A construction method for dual tunnels with small clearance in shallow-buried, bias-pressure fractured strata.

Abstract

This invention relates to the field of tunnel and bridge construction technology, specifically to a construction method for shallow-buried, biased, fractured strata with small clearance for dual tunnels with entrances and exits. The method comprises five steps: formulating corresponding measures for tunnel entrances and exits, determining the construction plan, implementing entrance construction techniques, developing construction plans for cross passages and active intersection areas, and determining the main tunnel exit construction techniques. This method is widely applicable to projects where shallow-buried, biased tunnel entrances cross steep slopes with poor overall stability and high construction difficulty, especially in projects with severe bias, fractured rock masses, and dual tunnel construction with small clearances. It solves the major engineering accidents such as tunnel entrance collapses and roof falls that are easily caused by key technical challenges such as steep mountain terrain, shallow-buried bias at the tunnel entrance, and harsh construction conditions, greatly improving construction safety.

Description

Technical Field

[0001] This invention relates to the field of tunnel and bridge construction technology, and in particular to a construction method for a double tunnel with small clearance in shallow buried, biased, fractured strata. Background Technology

[0002] In recent years, my country has continuously increased its investment in expressways in the mountainous areas of Southwest China. The construction of expressways in the challenging mountainous areas of my country is in full swing. The continuous rapid economic development has brought a new construction phase to the transportation in Southwest China. As an essential and convenient mode of transportation, expressways have become a top priority for the development of the transportation industry in Southwest China.

[0003] Construction of the tunnel portal section in mountainous highway tunnels is often a key and challenging aspect of tunnel construction. Due to long-term weathering, the surrounding rock at the tunnel portal is of low grade and extremely unstable, especially in steep mountain slopes like those in Sichuan and Yunnan provinces of my country. The shallow burial depth of the tunnel portal further reduces the arching capacity of the overlying rock, resulting in highly unstable surrounding rock and significantly increasing construction difficulty. Among the common design forms of highway tunnels in my country's existing mountainous highways, using separated tunnels with smaller clearances can optimize the alignment of the portal section, but this carries greater construction risks and increases the risk of eccentric pressure at the portal.

[0004] Shallow-buried, biased-pressure, and small-clearance tunnels differ from ordinary small-clearance tunnels. Due to their complex geological conditions and special construction environment, they are extremely prone to engineering accidents, especially in the most critical section of tunnel construction—the tunnel portal section. This is because the tunnel portal section is mostly under biased pressure, the surrounding rock is highly weathered, and the burial depth is relatively shallow. The terrain at the portal is steep, and there are no conditions for setting up a construction site. Therefore, if the construction methods or support measures are inappropriate, it is easy to cause major engineering accidents such as portal collapse and roof fall, which seriously threaten construction safety. Summary of the Invention

[0005] This invention aims to solve the above-mentioned problems and provides a construction method for dual tunnels with small clearance in shallow buried, biased, and fractured strata. This method solves the major engineering accidents such as tunnel entrance collapse and roof fall that are easily caused by key technical problems such as steep mountain terrain, shallow buried biased pressure at the tunnel entrance, and harsh construction conditions during the construction process, and greatly improves construction safety.

[0006] A construction method for dual-tunnel entry and exit tunnels with small clearance in shallow-buried, bias-pressure fractured strata includes the following steps:

[0007] Step 1: Based on the actual situation of the shallow buried bias-pressure fractured strata and small clearance double tunnel project, conduct engineering geology and surrounding environment surveys, conduct risk analysis around the tunnel entrance, and formulate corresponding measures for tunnel entry and exit.

[0008] Step 2: By establishing a high-precision three-dimensional numerical model, simulate and analyze the tunnel deformation, stress and stratum deformation caused by different construction methods, and select the most reasonable construction scheme;

[0009] Step 3: Determine a reasonable tunnel entry advance support technology. Before entering the left tunnel, the slope protection at the tunnel entrance and the pile foundation support beam retaining wall construction should be completed. Before entering the right tunnel, the anti-slide pile construction should be completed.

[0010] Step 4: The direct top-lifting method is used for construction at the intersection of the transverse tunnel and the main tunnel. Construction is carried out in strict accordance with the technical instructions to avoid excessive local deformation.

[0011] Step 5: When the main tunnel is excavated to 30 meters before the end of the transition between open and closed sections, the tunnel exit will first use a small pilot tunnel with a single side wall, and temporary support will be provided while excavating. After the pilot tunnel is completed and the conditions for transporting machinery, equipment and materials are met, the guide wall and tunnel portal will be constructed. After the tunnel portal is completed, the main tunnel will be used as the center to make repairs to the surrounding area until the construction requirements are met.

[0012] Preferably, the construction plan in step two, combined with the mechanical response of the surrounding rock and the grey relational analysis, determines that the preferred technology is the single-sidewall pilot tunnel method.

[0013] Preferred, the tunnel entry construction technology in step three includes surface treatment, anti-slide pile construction at the tunnel entrance, slope protection construction, and pile foundation support beam retaining wall construction.

[0014] Preferably, the construction of anti-slide piles at the tunnel entrance should first involve grouting and reinforcing the surface at the entrance, followed by excavation of the tunnel entrance. Grouting reinforcement using steel pipe piles is employed, with hot-rolled seamless steel pipes used as the grouting guide. Grouting holes are drilled on the guide pipes in a staggered pattern. The grouting material is pure cement slurry (w / c = 0.6–0.8), the strength grade of the grouting cement is 42.5, and the grouting pressure is 1–1.5 MPa.

[0015] Preferably, the slope protection should be carried out after the slope is excavated by hanging wire mesh and spraying. It should be carried out from top to bottom, layer by layer, in a timely manner as the excavation progresses. Anchor bolts should be installed on the slope, and wire mesh and spraying should be used for support. Isolated boulders and dangerous rocks on the top of the tunnel should be treated. To prevent surface water from seeping down from the platform between the slope and the intercepting ditch, a 5cm plain concrete seal should be used between the top of the slope and the ditch.

[0016] Preferably, in the construction of the pile foundation beam retaining wall, a pile foundation beam inclined retaining wall is set at the tunnel entrance, with the retaining wall set close to the right side slope. The upper part of the beam is a gravity retaining wall with a height of 6-10 meters. The beam consists of 5 sections, each 12 meters long, 3.5-5 meters wide, and 1.2-1.5 meters high. The pile foundation uses a single row of round piles with a diameter of 2 meters and a length of 8-12 meters. The pile foundation holes are drilled using an impact drill, and after the steel cage is installed, C30 underwater concrete is poured. After the pile foundation construction is completed, a C30 reinforced concrete cap beam is poured. The retaining wall uses C20 rubble concrete material. The slope ratio of the excavation slope of the beam is 1:0.3. The temporary slope behind the beam retaining wall is protected with wire mesh and shotcrete. Drainage pipes are reserved inside the retaining wall as required.

[0017] Preferably, the construction technology for the intersection area of ​​the transverse tunnel and the main tunnel in step four includes transverse tunnel entry technology and cross-section roof lifting construction technology;

[0018] Among them, the transverse tunnel entry technology involves anchoring and spraying (mesh) to reinforce the slope 1-3m outside the transverse tunnel entrance before construction; the transverse tunnel entrance section is initially excavated using the single-side wall pilot tunnel method, and the subsequent construction can adopt the full-section method excavation according to the actual site conditions; when the transverse tunnel is excavated to the junction with the main tunnel, the arch of the transverse tunnel is reinforced with advanced small guide pipes.

[0019] Among them, the cross-section cantilever construction technology involves first using a double-layer steel arch frame to lock the cross-section of the branch tunnel when it intersects with the maximum span of the secondary lining of the main tunnel during the construction of the branch tunnel; then, advanced support is carried out; and then, the main tunnel is vertically entered through the upper and lower step excavation method and the arc-shaped cantilever excavation is carried out, and the initial support and temporary gantry support of the main tunnel are set up.

[0020] Preferably, the fifth step of the tunnel construction technology includes small pilot tunnel construction and large pipe shed construction;

[0021] During the construction of the pilot tunnel, 30cm before the main tunnel reaches the boundary between open and closed sections, a small tunnel with a clearance of 4×5m is first opened in the pilot tunnel, and temporary support is provided while excavating. After the pilot tunnel is completed and the conditions for transporting machinery, equipment, and materials are met, the guide wall and tunnel portal are constructed.

[0022] The large pipe shed is located at the tunnel entrance, with a length of 30m, and is made of hot-rolled seamless steel pipe. The pipe joints are connected with threaded sections longer than 15cm, and adjacent pipe joints are staggered by more than 1m. The number of joints in the same longitudinal section of the tunnel does not exceed 50%. Grouting holes with a diameter of Φ12mm are drilled on the steel pipes, with a diameter spacing of 15cm, arranged in a quincunx pattern. The exposed part of the steel pipe arch is 3.0m without drilled holes, with a circumferential spacing of 40cm. During the construction of the advanced large pipe shed, the steel pipes are parallel to the tunnel centerline, with an elevation angle of 1-2° (excluding the longitudinal slope of the route), which can be adjusted according to the actual situation.

[0023] Preferably, the tunnel entrance is situated on a cliff face with no construction conditions or road access, making it impossible to directly transport major machinery, equipment, and materials to the tunnel entrance. The shallow-buried, bias-pressure overburden layer at the tunnel entrance is 4–10 m thick, and the minimum clearance between the left and right monitoring lines is 7.81 m. Enhanced pre-support and initial support measures are implemented, with Φ42×4 grouting pipes used for reinforcement in the central rock wall area. If necessary, the foundation may be replaced or reinforced with grout. Close monitoring and measurement of the tunnel surrounding rock and support structure are crucial, with timely analysis of deformation at key tunnel components to determine the stability of the surrounding rock and support structure.

[0024] The present invention has the following beneficial effects:

[0025] 1. This construction method overcomes the tunnel construction difficulties caused by shallow burial, bias pressure, and cliff-like cliffs at the tunnel entrance, and the lack of on-site construction conditions due to the construction scheme optimization technology for shallow buried, bias pressure, and fractured strata with small clearance, tunnel entry advance support technology, cross tunnel and main tunnel intersection area construction technology, and main tunnel exit construction technology.

[0026] 2. This construction method effectively reduces construction risks by formulating the optimal construction scheme and the optimal portal support scheme for shallow buried, biased, fractured strata with small clearance for double tunnels. This avoids the subjectivity and blindness of experience-based decision-making.

[0027] 3. This construction method effectively reduces the possibility of collapse due to the complex three-dimensional stress of the surrounding rock space at the intersection of the main tunnel and the cross tunnel by adopting the "direct top lifting method". Attached Figure Description

[0028] 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 one embodiment of the present invention. For those skilled in the art, other embodiments can be derived from the provided drawings without creative effort.

[0029] Figure 1 : Construction method process flow diagram of this invention;

[0030] Figure 2 : Cross-sectional views of different construction schemes in step two of this invention;

[0031] Figure 3 : Step 3 of this invention: Pre-reinforcement of the surrounding rock at the tunnel entrance, displacement variation diagram;

[0032] Figure 4 Stress diagram of the pre-reinforcement of the lower tunnel support structure in step three of this invention;

[0033] Figure 5Step 4 of this invention: Design drawing for the horizontal tunnel support;

[0034] Figure 6 : Plan layout diagram of the construction cross tunnel in step four of this invention;

[0035] Figure 7 : Step 4 of this invention: Pre-operative small catheter support diagram;

[0036] Figure 8 Step four of this invention: Design drawing for the support structure during the cantilever construction;

[0037] Figure 9 Step 5 of this invention: Diagram showing the positional relationship between the tunnel and the pilot tunnel;

[0038] Figure 10 Step 5 of this invention: Pipe shed cross-section design drawing:

[0039] Figure 11 Step 5 of this invention: cross-sectional view of the pipe shed excavation. Detailed Implementation

[0040] The present invention will be further described below with reference to the accompanying drawings and examples:

[0041] The present invention will now be described in detail with reference to the accompanying drawings and embodiments. It should be noted that, unless otherwise specified, the embodiments and features described herein can be combined with each other.

[0042] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "joining" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a direct connection or an indirect connection through an intermediate medium. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0043] In the description of this invention, it should be understood that the terms "longitudinal", "lateral", "up", "down", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0044] In shallow-buried tunnel sections with small clearance and biased pressure, the terrain at the tunnel entrance is steep, and there are no suitable construction sites. The surrounding rock is classified as Class V, and the rock type is topsoil. The surrounding rock is relatively broken, with complex and variable geology and well-developed joints. Prevention and control measures for shallow-buried sections are mainly implemented from three aspects: support structure, construction procedures, and monitoring and measurement.

[0045] A construction method for dual-tunnel entry and exit tunnels with small clearance in shallow-buried, bias-pressure fractured strata includes the following steps:

[0046] Step 1: Formulate corresponding measures for tunnel entry and exit: Based on the actual situation of the shallow buried bias-pressure fractured strata and small clearance double tunnel on site, conduct engineering geology and surrounding environment surveys, conduct risk analysis around the tunnel entrance, and formulate corresponding measures for tunnel entry and exit.

[0047] Step 2: Determine the construction plan: By establishing a high-precision three-dimensional numerical model, simulate and analyze the tunnel deformation, stress and stratum deformation caused by different construction methods, and select the most reasonable construction plan;

[0048] Step 3: Tunneling construction technology: Determine reasonable tunneling advance support technology. Before entering the left tunnel, the slope protection at the tunnel entrance and the pile foundation support beam retaining wall construction shall be completed. Before entering the right tunnel, the anti-slide pile construction shall be completed.

[0049] Step 4: Determine the construction method for the intersection of the transverse tunnel and the main tunnel using the direct top-lifting method, and strictly follow the technical instructions during construction to avoid excessive local deformation;

[0050] Step 5: Determine the main tunnel exit construction technology. When the main tunnel is excavated to 30 meters before the end of the light-dark boundary, the exit will first be carried out using a small pilot tunnel with a single side wall, with temporary support being installed while excavating. After the pilot tunnel is completed and the conditions for transporting machinery, equipment and materials are met, the guide wall and tunnel portal will be constructed. After the tunnel portal is completed, the construction will be carried out from the central pilot tunnel outwards until the construction requirements are met.

[0051] The following sections provide a detailed explanation of the construction steps for a shallow-buried, bias-pressure fractured strata, small-clearance double-tunnel entry and exit method.

[0052] Step 1: Formulate corresponding measures for tunnel entry and exit: Based on the actual situation of the shallow buried bias-pressure fractured strata and small clearance double tunnel on site, conduct engineering geology and surrounding environment surveys, risk analysis around the tunnel entrance, and formulate corresponding measures for tunnel entry and exit.

[0053] This construction method can be widely applied to projects where the shallow-buried biased tunnel entrance section crosses a steep slope with poor overall stability and great construction difficulty, especially projects with severe shallow bias, fractured rock mass, and small-clearance double tunnel construction.

[0054] Step 2: Determine the construction plan: By establishing a high-precision three-dimensional numerical model, simulate and analyze the tunnel deformation, stress and stratum deformation caused by different construction methods, and select the most reasonable construction plan;

[0055] The tunnel entrance section was selected as the simulation section. Numerical models simulating four excavation methods all used the same cross-sectional dimensions. See details below. Figure 2, Figure 2 The numbers in the middle indicate the construction sequence. Due to the existence of topographic bias, the surrounding rock of the tunnel with a burial depth of 9 to 18 m adopts the Mohr-Coulomb elastoplastic model. The surrounding rock of the tunnel follows the Mohr-Coulomb criterion. The initial support and secondary lining adopt the solid elastic model. The advanced support is simulated by increasing the parameters of its surrounding rock reinforcement zone. The anchor bolt adopts the cable element.

[0056] Through simulation analysis of four construction schemes—step method, core soil retention method, single-side wall pilot tunnel method, and double-side wall pilot tunnel method—the analysis was conducted from aspects such as stress, displacement, and plastic zone. The impact of different excavation methods on the surrounding rock disturbance was considered. Combined with grey relational analysis, the single-side wall pilot tunnel method was finally selected as the optimal excavation method.

[0057] Step 3: Tunneling Construction Technology: Determine reasonable tunneling advance support technology. Before entering the left tunnel, the slope protection at the tunnel entrance and the pile foundation support beam retaining wall construction should be completed. Before entering the right tunnel, the anti-slide pile construction should be completed.

[0058] To verify the effectiveness of pre-reinforcement, the numerical calculation results of the pre-grouting reinforcement condition were compared and analyzed with those before reinforcement, and the patterns were summarized:

[0059] 3.1 Analysis of the Plastic Zone

[0060] Numerical calculations were performed on the tunnel arch after pre-reinforcement to determine the development and distribution of the plastic zone in the surrounding rock. The results showed a significant reduction in the plastic zone distribution within the pre-reinforced area, particularly at the tunnel crown, where almost no plastic failure occurred. This is attributed to the fact that pre-reinforcement improved the overall stability of the surrounding rock above the tunnel, reduced its sensitivity, and significantly weakened the disturbance caused by tunnel excavation. This, in turn, reduced deformation of the surrounding rock, resulting in a smaller area of ​​plastic failure. Furthermore, the "supporting" effect of the pre-reinforced tunnel arch surrounding rock on the strata significantly reduced consolidation settlement at the tunnel crown, further minimizing plastic failure.

[0061] 3.2 Deformation of surrounding rock

[0062] from Figure 3The displacement changes of the surrounding rock at the tunnel entrance under pre-reinforcement show that as the tunnel is excavated, the stress in the surrounding rock is gradually released. The maximum settlement occurs at the crown, the maximum heave occurs at the bottom, and the maximum convergence occurs at the foot of the arch. After reinforcement, the settlement of the crown did not exceed the warning value. This is because the pre-grouting reinforcement of the surrounding rock at the tunnel crown changed the lithology of the surrounding rock around the crown, improved the overall stability of the surrounding rock, and greatly reduced the disturbance of the tunnel excavation to the surrounding rock at the tunnel crown, thereby reducing the settlement and deformation of the crown. The "supporting" effect of the pre-reinforcement at the tunnel entrance caused a significant reduction in the consolidation settlement of the top surrounding rock. It can be seen that the pre-reinforcement effect is good, which can effectively reduce the surrounding rock at the tunnel entrance and ensure construction safety.

[0063] 3.3 Stress in the support structure

[0064] The stress of the initial support at the tunnel portal under advanced reinforcement conditions is as follows: Figure 4 As shown. The maximum principal stress of the initial tunnel support is under tension. Stress concentration occurs at the arch crown, with the maximum tensile stress occurring there and the minimum at the arch waist. The maximum tensile stress is 1.11 MPa. At this point, the maximum tensile stress of the initial support under the advanced reinforcement condition at the tunnel entrance arch crown is less than the tensile strength of C25 concrete (1.78 MPa), which is within a safe range. The minimum principal stress of the initial tunnel support is under compression. Stress concentration occurs at the arch foot, with the maximum compressive stress occurring there and the minimum at the arch crown. The maximum compressive stress is 7.94 MPa. At this point, the maximum compressive stress of the initial support at the tunnel entrance is less than the tensile strength of C25 concrete (16.7 MPa), which is within a safe range.

[0065] The advanced support technology for tunnel entry includes surface treatment, construction of anti-slide piles at the tunnel entrance, protection of the side slopes, and construction of the pile foundation support beam retaining wall before entering the tunnel. The right tunnel is entered after the construction of the anti-slide piles is completed.

[0066] 3.3.1 Surface treatment and intercepting gutters, wind and water power construction

[0067] Before the tunnel portal construction, surveying and setting out are carried out. Based on the surveying and setting out, the outline of the slope excavation and the intercepting ditch are prepared. In order to effectively intercept surface water and prevent surface water from eroding and endangering the stability of the tunnel portal structure and the slope, and to facilitate drainage, an intercepting ditch is set up 10m away from the excavation line. At the same time, funnels, depressions, and loose rocks within 10-15 meters outside the excavation line of the tunnel portal section are treated to prevent surface water from seeping downwards or sinkholes from continuing to expand and affecting tunnel safety. High-altitude water pool facilities are built and pipelines are laid to meet the water demand for construction. The main power supply is connected and the air compressor is installed to prepare for entering the tunnel.

[0068] 3.3.2 Construction of anti-slide piles at the tunnel entrance

[0069] 1) To prevent the excavation of the tunnel entrance and the slope from affecting the stability of the accumulation above the tunnel, the surface at the entrance end should be reinforced by grouting before the tunnel entrance excavation is carried out.

[0070] 2) Grouting reinforcement using Φ75 steel pipe piles is employed for soil treatment. The guide pipe is a hot-rolled seamless steel pipe with an outer diameter of 75mm and a wall thickness of 6mm. Grouting holes of 12mm diameter are drilled on the guide pipe, spaced 1m x 1m in a staggered pattern. The last 1m of the guide pipe is left undrilled as a grout stop section. The grouting material is pure cement slurry (w / c = 0.6~0.8), with a cement strength grade of 42.5. The grouting pressure is 1~1.5MPa. For slopes with soil, excavation and protection should be carried out simultaneously.

[0071] 3) Drilling shall stop when the Φ75 steel pipe is buried to any of the following depths: a. Tunnel excavation outline; b. 0.5m below the bedrock surface; c. 0.5m below the tunnel bottom.

[0072] 4) Grouting shall be terminated immediately when the grouting volume or grouting pressure reaches the design value.

[0073] 3.3.3 Side slope protection

[0074] After the slope excavation, timely application of wire mesh, anchors, and shotcrete protection to the tunnel entrance slope is carried out. This is done from top to bottom, layer by layer, as excavation progresses. Anchor bolts are installed on the slope, and wire mesh and shotcrete are applied to prevent weathering and erosion. Isolated boulders and unstable rocks on the tunnel roof are also addressed. To prevent surface water from seeping down the platform between the slope and the intercepting ditch, a 5cm layer of plain concrete is used to seal the area between the slope top and the ditch.

[0075] Temporary slope protection was strengthened by using Φ22 mortar anchor bolts, 3.5m long and spaced 1.2m apart, arranged in a quincunx pattern. A Φ8 steel mesh, 20*20cm, was then installed; followed by C20 shotcrete reinforcement.

[0076] Anchor bolts: Before anchor bolt construction, the bolt hole positions should be determined according to design requirements and allowable adjustment principles. The inclination angle of the anchor bolts should meet the design and construction requirements, with an error of ±5°. If the requirements are not met, readjustment can be made. For steep slopes with an elevation angle greater than 45°, it is advisable to erect construction scaffolding or working platforms during construction.

[0077] Anchoring: Different anchoring methods are selected according to the lithology of the strata where the foundation is located.

[0078] Shotcrete: Prepare C20 shotcrete material according to design requirements, spray a 10cm layer of concrete onto the rock surface, add accelerator to the feed, and operate by professional and skilled workers to ensure that the shotcrete surface is flat and free of dry spots or peeling.

[0079] The tunnel entrance section has a thick layer of gravel and rock, so loose rock on the tunnel roof must be removed before construction, and active and passive protective nets must be installed.

[0080] Strictly follow the above work procedures to create favorable conditions for entering the tunnel.

[0081] 3.3.4 Pile Foundation Supporting Beam Retaining Wall

[0082] A pile-supported beam inclined retaining wall is constructed at the tunnel entrance, with the retaining wall set against the right-side slope. The upper part of the beam is a gravity retaining wall, 6-10 meters high. The beam consists of 5 sections, each 12 meters long, 3.5-5 meters wide, and 1.2-1.5 meters high. The pile foundation uses a single row of round piles, 2 meters in diameter and 8-12 meters long. Piles are drilled using impact drilling, and after installing the reinforcing cage, C30 underwater concrete is poured. After the pile foundation construction is completed, a C30 reinforced concrete cap beam is poured. The retaining wall uses C20 rubble concrete. The slope of the excavated side of the beam is 1:0.3. The temporary slope behind the beam retaining wall is protected with wire mesh and shotcrete. Drainage pipes are pre-installed inside the retaining wall as required.

[0083] Step 4: Construction technology at the intersection of the transverse tunnel and the main tunnel; The direct top-lifting method is adopted for construction at the intersection of the transverse tunnel and the main tunnel, and construction is carried out in strict accordance with the technical instructions to avoid excessive local deformation;

[0084] 4.1 Horizontal tunnel entry technology

[0085] Before construction, the slope 1-3m outside the tunnel entrance was reinforced with anchor spraying (mesh). The initial excavation of the tunnel entrance section adopted the single-side wall pilot tunnel method. Subsequent construction, depending on site conditions, may adopt the full-face excavation method, with V-grade cast-in-place lining, supported by I18 type steel arch frames with a spacing of 0.8m / frame; Φ22 explosive coil anchors 0.8m / ring, 8 anchors per ring, each 3m long; Φ6.5 steel mesh, 20*20cm mesh, was used for shotcreting. When the tunnel reached the junction with the main tunnel, the tunnel arch was reinforced with a single layer of Φ42 advanced small guide pipes, spaced 40cm circumferentially, each 4.5m long, with an inclination angle of 10-15°. During construction, monitoring and measurement, and advanced geological forecasting were carried out, and initial support and secondary lining construction were carried out promptly based on feedback. The support structure design is as follows: Figure 5 As shown.

[0086] The transverse tunnel is excavated from the highway location to the main tunnel, then from the main tunnel towards the entrance to the main tunnel entrance, and towards the exit to the tunnel exit. Its plan layout is as follows: Figure 6 As shown.

[0087] 4.2 Construction technology for cantilevered sections at intersections

[0088] Due to the large span, complex stress, and poor surrounding rock conditions at the intersection of the adit and the main tunnel, and the fact that the left tunnel line intersects the adit perpendicularly with a pedestrian cross passage at this station, forming a crossroads, the surrounding rock at the intersection experiences complex three-dimensional stress. Improper excavation and support could easily lead to subsidence, deformation, or even collapse, resulting in safety and quality accidents. When formulating the construction plan, safety was the primary principle. Referring to past experience with similar projects, and through repeated research and comparison, the "direct cantilever" reinforcement method was chosen for the main tunnel intersection. This involves first securing the adit intersection with a double-layered steel arch frame when the adit construction reaches the maximum span of the main tunnel's secondary lining; then implementing advance support; and finally, vertically entering the main tunnel using a stepped excavation method and performing curved cantilever excavation, setting up initial support and temporary gantry support for the main tunnel. The advantage of this method is that it allows for rapid, one-time construction of the main tunnel using the stepped method, avoiding secondary construction, reducing surrounding rock disturbance, and significantly shortening the construction period.

[0089] The excavation process for the intersection section is as follows: advance support—top excavation—initial shotcrete (anchor bolts, mesh)—main tunnel steel arch frame (single section)—temporary gantry frame (supported on the side of the tunnel face)—re-shotcrete—next cycle.

[0090] The construction method is as follows:

[0091] 4.2.1 Pre-operative small catheter support

[0092] The surrounding rock at the intersection of the side tunnel and the main tunnel is classified as Class V, such as... Figure 7 As shown, the arch of the branch tunnel uses a single layer of Φ42 advanced small guide pipes with a circumferential spacing of 40cm, each pipe is 4.5m long and has an inclination angle of 10-15°.

[0093] 4.2.2 Stepped Excavation

[0094] After the pre-support is completed, excavation is carried out in stages. First, excavation is carried out at a height of 5.5m for the upper stage to verify the top elevation. Holes are drilled at a 44° upward inclination angle at the arch crown, with the bottom raised by 50cm (to create an upward slope). The drilling depth is controlled at 2m, and the spacing between boreholes is controlled at 40cm. Weak-face blasting is used, and each borehole employs a decoupled charge structure longitudinally. The damage to the surrounding rock from blasting is minimized. After entering the left main tunnel area, the highest point of the arch is increased by 50cm above the design elevation to facilitate arch formation and improve stress conditions. Excavation progress is strictly controlled; the impact of blasting on the vibration velocity of adjacent sections should be controlled within 5cm / s.

[0095] Low-velocity, small-diameter explosives, such as smooth blasting explosives with a diameter of 20mm or 25mm, are used. Non-electric millisecond detonators with a sufficient number of segments are selected; if conditions permit, 100ms, 200ms, or half-second level differential detonators can be added to further improve the blasting effect and reduce blasting vibration intensity. The cut-out blast holes are arranged in a wedge shape, with hollow holes added to the mold body. The cut-out holes should be arranged as close as possible to the bottom of the excavation face. When there are enough detonators with a sufficient number of segments, the cut-out can be carried out in layers and stages to minimize the maximum common charge in the cut-out area and reduce vibration intensity. Blasting holes in other parts of the tunnel are arranged according to the principle of shallow and dense blasting, meaning that the depth (scale) of a single blast should not be large, and the explosive should be distributed as evenly as possible in the densely arranged blast holes.

[0096] During construction, blasting vibrations are monitored, mainly by setting up measuring points on beams, columns, and different floors of buildings in the affected area. The blasting plan is adjusted as needed based on the measurement data and the building vibration velocity requirements of the "Safety Regulations for Blasting" (GB 6722-2014).

[0097] 4.2.3 Construction of the cantilevered roof

[0098] After the branch tunnel is excavated to the outline of the main tunnel, a reinforced support ring is installed at the intersection; smooth blasting is used to excavate in a direction perpendicular to the centerline of the main tunnel.

[0099] The first cycle advances 1.6m, with an initial shotcrete thickness of 5cm. I18 steel sections are installed perpendicular to the tunnel axis, one end welded to the support beam, the other end first anchored to the surrounding rock using a locking anchor rod. Two I18 sections are then connected in parallel to form a temporary "gate-shaped" scaffold. Explosive cartridge anchor rods, 2.5m long, are installed at 120×60cm intervals. A second shotcrete layer of 24cm is applied to the designed thickness. Specific design details are as follows... Figure 8 As shown.

[0100] After the second cycle, the steel arch frame is set according to the pre-set arc. After completion, the arch elevation is checked. In the third cycle, the arch is advanced by 2.7 meters, reaching the highest point of the left-side cantilever. The cantilever work is completed in the sixth cycle.

[0101] A gantry arch frame is installed in the pedestrian cross passage and welded to the arched steel section of the main tunnel to form a closed and complete load-bearing system for the main tunnel's roof arch. The temporary supports on the middle side are removed, and advanced support is constructed according to the design. The upper step of the main tunnel is excavated, with an advance of 1.2m per cycle. When the upper step is 30m away from the lower step, the lower step and sidewalls are excavated and supported. The pedestrian cross passage is excavated according to the design drawings.

[0102] The construction of the cantilevered structure should be carried out in strict accordance with the construction procedures of the plan. After excavation, the top should be found in time, and anchor rods should be installed after initial spraying of concrete. Then, the steel arch frame should be installed. The steel arch frame should be connected to the supporting arch frame and welded firmly. Each cycle of excavation can only proceed after the temporary gantry frame is installed to "lock" the arch frame.

[0103] Step 5: Exit Construction Technology: When the main tunnel is excavated to 30 meters before the end of the transition between open and closed sections, the exit will be carried out by first using a small pilot tunnel with a single side wall, and temporary support will be provided while excavating. After the pilot tunnel is completed and the conditions for transporting machinery, equipment and materials are met, the guide wall and tunnel portal will be constructed. After the tunnel portal is completed, the construction will be carried out from the central pilot tunnel outward until the construction requirements are met.

[0104] 5.1 Construction of the pilot tunnel

[0105] Because the tunnel's entrance and exit are both located on cliffs, there are no suitable conditions for construction operations, nor are there any roads leading to the tunnel. Major machinery, equipment, and materials cannot be directly transported to the tunnel entrance. When the main tunnel is excavated to 30 meters before the boundary between open and closed sections, the exit will be achieved by first excavating a small, 4×5m clear tunnel through the central pilot tunnel, with temporary support being installed while excavating. Once the central pilot tunnel is completed, allowing for the transport of machinery, equipment, and materials, the guide walls and tunnel portal will be constructed. After the tunnel portal is completed, further refinement will be carried out from the central pilot tunnel outwards until the design drawings are met.

[0106] When the tunnel body is excavated to 30m from the boundary between the open and closed sections at the entrance, a small pilot tunnel is excavated using the tunnel muck pad access road. Figure 9 Each excavation advance was less than 1.5m, and initial support was promptly implemented. The pilot tunnel measures 4m (width) × 5m (height), with a semi-circular arch of r = 2m at the top, located on the right side of the upper bench of the tunnel. Smooth, weak blasting was used for the pilot tunnel excavation, with reduced explosive charge and manual slag removal. For the pilot tunnel's advance support, Φ42 hot-rolled seamless steel pipes were used as grouting guides, with a diameter of 42mm and a wall thickness of 4mm. Initial support for the tunnel used Φ22 explosive cartridge anchors, Φ6 steel mesh, and shotcrete C25. The steel frame was fabricated outside the tunnel, with allowable lateral and vertical errors within ±5cm and verticality within ±2°. The lower end of the steel frame was supported on stable rock. The tunnel arch foot was positioned 15–20cm below the excavation line of the upper bench. When over-excavation is excessive, concrete of the same strength should be sprayed onto the arch back to ensure close contact between the support and the surrounding rock, thus controlling the continuous increase in deformation of the tunnel support. When the excavation at the tunnel arch foot is too deep, steel plates or concrete pads should be added, and Φ22 steel reinforcement rods should be added between the two rows of steel frames for secure connection, making it a structurally integrated load-bearing structure.

[0107] 5.2 Construction of large pipe sheds

[0108] A 30m long pre-excavation pipe roof, constructed from Φ108mm×6mm hot-rolled seamless steel pipes, is installed at the tunnel entrance. The pipe joints use threaded connections with a length greater than 15cm, and adjacent pipe joints are staggered by more than 1m. The number of joints within the same longitudinal section of the tunnel does not exceed 50%. Grouting holes with a diameter of Φ12mm are drilled on the steel pipes, spaced 15cm apart in a quincunx pattern. The exposed portion of the steel pipe arch is 3.0m without quincunx holes, with a circumferential spacing of 40cm. During construction, the steel pipes are parallel to the tunnel centerline, with an elevation angle of 1-2° (excluding the longitudinal slope of the route), which can be adjusted according to actual conditions. The pipe roof cross-section design and excavation cross-section diagram are shown below. Figure 10 and Figure 11 As shown.

[0109] 5.3 Main tunnel exit technical plan

[0110] Because the tunnel entrance is situated on sheer cliffs, lacking both construction conditions and road access, major machinery and materials cannot be directly transported to the tunnel entrance. When the main tunnel reaches 30 meters before the boundary between open and closed sections, the exit will initially utilize a small pilot tunnel with a single sidewall, employing temporary support while excavating. Once the pilot tunnel is completed and conditions for transporting machinery and materials are met, the guide wall and tunnel portal will then be constructed.

[0111] The tunnel entrance is shallowly buried and biased, with a cover layer thickness of only 4-10m. The surface Quaternary landslide deposits are thick, and the minimum clear distance between the left and right monitoring lines is only 7.81m. Strengthening advanced and initial support measures is crucial. For the middle rock wall reinforcement area, Φ42×4 grouting pipes are used for reinforcement. If necessary, the foundation should be replaced or reinforced with grout. Enhanced monitoring and measurement of the tunnel surrounding rock and support structure are essential. Timely analysis of deformation at key tunnel components is necessary to determine the stability of the surrounding rock and support structure.

[0112] The main materials and equipment used are as follows:

[0113] 6.1 Main Materials

[0114] 6.1.1 Pipe Shed

[0115] Φ108mm×6mm hot-rolled seamless steel pipe is selected. The pipe body has Φ12 overflow holes around its perimeter, and the pipe wall thickness is 6mm. Grouting material: 1:1 pure cement grout, grouting pressure 1.0~2.0MPa. Φ89 medium-sized pipe roof uses Φ89 hot-rolled steel pipe, with Φ12 overflow holes around its perimeter, and a pipe wall thickness of 6mm. Grouting material: W / C=0.6~0.8 pure cement grout, grouting pressure not less than 1.0MPa.

[0116] 6.1.2 Grouting Small Pipe

[0117] Φ12 hot-rolled seamless steel pipe with Φ8 overflow holes around the pipe body and a wall thickness of 4mm. Grouting material: 0.6~0.8 cement grout, grouting pressure: 0.5~1.0MPa.

[0118] 6.1.3 Arch frame: I-beams are used for construction.

[0119] 6.1.4 System Anchor Bolts

[0120] The Φ22 explosive cartridge anchor bolt is made of HRB335 threaded steel bar and HPB235 steel plate. The plate size is 15cm×15cm×0.6cm (length×width×thickness). The explosive loading length is not less than 40cm. The tensile strength of the anchor bolt is ≥50KN and the pull-out strength of the anchor bolt is ≥70KN.

[0121] 6.2 Machinery and Equipment

[0122] For detailed specifications of the machinery and equipment, please refer to Table 1:

[0123] Table 1. Machinery and Equipment List

[0124]

[0125]

[0126] The present invention has been described above by way of example, but the present invention is not limited to the specific embodiments described above. Any modifications or variations made based on the present invention shall fall within the scope of protection claimed by the present invention.

Claims

1. A shallow-buried bias broken stratum small clear spacing double tunnel entrance and exit hole construction method, characterized in that, Includes the following steps: Step 1: Based on the actual situation of the shallow buried bias-pressure fractured strata and small clearance double tunnel project, conduct engineering geology and surrounding environment surveys, conduct risk analysis around the tunnel entrance, and formulate corresponding measures for tunnel entry and exit. Step 2: By establishing a high-precision three-dimensional numerical model, simulate and analyze the tunnel deformation, stress and stratum deformation caused by different construction methods, and determine a reasonable construction plan by combining the surrounding rock mechanical response and grey relational analysis. Step 3: Determine a reasonable tunnel entry advance support technology. Before entering the left tunnel, the slope protection at the tunnel entrance and the pile foundation support beam retaining wall construction should be completed. Before entering the right tunnel, the anti-slide pile construction should be completed. Step 4: Determine the intersection area of ​​the cross tunnel and the main tunnel and construct using the direct top-lifting method, strictly following the technical instructions; the cross tunnel is excavated from the outside to the main tunnel, then from the main tunnel body towards the entrance to the main tunnel entrance, and towards the exit to the tunnel exit; Construction techniques for the intersection of the transverse tunnel and the main tunnel include transverse tunnel entry techniques and cross-section roof lifting construction techniques. The aforementioned transverse tunnel entry technology involves anchoring and spraying reinforcement of the slope 1-3m outside the transverse tunnel entrance before construction; the initial excavation of the transverse tunnel entrance section adopts the single-side wall pilot tunnel method, and subsequent construction can adopt the full-section method based on the actual site conditions; when the transverse tunnel is excavated to the junction with the main tunnel, the arch of the transverse tunnel is reinforced with advanced small guide pipes. The aforementioned cross-section cantilever construction technology involves, when the branch tunnel is constructed to the point where it intersects with the maximum span of the secondary lining of the main tunnel, firstly, using a double-layer steel arch frame to lock the intersection of the branch tunnel; secondly, providing advance support; and then, vertically entering the main tunnel using an up-and-down step excavation method and carrying out arc-shaped cantilever excavation, setting up initial support and temporary gantry support for the main tunnel. Step 5: Determine the main tunnel exit construction technology. When the main tunnel is excavated to 30 meters before the end of the light-dark boundary, the exit will first be carried out using a small pilot tunnel with a single side wall, with temporary support being installed while excavating. After the pilot tunnel is completed and the conditions for transporting machinery, equipment and materials are met, the guide wall and tunnel portal will be constructed. After the tunnel portal is completed, the construction will be carried out from the central pilot tunnel outwards until the construction requirements are met.

2. The construction method for a shallow-buried, bias-pressure fractured strata with small clearance dual-tunnel entry and exit as described in claim 1, characterized in that: The construction plan for step two was determined to be the single-sidewall pilot tunnel method based on the analysis of surrounding rock mechanical response and grey relational analysis.

3. The construction method for a shallow-buried, bias-pressure fractured strata with small clearance dual-tunnel entry and exit as described in claim 1, characterized in that: The tunnel entry construction technology in step three includes surface treatment, anti-slide pile construction at the tunnel entrance, slope protection construction, and pile foundation support beam retaining wall construction.

4. The construction method for a shallow-buried, bias-pressure fractured strata with small clearance dual-tunnel entry and exit as described in claim 3, characterized in that: The construction of the anti-slide piles at the tunnel entrance should first involve grouting and reinforcing the surface at the entrance, followed by excavation of the tunnel entrance. Grouting reinforcement using steel pipe piles is employed, with hot-rolled seamless steel pipes used as the grouting guide. Grouting holes are drilled on the guide pipes in a quincunx pattern. The grouting material is pure cement slurry, with a cement strength grade of 42.5 and a grouting pressure of 1~1.5 MPa.

5. The construction method for a shallow-buried, biased, fractured strata with small clearance for dual tunnels at the entrance and exit, as described in claim 3, is characterized in that: The slope protection should be carried out after the slope is excavated, with wire mesh and shotcrete installed at the tunnel entrance. The protection should be carried out from top to bottom, layer by layer, in a timely manner as the excavation progresses. Anchor bolts and wire mesh shotcrete support should be installed on the slope, and isolated boulders and dangerous rocks on the tunnel top should be treated. The area between the slope top and the ditch should be sealed with 5cm plain concrete to prevent surface water from seeping down from the platform between the slope and the intercepting ditch.

6. The construction method for a shallow-buried, bias-pressure fractured strata with small clearance for dual tunnels at the entrance and exit, as described in claim 3, is characterized in that: The construction of the pile foundation beam retaining wall involves setting up an inclined retaining wall at the tunnel entrance, with the retaining wall closely attached to the right side slope. The upper part of the beam is a gravity retaining wall, 6-10 meters high. The beam consists of 5 sections, each 12 meters long, 3.5-5 meters wide, and 1.2-1.5 meters high. The pile foundation uses a single row of round piles with a diameter of 2 meters and a length of 8-12 meters. The pile holes are drilled using an impact drill, and after the steel cage is installed, C30 underwater concrete is poured. After the pile foundation construction is completed, a C30 reinforced concrete cap beam is poured. The retaining wall uses C20 rubble concrete. The slope ratio of the excavated side of the beam is 1:0.

3. The temporary slope behind the beam retaining wall is protected with wire mesh and shotcrete. Drainage pipes are reserved inside the retaining wall as required.

7. The construction method for a shallow-buried, bias-pressure fractured strata with small clearance dual-tunnel entry and exit as described in claim 1, characterized in that: The fifth step of the tunnel construction technology includes small pilot tunnel construction and large pipe shed construction; In the construction of the small pilot tunnel, 30 meters before the main tunnel is excavated to the end of the transition between open and closed sections, the tunnel exits by first opening a small tunnel with a clearance of 4×5m within the central pilot tunnel, and excavating while making temporary support. After the central pilot tunnel is completed and the conditions for transporting machinery, equipment and materials are met, the guide wall and tunnel portal are constructed. The large pipe shed is located at the tunnel entrance, with a length of 30m. It is made of hot-rolled seamless steel pipes. The steel pipe joints are connected by threaded sections with a length greater than 15cm. Adjacent steel pipe joints are staggered by more than 1m. The number of joints in the same longitudinal section of the tunnel is no more than 50%. Grouting holes are drilled on the steel pipes with a diameter of Φ12mm and a diameter spacing of 15cm, arranged in a quincunx pattern. The exposed part of the steel pipe arch is 3.0m without drilled holes, with a circumferential spacing of 40cm.

8. The construction method for a shallow-buried, biased, fractured strata with small clearance for dual tunnels at the entrance and exit, as described in claim 1, is characterized in that: The tunnel entrance is located on a cliff, with no conditions for construction operations and no road access. Major machinery, equipment, and materials cannot be directly delivered to the tunnel entrance. The shallow buried bias overburden at the tunnel entrance is 4-10m thick, and the minimum clear distance between the left and right monitoring lines is 7.81m. To strengthen the advance support and initial support measures, Φ42×4 grouting pipes are used to reinforce the middle rock wall area, and the foundation is replaced or grouted for reinforcement.

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