A method for deep foundation pit support based on prestressed bored piles

Abstract

A prestressed bored pile and a deep foundation pit support method based on the pile are disclosed. The prestressed bored pile includes a solid cylindrical pile body; the pile body further includes an internal hollow cylindrical reinforcing cage and a concrete pile body, wherein: prestressing tendons are provided on the inner side of the reinforcing cage, extending along the length of the pile body and eccentrically and asymmetrically arranged relative to the center of the pile cross-section, and the prestressing tendons are arranged on the side of the concrete pile body away from the foundation pit. This invention takes into account both deformation control and construction space requirements, and can solve the problems of traditional internal supports occupying space in the pit, anchor rods being limited by external anchoring conditions, and the insufficient active deformation control capability of ordinary cast-in-place piles.

Description

Technical Field

[0001] This invention relates to the field of foundation pit support and geotechnical engineering technology, and more particularly to a prestressed bored pile and a deep foundation pit support method based on the pile, which is applicable to the construction of building foundation pit support under medium excavation depth conditions. Background Technology

[0002] With the increasing number of high-rise buildings and underground space developments, the requirements for support safety and deformation control in deep foundation pit engineering are becoming increasingly stringent. Traditional bored piles, under cantilever stress, are prone to significant lateral deformation during excavation, which may seriously affect the safety of surrounding buildings, roads, and underground pipelines.

[0003] To address the aforementioned issues, existing engineering projects typically employ methods such as internal bracing or prestressed anchor bolts to control foundation pit deformation.

[0004] However, among these, the support structures and methods based on internal bracing, such as the deep foundation pit support structure for bored piles disclosed in CN216948335U, although having greater overall rigidity and better deformation control, will occupy the construction space inside the foundation pit, affecting earthwork excavation, material transportation and main structure construction; while the support structures and methods based on prestressed anchor rods, such as the deep foundation pit support structure for bored piles disclosed in CN223867260U, have high requirements for external anchoring conditions and are often difficult to implement in conditions of dense surrounding buildings, restricted red lines or soft strata.

[0005] Therefore, there is an urgent need for a new type of support method that takes into account both deformation control and construction space requirements.

[0006] The information disclosed in this background section is intended only to enhance the understanding of the overall background of the invention and should not be construed as an admission or in any way implying that the information constitutes prior art known to those skilled in the art. Summary of the Invention

[0007] This invention aims to at least partially solve one of the technical problems in related technologies. To this end, this invention proposes a prestressed bored pile and a deep foundation pit support method based on the pile, which does not require a large construction space inside the pit, has less dependence on external anchoring conditions, and has the advantages of flexible construction organization, strong applicability, and good deformation control.

[0008] In one aspect of the present invention, a prestressed bored pile is provided, comprising a solid cylindrical bored pile body; the bored pile body further comprises an internal hollow cylindrical reinforcing cage and a concrete pile body, wherein:

[0009] The inner side of the steel cage is provided with prestressed tendons, which extend along the length of the pile body and are eccentrically and asymmetrically arranged relative to the center of the pile section. At the same time, the prestressed tendons are arranged on the side of the concrete pile body away from the foundation pit.

[0010] In some embodiments, the upper and lower ends of the prestressed tendons are fixed to the concrete pile body by the pile top anchorage and the pile bottom anchorage, respectively.

[0011] In some embodiments, the reinforcing cage includes longitudinal reinforcing bars and stirrups, and the prestressed tendons are fixed at a position between the longitudinal reinforcing bars and the center of the pile section by positioning guides evenly arranged along the pile body, while maintaining a preset eccentricity.

[0012] In some embodiments, the positioning guide includes a positioning frame and a guide pad embedded in the positioning frame. The positioning frame is fixedly mounted on the reinforcing cage, and the prestressed tendons pass through the guide pad.

[0013] In some embodiments, the prestressing tendon consists of four prestressed steel strands, each with a cross-sectional area of ​​140 mm², a control stress of 1100 MPa, a combined prestressing force of 616 kN, and an eccentricity of 0.25 m.

[0014] In another aspect of the present invention, a method for deep foundation pit support based on prestressed bored piles is also provided, comprising the following steps:

[0015] S10. Determine the parameters for the foundation pit and support;

[0016] S20. After drilling and cleaning the hole, fabricate and lower the steel cage containing prestressed tendons and install positioning guides.

[0017] S30, Concrete pouring to form retaining piles:

[0018] Concrete is poured into each borehole to form a prestressed bored pile in any technical solution of the present invention. All the prestressed bored piles together constitute the support pile.

[0019] S40, tensioning and anchoring prestressing tendons:

[0020] For each prestressed bored pile, after the pile concrete reaches the set strength, tension is applied to the prestressing tendons in the pile and they are anchored and locked, so that the pile body forms an initial bending tendency away from the foundation pit side.

[0021] S50, excavate in layers and monitor deformation.

[0022] According to the present invention, a prestressed bored pile and a deep foundation pit support method based on the pile can utilize built-in eccentric, asymmetrical prestressing tendons located away from the foundation pit. This allows the prestressed bored pile to be subjected to eccentric prestress within the pile body, thereby enabling the pile body to obtain an initial internal force state opposite to the direction of the active earth pressure on the foundation pit before excavation. Therefore, during subsequent earthwork excavation, as the earth pressure is gradually released and acts on the support pile, a portion of the lateral displacement of the pile body is offset by this initial reverse bending, thus reducing the pile top displacement and pile body deformation. Compared to ordinary cantilevered piles that rely solely on passive force, the present invention is an active control support method, which is less dependent on external anchoring conditions, has wider applicability, and better deformation control. Compared to internal support systems, the present invention does not require the installation of large-span support components within the foundation pit, freeing up construction space inside the foundation pit, facilitating mechanical operations and main structure construction, and making construction organization more flexible. It is particularly suitable for deep foundation pit support projects under medium excavation depth conditions.

[0023] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0024] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0025] Figure 1 This is a schematic diagram of the cross-sectional structure of a prestressed bored pile according to an embodiment of the present invention.

[0026] Figure 2 This is a schematic diagram of the longitudinal section structure of a prestressed bored pile according to an embodiment of the present invention.

[0027] Figure 3 This is a flowchart illustrating a deep foundation pit support method according to an embodiment of the present invention.

[0028] Meaning of the labels in the attached diagram:

[0029] 1- The pile body of the bored cast-in-place pile;

[0030] 11-Reinforcing cage;

[0031] 111 - Longitudinal reinforcing bars;

[0032] 112-Stirrups;

[0033] 113-Prestressed tendon; 1131-Pile top anchorage; 1132-Pile bottom anchorage end; 1133-Positioning guide;

[0034] 12-Concrete pile body. Detailed Implementation

[0035] Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0036] In this article, the term "prestressed eccentricity" refers to the distance from the line of action of the resultant force of the prestressing tendons to the geometric center of the pile section; "reverse pre-bending" refers to the initial bending deformation of the pile body towards the outside of the foundation pit after tensioning, which is opposite to the bending direction caused by subsequent earth pressure.

[0037] The support structures and methods used in related technologies to control foundation pit deformation, such as internal bracing or prestressed anchors, all have significant limitations in applicability: while internal bracing systems offer high rigidity, they severely encroach on the working space inside the foundation pit, affecting the construction efficiency of the main structure; prestressed anchors require sufficient anchorage length and soil strength outside the pit, which is often impossible to implement in soft soil, gravel, or under conditions where the boundary line is restricted. Therefore, how to effectively control the lateral deformation of the foundation pit without occupying a large area of ​​construction space inside the pit and without over-reliance on external anchoring conditions has become an important issue in deep foundation pit support technology.

[0038] In view of this, the embodiments of this invention aim to provide a prestressed bored pile and a deep foundation pit support method based on the pile. By setting up prestressed bored piles and utilizing built-in eccentric, asymmetrical prestressing tendons located away from the foundation pit, a reverse internal force state can be pre-established before excavation, offsetting part of the lateral deformation caused by soil pressure during excavation. This improves the active deformation control capability of the support structure composed of the prestressed bored piles (i.e., the support piles below). Compared with traditional support structures and methods based on internal bracing or anchor bolts, this invention does not require a large construction space within the pit, has less dependence on external anchoring conditions, and has the advantages of flexible construction organization, strong applicability, and good deformation control effect. Thus, it solves the above-mentioned problems.

[0039] The following is based on Figures 1-2 A prestressed bored pile according to the present invention will be described in detail.

[0040] like Figure 1 and Figure 2 As shown in the figure, an embodiment of the present invention provides a prestressed bored pile, including a solid cylindrical bored pile body 1; the bored pile body 1 further includes an internal hollow cylindrical steel cage 11 and a concrete pile body 12, wherein: the inner side of the steel cage 11 is provided with prestressing tendons 113, the prestressing tendons 113 extend along the length of the pile body and are eccentrically and asymmetrically arranged relative to the center of the pile section, and the prestressing tendons 113 are arranged on the side of the concrete pile body 12 away from the foundation pit, so that the pile body forms an initial bending tendency away from the direction of earth pressure after tensioning.

[0041] In specific implementation, the present invention applies eccentric prestress to the pile body 1 by extending along the length of the pile body and being eccentrically and asymmetrically arranged relative to the center of the pile cross section, and simultaneously arranging prestressing tendons 113 on the side of the concrete pile body 12 away from the foundation pit. This causes the support pile to form a certain reverse pre-bending before the soil pressure of the foundation pit acts, thereby offsetting some of the lateral deformation generated during the excavation process and improving the active control capability of the foundation pit support system. It is especially suitable for deep foundation pit support projects under medium excavation depth conditions.

[0042] For example, refer to Figure 2 In this embodiment of the invention, the upper and lower ends of the prestressed tendon 113 are fixed to the concrete pile body 12 by the pile top anchor 1131 and the pile bottom anchor 1132, respectively.

[0043] In practice, the tensioning and anchoring locking process is completed by the pile top anchor 1131 and the pile bottom anchoring end 1132, so that the pile body forms a preset reverse bend before excavation, thereby improving the active deformation control capability of the support pile in the subsequent excavation stage.

[0044] For example, refer to Figure 1 and Figure 2 The steel cage 11 of the present invention may include longitudinal reinforcing bars 111 and stirrups 112. The prestressed tendons 113 are fixed at the position between the longitudinal reinforcing bars 111 and the center of the pile section by positioning guides 1133 evenly arranged along the pile body and maintain a preset eccentricity.

[0045] Furthermore, the positioning guide 1133 in this embodiment of the invention may include a positioning frame and a plurality of guide pads embedded in the positioning frame. The positioning frame is fixedly mounted on the reinforcing cage, and the prestressing tendons 113 pass through the guide pads. More specifically, the positioning frame is fixed to the stirrups 112 or the longitudinal reinforcing bars 111 by binding. The plurality of guide pads are installed on the inner side of the positioning frame and maintain the same fixed spacing with respect to the geometric center of the pile section, which is used to limit the radial position of the prestressing tendons 113. That is, the prestressing tendons 113 pass through the central holes of all the guide pads in sequence, thus ensuring that the eccentricity e of the prestressing tendons relative to the geometric center of the pile section is within the design range.

[0046] In addition to the conventional longitudinal reinforcing bars 111 and stirrups 112, the reinforcing cage 11 also includes a space for installing prestressing tendons 113 and a positioning guide 1133. During construction, the skeleton of the reinforcing cage 11 is first constructed using the longitudinal reinforcing bars 111 and stirrups 112, and then the prestressing tendons 113 are installed. Specifically, the eccentric position of the prestressing tendons 113 is controlled by positioning frames and guide blocks as positioning guides 1133 to ensure that the prestressing tendons 113 do not shift significantly during concrete pouring and vibration.

[0047] Preferably, in this embodiment of the invention, the prestressing tendon 113 consists of four prestressed steel strands, each with a cross-sectional area of ​​140 mm², a control stress of 1100 MPa, a combined prestressing force of 616 kN, and an eccentricity of 0.25 m.

[0048] like Figure 3 As shown in the figure, this embodiment of the invention also provides a deep foundation pit support method based on prestressed bored piles, including the following steps:

[0049] S10. Determine the parameters for the foundation pit and support;

[0050] S20. After drilling and cleaning the hole, fabricate and lower the steel cage containing prestressed tendons, and install positioning guides.

[0051] S30, Concrete pouring to form retaining piles:

[0052] Concrete is poured into each borehole to form a prestressed bored pile in any of the schemes of this invention. All the prestressed bored piles together constitute the support pile.

[0053] S40, Tensioning and anchoring prestressing tendons (i.e., applying prestress):

[0054] For each prestressed bored pile, after the pile concrete reaches the set strength, tension is applied to the prestressing tendons in the pile and they are anchored and locked, so that the pile body forms an initial bending tendency away from the foundation pit side.

[0055] S50, excavate in layers and monitor deformation.

[0056] In this embodiment of the invention, step S10 specifically includes: determining the arrangement form, pile diameter, pile spacing, pile length and prestress parameters of the support piles based on the excavation depth, plan dimensions, surrounding environmental conditions and site soil parameters.

[0057] In step S10, regarding the design of the support pile parameters...

[0058] The support structure (i.e., the support piles) can be arranged in the form of pile rows:

[0059] Optionally, the pile diameter can be adjusted within the range of 0.6m to 1.2m, the ratio of pile spacing to pile diameter should preferably be 2.0 to 2.5, and the pile length is determined according to the depth of the foundation pit and the geological conditions, usually 2.5 to 3.5 times the excavation depth. More preferably, the pile diameter is 0.8m; the pile spacing is 1.8m; the pile length is 20m; the foundation pit excavation depth is 6m; and the top of the support piles is flush with the natural ground level.

[0060] In step S10, regarding the design of prestressing parameters

[0061] Asymmetrical prestressing tendons are installed inside the prestressed bored pile. Preferably, each pile has 4 prestressed steel strands; the cross-sectional area of ​​a single prestressing tendon is 140 mm²; the control stress is 1100 MPa; the resultant prestressing force of a single pile is 616 kN; and the eccentricity is 0.25 m. Therefore, the equivalent bending moment formed by a single pile is:

[0062]

[0063] In the formula:

[0064] P represents the resultant force of prestressed single pile;

[0065] e is the eccentricity of the prestressing action;

[0066] M is the equivalent bending moment generated by prestressing.

[0067] Therefore, in the preferred embodiment, the equivalent bending moment is:

[0068]

[0069] The pre-bending rate of the pile body generated by the equivalent bending moment is verified by the elastic foundation beam model, which can ensure that the tensile stress of the pile concrete does not exceed the design value of tensile strength under various excavation conditions.

[0070] According to mechanics of materials, under elastic working conditions, the lateral displacement of the pile top is approximately linearly inversely proportional to the applied pre-bending moment. Therefore, by reasonably selecting the eccentricity e and the resultant prestressing force P, the maximum horizontal displacement caused by excavation can be reduced by 30% to 50% without increasing the pile diameter or pile length.

[0071] In this embodiment of the invention, step S20 specifically includes: positioning the drilling rig, drilling holes according to the designed pile positions, cleaning the holes, fabricating and lowering a steel cage containing prestressed tendons, and installing positioning guides.

[0072] In step S20, regarding the hole-forming construction process...

[0073] During the hole-forming stage, rotary drilling rigs or slewing drilling rigs are used to vertically drill holes according to the designed pile positions. During construction, the hole diameter, depth, and verticality are controlled to meet design requirements. Specifically, the verticality deviation should not exceed 1 / 200; otherwise, the actual eccentricity of the prestressing tendons will deviate from the design value, affecting the pre-bending effect. The hole diameter deviation should not exceed the design allowable value, and the hole depth should reach the designed embedment depth. After hole formation, hole cleaning is performed. For example, the thickness of the sediment at the bottom of the hole should meet the requirements of the cast-in-place pile construction specifications, and the hole inclination should be controlled within the allowable range. Hole cleaning and inspection are conducted to reduce the thickness of the sediment at the bottom of the hole and ensure the quality of the pile tip and side.

[0074] Step S10 and step S20 may also include a support structure construction preparation step:

[0075] Complete site leveling and surveying, pile location layout and positioning, construction platform preparation, reinforcement cage and prestressed tendon fabrication, and the entry of construction machinery.

[0076] In step S30, regarding the grouting pile construction process...

[0077] After the installed steel cage and prestressed tendons are hoisted into the borehole as a whole, continuous concrete pouring is carried out using the tremie pipe method to avoid quality problems such as pile breakage, necking, and mud inclusion. After the concrete pouring is completed, the pile head is treated and the pile enters the curing stage. Preferably, the concrete strength grade meets the design requirements, the pouring process is continuous, the tremie pipe embedment depth meets the specification requirements, and the over-pour height at the pile top meets the requirements for subsequent chiseling and pile head treatment.

[0078] In step S40, regarding the prestressing tensioning and locking process...

[0079] After the concrete strength of the pile body reaches the design tensioning conditions, the prestressing tendons are tensioned. During tensioning, a staged loading and multiple tensioning methods are used to control the tension force and elongation, preventing localized stress concentration or pile cracking. The preferred tensioning process includes:

[0080] (1) Install jacks, oil pumps and measuring devices;

[0081] (2) Initial tensioning of the prestressed tendons is carried out one by one or in groups;

[0082] (3) Perform final tensioning according to the design control stress, with the preferred tension control stress being 1100 MPa;

[0083] (4) After reaching the target tension, anchor and lock the anchor.

[0084] (5) After locking is completed, anchor sealing, anti-corrosion and protection treatment shall be carried out.

[0085] In this embodiment of the invention, step S50 specifically includes:

[0086] S51. Layered excavation and deformation control steps: Excavate the earthwork in layers and sections according to the predetermined excavation conditions, and monitor the horizontal displacement of the pile top, the deformation of the pile body and the displacement of the soil outside the pit simultaneously.

[0087] S52. Support effect evaluation steps: Evaluate the support effect of prestressed bored piles based on monitoring results, and adjust construction parameters based on monitoring feedback.

[0088] In step S51, regarding the layered excavation method, it is preferable to proceed in two stages:

[0089] The first stage of excavation is 3m deep; the second stage of excavation is carried out to a depth of 6m to the bottom of the pit. During the excavation process, zoned, segmented, symmetrical, or balanced excavation methods are adopted to reduce support deformation caused by local over-excavation and uneven unloading.

[0090] In step S51, the monitoring process for earthwork excavation is described.

[0091] During the layered and segmented excavation of the foundation pit, deformation monitoring is conducted at each excavation stage to determine the working status of the support system. Monitoring includes the horizontal displacement of the pile tops, the distribution of horizontal displacement along the depth of the pile body, the horizontal displacement of the soil outside the pit, surface settlement, and deformation of surrounding buildings, structures, and underground pipelines. When monitoring values ​​approach warning levels, control measures can be implemented by adjusting the excavation rate, shortening the segment length, adding temporary supports, or supplementing reinforcement.

[0092] In summary, the construction process of the deep foundation pit support method of the present invention can be as follows: The pile diameter, pile length, pile spacing, and prestressing parameters of the bored piles are determined according to the excavation depth of the foundation pit, the site soil conditions, and the surrounding environment; holes are drilled and cleaned according to the designed pile positions; a steel cage with eccentrically arranged prestressed tendons is fabricated and lowered, and concrete is poured to form the support piles; after the pile concrete reaches the design strength, the prestressed tendons are tensioned and anchored, causing the pile to form an initial reverse bending away from the foundation pit side; subsequently, layered and segmented earthwork excavation is carried out, and the horizontal displacement of the pile top, the displacement of the pile body, and the deformation of the soil outside the pit are monitored simultaneously. More specifically:

[0093] Taking a rectangular foundation pit as an example, the pit's plan dimensions are 19.8m × 14.4m, and the excavation depth is 6m. The support structure uses bored cast-in-place piles with a pile diameter of 0.8m, a pile spacing of 1.8m, and a pile length of 20m, totaling 38 piles.

[0094] During construction, the site was first leveled and surveyed, and holes were drilled according to the designed pile positions. After drilling, the holes were cleaned, and a pre-fabricated reinforcing cage was lowered. The reinforcing cage contained four prestressed steel strands, each with a cross-sectional area of ​​140 mm², a control stress of 1100 MPa, a resultant prestressing force of 616 kN, and an eccentricity of 0.25 m relative to the center of the pile section. After the concrete was poured and reached the designed tension strength, the prestressing tendons were tensioned and anchored, resulting in an equivalent bending moment of approximately 154 kN·m for each pile.

[0095] After the prestressing was applied, the foundation pit was excavated in two stages: first to 3m, then to 6m. Displacement, settlement, and internal forces were continuously monitored during the excavation process. After the design elevation was reached, the foundation layer and subsequent main structure construction were carried out.

[0096] Verification has shown that this specific example scheme, through the reverse pre-bending effect generated by prestressing, reduces the lateral deformation of the pile body caused by subsequent earth pressure, thereby achieving effective control of the foundation pit stability.

[0097] In summary, the prestressed bored pile and deep foundation pit support method based on the pile provided by the embodiments of the present invention can demonstrate at least the following technical advantages in practical applications:

[0098] 1) Good deformation control effect: By applying eccentric prestress to the bored pile, the present invention can effectively reduce the horizontal displacement of the pile top and the lateral deformation of the pile body after excavation by forming a reverse pre-bending of the pile body before excavation.

[0099] 2) Can replace part of the internal support function: Under medium excavation depth conditions, the present invention can achieve a deformation control effect similar to that of a horizontal internal support, thereby reducing or replacing the setting of internal support in the pit.

[0100] 3) It does not occupy the construction space inside the pit: Since it is not necessary to set up large-volume support rods inside the foundation pit, it is conducive to earthwork excavation, machinery passage, material transportation and main structure construction.

[0101] 4) Low dependence on external anchoring conditions: This invention mainly achieves active support by applying prestress to the pile itself, without relying on the anchoring space outside the pit, and is suitable for sites with limited red lines or complex surrounding environments.

[0102] 5) Better control effect in the early stage of excavation: Prestress can be applied before excavation, which can play a role in the early stage of excavation, reduce the development of initial displacement, and is conducive to the safety of foundation pit construction in adjacent sensitive environments.

[0103] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this invention and simplifying the description, and are not intended to 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 invention.

[0104] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0105] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection, an electrical connection, or a connection that allows communication between them; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components, unless otherwise explicitly limited. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0106] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention in any way. Any simple modifications and equivalent changes made to the above embodiments based on the technical essence of the present invention shall fall within the protection scope of the present invention.

Claims

1. A prestressed bored pile, characterized in that, The pile body comprises a solid cylindrical bored pile; the bored pile body further comprises an internal hollow cylindrical steel cage and a concrete pile body, wherein: The inner side of the steel cage is provided with prestressed tendons, which extend along the length of the pile body and are eccentrically and asymmetrically arranged relative to the center of the pile section. At the same time, the prestressed tendons are arranged on the side of the concrete pile body away from the foundation pit.

2. A prestressed bored pile according to claim 1, characterized in that, The upper and lower ends of the prestressed tendon are fixed to the concrete pile body by the pile top anchor and the pile bottom anchor, respectively.

3. A prestressed bored pile according to claim 2, characterized in that, The steel cage includes longitudinal reinforcing bars and stirrups. The prestressed tendons are fixed at the position between the longitudinal reinforcing bars and the center of the pile section by positioning guides evenly arranged along the pile body, and maintain a preset eccentricity.

4. A prestressed bored pile according to claim 3, characterized in that, The positioning guide includes a positioning frame and a guide pad embedded in the positioning frame. The positioning frame is fixedly mounted on the reinforcing cage, and the prestressed tendons pass through the guide pad.

5. A prestressed bored pile according to claim 4, characterized in that, The prestressing tendon consists of four prestressed steel strands, each with a cross-sectional area of ​​140 mm², a control stress of 1100 MPa, a combined prestressing force of 616 kN, and an eccentricity of 0.25 m.

6. A deep foundation pit support method based on prestressed bored piles, characterized in that, Includes the following steps: S10. Determine the parameters for the foundation pit and support; S20. After drilling and cleaning the hole, fabricate and lower the steel cage containing prestressed tendons, and install positioning guides. S30, Concrete pouring to form retaining piles: Concrete is poured into each borehole to form a prestressed bored pile as described in any one of claims 1 to 5, and all the prestressed bored piles together constitute a support pile. S40, tensioning and anchoring prestressed tendons: For each prestressed bored pile, after the pile concrete reaches the set strength, tension is applied to the prestressing tendons in the pile and they are anchored and locked, so that the pile body forms an initial bending tendency away from the foundation pit side. S50, excavate in layers and monitor deformation.

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