Construction method for karst tunnel to extend through karst cave

Abstract

Provided in the present invention is a construction method for a karst tunnel to extend through a karst cave. The construction method comprises the following steps: (1) excavation expansion and support for a karst cave; (2) dredging of the base of the karst cave, and backfilling; (3) inverted-arch and filling construction; (4) water-stop wall construction, and concrete backfilling on two sides; (5) initial support construction; (6) initial-support top backfilling and waterproof-layer construction; (7) secondary-lining waterproofing and drainage construction; (8) mounting and debugging of a water pressure monitoring system; and (9) implementation of a monitoring and early-warning scheme. The construction method is suitable for engineering in unfavorable geological conditions, for example, a water-rich karst tunnel extending through large underground karst caves, karst troughs and underground rivers. In the construction method, the operation difficulty of the steps is relatively low, the construction quality is controllable, there are few construction blind areas, and the safety of construction is relatively high; the initial runoff balance of karst cave underground water can be maintained, and thus the surrounding ecological environment can be protected to the maximum extent.

Description

A construction method for a karst tunnel passing through a karst cave Technical Field

[0001] The present invention relates to the technical field of road, bridge and tunnel engineering construction, and in particular to a construction method for a karst tunnel passing through a karst cave. Background Art

[0002] Karst tunnels are underground caves or river tunnels, typically formed through long periods of rock erosion and dissolution. These formations are caused by surface and groundwater eroding the rock surface vertically. As erosion deepens, a vast system of cavities forms within the cracks and holes in the rock formations. These cavities expand over time, forming caves and cavities of varying sizes and shapes.

[0003] As the coverage of transportation networks continues to expand into areas dotted with karst landforms, the process will inevitably involve numerous water-rich karst tunnels traversing large underground caves, karst troughs, and subterranean rivers, among other unfavorable geological conditions. Grouting and sealing techniques are often employed to address these situations. However, the traditional grouting and sealing techniques currently used have disadvantages during on-site construction: The construction period is long and challenging, and grouting and sealing require waiting for the grout to fully solidify before the next task can proceed, which prolongs the construction process. Grouting also requires specialized skills and experience, is difficult to operate, and carries certain construction difficulties and risks. Furthermore, the concealed nature of the construction makes it difficult to control construction quality, while also disrupting the groundwater runoff balance in the cave and the surrounding ecological environment.

[0004] Summary of the Invention

[0005] The main technical problem to be solved by the present invention is to provide a construction method for a karst tunnel passing through a karst cave. The construction method has low operating difficulty, controllable construction quality, reduces construction blind spots, and improves the safety of the construction process.

[0006] In order to solve the above technical problems, the present invention provides a construction method for a karst tunnel passing through a cave, comprising the following steps:

[0007] (1) Cave expansion and support: Excavation of the cave and support of the cave top;

[0008] (2) Desilting and backfilling of cave base: excavation at the cave base and backfilling with concrete;

[0009] (3) Inverted arch and filling construction: carry out inverted arch secondary lining construction and inverted arch filling construction;

[0010] (4) Construction of waterstop wall and concrete backfill on both sides: a waterstop wall is constructed in the tunnel and concrete backfill is carried out on both sides of the waterstop wall;

[0011] (5) Initial support construction: Carry out initial tunnel support construction according to the design drawings;

[0012] (6) Construction of backfill and waterproof layer at the top of primary support: construction of backfill at the top of primary support, construction of drainage blind pipe, construction of earthwork cloth, construction of PVC waterproof board and construction of water stop;

[0013] (7) Secondary lining waterproofing and drainage construction: carry out water pressure resistant secondary lining construction and inspection channel construction;

[0014] (8) Installation and commissioning of water pressure monitoring system: Before installing the water pressure monitoring system, ensure that the on-site equipment meets the design pressure requirements;

[0015] (9) Implementation of monitoring and early warning scheme: When the external water pressure of the tunnel lining measured by the water pressure gauge exceeds the limit, the electronically controlled pressure control valve is opened through the pressure control system to reduce the pressure to below the limit.

[0016] In a preferred embodiment, the expansion operation of the cave in step (1) includes:

[0017] Before expanding the cave, clear away the dangerous rocks within the operating range at the top of the cave and the sediment within the cave cavity;

[0018] Drilling and blasting are used to expand the cave top, and machinery is used for local artificial rock drilling; the expanded cave is measured;

[0019] In a preferred embodiment, the support operation for the cave top in step (1) includes:

[0020] After the cave expansion operation is completed, support should be carried out in a timely manner according to the integrity of the surrounding rock;

[0021] When the surrounding rock is intact and there is no falling block, mortar anchor rods, steel mesh and shotcrete are used to support it; when the surrounding rock has falling blocks, anchor rods, steel mesh, steel support and shotcrete support are used to support it.

[0022] During the steel support construction, cross beams and locking anchor rods are used to temporarily fix it on the solid surrounding rock. When the water stop walls on both sides are constructed to the designed elevation, the whole structure is poured inside.

[0023] In a preferred embodiment, the dredging and backfilling of the cave base in step (2) includes:

[0024] Use an excavator to excavate and remove silt layer by layer to the base. After the silt is removed, conduct a foundation bearing capacity test. After it meets the requirements, use high-pressure water to flush the rock wall. After the flushing is completed, use a water pump to drain the accumulated water at the base.

[0025] After the cave base is excavated, concrete is backfilled layer by layer to the designed elevation. The backfilling is carried out in two steps, and each layer is vibrated and compacted.

[0026] Before the second concrete pouring, the contact surface should be roughened and cleaned, and the roughened concrete surface should be rinsed with clean water.

[0027] In a preferred embodiment, the inverted arch and filling construction in step (3) includes:

[0028] Before pouring the secondary lining of the inverted arch, geotextile and fully covered waterproof board shall be laid, and the overlap portion with the waterproof layer of the arch wall shall be reserved;

[0029] Before the construction of the inverted arch reinforcement, reserve the circumferential lap reinforcement for welding with the secondary lining reinforcement of the arch wall, and stagger the lap joints of adjacent reinforcements;

[0030] Before pouring the inverted arch concrete, a double-layer embedded waterstop is installed and fixed between the two layers of steel bars with steel bar clamps. The inverted arch and the bottom are constructed with a fully hydraulic self-propelled inverted arch trestle to facilitate concrete construction.

[0031] When pouring the second lining of the inverted arch, the entire section is poured together, and the filling concrete and the inverted arch concrete are poured separately;

[0032] The inverted arch filling construction shall be carried out after the pouring of the inverted arch secondary lining is completed, and the surface water and debris of the inverted arch secondary lining shall be cleaned before construction. The transverse construction joints of the inverted arch filling concrete and the construction joints of the inverted arch secondary lining shall be staggered.

[0033] In a preferred embodiment, the water stop wall construction in step (4) includes:

[0034] The wall and foundation of the waterstop wall are made of reinforced concrete, and pre-buried I-beams or channel steels are used between the foundation and the base;

[0035] Carry out a base bearing capacity test before construction, and pre-bury drainage pipes during construction;

[0036] Before pouring the foundation, measurement and layout are carried out to determine the position of the formwork. After the formwork is erected, reinforcement is tied. The foundation reinforcement reserves a portion for overlapping with the wall reinforcement. The foundation is poured in one go, and adjacent joints are staggered during wall construction.

[0037] The formwork uses customized steel formwork, which is fixed with tension bolts and has diagonal braces added on the outside. The formwork is polished and coated with release agent before erecting the formwork. The wall is poured in sections, layers, and vibrated layer by layer. After each section is poured, the back of the wall is backfilled with concrete in a timely manner. At the same time, the concrete backfill surface is used as the construction platform for the next retaining wall, and the pouring construction is carried out section by section until the design elevation.

[0038] In a preferred embodiment, the concrete backfilling construction on both sides of the water stop wall in step (4) includes:

[0039] Concrete is used to backfill both sides of the waterstop wall. The backfill construction is carried out in parallel with the pouring of the waterstop wall until the design elevation is reached.

[0040] The construction of the last layer of concrete backfill is controlled according to the designed slope of 1:2, with the high side flush with the top of the water stop wall and the low side following the terrain inside the cave.

[0041] In a preferred embodiment, the backfill construction at the top of the primary support in step (6) includes:

[0042] After the initial support is completed, pumped concrete is used for arch protection construction; the top surface is covered with a double-layer steel mesh. After pumping is completed, it is fully wrapped with waterproof board and a layer of emulsified asphalt waterproof coating is applied as a waterproof layer.

[0043] In a preferred embodiment, the construction of the drainage blind pipe in step (6) includes:

[0044] Before installing the annular drainage pipe, first measure and mark the specific position of the annular drainage pipe on the initial support structure surface, drill holes to locate it, drive expansion anchors into the positioning holes or use anchoring agents to embed the steel bar heads in the positioning holes, use steel bar clips to drive into the positioning holes to fix the annular blind pipe, wrap the blind pipe with non-woven fabric, and tie it with wire; clamp the blind pipe with clips and then fix it to the expansion bolts; the longitudinal drainage pipe is constructed according to the design drawings with reference to the annular drainage blind pipe.

[0045] In a preferred embodiment, the earthwork cloth construction in step (6) includes:

[0046] Use a laser profiler or laser 3D scanner to detect the inner contour of the initial support of the tunnel. When the flatness of the initial support surface meets the D / L ratio of no more than 1 / 6, geotextile laying is carried out. D is the depth of the concave between two adjacent convex surfaces of the initial support surface, and L is the distance between two adjacent convex surfaces of the initial support surface. During the laying process, nails are used to heat hot-melt pads for fixation, and appropriate fixing points are added in the concave areas.

[0047] In a preferred embodiment, the construction of the PVC waterproof board in step (6) includes: before the waterproof board is constructed, checking the smoothness of the concrete surface and then constructing it after it meets the requirements; using an automatic hot melt climbing welding machine for construction, and using a handheld welding gun for detailed processing or repair; laying the waterproof layer in a circumferential direction, without longitudinal overlap between the arch and the side wall.

[0048] In a preferred embodiment, the waterstop construction in step (6) includes: construction joints are constructed using embedded waterstops, and expansion joints are constructed using embedded steel-edged rubber waterstops in combination with back-attached rubber waterstops, and the back-attached waterstops are fully enclosed.

[0049] In a preferred embodiment, the water pressure resistant secondary lining construction in step (7) includes: secondary lining steel mesh construction, secondary lining concrete construction and water blocking wall construction;

[0050] The secondary lining steel mesh construction includes: mechanical connection of the secondary lining stress-bearing steel bars; wrapping the steel bar ends with customized plastic shells or geotextiles; installing protective layer pads for the steel bars; and reserving the steel bar extension length at the invert arch where the invert arch connects to the side wall;

[0051] The secondary lining concrete construction includes: the secondary lining concrete construction uses a fully hydraulic automatic walking integral lining trolley, adopts a chute to symmetrically pour concrete, and pours concrete symmetrically in layers from the side walls on both sides to the vault from bottom to top; the concrete is poured continuously with manual auxiliary vibration, the vault concrete is poured densely in one go, and grouting holes are reserved in the vault. After the secondary lining concrete pouring is completed, the vault grouting is carried out;

[0052] The water blocking wall construction includes: constructing a longitudinal secondary lining water blocking wall at the contact position between the pressure-bearing side and the non-pressure-bearing side of the tunnel, using a waterproof layer, waterproof concrete, waterproof layer and waterproof concrete to block the end water; the water blocking wall is excavated in advance, and the waterproof layer and secondary lining concrete are integrally poured with the secondary lining of the main tunnel;

[0053] The construction of the inspection channel includes: after the second lining construction of the inspection channel is completed, the road surface construction in the well is carried out, and the road surface construction is in a stepped shape; a concrete retaining wall is set at the intersection of the inspection channel and the cave, and a pressure-resistant manhole cover and valve are embedded in the wall. During the dry season, the manhole cover can be opened through the valve to enter the cave.

[0054] In a preferred embodiment, the water pressure monitoring system in step (8) includes: a water pressure gauge, a concrete strain gauge, a concrete surface strain gauge, a strain transmitter, an electric pressure regulating valve, a valve control system, and a seamless steel pipe; the strain gauge is pre-buried and installed synchronously during the concrete pouring process, and the remaining control system is installed after the tunnel lining is fully constructed and the cavity treatment construction is completed.

[0055] In a preferred embodiment, during the implementation of the monitoring and early warning scheme in step (9):

[0056] When the water pressure outside the tunnel lining monitored by the water pressure gauge exceeds the limit, but the rising rate does not exceed the limit, the small-diameter seamless steel pipe will be automatically opened to divert the groundwater outside the pressure-bearing section lining to the non-pressure section for discharge;

[0057] When the water pressure outside the tunnel lining monitored by the water pressure gauge exceeds the limit and the rising rate also exceeds the limit, the large-diameter seamless steel pipe will be automatically opened, and the groundwater outside the pressure-bearing section lining will be drained to the drainage ditch inside the tunnel for discharge through the large-diameter seamless steel pipe.

[0058] Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0059] 1. The present invention provides a construction method for a karst tunnel passing through a cave. Compared with traditional grouting and plugging, the related processes such as cave top excavation and support, bottom sediment cleaning and concrete backfilling, waterstop wall construction and concrete backfilling behind the wall, and pumped concrete arch construction are all non-concealed projects, with relatively low operational difficulty, controllable construction quality, fewer construction blind spots, and higher construction safety.

[0060] 2. The present invention provides a construction method for a karst tunnel passing through a karst cave. By constructing water-stop walls and concrete backfill behind the walls on both sides of the tunnel, and coordinating the tunnel vault excavation, support, and arch protection to restore the original runoff channel, the initial runoff balance of the karst cave groundwater is maintained, the water flow channel near the karst cave is not blocked, and the surrounding ecological environment is protected to the greatest extent.

[0061] 3. The present invention provides a construction method for a karst tunnel passing through a karst cave. An inspection passage leading to the karst cave from a pedestrian cross tunnel is constructed in the non-pressure-bearing section of the tunnel. The end of the passage is closed with a water pressure-resistant manhole cover and a valve. During the dry season, the water stored on the cave roof is discharged into the ditch through a pressure-limiting discharge pipe. Inspectors can open the pressure-resistant manhole cover and enter the cave to check the stability of the cave and whether it is silted up. If necessary, manual cleaning can be carried out to ensure the safety of tunnel operation. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] FIG1 is a schematic flow chart of the construction method in the preferred embodiment 1 of the present invention. DETAILED DESCRIPTION

[0063] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention; it is obvious that the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0064] In the description of the present invention, it should be noted that the terms "upper," "lower," "inner," "outer," "top / bottom," and the like, indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings and are intended solely to facilitate and simplify the description of the present invention. They are not intended to indicate or imply that the devices or components referred to must have, be constructed, or operate in a specific orientation, and therefore should not be construed as limitations on the present invention. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0065] In the description of the present invention, it should be noted that, unless otherwise clearly stipulated and limited, the terms "installed", "provided with", "set / connected", "connected", etc. should be understood in a broad sense. For example, "connection" can be a wall-mounted connection, a detachable connection, or an integral connection. It can be a mechanical connection or an electrical connection. It can be a direct connection or an indirect connection through an intermediate medium. It can be the internal connection of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to the specific circumstances.

[0066] Example 1

[0067] 1 , this embodiment provides a method for constructing a karst tunnel passing through a cave, comprising the following steps:

[0068] (1) Cave expansion and support: Excavation of the cave and support of the cave top;

[0069] (2) Desilting and backfilling of cave base: excavation at the cave base and backfilling with concrete;

[0070] (3) Inverted arch and filling construction: carry out inverted arch secondary lining construction and inverted arch filling construction;

[0071] (4) Construction of waterstop wall and concrete backfill on both sides: a waterstop wall is constructed in the tunnel and concrete backfill is carried out on both sides of the waterstop wall;

[0072] (5) Initial support construction: Carry out initial tunnel support construction according to the design drawings;

[0073] (6) Construction of backfill and waterproof layer at the top of primary support: construction of backfill at the top of primary support, construction of drainage blind pipe, construction of earthwork cloth, construction of PVC waterproof board and construction of water stop;

[0074] (7) Secondary lining waterproofing and drainage construction: carry out water pressure resistant secondary lining construction and inspection channel construction;

[0075] (8) Installation and commissioning of water pressure monitoring system: Before installing the water pressure monitoring system, ensure that the equipment on site meets the design pressure requirements and arrange for professional personnel to carry out the construction;

[0076] (9) Implementation of monitoring and early warning scheme: When the external water pressure of the tunnel lining measured by the water pressure gauge exceeds the limit, the electronically controlled pressure control valve is opened through the pressure control system to reduce the pressure to below the limit.

[0077] Specifically, the key points of the construction method of a karst tunnel passing through a cave described in this embodiment are as follows:

[0078] 1. Excavation and support of caves.

[0079] Cave Excavation: First, before cave excavation, dangerous rocks within the cave's top and sediment within the cavity must be cleared. Then, the cave's top is excavated using drilling and blasting, with localized rock drilling performed using machinery. Finally, the excavated cave is measured to ensure that the distance between the excavated cave's top and the top of the tunnel's designed vault backfill meets the minimum cross-section for groundwater runoff.

[0080] Cave top support: After cave excavation, support is promptly implemented based on the integrity of the surrounding rock. When the surrounding rock is intact and free of loose blocks, support is achieved using mortar anchors, steel mesh, and shotcrete. When loose blocks are present, support is achieved using anchors, steel mesh, steel supports, and shotcrete. During steel support construction, crossbeams and locking anchors are used to temporarily secure the steel supports to the solid surrounding rock. Once the waterstop walls on both sides have been constructed to the designed elevation, the steel supports are then integrally cast within.

[0081] 2. Dredging and backfilling of cave base.

[0082] Excavation of the cave base: Use an excavator to excavate and dredge the silt layer by layer to the base. After the silt is cleared, conduct a foundation bearing capacity test. If it meets the requirements, use high-pressure water to flush the rock wall to ensure that there is no debris and residual loose soil. After the flushing is completed, use a water pump to drain the accumulated water at the base.

[0083] Concrete backfill: After excavation of the cave base, concrete is backfilled layer by layer to the designed elevation. Backfilling is carried out in two steps, with each backfill height not exceeding 3.0m. The thickness of each layer is controlled at 30cm to 50cm, and each layer is vibrated and compacted layer by layer. Before the second concrete pouring, the contact surface needs to be cleaned by chiseling. This can be done by water washing, manual chiseling, or pneumatic chiseling to remove the loose and weak layers on the concrete surface until the coarse aggregate is exposed. The chiseled concrete surface should be rinsed with clean water. When using water washing and chiseling, the strength of the treated layer should reach 0.5MPa; when chiseling is done manually, it should reach 2.5MPa; and when chiseling is done with a pneumatic machine, it should reach 10MPa.

[0084] 3. Inverted arch and filling construction.

[0085] Construction of the secondary arch lining: Before pouring the secondary arch lining, geotextile and fully enclosed waterproofing sheeting should be laid, and an overlap with the arch wall waterproofing layer should be reserved. Before constructing the inverted arch reinforcement, circumferential lap reinforcement should be reserved for welding to the secondary arch lining reinforcement, and adjacent lap joints should be staggered. Before pouring the inverted arch concrete, a double-layer embedded waterstop should be installed and secured between the two layers of rebar using rebar clamps. A fully hydraulic, self-propelled inverted arch trestle should be used to facilitate concrete construction for the inverted arch and the base. The secondary arch lining should be poured as a single section; separate pours on the left and right sides are strictly prohibited. The single pour length should not exceed 5 meters. The fill concrete should be poured separately from the inverted arch concrete.

[0086] Inverted arch filling construction: The inverted arch filling construction can be carried out after the inverted arch secondary lining is poured. Before construction, the surface water and debris of the inverted arch secondary lining should be cleared. The transverse construction joints of the inverted arch filling concrete should be staggered with the construction joints of the inverted arch secondary lining, and the staggered distance should not be less than 0.5m.

[0087] 4. Construction of waterstop wall.

[0088] The waterstop wall and foundation are both reinforced concrete. Pre-buried I-beams or channel steel are used between the foundation and the base to ensure a tight connection. A bearing capacity test of the base is conducted before construction. During construction, drainage pipes are pre-buried according to design requirements. Before pouring the foundation, measurements and layout should be carried out to determine the formwork position. After the formwork is erected, the reinforcement should be tied. The foundation reinforcement should allow for overlap with the wall reinforcement, and the foundation should be poured in a single step. During wall construction, attention should be paid to welding the reserved foundation reinforcement, and adjacent joints should be staggered. The formwork should be custom-made steel, secured with tension bolts and with appropriate external bracing. Before erecting the formwork, the formwork should be polished and coated with a release agent. The wall should be poured in sections, layers, and vibrated layer by layer. Each section should be poured no more than 2 meters in height, and each layer should be between 30 and 50 cm thick. After each section is poured, backfill concrete should be applied promptly. The backfill surface should be used as a working platform for the next retaining wall. Pouring should continue section by section until the design elevation is reached.

[0089] 5. Concrete backfill construction on both sides of the waterstop wall.

[0090] Concrete backfill is used on both sides of the waterstop wall. This backfill construction should be carried out in layers simultaneously with the waterstop wall pouring until the designed elevation is reached. The final layer of concrete backfill should be constructed according to the designed 1:2 slope, with the higher side flush with the top of the waterstop wall and the lower side conforming to the topography of the cave.

[0091] 6. Initial support construction.

[0092] Carry out initial tunnel support construction according to the design drawings. It should be noted that after the initial support steel arch is installed, the pumping pipe should be pre-buried, welded to the steel arch, and temporarily sealed with hemp to prevent clogging during shotcrete construction.

[0093] 7. Backfill the primary support top arch.

[0094] After the initial support is completed and its strength reaches 75% of the design value, the arch protection is constructed using pumped concrete. The thickness of the arch protection should be no less than 0.5m, and the top surface should be covered with a double layer of steel mesh. The top surface crack prevention layer should be no less than 5cm thick. After the pumping is completed, the entire arch protection layer should be covered with waterproof board and a layer of emulsified asphalt waterproof coating should be applied as a waterproof layer.

[0095] 8. Drainage and waterproofing construction.

[0096] Drainage blind pipe construction: Before installing the annular drainage pipe, measure and mark its specific location on the initial support structure. Drill holes to locate it, drive expansion anchors into the holes, or embed rebar heads in the holes with anchoring agent. Secure the annular blind pipe with rebar clips driven into the holes. Wrap the blind pipe with non-woven fabric and tie it with wire. Secure the blind pipe with clips and secure it to the expansion bolts. Longitudinal drainage pipes should be constructed according to the design drawings, referring to the annular drainage blind pipe.

[0097] Geotextile Construction: Use a laser profiler or laser 3D scanner to inspect the inner contour of the tunnel's primary support. When the initial support surface flatness satisfies the D / L ratio of no more than 1 / 6 (D is the depth of the concave between two adjacent convex surfaces of the initial support surface, and L is the distance between two adjacent convex surfaces of the initial support surface), geotextile laying can be performed. During the laying process, nails are used to secure the geotextile with heated hot-melt pads, and appropriate fixing points are added in concave areas.

[0098] PVC Full-Wrap Waterproofing Installation: Before installing the waterproofing, check the concrete surface for smoothness and proceed only after it meets the requirements. Use an automatic hot-melt climbing welder for installation; detailed processing or repairs may be performed with a handheld welding torch. The waterproof layer should be laid circumferentially, with no longitudinal overlaps on the arches or side walls. The overlap width between two waterproofing sheets should be no less than 120mm, and the effective width of a single weld should be no less than 10mm. Welds should be continuous and uninterrupted, with no leaks, false welds, burns, or penetrations, and should be inspected for air insulated surfaces. The overlap between the waterproofing sheets and the construction joint should be no less than 50cm apart.

[0099] Waterstop construction: Construction joints are constructed with embedded waterstops. Expansion joints are constructed with embedded steel-edged rubber waterstops and back-attached rubber waterstops, with the back-attached waterstops being fully enclosed.

[0100] 9. Construction of water pressure resistant secondary lining.

[0101] Secondary lining reinforcement mesh construction: Mechanical connections are used for the secondary lining's load-bearing reinforcement. When welding, the overlap length for single-sided welding should be no less than 10d, and for double-sided welding, no less than 5d. When binding, the overlap length should be no less than 35d. The joint ratio for rebar of the same cross-section should not exceed 50%. Rebar ends should be wrapped with a custom plastic shell or geotextile to prevent them from piercing the waterproofing. Protective spacers should be provided during rebar installation, preferably through-hole spacers. The extended length of the reserved rebar at the junction of the invert and side walls should meet connection requirements.

[0102] Secondary lining concrete construction: The secondary lining concrete construction should utilize a fully hydraulic, automated, integral lining trolley. The trolley should be equipped with a built-in function for simultaneously pouring the side walls within the cable trench, using a chute for symmetrical pouring. Concrete should be poured symmetrically in layers from the side walls toward the vault, from bottom to top. The height difference between the concrete pouring surfaces on both sides should not exceed 1.0m, and the height difference between the concrete pouring surfaces on the same side should not exceed 0.5m. Concrete should be poured continuously with manual vibration. The vault concrete should be poured densely in one go. Grouting holes should be reserved for the vault, and grouting should be performed after the secondary lining concrete pouring is completed.

[0103] Water-blocking wall construction: A longitudinal secondary lining water-blocking wall will be constructed at the interface between the pressure-bearing and non-pressure-bearing sides of the tunnel. The wall should be at least 1.0m long and utilize a waterproof layer + waterproof concrete + waterproof layer + waterproof concrete structure to block water at the ends. The water-blocking wall must be excavated in advance, and the waterproof layer and secondary lining concrete should be poured integrally with the secondary lining of the main tunnel.

[0104] 10. Check channel construction.

[0105] After the second lining construction of the inspection channel is completed, the road surface construction in the well will be carried out. The road surface construction is in a stepped shape. A 1m thick concrete retaining wall is set at the intersection of the inspection channel and the cave. A pressure-resistant manhole cover and valve are embedded in the wall. The pressure resistance is 2MPa. During the dry season, the manhole cover can be opened through the valve to enter the cave.

[0106] 11. Construction of water pressure monitoring system.

[0107] The water pressure monitoring system primarily consists of a water pressure gauge, concrete strain gauges, concrete surface strain gauges, strain transmitters, electric pressure regulating valves, valve control systems, and seamless steel pipes. The strain gauges are pre-buried and installed during the concrete pouring process, while other control systems will be installed after the tunnel lining and cavity treatment are complete. Before installation, ensure that the on-site equipment meets the design pressure requirements, and arrange for specialized personnel to perform the work. The connection between the seamless steel pipe and the side ditch must be tight and leak-proof, and the steel pipe should be curved at bends. To prevent instantaneous erosion of the side ditch, the outlet side of the steel pipe is covered with steel plates, and a layer of waterproof mortar at least 2 cm thick is applied to the top of the side ditch cover.

[0108] 12. Implementation of monitoring and early warning plans.

[0109] When the external water pressure of the tunnel lining monitored by the water pressure gauge exceeds the limit, the electronically controlled pressure control valve is opened through the pressure control system to reduce the pressure to below the limit (manually opened if necessary), and normal operation is then resumed.

[0110] When the water pressure outside the tunnel lining monitored by the water pressure gauge exceeds the limit but the rising rate does not exceed the limit, the small-diameter seamless steel pipe will be automatically opened, and the groundwater outside the pressure-bearing section lining will be drained to the non-pressure section through the small-diameter seamless steel pipe for discharge.

[0111] When the water pressure outside the tunnel lining monitored by the water pressure gauge exceeds the limit and the rising rate also exceeds the limit, the large-diameter seamless steel pipe will be automatically opened, and the groundwater outside the pressure-bearing section lining will be drained to the drainage ditch inside the tunnel for discharge through the large-diameter seamless steel pipe.

[0112] In summary, the construction method described above primarily uses water-stop walls on both sides of the tunnel in conjunction with arch protection to maintain the initial runoff balance of the cave's groundwater, thereby preserving the ecological balance near the cave. A hierarchical management and monitoring system with pressure-limited discharge dynamically maintains the water pressure outside the tunnel support structure, ensuring that it remains within limits, thereby maximizing safety during tunnel construction and operation. This method is primarily suitable for water-rich karst tunnels traversing unfavorable geological conditions such as large underground caves, karst troughs, and underground rivers. When a tunnel passes through a large cave, the top of the cave is first excavated and supported, and the sediment at the bottom of the cave is cleaned and backfilled with concrete; after the construction of the tunnel invert arch and its filling, the water-stop walls on both sides of the tunnel are constructed and the concrete backfill behind the wall is backfilled, and seamless steel pipes for emergency drainage are pre-buried; after the initial support construction of the tunnel, the arch protection is constructed by pumping concrete, and two layers of steel mesh are laid on the top surface of the arch protection. After the pumping is completed, the whole arch is covered with waterproof board and a layer of emulsified asphalt waterproof coating is applied on the top surface as a water-sealing layer; then, the secondary lining of the tunnel is promptly constructed and an inspection channel is constructed at a suitable location; finally, the monitoring equipment for pressure-limited emission classification management is installed and debugged.

[0113] The above is only a preferred specific embodiment of the present invention, but the design concept of the present invention is not limited to this. Any technician familiar with this technical field who uses this concept to make non-substantial changes to the present invention within the technical scope disclosed by the present invention shall be deemed to infringe the scope of protection of the present invention. Industrial Applicability

[0114] The present invention provides a construction method for a karst tunnel passing through a cave, comprising the following steps: (1) cave excavation and support; (2) dredging and backfilling of the cave base; (3) invert arch and backfill construction; (4) water stop wall construction and concrete backfilling on both sides; (5) initial support construction; (6) backfilling of the primary support top and waterproofing layer construction; (7) secondary lining drainage construction; (8) installation and commissioning of a water pressure monitoring system; and (9) implementation of a monitoring and early warning plan. This construction method is suitable for water-rich karst tunnels passing through large underground caves, karst troughs, underground rivers, and other unfavorable geological projects. In this construction method, the steps are relatively easy to operate, the construction quality is controllable, there are fewer construction blind spots, and the construction safety is high; it can maintain the initial runoff balance of the cave groundwater, protect the surrounding ecological environment to the greatest extent, and has good industrial applicability.

Claims

1. A construction method for a karst tunnel passing through a karst cave, characterized in that: The steps include: (1) Cave expansion and support: expansion of the cave and support of the cave top; (2) Desilting and backfilling of cave base: excavation at the cave base and backfilling with concrete; (3) Invert and filling construction: carry out invert secondary lining construction and invert filling construction; (4) Construction of water stop wall and concrete backfill on both sides: a water stop wall is built in the tunnel and concrete backfill is carried out on both sides of the water stop wall; (5) Initial support construction: Carry out initial support construction of the tunnel according to the design drawings; (6) Construction of backfill and waterproof layer at the top of primary support: backfill construction of arch protection at the top of primary support, construction of drainage blind pipe, construction of civil engineering cloth, construction of PVC waterproof board and construction of water stop strip; (7) Secondary lining waterproofing and drainage construction: carry out water pressure resistant secondary lining construction and inspection channel construction; (8) Installation and commissioning of water pressure monitoring system: Before installing the water pressure monitoring system, ensure that the equipment on site meets the design pressure requirements; (9) Implementation of monitoring and early warning scheme: When the external water pressure of the tunnel lining monitored by the water pressure gauge exceeds the limit, the pressure control system will open the electronically controlled pressure control valve to reduce the pressure to below the limit.

2. The construction method of a karst tunnel passing through a karst cave according to claim 1, characterized in that: The excavation of the cave in step (1) includes: Before expanding the cave, the dangerous rocks within the operating range at the top of the cave and the sediments in the cave should be cleared; The top of the cave is expanded by drilling and blasting, and local rock drilling is performed by machinery; the expanded cave is measured; 3. The construction method of a karst tunnel passing through a karst cave according to claim 1, characterized in that: In step (1), the support operation for the cave top includes: After the cave expansion operation is completed, support should be carried out in a timely manner according to the integrity of the surrounding rock; When the surrounding rock is intact and has no falling blocks, mortar anchor rods, steel mesh and shotcrete are used to support it; when the surrounding rock has falling blocks, anchor rods, steel mesh, steel support and shotcrete support are used to support it; During the steel support construction, cross beams and locking anchor rods are used to temporarily fix it on the solid surrounding rock. When the water stop walls on both sides are constructed to the designed elevation, the whole structure is poured inside.

4. The construction method of a karst tunnel passing through a karst cave according to claim 1, characterized in that: The dredging and backfilling of the cave base in step (2) includes: Use an excavator to excavate and dredge the silt layer by layer to the base. After the silt is cleared, carry out a foundation bearing capacity test. After it meets the requirements, use high-pressure water to wash the rock wall. After the washing is completed, use a water pump to drain the accumulated water in the base. After the cave base is excavated, concrete is used to backfill in layers to the designed elevation. The backfilling is carried out in two steps, and each layer is vibrated and compacted; Before the second concrete pouring, the contact surface should be roughened and cleaned, and the roughened concrete surface should be rinsed with clean water.

5. The construction method of a karst tunnel passing through a karst cave according to claim 1, characterized in that: The invert and filling construction in step (3) includes: Before pouring the second lining of the inverted arch, geotextile and fully covered waterproof board shall be laid, and the overlapping part with the waterproof layer of the arch wall shall be reserved; Before the construction of the inverted arch reinforcement, the reserved annular lap reinforcement is welded with the secondary lining reinforcement of the arch wall, and the lap joints of adjacent reinforcements are staggered; Before pouring the invert concrete, install a double-layer buried water stop and use steel bar clips to firmly position and connect the two layers of steel bars. In the middle, the invert and the bottom are constructed with fully hydraulic self-propelled invert trestle and concrete; When pouring the second lining of the invert, the whole section is poured together, and the filling concrete and the invert concrete are poured separately; The invert filling construction shall be carried out after the pouring of the second lining of the invert is completed, and the surface water and debris of the second lining of the invert shall be cleaned before construction. The transverse construction joint of the invert filling concrete shall be staggered with the construction joint of the second lining of the invert.

6. The construction method of a karst tunnel passing through a karst cave according to claim 1, characterized in that: The water wall construction in step (4) includes: The wall and foundation of the water stop wall are made of reinforced concrete, and pre-buried I-beams or channel steels are used between the foundation and the base; Carry out a base bearing capacity test before construction, and pre-bury the drainage pipes during construction; Before pouring the foundation, measurement and layout are carried out to determine the position of the formwork. After the formwork is erected, the steel bars are tied. The foundation steel bars are reserved for overlapping with the wall steel bars. The foundation is poured in one go, and adjacent joints are staggered during wall construction. The formwork is made of customized steel formwork, fixed with tension bolts, with diagonal braces added on the outside. The formwork is polished and coated with a release agent before erecting the formwork. The wall body is cast in sections, layers, and vibrated layer by layer. After each section is cast, the back of the wall is backfilled with concrete in a timely manner. At the same time, the concrete backfill surface is used as the construction platform for the next retaining wall, and the casting construction is carried out section by section until the designed elevation.

7. The construction method of a karst tunnel passing through a karst cave according to claim 1, characterized in that: The concrete backfill construction on both sides of the water stop wall in step (4) includes: Concrete is used to backfill the two sides of the water stop wall, and the backfill construction is carried out in layers simultaneously with the pouring of the water stop wall until the designed elevation; The last layer of concrete backfill is constructed according to the designed slope of 1:2, with the high side flush with the top of the water stop wall and the low side following the terrain inside the cave.

8. The construction method of a karst tunnel passing through a karst cave according to claim 1, characterized in that: In step (6), the backfill construction at the top of the primary support includes: After the initial support is completed, pumped concrete is used for arch protection construction; the top surface is covered with a double-layer steel mesh. After pumping, it is fully wrapped with waterproof board and a layer of emulsified asphalt waterproof coating is applied as a waterproof layer.

9. The construction method of a karst tunnel passing through a karst cave according to claim 1, characterized in that: In step (6), the construction of drainage blind pipe includes: Before installing the annular drainage pipe, first measure and mark the specific position of the annular drainage pipe on the initial support structure surface, drill holes to locate it, drive expansion anchors into the positioning holes or use anchors to embed the steel bar heads in the positioning holes, use steel bar clips to drive into the positioning holes to fix the annular blind pipe, wrap the blind pipe with non-woven fabric, and tie it with wire; clamp the blind pipe with clips and then fix it on the expansion bolts; the longitudinal drainage pipe is constructed according to the design drawings and refer to the annular drainage blind pipe.

10. The construction method of a karst tunnel passing through a karst cave according to claim 1, characterized in that: In step (6), the civil engineering fabric construction includes: A laser profiler or a laser 3D scanner is used to detect the inner contour of the initial support of the tunnel. When the flatness of the initial support surface meets the D / L ratio of no more than 1 / 6, the geotextile is laid; D is the depth of the concave between two adjacent convex surfaces of the initial support surface, and L is the distance between two adjacent convex surfaces of the initial support surface; during the laying process, nails are used to heat the hot-melt pads for fixing, and appropriate fixing points are added in the concave parts.

11. The construction method of a karst tunnel passing through a karst cave according to claim 1, characterized in that: The construction of the PVC waterproof board in step (6) includes: before the construction of the waterproof board, check the smoothness of the concrete surface and then construct it after meeting the requirements; use an automatic hot melt climbing welding machine for construction, and use a handheld welding gun for detail processing or repair; the waterproof layer is laid in a circumferential manner, and there is no longitudinal overlap between the arch and the side wall.

12. The construction method of a karst tunnel passing through a karst cave according to claim 1, characterized in that: The waterstop construction in step (6) includes: construction joints are constructed using embedded waterstops, and expansion joints are constructed using embedded steel-edged rubber waterstops in combination with back-attached rubber waterstops, with the back-attached waterstops being fully enclosed.

13. The construction method of a karst tunnel passing through a karst cave according to claim 1, characterized in that: The water pressure resistant secondary lining construction in step (7) includes: secondary lining steel mesh construction, secondary lining concrete construction and water blocking wall construction; The secondary lining steel mesh construction includes: the secondary lining stress steel bars are mechanically connected; the steel bar ends are wrapped with customized plastic shells or geotextiles, protective layer pads are set for steel bar installation, and the steel bar extension length is reserved for the invert at the connection between the invert and the side wall; The secondary lining concrete construction includes: the secondary lining concrete construction adopts a fully hydraulic automatic walking integral lining trolley, adopts a chute for symmetrical pouring, and the concrete is poured symmetrically in layers from the side walls on both sides to the vault from bottom to top; the concrete is poured continuously and vibrated manually, the vault concrete is poured densely at one time, and grouting holes are reserved for the vault, and the vault grouting is carried out after the secondary lining concrete pouring is completed; The water blocking wall construction includes: constructing a longitudinal secondary lining water blocking wall at the contact position between the pressure-bearing side and the non-pressure-bearing side of the tunnel, and using a waterproof layer, waterproof concrete, waterproof layer and waterproof concrete in combination to block water at the end; expanding the water blocking wall in advance, and integrally pouring the waterproof layer and secondary lining concrete with the secondary lining of the main tunnel; The construction of the inspection channel includes: after the second lining construction of the inspection channel is completed, the road surface construction in the well is carried out, and the road surface construction is in a stepped shape; a concrete retaining wall is set at the intersection of the inspection channel and the cave, and a pressure-resistant manhole cover and valve are embedded in the wall. During the dry season, the manhole cover can be opened through the valve to enter the cave.

14. The construction method of a karst tunnel passing through a karst cave according to claim 1, characterized in that: The water pressure monitoring system in step (8) includes: a water pressure gauge, a concrete strain gauge, a concrete surface strain gauge, a strain transmitter, an electric pressure regulating valve, a valve control system, and a seamless steel pipe; the strain gauge is pre-buried and installed synchronously during the concrete pouring process, and the remaining control system is installed after the tunnel lining is fully constructed and the cavity treatment construction is completed.

15. The construction method of a karst tunnel passing through a karst cave according to claim 1, characterized in that: In the implementation of the monitoring and early warning scheme in step (9): When the water pressure outside the tunnel lining monitored by the water pressure gauge exceeds the limit, but the rising rate does not exceed the limit, the small-diameter seamless steel pipe is automatically opened to divert the groundwater outside the pressure-bearing section lining to the non-pressure section for discharge through the small-diameter seamless steel pipe; When the water pressure outside the tunnel lining monitored by the water pressure gauge exceeds the limit and the rising rate also exceeds the limit, the large-diameter seamless steel pipe will be automatically opened, and the groundwater outside the pressure-bearing section lining will be drained to the drainage ditch in the tunnel for discharge.

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