Roadbed structure suitable for large-span thin roof karst cave and construction method thereof

Abstract

According to the roadbed structure suitable for the large-span thin-top-plate karst cave and the construction method of the roadbed structure, the variable-diameter concrete cast-in-place pile group with the thick upper portion and the thin lower portion is adopted, the upper portions of the pile bodies are tightly bonded with the karst cave top plate through concrete, and part of roadbed loads are transferred to the pile bodies from the top plate through pile side friction resistance; and finally, the pile end embedded into the bottom of the karst cave transmits to a stable rock stratum at the bottom of the karst cave. Compared with a traditional method, a force transmission path is simplified, a force transmission cross beam does not need to be arranged, the structure is simpler, according to the construction method, the forming quality of the variable cross-section pile and reliable bonding of the variable cross-section pile and the top plate are ensured through the steps of drilling, lowering an outer mold with a blocking piece, cleaning the hole wall, pouring concrete and the like, and the adaptability to karst cave geological conditions is enhanced. The bearing capacity of the karst cave top plate and the deep bearing capacity of the pile foundation are comprehensively utilized, and the problems of insufficient bearing capacity and potential stability hazards of roadbeds of the road sections are economically and effectively solved.

Description

Technical Field

[0001] This invention relates to the field of road engineering technology, specifically to roadbed structures and their construction methods. Background Technology

[0002] Karst caves are underground spaces formed by karst processes, with spans ranging from a few meters to hundreds of meters. Caves with spans exceeding 15 meters are generally considered large-span caves. Large-span caves with thin roofs, especially those with relatively thin roofs, significantly weaken the bearing capacity of the foundation, easily leading to roadbed collapse, uneven settlement, and other safety accidents. In my country, the distribution of karst caves highly overlaps with mountainous areas. Due to the constraints of mountainous terrain, an increasing number of highways need to cross large-span caves with thin roofs, posing a significant potential threat to roadbed construction and operation.

[0003] Currently, the main methods for addressing the threat posed by karst caves to highway subgrade stability fall into two categories: grouting and pile foundations. Grouting solutions involve filling the interior space of the karst cave with concrete or other materials through grouting, or reinforcing the cave's roof slab through grouting. The former requires a large amount of concrete, resulting in high costs and is unsuitable for large-span karst caves; the latter has limited reinforcement effects and is unsuitable for karst caves with thin roofs. Pile foundation solutions transfer the vertical load of the subgrade to the bottom of the karst cave through pile foundations, reducing the vertical load borne by the cave's roof slab and thus preventing subgrade collapse and uneven settlement. Pile foundation solutions offer simple structure, clear force transmission, and good treatment results. Existing pile foundation schemes typically involve setting up force-transfer beams at the bottom of the subgrade before laying piles, resulting in complex force transmission paths, high construction difficulty, and failure to utilize the bearing capacity of the cave's roof slab, leading to high construction costs. Summary of the Invention

[0004] To address the problem of reinforcing large-span thin-roof karst caves as described in the background art: grouting solutions involve large amounts of concrete, are costly, and are unsuitable for large-scale karst caves, or have limited reinforcement effects and are unsuitable for thin-roofed karst caves; existing pile foundation solutions have complex force transmission paths, are difficult to construct, and do not utilize the bearing capacity of the karst cave roof, resulting in high construction costs; this application provides a roadbed structure and its construction method suitable for large-span thin-roof karst caves. This roadbed structure can transfer the vertical load of the roadbed to the piles and the bottom of the karst cave through the pile side friction between the karst cave roof and the cast-in-place piles, thereby greatly improving the roadbed bearing capacity and solving the threat posed by large-span thin-roof karst caves to the stability of highway roadbeds.

[0005] The first aspect of this application provides a construction method for roadbed structures suitable for large-span, thin-roofed karst caves, including: Below the roadbed, boreholes are drilled from top to bottom from the top of the cave to a certain depth at the bottom of the cave, wherein the borehole diameter within the cave top area is larger than the borehole diameter within the cave cavity and the cave bottom area; The outer mold is hoisted into the borehole, with the lower part of the outer mold inserted into the borehole within the bottom area of ​​the cave and the upper part of the outer mold placed in the borehole within the top area of ​​the cave; wherein the outer diameter of the outer mold is equal to the diameter of the borehole within the cave cavity and the bottom area of ​​the cave. Clean the borehole walls within the area of ​​the cave ceiling; A barrier is installed on the top outer side of the outer mold to abut against the inner wall of the drilled hole in the top plate of the sinkhole; Concrete is poured into the borehole to form a concrete pile with a variable diameter that is thicker at the top and thinner at the bottom. The upper part of the concrete pile is tightly bonded to the top of the cave, and the lower part of the concrete pile is embedded in the bottom of the cave cavity. Repeat the above steps to construct multiple concrete piles under the roadbed to form a pile group foundation.

[0006] Furthermore, based on parameters including the size, shape, extent, roof thickness, and strength of the karst cave, and considering the roadbed load conditions, the layout scheme of the cast-in-place concrete piles is determined, and the pile locations are identified. Since the pile locations are below the roadbed, holes are drilled from top to bottom from the karst cave roof to a certain depth at the cave bottom. The arrangement of the cast-in-place concrete piles can be either a staggered or matrix pattern, depending on the specific circumstances.

[0007] Furthermore, the concrete cast-in-place pile comprises two parts: the pile body and the pile top enlargement section. The diameter of the pile top enlargement section is 1.2-2.0 times the diameter D of the pile body. The top of the pile top enlargement section is flush with the top of the karst cave ceiling, the side of the pile top enlargement section is bonded to the karst cave ceiling, and the bottom of the pile top enlargement section is flush with the bottom of the karst cave ceiling and connected to the pile body. The top of the pile body is connected to the pile top enlargement section, and the bottom of the pile body is embedded in the bottom rock of the karst cave cavity.

[0008] Furthermore, the depth of the bottom rock embedded in the karst cavity at the bottom of the pile section shall not be less than 5D.

[0009] Furthermore, the inner diameter of the outer mold is the same as the diameter D of the pile section, and the length of the outer mold is slightly longer than the length of the pile section.

[0010] Furthermore, an outer mold top cover is provided at the top of the outer mold. The outer mold top cover is conical in shape, with a semi-apex angle of 60°-75°. When cleaning the borehole walls within the area of ​​the cave ceiling, the barrier on the outer side of the top of the outer mold does not abut against the inner wall of the borehole in the cave ceiling. The outer mold top cover covers the top of the outer mold, and the conical outer mold top cover guides the cleaning wastewater and debris into the cave cavity along the perimeter of the outer mold.

[0011] Furthermore, an inflatable sealing ring is bonded to the top outer side of the outer mold. When the inflatable sealing ring is not inflated, it collapses and contracts, covering the top of the outer mold and cleaning the borehole walls within the cave ceiling area. The resulting cleaning wastewater and debris enter the cave cavity from around the outer mold. Then, the inflatable sealing ring is inflated and expands, abutting against the inner wall of the borehole in the cave ceiling, separating the internal space of the outer mold from the cave cavity, and then concrete is poured into the borehole.

[0012] An inflatable sealing ring and an outer mold top cover are installed on the top of the outer mold, which can flexibly adjust the connection and separation of the space inside the mold and the space of the karst cave, ensuring the orderly discharge of cleaning wastewater and debris, and the effective pouring of concrete, thus ensuring the quality of the cast-in-place pile.

[0013] Furthermore, multiple positioning protrusions are provided below the outer mold top cover. When the outer mold top cover is placed on top of the outer mold, the positioning protrusions are fitted with the outer mold with a clearance, thereby ensuring that the outer mold top cover is located in the center position of the outer mold.

[0014] Furthermore, a top cover hook is installed on the outer top of the outer mold top cover. Before cleaning the borehole wall within the area of ​​the karst cave top plate, the top cover of the outer mold is suspended on top of the outer mold using the top cover hook. After cleaning is completed and before lowering the steel cage, the top cover of the outer mold is removed from the top of the outer mold using the top cover hook.

[0015] Based on the above construction method, the second aspect of this application provides a roadbed structure suitable for large-span thin-roof karst caves, including a roadbed with a plurality of concrete cast-in-place piles arranged below it; the concrete cast-in-place piles have a variable diameter structure with a thicker upper section and a thinner lower section, and the diameter of the concrete cast-in-place piles within the karst cave roof area is larger than the diameter of the concrete cast-in-place piles within the karst cave cavity and the karst cave bottom area; the upper concrete of the concrete cast-in-place piles is tightly bonded to the karst cave roof, and the lower part of the concrete cast-in-place piles is embedded in the bottom of the karst cave cavity.

[0016] Compared with the prior art, this application has the following advantages: (1) Clear and efficient load transfer path: By tightly bonding the upper part (pile top enlargement section) of the concrete cast-in-place pile to the top plate of the karst cave, the pile side friction generated by the pile-rock interface is used to transfer part of the roadbed load that was originally directly acting on the top plate of the karst cave to the pile body; the load is then transferred down the pile body to the stable rock bearing layer at the bottom of the karst cave, forming a clear load transfer path of "top plate (friction transfer) → pile body → pile end (bearing layer)", which not only utilizes the bearing capacity of the top plate, but also effectively transfers most of the load to the stable rock layer at the bottom of the karst cave, thereby significantly improving the overall bearing capacity and stability of the roadbed; (2) Simplified structure and cost optimization: Compared with the complex structure of traditional pile foundation schemes that require setting up force transmission beams at the bottom of the roadbed before laying piles, the scheme of this application directly bonds the variable diameter cast-in-place piles to the top plate of the karst cave, eliminating the need for traditional force transmission beams and other intermediate force transmission components, making the substructure of the roadbed simpler; this not only reduces the complexity of the structure and the amount of materials used, but also simplifies the construction process, which helps to reduce the difficulty of construction and the cost of the project. (3) Strong construction feasibility and adaptability: The construction method of "drilling first, then lowering the outer mold, and then pouring" is adopted, and the pouring form and quality of different sections are controlled by the outer mold and the top barrier. This process can ensure that in the special underground space of the karst cavity (whether or not there is filling material), a variable cross-section cast-in-place pile with a large upper diameter and a small lower diameter is formed. After drilling, the borehole wall of the karst top plate section is cleaned, which enhances the bonding strength between concrete and rock mass and ensures the quality of key force transmission interface. This application has strong adaptability to the internal conditions of the karst cavity and expands the applicability of the roadbed structure under complex geological conditions.

[0017] (4) Effectively addressing the core risks of large-span thin-roof karst caves: In response to the core problems of low bearing capacity and easy collapse or uneven settlement of the foundation of large-span thin-roof karst caves, this application supports the roadbed through a "group pile" of multiple concrete cast-in-place piles; the design of the variable diameter concrete cast-in-place piles increases the contact area of ​​the piles at the top slab, enhancing the load transfer capacity; the pile body penetrates the karst cave cavity and transfers the load to the bottom stable rock layer, fundamentally avoiding safety accidents caused by insufficient strength or failure of the top slab itself; this application directly and specifically improves the long-term stability and safety of the roadbed crossing such adverse geological bodies.

[0018] In summary, this application employs a variable-diameter cast-in-place concrete pile group with a thicker upper section and a thinner lower section. The upper part of the pile body is tightly bonded to the top slab of the karst cave through concrete. The pile's side friction transfers part of the roadbed load from the top slab to the pile body, ultimately transmitting it to the stable rock bearing layer at the bottom of the karst cave. Compared to traditional methods, this application simplifies the force transmission path, eliminates the need for force-transfer beams, and has a simpler structure. Its construction method, through drilling, lowering an external mold with obstructions, cleaning the borehole walls, and pouring concrete, ensures the forming quality of the variable-section piles and reliable bonding with the top slab, enhancing adaptability to karst cave geological conditions. This application comprehensively utilizes the residual bearing capacity of the karst cave top slab and the deep bearing capacity of the pile foundation, aiming to economically and effectively solve the problems of insufficient bearing capacity and potential stability hazards in roadbeds in such sections. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of this application, 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 of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 This is a schematic cross-sectional view of the roadbed structure in this application. Figure 2 This is a schematic diagram of the concrete cast-in-place pile and external formwork structure in this application. Figure 3 This is a schematic diagram of the upper part of the mold in this application and its connection with the drilled hole in the top plate of the karst cave. Explanation of reference numerals in the attached drawings: 1-cave cavity, 2-cave roof, 3-rock mass, 4-roadbed, 5-drilled pile, 6-pile body section, 7-pile top enlargement section, 8-outer mold, 9-inflatable sealing ring, 10-outer mold top cover, 11-positioning protrusion, 12-top cover hook. Detailed Implementation

[0021] In the following description, specific details such as particular system architectures and techniques are set forth for illustrative purposes and not for limitation, in order to provide a thorough understanding of the embodiments of this application. However, those skilled in the art will understand that this application may also be implemented in other embodiments without these specific details. In other instances, detailed descriptions of well-known systems, apparatuses, circuits, and methods have been omitted so as not to obscure the description of this application with unnecessary detail.

[0022] It should be noted that when a component is referred to as being "fixed to" or "set on" another component, it can be directly on or indirectly on that other component. When a component is referred to as being "connected to" another component, it can be directly connected to or indirectly connected to that other component.

[0023] It should be understood that the terms "length", "width", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this application.

[0024] It should be understood that, when used in this application specification and the appended claims, the term "comprising" indicates the presence of the described features, integrals, steps, operations, elements and / or components, but does not exclude the presence or addition of one or more other features, integrals, steps, operations, elements, components and / or a collection thereof.

[0025] It should also be understood that the term “and / or” as used in this application specification and the appended claims means any combination of one or more of the associated listed items and all possible combinations, and includes such combinations.

[0026] Furthermore, in the description of this application and the appended claims, the terms "first," "second," "third," etc., are used only to distinguish descriptions and should not be construed as indicating or implying relative importance.

[0027] References to "one embodiment" or "some embodiments" as described in this specification mean that one or more embodiments of this application include a specific feature, structure, or characteristic described in connection with that embodiment. Therefore, the phrases "in one embodiment," "in some embodiments," "in other embodiments," "in still other embodiments," etc., appearing in different parts of this specification do not necessarily refer to the same embodiment, but rather mean "one or more, but not all, embodiments," unless otherwise specifically emphasized. The terms "comprising," "including," "having," and variations thereof mean "including but not limited to," unless otherwise specifically emphasized.

[0028] like Figure 1 As shown in the embodiment of this application, a roadbed structure suitable for large-span thin-roof karst caves is provided, including a roadbed 4, with multiple cast-in-place concrete piles 5 arranged below the roadbed 4; the upper concrete of the cast-in-place concrete piles 5 (the expanded diameter section at the pile top) is tightly bonded to the karst cave roof 2, and the lower part of the cast-in-place concrete piles 5 (the pile body section) is embedded in the bottom of the karst cave cavity 2. Specifically, the cast-in-place concrete piles 5 have a variable diameter structure that is thicker at the top and thinner at the bottom, with the diameter of the expanded diameter section 7 at the pile top within the karst cave roof 2 being larger than the diameter of the pile body section 6 within the karst cave cavity 1 and the karst cave bottom. This roadbed structure can transfer the vertical load of the roadbed through the pile side friction between the karst cave roof 2 and the expanded diameter section 7 to the pile body section 6 and the stable rock layer at the bottom of the karst cave, thereby greatly improving the bearing capacity of the roadbed and solving the threat posed by large-span thin-roof karst caves to the stability of the highway roadbed.

[0029] In one embodiment, such as Figure 2As shown, the concrete cast-in-place pile 5 consists of two parts: the pile body section 6 and the pile top enlargement section 7. The diameter D of the pile body section 6 is calculated and determined based on factors such as the roadbed load, pile length, and the stability requirements of the compression member. The diameter of the pile top enlargement section 7 is 1.2-2.0 times the diameter D of the pile body section 6, and its specific dimensions are calculated and determined based on the roadbed load and the bond strength between the cast-in-place pile and the top plate of the karst cave.

[0030] In one embodiment, such as Figure 1 and Figure 2 As shown, the top of the pile top enlargement section 7 is flush with the top of the karst cave roof 2, the side of the pile top enlargement section 7 is bonded to the karst cave roof 2, the bottom of the pile top enlargement section 7 is flush with the bottom of the karst cave roof 2 and connected to the pile body section 6; the top of the pile body section 6 is connected to the pile top enlargement section 7, and the bottom of the pile body section 6 is embedded in the bottom rock of the karst cave cavity 2, with an embedding depth of not less than 5D.

[0031] To prepare the aforementioned roadbed structure, embodiments of this application provide a construction method for roadbed structures suitable for large-span, thin-roofed karst caves, including: Below the roadbed 4, boreholes are drilled from top to bottom from the top plate 2 of the karst cave to a certain depth at the bottom of the karst cave. The diameter of the borehole within the range of the top plate 2 of the karst cave is larger than the diameter of the borehole within the range of the karst cave cavity 1 and the bottom of the karst cave. The outer mold 8 is hoisted into the borehole, with the lower part of the outer mold 8 inserted into the borehole within the bottom range of the cave, and the upper part of the outer mold 8 placed in the borehole within the range of the top plate 2 of the cave; wherein the outer diameter of the outer mold 8 is equal to the diameter of the borehole within the range of the cave cavity 1 and the bottom range of the cave. Clean the borehole walls within the area of ​​the cave ceiling 2; A barrier is provided on the top outer side of the outer mold 8 to abut against the inner wall of the drilled hole of the cave top plate 2; Concrete is poured into the borehole to form a concrete pile 5 with a variable diameter that is thicker at the top and thinner at the bottom. The upper part of the concrete pile 5 is tightly bonded to the top plate 2 of the karst cave, and the lower part of the concrete pile 5 is embedded in the bottom of the karst cave cavity 2. Repeat the above steps to prepare multiple concrete piles 5 under the roadbed 4 to form a pile group foundation.

[0032] In practical applications, based on parameters including the size, shape, extent, roof thickness, and strength of the karst cave, and considering the load conditions of the roadbed 4, the arrangement of the concrete-filled piles 5 is determined, and the pile locations are identified. Since the piles are located below the roadbed 4, holes are drilled from top to bottom from the karst cave roof 2 to a certain depth at the bottom of the cave. The arrangement of the concrete-filled piles 5 can be either a staggered or matrix pattern, depending on the specific circumstances.

[0033] In one embodiment, such as Figure 2As shown, the concrete cast-in-place pile 5 comprises two parts: a pile body section 6 and a pile top enlarged diameter section 7. The diameter of the pile top enlarged diameter section 7 is 1.2-2.0 times the diameter D of the pile body section 6. The top of the pile top enlarged diameter section 7 is flush with the top of the karst cave roof 2, the side of the pile top enlarged diameter section 7 is bonded to the karst cave roof 2, and the bottom of the pile top enlarged diameter section 7 is flush with the bottom of the karst cave roof 2 and connected to the pile body section 6. The top of the pile body section 6 is connected to the pile top enlarged diameter section 7, and the bottom of the pile body section 6 is embedded in the bottom rock of the karst cave cavity 2, with an embedding depth of not less than 5D. Therefore, in order to meet the above structural requirements of the concrete cast-in-place pile 5, the inner diameter of the outer mold 8 is the same as the diameter D of the pile body section, and the length of the outer mold 8 is slightly longer than that of the pile body section. That is, after the outer mold 8 is installed in place, its bottom is flush with the bottom of the borehole, and its top is slightly higher than the bottom of the karst cave roof 2.

[0034] In one embodiment, such as Figure 3 As shown, an outer mold top cover 10 is provided at the top of the outer mold 8. The outer mold top cover 10 is conical in shape, and the semi-apex angle of the cone is 60°-75°. When cleaning the borehole wall within the range of the karst cave top plate 2, the barrier on the outer side of the top of the outer mold 8 does not abut against the inner wall of the borehole in the karst cave top plate 2. The outer mold top cover 10 covers the top of the outer mold 8. The conical outer mold top cover 10 guides the cleaning wastewater and debris into the karst cave cavity 1 along the perimeter of the outer mold, thereby preventing the cleaning wastewater and debris from entering the outer mold 8 and avoiding affecting the quality of the concrete cast-in-place pile 5.

[0035] In one embodiment, such as Figure 3 As shown, an inflatable sealing ring 9 is bonded to the outer top of the outer mold 8. When the inflatable sealing ring 9 is not inflated, it collapses and contracts, covering the top of the outer mold 8 with the top cover 10. The borehole walls within the area of ​​the cave ceiling 2 are cleaned, and the resulting cleaning wastewater and debris enter the cave cavity 1 from all sides of the outer mold 8. Then, the inflatable sealing ring 9 is inflated and expands, and the inflatable sealing ring 9 abuts against the inner wall of the borehole in the cave ceiling 2, separating the internal space of the outer mold 8 from the cave cavity 1. This prevents concrete from flowing into the cave cavity 2 during concrete pouring, and then concrete is poured into the borehole.

[0036] In one embodiment, such as Figure 3 As shown, multiple positioning protrusions 11 are provided below the outer mold top cover 10. When the outer mold top cover 10 is placed on top of the outer mold 8, the positioning protrusions 11 are in clearance fit with the outer mold 8, thereby ensuring that the outer mold top cover 10 is located at the center of the outer mold 8. Specifically, four positioning protrusions are provided below the outer mold top cover 10.

[0037] In one embodiment, such as Figure 3As shown, a top cover hook 12 is provided on the top outer side of the outer mold top cover 10. Before cleaning the borehole walls within the area of ​​the karst cave top plate 2, the outer mold top cover 10 is suspended on top of the outer mold 8 using the top cover hook 12. After cleaning and before lowering the reinforcing cage, the outer mold top cover 10 is removed from the top of the outer mold 8 using the top cover hook 12. Specifically, a top cover hook 12 is provided on the top outer side of the outer mold top cover 10.

[0038] Example 1 Geological survey methods were used to determine the basic parameters of the karst cave, such as its size, shape, range, thickness and strength of the cave roof, and the cave contains filling material.

[0039] Based on the basic parameters such as the size, shape, range, thickness and strength of the cave roof, and the roadbed load, the pile foundation layout scheme is calculated and determined, including pile foundation D, diameter of the expanded section at the pile top, pile spacing, layout form (quincunx or matrix), and pile location is determined.

[0040] The construction site was leveled, and mud slurry wall drilling was carried out according to the pile position. The drilling was divided into two stages: in the first stage, a drill bit matching the diameter of the expanded section at the top of the pile was used to drill through the top plate of the karst cave; in the second stage, the drill bit was changed, and a drill bit matching the pile diameter was used to continue drilling to the designed position at the bottom of the karst cave.

[0041] After drilling is completed, the outer mold is hoisted into the borehole to support the borehole wall, replacing the mud slurry's wall protection effect. The mud slurry used to prevent the filling material in the karst cave from collapsing during drilling is then extracted, and the top cover of the outer mold is placed on top of the outer mold.

[0042] The borehole sidewalls within the area of ​​the karst cave roof are flushed to ensure the bonding strength, side friction resistance, and load transfer capacity with the later expanded diameter section of the pile top. During the flushing process, the top cover of the outer mold prevents cleaning wastewater and debris from entering the interior of the outer mold and affecting the quality of the cast-in-place pile.

[0043] After the borehole sidewalls within the top slab area of ​​the karst cave are flushed, the top air sealing ring of the outer mold is inflated to separate the internal space of the outer mold from the cavity of the karst cave. This prevents concrete from flowing into the karst cave and also ensures that the casing is centered, thus ensuring the verticality of the pile.

[0044] Remove the top cover of the outer mold, place the reinforcing cage inside, and pour concrete from the bottom up. After the concrete has solidified, a variable diameter concrete pile with a thicker top and a thinner bottom is formed.

[0045] Repeat the above steps to prepare multiple cast-in-place concrete piles under the roadbed according to the predetermined pile foundation layout, forming a pile group foundation.

[0046] Example 2 Geological survey methods were used to determine the basic parameters of the cave, such as its size, shape, range, thickness and strength of the cave roof. There was no filling material inside the cave.

[0047] Based on the basic parameters such as the size, shape, range, thickness and strength of the cave roof, and the roadbed load, the pile foundation layout scheme is calculated and determined, including pile foundation D, diameter of the expanded section at the pile top, pile spacing, layout form (quincunx or matrix), and pile location is determined.

[0048] The construction site was leveled, and dry drilling was carried out according to the pile location. The drilling was divided into two stages: in the first stage, a drill bit matching the diameter of the expanded section at the top of the pile was used to drill through the top of the karst cave; in the second stage, the drill bit was changed, and a drill bit matching the pile diameter was used to continue drilling to the designed position at the bottom of the pile.

[0049] After drilling is completed, the outer mold is hoisted into the drill hole, and the top cover of the outer mold is placed on top of the outer mold.

[0050] The borehole sidewalls within the area of ​​the karst cave roof are flushed to ensure good adhesion, side friction resistance, and load transfer capacity with the later expanded diameter section of the pile. During flushing, the outer mold cover is used to prevent cleaning wastewater and debris from entering the interior of the outer mold and affecting the quality of the cast-in-place pile.

[0051] After the borehole sidewalls within the top slab area of ​​the karst cave are flushed, the top air sealing ring of the outer mold is inflated to separate the internal space of the outer mold from the cavity of the karst cave. This prevents concrete from flowing into the karst cave and also ensures that the casing is centered, thus ensuring the verticality of the pile.

[0052] A reinforcing cage is placed in the pile, and concrete is poured from the bottom up. After the concrete has solidified, a variable-diameter concrete pile with a thicker top and a thinner bottom is formed.

[0053] Repeat the above steps to prepare multiple cast-in-place concrete piles under the roadbed according to the predetermined pile foundation layout, forming a pile group foundation.

[0054] The above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application, and should all be included within the protection scope of this application.

Claims

1. A construction method for roadbed structures applicable to large-span, thin-roofed karst caves, characterized in that, include: Below the roadbed (4), drill holes from top to bottom from the top plate (2) of the karst cave to a certain depth at the bottom of the karst cave, wherein the diameter of the drill hole within the range of the top plate (2) of the karst cave is greater than the diameter of the drill hole within the range of the karst cave cavity (1) and the bottom of the karst cave; The outer mold (8) is hoisted into the borehole. The lower part of the outer mold (8) is inserted into the borehole within the bottom range of the cave, and the upper part of the outer mold (8) is placed in the borehole within the range of the top plate (2) of the cave. The outer diameter of the outer mold (8) is equal to the diameter of the borehole within the range of the cave cavity (1) and the bottom range of the cave. Clean the borehole walls within the area of ​​the cave ceiling (2); A barrier is provided on the top outer side of the outer mold (8) that can abut against the inner wall of the drill hole of the cave top plate (2); Concrete is poured into the borehole to form a concrete pile (5) with a variable diameter that is thicker at the top and thinner at the bottom. The upper part of the concrete pile (5) is tightly bonded to the top plate (2) of the karst cave, and the lower part of the concrete pile (5) is embedded in the bottom of the karst cave cavity (2). Repeat the above steps to construct multiple concrete piles (5) under the roadbed (4) to form a pile group foundation.

2. The construction method for roadbed structures suitable for large-span thin-roof karst caves according to claim 1, characterized in that: Based on parameters including cave size, cave shape, cave range, cave top plate thickness and cave top plate strength, and combined with the load conditions of the roadbed (4), the arrangement scheme of concrete cast-in-place piles (5) is determined, and the pile positions are determined. Based on the pile positions being below the roadbed (4), holes are drilled from top to bottom from the cave top plate (2) to a certain depth at the bottom of the cave.

3. The construction method for roadbed structures suitable for large-span thin-roof karst caves according to claim 1, characterized in that: The concrete cast-in-place pile (5) includes two parts: the pile body section (6) and the pile top enlargement section (7); the diameter of the pile top enlargement section (7) is 1.2-2.0 times the diameter D of the pile body section (6), the top of the pile top enlargement section (7) is flush with the top of the karst cave roof (2), the side of the pile top enlargement section (7) is bonded to the karst cave roof (2), the bottom of the pile top enlargement section (7) is flush with the bottom of the karst cave roof (2) and connected to the top of the pile body section (6); the top of the pile body section (6) is connected to the pile top enlargement section (7), and the bottom of the pile body section (6) is embedded in the bottom rock of the karst cave cavity (2).

4. The construction method for roadbed structures suitable for large-span thin-roof karst caves according to claim 3, characterized in that: The depth to which the bottom of the pile section (6) is embedded in the bottom rock of the karst cavity (2) is not less than 5D.

5. The construction method for roadbed structures suitable for large-span thin-roof karst caves according to claim 4, characterized in that: The inner diameter of the outer mold (8) is the same as the diameter D of the pile body section, and the length of the outer mold (8) is slightly longer than the length of the pile body section.

6. The construction method for roadbed structures suitable for large-span thin-roof karst caves according to claim 5, characterized in that: The top of the outer mold (8) is provided with an outer mold top cover (10). The outer mold top cover (10) is conical in shape, and the semi-apex angle of the cone is 60°-75°. When cleaning the borehole wall within the range of the cave top plate (2), the barrier on the outer side of the top of the outer mold (8) does not abut against the inner wall of the borehole of the cave top plate (2). The outer mold top cover (10) covers the top of the outer mold (8), and the conical outer mold top cover (10) guides the cleaning wastewater and debris into the cave cavity (1) along the periphery of the outer mold.

7. The construction method for roadbed structures suitable for large-span thin-roof karst caves according to claim 6, characterized in that: An inflatable sealing ring (9) is bonded to the top outer side of the outer mold (8). When the inflatable sealing ring (9) is not inflated, it collapses and contracts, covering the top of the outer mold (8) with the top cover (10). The borehole wall within the range of the cave top plate (2) is cleaned. The resulting cleaning wastewater and debris enter the cave cavity (1) from the periphery of the outer mold (8). Then, the inflatable sealing ring (9) is inflated and expands. The inflatable sealing ring (9) abuts against the inner wall of the borehole of the cave top plate (2), separating the internal space of the outer mold (8) from the cave cavity (1). Then, concrete is poured into the borehole.

8. The construction method for roadbed structures suitable for large-span thin-roof karst caves according to claim 6, characterized in that: Multiple positioning protrusions (11) are provided below the outer mold top cover (10). When the outer mold top cover (10) is placed on top of the outer mold (8), the positioning protrusions (11) and the outer mold (8) are fitted with a clearance.

9. The construction method for roadbed structures suitable for large-span thin-roof karst caves according to claim 6, characterized in that: A top cover hook (12) is set on the top of the outer side of the top cover (10). Before cleaning the hole wall within the range of the top plate (2) of the karst cave, the top cover (10) of the outer mold is suspended and placed on the top of the outer mold (8) by the top cover hook (12). After cleaning and before lowering the steel cage, the top cover (10) of the outer mold is removed from the top of the outer mold (8) by the top cover hook (12).

10. A roadbed structure suitable for large-span thin-roof karst caves, prepared by the construction method according to any one of claims 1-9, comprising a roadbed (4), characterized in that: Multiple concrete piles (5) are installed below the roadbed (4); the concrete piles (5) have a variable diameter structure with a thicker top and a thinner bottom. The diameter of the concrete piles (5) within the range of the top plate (2) of the karst cave is larger than the diameter of the concrete piles (5) within the range of the karst cave cavity (1) and the bottom of the karst cave; the upper concrete of the concrete piles (5) is tightly bonded to the top plate (2) of the karst cave, and the lower part of the concrete piles (5) is embedded in the bottom of the karst cave cavity (2).

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