Methods for treating and preventing pollution from backfilling karst caves in cable-stayed bridge pile foundations

Abstract

This invention relates to the field of engineering seepage prevention and barrier technology, and particularly to a method for treating and preventing pollution from backfilling beaded karst caves in cable-stayed bridge pile foundations. The method includes the following steps: surveying the karst cave and assessing risk factors based on the survey results to determine subsequent construction process parameters; constructing an outer structure formed by a first backfill material in a selected section of the karst cave according to the predetermined process parameters; the first backfill material includes large stones, clay, stone powder, and cement; after the outer structure is formed, completely filling and constructing an inner impermeable body formed by a second backfill material in the internal area enclosed by the outer structure according to the predetermined process parameters; the second backfill material includes montmorillonite, small stones, clay, and sawdust; and directly drilling pile holes in the solidified inner impermeable body to complete the pile foundation construction. This method solves the problems of incomplete sealing, easy pollution diffusion, poor material compatibility, environmental and economic imbalance, and insufficient structural stability in traditional beaded karst cave treatment processes.

Description

Technical Field

[0001] This invention relates to the field of engineering seepage prevention and barrier technology, and in particular to a method for treating and preventing pollution backfilling of beaded karst caves in karst pile foundations of cable-stayed bridges. Background Technology

[0002] In the field of bridge construction in karst areas, bored piles have become the mainstream construction method for bridge pile foundations crossing karst cave development areas due to their high bearing capacity and strong adaptability to geological formations. However, beaded karst caves, as a typical complex geological feature in karst strata, are a core challenge for pile foundation construction safety and surrounding ecological protection because they are interconnected longitudinally or laterally, have active internal water flow, and are prone to accumulating construction pollutants. If beaded karst caves are not effectively sealed during pile foundation construction, it can lead to mud loss and borehole wall collapse, and can also cause construction pollutants (such as waste mud and drill cuttings) to spread to the surrounding groundwater system through the cave channels, causing irreversible environmental damage to water source protection areas, ecologically sensitive areas, and other areas.

[0003] Traditional treatments for beaded karst caves often employ single-layer clay backfilling or ordinary concrete pouring. However, these methods have significant limitations in practical applications: First, it's difficult to balance pollution prevention and structural stability. While single-layer clay backfilling offers some impermeability, its structural strength is insufficient, making it susceptible to erosion by water flow within the cave, creating seepage channels and failing to prevent pollutant diffusion. Ordinary concrete pouring, while improving structural strength, becomes brittle after hardening, making it difficult to adapt to the irregular spaces and slight deformations of the strata in beaded karst caves. This leads to cracking later on, and the concrete's bond with the cave walls is not tight, still posing a risk of pollution leakage. Second, material adaptability is poor. Traditional materials cannot dynamically adjust their performance based on the water flow velocity and pollution type of the beaded karst cave. For high-velocity beaded karst caves, insufficient erosion resistance often leads to sealing failure. For high-concentration pollutant scenarios, there is a lack of specific impermeable components, making it difficult to meet the pollution prevention requirements of ecologically sensitive areas. Third, there is an imbalance between environmental protection and economic efficiency. In traditional processes, unused backfill materials are often directly discarded, which not only wastes resources but also increases the cost of solid waste treatment. Some modified backfill materials contain irritating chemical additives, which can easily aggravate the pollution of surrounding water bodies once they leak, and do not meet the current standards for green engineering construction.

[0004] With the increasing demands for pollution prevention, high adaptability, and strong environmental protection in engineering construction of pile foundations in karst areas, traditional beaded karst cave treatment processes can no longer meet the construction needs of complex karst strata. There is an urgent need to develop a mixed backfill material preparation and construction method that combines pollution prevention, structural stability, and environmental protection and energy saving to solve the problems of incomplete sealing of beaded karst caves, easy pollution diffusion, and material waste, and to achieve the synergistic operation of efficient karst cave sealing, pollution prevention, and resource recycling.

[0005] The information disclosed in this background section is intended only to enhance the understanding of the overall background of the present invention and should not be construed as an admission or in any way implying that the information constitutes prior art known to those skilled in the art. Summary of the Invention

[0006] The first objective of this invention is to provide a method for treating and preventing pollution in beaded karst caves in cable-stayed bridge pile foundations. This method addresses the problems of incomplete sealing, easy diffusion of pollution through connecting channels, poor material compatibility, imbalance between environmental protection and economic efficiency, and insufficient structural stability in traditional beaded karst cave treatment processes. It achieves a unified approach to efficient karst cave sealing, precise pollution prevention, material recycling, and green construction, meeting the stringent requirements for pollution prevention, erosion resistance, and structural stability in pile foundation construction in ecologically sensitive karst areas.

[0007] The above-mentioned technical objective of the present invention is achieved through the following technical solution:

[0008] The method for treating and preventing pollution from backfilling beaded karst caves in karst pile foundations of cable-stayed bridges includes the following steps:

[0009] S1 conducted a survey of the beaded karst caves at the pile location, assessed the risk factors affecting pollution prevention and structural stability based on the survey results, and determined the appropriate process parameters for subsequent construction accordingly.

[0010] This invention upgrades construction from a fixed formula to an intelligent system of perception, analysis, decision-making, and execution. Through preliminary surveying, it proactively identifies four major risk factors: water flow, pollution, deformation, and support. It also establishes a precise mapping relationship between each risk and specific material process parameters. Furthermore, multiple risk factors can be combined to trigger and respond in a coordinated manner, thus solving the unique complex working conditions of beaded karst caves.

[0011] S2 selects a section of the karst cave and constructs an outer structure formed by the first backfill material according to the process parameters determined by S1; the first backfill material includes large pieces of stone, clay, stone powder and cement;

[0012] Based on the geological survey data of the beaded karst cave, the splicing dimensions and installation position of the steel casing were determined. The steel casing was lowered using lifting equipment, and after being calibrated for verticality with a level, it was fixed to ensure that the casing fit snugly against the karst cave wall. The first backfill material was mixed according to the specified ratio and poured into the steel casing in layers. After each layer was poured, it was compacted with an immersion vibrator to avoid voids. After the outer structure hardened to the set strength, it was watered for curing. After curing, the steel casing was disassembled in sections. During the disassembly process, the steel plates were pulled with soft ropes to avoid collisions with the outer structure.

[0013] After the outer structure is formed, according to the process parameters determined in S1, the inner impermeable body formed by the second backfill material is completely filled and constructed in the internal area enclosed by the outer structure. The second backfill material includes montmorillonite, small stones, clay and sawdust.

[0014] Specifically, clean the residual impurities on the inner wall of the outer structure, repair any depressions, mix the second backfill material according to the specified ratio, and use a layered filling and light compaction process to ensure the inner impermeability is dense. After the inner impermeability is filled, water is sprayed for curing. After the curing period, the impermeability is tested with an impermeability detector to ensure there is no risk of leakage.

[0015] S4 drills pile holes directly in the solidified inner impermeable body to complete the pile foundation construction.

[0016] The core of this invention lies in the use of a flexible composite material, with montmorillonite as the core and sawn timber as the crack-resistant fiber, as a permanent annular thin-layer impermeable body to permanently encase the pile in the anti-pollution structure of beaded karst cave pile foundations. Addressing the two specific risks of high pollution and ground deformation in beaded karst caves, the pre-hydrated montmorillonite and dried, impurity-removed sawn timber undergo directional material performance activation to further improve bonding performance, simultaneously meeting the requirements of extremely low permeability, flexibility to adapt to ground deformation, and economic efficiency. A rigid outer shell must be constructed first to provide shaping protection and support, allowing for layered compaction to a density of over 95%. This transforms the inner layer material from loose filler into a machinable engineering medium. Through specific drilling methods, the pile body and the inner impermeable body are directly bonded at a mechanically well-interlocked borehole wall interface, achieving a seamless integration of the structural pile and the functional impermeable body.

[0017] The outer structure of this invention uses concrete with large stone slabs as a framework. Its core function is to create a dry and undisturbed internal environment for the precise construction of the inner impermeable layer during the construction phase. During operation, even if trace amounts of seepage penetrate the outer layer, their hydraulic gradient and flow velocity are greatly weakened, transforming into slow seepage that poses no threat to the inner flexible impermeable layer. The inner layer, a composite material with montmorillonite as its core, can achieve near-absolute physical barrier and ion adsorption for trace amounts of water and dissolved pollutants that seep in at extremely slow rates. The incorporated sawdust fibers give this layer flexibility to adapt to geological deformation, ensuring it does not crack during long-term service.

[0018] Furthermore, in step S1, the risk factors assessed include at least one of the following: water scour intensity, support strength requirements for the outer structure, contaminant concentration, and formation deformation risk; wherein, the assessment of water scour intensity or support strength requirements is related to process adjustments for the first backfill material; the assessment of contaminant concentration or formation deformation risk is related to process adjustments for the second backfill material.

[0019] Further process adjustments include:

[0020] If the water flow scouring intensity is not lower than the strong scouring threshold, the mixing time of the first backfill material shall be extended and / or an interface agent shall be added to ensure that the large stones are tightly bonded to other components.

[0021] Furthermore, the strong scour threshold is a critical indicator used to determine whether the water flow within a karst cave causes significant hydraulic scour of the structure. Preferably, the average flow velocity v within the karst cave is measured using a flow meter or tracer method. When the measured average flow velocity v ≥ 0.5 m / s, it is determined that the strong scour threshold has been reached. In addition, when fresh scour marks are observed on the cave walls, the water flow carries a large amount of suspended matter, or obvious turbulence, disturbances, or eddies are formed, even if the flow velocity is slightly lower than 0.5 m / s, it is also comprehensively determined that the scour is strong.

[0022] If the required support strength is not lower than the strength threshold, then optimize the vibration frequency and cement hydration curing conditions to improve the density of the outer layer material and adapt to the erosion resistance and structural stability requirements under different working conditions.

[0023] The strength threshold refers to the minimum required standard for the compressive strength or overall stability of the outer structure. Specifically, it is determined based on the geological survey report, obtaining information such as the thickness of the cave roof, rock mass quality grade (RQD), integrity factor, and superstructure load. This information determines the additional support force or safety factor that the outer structure needs to provide to ensure the safety of pile foundation construction and long-term operation. For example, if calculations and analysis show that there is a localized load on the cave roof, the cave roof's own stability safety factor is less than 1.3, or the design explicitly requires the outer structure to bear an additional stress of no less than 200 kPa, then the required support strength is determined to be higher than the preset strength threshold.

[0024] If the pollutant concentration is not lower than the high concentration threshold, the montmorillonite should be pre-hydrated before preparation to enhance its impermeability.

[0025] The high concentration threshold is a critical value for determining whether pollutants in groundwater or filling materials within a karst cave pose a significant environmental risk. Specifically, water or soil samples are collected from the karst cave, and the concentration C of the target pollutant (such as suspended solids, petroleum hydrocarbons, heavy metal ions, etc.) is detected. When the pollutant concentration C exceeds five times the limit of Class III water quality standard in the "Groundwater Quality Standard" (GB / T14848), or exceeds the relevant control standards stipulated by the environmental protection authority of the project location, it is determined to have reached the high concentration threshold.

[0026] If there is a risk of ground deformation, the sawn timber surface is dried and cleaned before preparation to improve crack resistance and meet the needs of pollution prevention and crack resistance under different working conditions.

[0027] The presence of ground deformation risk refers to the high probability of harmful deformation of the rock and soil surrounding the karst cave during construction or use, such as settlement, slippage, or convergence. This can be judged by one or more of the following signs:

[0028] The geological report clearly indicates that the rock mass of the cave roof or sidewalls is fractured, such as rock mass quality grade RQD < 25%, presence of weak interlayers, cave span > 5m, or located in a tectonic fracture zone;

[0029] Early-stage monitoring revealed minor displacements around the cave, such as crack expansion, or surveys revealed a large number of recently fallen rocks and deposits inside the cave.

[0030] Based on regional engineering experience, other karst caves under similar geological conditions have experienced deformation and instability.

[0031] When one of the above situations exists, and the engineering assessment determines that the deformation may affect the structural integrity of the composite barrier or the long-term safety of the pile foundation, it is determined that there is a risk of ground deformation.

[0032] Furthermore, in the process adjustment of the first backfill material, the mixing time is extended to be controlled at no less than 120 seconds; an interface agent is added by adding 1% to 5% organic polymer emulsion or cement-based binder by the mass of cement to enhance the bonding force between large stones and clay and stone powder; the vibration frequency and cement hydration curing conditions are optimized by optimizing the vibration frequency, controlling the humidity of the curing environment to be no less than 90%, and extending the curing time to ensure that the structural support strength meets the standards.

[0033] It should be noted here that when assessing a strong scouring condition, the core objective of adjusting the first backfill material is to construct a super-strong aggregate-slurry interface to resist water stripping. By extending the mixing time, it is ensured that a high proportion of large stone aggregate surfaces are fully and densely coated with cement slurry, eliminating aggregate-deficient zones caused by insufficient mixing. On this basis, an interface agent is introduced into the slurry to actively enhance the chemical adhesion and mechanical interlocking force between the cement slurry and the aggregate surface, making the aggregate more difficult to pull out when subjected to water shear.

[0034] When the required support strength for the outer structure exceeds the preset strength threshold, the vibration and curing processes are optimized to ensure that the first backfill material reaches and maintains its design compressive strength. Specifically, based on the thickness of the pouring layer and the aggregate particle size, layered, multiple, immersion vibration is used to ensure that the grout fully fills the gaps between the aggregates and coats them, thereby improving the density of the grout. At the same time, the humidity of the curing environment is controlled to be no less than 90% and maintained for more than 7 days to ensure continuous cement hydration, avoid early drying shrinkage cracks, and allow the dense skeleton to solidify into a high-strength whole.

[0035] Preferred organic polymer emulsions include styrene-butadiene emulsion (SBR), acrylic emulsion (PAE), or ethylene-vinyl acetate copolymer emulsion (EVA), which can form an interwoven polymer film in cement, significantly improving cohesion and interfacial adhesion. Cement-based binders, such as modified mortars containing redispersible latex powder, achieve rapid and tight bonding through higher activity or polymer components, ensuring rapid formation of early strength and integrity in damp karst caves.

[0036] Furthermore, in the process adjustment of the second backfill material, the pre-hydration treatment involves mixing montmorillonite with water and allowing it to stand for aging. The amount of water added is 50% to 100% of the mass of montmorillonite, and the aging time is 12 to 24 hours. This provides sufficient moisture and time for the montmorillonite to fully expand its layered structure, forming a stable gel network and ensuring that the montmorillonite forms a uniformly expanded layer. After pre-hydration, the montmorillonite particles can act as activated seepage prevention units when subsequently mixed with clay and small stones and compacted. They can be more evenly distributed and form a continuous, low-permeability seepage prevention matrix, which can immediately and effectively block and adsorb pollutants.

[0037] The drying and impurity removal process involves drying the sawn wood surface at 80-110℃ until the moisture content is below 10%, followed by fiber dispersion treatment and sieving to remove lumps and impurities, thereby further enhancing its stress dispersion effect in the material matrix.

[0038] The flexible properties of the second backfill material prepared by this invention are specifically designed to absorb minor deformations of the strata and pile foundations, ensuring that it does not crack and thus achieving long-lasting seepage prevention. The sawdust surface can solve the crack resistance problem of montmorillonite-based materials under constrained shrinkage and deformation. Drying removes moisture to prevent the sawdust surface from rotting in the material, while maintaining its fiber elasticity and preventing it from competing with montmorillonite and clay for moisture during the curing process, thus affecting overall hydration and bonding. Finally, removing lumps and impurities ensures that the fiber monomers can be evenly and fully dispersed in the second backfill material, forming an effective three-dimensional network support, which more efficiently transfers stress and inhibits the generation and development of cracks when the material is under tension.

[0039] Furthermore, in step S2, modular steel casings are used as templates for constructing the outer structure; the mass ratio of large stones, clay, stone powder, and cement is 12~14:5~7:1~3:1, preferably 13:6:2:1. The preparation of the first backfill material requires strict control of the water addition, with a target water-cement ratio controlled between 0.35 and 0.45, and a corresponding total moisture content of approximately 8%~12%, to meet the chemical water requirement for complete cement hydration and avoid material segregation, thus adapting to the construction of the steel casing.

[0040] This invention selects high-compressive-strength stone with continuous particle size distribution to form a stable skeleton structure that resists the erosion of water flow within the karst cave. Clay and stone powder work together as filling and binding components. Clay fills the gaps between the stone pieces and improves the overall integrity of the material, while stone powder optimizes the particle size distribution and reduces shrinkage gaps in the outer layer. A small amount of cement is used as a cementing material to enhance the structural strength and anti-loosening ability of the hardened outer layer, preventing it from falling off under external forces. All four components are mixed using a forced mixing device to ensure that the cement is fully hydrated and the stone powder evenly coats the surface of the stone pieces, without material segregation or insufficient cementation. This ensures that the outer layer meets the standards for erosion resistance and support performance. The structure is then cast and formed using a steel casing. After the outer structure hardens, the steel casing is removed, and the inner impermeable material is then filled in. This achieves dual protection against pollution diffusion in the beaded karst cave, enhancing structural stability and resistance to water erosion.

[0041] During the casting process using the steel casing, the pouring height and speed are controlled in each pour. After pouring, an immersion vibrator is used to assist in compaction, enhancing the structural strength of the hardened outer material. The steel casing adopts a modular, detachable steel structure, composed of multiple steel plates spliced ​​together with flange bolts. The steel plates are made of high-strength steel, with a thickness adapted to the lateral pressure of the outer material during casting. The inner wall is coated with a wear-resistant coating to reduce wear during material casting. Weather-resistant sealing strips are embedded at the joints to prevent leakage of the outer material. Evenly spaced reinforcing ribs are welded to the outside of the steel casing to improve the structural stability of the outer structure during casting and prevent deformation of the steel casing under lateral pressure from the material. The steel casing can be flexibly adjusted in terms of the number and combination of steel plates according to the cross-sectional dimensions and orientation of the beaded karst cave, providing a stable forming space for the pouring of the outer layer material. At the same time, it is easy to dismantle and recycle later. The steel casing is dismantled and recycled after the construction is completed and the outer structure reaches the set strength. The dismantling process adopts a segmented dismantling method from the corners to the middle to avoid structural disturbance to the formed outer structure. After the dismantled steel casing is cleaned of wear-resistant coating stains and the aged sealing strips are replaced, it can be reused for the treatment of other beaded karst caves.

[0042] Furthermore, in step S3, the inner impermeable layer is formed by backfilling in layers and compacting the second backfill material; the mass ratio of montmorillonite, small pebbles, clay, and sawdust is 10~12:4~6:2~4:1, preferably 11:5:3:1. During mixing, montmorillonite is mixed with other components, and a small amount of mixing water is added to make the moisture content of the mixture reach 15%~22%. Compaction at this moisture content allows the clay particles to form a dense arrangement between the montmorillonite gel and the small pebbles, generating the maximum dry density and the strongest cohesion, thereby achieving an extremely low permeability coefficient and excellent crack resistance.

[0043] Montmorillonite is the core seepage barrier component. Relying on its interlayer water absorption and expansion characteristics, it fills the gaps in the material to form a dense seepage barrier, blocking the migration channels of construction pollutants such as waste mud and drill cuttings. Sawdust is made from waste wood processing fibers, which are evenly dispersed in the material matrix to form a flexible stress skeleton, dispersing the tensile stress generated by slight deformation of the strata and preventing cracking of the inner seepage barrier. Clay serves as a bonding medium, enhancing the interfacial bonding force between montmorillonite and the boulders, reducing material delamination, and providing basic support for the small boulders. All four components are mixed using a twin-shaft mixing device, with the mixing speed and time controlled to ensure homogeneous mixing of montmorillonite, small boulders, clay, and sawdust. During molding, the layered filling method is selected according to the size of the karst cave space. After filling, a lightweight compaction device is used to assist in compaction, ensuring that the density of the inner layer meets the requirements for preventing pollution diffusion.

[0044] Furthermore, the large stones in the first backfill material have a particle size of 60~100mm, with continuous particle gradation and no weathering or damage, ensuring a dense outer structure and providing a stable external support space for the inner impermeable body.

[0045] Furthermore, the small stones in the second backfill material have a particle size of 20-40mm and their surfaces are treated to have no sharp edges, which not only provides basic support for the inner layer but also prevents them from tearing through the impermeable layer and forming leakage channels. The difference in particle size between the small stones and the large stones can also create an internal and external support gradient, improving the overall structural stability.

[0046] Furthermore, after pile construction, the radial thickness of the inner impermeable body is 0.1~0.3m; and the radial thickness of the outer structure is 0.1~0.3m.

[0047] The minimum thickness of the inner impermeable layer is the critical engineering thickness to ensure that the montmorillonite-based flexible material can perform continuous and reliable impermeability. Below this value, the material is prone to penetrating defects during compaction and subsequent geological deformation, making the impermeability system fragile. Montmorillonite material has low strength, and an excessively thick inner layer will weaken the overall structure of the composite wall and is uneconomical. The upper limit of 0.3m thickness can provide sufficient impermeability redundancy and can effectively accommodate small stone blocks and sawn wood fibers, ensuring its flexible crack resistance.

[0048] The minimum thickness of the outer structure is the minimum structural thickness required to form a concrete shell with sufficient rigidity and integrity to effectively resist the erosion of karst water flow, soil pressure, and protect the inner layer. The maximum thickness can provide sufficient erosion resistance and load-bearing cross section while ensuring the use of 60-100mm large stone aggregate and sufficient vibration compaction. Thicker designs have limited performance improvement but will significantly increase material usage and construction difficulty.

[0049] There is at least a 0.1m annular space between the pile diameter and the inner wall of the outer structure to ensure that the inner impermeable body will not be completely removed. This physically separates the rigid scour-resistant outer structure from the pile body, and is filled with a flexible inner impermeable body. This effectively absorbs and buffers the direct impact of the vertical load and vibration of the pile foundation on the outer structure, as well as the direct influence of ground deformation on the pile body.

[0050] Furthermore, in step S3, the compaction degree of the inner impermeable layer shall not be less than 95%.

[0051] Compaction greatly reduces the porosity between particles. Under high pressure, montmorillonite and clay particles rearrange to form a dense matrix with surface-to-surface contact, achieving extremely low permeability. Under high compaction, small pieces of stone are firmly embedded in the montmorillonite-clay matrix, becoming a load-bearing skeleton. Sawdust fibers are pressed in and evenly dispersed to form a three-dimensional reinforced network. The interparticle friction and interlocking generated during compaction endow the material with a certain cohesive force and initial shear strength, enabling it to have short-term self-stabilizing ability and creating drilling-ready medium conditions.

[0052] In step S4, directly drilling the pile hole refers to using a mechanical drilling method adapted to the properties of the cured flexible composite material. Given that the inner impermeable body has formed a self-stabilizing engineering medium, a dry drilling process that does not require mud wall protection is preferred, such as rotary drilling or casing follow-up drilling. The core is to maintain the integrity and cleanliness of the hole wall and minimize damage to the inner impermeable body structure while forming a regular pile hole, thereby ensuring that the subsequently poured pile body concrete can form a high-quality bond with the impermeable body.

[0053] Furthermore, step S5 involves monitoring the seepage prevention performance and structural stability of the composite barrier formed by the inner and outer layers during construction and after pile formation. The monitoring components include a seepage prevention performance tester and a structural strength sensor. The seepage prevention performance tester is used to detect the pollution diffusion prevention effect of the inner seepage prevention body, while the structural strength sensor is used to monitor the structural stability of the outer structure, ensuring that the composite barrier meets the construction requirements for pollution diffusion prevention in beaded karst caves.

[0054] Specifically, a seepage detection instrument is used to test the seepage prevention performance of the inner seepage prevention body. The probe of the instrument is attached to the surface of the inner layer, compressed air is introduced and the pressure change is monitored. If the pressure does not drop within a set time, the seepage prevention is deemed qualified. If there are leakage points, a small amount of inner layer material is injected through the grouting pipe to repair them until the seepage prevention performance meets the standard.

[0055] The structural strength sensor is installed at the contact surface between the structure and the surrounding rock of the karst cave. By monitoring the strain, stress and contact pressure of the concrete, it assesses whether the outer structure is in a safe stress state and whether abnormal deformation or stress concentration has occurred.

[0056] High-frequency monitoring is conducted during the completion of the structure and the construction of the pile foundation to provide real-time feedback on the process effect, which is used to guide or adjust subsequent procedures. After the pile is completed, long-term, low-frequency monitoring is implemented to establish a health record. When data anomalies occur, such as a sudden increase in water pressure at the inner interface or an outer stress exceeding the warning value, an early warning can be triggered to achieve predictive maintenance.

[0057] Compared with the prior art, the present invention has the following beneficial effects:

[0058] This invention systematically solves the problems of pollution diffusion and structural stability during pile foundation construction caused by the strong connectivity, complex water flow paths, and irregular spatial morphology of beaded karst caves. First, through precise surveying and risk assessment of the karst caves, adaptive process parameters are customized for subsequent construction. This allows the solution to intelligently respond to the specific risks of different karst caves, ensuring the targeted and economical nature of the measures. Then, a phased construction process, starting from the outside and working in stages, is adopted to form a functionally complementary double-layer composite barrier. The outer structure, positioned using a steel casing, is composed of large stones, clay, stone powder, and a small amount of cement, forming an erosion-resistant and highly stable supporting shell. The inner impermeable layer uses montmorillonite as its core, combined with small stones, clay, and sawdust, which, after compaction, forms a flexible sealing layer with both high impermeability and crack resistance. The two layers work together to form a double barrier that cuts off the pollution diffusion path.

[0059] The outer structure of this invention utilizes a steel casing to ensure molding quality and dimensional accuracy. The steel casing auxiliary system adapts to irregular karst cave spaces, and the resulting regular cavity after removal creates ideal conditions for homogeneous filling and compaction of the inner layer. The process of directly drilling piles into the already solidified and dense inner impermeable body allows the pile concrete to form a tight bond with the inner impermeable body, fundamentally eliminating the problem of easy leakage at the pile-soil interface in traditional processes. This method uses environmentally friendly and readily available materials, is convenient and efficient in construction, and can quickly achieve karst cave sealing and pollution prevention. It is suitable for pile foundation construction scenarios with stringent pollution prevention requirements, such as ecologically sensitive areas and water source protection zones in karst regions, and has broad engineering application value. Attached Figure Description

[0060] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0061] Figure 1 This is a schematic diagram of the structure of the double-layer barrier and pile foundation after pile formation provided in an embodiment of the present invention;

[0062] Figure 2 This is a schematic diagram of the structure of the double-layer barrier before pile formation provided in an embodiment of the present invention;

[0063] Figure 3 This is a schematic diagram of the connection between the outer structure and the steel casing provided in an embodiment of the present invention;

[0064] Attached reference numerals: 01, pile foundation; 02, inner impermeable layer; 03, outer structure; 04, steel casing. Detailed Implementation

[0065] Exemplary embodiments of the present disclosure will now be described in more detail with reference to the accompanying drawings. While exemplary embodiments of the present disclosure are shown in the drawings, it should be understood that the present disclosure may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art.

[0066] In the following description, certain specific details are set forth for the purpose of illustrating various disclosed embodiments in order to provide a thorough understanding of the various disclosed embodiments. However, those skilled in the art will recognize that embodiments may be practiced without one or more of these specific details. In other instances, well-known apparatuses, structures, and techniques associated with this application may not have been shown or described in detail to avoid unnecessarily obscuring the description of the embodiments.

[0067] Throughout this specification, references to "an embodiment" or "an embodiment" indicate that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Therefore, the appearance of "in an embodiment" or "an embodiment" in various places throughout the specification does not necessarily refer to the same embodiment. Furthermore, a particular feature, structure, or characteristic may be combined in any manner in one or more embodiments.

[0068] In the following description, in order to clearly demonstrate the structure and working method of the present invention, a number of directional terms will be used. However, terms such as "front", "back", "left", "right", "outside", "inside", "outward", "inward", "up", and "down" should be understood as convenient terms and not as limiting terms.

[0069] Please see Figures 1 to 3This invention proposes a method for treating and preventing pollution in beaded karst caves in cable-stayed bridge pile foundations. The method comprises pile foundation 01, an inner impermeable layer 02, and an outer structural layer 03, arranged sequentially from the inside out. It aims to solve the problems of incomplete sealing, easy pollution diffusion, poor material compatibility, and environmental and economic imbalances in traditional beaded karst cave treatment processes. The method achieves a collaborative operation throughout the entire process, including precise material preparation, double-layer barrier molding, and comprehensive quality control. The core structure of this method is a double-layer barrier consisting of an inner impermeable and crack-resistant layer and an outer erosion-resistant and foundation-stabilizing layer. It is combined with a steel casing 04 auxiliary construction system and a material preparation system. Through a step-by-step molding process, it adapts to the complex conditions of beaded karst caves with strong connectivity and irregular spatial patterns. Simultaneously, it relies on the resource utilization of sawdust solid waste and precise material proportioning to balance pollution prevention and green construction requirements.

[0070] Example 1:

[0071] Specifically, the designed pile diameter D0 is 2m, and the treatment is carried out at one pile location that passes through the beaded karst cave.

[0072] To address this issue, the method for treating and preventing pollution from backfilling beaded karst caves in the karst pile foundation of this cable-stayed bridge includes the following steps:

[0073] S1 conducts a survey of the beaded karst caves at the pile location. Based on the survey results, it assesses at least one of the following: water scour intensity, support strength requirements for the outer structure 03, pollutant concentration, and ground deformation risk. Accordingly, it determines the appropriate process parameters for subsequent construction. The assessment of water scour intensity or support strength requirements is related to the process adjustment of the first backfill material; the assessment of pollutant concentration or ground deformation risk is related to the process adjustment of the second backfill material.

[0074] Process adjustments include:

[0075] If the water flow scouring intensity is not lower than the strong scouring threshold, the mixing time of the first backfill material shall be extended and / or an interface agent shall be added to ensure that the large stones are tightly bonded to other components.

[0076] If the required support strength is not lower than the strength threshold, then optimize the vibration frequency and cement hydration curing conditions to improve the density of the outer layer material and adapt to the erosion resistance and structural stability requirements under different working conditions.

[0077] If the pollutant concentration is not lower than the high concentration threshold, the montmorillonite should be pre-hydrated before preparation to enhance its impermeability.

[0078] If there is a risk of ground deformation, the sawn timber surface is dried and cleaned before preparation to improve crack resistance and meet the needs of pollution prevention and crack resistance under different working conditions.

[0079] Specifically, a combination of drilling and in-hole radar was used to explore the cave, which extends longitudinally for approximately 15 meters and consists of three cavities with diameters of 3-5 meters connected by narrow pipes. The cave floor has perennial water flow. A flow meter was used to measure the water flow erosion intensity; the average flow velocity inside the cave was 0.6 m / s, exceeding the preset strong erosion threshold of 0.5 m / s. Water samples were taken for pollutant concentration testing; the suspended solids (SS) concentration reached 1200 mg / L, far exceeding environmental control standards, exceeding the preset high concentration threshold. The geological report showed that the cave roof rock mass was relatively fractured (RQD=55%), indicating a risk of stratum deformation. Calculations showed that the roof had sufficient safety reserves and no special support requirements, thus the strength threshold was not exceeded.

[0080] Based on the above assessment, it has been determined that the following three adaptation adjustments are required for the construction of this embodiment:

[0081] For strong scouring: extend the mixing time of the first backfill material and add an interface agent.

[0082] For high pollution: Pre-hydrate the montmorillonite in the second backfill material.

[0083] To address the risk of deformation: the sawdust surfaces in the second backfill material are dried and cleaned of impurities.

[0084] S2 selects a section of the karst cave and constructs an outer structure 03 formed by the first backfill material according to the process parameters determined by S1; the first backfill material includes large pieces of stone, clay, stone powder and cement;

[0085] Based on the geological survey data of the beaded karst cave, a relatively narrow section with a width of approximately 4 meters was selected within the cave as the location of the barrier, and a modular steel casing 04 was installed as the outer template. For example... Figure 3 As shown, the inner diameter D1 of steel casing 04 is designed to be 2.4m, and the outer diameter is 2.8m. Its height matches the cross-sectional height of the karst cave. Steel casing 04 adopts a modular, detachable steel structure, assembled from multiple standard steel plates using flange bolts. The steel plates are made of high-strength steel, with a thickness suitable for the lateral pressure requirements during the pouring of the outer layer material, preventing deformation of the casing during pouring. The inner wall of steel casing 04 has a wear-resistant coating to reduce wear on the casing during material pouring; weather-resistant sealing strips are installed at the joints of the inner wall to prevent leakage during the pouring of the outer layer material; and uniformly spaced reinforcing ribs are welded to the outer wall to further enhance the overall rigidity of the casing. After determining the splicing dimensions and installation position of steel casing 04, it is lowered using lifting equipment, and fixed after being calibrated for verticality with a level. After splicing, the sealing and verticality of the casing must be checked to avoid affecting the quality of subsequent construction.

[0086] The first backfill material was prepared according to the decision. Its preparation required meeting the requirements for erosion resistance and structural stability. Large boulders were selected based on high compressive strength and continuous particle size distribution. Weathered and broken stones were removed during screening to form a stable framework to resist erosion from the water flow within the cave. Clay and stone powder were used synergistically as a binding component. Clay filled the gaps between the boulders and improved the overall integrity of the material, while stone powder optimized the particle size distribution and reduced shrinkage gaps in the outer layer. A small amount of ordinary Portland cement was used as a cementing material to enhance the structural strength and anti-loosening ability of the hardened outer layer, preventing it from falling off under external forces. All four components were mixed using a forced mixing device. During mixing, it was necessary to ensure that the cement was fully hydrated and the stone powder evenly coated the surface of the boulders, without material segregation or insufficient cementation. After mixing, the bonding state of the material was tested to ensure that the large boulders were tightly bonded to the other components, meeting the requirements for subsequent pouring.

[0087] The specific mix ratio of the first backfill material is as follows: 1300 kg of large stone (particle size 60~100 mm, good gradation), 600 kg of clay, 200 kg of stone powder, and 100 kg of cement. A forced mixer was used, with the mixing time extended to 150 seconds. Styrene-butadiene latex, accounting for 3% of the cement mass, was added as an interface agent. Water addition was controlled to maintain the mixture in a low-slump, dry-hard state. The mixture was poured into the steel casing 04 using a layered pouring method. After each layer was poured, it was compacted using an immersion vibrator. The vibration frequency and insertion depth were controlled during vibration to ensure the outer structure 03 was free of voids and pores, forming a stable erosion-resistant skeleton. After pouring, the casing was water-cured for 3 days, keeping the material surface moist until it hardened to sufficient early strength. After curing, the steel casing 04 was dismantled in sections. First, the flange bolts were loosened, and then the steel plates were lifted off one by one using lifting equipment. During dismantling, soft ropes were used to pull the steel plates to avoid collisions with the outer structure 03 and causing structural damage. After dismantling, the steel casing 04, after cleaning residual materials and repairing worn coatings, could be reused for subsequent construction. After dismantling, a cylindrical outer structure 03 with a regular inner wall and a diameter of 2.4m was exposed, with a designed thickness of 0.2m.

[0088] After the outer structure 03 is formed, S3 first cleans away any remaining impurities on the inner wall of the outer structure 03 and checks the smoothness of the inner wall (in conjunction with...). Figure 2 (As shown), if there are depressions, a small amount of outer layer mixed material is used for repair to ensure the inner wall is flat and to avoid gaps formed during the filling of the inner layer material, which could lead to pollution and leakage. Then, the inner layer material preparation system is activated. According to the process parameters determined in S1, the inner layer of the impermeable body 02, formed by the second backfill material, is completely filled and constructed within the area enclosed by the outer structure 03. The second backfill material includes montmorillonite, small pieces of gravel, clay, and sawdust.

[0089] The components of the second backfill material must be mixed strictly according to the specified ratio. Montmorillonite, as the core seepage-proof component, is selected based on its purity and stable expansion performance. Its water-swelling properties fill the gaps in the material, forming a dense seepage barrier and blocking the migration channels of construction pollutants. Sawdust is made from waste fiber material generated during wood processing, screened to remove impurities before use. Its fibrous structure can be evenly dispersed in the material matrix, forming a flexible stress skeleton to disperse the tensile stress generated by minor deformations of the stratum, preventing inner layer cracking. Clay serves as a bonding medium, enhancing the interfacial bonding force between montmorillonite and the sawdust fragments, reducing material delamination. Small sawdust fragments, with a particle size of 30mm, are selected from natural stone with rounded surfaces and no sharp edges, providing basic support for the inner layer while preventing damage to the seepage-proof layer and the formation of leakage channels. All four components must be mixed using a twin-shaft mixing device. During mixing, the speed and time must be controlled to ensure uniform dispersion of montmorillonite and no agglomeration of sawdust fragments, guaranteeing consistent overall seepage prevention and crack resistance of the inner layer. After mixing, the homogeneity of the material must be checked to avoid localized clumping or delamination.

[0090] Specifically, montmorillonite and water are first mixed at a mass ratio of 1:0.8, stirred into a uniform slurry, and allowed to stand for 18 hours to complete the pre-hydration.

[0091] The sawn wood surface is then dried at 100°C until the moisture content is below 8%, and then sieved to remove lumps and impurities.

[0092] After pretreatment, mix 1100 kg of pre-hydrated montmorillonite, 500 kg of small pebbles (20-40 mm in diameter, with rounded surfaces), 300 kg of clay, and 100 kg of treated sawdust according to the mix ratio, and add water to adjust the compaction moisture content to approximately 18%. Then, fill the mixture in layers into the annular cavity inside the outer structure 03, with each layer not exceeding 30 cm in thickness. Use a light plate compactor to repeatedly compact the mixture until the compaction degree is not less than 95%, forming the inner impermeable body 02 tightly attached to the outer layer. Allow it to cure for at least 24 hours to allow it to initially bond and stabilize. After the curing period, use an impermeability tester to test the impermeability performance of the inner impermeable body. Place the probe of the tester against the surface of the inner layer, introduce compressed air, and monitor the pressure change. If the pressure does not drop within the set time, the impermeability is deemed qualified. If there are leakage points, inject a small amount of inner layer material through a grouting pipe to repair them until the impermeability performance meets the standards.

[0093] S4 drills pile holes directly in the solidified inner impermeable body 02 to complete the construction of pile foundation 01.

[0094] like Figure 3, after confirming that the inner anti-seepage body 02 has been cured, a 2m-diameter pile hole is directly drilled at the concentric position by using a rotary drilling rig. The drilling process is smooth and the hole wall is intact without collapse. Subsequently, conventional hole cleaning, placing of steel reinforcement cages, and pouring of underwater concrete are carried out to complete the construction of the pile foundation 01. After the pile is formed, the pile body is completely wrapped by the inner anti-seepage body 02, and the radial thickness of the inner anti-seepage body 02 is 0.2m; the radial thickness of the outer structural body 03 is 0.2m.

[0095] During the construction closing stage, the equipment is removed in the order of from top to bottom and from auxiliary to main: first, the detection instruments and small tools are removed, then the mixing equipment and lifting equipment are removed, and finally the temporary pipelines and supports in the construction site are cleaned up. After all the equipment is removed, it is immediately rinsed clean with clean water. High-pressure water flow is introduced into the inner wall of the mixing kettle to remove the residual materials. The plates of the steel cofferdam 04 are wiped and stored separately by category. The sensor probes are cleaned with soft cloth to avoid the influence of mud residue on the next use. The waste materials generated during construction (such as broken pieces of stones and residual dry ash) are centrally collected. The recyclable parts are transported to the engineering fill area for subgrade backfilling or site leveling to achieve the resource utilization of solid waste; the non-recyclable waste is handed over to the unit designated by the environmental protection department for disposal to avoid environmental pollution. The accumulated water in the construction site is cleaned up. The accumulated water is discharged after sedimentation in the sedimentation tank to prevent the mud from spreading with rainwater. Finally, the site is leveled to restore the topography and geomorphology before construction.

[0096] During the construction period and after the pile is formed, the anti-seepage performance and structural stability of the composite barrier formed by the inner and outer layers are monitored. The monitoring components include an anti-seepage performance detector and a structural strength sensor. The anti-seepage performance detector is used to detect the anti-pollution diffusion effect of the inner anti-seepage body 02, and the structural strength sensor is used to monitor the structural stability of the outer structural body 03 to ensure that the composite barrier meets the construction requirements for preventing pollution diffusion in beaded karst caves. Specifically, an anti-seepage detector is used to detect the anti-seepage performance of the inner anti-seepage body 02. The detector probe is attached to the inner surface, compressed air is introduced, and the pressure change is monitored. If the pressure does not drop within the set time, it is determined that the anti-seepage is qualified; if there are leakage points, a small amount of inner layer material is injected through the supplementary grouting pipe for repair until the anti-seepage performance reaches the standard. The structural strength sensor is installed at its contact surface with the karst cave surrounding rock. By monitoring the strain, stress, and contact pressure of the concrete, it is evaluated whether the outer structural body 03 is in a safe stress state and whether abnormal deformation or stress concentration occurs.

[0097] The continuously monitored data shows that: no obvious water pressure difference is formed within the anti-seepage area, the strain value of the outer structural body 03 is always within the elastic safety range, and the anti-seepage performance and structural stability of the composite barrier both meet the long-term service requirements.

[0098] Example 2:

[0099] Specifically, the designed pile diameter D0 is 2m, and treatment is carried out at another pile position passing through the beaded karst cave.

[0100] To address this issue, the method for treating and preventing pollution from backfilling beaded karst caves in the karst pile foundation of this cable-stayed bridge includes the following steps:

[0101] S1 conducts a survey of the beaded karst caves at the pile location. Based on the survey results, it assesses at least one of the following: water scour intensity, support strength requirements for the outer structure 03, pollutant concentration, and ground deformation risk. Accordingly, it determines the appropriate process parameters for subsequent construction. The assessment of water scour intensity or support strength requirements is related to the process adjustment of the first backfill material; the assessment of pollutant concentration or ground deformation risk is related to the process adjustment of the second backfill material.

[0102] Process adjustments include:

[0103] If the water flow scouring intensity is not lower than the strong scouring threshold, the mixing time of the first backfill material shall be extended and / or an interface agent shall be added to ensure that the large stones are tightly bonded to other components.

[0104] If the required support strength is not lower than the strength threshold, then optimize the vibration frequency and cement hydration curing conditions to improve the density of the outer layer material and adapt to the erosion resistance and structural stability requirements under different working conditions.

[0105] If the pollutant concentration is not lower than the high concentration threshold, the montmorillonite should be pre-hydrated before preparation to enhance its impermeability.

[0106] If there is a risk of ground deformation, the sawn timber surface is dried and cleaned before preparation to improve crack resistance and meet the needs of pollution prevention and crack resistance under different working conditions.

[0107] Specifically, a combination of drilling and in-hole radar was used to explore the cave, which was found to be dry with no perennial water flow, but the cave roof was relatively thin and the rock mass integrity was moderate. There was no stable water flow inside the cave, only seasonal seepage. The water erosion intensity was determined to be far below the strong erosion threshold; the environmental background was good, and the pollutant concentration was determined not to reach the high concentration threshold; the RQD value of the roof rock mass was 60%, and the cave span was relatively large. A risk of stratum deformation was determined; due to the thin roof and the large overburden load, calculations required the outer structure 03 to provide higher additional support. It was determined that the required support strength was higher than the preset strength threshold.

[0108] Based on the above assessment, it has been determined that the following two adaptation adjustments are required for the construction of this embodiment:

[0109] To address the high support strength requirements: optimize the vibration frequency of the first backfill material and the cement hydration curing conditions;

[0110] To address the risk of ground deformation: the sawn timber surfaces in the second backfill material were dried and cleaned of impurities;

[0111] Due to the absence of strong water flow and high pollution risk, the corresponding optimization process will not be initiated, thus saving costs and construction time.

[0112] S2 selects a section of the karst cave and constructs an outer structure 03 formed by the first backfill material according to the process parameters determined by S1; the first backfill material includes large pieces of stone, clay, stone powder and cement;

[0113] Based on the geological survey data of the beaded karst cave, a relatively narrow section with a width of approximately 4m was selected within the cave as the location of the barrier, and a modular steel casing 04 was installed as the outer template. The inner diameter D1 of the steel casing 04 was designed to be 2.4m, the outer diameter to be 2.8m, and the height to match the cross-sectional height of the karst cave.

[0114] Using the same basic mix ratio as in Example 1 (large stone: clay: stone powder: cement = 13:6:2:1), but without adding an interface agent, and using standard mixing time, during pouring, strict layering (thickness ≤ 40cm) and high-frequency immersion vibration are implemented to ensure extremely high density without the aid of an interface agent. After pouring, immediate moisturizing and covering curing are implemented, with an ambient humidity > 90%, and the curing period is extended to 10 days to fully develop the later strength of the concrete and meet the high support requirements. After curing, the formwork is removed to form a high-quality outer structure 03.

[0115] After the outer structure 03 is formed (S3), first clean the residual impurities on the inner wall of the outer structure 03 and check the flatness of the inner wall. If there are depressions, repair them with a small amount of outer layer mixed material to ensure the inner wall is flat and avoid gaps that could lead to pollution and leakage during the filling of the inner layer material. Then, start the inner layer material preparation system. According to the process parameters determined in S1, completely fill and construct the inner layer impermeable body 02 formed by the second backfill material in the internal area enclosed by the outer structure 03. The second backfill material includes montmorillonite, small pieces of stone, clay, and sawdust.

[0116] Specifically, montmorillonite is not pre-hydrated. The sawn timber is deeply dried at 105°C until the moisture content is below 7%, and then strictly sieved to maximize its crack resistance. The same second backfill material base ratio as in Example 1 is used, and the deeply treated sawn timber is mixed and compacted. A heavy-duty hand roller is used for layered compaction, with a compaction degree of not less than 96%, forming an inner impermeable body 02 of the designed thickness.

[0117] S4 drills pile holes directly in the solidified inner impermeable body 02 to complete the construction of pile foundation 01.

[0118] In the solidified inner impermeable body 02, pile holes are drilled using the same process as in Example 1 to complete the construction of pile foundation 01.

[0119] During construction and after pile completion, S5 monitors the seepage prevention performance and structural stability of the composite barrier formed by the inner and outer layers. The monitoring components include a seepage prevention performance tester and a structural strength sensor. The seepage prevention performance tester is used to test the pollution diffusion prevention effect of the inner seepage barrier 02, and the structural strength sensor is used to monitor the structural stability of the outer structure 03, ensuring that the composite barrier meets the construction requirements for pollution diffusion prevention in beaded karst caves.

[0120] The monitoring focus of this embodiment is on structural stress. The data shows that the stress distribution of the outer structure 03 is uniform and within a safe range after the pile foundation 01 is loaded, and the inner seepage prevention body 02 is intact and without cracks.

[0121] Those skilled in the art should understand that this invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to this invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A method for treating and preventing pollution from backfilling karst caves in karst pile foundations of cable-stayed bridges, characterized in that... Includes the following steps: S1 conducted a survey of the beaded karst caves at the pile location, assessed the risk factors affecting pollution prevention and structural stability based on the survey results, and determined the process parameters for subsequent construction accordingly. S2 selects a section of the karst cave and constructs an outer structure formed by the first backfill material according to the process parameters determined by S1; the first backfill material includes large pieces of stone, clay, stone powder and cement; After the outer structure is formed, according to the process parameters determined in S1, the inner impermeable body formed by the second backfill material is completely filled and constructed in the internal area enclosed by the outer structure. The second backfill material includes montmorillonite, small stones, clay and sawdust. S4 drills pile holes directly in the solidified inner impermeable body to complete the pile foundation construction.

2. The method for treating and preventing pollution from backfilling beaded karst caves in cable-stayed bridge pile foundations according to claim 1, characterized in that, In step S1, the risk factors assessed include at least one of the following: water scour intensity, support strength requirements for the outer structure, contaminant concentration, and formation deformation risk; wherein, the assessment of water scour intensity or support strength requirements is related to process adjustments to the first backfill material; the assessment of contaminant concentration or formation deformation risk is related to process adjustments to the second backfill material.

3. The method for treating and preventing pollution from backfilling beaded karst caves in cable-stayed bridge pile foundations according to claim 2, characterized in that, The process adjustments include: If the water flow scouring intensity is not lower than the strong scouring threshold, then extend the mixing time of the first backfill material and / or add an interface agent; If the required support strength is not lower than the strength threshold, then optimize the vibration frequency and cement hydration curing conditions. If the pollutant concentration is not lower than the high concentration threshold, the montmorillonite is pre-hydrated before preparation. If there is a risk of geological deformation, the sawn timber surface should be dried and cleaned before preparation.

4. The method for treating and preventing pollution from backfilling beaded karst caves in cable-stayed bridge pile foundations according to claim 3, characterized in that, In the process adjustment of the first backfill material, the mixing time of the first backfill material is extended to control the mixing time to not less than 120s; the interface agent is added to add organic polymer emulsion or cement-based binder accounting for 1% to 5% of the cement mass; the optimized vibration frequency and cement hydration curing conditions are layered, multiple, immersion vibration, and the humidity of the curing environment is controlled to not less than 90%.

5. The method for treating and preventing pollution from backfilling beaded karst caves in cable-stayed bridge pile foundations according to claim 3, characterized in that, In the process adjustment of the second backfill material, the pre-hydration treatment is to mix montmorillonite with water and let it stand for aging, wherein the amount of water added is 50% to 100% of the mass of montmorillonite, and the standing aging time is 12 to 24 hours; the drying and impurity removal treatment is to dry the sawn wood surface at 80 to 110°C until the moisture content is less than 10%, and to remove the lumps and impurities therein.

6. The method for treating and preventing pollution from backfilling beaded karst caves in cable-stayed bridge pile foundations according to claim 1, characterized in that, In step S2, a modular steel casing is used as a template to construct the outer structure; the mass ratio of the large stone, the clay, the stone powder and the cement is 12~14:5~7:1~3:

1.

7. The method for treating and preventing pollution from backfilling beaded karst caves in cable-stayed bridge pile foundations according to claim 1, characterized in that, In step S3, the inner impermeable body is formed by backfilling in layers and compacting the second backfill material; the mass ratio of the montmorillonite, the small stone pieces, the clay and the sawn wood surface is 10~12:4~6:2~4:

1.

8. The method for treating and preventing pollution from backfilling beaded karst caves in cable-stayed bridge pile foundations according to claim 1, characterized in that, The large stones in the first backfill material have a particle size of 60-100mm and are continuously graded; the small stones in the second backfill material have a particle size of 20-40mm and are treated to have no sharp edges.

9. The method for treating and preventing pollution from backfilling beaded karst caves in cable-stayed bridge pile foundations according to claim 1, characterized in that, After pile construction, the radial thickness of the inner impermeable body is 0.1~0.3m; the radial thickness of the outer structure is 0.1~0.3m.

10. The method for treating and preventing pollution from backfilling beaded karst caves in cable-stayed bridge pile foundations according to claim 9, characterized in that, The compaction degree of the inner impermeable body in step S3 shall not be less than 95%.

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