Advanced small pilot tunnel construction method for high ground stress soft rock tunnel

Abstract

The invention discloses an advanced small pilot tunnel construction method for a high ground stress soft rock tunnel, belongs to the technical field of tunnel engineering construction, and adopts an excavation mode of combining a micro-step with a small pilot tunnel method and a support measure of combining a composite lining structure with a retractable U-shaped steel frame for temporary support. When crossing through a high-ground-stress soft rock stratum, by constructing the advanced small pilot tunnel, the effects of geological pre-judgment, risk pre-prevention and control, stress release in advance, support pressure relief, disturbance reduction and surrounding rock loosening circle reduction can be achieved. The surrounding rock deformation after the main tunnel is excavated is effectively reduced, so that the high-crustal-stress soft rock tunnel is efficiently and safely constructed.

Description

Technical Field

[0001] This invention relates to the field of tunnel engineering construction technology, specifically to a method for constructing a pilot tunnel for soft rock tunnels under high ground stress. Background Technology

[0002] With the increasing scale of railway construction, the proportion of tunnels in plateau and mountainous areas is growing, as are their design lengths and burial depths. The significant topographical variations and complex geological environments in these regions pose severe challenges to tunnel construction. When long, deep-buried tunnels traverse fault fracture zones, the high ground stress and tectonic movements result in a high degree of rock fragmentation, easily leading to engineering risks such as face instability and initial support deformation. Engineering practice shows that after excavation of high-stress soft rock tunnels, the stress redistribution process can easily cause chain reactions such as steel arch twisting and shotcrete cracking, resulting in large deformation and failure of the initial support. Improving the stress environment during tunnel excavation, reducing blasting disturbance, and minimizing the loosened zone are key to reducing initial support deformation and failure. Therefore, this invention proposes a micro-step construction method for the main tunnel, setting up advance pilot tunnels on the upper steps. These pilot tunnels reveal the geology ahead, releasing stress in advance and effectively reducing the deformation of the surrounding rock after the main tunnel excavation, enabling efficient and safe construction of high-stress soft rock tunnels. Meanwhile, the small pilot tunnel adopts a pressure-relief steel frame structure, which allows for deformation of the surrounding rock. When the deformation and damage are minor, it can be easily repaired and reused, saving costs. Summary of the Invention

[0003] In order to overcome the shortcomings of the prior art, the purpose of this invention is to provide a method for constructing a pilot tunnel for high-stress soft rock tunnels. The aim is to improve the stress environment during the excavation of high-stress fractured soft rock tunnels, reduce blasting disturbance, reduce the loosening zone, effectively improve the deformation of the main tunnel support structure, and at the same time have the functions of geological prediction and risk prevention.

[0004] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for constructing a pilot tunnel for high-stress soft rock tunnels, characterized by comprising the following steps: Step 1: Determine the excavation cross-section; Step 2: Determine the height and length of the main tunnel steps; Step 3: Excavation of the pilot tunnel: The construction method adopts a combination of micro-steps and small pilot tunnels. A small pilot tunnel is set up in the space of the upper step of the main tunnel. The small pilot tunnel is excavated first, and the ground stress is released in advance by utilizing the open surface of the small pilot tunnel. The small pilot tunnel is constructed 10m to 15m ahead of the main tunnel. The small pilot tunnel is excavated by a combination of milling and blasting, depending on the geological conditions. Step 31: Install the steel frame in the small guide hole: The pilot tunnel is temporarily supported by a retractable "U"-shaped steel frame, which can be reused as the pilot tunnel is excavated. Step 32: Shotcrete; Step 33: Monitor and measure. If there is an abnormality, proceed to comprehensive analysis and judgment, adjust the support measures, and then proceed to the next cycle of operation. If there is no abnormality, proceed directly to the next cycle of operation. Step 4: When the pilot tunnel advances the main tunnel by 10m to 15m, proceed to the main tunnel advance support; otherwise, proceed to Step 3. Step 41: The main tunnel advance support adopts φ76 medium pipe roof with φ42 small pipe, HW200 steel frame is set up around the whole ring, low prestressed hollow grouting anchor rod is set up in the arch wall, φ42 steel flower pipe is set up around the whole ring for shallow grouting, a waterproof layer is set between the initial support and the lining, and the secondary lining adopts cast-in-place reinforced concrete. Step 42: The reserved portion of the main tunnel is excavated using milling. Simultaneously with the excavation of the main tunnel, the support of the small pilot tunnel is removed. The removal of the small pilot tunnel support is adapted to the excavation height reserved for the main tunnel. Step 43: Construction of anchor bolts and support for the main tunnel; the main tunnel support structure adopts a composite lining structure; installation of steel frame and monitoring and measurement markers; Step 44: After shallow radial grouting of the main tunnel, monitor and measure. If there is an abnormality, conduct comprehensive analysis and judgment, adjust the support measures, and then proceed to the next cycle of operation. If there is no abnormality, proceed directly to the next cycle of operation.

[0005] The aforementioned small pilot tunnel support structure adopts a retractable "U"-shaped steel frame, divided into Unit A, Unit B, and Unit C, and is fabricated using "U"-shaped steel. The joint between Unit A and Unit B is connected by an lap joint and fixed with clamps. The joint is located at the junction of the arch wall where the convergence deformation is large. The clamp-fixed connection can retract to adapt to the convergence deformation after excavation, and the steel frame can be recycled. Unit B and Unit C are fixed by connecting plates connected by bolts. The connecting plates are welded to the steel frame. A non-perforated base plate is set at the arch foot of Unit B.

[0006] The height of the small pilot tunnel and the reserved excavation height are the height of the upper step.

[0007] The height of the small guide tunnel is sufficient for the excavator and loader to operate at the required height. The reserved excavation height needs to be determined based on the geological conditions and the range of the loosened zone.

[0008] The height of the lower step is the height from the arch foot of the upper step to the arch foot of the arch wall steel frame.

[0009] The length of the upper step should be sufficient to accommodate the working space of the mechanical equipment.

[0010] The advance length of the pilot tunnel is determined by combining the construction progress and the convergence deformation. The principle is that the excavation length during the period of greatest deformation after excavation is the advance length of the pilot tunnel.

[0011] The present invention has the following beneficial effects: The method described in this invention can effectively address the engineering conditions of large deformation in soft rock under high ground stress. By using a micro-step excavation method and a small pilot tunnel to release ground stress in advance, combined with optimized support measures, it forms a highly efficient and safe method for constructing tunnels in soft rock under high ground stress. Compared with traditional tunnel construction methods, it greatly reduces the disturbance to the surrounding rock during excavation, reduces the loosened zone, reduces the damage to the support caused by high ground stress, reduces the amount of deformation reserved during the main tunnel excavation, improves construction efficiency, and shortens the construction period. It has extremely wide application value. Attached Figure Description

[0012] Figure 1 A schematic diagram showing the step height setting for the micro-step method; Figure 2 A schematic diagram showing the step length setting for the micro-step method; Figure 3 A cross-sectional view of the support structure for the main tunnel is provided. Figure 4 A cross-sectional view of the support structure for the main tunnel; Figure 5 For the support structure of the advanced small pilot tunnel; Figure 6 Schematic diagram of a retractable U-shaped steel frame; Figure 7 This is a construction process flowchart.

[0013] The components include: 1. Upper step; 2. Small guide tunnel; 3. Lower step; 4. Middle pipe shed; 5. Steel frame; 6. Anchor bolt; 7. Steel flower pipe; 8. Waterproof layer; 9. Secondary lining; 10. "U" shaped steel frame; 11. Unit A; 12. Unit B; 13. Unit C; 14. Joint; 15. Clamp; 16. Connecting plate; 17. Holeless base plate. Detailed Implementation

[0014] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0015] like Figure 7 As shown, a method for constructing a pilot tunnel for high-stress soft rock tunnels includes the following construction process: determining the excavation cross-section → pilot tunnel excavation → pilot tunnel support → main tunnel advance support → main tunnel reserved height excavation (simultaneously dismantling the temporary steel frame of the pilot tunnel in the same section) → main tunnel support → shallow radial grouting of the main tunnel → monitoring and measurement → next cycle operation. Specifically, it includes the following steps: Step 1: Determine the excavation cross-section; Step 2: Determine the height and length of the main tunnel steps; Step 3: Excavation of the pilot tunnel: The construction method adopts a combination of micro-steps and small pilot tunnels. A small pilot tunnel is set up in the space of the upper step of the main tunnel. The small pilot tunnel is excavated first, and the ground stress is released in advance by utilizing the open surface of the small pilot tunnel. The small pilot tunnel is constructed 10m to 15m ahead of the main tunnel. The small pilot tunnel is excavated by a combination of milling and blasting, depending on the geological conditions. Step 31: Install the steel frame in the small guide hole: The pilot tunnel is temporarily supported by a retractable "U"-shaped steel frame, which can be reused as the pilot tunnel is excavated. Step 32: Shotcrete; Step 33: Monitor and measure. If there is an abnormality, proceed to comprehensive analysis and judgment, adjust the support measures, and then proceed to the next cycle of operation. If there is no abnormality, proceed directly to the next cycle of operation. Step 4: When the pilot tunnel advances the main tunnel by 10m to 15m, proceed to the main tunnel advance support; otherwise, proceed to Step 3. Step 41: The main tunnel advance support adopts φ76 medium pipe roof with φ42 small pipe, HW200 steel frame is set up around the whole ring, low prestressed hollow grouting anchor rod is set up in the arch wall, φ42 steel flower pipe is set up around the whole ring for shallow grouting, a waterproof layer is set between the initial support and the lining, and the secondary lining adopts cast-in-place reinforced concrete. Step 42: The reserved portion of the main tunnel is excavated using milling. Simultaneously with the excavation of the main tunnel, the support of the small pilot tunnel is removed. The removal of the small pilot tunnel support is adapted to the excavation height reserved for the main tunnel. Step 43: Construction of anchor bolts and support for the main tunnel; the main tunnel support structure adopts a composite lining structure; installation of steel frame and monitoring and measurement markers; Step 44: After shallow radial grouting of the main tunnel, monitor and measure. If there is an abnormality, conduct comprehensive analysis and judgment, adjust the support measures, and then proceed to the next cycle of operation. If there is no abnormality, proceed directly to the next cycle of operation.

[0016] This invention discloses a method for constructing a pilot tunnel for soft rock tunnels under high ground stress. In specific embodiments, its core contents mainly include excavation methods and support measures.

[0017] The excavation method mentioned refers to the excavation method combining micro-steps and small pilot tunnels; The aforementioned support measures refer to the support measures of the composite lining structure of the main tunnel combined with the temporary support of the retractable U-shaped steel frame of the small pilot tunnel.

[0018] like Figure 1As shown, the step height is set by comprehensively considering the design outline and on-site construction requirements. The height of the upper step 1 is the spatial height of the small guide tunnel 2 and the reserved excavation height. The spatial height of the small guide tunnel 2 meets the height of the excavator and loader for muck removal. The reserved excavation height needs to be determined in combination with the geological conditions to analyze the range of the loosening zone. The height of the lower step 3 is the height from the arch foot of the upper step to the arch foot of the arch wall steel frame.

[0019] like Figure 2 As shown, the excavation bench length is set according to the construction concept of "short advance, weak blasting, and early ring formation" for tunnels with high ground stress and large deformation. The length of the upper bench 1 is shortened, and the length of the upper bench 1 is sufficient to meet the operating space of mechanical equipment. The advance length of the small pilot tunnel 2 is determined by combining the construction progress and the convergence deformation. The principle is that the excavation length during the period of large deformation after excavation is the advance length of the small pilot tunnel 2. The purpose is to use the small pilot tunnel construction to release the high ground stress in advance and reduce the initial support convergence deformation caused by ground stress during the main tunnel excavation.

[0020] The excavation of the pilot tunnel was carried out using a combination of milling and blasting, with non-explosive machinery being prioritized. The excavation equipment consisted of a high-powered short-arm excavator, a breaker head, a milling head, and a hook. Milling was used in strata with uneven hardness. The main tunnel's reserved excavation height was excavated using milling. By optimizing the excavation method, disturbance to the rock mass was reduced, thereby minimizing the impact range of the loosened zone.

[0021] The support measures are divided into two parts: the main tunnel and the pilot tunnel. The main tunnel support structure adopts a composite lining structure; the pilot tunnel support adopts a retractable U-shaped steel frame for temporary support, which is reused as the pilot tunnel is excavated.

[0022] like Figure 3 , 4 As shown, the main tunnel support structure adopts a composite lining structure. The advanced support adopts φ76 medium pipe shed 4, HW200 type steel frame 5 is set in the whole ring, low prestressed hollow grouting anchor rod 6 is set in the arch wall, and φ42 steel flower pipe 7 is set in the whole ring for shallow grouting to reinforce the rock mass outside the excavation outline. A waterproof layer 8 is set between the initial support and the lining, and the secondary lining 9 adopts cast-in-place reinforced concrete.

[0023] like Figure 5 As shown, the pilot tunnel is supported by a retractable "U"-shaped steel frame 10. The pilot tunnel is supported ahead of the main tunnel excavation. The support of the pilot tunnel is removed at the same time as the main tunnel excavation. The removal of the pilot tunnel support is adapted to the excavation height reserved for the main tunnel.

[0024] like Figure 6As shown, the retractable "U"-shaped steel frame 10 is divided into Unit A 11, Unit B 12, and Unit C 13, and is made of "U"-shaped steel. The joint 14 of Unit A and Unit B is connected by an overlapping method and fixed by clamps 15. The joint position is set at the junction of the arch wall with large convergence deformation. The connection and setting position are fixed by clamps to adapt to the convergence deformation after excavation. The steel frame can be recycled. Unit B and Unit C are fixed by bolts using connecting plates 16. The arch foot position of Unit B is equipped with a non-perforated foot plate 17.

[0025] Design mechanism of the invention: When long, deep-buried tunnels traverse fault fracture zones, the rock mass is highly fractured due to high ground stress and tectonic movement. After tunnel excavation, the stress redistribution process easily causes a chain reaction, such as steel arch twisting and shotcrete cracking, leading to large deformation and failure of the initial support. A method for constructing pilot tunnels in high-stress soft rock tunnels has the following core mechanism: Geological Prediction and Risk Prevention: Pre-construction pilot tunnels act as "geological scouts" for tunnel construction. Before large-scale excavation of the main tunnel, pilot tunnels can detect key geological parameters such as the specific extent of fault fracture zones, the degree of rock fragmentation, and groundwater distribution. Compared to traditional indirect detection methods such as ground-penetrating radar, pilot tunnels provide direct geological samples and on-site data, avoiding risks such as unreasonable support parameters and sudden water or mud inrushes caused by inaccurate geological surveys.

[0026] Early stress release alleviates support pressure: High ground stress is the core cause of initial support deformation and failure. During the excavation of the pilot tunnel, the stress in the surrounding rock is initially released and redistributed in advance, dispersing the stress originally concentrated around the main tunnel excavation outline to the periphery of the pilot tunnel, thus reducing the stress concentration during the main tunnel excavation.

[0027] Reduced disturbance and smaller loosened zone of surrounding rock: The micro-step method itself has a step length of only 3-5 meters, and drilling and detonation can be carried out simultaneously on the upper and lower steps, and the amount of explosives can be controlled. Compared with the long-step method, it significantly reduces the exposure time of the surrounding rock and the blasting disturbance. In addition, with the use of a pilot tunnel, the surrounding rock has already undergone pre-disturbance and stress adjustment by the pilot tunnel when the main tunnel is excavated, which relatively improves the stability of the rock mass and further reduces the loosened zone, avoiding a chain reaction of problems such as cracking of shotcrete and twisting of steel arches caused by excessive loosening of the surrounding rock.

[0028] Based on the principle of "pressure relief support", the support structure of the advanced small pilot tunnel adopts a retractable "U" shaped steel frame. By setting clamps at the joint positions, the shrinkage deformation effect is achieved. At the same time, the steel frame with less deformation and damage can be reused as the small pilot tunnel is excavated.

[0029] This invention proposes a micro-step construction method for the main tunnel, which involves setting up a small pilot tunnel on the upper step to reveal the geology ahead, release stress in advance, improve the stress environment during tunnel excavation, reduce blasting disturbance, reduce the loosening zone, and effectively reduce the deformation of the surrounding rock after the main tunnel is excavated, enabling efficient and safe construction of high-stress soft rock tunnels.

Claims

1. A method for constructing a pilot tunnel in advance for tunnels in soft rock with high ground stress, characterized in that, Includes the following steps: Step 1: Determine the excavation cross-section; Step 2: Determine the height and length of the main tunnel steps; Step 3: Excavation of the pilot tunnel: The construction method adopts a combination of micro-steps and small pilot tunnels. A small pilot tunnel is set up in the space of the upper step of the main tunnel. The small pilot tunnel is excavated first, and the ground stress is released in advance by utilizing the open surface of the small pilot tunnel. The small pilot tunnel is constructed 10m to 15m ahead of the main tunnel. The small pilot tunnel is excavated by a combination of milling and blasting, depending on the geological conditions. Step 31: Install the steel frame in the small guide hole: The advance pilot tunnel support uses a retractable "U"-shaped steel frame for temporary support, which can be reused as the pilot tunnel is excavated. Step 32: Shotcrete; Step 33: Monitor and measure. If there is an abnormality, proceed to comprehensive analysis and judgment, adjust the support measures, and then proceed to the next cycle of operation. If there is no abnormality, proceed directly to the next cycle of operation. Step 4: When the pilot tunnel advances the main tunnel by 10m to 15m, proceed to the main tunnel advance support; otherwise, proceed to Step 3. Step 41: The main tunnel advance support adopts φ76 medium pipe roof with φ42 small pipe, HW200 steel frame is set up around the whole ring, low prestressed hollow grouting anchor rod is set up in the arch wall, φ42 steel flower pipe is set up around the whole ring for shallow grouting, a waterproof layer is set between the initial support and the lining, and the secondary lining adopts cast-in-place reinforced concrete. Step 42: The reserved portion of the main tunnel is excavated using milling. Simultaneously with the excavation of the main tunnel, the support of the rear small pilot tunnel is removed. The removal of the small pilot tunnel support is adapted to the excavation height reserved for the main tunnel. Step 43: Construction of anchor bolts and support for the main tunnel; the main tunnel support structure adopts a composite lining structure; installation of steel frame and monitoring and measurement markers; Step 44: After shallow radial grouting of the main tunnel, monitor and measure. If there is an abnormality, conduct comprehensive analysis and judgment, adjust the support measures, and then proceed to the next cycle of operation. If there is no abnormality, proceed directly to the next cycle of operation.

2. The method for constructing a pilot tunnel for a high-stress soft rock tunnel according to claim 1, characterized in that, The small guide tunnel support structure adopts a retractable "U" shaped steel frame, which is divided into Unit A, Unit B, and Unit C, and is processed from "U" shaped steel. The joint between Unit A and Unit B is connected by an lap joint and fixed with clamps. The joint is located at the junction of the arch wall with large convergence deformation. The clamp-fixed connection can retract. Unit B and Unit C are fixed by connecting plates with bolts. The connecting plates are welded to the steel frame. The arch foot of Unit B is equipped with a non-perforated base plate.

3. The method for constructing a pilot tunnel for a high-stress soft rock tunnel according to claim 1, characterized in that, The height of the small pilot tunnel and the reserved excavation height are the height of the upper step.

4. The method for constructing a pilot tunnel for a high-stress soft rock tunnel according to claim 1, characterized in that, The height of the small guide tunnel is sufficient for the excavator and loader to operate at the required height. The reserved excavation height needs to be determined based on the geological conditions and the range of the loosened zone.

5. The method for constructing a pilot tunnel for a high-stress soft rock tunnel according to claim 1, characterized in that, The height of the lower step is the height from the arch foot of the upper step to the arch foot of the arch wall steel frame.

6. The method for constructing a pilot tunnel for a high-stress soft rock tunnel according to claim 1, characterized in that, The length of the upper step should be sufficient to accommodate the working space of the mechanical equipment.

7. The method for constructing a pilot tunnel for a high-stress soft rock tunnel according to claim 1, characterized in that, The advance length of the pilot tunnel is determined by combining the construction progress and the convergence deformation. The principle is that the excavation length during the period of greatest deformation after excavation is the advance length of the pilot tunnel.

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