Double-landslide area tunnel portal protection structure and construction method thereof

Abstract

The invention discloses a double-landslide area tunnel portal protection structure and a construction method thereof, and the double-landslide area tunnel portal protection structure comprises a plurality of slide-resistant piles which are arranged on the left side and the right side of a tunnel portal, the slide-resistant piles are partially inserted into bedrock, and the other parts, located above the bedrock, of the slide-resistant piles are connected with the outer edge, close to the tunnel portal, of a landslide; one end of the pre-stressed anchor cable is anchored on the slide-resistant pile, and the other end of the pre-stressed anchor cable penetrates through a landslide and is anchored in bed rock; the crown beam is arranged on the slide-resistant pile, and one end of the pre-stressed anchor cable is close to the joint of the crown beam and the slide-resistant pile; the steel arch truss covers the tunnel portal, and the two ends of the bottom of the steel arch truss are arranged on crown beams on the left side and the right side of the tunnel portal correspondingly; and the protective plate is arranged on the upper surface of the steel arch truss. The outer-layer rigid supporting structure and the inner-layer flexible supporting structure resist landslide sliding force and absorb rock-soil body deformation energy at the same time, the deformation coordination capacity and the system response capacity of the whole structure can be improved, and the method is suitable for reinforcement of the tunnel penetrating through the left-right double-landslide body.

Description

Technical Field

[0001] This invention relates to the field of tunnel construction technology, and in particular to a tunnel portal protection structure and its construction method in a double landslide area. Background Technology

[0002] In mountain tunnel engineering, it is common for the tunnel entrance to traverse landslide areas. Existing projects often reinforce and manage single-sided landslides using measures such as anti-slide piles, prestressed anchor cables, or open-cut tunnels. However, when the tunnel entrance traverses a region with two symmetrically distributed landslides, the loads exerted on the tunnel structure by the landslides exhibit characteristics such as double-sided superposition, symmetrical pushing, and complex structural responses. The aforementioned measures are insufficient to cope with the loads from two-sided landslides, often resulting in uneven stress, stiffness mismatch, and stress concentration. This is particularly true in sections where the two landslides advance each other and their boundaries intersect, where structural instability and failure are likely to occur, posing significant engineering risks.

[0003] Therefore, how to provide a reinforcement measure applicable to tunnels crossing double landslides on both sides is a problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0004] The purpose of this invention is to provide a protective structure for tunnel entrances in double landslide areas and its construction method, so as to solve the problems existing in the prior art.

[0005] To achieve the above objectives, the present invention provides a tunnel entrance protection structure in a double landslide area, wherein there are landslides on both the left and right sides of the tunnel entrance, and the area below the tunnel entrance and the landslides is bedrock, comprising:

[0006] Multiple anti-slide piles are installed on the left and right sides of the tunnel entrance. Part of the anti-slide pile is inserted into the bedrock, and the other part located above the bedrock is connected to the outer edge of the landslide near the tunnel entrance.

[0007] The prestressed anchor cable is anchored at one end to the anti-slide pile, and at the other end passes through the landslide and is anchored in the bedrock.

[0008] Furthermore, it also includes:

[0009] A capping beam is installed on the anti-slide pile, and one end of the prestressed anchor cable is close to the connection between the capping beam and the anti-slide pile;

[0010] A steel arch truss is installed over the tunnel entrance, with its bottom ends set on the capping beams on the left and right sides of the tunnel entrance, respectively.

[0011] A protective plate is installed on the upper surface of the steel arch truss.

[0012] Furthermore, it also includes:

[0013] The inter-pile hanging plate is made of waterproof material and is installed between adjacent anti-slide piles.

[0014] Furthermore, the tunnel entrance includes:

[0015] The tunnel lining is spaced out on the inner side of the steel arch truss, and the outer surface of the tunnel lining is provided with a waterproof layer; a foamed concrete layer is provided between the waterproof layer and the steel arch truss and between the waterproof layer and the anti-slide pile.

[0016] Furthermore, it also includes:

[0017] A crushed stone drainage cushion layer is placed on the upper surface of the landslide and at the outer edge of the landslide near the anti-slide piles;

[0018] The intercepting blind ditch is located on the side of the anti-slide pile closest to the landslide. The intercepting blind ditch is set on the bedrock and is connected to the crushed stone drainage cushion layer.

[0019] Furthermore, the intercepting blind ditch includes:

[0020] Impermeable geotextile is laid on the side of the anti-slide pile closest to the landslide and on the bedrock;

[0021] A permeable geotextile is connected to the impermeable geotextile and corresponds to the crushed stone drainage cushion layer. The impermeable geotextile and the permeable geotextile form a water-cutting chamber, which is filled with sand and gravel filler.

[0022] The permeable pipe has one end connected to the water interception chamber and the other end extending outward.

[0023] Furthermore, it also includes:

[0024] Counter-pressure earthwork is laid on the protective plate and the crushed stone drainage cushion layer.

[0025] Furthermore, the bedrock extends to a position near the top of the landslide and forms an upper rock layer, and a water interception ditch is formed on the upper surface of the upper rock layer, with the water interception ditch located near the top of the landslide.

[0026] This invention also provides a construction method for a tunnel portal protection structure in a double landslide area, comprising the following steps:

[0027] Piling holes are drilled on both sides of the tunnel entrance, with multiple pile holes arranged along the length of the tunnel entrance; steel cages are placed into the pile holes and positioning pads, reinforcing hoops and support frames are installed; concrete is poured to form anti-slide piles; prestressed anchor cable holes are reserved near the top of the anti-slide piles; the formed anti-slide piles are connected to the crushed stone drainage cushion layer; and inter-pile hanging plates made of waterproof material are installed between adjacent anti-slide piles.

[0028] A cap beam is constructed on top of the anti-slide piles;

[0029] When the strength of the anti-slide pile reaches more than 75% of the preset value, drill holes and grout at the prestressed anchor cable holes of the anti-slide pile. After curing, install the prestressed anchor cable and perform graded tensioning. One end of the prestressed anchor cable is anchored to the anti-slide pile, and the other end passes through the landslide and is anchored in the bedrock.

[0030] Install a steel arch truss on top of the cap beam, and install a protective plate on the upper surface of the steel arch truss;

[0031] Construct a water interception ditch on the side of the anti-slide pile closest to the landslide, and cut a trench in the bedrock and place the water interception ditch in the trench;

[0032] An arc-shaped steel formwork is installed below the steel arch truss, and a foamed concrete layer is poured on the lower surface of the steel arch truss; a waterproof layer is installed on the lower surface of the foamed concrete layer, and the open-cut lining of the tunnel entrance is constructed below the waterproof layer.

[0033] When the strength of the tunnel lining reaches more than 90% of the preset strength, a crushed stone drainage cushion layer is arranged on the upper surface of the landslide and the outer edge of the landslide near the tunnel entrance, so that the water interception blind ditch is connected with the crushed stone drainage cushion layer.

[0034] On the protective slab and crushed stone drainage cushion layer, the construction forms a counterweight earthwork, which covers the protective slab and crushed stone drainage cushion layer.

[0035] The present invention discloses the following technical effects:

[0036] 1. This invention involves arranging anti-slide piles on both sides of the tunnel entrance. One end of a prestressed anchor cable is anchored to the anti-slide pile, while the other end passes through the landslide and is anchored in the bedrock. Combined with a cap beam, steel arch truss, and protective plate, this forms an outer rigid support structure. A waterproof layer and a foamed concrete layer are installed on the outer side of the open-cut tunnel lining, forming an inner flexible support structure. By using both the outer rigid support structure and the inner flexible support structure to resist the landslide force while absorbing the deformation energy of the soil and rock, the overall structure's coordinated deformation capacity and system response capability are improved. This invention is suitable for reinforcing tunnels crossing double landslide bodies. It possesses good geological adaptability, structural versatility, and construction feasibility, meeting the high-standard stability requirements of mountain tunnel engineering.

[0037] 2. This invention breaks away from the traditional model of tunnel entrances serving only as passageways, enabling them to actively bear landslide loads, thereby enhancing the structural functionality and optimizing the spatial arrangement of the load-bearing structure.

[0038] 3. Counterpressure measures are taken above the tunnel entrance to effectively resist the combined thrust of the landslides on both sides, significantly improving the overall anti-sliding stability and safety reserve of the tunnel entrance section. Attached Figure Description

[0039] 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.

[0040] Figure 1 This is a front view of the structure of the present invention;

[0041] Figure 2 This is a schematic diagram illustrating the application of the present invention in a double landslide area;

[0042] Figure 3 This is a schematic diagram of the structure of the present invention;

[0043] Figure 4 This is a schematic diagram of a steel arch truss.

[0044] Figure 5 This is a schematic diagram of the protective plate;

[0045] Figure 6 This is a schematic diagram of a drainage ditch.

[0046] Figure 7 This is a schematic diagram of prestressed anchor cable installation;

[0047] Among them, 1. landslide; 2. bedrock; 3. anti-slide piles; 4. prestressed anchor cables; 5. capping beam; 6. steel arch truss; 7. protective plate; 8. pile-to-pile hanging plate; 9. open-cut tunnel lining; 10. waterproof layer; 11. foamed concrete layer; 12. crushed stone drainage cushion layer; 13. intercepting blind ditch; 1301. impermeable geotextile; 1302. permeable geotextile; 1303. sand and gravel filler; 1304. permeable pipe; 14. counterweight earthwork; 15. intercepting ditch; 16. anchor plate; 17. anchorage. Detailed Implementation

[0048] 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.

[0049] 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.

[0050] like Figures 1-7As shown, this embodiment of the invention provides a tunnel entrance protection structure in a double landslide area. There are landslides 1 on both the left and right sides of the tunnel entrance, and bedrock 2 lies beneath the tunnel entrance and the landslides 1. The structure includes:

[0051] Multiple anti-slide piles 3 are set on the left and right sides of the tunnel entrance. Part of the anti-slide pile 3 is inserted into the bedrock 2, and the other part located above the bedrock 2 is connected to the outer edge of the landslide 1 near the tunnel entrance. The anti-slide piles 3 can be in the form of manually excavated rectangular piles, mechanically drilled circular piles, micro pile groups, etc. The pile body can be friction piles or end-bearing piles.

[0052] The prestressed anchor cable 4 is anchored at one end to the anti-slide pile 3, and the other end passes through the landslide 1 and is anchored in the bedrock 2.

[0053] In this embodiment, it also includes:

[0054] The capping beam 5 is set on the anti-slide pile 3. One end of the prestressed anchor cable 4 is close to the connection between the capping beam 5 and the anti-slide pile 3, and is fixed to the anti-slide pile 3 by the anchor plate 16 and the anchor 17; Figure 2 As shown, the prestressed anchor cable 4 can simultaneously penetrate the cap beam 5 and the anti-slide pile 3, or it can be as follows: Figure 7 As shown, only the anti-slide pile 3 is penetrated; the capping beam 5 is set as a rectangle, and its thickness should not be less than 0.8m, and it is made of C30 concrete;

[0055] A steel arch truss 6 is installed over the tunnel entrance, with its bottom ends installed on the capping beams 5 on the left and right sides of the tunnel entrance, respectively.

[0056] The protective plate 7 is installed on the upper surface of the steel arch truss 6. The prestress of the prestressed anchor cable 4 cuts off the downward force of the landslide 1 and prevents the slope from loosening further under engineering disturbance.

[0057] In this embodiment, it also includes:

[0058] The pile-to-pile hanging plate 8 is made of waterproof material and is installed between adjacent anti-slide piles 3. The pile-to-pile hanging plate 8 is integrally cast with concrete and has the functions of retaining soil and blocking water.

[0059] In this embodiment, the tunnel entrance includes:

[0060] The tunnel lining 9 is spaced out on the inner side of the steel arch truss 6, and a waterproof layer 10 is provided on the outer surface of the tunnel lining 9. A foamed concrete layer 11 is provided between the waterproof layer 10 and the steel arch truss 6, and between the waterproof layer 10 and the anti-slide piles 3. The foamed concrete layer 11 is formed by filling with foamed concrete, which has the characteristics of being lightweight, energy-absorbing, high-strength, and having good self-compacting properties. It can adjust the stiffness difference between the anti-slide piles 3 and the tunnel lining 9. At the same time, the foamed concrete can serve as the inner flexible support structure of the protection system, and has a good absorption and dispersion effect on external loads. The waterproof layer 10 and the foamed concrete layer 11 can prevent groundwater from seeping into the tunnel clearance.

[0061] In this embodiment, it also includes:

[0062] A crushed stone drainage cushion layer 12 is provided on the upper surface of landslide 1 and on the outer edge of landslide 1 near the anti-slide pile 3;

[0063] The water interception blind ditch 13 is located on the side of the anti-slide pile 3 near the landslide 1. The water interception blind ditch 13 is set on the bedrock 2 and is connected to the crushed stone drainage cushion layer 12.

[0064] In this embodiment, the intercepting blind drain 13 includes:

[0065] The impermeable geotextile 1301 is laid on the side of the anti-slide pile 3 near the landslide 1 and on the bedrock 2;

[0066] The permeable geotextile 1302 is connected to the impermeable geotextile 1301 and corresponds to the crushed stone drainage cushion layer 12. The impermeable geotextile 1301 and the permeable geotextile 1302 form a water interception chamber, which is filled with sand and gravel filler 1303.

[0067] The permeable pipe 1304 has one end connected to the water interception chamber and the other end extending outward.

[0068] In this embodiment, it also includes:

[0069] The counterweight earthwork 14, which is placed on the protective plate 7 and the crushed stone drainage cushion 12, can effectively increase the anti-sliding force of the landslide 1 and improve its stability. The crushed stone drainage cushion 12 and the intercepting blind ditch 13 can drain the groundwater in the counterweight earthwork 14.

[0070] In this embodiment, the bedrock 2 extends to a position near the top of the landslide 1 and forms an upper rock layer. A water interception ditch 15 is provided on the upper surface of the upper rock layer. The water interception ditch 15 is set near the top of the landslide 1, which can effectively intercept and drain water after the slope and prevent it from entering the slope and causing diseases such as erosion, scouring, and undercutting.

[0071] This invention also provides a construction method for a tunnel portal protection structure in a double landslide area, comprising the following steps:

[0072] Step 1: Pre-construction preparation

[0073] 1) Geological exploration: Using drilling, geophysical exploration and other methods, the extent of landslide 1, the depth of the sliding surface and the groundwater situation were clarified.

[0074] 2) Slope stability assessment: Calculate the thrust of landslide 1 and determine the number, spacing, and cross-sectional dimensions of the anti-slide piles 3. At the same time, mark the excavation locations of the anti-slide piles 3 and determine the lengths of the free section and anchorage section of the prestressed anchor cables 4.

[0075] 3) Slope Drainage System: Before construction, the location of the intercepting blind ditch 13 should be determined based on the terrain and design elevation, and its direction and slope (generally controlled at 2%–5%) should be established. Mechanical excavation and manual trimming of the ditch bottom and slopes are employed. The ditch bottom width is generally 0.4–0.6m, and the depth is 0.5–0.8m. After the sidewall formwork is erected, C20 concrete is poured to form the ditch structure, using segmented pouring with expansion joints. After completion, timely curing is carried out, and drainage outlets and scour channels are installed to ensure smooth water flow away from landslide area 1.

[0076] Step 2: Leveling the construction site

[0077] Based on the design drawings, the layout area for anti-slide pile 3 was marked out to clarify the pile positions and work boundaries. Subsequently, mechanical excavation, with partial manual assistance, was used to remove loose surface soil and obstacles, and to remove unstable accumulation layers at the slope toe. During site leveling, construction ramps and equipment transport channels were rationally designed according to the terrain to ensure stable operation of construction equipment. The final result was a work platform with sufficient bearing capacity, unobstructed drainage, and meeting safe construction requirements, creating stable conditions for the drilling of anti-slide pile 3 and subsequent construction.

[0078] Step 3: Construction of prestressed anchor cables 4, anti-slide piles 3, and capping beam 5

[0079] 1) Pile Hole Construction: Anti-slide pile 3 uses reinforced concrete bored piles with a diameter of 1.2m, a length of 25-35m, and a pile spacing of 4.5m, with the pile top elevation 2.27m above the rail surface. Before pile hole construction, the pile positions are accurately laid out according to the design drawings, and the site is leveled and the casing is installed. Drilling is carried out using reverse circulation drilling or rotary drilling equipment, and the hole depth, verticality, and diameter are monitored in real time. If complex geological conditions such as loose strata or unstable hole walls are encountered, cast-in-place C20 concrete retaining walls (15-20cm thick, reinforced with φ8-10mm structural steel mesh) or temporary steel casings should be used to reinforce the hole opening, and drilling operations should be carried out according to the cyclic excavation process. The excavation advance should be determined according to the stratum characteristics. When the soil layer is basically stable, it can be 0.5m; when the soil layer is in a fluid plastic state, it can be 0.2-0.3m. After each section of advance is completed, the sediment at the bottom of the hole should be removed in time to ensure that the hole quality meets the requirements of subsequent pile construction. During construction, it is necessary to combine the drainage system of landslide area 1 with timely drainage to avoid secondary disasters caused by hole collapse and surface water backflow.

[0080] 2) Pile Construction: After the pile hole is formed and passes inspection, prestressed anchor cable 4 holes are first reserved at a predetermined height in the upper part of the pile body (generally 0.5-1m below the pile top) according to design requirements. The hole arrangement should be consistent with the design angle of the prestressed anchor cable 4, usually 20°-30°. Foam boards or steel pipes are used for pre-embedding and positioning before the installation of the reinforcing cage to ensure smooth construction of the prestressed anchor cable 4 later. Subsequently, the custom-made reinforcing cage is hoisted and installed in sections. The length of the reinforcing cage is usually more than 95% of the pile length. Positioning pads, reinforcing hoops, and support frames are set to ensure that the reinforcing cage is centered and stable in the hole. Then, C30 concrete is poured continuously in sections, and an immersion vibrator is used for timely vibration to prevent quality defects such as pile breakage, honeycombing, or slag inclusion.

[0081] 3) Construction of the capping beam 5: A 16m × 0.8m × 1.5m capping beam 5 will be constructed along the longitudinal direction of the tunnel at the top of the anti-slide piles 3 to enhance the overall stability of the anti-slide piles 3 structure. Before construction, the pile tops should be roughened to remove laitance and loose layers, and the connecting bars should be tied to the main bars of the capping beam 5 to ensure that the anchorage length of the bars meets the design requirements. After the bars are tied, formwork should be erected. The formwork must be tight and firm to prevent grout leakage and deformation. Subsequently, C30 concrete will be poured, and layered vibration will be used to ensure that the concrete is dense and free of honeycomb pits, providing a solid foundation for the subsequent tensioning of prestressed anchor cables 4 and the connection of the steel arch truss 6.

[0082] 4) Installation of Prestressed Anchor Cable 4: After the concrete strength of the anti-slide pile 3 reaches more than 75% of the design value, the installation of the prestressed anchor cable 4 will begin. First, a down-the-hole drill is used to drill at the reserved prestressed anchor cable 4 hole location at the design angle (20°~30°), with a hole diameter ≥130mm and a hole depth exceeding the design anchoring section by 0.5m to ensure complete penetration into the stable bedrock 2. After drilling, high-pressure air is used for hole cleaning to ensure that the thickness of sediment in the hole is ≤5cm. Then, grouting is performed using the bottom-of-hole grouting method, with the grouting pressure controlled at 0.5~1.0MPa, and the cement grout must meet a compressive strength of more than 30MPa. After grouting is completed and cured for a certain period of time, the prestressed anchor cable 4 is installed and staged cyclic tensioning is performed. The prestressed anchor cable 4 is observed to recoil at each stage of tension for a certain period of time. Finally, after tensioning to the design prestress value, the anchor 17 is locked.

[0083] Step 4: Installation of steel arch truss 6-guard plate 7 structure

[0084] 1) Preliminary preparation and arch seat construction: The arching line of the steel arch truss 6 is accurately laid out on the top of the cap beam 5. According to the design drawings, the tops of the anti-slip piles 3 on both sides are measured and laid out to ensure that the flatness error of the arch seat connection surface is ≤3mm. Then, the pile tops are roughened, the laitance is removed, and anchor steel plates are pre-embedded as the anchor foundation of the steel arch truss 6 base.

[0085] 2) Installation of Steel Arch Truss 6: The steel arch truss 6 is prefabricated in sections in the factory. Each section consists of four I20a steel sections, spaced 60cm apart, and connected by welding. After the finished product is transported to the site, it is hoisted and spliced ​​in sections, lifted one section at a time, and fixed in the design position with temporary supports. The base of the steel arch truss 6 is connected to the pre-embedded anchor steel plate of the cap beam 5 using high-strength bolts (torque controlled at 300 N·m). Adjacent sections of the steel arch truss 6 are welded together. During the installation of the steel arch truss 6, measuring instruments are used to control the arch axis, mid-span elevation, and segment splicing accuracy. After the steel arch truss 6 segments are connected, weld flaw detection and anti-corrosion treatment are performed. Finally, the temporary supports are removed, and the stiffness and stability of the entire steel arch frame are verified to ensure reliable stress on the components and provide upper constraints for subsequent structural linkage and the stability of the open tunnel.

[0086] 3) Installation of Guard Plate 7: Before construction, accurately lay out the curvature of the steel arch truss 6 and customize pre-bent steel guard plates 7 with a thickness of 5mm according to the drawings. Hoist the guard plate 7 to the outside of the steel arch truss 6 and temporarily fix it with clamps to ensure tight fit with the steel arch truss 6. Then, perform intermittent or full welding according to the preset weld positions. The welds should be continuous and uniform. After welding, perform visual inspection and weld flaw detection. After welding, grind the weld joints and uniformly apply an anti-corrosion coating to ensure that the guard plate 7 is firmly connected to the steel arch truss 6, has good anti-corrosion performance, and forms an integral load-bearing arch shell.

[0087] Step 5: Construction of Drainage Ditch 13

[0088] According to the design requirements, the trenches are laid out and positioned. The trench width is generally 0.4–0.6m, the depth is 0.6–0.8m, and the longitudinal slope is controlled at 2%–4%. Blind trenches are excavated mechanically or manually, followed by the installation of permeable geotextile 1302 and impermeable geotextile 1301, and then filled with 20–40mm diameter sand and gravel filler 1303, ensuring the drainage outlets are positioned along the slope. After the permeable pipes 1304 are laid, backfilling with sand and gravel continues to the design elevation, and finally, the soil is covered and compacted to form a closed permeable drainage system.

[0089] Step Six: Construction of Foamed Concrete Layer 11

[0090] Before construction, clean the gaps in the steel arch truss 6 to ensure the contact surfaces are clean and free of dust. Use curved steel formwork to construct a closed formwork system within the tunnel clearance to ensure good adhesion between the poured surface and the subsequent waterproofing layer 10. Based on the designed thickness of 50cm, pour foamed concrete in layers or all at once. Prepare the concrete on-site or transport it to the construction location by truck before pouring, and then deliver it to the filling area via hoses. The foamed concrete should have a dry density of 300–600 kg / m³. 3The compressive strength after 28 days should be ≥0.6MPa. During the pouring process, a plate vibrator should be used to gently vibrate the surface to ensure compaction and uniformity. After the filling layer is formed, its thickness and flatness should be checked to provide buffering and support for the stress on the open-cut structure.

[0091] Step 7: Install the waterproof layer 10

[0092] Before laying the waterproof layer 10 on the surface of the foamed concrete layer 11, it must be cleaned and smoothed to ensure there are no sharp protrusions or debris. A composite waterproof layer 10 with a thickness of 1.5–2.0 mm (such as EVA or HDPE) should be selected, cut into sections according to the design width, and laid out sequentially from bottom to top. The waterproof layer 10 is anchored to the foamed concrete surface using special expansion bolts or strips, with an overlap width of not less than 10 cm. Hot-melt welding or adhesive sealing is used to ensure the joints are tight and leak-proof.

[0093] Step 8: Construction of the tunnel lining 9

[0094] Before constructing the secondary lining (9th lining of the tunnel), the quality of the waterproof layer (10) should be inspected to ensure there is no damage or hollow areas. Based on the cross-sectional characteristics of a straight wall and a circular arch, modular steel formwork should be assembled and installed in sections. The bottom of the formwork should be laid along the straight wall, and the upper circular arch section should precisely align with the outline of the waterproof layer (10) to ensure a smooth overall shape. After the formwork is installed, the lining reinforcement should be tied, and a support and limiting system should be installed to prevent deformation. C30 concrete should be poured in sections from the arch foot to the arch crown, using an immersion vibrator to ensure the concrete is compacted.

[0095] Step Nine: Construction of Counterweight Earthwork 14

[0096] After the concrete strength of the tunnel lining reaches 90% of its design strength, the backfilling and counter-pressure construction of the tunnel will commence. The backfilling will employ a layered method, consisting of a drainage layer and a counter-pressure layer from bottom to top.

[0097] Drainage layer: First, lay a 50cm thick gravel drainage layer 12 (particle size 2-4cm) and connect it with the intercepting blind ditch 13.

[0098] Counterweight layer: The excavated waste material from the pile holes and open-cut tunnel lining 9 is compacted in layers to the design elevation to form counterweight soil 14. Each layer is ≤30cm thick and has a compaction degree ≥93%. A 6% herringbone drainage slope is set on the top, covered with 50cm of planting soil and planted with grass for protection.

[0099] 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.

[0100] 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 tunnel entrance protection structure in a double landslide area, wherein there are landslides (1) on both the left and right sides of the tunnel entrance, and bedrock (2) lies below the tunnel entrance and the landslides (1), characterized in that, include: Multiple anti-slide piles (3) are set on the left and right sides of the tunnel entrance. Part of the anti-slide piles (3) are inserted into the bedrock (2), and the other part located above the bedrock (2) is connected to the outer edge of the landslide (1) near the tunnel entrance. The prestressed anchor cable (4) is anchored at one end to the anti-slide pile (3) and at the other end passes through the landslide (1) and is anchored in the bedrock (2).

2. The tunnel entrance protection structure in a double landslide area according to claim 1, characterized in that, Also includes: A capping beam (5) is set on the anti-slide pile (3), and one end of the prestressed anchor cable (4) is close to the connection between the capping beam (5) and the anti-slide pile (3); A steel arch truss (6) is installed over the tunnel entrance, with its bottom ends installed on the cap beams (5) on the left and right sides of the tunnel entrance respectively. A protective plate (7) is provided on the upper surface of the steel arch truss (6).

3. The tunnel entrance protection structure in a double landslide area according to claim 2, characterized in that, Also includes: The inter-pile hanging plate (8) is made of waterproof material and is installed between adjacent anti-slide piles (3).

4. The tunnel entrance protection structure in a double landslide area according to claim 3, characterized in that, The tunnel entrance includes: The tunnel lining (9) is spaced out on the inner side of the steel arch truss (6), and the outer surface of the tunnel lining (9) is provided with a waterproof layer (10); a foamed concrete layer (11) is provided between the waterproof layer (10) and the steel arch truss (6) and between the waterproof layer (10) and the anti-slide pile (3).

5. The tunnel entrance protection structure in a double landslide area according to claim 3, characterized in that, Also includes: A crushed stone drainage cushion layer (12) is provided on the upper surface of the landslide (1) and at the outer edge of the landslide (1) near the anti-slide pile (3); The intercepting blind ditch (13) is located on the side of the anti-slide pile (3) near the landslide (1). The intercepting blind ditch (13) is set on the bedrock (2) and is connected to the crushed stone drainage cushion layer (12).

6. The tunnel entrance protection structure in a double landslide area according to claim 5, characterized in that, The intercepting blind drain (13) includes: Impermeable geotextile (1301) is laid on the side of the anti-slide pile (3) near the landslide (1) and on the bedrock (2); A permeable geotextile (1302) is connected to the impermeable geotextile (1301) and corresponds to the crushed stone drainage cushion layer (12). The impermeable geotextile (1301) and the permeable geotextile (1302) form a water-cutting chamber, which is filled with sand and gravel filler (1303). The permeable pipe (1304) is connected at one end to the water interception chamber and extends outward at the other end.

7. A tunnel entrance protection structure in a double landslide area according to claim 5, characterized in that, Also includes: Counter-pressure earthwork (14) is laid on the protective plate (7) and the crushed stone drainage cushion layer (12).

8. The tunnel entrance protection structure in a double landslide area according to claim 7, characterized in that, The bedrock (2) extends to a position near the top of the landslide (1) and forms an upper rock layer. A water interception ditch (15) is provided on the upper surface of the upper rock layer. The water interception ditch (15) is located near the top of the landslide (1).

9. A construction method for a tunnel portal protection structure in a double landslide area, characterized in that, Includes the following steps: Piling holes are drilled on the left and right sides of the tunnel entrance, and multiple pile holes are arranged along the length of the tunnel entrance; steel cages are placed in the pile holes and positioning pads, reinforcing hoops and support frames are set up, and concrete is poured to form anti-slide piles (3). Prestressed anchor cable (4) holes are reserved near the top of the anti-slide pile (3). The formed anti-slide pile (3) is connected to the crushed stone drainage cushion layer (12), and a pile-to-pile hanging plate (8) made of waterproof material is installed between adjacent anti-slide piles (3). A cap beam (5) is constructed on top of the anti-slide pile (3); When the strength of the anti-slide pile (3) reaches more than 75% of the preset value, drill holes and grout in the prestressed anchor cable (4) hole of the anti-slide pile (3), install the prestressed anchor cable (4) after curing and perform graded tensioning. One end of the prestressed anchor cable (4) is anchored on the anti-slide pile (3), and the other end passes through the landslide (1) and is anchored in the bedrock (2). A steel arch truss (6) is installed on the top of the cap beam (5), and a protective plate (7) is installed on the upper surface of the steel arch truss (6); Construct a water interception ditch (13) on the side of the anti-slide pile (3) close to the landslide (1), and cut a trench in the bedrock (2) and arrange the water interception ditch (13) in the trench; An arc-shaped steel template is installed below the steel arch truss (6), and a foamed concrete layer (11) is poured on the lower surface of the steel arch truss (6); a waterproof layer (10) is installed on the lower surface of the foamed concrete layer (11), and the open-cut lining (9) of the tunnel entrance is constructed below the waterproof layer (10). When the strength of the tunnel lining (9) reaches more than 90% of the preset strength, a gravel drainage cushion layer (12) is arranged on the upper surface of the landslide (1) and the outer edge of the landslide (1) near the tunnel entrance, so that the water interception blind ditch (13) is connected with the gravel drainage cushion layer (12). On the protective plate (7) and the crushed stone drainage cushion (12), a counterweight earthwork (14) is constructed, which covers the protective plate (7) and the crushed stone drainage cushion (12).

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