Construction method for micro pile group foundation reinforcement and foundation underpinning in subsurface tunnel

Abstract

The invention discloses a construction method for micro pile group foundation reinforcement and foundation underpinning in a subsurface tunnel, which comprises the following steps of: 1) pre-burying steel sleeves: pre-burying the steel sleeves at the front end and the rear end of a primary support inverted arch grid and between two adjacent grid steel frames during construction of a primary support structure; 2) improving the down-the-hole drill; (3) drilling a micro pile hole in the pre-buried steel sleeve; (4) forming a micro pile; (5) carrying out bearing capacity acceptance inspection on the micro piles; (6) waterproof treatment is conducted, specifically, waterproof treatment is conducted on the pile head after the tunnel waterproof isolation layer is constructed; and (7) tunnel secondary lining construction is conducted, the pile heads of the micro piles are anchored into a secondary lining bottom plate, and the secondary lining bottom plate is supported by the micro pile groups. The tunnel primary lining can be effectively prevented from being damaged and disturbed; main body structure sinking and cracking caused by foundation problems in the later period can be avoided, and the durability and the service life of a building are improved; and flexible construction can be carried out in a limited space in the tunnel by adopting a standardized construction process.

Description

Technical Field

[0001] The present invention relates to the technical field of dark-excavated tunnel construction, and in particular to a construction method suitable for micro-pile group foundation reinforcement and foundation underpinning in a dark-excavated tunnel. Background Art

[0002] When a tunnel is bored through unfavorable ground, such as ground with insufficient bearing capacity or collapsible soil, foundation reinforcement is required. Conventional methods include lime-soil compaction piles, cement-soil mixing piles, CFG piles, high-pressure jet grouting piles, and grouting. All of these foundation treatment methods must be implemented after the tunnel's primary lining is penetrated and before construction of the secondary lining. The typical diameter of lime-soil compaction piles, cement-soil mixing piles, CFG piles, and high-pressure jet grouting piles is 400-800mm. Pile holes are drilled through the primary substrate slab and then mechanically drilled. After cleaning, the pile body material is filled in according to the designed pile type and construction process.

[0003] Disadvantages of existing technology

[0004] 1. The pile diameter is large, and the drilling process will damage the primary lining concrete structure and the underlying grid steel frame, affecting the overall safety of the underground tunnel structure;

[0005] 2. The internal space of the tunnel is narrow and the clearance is low, making it difficult for conventional pile-driving machines to construct;

[0006] 3. The construction efficiency is low and it is difficult to adapt to the requirements of short, flat and fast construction;

[0007] 4. Cement-soil mixing, primary lining removal, etc. will cause environmental protection, construction waste disposal, resource consumption and environmental protection and water conservation issues.

[0008] 5. Grouting reinforcement is limited by geological influences. Sand and other soil types may cause the diffusion range to be too large, affecting surrounding underground pipelines or structures. The slurry is difficult to diffuse in the collapsible loess area, and the expected foundation reinforcement treatment effect cannot be achieved. Summary of the Invention

[0009] In order to solve the technical problems existing in the known technology, the present invention provides a construction method for micro-pile group foundation reinforcement and foundation replacement in a dark tunnel. This method does not damage the primary lining structure, has high construction efficiency, and performs well in environmental protection. It can improve the durability and service life of the tunnel.

[0010] The technical solution adopted by the present invention to solve the technical problems existing in the known technology is: a construction method for foundation reinforcement and foundation replacement of micro pile groups in a dark tunnel, adopting the following steps: 1) pre-buried steel casing: during the construction of the primary support structure, pre-buried steel casing according to the pile position and pile spacing, the steel casing is arranged at the front and rear ends of the primary support arch grid and between two adjacent grid steel frames, the inner diameter is the same as the diameter of the micro pile, and the diameter of the micro pile is 100-200mm; 2) improved down-the-hole drill: a hole expansion angle head coaxial with the drill bit is added to the drill bit; 3) after the primary support concrete reaches the design strength, follow the principle of skipping holes every other hole and use the improved down-the-hole drill in step 2) to drill micro pile holes in the pre-buried steel casing, and the bottom of the hole extends into the bearing layer below the soft soil layer; 4) micro pile formation: in the micro pile hole, Pour concrete into H-shaped steel to form micropiles. The pile body passes through the soft soil layer and is anchored in the bearing layer. The H-shaped steel protrudes from the tunnel waterproof isolation layer and extends into the secondary lining structure. 5) Pile foundation inspection: A single pile vertical compressive static load test is used for bearing capacity acceptance inspection. 6) H-shaped steel waterproofing treatment: After the pile inspection is passed and the tunnel waterproofing isolation layer is completed, two E-shaped steel plate waterstops are installed on the H-shaped steel. The two steel plate waterstops are respectively arranged on both sides of the H-shaped steel and welded. The steel plate waterstops are prefabricated in advance. 7) Tunnel secondary lining construction: After the waterproofing treatment is completed, the tunnel secondary lining steel bars are tied. The H-shaped steel at the micropile head is anchored into the secondary bottom plate and welded to the main reinforcement. The tunnel secondary lining structure concrete is poured. The secondary bottom plate and all micropiles are cast into a whole. The secondary bottom plate is supported on the micropile group.

[0011] In the step 1), the primary support grid steel frame structure is improved. When the primary support inverted arch grid steel frame is processed, the steel casing is welded to the main reinforcement side of the grid steel frame and the pipe opening is sealed with a plug. In the step 3), the pipe opening plug is removed before drilling.

[0012] In step 1), the micropiles are arranged in a plum blossom pattern with a circumferential spacing of 400 to 600 mm.

[0013] In step 3), dry drilling is performed.

[0014] In the step 4), the concrete poured into the micropile hole is slightly expanded fine stone concrete.

[0015] The advantages and positive effects of the present invention are as follows: after the initial support of the underground tunnel is connected and the deformation is stable, a small improved down-the-hole drill is used to drill holes in the steel casing pre-buried between the primary lining grid steel frames. After the holes are formed, micro-expanding fine stone concrete is poured, and H-shaped steel is inserted to form micro piles. The use of micro-expanding fine stone concrete can compensate for the shrinkage of the concrete, ensure that the pile body is in close contact with the surrounding soil, and increase the friction resistance. A pre-processed steel plate water stop is welded on the H-shaped steel extending into the secondary lining for waterproofing. This method can simplify the conventional pile head waterproofing process, quickly complete the waterproofing construction, achieve a good waterproofing effect, and improve construction efficiency. It also performs well in terms of environmental protection and reduces material loss. Micropiles follow the principle of skipping holes every other hole, which can effectively avoid damaging and disturbing the primary lining of the tunnel; a large number of micropiles are laid out at the bottom of the tunnel to form a micropile group, and a root-like structure is formed through the pile group effect. The pile body and the soil have a strong synergistic effect. The pile body passes through the soft soil layer and is anchored in the deep stable bearing layer, such as the rock layer and the dense sand layer. The pile end resistance, pile side friction resistance and the bending / shearing resistance of the pile body jointly bear the load to carry out foundation reinforcement and foundation replacement, which can avoid the main structure from sinking and cracking due to foundation problems in the later period, thereby improving the durability and service life of the building. The present invention adopts an in-hole micropile group to reinforce the foundation and replace the foundation of the dark excavation channel, and uses small machinery to dry-operate bored piles. It can be flexibly constructed in the limited space of the tunnel using standardized construction technology, with low energy consumption, less supporting personnel and material consumption, high construction efficiency, safe construction, stable and reliable construction quality, and good performance in environmental protection. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 A schematic diagram of the facade structure of a dark-excavated tunnel to which the present invention is applied;

[0017] Figure 2 A schematic diagram of the planar structure of a dark-excavated tunnel to which the present invention is applied;

[0018] Figure 3 A BIM model diagram of a dark tunnel to which the present invention is applied;

[0019] Figure 4 Schematic diagram of the improved primary support grid steel frame structure in the present invention;

[0020] Figure 5 Schematic diagram of the H-shaped steel treatment of the micro pile head in the present invention;

[0021] Figure 6 Schematic diagram of a single steel plate water stop in step 6) of the present invention;

[0022] Figure 7 Schematic diagram of the steel plate water stop in step 6) of the present invention.

[0023] In the figure: 1. Primary support structure; 2. Steel casing; 3. Grid steel frame; 4. Micro piles; 5. Secondary lining structure; 6. H-shaped steel; 7. Steel plate water stop. DETAILED DESCRIPTION

[0024] In order to further understand the content, features and effects of the present invention, the following embodiments are given as examples and described in detail with reference to the accompanying drawings:

[0025] See also Figures 1 to 7 A construction method for micro-pile group foundation reinforcement and foundation underpinning in a dark tunnel, comprising the following steps:

[0026] 1) Pre-embedded Steel Casing: During construction of the primary support structure 1, pre-embedded steel casing 2 is installed based on the pile positions and spacing. These casings are placed at the front and rear ends of the primary support inverted arch grid and between two adjacent grid steel frames 3 within the primary support inverted arch grid. Their inner diameter is the same as that of the micropiles 4, which range in diameter from 100 to 200 mm. Pre-embedded steel casings in the gaps between the grid steel frames serve as pile casings, effectively preventing damage and disturbance to the tunnel's primary lining and improving construction efficiency.

[0027] 2) Improved DTH Drill: A coaxial expansion head is added to the drill bit. This improves the common small-scale DTH drill by welding the expansion head to the drill bit, enabling it to complete pile hole drilling, ultimately achieving the goal of rapid and efficient drilling.

[0028] 3) After the primary support concrete reaches the design strength, follow the principle of skipping holes and use the improved down-the-hole drill in step 2) to drill micro pile holes in the pre-buried steel casing 2. The bottom of the hole extends into the bearing layer below the soft soil layer, which has a high drilling efficiency.

[0029] 4) Micropile Construction: Micro-expanding fine-stone concrete is poured into the micropile hole and H-shaped steel bars 6 are inserted to form micropile 4. The pile penetrates the soft soil layer and is anchored in the bearing stratum. The H-shaped steel bars 6 extend above the tunnel's waterproof isolation layer and into the secondary lining structure 5. This bored and cast-in-place column construction method is environmentally friendly.

[0030] 5) Pile Foundation Testing: A single pile vertical compressive static load test is used for bearing capacity acceptance testing. After micropile construction is completed, the grout at the pile head should be removed. In accordance with the "Technical Specification for Testing of Building Foundation Pile" (JGJ 106-2014), a single pile vertical compressive static load test is used for bearing capacity acceptance testing. The number of piles sampled for inspection should not be less than 1% of the total number of piles in the pile foundation sub-project under the same conditions, and should not be less than three. If the total number of piles in the pile foundation sub-project under the same conditions is less than 50, the number of piles sampled should not be less than two. The compressive bearing capacity of the pile foundation is determined through on-site static load testing.

[0031] 6) H-beam waterproofing: After the piles have passed the inspection and the tunnel waterproofing isolation layer has been completed, two E-shaped steel plate waterstops 7 are installed on the H-beam 6. The two steel plate waterstops 7 are respectively installed on both sides of the H-beam 6 and welded. The steel plate waterstops 7 are prefabricated in advance. This method can simplify the conventional pile head waterproofing process, improve the waterproofing construction efficiency, and achieve better waterproofing effect.

[0032] 7) Tunnel Secondary Lining Construction: After waterproofing is complete, the tunnel secondary lining reinforcement is tied. The micropile head H-shaped steel (6) is anchored into the secondary lining slab and welded to the secondary slab main reinforcement. The tunnel secondary lining structure (5) is concreted. The secondary slab and all micropiles are cast as a single unit, supported by the micropile cluster. Typically, the pile head reinforcement should be anchored into the secondary slab for at least 35d. The tunnel secondary lining structure is constructed in sections, where d is the diameter of the main reinforcement cage.

[0033] The more preferred solution of this embodiment is as follows:

[0034] In the step 1), the primary support grid steel frame structure is improved. When the primary support inverted arch grid steel frame is processed, the steel casing 2 is welded to the side of the main reinforcement of the grid steel frame, and the pipe opening is sealed with a plug to prevent the primary support shotcrete from pouring into the steel pipe. After the primary support concrete reaches the designed strength, the pipe opening plug is removed before drilling, and piles are drilled in the steel casing. This can ensure the safety of the underground excavation construction and shorten the construction period and reduce the on-site operation time.

[0035] In step 1), the micropiles 4 are arranged in a plum blossom pattern with a circumferential spacing of 400 to 600 mm.

[0036] In step 3), dry drilling is used, which makes construction simpler.

[0037] In step 4), the concrete poured into the micropile hole is micro-expanded fine stone concrete. The use of micro-expanded fine stone concrete facilitates concrete transportation and facilitates the insertion of H-shaped steel. At the same time, it can compensate for concrete shrinkage and increase frictional resistance.

[0038] This method reduces the pile diameter and uses small machinery to drill holes, enabling construction within the confined space of a tunnel. Drilling is performed through steel casing welded to the main reinforcement of the grid steel frame before construction of the tunnel's primary lining, preventing subsequent construction from disrupting the primary lining. After drilling, micro-expanding fine-stone concrete is poured and inserted into H-shaped steel sections, completing the construction of individual micropiles. The micropiles are spaced regularly to form a micropile cluster, effectively reinforcing the foundation.

[0039] Although the preferred embodiments of the present invention have been described above in conjunction with the accompanying drawings, the present invention is not limited to the above-mentioned specific embodiments. The above-mentioned specific embodiments are merely illustrative and not restrictive. Under the guidance of the present invention, ordinary technicians in this field can also make many forms without departing from the scope of protection of the present invention and the claims, which all fall within the scope of protection of the present invention.

Claims

1. A construction method for micro pile group foundation reinforcement and foundation underpinning in a dark tunnel, characterized in that: Use the following steps: 1) Pre-buried steel casing: During the construction of the primary support structure, pre-buried steel casing is installed according to the pile position and pile spacing. The steel casing is arranged at the front and rear ends of the primary support inverted arch grid and between two adjacent grid steel frames. The inner diameter of the steel casing is the same as the diameter of the micropile, which is 100-200mm. 2) Improved down-the-hole drill: add a coaxial reaming angle head to the drill bit; 3) After the primary concrete reaches the designed strength, follow the principle of skipping holes every other hole and use the improved down-the-hole drill in step 2) to drive micropile holes in the pre-buried steel casing, with the bottom of the hole extending into the bearing stratum below the soft soil layer; 4) Micropile construction: Concrete is poured into the micropile hole and H-shaped steel is inserted to form the micropile. The pile body penetrates the soft soil layer and is anchored in the bearing stratum. The H-shaped steel protrudes above the tunnel waterproof isolation layer and extends into the secondary lining structure. 5) Pile foundation testing: A single pile vertical compressive static load test is used for bearing capacity acceptance testing; 6) H-beam waterproofing: After the piles have passed inspection and the tunnel waterproofing isolation layer has been completed, two E-shaped steel plate waterstops are installed on the H-beam. The two steel plate waterstops are installed on both sides of the H-beam and welded together. The steel plate waterstops are prefabricated in advance. 7) Tunnel secondary lining construction: After waterproofing is completed, the tunnel secondary lining reinforcement is tied, the H-shaped steel at the micropile head is anchored into the secondary lining base plate and welded to the main reinforcement, and the tunnel secondary lining structural concrete is poured. The secondary lining base plate and all micropiles are cast into a whole, and the secondary lining base plate is supported on the micropile group.

2. The construction method for micro pile group foundation reinforcement and foundation underpinning in a dark tunnel according to claim 1 is characterized in that: In the step 1), the primary support grid steel frame structure is improved. When the primary support inverted arch grid steel frame is processed, the steel casing is welded to the main reinforcement side of the grid steel frame and the pipe opening is sealed with a plug. In the step 3), the pipe opening plug is removed before drilling.

3. The construction method for micro pile group foundation reinforcement and foundation underpinning in a dark tunnel according to claim 1 is characterized in that: In step 1), the micropiles are arranged in a plum blossom pattern with a circumferential spacing of 400 to 600 mm.

4. The construction method for micro pile group foundation reinforcement and foundation underpinning in a dark tunnel according to claim 1 is characterized in that: In step 3), dry drilling is performed.

5. The construction method for micro pile group foundation reinforcement and foundation underpinning in a dark tunnel according to claim 1 is characterized in that: In the step 4), the concrete poured into the micropile hole is slightly expanded fine stone concrete.

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