A composite foundation pit support method combining bored cast-in-place piles with bottom-mounted rebar grouting.

Abstract

This invention relates to the field of foundation pit construction technology, and in particular to a composite foundation pit support method combining bored cast-in-place piles with bottom-mounted rebar grouting. The method includes drilling until water is found in the hole, then driving reinforcing bars into the soil at the bottom of the hole, lowering a reinforcing cage and connecting it to the driven bars, inserting a grouting pipe at the bottom of the hole for grouting, adding cement to the water-accumulated area in the hole and mixing it to form a hardened zone at the pile bottom, and finally pouring concrete into the hole to combine with the reinforcing cage to form a cast-in-place pile area. The rebar grouting reinforcement area, the hardened zone at the pile bottom, and the cast-in-place pile area are connected to form a variable-stiffness pile. After the foundation pit is excavated, shotcrete and anchor support is applied to the soil surface. This invention forms a variable-stiffness pile vertically and shotcrete and anchor support horizontally, fully utilizing the self-stabilizing capacity of the foundation pit soil. Compared with traditional bored cast-in-place pile construction methods, the pile depth is significantly reduced, solving the problem of difficulty in pile formation due to water accumulation in the borehole, and effectively reducing project costs.

Description

Technical Field

[0001] This invention relates to the field of foundation pit construction technology, and in particular to a composite foundation pit support method that combines bored cast-in-place piles with bottom-mounted rebar grouting. Background Technology

[0002] Foundation pit support refers to temporary or permanent measures taken during the construction of underground engineering 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, sheet piles, cement-soil mixing piles, and diaphragm walls. Bored piles are widely used in foundation pit engineering due to their simple construction, fast speed, and low cost.

[0003] However, when encountering soft soil, gravel, or pebble strata with well-developed groundwater, the increased drilling depth of bored piles presents challenges such as water accumulation within the borehole and difficulties in pile formation. Setting up dewatering wells around the foundation pit results in slow dewatering rates and low construction efficiency. Replacing bored piles with diaphragm walls, which offer better water-stopping properties, significantly increases project costs and reduces economic viability. Therefore, this patent proposes a composite foundation pit support method combining bored piles with bottom-mounted rebar grouting, significantly reducing drilling depth to address the difficulty of pile formation in water-rich strata. Summary of the Invention

[0004] This invention addresses the aforementioned technical problems by improving existing methods for supporting bored pile foundation pits. It involves planting reinforcing bars at the bottom of the borehole and connecting them to a reinforcing cage, grouting at the bottom of the borehole for reinforcement, mixing and hardening cement in the water inside the borehole, and then inserting concrete into the upper part of the borehole. This sequentially connects the bottom reinforcement and grouting zone, the pile bottom hardening zone, and the cast-in-place pile zone into a single pile with varying stiffness. Combined with shotcrete and anchor support applied to the soil surface and between piles after excavation, this creates a composite foundation pit support method to solve the problem of difficult pile formation in water-rich sandy and gravelly strata.

[0005] The composite foundation pit support method combining bored cast-in-place piles with bottom-mounted rebar grouting provided by this invention specifically includes the following steps: Excavate and drill holes, stopping once the water surface is reached, and then lower a steel cage into the excavated hole. Several steel bars are inserted into the bottom surface of the hole, and the inserted steel bars are tied to the steel cage. Grouting is performed by drilling holes at the bottom of the hole to form a mixture of grout and soil in the bottom area of ​​the hole, which constitutes a grouting reinforcement zone. Adding cement to the water outlet area and mixing it hardens the water and soil at the bottom of the hole, forming a hardened zone at the bottom of the pile, which solves the problem of a small amount of water accumulating at the bottom of the hole and also improves the strength of the area. Concrete is then poured into the borehole to form a cast-in-place pile zone, connecting the grouting reinforcement zone, the pile bottom hardening zone, and the cast-in-place pile zone into a unified whole. This creates a variable-stiffness composite pile body from bottom to top, capable of withstanding the soil pressure after excavation disturbance. Specifically, grouting reinforces a certain area of ​​the stratum at the bottom of the borehole, reducing permeability. The inserted reinforcing steel bars are connected to the reinforcing steel bars of the bored cast-in-place pile, which not only increases the strength of the grouting reinforcement zone but also integrates it with the upper cast-in-place pile into a unified structure, sharing the load. Simultaneously, the reinforcement zone formed by the inserted reinforcing steel bars and grouting effectively reduces the embedment length of the bored cast-in-place pile, lowering construction difficulty and project cost. The grouting reinforcement zone, the pile bottom hardening zone, and the cast-in-place pile are connected to form a complete composite pile body, effectively solving the problem of difficult pile construction in water-rich sandy and gravelly strata.

[0006] Optionally, the foundation pit support method further includes forming a plurality of composite piles around the foundation pit to be excavated as foundation pit support, and the distance between the excavation outline of the pile and the excavation outline of the foundation pit is a, where a is 0.5-1m.

[0007] Optionally, the soil inside the foundation pit is excavated in stages until the bottom elevation of the foundation pit is reached. At the same time as the soil inside the foundation pit is excavated, shotcrete and anchor support is applied to the side walls of the foundation pit.

[0008] Optionally, the foundation pit support method further includes, when performing shotcrete and anchor support, the anchor rods penetrate deep into the soil between the composite piles, and the shotcrete forms a shotcrete layer, which, together with the anchor rods, improves the strength of the soil layer on the sidewall of the foundation pit.

[0009] Optionally, after excavating to the bottom elevation of the foundation pit, spray concrete is applied to the bottom of the foundation pit to form a sealing concrete layer, which is connected with the spray concrete layer formed on the sidewall of the foundation pit to form an integral whole, thereby improving the stability of the foundation pit.

[0010] Optionally, when grouting is performed by driving a grouting pipe into the bottom surface of the hole, the length of the grouting pipe is the same as the length of the reinforcing bar implanted into the bottom surface of the hole.

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

[0012] Optionally, when inserting several reinforcing bars, the length of the reinforcing bars at the bottom of the insertion hole is H2, forming the height of the composite pile H = H1 + H2, and the length of the area where the inserted reinforcing bars overlap with the upper reinforcing cage is... l ,in l =40D, where D is the diameter of the inserted steel bar.

[0013] The beneficial effects of this invention are: (1) In this invention, three areas are formed into an integral variable stiffness composite pile in the vertical direction of the foundation pit, and shotcrete support is formed in the horizontal direction. This fully mobilizes the self-stabilizing ability of the soil layer and combines soil layer reinforcement with pile structure bearing. Compared with the traditional foundation pit drilling and grouting pile construction method, the depth of the drilling and grouting pile is significantly reduced (the embedment depth is reduced), the pile formation speed is faster, and the problem of water accumulation in the borehole causing difficulty in pile formation is solved, which can effectively reduce the project cost.

[0014] (2) The foundation pit support method provided by the present invention is applicable to foundation pit projects with abundant groundwater. It can efficiently complete the foundation pit support work, especially applicable to the support of foundation pit projects in water-rich sandy gravel strata. It can effectively improve the problems of long pile construction period and high cost in water-rich sandy gravel strata. Attached Figure Description

[0015] Figure 1 This is a schematic diagram illustrating the application of the composite foundation pit support method to a foundation pit in a specific embodiment of the present invention. Figure 2 This is a cross-sectional view of the foundation pit and bored piles in a specific embodiment of the present invention. Figure 3 This is a schematic diagram of the structure of a single bored pile in a specific embodiment of the present invention; Figure 4 This is a schematic diagram of the reinforcement of a single bored pile in a specific embodiment of the present invention; Figure 5 This is a top view of the reinforcement of a single bored cast-in-place pile according to the present invention. Figure 6 This is a detailed structural diagram of the steel bar binding in a specific embodiment of the present invention; Figure 7 This is a schematic diagram of the arrangement of the reinforcing bars and grouting pipes in a specific embodiment of the present invention; In the diagram: 1. Grouting reinforcement zone; 2. Pile bottom hardening zone; 3. Cast-in-place pile zone; 4. Anchor bolt; 5. Shotcrete layer; 6. Sealing concrete layer. Detailed Implementation

[0016] To make the present invention clearer, the following description, in conjunction with the accompanying drawings, further illustrates the composite foundation pit support method of the present invention, which combines bored cast-in-place piles with bottom-mounted rebar grouting. The specific embodiments described herein are for illustrative purposes only and are not intended to limit the present invention.

[0017] The foundation pit support method of combined bored pile and foundation grouting reinforcement in this invention specifically includes the following steps: (1) Drilling and excavation: First, drill holes at the predetermined drilling locations, and stop drilling when the water surface is reached; (2) Lowering the steel cage: Lower the steel cage into the pre-drilled hole, with the bottom of the steel cage level with the bottom of the hole; (3) Inserting steel bars: Insert steel bars into the soil at the bottom of the hole, with the length of the steel bars reaching the pre-set depth of the cast-in-place pile, and tie the inserted steel bars to the steel cage; (4) Drilling and grouting: The length of the grouting pipe is the same as the length of the inserted steel bar. Grout is injected to reinforce the surrounding soil. After grouting, a mixture of grout and soil is formed in a certain range at the bottom of the hole. Its strength and permeability are higher than the original soil layer, forming the base grouting reinforcement zone 1. (5) Hardening of pile bottom: After the grouting reinforcement is completed, there is still some water in the hole. Cement can be added to the water outlet surface at the bottom of the hole and stirred to harden the water and soil at the bottom of the hole, forming the hardened area 2 at the bottom of the pile. (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 the grouting pile area 3, and connect it with other parts to form a composite pile body, forming a complete bored grouting pile as foundation pit support. (7) Reinforcement of soil around piles: Grouting is carried out around the piles after the concrete is poured to increase the friction of the piles; (8) Constructing tie beams between piles: Constructing tie beams below the head of the piles to connect the upper parts of the piles into a whole, forming an effective anti-overturning measure; (9) Excavation of the foundation pit: The soil inside the foundation pit is excavated in a distributed manner 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 0.5m during the excavation. (10) Shotcrete and anchor support: Shotcrete and anchor support shall be installed on the side wall of the foundation pit at the same time as the foundation pit is excavated. The anchor rod 4 shall be at an upward angle of 15°~25° and penetrate into the soil between the piles 2.5m~3.0m. After the foundation pit is excavated, shotcrete shall be sprayed at the bottom of the foundation pit to form a shotcrete layer 5 and a bottom sealing concrete layer 6 on the side wall and bottom of the foundation pit, respectively.

[0018] It should be noted that, in the specific embodiments 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) area between 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 opening, and the diameter after diffusion is between D1 and D2, where 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 must not be greater than H1.

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

[0020] See Figure 4 and Figure 5 This is a schematic diagram of the reinforcement of a single pile. The pile diameter is d, the height of the reinforcing cage is H1, and the height of the inserted reinforcing bars is H2. The length connecting the lower inserted reinforcing bars to the upper reinforcing cage is... l Within this area, the stirrups need to be densified, and the stirrup spacing should be determined according to relevant specifications.

[0021] See Figure 6 This is a detailed structural diagram of the reinforcing steel reinforcement binding, showing the lap length between the lower embedded reinforcing steel and the upper reinforcing steel cage. l Generally, 40D is used (D is the diameter of the inserted steel bar).

[0022] See Figure 7 This is a schematic diagram showing the arrangement of the lower-level reinforcing bars and grouting pipes.

[0023] As one of the optional embodiments of the present invention, the aforementioned composite foundation pit support method combining bored cast-in-place piles with bottom reinforcement grouting is used to obtain a grouting reinforcement zone 1, a pile bottom hardening zone 2, and a cast-in-place pile zone 3, which are then connected in sequence to form a composite pile body. This composite pile body is used as foundation pit support, especially in foundation pit excavation projects in water-rich sandy gravel strata.

[0024] It is understandable that, such as Figure 2 As shown, in the traditional bored pile construction method, the hole needs to be excavated to a depth H during drilling. However, in actual construction, the water surface is reached when the hole is excavated to a depth of H1. Therefore, there are problems of water accumulation and difficulty in pile formation in the hole at a depth of H2 below the water surface. It is necessary to add dewatering wells or underground continuous walls, which leads to high project costs and long construction period. To address this, this application involves inserting steel bars into the part below the water surface and connecting them with the steel cage, followed by grouting reinforcement to form a grouting reinforcement zone 1. Cement is added and mixed at the water surface to form a pile bottom hardening zone 2. Finally, concrete is poured to obtain a bored pile body with a composite support structure, thereby achieving effective support for the foundation pit at a depth of h.

[0025] Compared with traditional bored pile support methods, this invention involves inserting reinforcing bars into the bottom of the hole and grouting them, connecting them with the reinforcing bars in the upper bored pile area. Vertically, this forms a unified variable stiffness composite pile body with three areas, reducing the required drilling depth for bored piles. Simultaneously, horizontal shotcrete and anchor support is formed in the soil between the piles. The synergistic effect of these two support methods fully mobilizes the self-stabilizing capacity of the soil layer, combining soil reinforcement with the bearing capacity of the support structure. This significantly improves the strength and stability of the foundation pit support, effectively addresses the problem of long pile construction cycles in water-rich sandy gravel strata, and greatly reduces project costs. This method is applicable to urban foundation pit engineering, can accelerate pile construction in water-rich sandy gravel strata, and is safe and efficient.

[0026] In addition to the embodiments described above, the present invention may have other implementations. All technical solutions formed by equivalent substitution or equivalent transformation fall within the protection scope claimed by the present invention.

Claims

1. A composite foundation pit support method combining bored cast-in-place piles with bottom-mounted rebar grouting, characterized in that, Includes the following steps: Drilling and excavation continues until the water surface is reached, at which point a steel cage is lowered into the excavated hole. Vertical steel bars are inserted into the bottom surface of the hole, and the inserted steel bars are tied to the steel cage. Grouting is performed by driving a grouting pipe into the bottom of the hole to form a mixture of grout and soil in the bottom area of ​​the hole, which constitutes the grouting reinforcement zone (1). Cement was added to the water outlet area and mixed to harden the water and soil at the bottom of the hole, forming a hardened area at the bottom of the pile (2). Then concrete is poured into the borehole to combine with the steel cage to form a cast-in-place pile area (3), so that the grouting reinforcement area (1), the pile bottom hardening area (2) and the cast-in-place pile area (3) are connected in sequence to form a composite pile with variable stiffness. The height of the reinforcing cage is H1, and the bottom elevation of the foundation pit is h, where h ≤ H 1； When several reinforcing bars are inserted, the length of the reinforcing bars at the bottom of the insertion hole is H2, the height of the composite pile is H = H1 + H2, and the length of the area where the inserted reinforcing bars overlap with the upper reinforcing cage is... l ,in l =40D, where D is the diameter of the inserted steel bar.

2. The composite foundation pit support method combining bored cast-in-place piles with bottom-mounted rebar grouting as described in claim 1, characterized in that, Multiple composite piles are formed around the foundation pit to be excavated as foundation pit support, and the distance between the edge of the pile and the outline of the foundation pit excavation is a, where a is 0.5-1m.

3. The composite foundation pit support method combining bored cast-in-place piles with bottom-mounted rebar grouting as described in claim 2, characterized in that, The soil inside the foundation pit is excavated in stages until the bottom elevation of the foundation pit is reached. At the same time as the soil inside the foundation pit is excavated, shotcrete and anchor support is applied to the side walls of the foundation pit.

4. The composite foundation pit support method combining bored cast-in-place piles with bottom-mounted rebar grouting as described in claim 3, characterized in that, When the shotcrete support is applied, the anchor rod (4) penetrates into the soil between the composite piles, and the shotcrete forms a shotcrete layer (5).

5. The composite foundation pit support method combining bored cast-in-place piles with bottom-mounted rebar grouting as described in claim 4, characterized in that, After excavating to the bottom elevation of the foundation pit, spray concrete at the bottom of the foundation pit to form a bottom sealing concrete layer (6).

6. The composite foundation pit support method combining bored cast-in-place piles with bottom-mounted rebar grouting as described in claim 1, characterized in that, When grouting is performed by driving a grouting pipe into the bottom surface of the hole, the length of the grouting pipe is the same as the length of the reinforcing bar implanted into the bottom surface of the hole.

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