Tunnel reverse hole construction method without arrival road condition

Abstract

The present application relates to the technical field of tunnel construction, and particularly refers to a tunnel reverse hole construction method without reaching road conditions, which solves the problems of no external road at the tunnel exit in the mountain and canyon area, inability to construct hole protection, easy collapse of broken surrounding rock and mud-rock flow risk. Specifically, the method comprises the following steps: S1: advance geological prediction; S2: segmented composite advance support; S3: small pilot hole first exit; S4: step-by-step controlled excavation; S5: close type initial support; S6: hole slope protection; S7: dynamic monitoring and feedback. The present application does not need external construction road and hole pre-provision facilities, the segmented pipe shed adapts to the construction angle in the hole, the pilot hole reduces the risk of hole exit, the controlled blasting reduces the disturbance of surrounding rock, and the whole process monitoring ensures the safety of construction; the deformation of surrounding rock is controllable, there is no safety accident, and the construction efficiency is high, especially suitable for the tunnel exit engineering construction in the mountain and canyon, steep terrain and no accessible road, and has significant practical value and popularization prospect.

Description

Technical Field

[0001] This invention relates to the field of tunnel construction technology, specifically to a method for reverse tunnel exit construction under conditions where there is no accessible road. Background Technology

[0002] In complex terrains such as high mountains, deep valleys, and remote mountainous areas, tunnel engineering often faces the predicament of having no accessible road to the exit. The conventional tunnel exit construction process is to proceed from the outside in and from the inside out, and from protection to excavation. That is, first, an external road is used to reach the tunnel entrance, and after completing safety measures such as clearing the slopes around the entrance, constructing drainage systems, and installing segmented long pipe roof supports, excavation is gradually carried out from the entrance. This process relies on the accessibility of the construction site outside the tunnel entrance and the available space for work. For tunnel exits without accessible roads, if the conventional process is forcibly adopted, several kilometers or even tens of kilometers of winding mountain roads must be built first. Not only are the construction costs 20 to 30 times higher than those of roads in plains areas, but the roads are also often located in areas prone to landslides and debris flows, making it extremely easy for roads to be damaged and for personnel to be injured during construction. The subsequent maintenance costs are also incalculable. In some extremely steep terrains, it is not even possible to build a road, causing the tunnel exit project to stall.

[0003] At the same time, tunnel exit sections without accessible roads generally suffer from multiple overlapping geological hazards. Exit sections are mostly shallowly buried (usually less than 33m deep), with the surface cover consisting of loosely deposited gravelly soil and rounded gravel, 0.5 to 3m thick. Affected by the disturbance of tunnel excavation, surface subsidence and slope collapse are very likely to occur. In addition, the catchment area above the tunnel entrance is large, which can easily lead to debris flows during the rainy season, directly threatening construction safety.

[0004] In response to the aforementioned special scenarios, the engineering community has attempted various reverse excavation techniques, namely, reverse excavation from the already excavated section inside the tunnel towards the exit. However, existing technologies still have many insurmountable drawbacks: First, traditional segmented long pipe roofs require horizontal construction from outside the tunnel entrance along the tunnel axis. However, due to tunnel clearance limitations, pipe roof drilling rigs cannot achieve horizontal drilling during reverse construction inside the tunnel. Instead, a steep upward tilting reverse construction method is used, resulting in limited pipe roof support length (usually no more than 15m). Furthermore, the pipe roof ends cannot extend beyond the tunnel excavation outline, failing to effectively cover the shallow buried section at the tunnel entrance, significantly reducing the support effect. Second, existing reverse excavation methods mostly employ full-face or bench excavation without setting up pilot tunnels for risk assessment and stress release. In situations where there is no tunnel entrance... Under external protection conditions, an excessively large excavation cross-section can easily lead to stress concentration in the surrounding rock, causing instability at the tunnel face and collapse of the tunnel entrance slope. Third, existing blasting techniques have not been specifically optimized for Class V fractured surrounding rock, and conventional charge structures and detonation methods are generally used, with blasting vibration velocities typically exceeding 3 cm / s, which can easily disperse the already fractured surrounding rock and induce secondary disasters. Fourth, there is a lack of a specific prevention and control system for debris flow risks, and most projects only implement slope protection after exiting the tunnel, which cannot cope with the debris flow impact that may occur during the exit process. Fifth, existing technologies are mostly fragmented summaries of construction experience, and have not formed a complete standardized technical system covering geological forecasting, advanced support, excavation and blasting, initial support, slope protection, and dynamic monitoring, resulting in poor controllability of the construction process and frequent safety accidents.

[0005] Currently, domestic and international research on reverse tunnel exit construction mainly focuses on Class II to IV surrounding rock scenarios with relatively good surrounding rock conditions, or only addresses the single problem of no road access. For extremely complex and superimposed scenarios with no external access roads, there is still no mature, reliable, and quantifiable complete set of construction technologies. Summary of the Invention

[0006] In view of the shortcomings of the existing technology, the purpose of this invention is to provide a safe, efficient and controllable method for tunnel reverse exit construction under conditions of no accessible road that can be adapted to this special scenario.

[0007] To achieve the above objectives, the present invention provides a method for reverse tunnel exit construction under conditions of no accessible road, comprising the following steps: S1. Advanced geological prediction: Conduct geological exploration of the surrounding rock at the tunnel exit section to obtain the degree of rock fragmentation and water content. S2. Segmented composite advance support: From the excavated section of the tunnel towards the exit, an advance support system is constructed in at least two segments to form a continuous surrounding rock reinforcement layer covering the exit section. S3. Small pilot tunnel exits first: A small pilot tunnel with a cross-section smaller than the design cross-section is excavated at the tunnel face, so that the pilot tunnel can be connected to the tunnel exit first. S4. Step-by-step controlled excavation: Using the pilot tunnel as the open face, the tunnel cross-section is gradually expanded to the design size using a step-by-step excavation method; S5. Immediate initial support: Immediately after excavation, initial support is applied to seal the exposed surrounding rock; S6. Slope protection at the tunnel entrance: After exiting the pilot tunnel, the slope at the tunnel entrance and the surrounding area shall be protected. S7. Dynamic monitoring and feedback: Monitor the deformation of the surrounding rock and the stability of the slope throughout the process, and adjust the construction parameters in real time based on the monitoring results.

[0008] To better implement the method of the present invention, further, in step S1, the advanced geological prediction adopts a combination of advanced horizontal drilling and ground-penetrating radar detection; the depth of the advanced horizontal drilling covers all key sections of the tunnel exit, and the detection range of the ground-penetrating radar includes the surrounding rock area not less than 20m in front of the tunnel face.

[0009] To better implement the method of the present invention, further, in step S2, the segmented composite advanced support system includes segmented long pipe roofs and advanced small guide pipes; the segmented long pipe roofs are constructed in at least two sections along the tunneling direction, with adjacent pipe roof sections overlapping each other, and the pipe roofs are set within a predetermined range of the tunnel arch; the advanced small guide pipes are arranged circumferentially between the pipe roofs, forming a spatial composite reinforcement structure with the segmented long pipe roofs.

[0010] To better implement the method of the present invention, the segmented long pipe roof is further constructed at an upward angle of 5° to 8° with the horizontal direction, and the end of the pipe roof extends out of the tunnel excavation outline by not less than 1.5m; reinforcing grout is injected into both the segmented long pipe roof and the advanced small pipe, so that the grout spreads to the surrounding rock and cements the broken surrounding rock into a whole with self-stabilizing ability.

[0011] To better implement the method of the present invention, the specific process of step S3 is as follows: when the tunnel construction reaches 5-10m from the exit, the full-section excavation construction is suspended; the existing tunnel arch frame is used to weld the middle partition steel frame to form the support skeleton of the small pilot tunnel; the pilot tunnel excavation follows the principles of short advance, weak blasting, and fast closure, with a cycle advance of no more than 0.8m; after the pilot tunnel exits the tunnel, the dangerous rocks and boulders within a radius of no less than 5m on the slope of the tunnel entrance are immediately cleared, and a temporary retaining structure is set up at the tunnel entrance.

[0012] To better implement the method of the present invention, the intermediate partition steel frame is further made of I18 I-beams and is connected to the original tunnel arch frame B unit by double-sided welding, with a weld height of not less than 8mm; the spacing between the intermediate partition steel frames is 0.5m to 1.2m, and a φ8 steel mesh with a grid size of 20cm×20cm is hung on the inner side; adjacent steel frames are connected by φ22 longitudinal connecting bars, with a circumferential spacing of 1.0m between the longitudinal connecting bars.

[0013] To better implement the method of the present invention, further, in step S4, the step-by-step excavation method adopts a two-stage excavation method, the height of the upper stage covers the entire range of the arch pre-support, and the height of the lower stage is the remaining height of the tunnel; the excavation process adopts controlled blasting technology to control the blasting vibration velocity within a safe limit.

[0014] To better implement the method of this invention, the controlled blasting technology further employs a single-stage wedge-shaped cut-out combined with peripheral smooth blasting; the borehole parameters are: peripheral hole spacing 50cm, minimum resistance line 60cm, excavation hole spacing 100cm, and bottom plate borehole spacing 100cm; peripheral hole depth 1.0m, cut-out hole depth 1.4m, and auxiliary hole depth 1.2m; the charge control is as follows: peripheral hole charge coefficient 0.2, single hole charge 0.3kg; cut-out hole charge coefficient 0.65, single hole charge 1.35kg; auxiliary hole charge coefficient 0.55~0.6, single hole charge 0.6~0.75kg; electronic digital detonators are used for initiation, the single hole delay of the cut-out hole is not less than 15ms, and the peripheral holes detonate every 3~5 holes, ensuring a smooth blasting effect while using the phase-shifting vibration reduction principle to control the blasting vibration velocity to no more than 1.5cm / s.

[0015] To better implement the method of the present invention, the specific process of step S5 is as follows: the initial support is constructed immediately after the excavation face, and the distance between the excavation face and the working face of the initial support does not exceed 2m, reducing the exposure time of the surrounding rock; the initial support system consists of an I18 I-beam steel frame, φ25 low prestressed resin roll hollow grouting anchor rods, φ8 steel mesh, and C30 early high-strength shotcrete, with a shotcrete thickness of 25cm; the steel frame is firmly welded to a 3m long anchor pipe, and the grouting pressure of the anchor pipe is 0.8~1.2MPa; the steel mesh is laid close to the surface of the surrounding rock and firmly connected to the steel frame; the shotcrete is constructed using a wet spraying process to ensure the spray thickness and strength.

[0016] To better implement the method of the present invention, further, in step S6, the slope protection of the tunnel entrance includes temporary protection and permanent protection; the temporary protection adopts a structure of anchor bolts, steel mesh, and shotcrete; the permanent protection is set with a passive protection net with a length of not less than 60m and anchor bolt frame beams with a spacing of 3m×3m; at the same time, a drainage system is set on the outside of the tunnel entrance and the top of the slope to divert surface water away from the tunnel entrance area and prevent rainwater from eroding the slope.

[0017] Compared with the prior art, the present invention has the following advantages and beneficial effects: (1) This invention does not require the construction of high-risk mountain access roads of several kilometers to tens of kilometers. It directly pushes out of the tunnel from inside the tunnel in reverse, which not only avoids the geological disaster risks such as landslides and debris flows along the access road, but also solves the construction bottleneck of cliffs, deep valleys and other places where access roads are not feasible. It greatly expands the site selection range of tunnel projects. (2) This invention is adapted to the reverse-cantilever construction angle inside the tunnel, which solves the problem that traditional long pipe roofs cannot be constructed horizontally inside the tunnel, and can achieve full coverage support for the shallow buried section of the tunnel entrance; the pipe roof and small pipe form a spatial composite reinforcement structure, which increases the strength of the broken surrounding rock by 30% to 50%, effectively preventing collapse, water inrush and mud inrush accidents; the excavation cross section is reduced to 1 / 5 to 1 / 4 of the original cross section, releasing the stress of the surrounding rock in advance; a temporary retaining structure is set up immediately after exiting the tunnel, which solves the construction safety problem under the condition of no external protection of the tunnel entrance; the electronic digital detonator delayed detonation technology is adopted to strictly control the blasting vibration velocity within 1.5cm / s; a complete process of advance prediction, real-time monitoring and dynamic adjustment is established, and the deformation of the surrounding rock is controllable throughout the process; (3) This invention does not require waiting for the construction of access roads and pre-construction facilities at the entrance of the tunnel. The various processes are closely connected, which greatly shortens the construction period of exiting the tunnel and saves tens of millions of yuan in construction costs for the mountain access road. At the same time, it avoids the handling of safety accidents and losses due to construction delays. It does not require large-scale excavation of the mountain to build access roads, thus protecting the fragile ecological environment of the western mountainous area to the greatest extent. (4) This invention enables safe and efficient construction of tunnels exiting in reverse under conditions of no access road, and also has significant economic, social and ecological benefits, which is of great significance for promoting the construction of transportation infrastructure in the mountainous areas of western my country. Attached Figure Description

[0018] Other features, objects, and advantages of the invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings: Figure 1 This is a flowchart of the method described in this invention; Figure 2 This is a schematic diagram of the pre-support method for the tunnel exit of the present invention. Figure 3 This is a blasting design diagram for the method described in this invention. Detailed Implementation

[0019] Embodiments of the present invention are described in detail below. Examples of these embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0020] To make the objectives, process conditions, and advantages of the present invention clearer, the present invention will be further described in detail with reference to the following embodiments. However, the embodiments of the present invention are not limited thereto. Various substitutions and modifications can be made based on common technical knowledge and conventional means in the art without departing from the above-described technical concept of the present invention, and all such substitutions and modifications should be included within the scope of the present invention. The specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0021] Example 1: This embodiment provides a method for tunnel reverse exit construction under conditions where there is no accessible road. The specific process is as follows: Figure 1 As shown, it includes the following steps: Step S1: Conduct geological exploration of the surrounding rock at the tunnel exit section to obtain the degree of rock fragmentation and water content. Step S2: From the excavated section of the tunnel towards the exit, construct an advanced support system in multiple sections to form a continuous surrounding rock reinforcement layer; Step S3: Excavate a pilot tunnel at the tunnel face, so that the pilot tunnel is the first to be connected to the tunnel exit; Step S4: Using the pilot tunnel as the free face, expand the tunnel cross-section to the design size by using a step-by-step excavation method; Step S5: Immediately after excavation, implement initial support to seal the surrounding rock; Step S6: After exiting the pilot tunnel, the slope at the tunnel entrance is protected. Step S7: Monitor the deformation of the surrounding rock and the stability of the slope throughout the process, and adjust the construction parameters in real time based on the monitoring results.

[0022] in, 1. Accurate advance geological forecasting is a prerequisite for safe reverse tunneling construction. It can effectively identify key information such as the lithology, degree of fracture, and water content of the surrounding rock ahead, providing a basis for adjusting the construction plan.

[0023] (1) Selection of forecasting methods Based on geological conditions, a comprehensive forecasting method combining advanced horizontal drilling and core sampling with ground-penetrating radar (GPR) detection was adopted. Three advanced horizontal boreholes were drilled at the tunnel face, covering the key section leading out of the tunnel. Core analysis was used to determine the lithology, rock mass fracture degree, and stratigraphic boundaries. Simultaneously, GPR was used to detect the water-bearing capacity of the surrounding rock within a 20m range in front of the tunnel face, achieving complementary verification of geological information.

[0024] (2) Drilling technical requirements Advanced horizontal drilling must be carried out in strict accordance with the core drilling standards. The core recovery rate of different strata must meet the following requirements: not less than 70% within 5m above and below the bedrock sliding surface and the bottom of important strata; not less than 70% for slightly weathered and weakly weathered strata; not less than 65% for strongly weathered, completely weathered and tectonic fracture zones; and not less than 80% for intact bedrock.

[0025] During the drilling process, the water output changes are monitored in real time. Water pressure and water volume measuring instruments are installed to record the mileage of water output changes. Time-water pressure-water volume change curves are plotted to accurately determine the water-rich areas of the surrounding rock.

[0026] (3) Application of forecast results Based on advanced geological forecasts, construction parameters were adjusted promptly. The original design support parameters were optimized to suit the characteristics of the surrounding rock, strengthening the advanced support, shortening the cycle advance, and ensuring construction safety.

[0027] 2. Segmented advanced support The surrounding rock stability of the exit section is poor, and the quality of the advance support directly determines the success or failure of the reverse exit. Considering the constraint that there is no access road and the pipe roof cannot be constructed from the tunnel entrance, a composite advance support system of long pipe roof and advance small guide pipe is adopted, and segmented pipe roof construction technology is used.

[0028] (1) Support parameter design, such as Figure 2 As shown D89 segmented long pipe roof: The designed length is 30m. Considering the installation angle limitations for the inverted tunnel construction, it was adjusted to be constructed in two segments. The first segment is 18m long, and the second segment is 15m long, with an overlap length of 3m. The circumferential spacing of the pipe roof is 40cm, and the installation range is 120° of the arch. It is reinforced by single-component cement grout injection.

[0029] D42 advanced small guide pipe: 4m in length, 40cm in circumferential spacing, 3.0m in longitudinal spacing, and 1m in overlap length. It uses single-component cement grouting to form a synergistic support effect with the long pipe roof, thereby enhancing the integrity of the surrounding rock.

[0030] (2) Segmented pipe roof construction technology Drilling: Use a geological drilling rig to drill holes within a 120° range of the arch of the working face. The hole diameter is slightly larger than the pipe roof diameter to ensure smooth installation of the pipe roof.

[0031] Pipe roof installation: After the first section of pipe roof is completed, grouting is carried out. After the grout strength reaches the design requirements, the second section of pipe roof is constructed to ensure that the overlap length of the two sections of pipe roof meets the design requirements and forms a continuous support structure.

[0032] Grouting control: The grouting pressure is controlled at 0.5-1.0 MPa to ensure that the grout is evenly diffused into the surrounding rock fissures. After the grouting is completed, the grouting effect is checked. If it does not meet the standard, grouting treatment is required.

[0033] 3. Small pilot tunnel exit technique To address the safety issues associated with exiting the tunnel in the absence of access facilities, a pilot tunnel approach was adopted. A temporary protective structure was formed using the intermediate partition wall to gradually achieve the breakthrough of the upper step, followed by the construction of the middle and lower steps.

[0034] (1) Design parameters of small pilot tunnel The cross-sectional dimensions of the pilot tunnel are controlled at 2m × 3m, supported by I18 I-beam steel frames with an arch spacing of 0.8m. The central diaphragm steel frame is connected to the existing arch frame B unit of the tunnel to form a stable load-bearing system; a 20cm × 20cm φ8 steel mesh is installed, using φ22 longitudinal connecting bars with a circumferential spacing of 1.0m to enhance the integrity of the support structure.

[0035] (2) Construction process When the tunnel construction reaches 6m from the tunnel entrance, the face construction is suspended, and the existing arch frame B unit is used to set up a middle partition wall to form a small pilot tunnel; During the excavation of the pilot tunnel, the principles of short advance, weak blasting, and rapid closure were strictly followed, and the advance per cycle was controlled within 0.8m. After exiting the small tunnel, immediately clear the dangerous rocks and boulders on the slope of the tunnel entrance to create a safe environment for subsequent construction; then proceed with the protection of the slope of the tunnel entrance, the exit of the middle and lower steps, and the lining construction in sequence.

[0036] 4. Step-by-step controlled blasting The exit section consists of Class V fractured rock, requiring strict control of vibration intensity during blasting to avoid exacerbating the risk of collapse due to rock disturbance. A bench excavation method combined with controlled blasting technology was employed, optimizing blasting parameters and initiation methods to achieve gentle rock fracturing.

[0037] (1) Excavation step division The upper step is 4.17m high, and the lower step is 3.33m high, using a two-step excavation method. Section ZK0+185-205 will be constructed according to the original design step height, while section ZK0+205-215 will use mechanical excavation of small pilot tunnels, and large-scale excavation is prohibited.

[0038] (2) Optimization of blasting parameters Hole design: A single-stage wedge-shaped cut is adopted, with smooth blasting around the perimeter. The spacing between peripheral holes is 50cm, and the minimum resistance line is 60cm; the spacing between excavation holes is 100cm; the spacing between floor blast holes is 100cm. The depth of peripheral holes is 1.0m, the depth of cut holes is 1.4m, and the depth of auxiliary holes is 1.2m.

[0039] Charge control: Limit the maximum charge per section. The charge coefficient for peripheral holes is 0.2, and the charge per hole is 0.3 kg. The charge coefficient for slotted holes is 0.65, and the charge per hole is 1.35 kg. The charge coefficient for auxiliary holes is 0.55 to 0.6, and the charge per hole is 0.6 to 0.75 kg.

[0040] Detonation method: Electronic digital detonators are used for detonation, with a single hole delay of not less than 15ms; 3 to 5 peripheral holes detonate at a time, ensuring the light blasting effect while reducing the blasting vibration velocity by using the phase-shifting vibration reduction principle.

[0041] (3) Principles of blasting construction The principles of "weak blasting, short advancing footage, strong support, early closure and frequent measurement" shall be strictly followed, the charging amount shall be controlled, and the number of perimeter holes and auxiliary holes shall be increased. For grade V surrounding rock, the cyclic footage shall be controlled as 1 bay for the upper bench and 2 bays for the lower bench, so as to avoid excessive disturbance caused by single excavation.

[0042] 5. Primary support The primary support shall be constructed in time closely following the excavation face to reduce the exposure time of the surrounding rock. The support system is composed of I18 I-steel frames, φ25 low-prestress resin coil hollow grouting anchor rods, φ8 reinforcement meshes and C30 early high-strength shotcrete, and the thickness of the shotcrete is 25 cm.

[0043] The steel frames are installed manually, and are firmly connected with foot-locking anchor pipes, and the length of the foot-locking anchor pipes is 3 m; the reinforcement mesh is hung closely against the surface of the surrounding rock and connected firmly with the steel frames; the shotcrete is constructed by a wet spraying machine to ensure the spraying thickness and strength.

[0044] 6. Slope protection After the pilot tunnel exits, the portal section shall be cleared of surface vegetation and graded in time, the surface loose gravel soil and loose rock mass shall be removed, and the slope shall be graded at a slope ratio of 1:1. The temporary edge and crown slope protection adopts a system of φ22 mortar anchor rods + φ8 reinforcement mesh + C30 shotcrete, the anchor rods are 3 m in length and arranged in a quincuncial pattern with a spacing of 1.2 m×1.2 m; permanent protection is provided with 60 m passive protective nets and anchor rod frame beams to ensure the long-term stability of the slope.

[0045] 7. Construction process control and monitoring (1) Principles of construction process control Principle of dynamic adjustment: the excavation condition of the tunnel face and the advance prediction results shall be closely concerned during construction; if the stability of the surrounding rock or the monitoring data is abnormal, the situation shall be reported to all participating parties in time, and the surrounding rock grade and support parameters shall be adjusted.

[0046] Principle of cooperative operation: all working procedures shall be connected strictly in accordance with the construction process, and excavation, support and monitoring shall be carried out synchronously, so as to avoid safety risks caused by disconnected working procedures.

[0047] Principle of safety priority: special emergency plans shall be formulated for potential risks such as surrounding rock collapse and debris flow, sufficient emergency materials and equipment shall be equipped, and emergency drills shall be carried out regularly.

[0048] (2) Design of monitoring scheme 1) Monitoring contents and measuring point arrangement The main monitoring contents include surface settlement, surrounding rock convergence in the tunnel and vault settlement. The measurement range of surface settlement measuring points is not less than H0+B (H0 is the buried depth of the tunnel, B is the excavation width of the tunnel), the transverse spacing is 2 to 5 m, and proper densification shall be carried out near the central line; the longitudinal spacing is determined according to the buried depth of the tunnel, which is 5 to 10 m when H0≤B, 10 to 15 m when B<H0≤2B, and 15 to 30 m when 2B<H0≤2(B+H).

[0049] The measuring points are anchored with φ22 steel bars. The pit is 1.5m deep (frost depth + 0.3m), 40cm wide and 40cm long, and anchored with concrete. The steel bar ends protrude 20-30mm above the ground, are painted red, and reflective sheets are attached.

[0050] 2) Monitoring frequency and data processing Surface monitoring is conducted once a day, with the frequency increased if any abnormalities are detected. Monitoring data is promptly processed and analyzed to generate settlement-time curves, assessing the stability of the surrounding rock. If the settlement rate exceeds the standard limit (2 mm / d), emergency measures such as reinforced support are immediately implemented.

[0051] (3) Risk prevention and control plan Handling of abnormal surrounding rock: If there are signs of rockfall or collapse at the working face, construction should be stopped immediately, personnel and equipment should be evacuated, and radial grouting should be used to reinforce the surrounding rock before construction can continue.

[0052] Support parameter adjustment: When the construction of No. 1 adit reached ZK0+205 (10m from the tunnel entrance), the excavation of the middle and lower benches was stopped, and the upper benches were excavated one by one to open the tunnel. The spacing of the steel frames was adjusted from 1m to 0.8m, and a set of locking feet was added to the upper benches for reinforcement.

[0053] Single-sided exit plan: Based on the surrounding rock conditions of the upper step, a central partition wall can be added to the last 10m section before single-sided exit to ensure construction safety.

[0054] Grouting reinforcement plan: Based on the surrounding rock conditions, the lithology within 10m of the tunnel entrance will be determined, and radial grouting will be carried out if necessary to enhance the bearing capacity of the surrounding rock.

[0055] Example 2: This embodiment provides a more detailed description of the invention in conjunction with specific engineering examples.

[0056] 1. Basic Project Information A tunnel is located in Changdu City, Tibet Autonomous Region. The No. 1 branch tunnel intersects with the cross tunnel at kilometer marker H0+624, with an intersection angle of 63°38′56″. The branch tunnel extends from ZK0+022.06 to ZK0+215, with a total length of 192.94m. It employs a reverse slope construction method, with the exit kilometer marker ZK0+215. The tunnel entrance is at an altitude of 3670.52m. Because the branch tunnel exit is located in a high mountain canyon area with no accessible roads, preliminary work such as site leveling and access road construction cannot be carried out. Furthermore, slope protection and portal facilities cannot be prioritized at the tunnel entrance. Therefore, a reverse exit construction scheme was chosen.

[0057] 2. Engineering geological and hydrogeological conditions (1) Engineering geological characteristics The exit section of branch tunnel #1 traverses Quaternary overburden and weathered bedrock, primarily consisting of Class V surrounding rock. The tunnel body itself is mainly composed of gneiss and granite, predominantly Class III surrounding rock. Near the entrance, the surface is covered with Quaternary Holocene colluvial fine breccia and alluvial fine breccia, both in a medium-dense state, Class II ordinary soil, with a bearing capacity standard value δ0 = 250 kPa. The underlying granite has a fine- to medium-coarse-grained structure and massive texture. The strongly weathered layer has a bearing capacity standard value δ0 = 550 kPa, class IV soft rock, corresponding to Class IV surrounding rock; the weakly weathered layer has a bearing capacity standard value δ0 = 1200 kPa, class V secondary hard rock.

[0058] According to advanced geological forecasts, the strata in the ZK0+170–ZK0+205 section are Yanshanian granite, mainly strongly weathered rock, soft in texture, with well-developed joints and fissures, fractured rock mass, and generally damp tunnel face. Point-to-line water seepage is observed in the surrounding rock fissures, indicating poor stability. The ZK0+174–ZK0+189 section is a shallow-buried tunnel section with extremely poor surrounding rock integrity and stability, posing a risk of rockfall and collapse. Construction control is implemented according to Class V surrounding rock standards.

[0059] (2) Hydrogeological conditions The groundwater is mainly bedrock fissure water, with good quality and no corrosiveness. However, the exit section of the branch tunnel presents a risk of debris flow, requiring special attention to prevent groundwater inrush and debris flow hazards during construction.

[0060] The specific implementation steps of the construction using the method of this invention are as follows: 1. Advanced geological forecasting Three advanced horizontal boreholes, each 35m deep, were drilled along the tunnel face (ZK0+185) to cover the entire critical exit section (ZK0+170~ZK0+205). The rock core fragmentation was recorded in real time during drilling, categorized into four levels: intact, relatively intact, fragmented, and extremely fragmented. Simultaneously, ground-penetrating radar was used to detect water abundance in the surrounding rock within a 20m radius ahead of the tunnel face, generating time-pressure-volume curves and identifying three water-rich areas with a single-section water outflow ≥5L / min. Based on the detection results, the section from ZK0+175 to ZK0+185 was determined to be a high-risk section due to both fragmentation and water abundance, and subsequent support parameters were accordingly strengthened.

[0061] 2. Segmented composite advanced support A composite support system using D89 segmented long pipe roofs and D42 advanced small pipes, such as... Figure 2As shown. The D89 segmented long pipe roof was constructed in two sections: the first section was 18m long, and the second section was 15m long, with a 3m overlap. The pipe roof was constructed at a 6° angle upwards to the horizontal, with a circumferential spacing of 40cm, extending 120° from the arch. The end of the pipe roof extended 1.5m beyond the tunnel excavation outline. The D42 advanced small guide pipe was 4m long, with a circumferential spacing of 40cm, a longitudinal spacing of 3.0m, a 1m overlap, and an external insertion angle of 12°, positioned circumferentially between the pipe roofs. Both the pipe roof and the small guide pipes were grouted using single-component cement grout at a pressure of 0.5–1.0MPa, with the pressure increased to 0.8–1.2MPa in high-risk sections. After grouting, core drilling was used to verify that the grout diffusion radius was ≥0.8m; sections that did not meet the standard were supplemented with grout.

[0062] 3. A small pilot tunnel will be used to exit the main tunnel first. When the tunnel construction reached 6m from the exit (ZK0+199), full-face excavation was suspended. An I18 I-beam steel diaphragm frame was welded to the existing arch frame B unit to form a small pilot tunnel with a cross-sectional size of 2m × 3m. The diaphragm frame was double-sided welded to the existing arch frame, with a weld height of 8mm and a frame spacing of 0.8m. A 20cm × 20cm φ8 steel mesh was installed on the inner side, and adjacent frames were connected by φ22 longitudinal connecting bars with a circumferential spacing of 1.0m. The pilot tunnel excavation followed the principles of short advance, weak blasting, and rapid closure, with a cycle advance of 0.8m and a single-hole charge ≤0.2kg. Within 2 hours of excavation, a 25cm thick C30 concrete layer was sprayed to seal the rock surface. After the pilot tunnel exited, mechanical and manual methods were immediately used to clear any loose rocks within a 5m radius of the tunnel entrance slope. A temporary steel arch frame was installed at the entrance, and a 25cm thick C30 concrete layer was sprayed to form a temporary retaining structure.

[0063] 4. Control excavation in stages A two-stage excavation method was adopted. The upper stage was 4.17m high (covering the 120° support area of ​​the arch), and the lower stage was 3.33m high. The upper and lower stages were longitudinally staggered by 6m, and the left and right excavation faces of the lower stage were laterally staggered by 2.5m. A single-stage wedge cut was used in conjunction with smooth blasting around the perimeter. The borehole parameters were strictly implemented according to the design. The specific blasting design is as follows: Figure 3 As shown: the spacing between peripheral holes is 50cm, the minimum resistance line is 60cm, the spacing between excavation holes is 100cm, and the spacing between floor blast holes is 100cm; the depth of peripheral holes is 1.0m, the depth of cut holes is 1.4m, and the depth of auxiliary holes is 1.2m. The charge quantity is controlled as follows: 0.3kg per peripheral hole, 1.35kg per cut hole, and 0.6-0.75kg per auxiliary hole. Electronic digital detonators are used for initiation, with a 15ms delay per cut hole and a 4-hole detonation sequence for peripheral holes. The measured blasting vibration velocity is 1.2-1.4cm / s, meeting safety requirements.

[0064] 5. Close-fitting initial support Initial support is constructed immediately following the excavation face, with the distance between the excavation face and the initial support face controlled within 1.5m. The initial support system consists of an I18 I-beam steel frame, φ25 low-prestressed resin roll hollow grouting anchors, φ8 steel mesh, and C30 early high-strength shotcrete, with a shotcrete thickness of 25cm. Each steel frame is equipped with four 3m long anchor pipes, which are double-sided welded to the steel frame using L-shaped φ22 steel bars, with a weld length of 15cm. The grouting pressure of the anchor pipes is 0.8~1.2MPa. The steel mesh is laid tightly against the surrounding rock surface and securely tied to the steel frame. Shotcrete is applied using a wet shotcrete machine to ensure uniform thickness and adequate strength.

[0065] 6. Slope protection at the tunnel entrance After exiting the pilot tunnel, the slope at the tunnel entrance is cleaned and brushed at a 1:1 gradient, removing the top layer of loose, rounded gravel 0.5–1.5 m thick. Temporary slope protection uses a structure of φ22 mortar anchors (3 m long, spaced 1.2 m × 1.2 m in a quincunx pattern), φ8 steel mesh, and C30 shotcrete (15 cm thick). Permanent protection consists of a 60 m long passive protective net (30 cm × 30 cm mesh size) and anchor frame beams (8 m anchor length, spaced 3 m × 3 m). A ring-shaped intercepting ditch (0.6 m × 0.6 m cross-section) is constructed at the top of the slope, and a longitudinal drainage ditch is installed every 10 m along the slope. A temporary intercepting ditch (0.5 m × 0.5 m cross-section) is installed 5 m outside the tunnel entrance. All drainage ditches are interconnected to divert surface water away from the tunnel entrance area.

[0066] 7. Dynamic monitoring and feedback Surface settlement, tunnel surrounding rock convergence, and arch subsidence were monitored throughout the entire process. The surface settlement monitoring points covered an area of ​​H0+B (H0 being the tunnel depth and B the tunnel excavation width), with a lateral spacing of 2–5 m (reinforcing to 2 m near the centerline) and a longitudinal spacing of 5–10 m. Monitoring was conducted once daily. When the settlement rate exceeded 2 mm / d, radial grouting anchors (4 m long, spaced 0.8 m × 0.8 m) were immediately installed for reinforcement. During construction, the maximum surrounding rock convergence was 12 mm, and the maximum arch subsidence was 18 mm, both within the specified limits.

[0067] 8. Construction process control and monitoring (1) Construction process control principles Dynamic adjustment principle: During construction, closely monitor the excavation of the working face and the results of advance forecasts. If the stability of the surrounding rock or the monitoring data is abnormal, report to all parties involved in the construction in a timely manner and adjust the surrounding rock level and support parameters.

[0068] Collaborative work principle: Each process should be strictly connected according to the construction process, with excavation, support and monitoring carried out simultaneously to avoid safety risks caused by disconnection between processes.

[0069] Safety priority principle: Specialized emergency plans are formulated for potential risks such as surrounding rock collapse and debris flow, sufficient emergency materials and equipment are equipped, and emergency drills are carried out regularly.

[0070] (2) Monitoring scheme design 1) Monitoring contents and measuring point arrangement The main monitoring contents include surface settlement, tunnel surrounding rock convergence and vault settlement. The measurement range of surface settlement measuring points is not less than H0+B (H0 is the buried depth of the tunnel, B is the excavation width of the tunnel), the transverse spacing is 2~5m, and the spacing is appropriately increased near the central line; the longitudinal spacing is determined according to the tunnel buried depth: it is 5~10m when H0≤B, 10~15m when B<H0≤2B, and 15~30m when 2B<H0≤2(B+H).

[0071] The measuring points are anchored with φ22 steel bars, the pit digging depth is 1.5m (frozen soil depth +0.3m), both the width and length are 40cm, the measuring points are anchored with concrete, the steel bar head is exposed 20~30mm above the ground, coated with red paint and pasted with reflective sheets.

[0072] 2) Monitoring frequency and data processing The surface monitoring frequency is once per day, and the monitoring frequency shall be increased when abnormal conditions occur. The monitoring data shall be sorted and analyzed in time, the settlement-time curve shall be drawn, and the stability of surrounding rock shall be judged. If the settlement rate exceeds the specification limit (2mm / d), emergency measures such as strengthening support shall be taken immediately.

[0073] (3) Risk prevention and control plan Treatment for abnormal surrounding rock: If there are signs of block falling and sliding collapse on the working face, construction shall be stopped immediately, personnel and equipment shall be evacuated, and construction shall be continued after the surrounding rock is reinforced by radial grouting.

[0074] Adjustment of support parameters: When the 1# branch adit is constructed to ZK0+205 (10m away from the tunnel portal), the excavation of the middle and lower steps shall be stopped, the upper step shall be excavated portal-outward frame by frame, the spacing of steel frames is adjusted from 1m to 0.8m, and an additional set of foot locking reinforcement is added to the upper step.

[0075] Unilateral portal exit plan: According to the revealed condition of the surrounding rock on the upper step, a middle partition can be added to the last 10m section before exiting the portal unilaterally first, so as to ensure construction safety.

[0076] Grouting reinforcement plan: According to the surrounding rock condition, the lithology within the 10m range of the tunnel portal shall be distinguished, and radial grouting shall be carried out if necessary to improve the bearing capacity of the surrounding rock.

[0077] 9. Construction effect and verification (1) Construction safety effect During the reverse exit construction of the No. 1 adit of the construction tunnel, no accidents such as surrounding rock collapse, slope instability, or blasting safety occurred. Three potential water-rich areas were accurately identified through advanced geological forecasting, and targeted grouting reinforcement measures effectively controlled the risk of groundwater inrush. The application of segmented advanced support and small pilot tunnel technology ensured the stability of the Class V fractured surrounding rock, with no significant rockfall or deformation at the working face.

[0078] (2) Effect of surrounding rock deformation control Monitoring data shows that the maximum surface settlement was 18 mm, the maximum convergence of the surrounding rock inside the tunnel was 12 mm, and the maximum subsidence of the arch was 15 mm, all within the limits specified in the "Technical Specification for Monitoring and Measurement of Railway Tunnels" (Q / CR9218-2015). The settlement rate was less than 2 mm / d and tended to stabilize in the later stages, indicating that the reverse exit construction technology had a good effect on controlling the deformation of the surrounding rock.

[0079] (3) Construction efficiency and quality The total construction period for the reverse exit tunnel was 20 days, which was completed 5 days ahead of schedule, meeting the project requirements in terms of construction efficiency. The support structure showed no cracks, leaks, or other quality issues, and the project quality met the design standards.

[0080] This invention uses a construction tunnel adit as an engineering basis to study the reverse exit construction technology of tunnels under conditions where there is no accessible road, and draws the following conclusions: (1) Under the condition of no access road, the reverse exit technology advances in reverse inside the tunnel without relying on external roads and tunnel entrance sites. It is an effective way to solve the problem of tunnel exit in special terrain and is applicable to tunnel projects in high mountain and canyon areas where there is no external access.

[0081] (2) The technical system of advanced geological prediction, segmented advanced support, pilot tunnel first, bench blasting control, and dynamic monitoring can effectively control the stability of Class V fractured surrounding rock and achieve safe and efficient exit from the tunnel. Among them, the segmented D89 pipe roof solves the angle limitation problem of reverse cantilever construction in the tunnel, the pilot tunnel technology reduces the risk of exiting the tunnel under the condition of no tunnel entrance protection, and the controlled blasting technology reduces the disturbance of the surrounding rock.

[0082] (3) The dynamic adjustment mechanism and risk prevention and control plan adopted during the construction process can respond to changes in geological conditions in a timely manner and ensure construction safety. The surface settlement and tunnel monitoring data show that the control effect of this technology on the deformation of the surrounding rock meets the requirements of the specifications, and the project quality and construction efficiency have reached the expected goals.

[0083] Although embodiments of the invention have been shown and described, those skilled in the art will understand that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the claims and their equivalents.

Claims

1. A method for reverse tunnel exit construction under conditions of no accessible road, characterized in that, Includes the following steps: Step S1: Conduct geological exploration of the surrounding rock at the tunnel exit section to obtain the degree of rock fragmentation and water content. Step S2: From the excavated section of the tunnel towards the exit, construct an advanced support system in multiple sections to form a continuous surrounding rock reinforcement layer; Step S3: Excavate a pilot tunnel at the tunnel face, so that the pilot tunnel is the first to be connected to the tunnel exit; Step S4: Using the pilot tunnel as the free face, expand the tunnel cross-section to the design size by using a step-by-step excavation method; Step S5: Immediately after excavation, implement initial support to seal the surrounding rock; Step S6: After exiting the pilot tunnel, the slope at the tunnel entrance is protected. Step S7: Monitor the deformation of the surrounding rock and the stability of the slope throughout the process, and adjust the construction parameters in real time based on the monitoring results.

2. The method for reverse tunnel exit construction under conditions of no accessible road, as described in claim 1, is characterized in that, In S1, advanced horizontal drilling combined with ground-penetrating radar is used to conduct geological exploration of the surrounding rock at the tunnel exit section. The drilling depth covers the key section at the tunnel exit, and the exploration range includes the surrounding rock within 20m in front of the tunnel face.

3. A method for reverse tunnel exit construction under conditions of no accessible road, as described in claim 1 or 2, characterized in that, The advanced support system constructed in multiple segments in step S2 includes segmented long pipe roofs and advanced small guide pipes; the segmented long pipe roofs are constructed in at least two segments, with adjacent segments overlapping each other, and the pipe roofs are set in the tunnel arch; the advanced small guide pipes are set between the pipe roofs to form a composite reinforcement structure with the pipe roofs.

4. A method for reverse tunnel exit construction under conditions of no accessible road, as described in claim 3, is characterized in that... The segmented long pipe roof is constructed at an upward angle of 5° to 8° with the horizontal direction, and the end of the segmented long pipe roof extends not less than 1.5m beyond the tunnel excavation outline. Grout is injected into both the segmented long pipe roof and the small pipe, so that the grout spreads to the surrounding rock and cements the broken surrounding rock into a whole.

5. A method for reverse tunnel exit construction under conditions of no accessible road, as described in claim 1 or 2, characterized in that, The specific process of step S3 is as follows: When the tunnel construction reaches 5 to 10 meters from the exit, the full-section construction is suspended. The cross-sectional size of the pilot tunnel is smaller than that of the tunnel. The cyclic advance is no more than 0.8 meters. The steel frame of the middle partition wall is welded using the original tunnel arch frame to support the excavated pilot tunnel. After the pilot tunnel exits the tunnel, the dangerous rocks and falling rocks on the slope of the tunnel entrance are immediately cleared, and a temporary retaining structure is set up at the tunnel entrance.

6. A method for reverse tunnel exit construction under conditions of no accessible road, as described in claim 5, is characterized in that, The central diaphragm steel frame is welded from I18 I-beams to the original tunnel arch frame. The spacing between the central diaphragm steel frames is 0.5m to 1.2m, and the inner side is equipped with a φ8 steel mesh with a grid size of 20cm×20cm. φ22 longitudinal connecting bars are used, with a circumferential spacing of 1.0m.

7. A method for reverse tunnel exit construction under conditions of no accessible road, as described in claim 1 or 2, characterized in that, In step S4, the excavation method adopts a two-stage method, with the upper stage height covering the arch pre-support range and the lower stage height being the remaining height of the tunnel; the blasting vibration velocity is controlled within a safe range.

8. A method for reverse tunnel exit construction under conditions of no accessible road, as described in claim 7, is characterized in that, The specific process for controlling the blasting vibration velocity within a safe range is as follows: A single-stage wedge-shaped cut is used, with smooth blasting around the perimeter. The distance between peripheral holes is 50cm, and the minimum resistance line is 60cm. The distance between excavation holes is 100cm; the distance between bottom blast holes is 100cm; the depth of peripheral holes is 1.0m, the depth of cut holes is 1.4m, and the depth of auxiliary holes is 1.2m. The maximum charge per section is limited: the charge coefficient for peripheral holes is 0.2, and the charge per hole is 0.3kg; the charge coefficient for cut holes is 0.65, and the charge per hole is 1.35kg; the charge coefficient for auxiliary holes is 0.55-0.6, and the charge per hole is 0.6-0.75kg. Electronic digital detonators are used for initiation, with a delay of no less than 15ms for each cut hole; 3-5 peripheral holes are detonated at a time.

9. A method for reverse tunnel exit construction under conditions of no accessible road, as described in claim 1 or 2, characterized in that, The specific process of step S5 is as follows: the initial support is constructed in a timely manner following the excavation face, and the distance between the excavation face and the initial support face does not exceed 2m; the initial support system consists of an I18 I-beam steel frame, φ25 low prestressed resin roll hollow grouting anchor rods, φ8 steel mesh and C30 early high-strength shotcrete, with a shotcrete thickness of 25cm; the steel frame is installed manually and firmly connected to the locking anchor pipe, with a locking anchor pipe length of 3m; The reinforcing mesh is hung close to the surface of the surrounding rock and is firmly connected to the steel frame; Shotcrete is applied using a wet spraying machine.

10. A method for reverse tunnel exit construction under conditions of no accessible road, as described in claim 1 or 2, characterized in that, The slope protection treatment at the tunnel entrance in step S6 includes temporary protection and permanent protection; the temporary protection adopts an anchor bolt and sprayed mesh structure, and the permanent protection is set with a passive protection net and anchor bolt frame beams; at the same time, a drainage system is set at the tunnel entrance.

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