Pile foundation-joist-arch protection structure for orthogonal shallow landslide section of tunnel portal and construction method of pile foundation-joist-arch protection structure

Abstract

The invention belongs to the technical field of tunnel construction, and particularly discloses a pile foundation-joist-arch protection structure for an orthogonal shallow landslide section of a tunnel portal and a construction method.According to the pile foundation-joist-arch protection structure for the orthogonal shallow landslide section of the tunnel portal, a pile foundation penetrates into bedrock, a pre-stressed anchor cable is arranged to actively cut off the sliding force of the landslide, and a joist is connected with the pile foundation and a protection arch to form an overall stress frame; the protective arch is formed by pouring a profile steel arch frame and reinforced concrete and covers back pressure earthwork to enhance the anti-sliding capacity. The matched water interception blind ditch and the gravel drainage cushion layer cooperatively drain underground water, and the grouting area forms a soil arching effect through high-pressure grouting to transmit sliding force to the pile foundation. The construction method comprises the steps of geological supplementary survey, retaining wall construction, pile foundation and joist pouring, protection arch segmented alternate warehouse construction, slip mass grouting and the like. According to the technical scheme, a point-line-plane integrated space frame system is constructed, the problem that a traditional support is difficult to resist bias load is effectively solved, accurate control over surrounding rock deformation is achieved, meanwhile, the space utilization rate is optimized through the straight wall type design, and a safe and reliable overall solution is provided for a tunnel portal to penetrate through a shallow landslide section.

Description

Technical Field

[0001] This invention relates to the field of tunnel construction technology, and in particular to a pile foundation-support beam-arch protection structure and construction method for a tunnel entrance in a shallow landslide area. Background Technology

[0002] Tunnel portal sections often traverse shallow landslides orthogonally. Due to their shallow depth, the surrounding rock is unlikely to form an effective arch effect, and the sliding force of the slope after excavation can directly act on the tunnel body, leading to lining cracking or even large-scale damage. Traditional solutions include pre-installed pipe roofs, pre-installed small guide pipes, surface anchors, or shotcrete sealing. These methods can provide temporary support when the slope is in a relatively stable state, but when the slope itself is unstable or destabilized due to aging deformation, the eccentric load increases significantly, which traditional supports cannot withstand, easily leading to structural defects during construction and operation. Therefore, how to propose systematic and reliable surrounding rock deformation control measures to address the engineering characteristics of soft, slippery strata—"difficult to form arches, easy to collapse"—and avoid eccentric loads on the tunnel portal has become a key technical bottleneck that urgently needs to be overcome in the design and construction of such projects. Summary of the Invention

[0003] This invention addresses the challenges of tunnel entrance construction in shallow landslide areas, where the arch effect of the surrounding strata is difficult to form, excavation is prone to landslides, and landslide reactivation can even be induced, leading to difficulties in tunnel entry. It provides a pile-beam-arch support structure and construction method for tunnel entrances in shallow landslide areas. By setting up a pile-beam-arch support structure at the tunnel entrance, a point-line-surface integrated spatial frame support system is formed. Utilizing the rigidity advantage of the anti-slide piles, combined with a landslide grouting modification process, the sliding force of the landslide is transferred to the pile foundation. This prevents the sliding force of the excavated slope from directly acting on the tunnel body and causing lining damage, effectively controlling the deformation during construction when the tunnel entrance orthogonally crosses a shallow landslide area.

[0004] To achieve the above objectives, the present invention provides the following solution:

[0005] A pile-support beam-arch structure for a shallow landslide at the tunnel entrance includes: piles, penetrating the bedrock and symmetrically arranged along the tunnel longitudinal direction, with prestressed anchor cables on the pile tops; a support beam, located on top of the piles, connecting the piles and serving as the supporting foundation for the arch; an arch, with the support beam as the arch seat, formed by steel arch frames and reinforced concrete casting to create an arch structure, topped with counter-pressure soil; a drainage ditch, located at the connection between the piles and the sliding surface on the mountainside, for draining groundwater; a grouting zone, formed by grouting at the top of the landslide body, transmitting the sliding force to the piles; and a retaining wall, located at the bottom of the landslide body, providing support for the landslide body on the mountainside of the tunnel.

[0006] Furthermore, a waterproof retaining plate is provided at the connection between the pile foundation and the tunnel clearance.

[0007] Furthermore, the tensioning end of the prestressed anchor cable is fixed inside the pile foundation by anchor plates and clamps, the anchor cable inclination angle is 20° to 30°, the anchoring section enters the bedrock and the grouting pressure is 0.5 to 1.0 MPa.

[0008] Furthermore, the supporting beam is a rectangular reinforced concrete structure with a thickness of not less than 0.8m and a concrete strength grade of C30. It is connected to the pile foundation by steel bars extending in and with a fixed length of ≥10cm.

[0009] Furthermore, the protective arch is composed of an initial support thin arch and cast-in-place reinforced concrete. The initial support includes an I20a type steel arch frame, a double-layer steel mesh, and shotcrete. The thickness of the cast-in-place protective arch is 80cm, and the concrete strength grade is C35.

[0010] Furthermore, the intercepting blind ditch comprises, from the inside out, a permeable geotextile, an impermeable geotextile, a sand and gravel filler, and a permeable pipe, and is connected to the crushed stone drainage cushion layer.

[0011] Furthermore, the grouting zone is formed by grouting through vertically drilled holes arranged in a quincunx pattern, with a grouting spacing of 1.5m and a depth of 1.2 to 1.5 times the thickness of the sliding body.

[0012] Furthermore, a 0.5m thick crushed stone drainage cushion layer is laid at the interface between the counterweight earthwork and the sliding body, and a 6% herringbone drainage slope and a planting soil protective layer are set on top.

[0013] A construction method for a pile foundation-support beam-arch protection structure in a shallow landslide area at the tunnel entrance includes the following steps:

[0014] S1. Pre-construction preparation: Geological exploration to determine the landslide range and sliding surface depth, calculate landslide thrust and design pile foundation parameters;

[0015] S2. Retaining wall construction: Excavate the foundation in sections and build retaining walls, and set up drainage ditches at the same time;

[0016] S3. Pile foundation construction: excavate pile holes in layers and pour protective walls, install prestressed anchor cables, grout and tension them, and pour pile body concrete.

[0017] S4. Support beam construction: Excavate the foundation trench at the top of the pile foundation, tie the steel bars and pour concrete to ensure a rigid connection with the pile foundation;

[0018] S5. Arch support construction: Erect steel arch frame and spray initial support thin arch, and after installing formwork, pour reinforced concrete arch support in sections.

[0019] S6. Backfilling of open tunnel: Backfilling of counter-pressure earthwork and gravel drainage cushion layer in layers, with grass planting on top for protection;

[0020] S7. Construction of the tunnel: The three-stage method is used for excavation, and a system of anchor bolts and temporary invert arches are constructed.

[0021] S8. Sliding body grouting: High-pressure grouting is performed within the sliding body area behind the pile foundation to form a grouting zone.

[0022] Furthermore, in step S5, the casting of the arch support adopts a segmented skip-pour process, with each segment having a length of ≤8m and the concrete slump controlled at 180±20mm. Grouting holes are reserved at the top of the arch for later backfilling and grouting.

[0023] Compared with the prior art, the present invention discloses at least the following beneficial effects:

[0024] This invention achieves highly efficient and stable engineering protection in shallow landslide areas at tunnel entrances through an innovative pile-beam-arch structure system. The structure uses piles driven into the bedrock and prestressed anchor cables to actively intercept landslide forces and prevent slope loosening. The beams, acting as rigid connecting components, tightly integrate the piles and arches, forming an integrated load-bearing frame. The arches, constructed with steel arch frames and reinforced concrete, are topped with counter-pressure soil, significantly enhancing anti-sliding capacity. Drainage ditches and gravel drainage layers work synergistically to effectively drain groundwater, reducing the negative impact of water pressure on the structure. High-pressure grouting in the grouting zone creates an arch effect, transferring the sliding force to the piles and preventing the tunnel from directly bearing the load. The retaining wall further improves the local anti-sliding capacity of the slope. This spatial frame system not only solves the problem of traditional support systems being unable to withstand eccentric loads but also achieves effective control of surrounding rock deformation through a point, line, and surface combined stress pattern, ensuring the safety and stability of tunnel construction and operation. Meanwhile, the straight-wall design increases tunnel clearance and optimizes space utilization efficiency, while the coordinated design of open and closed tunnel structures further enhances the long-term stability of the overall project. Attached Figure Description

[0025] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0026] Figure 1 This is a front view diagram of an embodiment of the present invention;

[0027] Figure 2 This is a side view of an embodiment of the present invention;

[0028] Figure 3 This is a schematic diagram of the pile foundation-supporting beam-arch structure according to an embodiment of the present invention;

[0029] Figure 4 This is a schematic diagram of a water interception blind ditch according to an embodiment of the present invention;

[0030] Figure 5 This is a schematic diagram of a pile foundation according to an embodiment of the present invention.

[0031] In the diagram: 1. Landslide boundary; 2. Bedrock; 3. Sliding surface; 4. Sliding body; 5. Pile foundation; 6. Prestressed anchor cable; 7. Anchor plate; 8. Clamp; 9. Waterproof retaining wall; 10. Tunnel pavement; 11. Support beam; 12. Arch support; 13. Drainage ditch; 1301. Permeable geotextile; 1302. Impermeable geotextile; 1303. Sand and gravel filler; 1304. Permeable pipe; 14. Counterweight earthwork; 15. Crushed stone drainage cushion; 16. Grouting zone; 17. Retaining wall. Detailed Implementation

[0032] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0033] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0034] Reference Figures 1 to 5 As shown, this embodiment provides a pile foundation-support beam-arch structure for a shallow landslide section at the tunnel entrance. This structure mainly consists of pile foundations 5, support beams 11, arches 12, a drainage ditch 13, counterweight earthwork 14, a gravel drainage cushion layer 15, a grouting zone 16, and a retaining wall 17. Pile foundations 5 penetrate into bedrock 2. Prestressed anchor cables 6 are installed at the top of pile foundations 5 to actively intercept the landslide force and prevent slope loosening. Support beams 11 are installed at the top of pile foundations 5 to connect the pile foundations and provide a foundation for the arches 12. A drainage ditch 13 is provided on the mountainside, comprising permeable geotextile 1301, impermeable geotextile 1302, gravel filler 1303, and permeable pipes 1304, for draining groundwater and protecting the pile foundations. The top of the arches 12 is covered with counterweight earthwork 14 to improve sliding resistance, and a 0.5m thick gravel drainage cushion layer 15 is laid at the interface between the arches and the landslide body 4. The top of the landslide body 4 is grouted with high pressure to form a grouting zone 16, which enhances the soil arching effect and transfers the sliding force to the pile foundation 5. A retaining wall 17 is installed at the bottom of the landslide body 4 to support the landslide body and improve local anti-sliding force. This structure effectively controls the deformation of the tunnel during landslide construction through a point-line-plane-body spatial framework system, ensuring the safety and stability of the tunnel.

[0035] It should be understood that in practical applications, bedrock 2 is a horizontal or near-horizontal sedimentary rock layer, and the landslide body 4 overlies bedrock 2. A 0.5m thick gravel drainage cushion layer 15 is provided at the interface between the landslide body 4 and the counterweight earthwork 14 to drain groundwater from the counterweight earthwork 14. High-pressure grouting is performed at the top of the landslide body 4 using vertical drilling to form a grouting zone 16, thereby ensuring an effective soil arch effect between the pile foundations 5 on the mountain-side of the tunnel, effectively transferring the sliding force of the landslide to the pile foundations 5 and reducing the compression of the arch 12 by the landslide body 4. A retaining wall 17 is provided at the bottom of the landslide body 4 to support the landslide body on the mountain-side of the tunnel, improving the local anti-sliding force of the slope, specifically as follows... Figure 1 and Figure 2 As shown.

[0036] In one specific embodiment, the pile foundation 5 penetrates the bedrock 2 and can be in the form of manually excavated piles, mechanically drilled piles, or micro pile groups. The pile type can be friction piles or end-bearing piles.

[0037] In one specific embodiment, such as Figure 3 As shown, a waterproof retaining plate 9 is provided at the connection between the pile foundation 5 and the tunnel clearance, and the tunnel road surface 10 is located between the pile foundations 5 on both sides of the tunnel clearance.

[0038] In one specific embodiment, such as Figure 5 As shown, a prestressed anchor cable 6 is provided at the top of the pile foundation 5. The tensioning end of the prestressed anchor cable 6 is fixed inside the pile foundation 5 by an anchor plate 7 and a clamp 8. In this way, the active force-bearing effect of the anchor cable anti-slide pile is utilized to cut off the downward force of the landslide at the rear and prevent the slope from further loosening under engineering disturbance.

[0039] In one specific embodiment, the supporting beam 11 has a rectangular cross section with a thickness of not less than 0.8m, is cast from C30 concrete, connects to the pile foundation 5, and provides a foundation for the protective arch 12.

[0040] In one specific embodiment, the arch support 12 uses the support beam 11 as the arch seat. After forming a thin arch through "steel frame + steel mesh + shotcrete", an 80cm thick C35 reinforced concrete arch support is poured. The top of the arch support 12 is covered with counter-pressure soil 14 to enhance the anti-sliding force.

[0041] In one specific embodiment, the drainage system is provided by a water-blocking blind ditch 13 and a gravel drainage cushion layer 15. The water-blocking blind ditch 13 is located at the connection between the pile foundation 5 on the mountainside and the sliding surface 3. The specific structure of the water-blocking blind ditch 13 is as follows: Figure 4 As shown, the intercepting blind ditch 13 is composed of permeable geotextile 1301, impermeable geotextile 1302, sand and gravel filler 1303 and permeable pipe 1304, which drains groundwater and protects the pile foundation 5; the crushed stone drainage cushion layer 15 is 0.5m thick and is laid at the interface between the sliding body 4 and the counterweight soil 14 for groundwater drainage.

[0042] The intercepting blind ditch 13 can lower the groundwater level within a certain depth range of the pile foundation 5 and quickly drain the groundwater inside the landslide body. Simultaneously, by installing an impermeable geotextile 1302 on the side of the intercepting blind ditch 13 closest to the pile foundation 5, groundwater infiltration can be prevented from threatening the safety of the pile foundation 5. A 0.5m thick crushed stone drainage cushion layer 15 is installed at the interface between the landslide body 4 and the counterweight earthwork 14 to drain the groundwater from the counterweight earthwork 14.

[0043] In one specific embodiment, a grouting zone 16 is formed by high-pressure grouting through vertical drilling at the top of the landslide 4, thereby ensuring an effective soil arch effect between the pile foundations 5 on the mountain side of the tunnel, thus effectively transferring the sliding force of the landslide to the pile foundations 5 and reducing the compression of the landslide 4 on the protective arch 12.

[0044] In one specific embodiment, the grouting zone 16 is formed by grouting through vertically drilled holes arranged in a quincunx pattern, with a grouting spacing of 1.5m and a depth of 1.2 to 1.5 times the thickness of the sliding body 4.

[0045] In one specific embodiment, the retaining wall 17 is located at the bottom of the sliding body 4, thereby supporting the sliding body 4 of the tunnel on the mountain side and improving the local anti-sliding force of the slope.

[0046] In one specific embodiment, such as Figure 3 As shown, the waterproof retaining plate 9 is located on the inner side of the pile foundation 5, that is, at the connection between the pile foundation 5 and the tunnel clearance. The waterproof retaining plate 9 is integrally cast with concrete and has a waterproof function.

[0047] The pile-beam-arch structure based on the above structure in shallow landslide areas at the tunnel entrance has the following technical advantages:

[0048] 1. High structural stability: By innovatively setting up a pile foundation-support beam-arch protection structure at the tunnel entrance in a shallow landslide area, a spatial frame support system of points-lines-surfaces-volumes is constructed. This system fully utilizes the stiffness advantage of anti-slide piles, effectively transferring the sliding force generated by the landslide to the pile foundation structure, thereby avoiding the risk of failure caused by the tunnel structure directly bearing shear forces.

[0049] 2. Outstanding geological adaptability: The proposed pile foundation-support beam-arch protection structure design has wide applicability, especially showing significant advantages for complex geological conditions such as loose, water-rich, soft and slippery strata, and can effectively solve the tunnel entry construction problems that are difficult to handle with traditional methods.

[0050] 3. Excellent safety protection performance: By extending the open-cut structure at the tunnel entrance and adopting counter-pressure measures, this technical solution not only resists mountain pressure and landslide thrust, but also has the function of intercepting falling rocks and debris, providing multiple safety protections for the tunnel entrance and significantly reducing the threat posed by natural disasters.

[0051] 4. High space utilization efficiency: The straight-wall tunnel structure design not only increases the effective clearance, but also saves on the construction investment of curved walls and inverted arch structures, realizing the efficient use of space resources.

[0052] 5. Good structural coordination: The open-cut structure based on the pile foundation-support beam-arch protection system has good coordination with the structure of the tunnel entrance to be built in terms of displacement characteristics. This synergy significantly improves the stability of the overall structure and provides a reliable guarantee for the long-term safe operation of the tunnel project.

[0053] This embodiment also provides a construction method for a pile foundation-support beam-arch protection structure in a shallow landslide area at the tunnel entrance, including the following steps:

[0054] Step S1: Pre-construction preparation

[0055] S101. Geological Supplementary Exploration: Using drilling, geophysical exploration and other methods, clarify the landslide range, the depth of the slip surface and the distribution of groundwater.

[0056] S102. Slope stability assessment: Based on the landslide thrust calculation results, determine the number, spacing and cross-sectional dimensions of pile foundation 5; mark the pile positions on site, and determine the length of the free section and anchorage section of the prestressed anchor cable 6 accordingly.

[0057] S103. Slope drainage system: Improve drainage facilities at the tunnel entrance to ensure rapid drainage of surface water and reduce the softening effect of rainwater on the landslide.

[0058] S104, Parameter determination of retaining wall 17: Calculate the length, height and top and bottom geometric dimensions of retaining wall 17 based on the sliding thrust at the slope toe, and then select the type according to the on-site material conditions; when stone is abundant, use rubble concrete retaining wall, and when stone is scarce, use reinforced concrete retaining wall.

[0059] Step S2, Retaining Wall 17 Construction

[0060] Based on the determined parameters of retaining wall 17, the foundation trench is excavated in sections. Soft soil is promptly replaced and compacted in layers. Once the foundation bearing capacity meets the design requirements, retaining wall 17 is poured from bottom to top. Simultaneously, a water-cutting blind ditch 13, a filter layer, and drainage holes are constructed behind the wall to prevent groundwater accumulation. Upon completion, retaining wall 17 will provide a stable foundation for the subsequent "pile foundation-supporting beam-arch protection" system.

[0061] Step S3, Construction of Pile Foundation 5

[0062] S301. Pile Hole Construction: On the outer edge of the horizontal projection of the open-cut tunnel structure, mark the center line of the pile positions and the excavation edge line according to the design drawings. Specifically, two rows of rectangular cross-section piles 5 can be excavated manually. These piles 5 serve as longitudinal and transverse anti-slip supports for the slope and the self-weight of the open-cut tunnel. The pile tops are 2.27m above the rail surface, with a cross-section of 1.5m × 2m and a spacing of 4.5m. Excavation is carried out in layers, with each cycle being 0.5m when the soil layer is stable and reduced to 0.2-0.3m when the soil is in a fluid state. Formwork is erected immediately after each layer is excavated, and a 15-20cm thick C20 concrete retaining wall is poured in place, reinforced with Φ8-10 structural steel mesh. Excavation continues only after the retaining wall has reached the required strength.

[0063] S302, Installation of prestressed anchor cable 6: After the pile hole is excavated to the preset position of the anchor cable and the wall strength reaches 75%, a down-the-hole drill is used at an inclination angle of 20° to 30°, with a hole diameter ≥130mm and a hole depth exceeding the design length by 0.5m (to ensure that the anchoring section enters the bedrock); after drilling, the hole is cleaned with high-pressure air, and the thickness of sediment in the hole is ≤5cm; after further installing the prestressed anchor cable 6, grouting is carried out using the bottom grouting method, with a pressure of 0.5 to 1.0MPa. After the grout strength reaches 30MPa, the anchor cable is tensioned using the staged cyclic tensioning method until the prestress of the anchor cable reaches the design requirements before the next cycle of pile hole excavation can be carried out.

[0064] S303, Pile Construction: After the bottom of the hole passes inspection, the reinforcing cage is lowered, and C30 to C35 concrete is poured in layers, with each layer vibrated to a thickness of ≤0.5m to prevent segregation; after the strength meets the standard, the construction of the support beam 11 begins; in addition, in areas with suitable site conditions, mechanically drilled circular piles (drilled cast-in-place piles) or micro piles can be used instead of manually excavated rectangular piles to reduce the life risks to construction workers; when using circular pile foundations, their diameter can be 1.8m.

[0065] Step S4, Construction of Support Beam 11

[0066] A 16m×0.8m×1.5m support beam 11 is constructed at the top of pile foundation 5 along the longitudinal direction of the tunnel to enhance the overall structural stability. After the strength of support beam 11 reaches the required level, the arch support 12 is constructed. The specific process is as follows: excavate the foundation trench along the top of pile foundation 5 to the design elevation (trench width = support beam width + 0.5m working space); further install the formwork inside the foundation trench, and after the formwork is formed, put in the steel cage and pour C30 concrete; it should be noted that the steel bars between pile foundation 5 and support beam 11 should be reliably connected, and a fixed connection is adopted in which the pile foundation 5 extends into the support beam 11 by not less than 10cm.

[0067] Step S5, Arch Protection 12 Construction

[0068] S501, Arch base construction: After the strength of the support beam 11 reaches the design strength, use a total station to locate the arch foot embedded parts, accurately lay out the arching line, and ensure that the flatness error of the arch base connection surface is ≤3mm; the arch foot part needs to be roughened and Φ22 connecting steel bars need to be inserted.

[0069] S502. Initial support thin arch construction: Install I20a type steel arch frame with a spacing of 50cm. The arch frame and the embedded steel plate of the supporting beam are connected with high-strength bolts with a torque of 300N·m. Hang Φ8@150×150 double-layer steel mesh and spray 80cm thick C25 early strength concrete to form initial support. During spraying, reserve Φ42 grouting pipes (circumferential spacing of 2m) for later backfilling grouting.

[0070] S503. Formwork System Construction: An adjustable arc-shaped formwork support system is erected on the inner side of the initial support, using channel steel as the circumferential back rib, with Φ48 steel pipes arranged at a longitudinal spacing of 60cm; the main reinforcement of the arch is tied, with the main reinforcement arranged in a double layer and bidirectional manner using Φ25@150, and the stirrups arranged in a quincunx pattern using Φ12@300, with the protective layer thickness controlled at 50mm.

[0071] S504 Concrete Pouring Control: A segmented skip-pour pouring process is adopted, with each segment not exceeding 8m in length, and the concrete slump controlled at 180±20mm; an immersion vibrator and an attached vibrator are used in combination, and three observation holes that also serve as grouting holes are reserved at the top of the arch; immediately after pouring, geotextile is covered for moisture retention and curing, and the side formwork is removed after 24 hours, while the load-bearing formwork is removed after the strength reaches 80%; finally, 1:1 cement grout is injected through the pre-embedded grouting pipe at a pressure of 0.5~1.0MPa to fill the gaps and complete the arch protection 12.

[0072] Step S6: Backfilling the tunnel

[0073] The backfilling and counter-pressure construction of the open tunnel can only begin after the concrete strength of the arch support 12 reaches 90% of its design strength. The backfilling will be carried out using a bottom-up, layered method, with the drainage layer and counter-pressure layer constructed sequentially.

[0074] Drainage layer: First, lay a 50cm thick gravel drainage layer 15 (particle size 2-4cm), and then construct a water interception blind ditch 13 to connect with the drainage system at the opening.

[0075] Counterweight layer: The excavated waste from the pile holes and foundation trenches is backfilled and compacted in layers to the design elevation. Each layer is no more than 30cm thick and the compaction degree is no less than 93%. A 6% herringbone drainage slope is set at the top, and the slope is covered with 50cm thick planting soil and planted with grass for protection.

[0076] Step S7, Construction of the Dark Cave

[0077] After the open-cut tunnel structure is completed, the soil and rocks inside the open-cut tunnel cavity are removed and the structure is closed. Then, the tunnel is excavated according to the design, usually using a three-stage method (upper, middle and lower stages are advanced in sequence) to control the deformation of the surrounding rock. After each stage is excavated, a temporary invert arch is immediately constructed to form a closed support. At the same time, systematic anchor bolts are installed in the arch and side walls to enhance the integrity of the surrounding rock and the initial support. During the construction process, the surrounding rock should be monitored in real time, and the support parameters should be dynamically adjusted to ensure construction safety and structural stability.

[0078] The junction between the open and closed tunnels is a transitional section with a sudden change in stiffness, which is prone to differential settlement and stress concentration. During construction, a settlement joint of about 2cm should be reserved at the junction to absorb the displacement difference caused by foundation deformation or uneven stress and prevent structural cracking. The settlement joint should be filled with flexible waterproof material, which allows deformation while ensuring waterproofing. At the same time, additional reinforcing steel bars and transition reinforcement measures should be added to improve the connection strength. Settlement and displacement monitoring points should be densely distributed in this section to track the structural response in real time. If necessary, the support or joint treatment plan should be adjusted to ensure the long-term safety and stability of the junction between the open and closed tunnels.

[0079] Step S8, Grouting of sliding body 4

[0080] Within the sliding body 4, take 1.5 times the spacing of the anti-slide piles, that is, about 20m behind the piles, and grout in a 1.5m quincunx pattern, with a grouting depth of 17m. Through grouting reinforcement, a reliable soil arch effect can be quickly formed between the adjacent piles on both sides of the tunnel, effectively transferring the sliding force of the sliding body to the more rigid pile foundation 5, thereby significantly reducing the compression of the sliding body 4 on the arch 12 and ensuring the safety of the tunnel structure.

[0081] 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, and are only for the convenience of describing this invention, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this invention.

[0082] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A pile foundation-support beam-arch protection structure for a tunnel entrance in a shallow landslide area, characterized in that, include: The pile foundation (5) penetrates the bedrock (2) and is symmetrically arranged along the longitudinal direction of the tunnel. The pile top is equipped with prestressed anchor cable (6). A support beam (11) is set on the top of the pile foundation (5), connecting the pile foundation (5) and serving as the supporting foundation for the protective arch (12); The arch support (12) is formed by using the supporting beam (11) as the arch seat, and by using steel arch frame and reinforced concrete to form an arch structure, with the top covered by counter-pressure earthwork (14). A water-cutting blind ditch (13) is set at the connection between the pile foundation (5) and the sliding surface (3) on the mountain side to drain groundwater; Grouting zone (16), grouting is formed at the top of the sliding body (4) to transmit the sliding force to the pile foundation (5); A retaining wall (17) is installed at the bottom of the sliding body (4) to support the sliding body (4) of the tunnel on the mountain side.

2. The pile foundation-support beam-arch structure for orthogonal shallow landslide areas at tunnel entrances according to claim 1, characterized in that, A waterproof retaining plate (9) is provided at the connection between the pile foundation (5) and the tunnel clearance.

3. The pile foundation-supporting beam-arch structure for orthogonal shallow landslide areas at tunnel entrances according to claim 1, characterized in that, The tensioning end of the prestressed anchor cable (6) is fixed inside the pile foundation (5) by the anchor plate (7) and the clamp (8). The anchor cable inclination angle is 20° to 30°, the anchoring section enters the bedrock (2) and the grouting pressure is 0.5 to 1.0 MPa.

4. The pile foundation-supporting beam-arch structure for orthogonal shallow landslide sections at tunnel entrances according to claim 1, characterized in that, The supporting beam (11) is a rectangular reinforced concrete structure with a thickness of not less than 0.8m and a concrete strength grade of C30. It is connected to the pile foundation (5) by steel bars with a fixed length of ≥10cm.

5. The pile foundation-supporting beam-arch structure for orthogonal shallow landslide sections at tunnel entrances according to claim 1, characterized in that, The arch support (12) consists of an initial support thin arch and cast-in-place reinforced concrete. The initial support includes an I20a type steel arch frame, a double-layer steel mesh and shotcrete. The cast-in-place arch support (12) has a thickness of 80cm and a concrete strength grade of C35.

6. The pile foundation-supporting beam-arch structure for orthogonal shallow landslide sections at tunnel entrances according to claim 1, characterized in that, The intercepting blind ditch (13) consists of, from the inside out, a permeable geotextile (1301), an impermeable geotextile (1302), a sand and gravel filler (1303), and a permeable pipe (1304), and is connected to the crushed stone drainage cushion layer (15).

7. The pile foundation-supporting beam-arch structure for orthogonal shallow landslide sections at tunnel entrances according to claim 1, characterized in that, The grouting zone (16) is formed by vertical drilling in a quincunx pattern, with a grouting spacing of 1.5m and a depth of 1.2 to 1.5 times the thickness of the sliding body (4).

8. The pile foundation-support beam-arch structure for orthogonal shallow landslide areas at tunnel entrances according to claim 1, characterized in that, A 0.5m thick gravel drainage cushion layer (15) is laid at the interface between the counter-pressure earthwork (14) and the sliding body (4), and a 6% herringbone drainage slope and a planting soil protective layer are set on top.

9. A construction method for a pile foundation-support beam-arch structure at the tunnel entrance in an orthogonal shallow landslide area as described in any one of claims 1-8, characterized in that, Includes the following steps: S1. Pre-construction preparation: Geological investigation to determine the landslide range and sliding surface depth, calculate landslide thrust and design pile foundation (5) parameters; S2, Retaining Wall (17) Construction: Excavate the foundation in sections and build the retaining wall, and set up the water interception blind ditch (13) at the same time; S3, Pile foundation (5) construction: excavate pile holes in layers and pour protective walls, install prestressed anchor cables (6) and then grout and tension, and pour pile body concrete; S4, Construction of support beam (11): Excavate the foundation trench at the top of the pile foundation (5), tie the steel bars and pour concrete to ensure a rigid connection with the pile foundation (5); S5, Arch Protection (12) Construction: Erect steel arch frame and spray initial support thin arch, install formwork and then pour reinforced concrete arch protection (12) in sections; S6. Backfilling of the tunnel: Backfilling the counter-pressure earthwork (14) and the crushed stone drainage cushion (15) in layers, with grass planting on top for protection; S7. Construction of the tunnel: The three-stage method is used for excavation, and a system of anchor bolts and temporary invert arches are constructed. S8. Grouting of sliding body (4): High-pressure grouting is performed within the sliding body (4) behind the pile foundation (5) to form a grouting zone (16).

10. The construction method of the pile foundation-support beam-arch structure in the orthogonal shallow landslide section at the tunnel entrance according to claim 9, characterized in that, In step S5, the casting of the arch support (12) adopts a segmented skip-filling process, with each segment having a length of ≤8m and the concrete slump controlled at 180±20mm. The arch top is reserved with grouting holes for later backfilling and grouting.

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