.

[0051] In an embodiment of the present invention, the step of drilling a second set of deep, small-diameter stress relief holes at the grid node locations formed by the first set of shallow, large-diameter stress relief holes includes: S21. After the first set of holes is completed and the working face is cleaned, use the same rock drill to replace the 45 mm diameter drill bit and carry out drilling operations at the center point of the marked diamond grid. The drilling direction is parallel to the tunnel axis or slightly inclined into the tunnel at a degree not greater than 3 degrees, and the drilling depth reaches 3.2 meters. S22. After drilling is completed, a PVC plastic pipe with an outer diameter of 42 mm is inserted into the hole. The front end of the PVC plastic pipe is closed, the pipe wall is drilled with overflow holes, and the rear end protrudes 10 cm above the rock surface without grouting, so that the rear end protruding above the rock surface serves as a channel for deep stress release.

[0052] The drilling of the second set of deep, small-diameter stress relief boreholes aims to extend the stress relief range from the shallow layer to the deeper surrounding rock, thus more comprehensively addressing the problem of large deformation in soft rock. After the completion of the first set of shallow, large-diameter stress relief boreholes, the working face was cleaned to ensure the accuracy and safety of subsequent drilling operations. Using the same rock drill with a 45mm diameter drill bit improves construction efficiency and ensures equipment compatibility. Drilling was conducted at the center points of the marked diamond grid, ensuring a specific geometric relationship between the second set of boreholes and the first set, thereby optimizing the stress relief effect. The drilling direction was parallel to the tunnel axis or slightly inclined inwards at no more than 3 degrees, aiming to guide the release of deep stress into the tunnel interior while avoiding adverse effects on the shallow support structure. The drilling depth reached 3.2 meters, significantly deeper than the 2.0 meters of the first set of boreholes, ensuring that the stress relief effect reached deeper layers of the surrounding rock.

[0053] The step of inserting a 42 mm outer diameter PVC plastic pipe into the borehole is used to create a stable, unfilled channel within the deep stress relief borehole to promote stress release in the surrounding rock. The 42 mm outer diameter PVC plastic pipe, slightly smaller than the borehole diameter, facilitates insertion and the formation of annular gaps. The front end of the PVC plastic pipe is sealed to prevent blockage by mud or gravel, ensuring unobstructed passage. Overflow holes are drilled in the pipe wall, allowing for slight adjustment of the surrounding rock mass under pressure deformation, or, in extreme cases, a limited balance between the internal pressure and the external environment. The rear end protrudes 10 cm above the rock surface for easy identification and subsequent monitoring by construction personnel. The key is "no grouting," meaning that the annular space inside and around the PVC plastic pipe remains open, unfilled with consolidation material, thus enabling it to serve as an effective channel for deep stress release. This allows the surrounding rock to deform into the channel, reducing the pressure of the surrounding rock on the support structure.

[0054] By drilling a second set of deep, small-diameter stress relief holes after completing the construction of the first set of shallow, large-diameter stress relief holes and inserting ungrouted PVC plastic pipes into them, this application effectively extends the stress relief range from the shallow layer near the tunnel excavation face to the deeper surrounding rock. Specifically, the first set of shallow, large-diameter stress relief holes mainly targets stress concentration near the excavation face, while the second set of deep, small-diameter stress relief holes provides additional stress relief space at a deeper level. When the deep surrounding rock deforms under its own weight and ground stress, it can creep or deform into the channels formed by these ungrouted PVC plastic pipes, thereby effectively reducing the squeezing effect of the deep surrounding rock on the initial support structure and the shallow rock mass. This combined shallow and deep stress relief mechanism not only more comprehensively controls large deformations in soft rock and reduces the overall displacement of the surrounding rock, but also avoids excessive stress concentration in the shallow support by providing deep deformation space, thereby improving the stability and durability of the initial support and creating more favorable conditions for subsequent secondary lining construction.

[0055] In an embodiment of the present invention, anchor bolt holes are drilled and grouting anchor bolts are installed around each group of stress relief holes. A staged, differential pressure grouting operation is performed. First, high-pressure grouting is applied to the anchor bolts surrounding the second group of deep, small-diameter stress relief holes to form a deep reinforcement ring. Then, low-pressure grouting is applied to the anchor bolts surrounding the first group of shallow, large-diameter stress relief holes to form a shallow coupling ring. S31. High-pressure grouting is performed using a screw-type grouting pump. Through the grouting joint connected to the anchor rods around the second set of holes, ultrafine cement grout with a water-cement ratio of 0.35:1 is injected into the holes. The initial grouting pressure is 0.5 MPa. The pressure is gradually increased to 1.2 MPa within 1 minute and maintained at this pressure value. The grouting is stopped when the grouting volume reaches 80% of the design value or the pressure rises to 1.5 MPa. S32, after an interval of at least 4 hours, perform low-pressure grouting using the same equipment but with ordinary cement grout. Grouting is carried out through the anchor rods around the first set of holes. The grouting pressure is always controlled at 0.4 MPa. When the grout seeps steadily from the adjacent anchor rod holes or rock fissures, maintain this pressure and continue grouting for another 30 seconds before ending the process.

[0056] Specifically, during high-pressure grouting, a screw-type grouting pump is used. Through grouting joints connected to anchor bolts around the second set of deep, small-diameter stress relief holes, ultrafine cement grout with a water-cement ratio of 0.35:1 is injected into the holes. The screw-type grouting pump provides stable and continuous grouting pressure, ensuring uniform grout diffusion, and is particularly suitable for ultrafine cement grout, which requires high pumpability. Ultrafine cement grout has excellent permeability, penetrating deep into micro-fractures in the rock mass to form a dense, solidified body. Its 0.35:1 water-cement ratio ensures both the fluidity of the grout and its strength after solidification. The grouting joints ensure the sealing of the grouting process, preventing grout leakage and ensuring that the grouting pressure effectively acts on the rock mass. The initial grouting pressure is set at 0.5 MPa, and the pressure is gradually increased to 1.2 MPa within 1 minute and maintained at this value. This gradual pressure increase method avoids the impact of instantaneous high pressure on the surrounding rock, allowing the grout to gradually penetrate. Simultaneously, the maintaining pressure of 1.2 MPa ensures that the grout can fully diffuse and compact the deep surrounding rock, forming an effective deep reinforcement ring. Grouting is stopped when the grout volume reaches 80% of the design value or the pressure rises to 1.5 MPa. This stopping criterion comprehensively considers both the grout volume and grout pressure, ensuring sufficient grout filling while avoiding over-grouting that could lead to surrounding rock fracturing or grout waste, thus ensuring the economy and safety of deep reinforcement.

[0057] Low-pressure grouting is performed at least 4 hours later. This interval allows sufficient initial setting time for the grout forming the deep reinforcement ring during high-pressure grouting, preventing disturbance or damage to the solidified deep grout during subsequent low-pressure grouting and ensuring the stability of the deep reinforcement effect. During low-pressure grouting, the same equipment is used, but ordinary cement grout is applied through anchor bolts around the first set of shallow, large-diameter stress relief holes. Reusing the grouting equipment improves construction efficiency, while ordinary cement grout is less expensive and sufficient for forming a shallow coupling ring, eliminating the need for highly permeable ultrafine cement grout. The grouting pressure is consistently controlled at 0.4 MPa. This lower grouting pressure aims to avoid excessive stress concentration or damage to the shallow surrounding rock and initial support structure, while ensuring the grout can uniformly fill shallow fissures and voids, achieving good coupling with the initial support. Once the grout has stably seeped from adjacent anchor bolt holes or rock fissures, this pressure is maintained for another 30 seconds before grouting ends. This stopping standard allows for a direct assessment of the saturation level of the shallow surrounding rock by observing the grout overflow, and an additional 30 seconds of grouting is maintained to ensure that the grout fully diffuses and fills the rock, forming a uniform and effective shallow coupling ring.

[0058] Through the above technical solution, this application provides refined grouting parameter control for the formation of deep reinforcement rings and shallow coupling rings. High-pressure grouting is performed using a screw-type grouting pump with ultrafine cement grout at a water-cement ratio of 0.35:1. Initial pressure, gradually increasing pressure, and maintaining pressure are set, along with a clear grouting stop standard. This ensures that the ultrafine cement grout can effectively penetrate into the micro-fractures of the deep surrounding rock, forming a high-strength, high-density deep reinforcement ring, thereby significantly improving the bearing capacity and stability of the deep surrounding rock and effectively suppressing large deformations. After the deep reinforcement ring has initially set, low-pressure grouting is performed using ordinary cement grout at a constant pressure of 0.4 MPa to grout the shallow surrounding rock. A stop standard of stable grout seepage and continuous grouting for 30 seconds ensures sufficient coupling between the shallow surrounding rock and the initial support structure, forming a uniform and effective shallow coupling ring. This phased, differentiated, and precisely controlled grouting strategy not only avoids the potential damage to the surrounding rock or uneven reinforcement that may result from traditional grouting, but also achieves effective reinforcement of deep surrounding rock and close integration of shallow surrounding rock with initial support. This results in the construction of a composite support system with strong integrity and excellent deformation control, significantly improving the safety and stability of tunnel construction in soft rock sections with large deformation.

[0059] In an embodiment of the present invention, the bottom support assembly consists of a fixed hinge seat, a two-stage telescopic sleeve, and a mechanical locking clamp; The steps of erecting an arch wall steel frame within the shallow coupling ring and simultaneously installing a bottom support assembly with a three-stage extension and locking function, wherein the three-stage extension and locking function includes a first-stage hinged deployment, a second-stage hydraulic extension, and a third-stage mechanical locking, to instantly form a spatially closed temporary support ring for a single steel frame include: S41, First, connect the fixed hinge seat to the arch foot base plate of the arch wall steel frame with high-strength bolts; S42, swing the first-stage sleeve to rotate it around the hinge axis to the designed angle, insert the limit pin to complete the first-stage hinge unfolding; S43, activate the micro hydraulic cylinder integrated in the secondary sleeve, push the secondary sleeve downward until the bearing plate at its bottom is in close contact with the rock surface of the tunnel floor, and monitor the contact pressure to reach 8 kN through the pressure gauge to complete the second stage of hydraulic extension; S44, tighten the handwheel of the mechanical locking clamp set on the secondary sleeve, so that the wedge block inside the clamp bites the sleeve, completing the third-stage mechanical locking.

[0060] The bottom support assembly is a key structure connecting the arch wall steel frame to the tunnel floor rock surface, designed to provide adjustable and stable vertical support, ensuring the steel frame forms a closed temporary support ring. The fixed hinge seat is the base connecting the bottom support assembly to the arch wall steel frame; it is typically a high-strength metal component, securely connected to the arch foot plate of the steel frame via high-strength bolts, and provides a rotation axis for initial positioning and deployment of the assembly. The two-stage telescopic sleeve is the core component enabling adjustable length of the bottom support assembly; it contains a relatively sliding sleeve structure to bridge the distance between the arch foot of the steel frame and the tunnel floor. The mechanical locking clamp is a device used to mechanically lock the telescopic sleeve after it reaches a predetermined length, ensuring fixed support length and stable transmission of support force.

[0061] The three-stage extension locking function is a phased and refined deployment and fixing mechanism for the bottom support component. The first-stage hinge deployment is the rapid coarse positioning stage of the bottom support component. By swinging the first-stage sleeve around the hinge axis to a preset angle and inserting a limit pin for initial fixing, the component is quickly deployed and roughly positioned, laying the foundation for subsequent accurate adjustments. The second-stage hydraulic extension is the accurate adjustment and pre-loading stage of the bottom support component. By activating the micro-hydraulic cylinder integrated in the second-stage sleeve, the sleeve is accurately extended until its bottom bearing plate is in close contact with the tunnel floor rock surface. During this process, the contact pressure is monitored in real time using a pressure gauge and controlled at 8 kN to ensure the immediate effectiveness of the support and the application of prestress, allowing the support component to function immediately upon contact and resist surrounding rock deformation. The third-stage mechanical locking is the final fixing stage of the bottom support component. By tightening the handwheel of the mechanical locking clamp on the second-stage sleeve, the wedge-shaped blocks inside the clamp tightly engage the sleeve, thereby achieving mechanical locking of the extended sleeve. This mechanical locking method provides durable and reliable support, ensuring the stable maintenance of support length and support force even if the hydraulic system is unloaded or fails.

[0062] By employing a bottom support assembly consisting of a fixed hinge, a two-stage telescopic sleeve, and a mechanical locking clamp, combined with a three-stage extension locking function, this application effectively solves the technical problems of low installation efficiency, weak contact, and inability to provide immediate stable support in the construction of tunnels in soft rock sections with large deformation. Specifically, the first-stage hinge deployment enables rapid coarse positioning of the bottom support assembly, significantly improving installation efficiency. The second-stage hydraulic extension accurately controls the extension amount through a micro hydraulic cylinder, ensuring close contact between the pressure plate and the tunnel floor rock surface, and applying a preset contact pressure of 8 kN, ensuring the immediacy and firmness of the support, effectively resisting the initial deformation of the surrounding rock. The third-stage mechanical locking firmly locks the extended sleeve through a mechanical locking clamp, providing durable and reliable support, avoiding support failure due to hydraulic system unloading or malfunction, thereby ensuring the overall rigidity and long-term stability of the temporary support ring that instantly forms a spatially closed structure, providing a solid foundation for subsequent construction.

[0063] In an embodiment of the present invention, the prestressed tie rod is a precision-rolled threaded steel bar with a diameter of 20 mm, equipped with dedicated tensioning end and fixing end anchors; As tunnel excavation progresses, after the longitudinal connecting bars of the temporary support rings are welded, the step of immediately using prestressed tie rods to cross-connect the bottom support assemblies of at least three adjacent temporary support rings in both the transverse and longitudinal directions to form a bottom support grid with overall rigidity includes: S51, During connection, the prestressed tie rod is passed through the pre-reserved transverse connection hole on the bottom support assembly of the three adjacent steel frames along the transverse direction of the tunnel; S52, along the longitudinal direction of the tunnel, another set of the prestressed tie rods is arranged in the same manner, and the two sets of prestressed tie rods intersect in an X-shape in the plane; S53 uses two hydraulic jacks to simultaneously tension both ends of the tie rod, with the tension force controlled at 80 kN. Once the predetermined force value is reached, it is immediately anchored with lock nuts, so that the temporary support rings in this area are connected into an integral load-bearing structure through the bottom grid.

[0064] Specifically, the prestressed tie rods are made of 20mm diameter precision-rolled threaded steel, a high-strength, high-ductility steel. Its unique threaded structure facilitates a tight connection with the anchorage, ensuring efficient prestress transfer and reliable anchoring. It can withstand large tension forces and maintain prestress for extended periods. Dedicated tensioning and fixing end anchorages are provided. These anchorages are designed based on the characteristics of the precision-rolled threaded steel and the requirements of prestressed construction. The tensioning end anchorages typically include wedges, anchor plates, and other components, used in conjunction with hydraulic jacks to apply tension. The fixing end anchorages are used to firmly anchor the tie rods to the structure, ensuring stable prestress within the tie rods and preventing relaxation.

[0065] During the connection process, prestressed tie rods are first passed through pre-drilled transverse connection holes on the bottom support assemblies of at least three adjacent steel frames, along the tunnel's transverse direction. These pre-drilled holes are precisely designed in position and size to ensure that the tie rods can pass through smoothly and be accurately positioned, achieving transverse connection between the steel frames. By connecting at least three steel frames, rather than just two, longer continuous connection units can be formed, thereby improving the integrity and stability of the transverse connection.

[0066] Subsequently, another set of prestressed tie rods is arranged in the same manner along the longitudinal direction of the tunnel. These two sets of prestressed tie rods are arranged in an X-shape in the plane. This cross connection method can form a stable triangular unit, which significantly enhances the shear and torsional resistance of the bottom support grid, effectively resists the surrounding rock loads from different directions, and improves the overall stiffness of the grid.

[0067] To ensure the quality and effectiveness of prestressing application, two hydraulic jacks were used to simultaneously tension both ends of the tie rod. Synchronous tensioning ensures uniform force distribution at both ends of the tie rod, avoiding eccentric stress or rod twisting caused by unilateral tensioning, thus ensuring uniform distribution of prestress along the entire length of the tie rod. The tension force was precisely controlled at 80 kN, a value determined comprehensively based on surrounding rock conditions, support structure design, and material properties, aiming to provide sufficient preload to the bottom support grid. Once the predetermined tension force was reached, the tie rod was immediately anchored using lock nuts. The lock nuts, through their self-locking mechanism, reliably lock the prestress within the tie rod, preventing prestress loss, thereby connecting the temporary support rings in this area into a unified load-bearing structure through the bottom grid.

[0068] The above technical solution utilizes high-strength precision-rolled threaded steel as prestressed tie rods, coupled with specialized anchors, ensuring the reliability and durability of prestress application. At least three temporary support rings are connected in a transverse and longitudinal X-shaped cross configuration to form their bottom support components, which are then accurately tensioned and anchored synchronously. This effectively integrates the originally independent bottom support components of the temporary support rings into a single, rigid bottom support grid. This bottom support grid significantly improves the overall stability and deformation resistance of the support structure, distributing the surrounding rock load from localized concentrated areas to a wider support range. It effectively suppresses the occurrence and development of large deformations in soft rock, preventing localized support structure instability and thus providing a safer and more stable working environment for subsequent lining construction.

[0069] In an embodiment of the present invention, the secondary lining is constructed using a pouring sequence of first the inverted arch and then the arch wall, with sections being poured in stages. A delayed expansion agent is added to the arch wall lining concrete to induce a controllable micro-expansion of the lining concrete within 7 to 14 days after pouring. This includes the following steps: S71, the first section of the invert arch and the invert arch filling concrete with a length of 6 meters were poured in one go using C35 concrete and covered with the bottom support grid; S72, after the compressive strength of the first section of the invert arch and the invert arch filling concrete reaches 25 MPa after testing the test block cured under the same conditions, skip the adjacent 6-meter section and pour the second section of the invert arch and the invert arch filling concrete in the next 6-meter section after the interval. S73 After the first section of the invert and the invert filling concrete and the second section of the invert and the invert filling concrete have reached the predetermined strength, an arch wall lining trolley is erected in the 6-meter jump section between the first section of the invert and the invert filling concrete and the second section of the invert and the invert filling concrete, steel bars are tied, and secondary lining concrete of the arch wall in this section is poured.

[0070] The "invert arch first, then arch wall, segmented skip-construction" pouring sequence is an effective method to optimize the secondary lining construction process, control deformation, and improve structural stability. Pouring the invert arch first, the arc-shaped structure at the bottom of the tunnel, provides a stable foundation for the subsequent arch wall lining and seals the tunnel bottom, forming a complete ring structure. This enhances the overall stability of the initial support and effectively resists the heave deformation of the surrounding rock at the bottom. Segmented skip-construction refers to pouring lining sections discontinuously, skipping one section after pouring the next. This method effectively reduces the concentration of concrete shrinkage stress, avoids cracks that may result from continuous pouring, and provides sufficient time for concrete curing and strength development, minimizing disturbance to adjacent newly poured sections. This construction organization method also helps improve construction efficiency, allowing different processes to be carried out simultaneously in different sections.

[0071] Adding a delayed expansion agent to the concrete lining of arch walls allows for controlled micro-expansion of the lining concrete within 7 to 14 days after pouring. This aims to compensate for concrete shrinkage through slight volume expansion, reducing the gap between the lining and the surrounding rock. This controlled micro-expansion enables a closer contact between the lining and the surrounding rock, forming a more effective support system, thereby improving the lining's load-bearing capacity and crack resistance. The delayed expansion characteristic ensures that the expansion occurs after the concrete has reached a certain strength, avoiding potential damage to the concrete structure caused by early expansion.

[0072] In practice, the first 6-meter-long section of the invert arch and its infill concrete was poured in a single pour using C35 concrete. This was intended to enclose the previously constructed bottom support grid, making the grid an integral part of the lining structure and sharing the load. The strength grade of C35 concrete meets the strength and durability requirements of tunnel lining, while the single-pour ensures the integrity and density of this section. Before pouring the next section, the first section of the invert arch and its infill concrete was tested using test blocks cured under the same conditions, ensuring its compressive strength reached 25 MPa. This serves as a key quality control indicator, guaranteeing that the concrete possesses sufficient load-bearing capacity and self-stability to withstand subsequent construction loads and ensure structural safety. Subsequently, the adjacent 6-meter section was skipped, and the second section of the invert arch and its infill concrete was poured in the next 6-meter section after the interval. This segmented skip-pour strategy aims to avoid disturbing the already poured but not fully hardened concrete with the newly poured concrete, and to provide space and time for the curing and strength development of the already poured sections. This also helps to disperse concrete shrinkage stress and reduce the risk of cracking. Finally, after the first and second sections of the invert arch and their filling concrete have reached their predetermined strength, a lining trolley is erected in the 6-meter skip section between these sections. Reinforcing steel is then tied, and the secondary lining concrete for this section is poured. This sequence ensures that the invert arch's strength is prioritized, providing reliable support for the arch wall, and is the standard procedure for arch wall lining construction.

[0073] By employing the above technical solutions, and adopting a segmented, skip-construction pouring sequence—first the invert arch, then the arch wall—the problem of effective integration between the invert arch and the bottom support grid can be effectively solved. By covering the grid with the invert arch, it becomes an integral part of the overall load-bearing structure, significantly enhancing the overall stability of the lining structure. The segmented, skip-construction strategy avoids stress concentration and cracking problems that may arise from continuous pouring. Using strength testing as a condition for pouring the next segment ensures construction safety and improves the overall quality and durability of the lining. Simultaneously, the addition of a delayed-expansion agent to the arch wall lining concrete effectively compensates for concrete shrinkage through controllable micro-expansion, ensuring a tight bond between the lining and the surrounding rock, forming a "zero-gap" support. This fully leverages the combined effect of the surrounding rock and the lining, improving the lining's load-bearing capacity and crack resistance. The delayed-expansion characteristic avoids the adverse effects of early expansion on concrete strength development, ensuring that expansion occurs after the lining has reached a certain strength. This refined secondary lining construction method can effectively control lining deformation in soft rock sections with large deformation, improve the synergistic support capacity between the lining and the surrounding rock, reduce the risk of lining cracking, and optimize construction efficiency and quality, thereby significantly improving the long-term stability and safety of tunnel engineering.

[0074] In an embodiment of the present invention, the delayed expansion agent is an ettringite-based expansion agent, which is added at a ratio of 7% of the total mass of cementitious materials during concrete mixing. When constructing the secondary lining, the pouring sequence is to first pour the inverted arch and then the arch wall, using a segmented, skip-construction method. A delayed expansion agent is added to the arch wall lining concrete to induce controlled micro-expansion of the lining concrete within 7 to 14 days after pouring. This process also includes: S74. After the concrete is poured into the formwork, the internal strain development is monitored by strain sensors embedded in the lining, and the expansion process is controlled by heat preservation and moisture retention curing. The expansion rate of the concrete is less than 0.002% / day from the 3rd to the 7th day after pouring, the main expansion occurs from the 7th to the 14th day, and the final restricted expansion rate is stable at 0.020%.

[0075] Specifically, the delayed expansion agent is preferably an ettringite-based expansion agent, whose main component generates ettringite crystals during hydration. These crystals exhibit volume expansion characteristics, and their expansion reaction is typically more rapid and controllable. Compared to other types of expansion agents, ettringite-based expansion agents can produce stable and predictable expansion behavior under specific conditions. Adding this expansion agent at a ratio of 7% of the total mass of the cementitious materials is an optimized dosage verified through experiments. This aims to ensure that the concrete produces moderate micro-expansion during the hardening process, effectively compensating for concrete shrinkage while avoiding unnecessary stress concentration or cracking in the lining structure due to excessive expansion, while simultaneously considering the strength development and workability of the concrete.

[0076] To accurately monitor the expansion state of the lining concrete, strain sensors embedded within the lining, such as vibrating wire strain gauges or resistance strain meters, are used to acquire real-time, continuous data on the internal strain of the lining concrete during its expansion and hardening process after the concrete is poured into the formwork. These sensors are typically fixed to the reinforcing steel frame or pre-embedded at specific locations within the lining before concrete pouring and are connected to an external data acquisition system via wires. By monitoring the strain data, construction personnel can intuitively understand the expansion initiation time, expansion rate, expansion volume, and uniformity of the lining concrete, providing a scientific basis and feedback for subsequent curing measures and ensuring that the expansion process meets design expectations.

[0077] Based on this, the expansion process is precisely controlled through thermal insulation and moisture retention curing measures. The formation and expansion of ettringite require suitable temperature and sufficient moisture. Thermal insulation curing maintains the internal temperature of the concrete by covering it with insulation materials (such as insulation blankets or plastic films) or setting up heating devices (such as steam curing or electric curing), promoting the hydration reaction of the expansive agent. Moisture retention curing ensures sufficient moisture within the concrete to participate in the expansion reaction through methods such as sprinkling water, covering with wet burlap sacks, and spraying curing agents, preventing insufficient expansion due to excessive evaporation. By accurately controlling the temperature and humidity of the curing environment, the formation rate and quantity of ettringite can be effectively regulated, thereby precisely controlling the expansion rate and final expansion amount of the lining concrete, ensuring it develops according to the preset expansion curve. These specific expansion rates and final limiting expansion rate indicators are carefully designed and optimized. From the 3rd to the 7th day after pouring, the expansion rate is controlled at a low level (less than 0.002% / day) to avoid excessive expansion when the early strength of the concrete is insufficient, thereby reducing the risk of early cracking and providing sufficient strength development time for the lining structure. The main expansion occurs between day 7 and day 14. This occurs after the concrete reaches a certain strength, releasing the expansion stress in a concentrated manner. This allows for good contact and prestress between the lining and the surrounding rock, effectively compensating for the drying shrinkage of the concrete. A final, stable restricted expansion rate of 0.020% ensures that the lining receives sufficient prestress, enhancing its crack resistance and overall stability, while preventing excessive expansion from damaging the structure. This achieves long-term collaborative work between the lining and the surrounding rock.

[0078] Through the above technical solution, this application achieves accurate control over the expansion process of the lining concrete. Specifically, by monitoring data feedback, curing conditions can be dynamically adjusted to ensure that the expansion rate of the concrete remains at an extremely low level (less than 0.002% / day) from day 3 to day 7 after pouring, effectively avoiding excessive expansion due to insufficient early strength, thereby significantly reducing the risk of early cracking of the lining. Subsequently, from day 7 to day 14, by optimizing curing conditions, the concrete undergoes major expansion, allowing the lining to fully compensate for its own shrinkage and establish a close contact and prestress with the surrounding rock. Ultimately, the restricted expansion rate of the lining can be stably controlled at 0.020%, which not only effectively improves the crack resistance and durability of the lining, but more importantly, by accurately controlling the expansion process, the interaction between the lining and the surrounding rock reaches an optimal state, thereby significantly enhancing the overall stability and load-bearing capacity of the secondary lining, effectively coping with the complex geological conditions of soft rock sections with large deformation, and ensuring the safety and long-term service performance of the tunnel structure.

[0079] In an embodiment of the present invention, prior to the step of immediately using prestressed tie rods to cross-connect the bottom support components of at least three adjacent temporary support rings in the transverse and longitudinal directions to form a bottom support grid with overall rigidity after the longitudinal connecting bars of the temporary support rings are welded during tunnel excavation, the tunnel construction method for soft rock large deformation sections further includes: S501, use a geological drilling rig to drill grouting holes with a depth of 2.5 meters and a diameter of 60 millimeters at the bottom plate position corresponding to the arch foot; S502, through the grouting valve installed at the orifice, water glass-cement dual-liquid grout with added quick-setting agent is injected into the hole at a pressure of 0.8-1.0 MPa. Grouting is stopped when the pressure rises to 1.5 MPa or when grout appears to precipitate on the surface of the bottom plate, forming a local reinforcement pad with a thickness of not less than 0.5 meters. The bottom support assembly is then supported on the local reinforcement pad.

[0080] Specifically, a geological drilling rig is a drilling device specifically designed for geotechnical engineering investigation and construction. Its characteristics include high drilling accuracy and efficiency, and the ability to precisely control the depth, diameter, and location of boreholes. In this embodiment, using a geological drilling rig ensures accurate drilling of grouting holes at the corresponding base plate location of the arch foot, providing accurate channels for subsequent base plate reinforcement. The arch foot is a critical force transmission point in the tunnel support structure, bearing both upper loads and lateral compressive forces. The base plate area below it experiences significant concentrated stress. Therefore, local reinforcement of this area can effectively improve its load-bearing capacity and ensure the overall stability of the support structure. A drilling depth of 2.5 meters aims to penetrate the weak rock mass on the surface of the base plate, achieving effective reinforcement within a certain depth range and forming a reinforced area of ​​sufficient thickness. The 60 mm diameter borehole balances the injection efficiency of the grouting material with the stability of the borehole wall, ensuring that the grout can smoothly diffuse and fill the target area.

[0081] The grouting valve installed at the borehole opening is a key control component in grouting operations. Its function is to prevent grout from overflowing or flowing back from the borehole opening during grouting, while also bearing and transmitting grouting pressure to ensure that the grout is effectively injected deep into the formation. Furthermore, the grouting valve is usually equipped with a pressure gauge interface for real-time monitoring of the grouting pressure. An initial grouting pressure of 0.8-1.0 MPa is a suitable pressure range, ensuring that the grout has sufficient penetration and fracturing ability in the soft rock base, allowing it to effectively diffuse to the target reinforcement area, while avoiding potential rock damage or grout loss due to excessive pressure. Water glass-cement two-component grout with added accelerator is a commonly used chemical grouting material with good permeability and setting and hardening properties. By adding an accelerator, the setting time of the grout can be significantly shortened, allowing it to quickly develop strength after injection, thereby rapidly improving the bearing capacity of the soft rock base. A key feature of the two-component grout is that the two components react only after mixing inside the borehole, allowing for better control of the setting time. Grouting should be stopped when the pressure reaches 1.5 MPa or when grout appears to seep out on the surface of the base plate. These are two important criteria for judging the grouting effect and terminating grouting. When the grouting pressure reaches 1.5 MPa, it indicates that the pores and cracks inside the base plate have been fully filled with grout, achieving the expected density. When grout appears to seep out on the surface of the base plate, it indicates that the grout has diffused to the ground surface, and further grouting may result in waste or unnecessary uplift; grouting should be stopped at this point. The locally reinforced cushion layer is a rock and soil mass with high strength and rigidity formed by the grouting operation, with a thickness of not less than 0.5 meters. It aims to provide a solid and uniform bearing foundation for the bottom support components. This cushion layer can effectively disperse the concentrated load transmitted by the bottom support components, preventing local stress concentration and plastic deformation of the soft rock base plate. Supporting the bottom support components on the locally reinforced cushion layer formed by grouting reinforcement is the core objective of this embodiment. This measure ensures that the foundation of the bottom support components is stable and reliable, avoiding instability or settlement of the support structure due to a weak base plate, thereby fully leveraging the overall support function of the temporary support ring.

[0082] By pre-grouting reinforcement at the base plate location corresponding to the arch foot to form a localized reinforcement cushion layer, this application effectively solves the problem of unstable foundation of the bottom support components due to insufficient bearing capacity of the soft rock base plate. Specifically, grouting holes are accurately drilled using a geological drilling rig, and water glass-cement dual-liquid grout with added quick-setting agent is injected to quickly and effectively form a high-strength, high-rigidity localized reinforcement cushion layer in the weak base plate. This cushion layer can significantly improve the local bearing capacity of the base plate and evenly distribute the concentrated load transmitted by the bottom support components, thereby avoiding the overall instability of the temporary support ring caused by local deformation or settlement of the base plate. This pre-reinforcement measure ensures that the bottom support components can be stably supported on a solid foundation, making the stress on the entire temporary support ring more uniform and stable, and greatly improving the reliability and safety of the temporary support system in tunnel construction in soft rock sections with large deformation.

[0083] In an embodiment of the present invention, after tunnel excavation and completion of the initial shotcrete, the step of drilling a first set of shallow, large-diameter stress relief holes in the arch and sidewall areas of the excavation profile, and placing highly compressible filler in the holes, further includes: S110 uses a wet spraying robot to spray C30 early-strength shotcrete mixed with nano-silica powder, wherein the amount of nano-silica is 3% of the cement mass; S120, the spraying operation is carried out in two layers. The thickness of the first layer of spraying is controlled at 30 mm. Immediately after spraying, a steel mesh with a grid spacing of 150 mm * 150 mm is hung and welded to the end of the anchor rod. S130, spray the second layer of concrete to increase the total thickness to 55 mm, and during the spraying process, make the concrete completely cover the steel mesh and anchor plate.

[0084] Specifically, wet-spraying robotic arms are used for shotcrete construction. These robotic arms are automated or semi-automated shotcrete construction equipment that achieves accurate and uniform spraying of concrete through robotic arms and spray heads. Compared to manual spraying, wet-spraying robotic arms significantly improve spraying efficiency, reduce rebound, decrease dust pollution, and ensure the uniformity of the sprayed layer thickness and density, thereby improving the construction quality and operational safety of shotcrete. They are typically equipped with high-pressure pumping systems and accurate flow control devices to ensure the uniformity of the concrete mixture before spraying and its stability during the spraying process. The sprayed concrete is C30 early-strength shotcrete incorporating nano-silica powder. C30 early-strength shotcrete refers to shotcrete with a design strength grade of C30 and characteristics of rapid setting and rapid early strength gain. In soft rock tunnels with large deformation, the surrounding rock deforms rapidly, requiring the support structure to provide load-bearing capacity as quickly as possible. The early-strength characteristic allows the shotcrete to reach sufficient strength in a short time, quickly resisting the pressure of the surrounding rock and providing a stable working face for subsequent construction. Nano-silica powder is an ultrafine siliceous material with extremely high specific surface area and activity. When incorporated into concrete, it can, on the one hand, exert a micro-aggregate filling effect, filling the tiny voids between cement particles and making the concrete structure more compact; on the other hand, nano-silica exhibits significant pozzolanic activity, reacting with calcium hydroxide, a cement hydration product, to generate more hydrated calcium silicate gel, thereby improving the early strength, later strength, toughness, impermeability, and bond strength with the surrounding rock of the concrete. The dosage of nano-silica is 3% of the cement mass. This proportion has been optimized to fully utilize the modifying effect of nano-silica while avoiding the potential decrease in workability or increase in cost due to excessive dosage.

[0085] Furthermore, the shotcrete operation is carried out in two layers. First, the thickness of the first layer is controlled at 30 mm. This initial thin layer of concrete serves primarily to provide a smooth, well-bonded base surface for the subsequent reinforcement mesh and to initially stabilize the surrounding rock surface. The 30 mm thickness ensures sufficient coverage and adhesion without hindering the rapid installation of the reinforcement mesh due to excessive thickness. Immediately after the first layer is sprayed, a 150 mm x 150 mm reinforcement mesh is installed and welded to the anchor bolt ends. The reinforcement mesh, acting as the tensile skeleton of the shotcrete, significantly improves its tensile and shear strength, enhancing its ductility. The 150 mm x 150 mm mesh spacing is designed to provide uniform reinforcement. Welding the reinforcement mesh to the anchor bolt ends ensures the overall synergistic work of the shotcrete and anchor bolt support system, forming a more stable composite support structure that effectively transmits and disperses surrounding rock pressure. Subsequently, the second layer of concrete is sprayed, increasing the total thickness to 55 mm. During the spraying process, the concrete completely encapsulates the reinforcement mesh and anchor bolt pads. After the reinforcing mesh is installed and secured, a second layer of shotcrete is applied, bringing the total thickness to 55 mm. This shotcrete layer aims to completely cover and encase the reinforcing mesh and anchor plates, forming a dense and uniform integral shotcrete layer. Completely encasing the reinforcing mesh and anchor plates not only prevents steel corrosion and extends the service life of the support structure, but more importantly, it ensures effective bonding between the reinforcing mesh and the concrete, allowing them to fully cooperate in resisting surrounding rock deformation and improving the overall load-bearing capacity and stability of the support structure.

[0086] By employing the aforementioned technical solutions and utilizing wet-spraying robots for shotcrete construction, construction efficiency and quality can be significantly improved. This ensures uniform shotcrete layer thickness and high density, while reducing rebound, thus providing a more stable and reliable initial support surface for subsequent processes. C30 early-strength shotcrete, incorporating nano-silica powder, possesses excellent early strength and enhanced toughness, enabling it to rapidly resist initial deformation of soft rock, effectively suppressing rock relaxation and displacement, significantly improving the bond between shotcrete and surrounding rock, and enhancing the integrity and durability of the support structure. Applying two layers of shotcrete with timely installation of reinforcing mesh, followed by the second layer of shotcrete, ensures accurate positioning and full coverage of the mesh. This creates a highly efficient composite load-bearing structure between the shotcrete and the mesh, better able to withstand tensile stress, improve shear resistance, and form an integral whole with the anchor bolt support. This effectively controls large deformations in soft rock, creating more favorable conditions for subsequent stress relief hole drilling and the formation of deep and shallow reinforcement rings, thereby enhancing the safety and stability of the entire tunnel construction method.

[0087] The following example will provide a more detailed explanation of the above technical solution: In a tunnel construction project involving a soft rock section with large deformation, the surrounding rock had low strength and poor self-stabilizing ability, making it prone to continuous large deformations after excavation, posing a challenge to the support structure. To effectively control the deformation of the surrounding rock and ensure construction safety and progress, the construction team adopted a systematic tunnel construction method.

[0088] First, after tunnel excavation and initial shotcreting, construction workers used a hydraulic rock drill with an 85mm diameter drill bit to drill the first set of shallow, large-diameter stress relief holes along the normal direction of the tunnel excavation outline in the arch and sidewall areas of the excavation profile. The drilling depth was strictly controlled at 2.0 meters. During drilling, the drill was withdrawn every 0.5 meters to clean the hole with high-pressure air, ensuring the hole wall was straight and free from collapse. After hole acceptance, pre-cut cylindrical perforated polyurethane foam with a length of 2.1 meters and a diameter of 83mm was selected as the highly compressible filler. A metal pusher rod with a guide cone at the front end was used to push the foam into the hole until the depth mark on the rod was flush with the hole opening, ensuring the foam completely filled the cavity. This measure aims to effectively absorb and release shallow surrounding rock stress by creating a compressible space within the rock, preventing excessive relaxation of the surrounding rock and solving the problem of poor performance of single-depth stress relief holes in existing technologies.

[0089] After the first set of holes was completed and the working face was cleaned, the construction workers used the same rock drill with a 45mm diameter drill bit to drill at the center point of the marked diamond grid. The drilling direction was parallel to the tunnel axis or slightly inclined into the tunnel at a degree not exceeding 3 degrees, reaching a depth of 3.2 meters. This was followed by drilling the second set of deep, small-diameter stress relief holes. After drilling, a 42mm outer diameter PVC pipe was inserted into the hole. The front end of the PVC pipe was sealed, and overflow holes were drilled in the pipe wall. The rear end protruded 10cm above the rock surface without grouting, allowing the exposed rear end to serve as a channel for deep stress release. By setting up two sets of stress relief holes with different diameters and depths, hierarchical control of stress release in the surrounding rock at different depths was achieved, avoiding the problem of insufficient deep stress release.

[0090] Subsequently, anchor bolt holes were drilled and grouting anchor bolts were installed around each set of stress relief holes, and grouting operations were carried out in stages with differential pressure. First, high-pressure grouting was performed using a screw-type grouting pump. Through the grouting joints connected to the anchor bolts around the second set of deep, small-diameter stress relief holes, ultrafine cement grout with a water-cement ratio of 0.35:1 was injected into the holes. The initial grouting pressure was 0.5 MPa, and the pressure was gradually increased to 1.2 MPa within 1 minute and maintained at this pressure value. Grouting was stopped when the grout volume reached 80% of the design value or the pressure reached 1.5 MPa, forming a deep reinforcement ring. After an interval of at least 4 hours, low-pressure grouting was performed using the same equipment but with ordinary cement grout. Grouting was carried out through the anchor bolts around the first set of shallow, large-diameter stress relief holes. The grouting pressure was always controlled at 0.4 MPa. When the grout stably seeped from adjacent anchor bolt holes or rock fissures, this pressure was maintained for another 30 seconds before grouting ended, forming a shallow coupling ring. This phased, differential pressure grouting method effectively reinforces the deep surrounding rock, while the shallow surrounding rock forms a flexible coupling with the support structure, further optimizing the stress release effect and improving the overall stability of the support system.

[0091] A steel frame for the arch wall is erected within the shallow coupling ring, and a bottom support assembly with a three-stage extension locking function is installed simultaneously. The bottom support assembly consists of a fixed hinge seat, a two-stage telescopic sleeve, and a mechanical locking clamp. Before erecting the steel frame, a geological drilling rig is used to drill grouting holes with a depth of 2.5 meters and a diameter of 60 mm at the base plate position corresponding to the arch foot. Through the grouting valve installed at the hole opening, water glass-cement dual-liquid grout with added quick-setting agent is injected into the hole at a pressure of 0.8-1.0 MPa. Grouting is stopped when the pressure rises to 1.5 MPa or when grout appears to precipitate on the surface of the base plate, forming a local reinforcement cushion layer with a thickness of not less than 0.5 meters. The bottom support assembly is then supported on the local reinforcement cushion layer. During installation, the fixed hinge seat is first connected to the arch foot plate of the arch wall steel frame using high-strength bolts; the first-stage sleeve is swung to rotate around the hinge axis to the designed angle, and the limit pin is inserted to complete the first-stage hinge deployment; the micro hydraulic cylinder integrated in the second-stage sleeve is activated to push the second-stage sleeve downward until its bottom bearing plate is in close contact with the tunnel floor rock surface, and the contact pressure is monitored by a pressure gauge to reach 8 kN, completing the second-stage hydraulic extension; the handwheel of the mechanical locking clamp set on the second-stage sleeve is tightened, so that the wedge block in the clamp bites the sleeve, completing the third-stage mechanical locking. This allows a single steel frame to instantly form a spatially closed temporary support ring, solving the problems of insufficient rigidity, complex installation, and difficulty in quickly forming an effective closed ring in traditional bottom support structures. Especially under conditions of weak floor slabs, the local reinforcement pad provides a reliable support foundation.

[0092] As tunnel excavation progresses, immediately after the longitudinal connecting bars of the temporary support rings are welded, prestressed tie rods are used to cross-connect the bottom support components of at least three adjacent temporary support rings in both the transverse and longitudinal directions, forming a bottom support grid with overall rigidity. The prestressed tie rods are 20 mm diameter precision-rolled threaded steel bars, equipped with dedicated tensioning and fixing end anchors. During connection, along the transverse direction of the tunnel, the prestressed tie rods are passed through the pre-reserved transverse connection holes on the bottom support components of the three adjacent steel frames; along the longitudinal direction of the tunnel, another set of prestressed tie rods is arranged in the same manner, with the two sets of prestressed tie rods intersecting in an X-shape in the plane; two hydraulic jacks are used to simultaneously tension the tie rods at both ends, with the tension force controlled at 80 kN. Once the predetermined force value is reached, the tie rods are immediately anchored with lock nuts, connecting the temporary support rings in this area into an integral load-bearing structure through the bottom grid. This bottom support grid formation significantly improves the overall rigidity of the support system, solves the problem of lack of integral bottom connection between adjacent steel frames, and enhances the collaborative working ability of the support structure.

[0093] During tunnel construction, based on the spatiotemporal evolution characteristics of surrounding rock deformation at monitoring sections, secondary lining is initiated when the deformation is determined to have entered the longitudinal attenuation and transmission period. The criterion for determining the longitudinal attenuation and transmission period is: among at least three consecutive monitoring sections arranged along the tunnel longitudinally, the deformation rate of the section closest to the tunnel face has passed its peak and begun to decline, while the deformation rate of the more distant sections has decreased to below 30% of its peak rate. This scientifically based selection of the timing for secondary lining construction avoids the problems of lining cracking or delays in construction that might arise from traditional experience-based judgments.

[0094] When constructing the secondary lining, a pouring sequence of first the invert arch and then the arch wall, using a segmented skip-construction method, is adopted. A delayed expansion agent is added to the arch wall lining concrete to allow for controlled micro-expansion within 7 to 14 days after pouring. Specifically, the first 6-meter section of the invert arch and its infill concrete is poured in one go using C35 concrete, covering the bottom support grid. After the compressive strength of the first section of the invert arch and its infill concrete reaches 25 MPa as tested by test blocks cured under the same conditions, the adjacent 6-meter section is skipped, and the second section of the invert arch and its infill concrete is poured in the next 6-meter section after the interval. After both the first and second sections of the invert arch and its infill concrete reach their predetermined strengths, an arch wall lining trolley is erected in the 6-meter skip-construction section between the first and second sections, reinforcing bars are tied, and the secondary lining concrete for this section of the arch wall is poured. The delayed expansion agent is an ettringite-based expansion agent, added at a ratio of 7% of the total mass of cementitious materials during concrete mixing. After the concrete is poured into the formwork, strain sensors embedded in the lining monitor its internal strain development, and the expansion process is controlled through heat preservation and moisture retention curing. This ensures that the expansion rate of the concrete is less than 0.002% / day from day 3 to day 7 after pouring, with major expansion occurring from day 7 to day 14, and the final restricted expansion rate stabilizing at 0.020%. This segmented, skip-pour pouring method combined with the application of the delayed expansion agent allows the lining concrete to undergo micro-expansion in the later stages, actively filling the tiny gaps between the lining and the surrounding rock. This achieves a tight fit between the lining structure and the surrounding rock, solving the problem of separation between the traditional lining and the surrounding rock, and improving the long-term stability and load-bearing capacity of the support system.

[0095] Through the synergistic effect of the above series of technical measures, this tunnel construction method effectively solves problems such as insufficient stress release of surrounding rock, insufficient stiffness of support structure, improper timing of secondary lining construction, and separation of lining from surrounding rock in the construction of soft rock large deformation sections, forming a systematic, scientific and efficient support system for soft rock large deformation tunnels.

[0096] 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 tunnel construction in a soft rock large deformation section, characterized in that, The tunnel construction method for soft rock sections with large deformation includes: After the tunnel excavation and initial shotcreting are completed, the first set of shallow, large-diameter stress relief holes are drilled in the arch and sidewall areas of the excavation profile, and highly compressible filler is placed in the holes. At the grid node locations formed by the first group of shallow, large-diameter stress relief holes, a second group of deep, small-diameter stress relief holes is drilled. Anchor bolt holes were drilled and grouting anchor bolts were installed around each group of stress relief holes. Grouting was carried out in stages with differential pressure. First, high-pressure grouting was performed on the anchor bolts around the deep, small-diameter stress relief holes of the second group to form a deep reinforcement ring. Then, low-pressure grouting was performed on the anchor bolts around the shallow, large-diameter stress relief holes of the first group to form a shallow coupling ring. High-pressure grouting was performed using a screw-type grouting pump. Ultrafine cement slurry with a water-cement ratio of 0.35:1 was injected into the holes through grouting joints connected to the anchor bolts around the second group of holes. The initial grouting pressure was 0.5 MPa, and the pressure was gradually increased to 1.2 MPa within 1 minute and maintained at this pressure value. Grouting was stopped when the grout volume reached 80% of the design value or the pressure reached 1.5 MPa. After an interval of at least 4 hours, low-pressure grouting was performed using the same equipment but with ordinary cement slurry, injected through the anchor bolts around the first group of holes. The grouting pressure was always controlled at 0.4 MPa. MPa, when the grout seeps steadily from the adjacent anchor bolt holes or rock fissures, maintain this pressure for another 30 seconds before ending the grouting process; An arch wall steel frame is erected within the shallow coupling ring, and a bottom support assembly with a three-level extension and locking function is installed simultaneously. The three-level extension and locking function includes a first-level hinged unfolding, a second-level hydraulic extension, and a third-level mechanical locking, so that a single steel frame can instantly form a spatially closed temporary support ring. As the tunnel is excavated, after the longitudinal connecting bars of the temporary support ring are welded, prestressed tie rods are immediately used to cross-connect the bottom support components of at least three adjacent temporary support rings in the horizontal and vertical directions to form a bottom support grid with overall rigidity. During tunnel construction, based on the spatiotemporal evolution characteristics of surrounding rock deformation at the monitoring section, secondary lining is started when it is determined that the deformation has entered the longitudinal attenuation and transmission period. The criteria for determining the longitudinal attenuation and transmission period are: among at least three continuous monitoring sections arranged along the longitudinal direction of the tunnel, the deformation rate of the section closest to the working face has passed the peak and begun to decrease, while the deformation rate of the more distant section has dropped to below 30% of the peak rate. When constructing the secondary lining, the pouring sequence of first the inverted arch and then the arch wall, with sections skipping sections, is adopted. A delayed expansion agent is added to the arch wall lining concrete to allow the lining concrete to undergo controllable micro-expansion within 7 to 14 days after pouring.

2. The method of tunnel construction for a soft rock large deformation section according to claim 1, characterized in that, After tunnel excavation and initial shotcreting, the steps of drilling the first set of shallow, large-diameter stress relief holes and placing highly compressible filler in the arch and sidewall areas of the excavation profile include: Using a hydraulic rock drill with an 85 mm diameter drill bit, drill holes along the normal direction of the tunnel excavation outline. The drilling depth is strictly controlled at 2.0 meters. During the drilling process, retract the drill every 0.5 meters and clean the hole with high-pressure air to ensure that the hole wall is straight and there is no collapse. After the hole-forming acceptance, a pre-cut cylindrical open-cell polyurethane foam with a length of 2.1 meters and a diameter of 83 millimeters was selected as the high compressibility filler. Use a metal push rod with a guide cone at the front end to push the foam into the hole until the depth mark on the rod is flush with the hole opening, ensuring that the foam completely fills the hole.

3. A method of tunnel construction for a soft ground large deformation section as claimed in claim 2, wherein, The steps for drilling a second set of deep, small-diameter stress relief holes at the grid node locations formed by the first set of shallow, large-diameter stress relief holes include: After the first set of holes is completed and the working face is cleaned, the same rock drill is used with a 45 mm diameter drill bit. Drilling is carried out at the center point of the marked diamond grid. The drilling direction is parallel to the tunnel axis or slightly inclined into the tunnel at a degree not greater than 3 degrees. The drilling depth reaches 3.2 meters. After drilling is completed, a PVC plastic pipe with an outer diameter of 42 mm is inserted into the hole. The front end of the PVC plastic pipe is closed, the pipe wall is drilled with overflow holes, and the rear end protrudes 10 cm above the rock surface without grouting, so that the rear end protruding above the rock surface serves as a channel for deep stress release.

4. The method of tunneling through a section of soft ground with large deformation as claimed in claim 1, wherein, The bottom support assembly consists of a fixed hinge seat, a two-stage telescopic sleeve, and a mechanical locking clamp. The steps of erecting an arch wall steel frame within the shallow coupling ring and simultaneously installing a bottom support assembly with a three-stage extension and locking function, wherein the three-stage extension and locking function includes a first-stage hinged deployment, a second-stage hydraulic extension, and a third-stage mechanical locking, to instantly form a spatially closed temporary support ring for a single steel frame include: First, connect the fixed hinge seat to the arch foot base plate of the arch wall steel frame with high-strength bolts; Swing the first-stage sleeve to rotate it around the hinge axis to the designed angle, and insert the limit pin to complete the first-stage hinge deployment; The micro hydraulic cylinder integrated in the secondary sleeve is activated, pushing the secondary sleeve downward until the pressure plate at its bottom is in close contact with the rock surface of the tunnel floor. The contact pressure is monitored by a pressure gauge to reach 8 kN, thus completing the second stage of hydraulic extension. Tighten the handwheel of the mechanical locking clamp set on the secondary sleeve, so that the wedge block inside the clamp bites the sleeve, thus completing the third-stage mechanical locking.

5. The method of tunneling through a section of soft ground with large deformation as claimed in claim 1, wherein, The prestressed tie rod is a 20 mm diameter precision rolled threaded steel bar, equipped with dedicated tensioning end and fixing end anchors; As tunnel excavation progresses, after the longitudinal connecting bars of the temporary support rings are welded, the step of immediately using prestressed tie rods to cross-connect the bottom support assemblies of at least three adjacent temporary support rings in both the transverse and longitudinal directions to form a bottom support grid with overall rigidity includes: During connection, the prestressed tie rod is passed through the pre-reserved transverse connection holes on the bottom support components of the three adjacent steel frames along the transverse direction of the tunnel; Along the longitudinal direction of the tunnel, another set of the prestressed tie rods is arranged in the same manner, and the two sets of prestressed tie rods intersect in an X-shape in the plane; Two hydraulic jacks are used to tension the rods simultaneously at both ends. The tension force is controlled at 80 kN. Once the predetermined force value is reached, the rods are immediately anchored with lock nuts, so that the temporary support rings in this area are connected into an integral load-bearing structure through the bottom grid.

6. The method of tunneling through a section of soft ground with large deformation as claimed in claim 1, wherein, When constructing the secondary lining, the following steps are taken: first the invert arch, then the arch wall, and the lining is poured in sections with skipped pours. A delayed expansion agent is added to the arch wall lining concrete to induce controlled micro-expansion of the lining concrete within 7 to 14 days after pouring. The first section of the invert arch, which is 6 meters long, and the invert arch filling concrete were cast in one go using C35 concrete and covered with the bottom support grid. After the compressive strength of the first section of the invert arch and the invert arch filling concrete reaches 25 MPa after testing the test blocks cured under the same conditions, skip the adjacent 6-meter section and pour the second section of the invert arch and the invert arch filling concrete in the next 6-meter section after the interval. After the first section of the invert and the invert filling concrete and the second section of the invert and the invert filling concrete have reached the predetermined strength, an arch wall lining trolley is erected in the 6-meter skip section between the first section of the invert and the invert filling concrete and the second section of the invert and the invert filling concrete, steel bars are tied, and secondary arch wall lining concrete is poured in this section.

7. The method of tunneling through a section of soft ground with large deformation as claimed in claim 1, wherein, The delayed expansion agent is an ettringite-based expansion agent, which is added at a ratio of 7% of the total mass of cementitious materials during concrete mixing. When constructing the secondary lining, the pouring sequence is to first pour the inverted arch and then the arch wall, using a segmented, skip-construction method. A delayed expansion agent is added to the arch wall lining concrete to induce controlled micro-expansion of the lining concrete within 7 to 14 days after pouring. This process also includes: After the concrete is poured into the formwork, the internal strain development is monitored by strain sensors embedded in the lining, and its expansion process is controlled by heat preservation and moisture retention curing. The expansion rate of the concrete is less than 0.002% / day from the 3rd to the 7th day after pouring, the main expansion occurs from the 7th to the 14th day, and the final restricted expansion rate is stable at 0.020%.

8. The tunnel construction method for soft rock sections with large deformation as described in claim 1, characterized in that, As tunnel excavation progresses, prior to the step of immediately using prestressed tie rods to cross-connect the bottom support components of at least three adjacent temporary support rings in both the transverse and longitudinal directions after the longitudinal connecting bars of the temporary support rings are welded, forming a bottom support grid with overall rigidity, the tunnel construction method for soft rock large deformation sections further includes: A geological drilling rig was used to drill grouting holes with a depth of 2.5 meters and a diameter of 60 millimeters at the base plate position corresponding to the arch foot. Through the grouting valve installed at the orifice, water glass-cement dual-liquid grout with added quick-setting agent is injected into the hole at a pressure of 0.8-1.0 MPa. Grouting is stopped when the pressure rises to 1.5 MPa or when grout appears to precipitate on the surface of the bottom plate, forming a local reinforcement cushion layer with a thickness of not less than 0.5 meters. The bottom support assembly is then supported on the local reinforcement cushion layer.

9. The tunnel construction method for soft rock sections with large deformation as described in claim 1, characterized in that, After tunnel excavation and initial shotcreting, the steps of drilling the first set of shallow, large-diameter stress relief holes and placing highly compressible filler in the arch and sidewall areas of the excavation profile also include: A wet spraying robot is used to spray C30 early-strength shotcrete mixed with nano-silica powder, wherein the amount of nano-silica is 3% of the cement mass. The spraying operation is carried out in two layers. The thickness of the first layer is controlled at 30 mm. Immediately after spraying, a steel mesh with a grid spacing of 150 mm * 150 mm is hung and welded to the end of the anchor bolt. A second layer of concrete is sprayed to increase the total thickness to 55 mm. During the spraying process, the concrete completely covers the steel mesh and anchor plate.

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