A pile-anchor support foundation pit deepening and reinforcement support structure and its construction method

Abstract

The present invention relates to a pile-anchor support foundation pit deepening reinforcement support structure and a construction method thereof. The reinforcement support structure comprises original support piles (1), waist beams (3) and original prestressed anchor cables in the original pile-anchor support system, as well as newly-grown support piles (2), newly-added prestressed anchor cables (4) and newly-installed crown beams (5); the newly-grown support piles (2) are arranged between the original support piles (1), and the newly-grown support piles (2) run through the entire new excavation depth of the foundation pit, with their tops flush with the tops of the original support piles (1) and connected to form a whole through the newly-installed crown beams (5); the newly-installed prestressed anchor cables (4) are distributed between the new and old piles, and their outer ends are fixed to the waist beams (3). The present invention adopts a reinforcement method of "inserting long support piles + prestressed anchor cables between the original support piles". Through the connection of the waist beams, anchor cables and newly-installed crown beams, the structure forms a unified whole with the original support structure, which can effectively increase the overall stiffness and cooperative deformation capacity of the enclosure structure and strengthen the bearing capacity of the enclosure structure.

Description

Technical Field

[0001] The present invention relates to the technical field of foundation pit engineering support and reinforcement, and in particular to a pile-anchor foundation pit deepening and reinforcement support structure and a construction method thereof. Background Art

[0002] With the rapid development of the economy and urbanization, available land for construction is becoming increasingly scarce. To meet people's diverse needs for living and working space, the utilization and development of urban underground space has become a hot topic of increasing concern. The scale and depth of foundation pit excavations are increasing, bringing with them increased difficulties in pit support and complexity in the surrounding environment. When the foundation pit support structure is already or is about to be completed, design changes, such as the addition of a basement floor, may necessitate deeper excavation than originally planned.

[0003] However, the internal forces of the foundation pit caused by the deepening of the foundation pit will exceed the internal force design values ​​of the original foundation pit support scheme, which will inevitably lead to the original foundation pit support design being unable to meet the support strength requirements, and will have a huge impact on the overall safety and stability of the foundation pit. This leaves the existing foundation pit with two options: one is to abandon the existing foundation pit and redesign the support, or to strengthen the design based on the original support. For the construction unit, foundation pit safety and economic benefits are the main considerations. Obviously, if the existing foundation pit support structure is abandoned, it will cause a lot of waste of resources and increase the cost of the project. However, if the existing foundation pit support structure can be used for design and strengthening, and at the same time, reasonable design and transformation can be carried out to reduce the cost, it will inevitably achieve good benefits for the project. Therefore, it is of great value to strengthen the design of the support structure based on the original support structure and ensure the safety and stability of the foundation pit during construction.

[0004] In the existing technology, the main method of reinforcing the support structure after the foundation pit is fully deepened and excavated, which was originally supported by pile-anchor support, is the anchor cable layer reinforcement method, that is, after the construction of the support piles and prestressed anchor cables in the original design is completed, a layer of prestressed anchor cables is added at the appropriate location. Among them, the traditional pile-anchor support system mainly includes: support piles, anchor cables, waist beams, crown beams, etc. The support piles are vertically driven into the soil layer on the side wall of the foundation pit; the crown beam and the support piles are cast together at the top of the piles; the prestressed anchor cables are driven into the soil layer through the waist beams, and the waist beams are set along the side wall of the foundation pit, so that the anchor cables, support piles, and crown beams form a whole that can deform synergistically.

[0005] Although the above-mentioned anchor cable reinforcement method can to some extent make up for the insufficient bearing capacity of the original support system caused by the insufficient embedding depth of the original support piles due to the deepening of the foundation pit, it has the following disadvantages:

[0006] (1) The purpose of adding anchor cables is to strengthen the connection strength between the supporting pile body and the pit wall soil. By indirectly limiting the displacement of the pit wall soil through the supporting pile, the excessive displacement of the embedded end of the supporting pile can be reduced to compensate for the problem of weakened bearing capacity. However, it still does not fundamentally solve the problem of insufficient embedded depth of the original supporting pile. In some working conditions, such as when the excavation depth of the foundation pit is large and the lateral soil pressure of the foundation pit is large, this reinforcement method is easy to fail, causing great safety hazards to the construction. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to provide a pile-anchor support foundation pit deepening reinforcement support structure with better reinforcement effect and its construction method, so as to solve the problem of insufficient support strength of the pile-anchor support system due to the deepening excavation of the foundation pit.

[0008] In order to solve the above problems, the present invention provides a pile-anchor support pit deepening and reinforcement support structure, which includes the original support piles in the original pile-anchor support system, the waist beam arranged on the original support piles and the original prestressed anchor cables with the outer ends fixed on the waist beam, as well as the newly increased support piles, the newly added prestressed anchor cables and the newly installed crown beam; the newly increased support piles are arranged between the original support piles and the two are kept in the same plane, the newly increased support piles run through the new excavation depth of the entire foundation pit, and the tops thereof are flush with the tops of the original support piles and are cast and connected as a whole through the newly installed crown beam; the newly added prestressed anchor cables are distributed between the original support piles and the newly increased support piles and the outer ends thereof are fixed on the waist beam.

[0009] Preferably, the diameter of the newly added support pile is equal to or greater than the diameter of the original support pile.

[0010] Preferably, the newly added support piles are arranged at intervals of one, two or more of the original support piles.

[0011] The present invention also provides a construction method for deepening and reinforcing a pile-anchor support pit support structure, the construction method comprising:

[0012] Remove the original crown beam in the original pile-anchor support system, and try to avoid breaking the main reinforcement of the original support pile during the removal process;

[0013] Insert new support piles as required;

[0014] After all the newly added support piles are implemented, the construction of the new prestressed anchor cables and the casting of the new crown beam are completed. During the casting process, the main reinforcement of the original support piles is straightened as much as possible and anchored and cast in the new crown beam according to the principle of proximity.

[0015] Compared with the prior art, the present invention has the following advantages:

[0016] 1. The present invention adopts the reinforcement method of "inserting long support piles + prestressed anchor cables between the original support piles". The newly added support piles run through the entire excavation depth of the foundation pit, and are connected with the original support structure through the waist beam and the newly installed crown beam to form a unified whole. It can effectively increase the overall stiffness and coordinated deformation capacity of the enclosure structure and strengthen the bearing capacity of the enclosure structure. In addition, the reinforcement form of the present invention has been proven to have a good effect in controlling foundation pit deformation through numerical simulation, field tests and monitoring. 2. The present invention utilizes the existing foundation pit support structure for design reinforcement, conducts reasonable design transformation and construction, reduces construction costs, and achieves good benefits for the project. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings.

[0018] Figure 1 A schematic diagram of a reinforcement support structure provided in an embodiment of the present invention.

[0019] Figure 2 A top view of the pile distribution provided by an embodiment of the present invention.

[0020] Figure 3 A cross-sectional view of the construction of a prestressed anchor cable provided in an embodiment of the present invention.

[0021] In the figure: 1—original support piles, 2—newly added support piles, 3—waist beam, 4—newly added prestressed anchor cables, 5—newly installed crown beam. DETAILED DESCRIPTION

[0022] First of all, it should be noted that although the long and short pile support system is well known in the prior art, this is a foundation pit support method, and the present invention proposes a pile-anchor support foundation pit deepening and reinforcement support structure, which is a reinforcement method of foundation pit support. The difference lies in that the premise is to deepen the excavation and reinforce the structure, and long support piles are inserted between the original support piles. Different premises involve different fields.

[0023] refer to Figure 1 and Figure 2 An embodiment of the present invention provides a pile-anchor support foundation pit deepening and reinforcement support structure, which mainly includes the original support pile 1 in the original pile-anchor support system, the waist beam 3 arranged on the original support pile 1 and the original prestressed anchor cable with the outer end fixed on the waist beam 3, as well as the newly increased support pile 2, the newly added prestressed anchor cable 4 and the newly set crown beam 5.

[0024] New support piles 2 are laid between the existing support piles 1, with their inner sides flush and aligned. They extend throughout the new excavation depth, with their tops aligned with the tops of the existing support piles 1 and connected by a newly installed cap beam 5. The new support piles 2 remain aligned with the existing support piles 1 and do not affect the excavation surface.

[0025] The newly added prestressed anchor cables 4 are distributed between the original support piles 1 and the newly added support piles 2, and their outer ends are fixed to the waist beam 3. In actual application, the specific arrangement, spacing, number and applied load of the prestressed anchor cables depend on the specific requirements of the project.

[0026] The diameter of the newly added support pile 2 is equal to or greater than the diameter of the original support pile 1. In practical applications, the newly added support pile 2 can increase the cross-sectional size according to the design to enhance the pile rigidity while ensuring sufficient embedding depth.

[0027] New support piles 2 are arranged at intervals of one, two, or more than one existing support pile 1. In practical applications, different spacings can be selected based on the actual conditions of the foundation pit, taking into account both pit safety and economic benefits. For example, they can be arranged at intervals of one or two existing support piles. The density can be appropriately increased in areas with weaker bearing capacity. Figure 1 The figure shows the situation of "adding a long support pile for every two original support piles".

[0028] Based on the reinforcement support structure disclosed in the above embodiment, an embodiment of the present invention provides a construction method, which mainly includes the following steps:

[0029] 1. Remove the original crown beam in the original pile-anchor support system, and try to avoid breaking the main reinforcement of the original support pile 1 during the removal process.

[0030] 2. Insert new support piles 2 as required.

[0031] 3. After all the newly added support piles 2 are implemented, complete the construction of the newly added prestressed anchor cables 4 and the pouring of the new crown beam 5. During the pouring process, try to straighten the main reinforcement of the original support piles 1 as much as possible and anchor and pour them in the new crown beam 5 based on the principle of proximity.

[0032] Except for the matters mentioned and emphasized above, the construction techniques for the newly added support piles 2 in step 2, the newly added prestressed anchor cables 4 in step 3, and the newly installed crown beam 5 can be referred to in the prior art. However, to facilitate understanding of the present invention by those skilled in the art, detailed implementation details are provided below.

[0033] In the above step 2, the implementation of the newly added support pile 2 can refer to the following:

[0034] ① Hole-forming process and technical requirements

[0035] (1) The drilling can be done by mechanical drilling with mud wall protection;

[0036] (2) Before drilling, adequate preparations should be made. The drilling operation should be completed in one go without interruption and drilling should not be stopped without reason. The interval between drilling and grouting should not exceed 24 hours.

[0037] (3) When drilling, the drilling speed should be controlled to prevent the hole from collapsing and tilting;

[0038] (4) Pile position deviation ≤ 50 mm, verticality deviation < 1%, pile bottom sediment thickness ≤ 200 mm;

[0039] ②Reinforcement cage production

[0040] (1) The external dimensions of the steel cage should meet the design requirements, and the allowable deviations are as follows:

[0041] Main reinforcement spacing: ±10mm Stirrup spacing: ±20mm

[0042] Steel cage diameter: ±10mm Steel cage total length: ±100mm

[0043] Steel cage protection layer: ±20mm

[0044] (2) The main reinforcement of the steel cage can be connected by lap welding. When lap welding is used, the lap length of single-sided welding should be no less than 10 times the diameter of the steel bar, and the length of double-sided welding should be no less than 5 times the diameter of the steel bar. The reinforcement and main reinforcement in the steel cage should be connected by welding, and the stirrups and main reinforcement should be connected by tying or spot welding;

[0045] (3) The steel cage should be made in whole sections. Measures should be taken to prevent deformation during lifting, transportation and installation. External or internal hoops should be installed to strengthen the steel bars at the lifting points. When sinking in sections, the longitudinal bars should be connected by welding. Special attention should be paid to the welding quality. The number of joints on the same section shall not exceed 50%, and the spacing between adjacent joints shall be 35d and not less than 500mm. The placement direction of the steel cage should be consistent with the design direction.

[0046] ③Pile body concrete pouring

[0047] (1) When the bored pile reaches the designed elevation at the bottom of the hole, it should be inspected and accepted in time. After passing the inspection, the pile body should be poured with concrete in time.

[0048] (2) The concrete mix ratio should be determined by calculation or trial mixing;

[0049] (3) The materials used should have a quality certificate and be inspected and qualified before use;

[0050] (4) When mixing concrete, the mix ratio should be strictly followed;

[0051] (5) The concrete pouring of the pile body should be done by the conduit method. The concrete must have good workability and fluidity, and the slump is generally 180~220mm. The concrete should be poured in one continuous pouring and the density should be guaranteed.

[0052] (6) The conduit should be buried in the concrete surface. It is strictly forbidden to lift the conduit out of the concrete surface. The time for removing the conduit should be strictly controlled. Concrete pouring should be continuous. A vibrator should be fixed at the bottom of the conduit to tamp the pile concrete while pouring. During the pouring of concrete, a dedicated person should record the buried depth of the conduit and the height difference of the concrete surface inside and outside the conduit so that the conduit can be lifted and removed in time.

[0053] (7) Before the construction of the crown beam, the floating slurry on the pile top should be chiseled off and cleaned, and the length of the exposed steel bars above the pile top should meet the anchoring requirements.

[0054] In the above step 3, the implementation of the newly added prestressed anchor cable 4 can refer to the following:

[0055] ① Prestressed anchor cable drilling

[0056] Prestressed anchor cables are drilled mechanically, with the holes tilted 12-20° from the horizontal. Pipe-drilling is used to prevent collapse. To ensure anchor cable depth, the actual drilling depth must be 0.50m greater than the designed depth, taking into account the impact of sediment. After drilling, the anchor holes must be completely cleaned of sediment.

[0057] ②Prestressed anchor cable setting

[0058] The prestressed anchor cable rod should be straight, rust-free and degreased before use, and the material should be cut strictly according to the design size; before the prestressed anchor cable is placed in the anchor hole, the remaining and disturbed waste soil in the hole should be removed as much as possible; the prestressed anchor cable and the grouting pipe should be placed in the anchor hole along the center line of the anchor hole to avoid disturbance and collapse of the hole wall soil. If the hole wall soil collapses, the anchor cable should be pulled out to remove the collapsed soil; after the prestressed anchor cable is placed in the anchor hole, it should be sealed and grouting should be done in time.

[0059] ③ Prestressed anchor cable grouting

[0060] The grouting material must be formulated strictly according to the design requirements and must not be arbitrarily changed. During grouting, the grouting pipe should be inserted to a depth of 50-100mm from the bottom of the hole. The grouting pressure should reach 0.60MPa and be maintained for 2 minutes until the grout overflows from the hole. Otherwise, additional grouting should be performed. After grouting, the anchor cable must not be disturbed until the grout strength reaches the design requirements.

[0061] ④Prestressed anchor cable tensioning and locking

[0062] When the slurry strength reaches 85% of the design strength, tensioning and locking are carried out. After tensioning and locking are normal, the exposed prestressed tendons shall not be removed to facilitate the subsequent supplementary tensioning.

[0063] Actual Case

[0064] In a certain foundation pit deepening project in Lanzhou, a pile-anchor support system was adopted. Due to a later design change (basement floor addition), it was discussed that the original support structure needed to be reinforced. Therefore, the "anchor cable reinforcement design" commonly used in existing technologies was adopted. However, the deformation control of the foundation pit is an important indicator to test whether the support structure is effective. This reinforcement method did not achieve the desired effect: after using this reinforcement method, the foundation pit deformation indicators such as horizontal displacement, vertical displacement, and deep soil displacement of the support piles did not meet the requirements. It can be seen that this reinforcement scheme is not applicable to some cases where the foundation pit is deep and the lateral soil pressure of the foundation pit is large.

[0065] Therefore, based on the actual situation of the foundation pit (the foundation pit is excavated deeply and the lateral soil pressure of the foundation pit is large), we explored that the essence of the problem is that the embedding depth of the original supporting piles is insufficient. After research, we adopted a reinforcement scheme that can provide greater bearing capacity, namely the reinforcement scheme of "inserting long supporting piles between the original supporting piles + prestressed anchor cables" of the present invention: First, a rigorous numerical simulation analysis was carried out in the early stage. The results showed that the foundation pit deformation indicators all met the requirements of the relevant foundation pit specifications, and the indicators were within the safe range. Later, field tests, i.e., on-site construction, were carried out, and in conjunction with on-site monitoring of foundation pit deformation, the reinforcement effect of this reinforcement method was further verified through monitoring data, and compared with the early simulation results, the conclusion was drawn that this "inserting long supporting piles between the original supporting piles + prestressed anchor cables" reinforcement structure can well control the deformation of the foundation pit and provide a strong guarantee for the deepening and reinforcement of the pile-anchor support foundation pit.

[0066] The technical solution provided by the present invention is introduced in detail above. Specific examples are used herein to illustrate the principles and implementation methods of the present invention. The description of the above embodiments is only used to help understand the method of the present invention and its core idea. It should be pointed out that for ordinary technicians in this technical field, without departing from the principles of the present invention, the present invention can also be improved and modified in a number of ways, and these improvements and modifications also fall within the scope of protection of the claims of the present invention.

Claims

1. A construction method for deepening and reinforcing a pile-anchor support pit support structure, characterized in that: The reinforced support structure comprises original support piles (1) in the original pile-anchor support system, a waist beam (3) arranged on the original support piles (1), and original prestressed anchor cables with their outer ends fixed on the waist beam (3), as well as newly grown support piles (2), newly added prestressed anchor cables (4) and newly established crown beams (5); the newly grown support piles (2) are arranged between the original support piles (1) and the two are kept in the same plane, the newly grown support piles (2) run through the entire new excavation depth of the foundation pit, and the top of the newly grown support piles (2) is fixed to the original foundation pit. It is flush with the top of the original supporting pile (1) and is cast and connected as a whole through the newly-installed crown beam (5); the newly-added prestressed anchor cable (4) is distributed between the original supporting pile (1) and the newly-added supporting pile (2) and its outer end is fixed on the waist beam (3); the pile diameter of the newly-added supporting pile (2) is equal to or greater than the pile diameter of the original supporting pile (1); the newly-added supporting pile (2) is arranged at intervals of one, two or more of the original supporting piles (1); The construction method includes: Remove the original crown beam in the original pile-anchor support system, and try to avoid breaking the main reinforcement of the original support pile (1) during the removal process; Insert new support piles as required (2); After all the newly added support piles (2) are completed, the construction of the newly added prestressed anchor cables (4) and the pouring of the newly set crown beam (5) are completed. During the pouring process, the main reinforcement of the original support piles (1) is straightened as much as possible and anchored and poured into the newly set crown beam (5) according to the principle of proximity.

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