Combined type foundation pit supporting method combining cast-in-situ bored pile with hole bottom bar planting and grouting

Abstract

The invention relates to the technical field of foundation pit construction, in particular to a combined type foundation pit supporting method combining cast-in-situ bored piles with hole bottom bar planting and grouting. The method comprises the steps that drilling excavation is stopped after water flows out of a hole, reinforcing steel bars are driven into a soil layer at the bottom of the hole, a reinforcement cage is hung and placed to be connected with the driven reinforcing steel bars, a grouting pipe is inserted into the bottom of the drilled hole for grouting, cement is added into a water accumulation area in the hole to be stirred to form a pile bottom hardening area, and finally concrete is poured into the drilled hole to be combined with the reinforcement cage to form a cast-in-place pile area. The embedded steel bar grouting reinforcement area, the pile bottom hardening area and the cast-in-place pile area are connected to form a variable-rigidity pile body, and after a foundation pit is excavated, shotcrete and anchor supporting is conducted on the surface of a soil layer. The variable-stiffness pile body is formed in the vertical direction, the spray anchor support is formed in the horizontal direction, the self-stabilization capacity of the foundation pit soil layer is fully mobilized, compared with a traditional cast-in-situ bored pile construction method, the depth of the pile is remarkably reduced, the problem that pile forming is difficult due to water accumulation in a drilled hole is solved, and the construction cost can be effectively reduced.

Description

Technical Field

[0001] The invention relates to the technical field of foundation pit construction, and in particular to a composite foundation pit supporting method combining bored cast-in-place piles with hole bottom reinforcement grouting. Background Art

[0002] Foundation pit support refers to temporary or permanent measures taken during the construction of underground projects or building foundations to prevent the collapse of the foundation pit soil and ensure construction safety and the stability of the surrounding environment. Common foundation pit support measures include cement retaining walls, bored piles, steel sheet piles, cement soil mixing piles, underground continuous walls, etc. Bored piles have been widely used in foundation pit projects due to their simple, fast and low cost.

[0003] However, when encountering soft soil, gravel, and pebble strata with developed groundwater, as the bored piles are drilled deeper, they will face problems such as water accumulation in the hole and difficulty in pile formation. If dewatering wells are set up around the foundation pit, there will be problems such as slow dewatering rate and low construction efficiency. If underground continuous walls with better water-stopping effects are used instead of bored piles, the project cost will increase significantly and the economy will be poor. To this end, this patent proposes a composite foundation pit support method that combines bored piles with bottom-hole reinforcement and grouting, which significantly reduces the drilling depth to solve the problem of difficulty in forming bored piles in water-rich strata. Summary of the invention

[0004] In view of the above technical problems, the present invention improves the existing bored cast-in-place pile foundation pit support method, by implanting steel bars at the bottom of the drill hole to connect with the steel cage, grouting reinforcement at the bottom of the hole, mixing and hardening cement in the hole, and injecting concrete into the upper part of the drill hole, the hole bottom planting steel bar grouting reinforcement area, the pile bottom hardening area and the cast-in-place pile area are connected in sequence to form a variable stiffness pile body, combined with spray anchor support on the soil surface and the soil between the piles after the foundation pit is excavated, to form a composite foundation pit support method, which is used to solve the problem of difficulty in forming bored cast-in-place piles in water-rich sand and gravel strata.

[0005] The composite foundation pit support method of bored cast-in-place piles combined with hole bottom reinforcement grouting provided by the present invention specifically comprises the following steps: Excavate a hole and stop when the hole is above water level, and hang a steel cage in the hole; Implant a plurality of steel bars on the bottom surface of the hole, and tie and connect the implanted steel bars to the steel cage; Drill holes and inject grout into the bottom surface of the hole, so that a mixture of slurry and soil is formed in the bottom area of ​​the hole to form a grouting reinforcement area; Add cement and stir in the water outlet area to harden the water and soil at the bottom of the hole to form a hardened area at the bottom of the pile, which can solve the problem of a small amount of water accumulation at the bottom of the hole and also improve the strength of this area; Then pour concrete into the borehole and combine it with the steel cage to form a bored pile area, so that the grouting reinforcement area, the pile bottom hardening area, and the bored pile area are connected in sequence into a whole, forming a composite pile body with variable stiffness from bottom to top to withstand the soil pressure after the foundation pit excavation disturbance. Specifically, the stratum in a certain range at the bottom of the hole is reinforced by grouting to reduce the permeability, and the implanted steel bars and the steel bars of the bored pile are connected together, which not only improves the strength of the grouting reinforcement area, but also makes the grouting reinforcement area and the upper bored pile combined as a whole to bear the force together. At the same time, the reinforcement area formed by implanting steel bars combined with grouting can effectively reduce the embedded length of the bored pile, reduce the construction difficulty and project cost. The three areas of grouting reinforcement area, pile bottom hardening area and bored pile are connected to form a complete composite pile body, which can effectively solve the problem of difficulty in pile formation in water-rich sand and gravel strata.

[0006] Optionally, the foundation pit support method further comprises forming a plurality of the composite pile bodies around the foundation pit to be excavated as foundation pit support, and the distance between the pile body excavation contour and the foundation pit excavation contour line is a, wherein a is 0.5-1m.

[0007] Optionally, the soil inside the foundation pit is excavated step by step until the bottom elevation of the foundation pit is reached, and sprayed anchor support is applied to the side walls of the foundation pit while the soil inside the foundation pit is excavated.

[0008] Optionally, the foundation pit support method also includes applying sprayed anchor support, where the anchor rods penetrate into the soil between the composite piles, and spray concrete to form a sprayed concrete layer, which cooperates with the anchor rods to improve the strength of the soil layer on the side wall of the foundation pit.

[0009] Optionally, after excavating to the bottom elevation of the foundation pit, concrete is sprayed at the bottom of the foundation pit to form a bottom concrete layer, which is connected to the sprayed concrete layer formed on the side wall of the foundation pit to form a whole, thereby improving the stability of the foundation pit.

[0010] Optionally, when a grouting pipe is driven into the inner bottom surface of the hole for grouting, the length of the grouting pipe is the same as the length of the steel bars implanted into the inner bottom surface of the hole.

[0011] Optionally, the height of the steel cage is H1, the elevation of the bottom of the foundation pit is h, and h≤H1.

[0012] Optionally, when several steel bars are implanted, the length of the steel bars implanted in the bottom surface of the hole is H2, the height of the composite pile is H=H1+H2, and the length of the area where the implanted steel bars overlap with the upper steel cage is l ,in l =40D, D is the diameter of the embedded steel bar.

[0013] The beneficial effects of the present invention are: (1) The present invention forms an integral variable-rigidity composite pile body in the vertical direction of the foundation pit in three areas, and forms a sprayed anchor support in the horizontal direction, fully mobilizing the self-stabilizing capacity of the soil layer, and combining the soil layer reinforcement with the pile structure bearing capacity; compared with the traditional foundation pit bored cast-in-place pile construction method, the depth of the bored cast-in-place pile is significantly reduced (the embedding depth is reduced), the pile forming speed is faster, and the problem of difficulty in pile forming caused by water accumulation in the drill hole is solved, which can effectively reduce the project cost.

[0014] (2) The foundation pit support method provided by the present invention is suitable for foundation pit projects with abundant groundwater, and can efficiently complete the foundation pit support work, especially suitable for supporting foundation pit projects in water-rich sand and gravel formations, and can effectively improve the problems of long pile formation period and high cost in water-rich sand and gravel formations. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 It is a schematic diagram of a composite foundation pit supporting method applied to a foundation pit in a specific embodiment of the present invention; Figure 2 It is a cross-sectional view of a foundation pit and bored piles in a specific embodiment of the present invention; Figure 3 It is a schematic diagram of the structure of a single bored pile in a specific embodiment of the present invention; Figure 4 A schematic diagram of the reinforcement of a single bored pile in a specific embodiment of the present invention; Figure 5 A top view of a single bored pile of the present invention during reinforcement arrangement; Figure 6 It is a schematic diagram of the detailed structure of steel bar binding in a specific implementation manner of the present invention; Figure 7 It is a schematic diagram of the arrangement of implanted steel bars and grouting pipes in a specific embodiment of the present invention; In the figure: 1. Grouting reinforcement area; 2. Pile bottom hardening area; 3. Cast-in-place pile area; 4. Anchor rods; 5. Sprayed concrete layer; 6. Bottom sealing concrete layer. DETAILED DESCRIPTION

[0016] To make the present invention more clear, the composite foundation pit support method of bored cast-in-place piles combined with bottom-hole reinforcement grouting of the present invention is further described below in conjunction with the accompanying drawings. The specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0017] The method for supporting a foundation pit by combining bored cast-in-place piles with base grouting reinforcement in the present invention specifically comprises the following steps: (1) Drilling: First, drill at the predetermined drilling location and stop drilling when the excavation reaches the water surface; (2) Hanging the steel cage: Hang the steel cage in the drilled hole, with the bottom of the steel cage level with the bottom of the hole; (3) Implanting steel bars: Implant steel bars in the bottom soil of the hole, with the length of the steel bars reaching the preset depth of the cast-in-place pile, and tie the implanted steel bars to the steel cage; (4) Drilling grouting: The length of the grouting pipe is the same as the length of the implanted steel bar. Grout is injected to reinforce the surrounding soil. After grouting, a mixture of grout and soil is formed within a certain range at the bottom of the hole. Its strength and permeability are higher than the original soil layer, forming a base grouting reinforcement area 1; (5) Pile bottom hardening: After grouting reinforcement is completed, there is still some water in the hole. Cement can be added to the water outlet of the hole to harden the water and soil at the bottom of the hole to form a pile bottom hardening zone 2; (6) Pile formation: clean the water in the hole, pour concrete into the hole, combine it with the steel cage to form a pile, form a cast-in-place pile area 3, and connect it with other parts to form an integral composite pile body, forming a complete bored cast-in-place pile as foundation pit support; (7) Reinforcement of soil around piles: grouting is performed around the pile foundation after pouring to increase the friction of the pile foundation; (8) Make tie beams between piles: Make connecting beams below the pile heads to connect the upper parts of the piles into a whole, forming an effective anti-overturning measure; (9) Excavation of foundation pit: Excavate the soil inside the foundation pit until the bottom elevation of the foundation pit is reached. At the same time, the distance a between the edge of the foundation pit and the edge of the pile is 0.5 m during the excavation. (10) Sprayed anchor support: During the excavation of the foundation pit, sprayed anchor support is applied on the side walls of the foundation pit. The anchor rod 4 is angled upward by 15°~25° and penetrates 2.5m~3.0m into the soil between the piles. After the excavation of the foundation pit is completed, concrete needs to be sprayed at the bottom of the foundation pit to form a sprayed concrete layer 5 and a bottom sealing concrete layer 6 on the side walls and bottom of the foundation pit respectively.

[0018] It should be noted that, in a specific embodiment of the present invention, the diameter of the borehole in step (1) can be designed according to relevant specifications, the overall design length of the bored pile is H, and the depth of the borehole is the distance H1 from the ground to the water surface; in step (3), the spacing of the stirrups in the overlap length (l) of the lower embedded steel bar and the upper steel cage needs to be determined according to relevant specifications; in steps (3) and (4), the length H2 of the lower embedded steel bar and the grouting pipe is the overall design length H of the bored pile minus the distance H1 from the ground to the water surface; in step (4), the diffusion radius at different positions during grouting is different due to the distance from the grouting pipe orifice, and the diameter after diffusion is between D1 and D2, D1 is the reinforcement diffusion diameter at the connection position with the bored pile, and D2 is the diffusion diameter after the grouting pipe penetrates into the stratum; in step (6), the pile diameter is d; in step (9), the excavation width of the foundation pit is b, the excavation depth is h, and h shall not be greater than H1.

[0019] See also Figure 1 and Figure 2The overall height of the bored pile is H, the height of the bored pile is H1, the height of the grouting reinforcement area 1 is H2, the excavation depth of the foundation pit is h, and h is not greater than H1, and the range of grouting reinforcement is D1-D2. Figure 3 Finally, the overall structure presents three parts from bottom to top: grouting reinforcement area 1, pile bottom hardening area 2 and cast-in-place pile area 3.

[0020] See also Figure 4 and Figure 5 , is a schematic diagram of the reinforcement of a single pile, the pile diameter is d, the height of the steel cage is H1, and the height of the embedded steel bar is H2. The length of the connection between the lower embedded steel bar and the upper steel cage is l In this area, the stirrups need to be denser and the stirrup spacing should be determined according to relevant specifications.

[0021] See also Figure 6 , is a detailed structural diagram of steel bar binding, the lap length of the lower embedded steel bar and the upper steel cage is l , generally take 40D (D is the diameter of the implanted steel bar).

[0022] See also Figure 7 , which is a schematic diagram of the layout of the lower embedded steel bars and grouting pipes.

[0023] As one of the optional embodiments of the present invention, a composite foundation pit support method using the aforementioned bored cast-in-place piles combined with bottom hole reinforcement and grouting is used to obtain a grouting reinforcement area 1, a pile bottom hardening area 2 and a cast-in-place pile area 3, which are sequentially connected into a composite pile body. The composite pile body is used as a foundation pit support, and is particularly used in foundation pit excavation projects in water-rich sand and gravel strata.

[0024] It is understandable that if Figure 2 As shown, in the traditional bored pile construction method, the hole needs to be excavated to a depth H when drilling, but in actual construction, the water surface is reached when the excavation reaches a depth of H1. Therefore, there is water accumulation in the hole at a depth of H2 below the water surface and the problem of difficulty in pile formation exists. It is necessary to carry out projects such as dewatering wells or underground continuous walls, resulting in high project costs and long construction periods. For this reason, the present application implants steel bars in the part below the water surface and connects it with the steel cage, and then performs grouting reinforcement to form a grouting reinforcement area 1, and adds cement to the water surface to form a pile bottom hardening area 2, and finally pours concrete to obtain a bored pile body with a composite support structure, thereby achieving effective support for the foundation pit of depth h.

[0025] Compared with the traditional bored cast-in-place pile supporting method, the present invention implants steel bars in the bottom of the hole and injects grouting, which are connected with the steel bars in the upper cast-in-place pile area, vertically forming an overall variable stiffness composite pile body in the three areas, reducing the depth of the drilling hole required for the bored cast-in-place pile, and at the same time forming a sprayed anchor support in the soil between the piles in the horizontal direction. The two support methods work synergistically to fully mobilize the self-stabilizing ability of the soil layer, combine the soil layer reinforcement with the bearing capacity of the support structure, greatly improve the strength and stability of the foundation pit support, effectively improve the problem of long pile formation period in water-rich sand and gravel strata areas, and greatly reduce the project cost. The method of the present invention is suitable for urban foundation pit projects, can accelerate the pile formation effect in water-rich sand and gravel strata areas, and is safe and efficient.

[0026] In addition to the above embodiments, the present invention may also have other implementation modes. Any technical solution formed by equivalent replacement or equivalent transformation falls within the protection scope required by the present invention.

Claims

1. A composite foundation pit support method combining bored piles with bottom-hole reinforcement grouting, characterized in that: The following steps are involved: Drilling and excavation, and stop when the excavation reaches the water surface, and hang the steel cage in the excavated hole; implanting vertical steel bars into the bottom surface of the hole, and tying and connecting the implanted steel bars to the steel cage; Driving a grouting pipe into the bottom surface of the hole to inject grouting, so that a mixture of slurry and soil is formed in the bottom area of ​​the hole to form a grouting reinforcement area (1); Add cement to the water surface area and mix to harden the water and soil at the bottom of the hole to form a hardened area at the bottom of the pile (2); Concrete is then poured into the borehole to form a cast-in-place pile area (3) combined with the steel cage, so that the grouting reinforcement area (1), the pile bottom hardening area (2) and the cast-in-place pile area (3) are sequentially connected into a whole, forming a composite pile body with variable stiffness.

2. The foundation pit supporting method according to claim 1, characterized in that: A plurality of the composite pile bodies are formed around the foundation pit to be excavated as foundation pit support, and the distance between the pile body excavation outline and the foundation pit excavation outline is a, wherein a is 0.5-1m.

3. The foundation pit supporting method according to claim 2, characterized in that: The soil inside the foundation pit is excavated step by step until the bottom elevation of the foundation pit is reached. While excavating the soil inside the foundation pit, sprayed anchor support is applied on the side walls of the foundation pit.

4. The foundation pit supporting method according to claim 3, characterized in that: When sprayed anchor support is applied, the anchor rods (4) penetrate into the soil between the composite pile bodies, and the sprayed concrete forms a sprayed concrete layer (5).

5. The foundation pit supporting method according to claim 4, characterized in that: After excavation reaches the bottom elevation of the foundation pit, concrete is sprayed at the bottom of the foundation pit to form a bottom concrete layer (6).

6. The foundation pit supporting method according to claim 1, characterized in that: When a grouting pipe is driven into the inner bottom surface of the hole for grouting, the length of the grouting pipe is the same as the length of the steel bar implanted in the inner bottom surface of the hole.

7. The foundation pit supporting method according to claim 6, characterized in that: The height of the steel cage is H1, the elevation of the bottom of the foundation pit is h, and h≤H1.

8. The foundation pit supporting method according to claim 7, characterized in that: When several steel bars are implanted, the length of the steel bars implanted in the bottom surface of the hole is H2, forming the height of the composite pile body H=H1+H2, and the length of the area where the implanted steel bars overlap with the upper steel cage is l ,in l =40D, D is the diameter of the embedded steel bar.

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