Fabricated foundation pit support structure capable of controlling top deformation and construction method of fabricated foundation pit support structure

Abstract

The invention discloses an assembly type foundation pit support structure for controlling top deformation and a construction method thereof.The foundation pit support structure comprises precast piles, pile top beams and shear walls with end bearing plates, the pile top beams are arranged on the upper sides of the precast piles, shear connectors are pre-buried in the pile top beams, the shear walls with the end bearing plates are arranged between every two adjacent precast piles, and the end bearing plates are arranged between the precast piles. The tops of the shear walls with the end bearing plates are fixed to the pile top beams through shear connectors. Pit top cast-in-place concrete layers are arranged on the upper sides of the pile top beams and the shear walls with the end bearing plates, and stiff steel beams are arranged in the pit top cast-in-place concrete layers. The pile top crown beam, the shear wall with the end bearing plate and the pit top cast-in-place concrete layer are in reinforced connection with a soil layer outside the foundation pit through anchor rods. By constructing a mechanical transmission path of'soil layer-anchor rod-wall beam integration-pile 'cooperative bearing, soil pressure on the outer side of the foundation pit is efficiently transmitted to the precast pile through the anchor rod, the shear wall and the top beam, and deformation of the top of the enclosure structure is effectively controlled.

Description

Technical Field

[0001] This invention relates to the field of foundation pit support technology, specifically to a prefabricated foundation pit support structure for controlling top deformation and its construction method. Background Technology

[0002] Traditional deep foundation pit retaining structures mostly use cast-in-place concrete structures, which have problems such as long construction periods, large amounts of on-site wet work, and significant impacts on quality from environmental and human factors, easily leading to safety hazards such as leakage, local instability, and even collapse. In recent years, prefabricated retaining structures have been gradually promoted and applied due to their advantages such as factory prefabrication and rapid on-site assembly. However, they still face prominent technical bottlenecks in practical applications: on the one hand, the reliability of component node connections is insufficient, making it difficult to effectively transfer bending moments and shear forces; on the other hand, the difficulty in controlling hoisting precision leads to the accumulation of assembly errors, affecting the overall load-bearing performance.

[0003] Furthermore, at the construction management level, improper coordination between earthwork excavation and support procedures often leads to over-excavation or delayed support, exacerbating the risk of deformation of the retaining structure. Complex coordination among multiple trades and inadequate supervision of key processes further amplify potential quality hazards. This is particularly prominent during deep foundation pit excavation, where excessive deformation of the top of the retaining structure is a significant problem. This not only threatens the safety of underground structure construction but also easily induces disturbance of surrounding strata, causing secondary disasters such as ground subsidence and cracking of adjacent buildings. Simultaneously, improper groundwater control can lead to water inrush and quicksand, while noise, vibration, and mud pollution generated during construction also significantly impact the urban environment and residents' lives.

[0004] Therefore, there is an urgent need for a new technical solution that can effectively control the deformation of the top of the prefabricated foundation pit retaining structure, improve the overall synergistic stress performance, and take into account both construction efficiency and environmental friendliness. Summary of the Invention

[0005] The purpose of this invention is to provide a prefabricated foundation pit retaining structure and its construction method for controlling top deformation, so as to solve the problems raised in the background art.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a prefabricated foundation pit retaining structure for controlling top deformation, comprising precast piles, a pile cap beam, and a shear wall with end bearing plates. The pile cap beam is located on the upper side of the precast piles, and shear-resistant connectors are pre-embedded inside the pile cap beam. The shear wall with end bearing plates is located between two adjacent precast piles, and the top of the shear wall with end bearing plates is fixed to the pile cap beam through shear-resistant connectors. A cast-in-place concrete layer for the pit top is provided on the upper side of the pile cap beam and the shear wall with end bearing plates, and a stiffening steel beam is provided inside the cast-in-place concrete layer for the pit top at the nodes of the precast piles, the pile cap beam, and the shear wall with end bearing plates. The pile cap beam, the shear wall with end bearing plates, and the cast-in-place concrete layer for the pit top are respectively reinforced and connected to the soil layer outside the foundation pit through anchor bolts.

[0007] Preferably, the shear connector consists of a threaded steel bar and anchor plates at both ends of the threaded steel bar. The diameter of the threaded steel bar is not less than 40mm. The thread pattern on the outer surface of the threaded steel bar is consistent with the thread pattern required for tightening the anchor plates at both ends. The number of shear connectors on the pile cap beam within the same span is not less than 2, with a spacing of not more than 1000mm. The length of the shear connector embedded in the pile cap beam is not less than 2 / 3 of the height of the pile cap beam. The top of the shear connector extends at least 120mm into the cast-in-place concrete layer at the top of the pit.

[0008] Preferably, all vertical reinforcing bars of the precast pile are anchored into the cast-in-place concrete layer at the top of the pit, with an anchorage length of not less than 40d and not less than 800mm, where d is the diameter of the vertical reinforcing bars of the precast pile, and the vertical reinforcing bars of the precast pile extend into the cast-in-place concrete layer at the top of the pit after passing through the top cap beam of the pile.

[0009] Preferably, the rigid steel beam consists of flanges on both sides and a web in the middle. Each rigid steel beam has no less than 4 anchor holes on its web. The diameter of the anchor holes is 20mm larger than the designed anchor diameter. The drilling direction of the anchor holes in the web is consistent with the designed anchor driving direction. The anchors inside the cast-in-place concrete layer at the top of the pit are driven diagonally through the anchor holes on the web of the rigid steel beam into the soil layer outside the pit. The anchors are anchored on the upper side of the cast-in-place concrete layer at the top of the pit using anchorages.

[0010] Preferably, the shear wall with end bearing plate consists of a vertical reinforced concrete shear wall panel and a horizontal reinforced concrete end bearing plate. The horizontal reinforced concrete end bearing plate is provided with a socket hole, into which the shear connector is inserted and the hole is filled with a micro-expansion, low-shrinkage, high-strength grout. The length of the outward-extending steel bars at the end of the horizontal reinforced concrete end bearing plate is not less than 450mm and all of them are anchored into the cast-in-place concrete layer at the top of the pit. The angle between the anchor rod installation direction and the horizontal plane is 45 degrees to 60 degrees.

[0011] This invention also provides a construction method for a prefabricated foundation pit retaining structure that controls top deformation. The overall construction process includes the following steps: Step S1: Measure and set out the pile positions, and construct the precast piles and cement-soil mixing piles separately; Step S2: Construction of the pile top cap beam and pre-embedding of shear connectors; Step S3: Adjustment of shear connectors, hoisting of shear wall with end bearing plate and grouting of socket holes; Step S4: High-pressure grouting to seal the joints and installing anchor bolts; Step S5: Tighten the top anchor plate of the shear connector and treat the vertical reinforcing steel bars at the top of the precast column; Step S6: Install the stiffening steel beam and install anchor bolt sleeves; Step S7: Pour the cast-in-place concrete layer at the top of the pit, and install and anchor the anchor bolts.

[0012] This invention also provides a construction method for a prefabricated foundation pit retaining structure that controls top deformation. The detailed construction process includes the following steps: Step S1: Measure and set out the pile positions, and construct precast piles and cement-soil mixing piles respectively. The precast piles are driven by static pressure method, and the vertical deviation is controlled to be ≤0.3% and the pile top elevation error is ≤10mm. The cement-soil mixing piles are set 1500-2000mm outside the corresponding precast piles, and their cross-sectional dimensions are not less than twice that of the precast piles, and correspond to the subsequent anchor positions. Step S2: Remove the top of the precast pile and treat the reinforcing steel, excavate the trench for the top cap beam, hoist the cap beam reinforcement cage and pre-embed shear connectors and anchor sleeves, anchor the top reinforcement into the cap beam, pour fine stone concrete with a strength of not less than C30 to form the top cap beam, and control the position deviation of the shear connectors to be ≤5mm and the elevation error to be ≤3mm. Step S3: After the concrete strength of the pile top cap beam reaches 100%, hoist the shear wall with end bearing plate, so that its end bearing plate is supported on the top surface of the pile top cap beam, align the socket hole with the shear connection piece, and fill the socket hole with high-strength grout. Step S4: Use the anchor holes at the bottom corner of the shear wall that do not have anchor sleeves as grouting holes to seal the gap between the shear wall and the soil outside the foundation pit in sections with high pressure grouting. After the grouting is completed, install the anchor sleeves for the holes. Step S5: After the grouting material reaches 85% or more of the design strength, tighten the anchor plate on the top of the shear connector and anchor the vertical reinforcing steel bar at the top of the precast pile into the reinforcing cage of the cast-in-place concrete layer at the top of the pit. The anchoring length shall not be less than 40d and not less than 800mm. Step S6: Install the stiffened steel beam, and drive the anchor sleeve obliquely into the soil layer outside the foundation pit through the anchor holes on its web. The direction of the anchor sleeve is consistent with the design anchor direction, and weld it to the steel cage for fixation. Step S7: Pour a cast-in-place concrete layer at the top of the pit with a strength of not less than C30. Pour the concrete symmetrically in one go without leaving construction joints, and control the top of the anchor sleeve to be exposed by 3-5mm. After the concrete strength reaches 100%, drive the anchor into the soil layer outside the pit through the anchor sleeve, and anchor the front end of the anchor into the corresponding cement-soil mixing pile.

[0013] Compared with the prior art, the beneficial effects of this invention are as follows: 1. This invention constructs a mechanical transmission path that coordinates the bearing capacity of "soil layer-anchor rod-wall beam-pile", which efficiently transmits the soil pressure on the outside of the foundation pit to the precast pile through anchor rods, shear walls and capping beams, significantly suppressing the horizontal displacement of the top of the retaining system and ensuring the safety of deep foundation pit construction and the stability of the surrounding environment; 2. This invention uses shear-resistant connectors to reliably connect the shear wall with end bearing plate to the pile cap beam, and combines it with the stiff steel beam and the cast-in-place concrete layer at the top of the pit to form a spatial overall frame, thereby realizing the effective transmission and distribution of force between prefabricated components and overcoming the defects of weak connection of traditional prefabricated nodes; 3. The shear connector of the present invention has both load-bearing function and auxiliary positioning function, guiding the shear wall socket hole to be accurately positioned, reducing the amount of on-site adjustment work, greatly shortening the construction period, and reducing the interference to the surrounding environment; 4. This invention uses pre-reserved anchor holes as grouting channels to perform high-pressure grouting to seal the gaps between the shear wall and the soil, forming a continuous and closed seepage barrier that effectively prevents groundwater leakage and soil loss. Attached Figure Description

[0014] Figure 1 This is a front view (viewed from inside the pit to the outside) of the foundation pit retaining structure of the present invention; Figure 2 yes Figure 1 Sectional view of AA; Figure 3 yes Figure 1 BB section view; Figure 4 yes Figure 1 CC section view; Figure 5 This is a schematic diagram of the structure of the stiffened steel beam of the present invention; Figure 6 This is a process flow diagram of the construction method of the present invention. Detailed Implementation

[0015] This invention discloses a prefabricated foundation pit retaining structure for controlling top deformation, such as... Figures 1 to 4 As shown, the foundation pit retaining structure includes precast piles 1, pile cap beams 2, shear walls with end bearing plates 3, shear connectors 4, stiff steel beams 5, cast-in-place concrete layer at the top of the pit 6, and anchor bolts 7.

[0016] A pile cap beam 2 is provided on the upper side of the precast pile 1. Shear connectors 4 are pre-embedded inside the pile cap beam 2. A shear wall 3 with end bearing plate is provided between two adjacent precast piles 1. The top of the shear wall 3 with end bearing plate is fixed to the pile cap beam 2 through the shear connectors 4. A stiffening steel beam 5 is provided at the joint of the precast pile 1, the pile cap beam 2, and the shear wall 3 with end bearing plate. Anchor rods 7 are used to reinforce the connection between the pile cap beam 2 and the shear wall 3 with end bearing plate and the soil layer outside the foundation pit. A cast-in-place concrete layer 6 with pit top is also provided on the upper side of the pile cap beam 2 and the shear wall 3 with end bearing plate. The cast-in-place concrete layer 6 with pit top is also driven into the soil layer outside the foundation pit at an angle using anchor rods 7.

[0017] The shear connector 4 inside the pile cap beam 2 consists of a threaded steel bar and anchor plates at both ends of the threaded steel bar. The diameter of the threaded steel bar is not less than 40mm. The thread pattern on the outer surface of the threaded steel bar is consistent with the thread pattern required for tightening the anchor plates at both ends. The number of shear connectors 4 inside the pile cap beam 2 in the same span is not less than 2, with a spacing of not more than 1000mm. The length of the shear connector 4 embedded inside the pile cap beam 2 is not less than 2 / 3 of the height of the pile cap beam 2. The top of the shear connector 4 extends at least 120mm into the cast-in-place concrete layer 6 at the top of the pit.

[0018] At the joint of the precast pile 1, the pile top cap beam 2, and the shear wall 3 with end bearing plate, all the vertical reinforcing bars of the precast pile 1 are anchored into the cast-in-place concrete layer 6 at the top of the pit, with an anchorage length of not less than 40d and not less than 800mm, where d is the diameter of the vertical reinforcing bars of the precast pile 1. The design of the vertical reinforcing bars of the precast pile 1 is to pass through the pile top cap beam 2 and then extend into the cast-in-place concrete layer 6 at the top of the pit with a 90-degree hook.

[0019] The stiffening steel beam 5 at the joint of the precast pile 1, the pile cap beam 2, and the shear wall with end bearing plate 3 consists of flanges 51 on both sides and a web 52 in the middle. (See reference) Figure 5 Each stiffened steel beam web 52 has at least four anchor bolt holes, the diameter of which is 20mm larger than the designed anchor bolt diameter. The drilling direction of the anchor bolt holes on the web 52 is consistent with the designed anchor bolt direction. The anchor bolts 7 inside the cast-in-place concrete layer 6 at the top of the pit are driven obliquely through the anchor bolt holes on the stiffened steel beam web 52 into the soil layer outside the pit. All anchor bolts 7 are anchored on the upper side of the cast-in-place concrete layer 6 at the top of the pit using anchorages. After the pit is backfilled, the stiffened steel beams 5, being on the top surface of the pit, can be recycled and reused after processing.

[0020] See Figure 3 The shear wall 3 with end bearing plate consists of a vertical reinforced concrete shear wall plate 31 and a horizontal reinforced concrete end bearing plate 32, which are integrally formed in a prefabricated PC factory. The horizontal reinforced concrete end bearing plate 32 is provided with a socket hole 33. The shear connector 4 provided inside the pile top cap beam 2 is inserted into the socket hole 33 of the horizontal reinforced concrete end bearing plate 32 at the top of the shear wall 3 with end bearing plate, and the hole is filled with micro-expansion, low shrinkage, high strength grout. The end of the horizontal reinforced concrete end bearing plate 32 is also provided with an extended steel bar. The extended steel bar is not less than 450mm in length and is all anchored into the cast-in-place concrete layer 6 at the top of the pit. The angle between the anchor rod 7 and the horizontal plane is 45 degrees to 60 degrees.

[0021] The overall plan follows the technical approach of "strictly controlling the deformation of the top of the foundation pit retaining structure, constructing an integrated soil-anchor-wall-beam-pile bearing capacity transfer path, strengthening the overall component's synergistic stress, using shear-resistant connectors to both bear the load and assist in the installation of shear walls, using stiff steel beams in conjunction with anchors for inclined soil stabilization, and fully pouring the cast-in-place concrete layer at the top of the pit".

[0022] Based on the above-mentioned prefabricated foundation pit retaining structure and corresponding technical solutions, the following construction methods are provided for reference. Figure 6 The overall construction process is as follows: Step S1: Measure and set out the pile positions, and construct the precast piles and cement-soil mixing piles separately; Step S2: Construction of the pile top cap beam and pre-embedding of shear connectors; Step S3: Adjustment of shear connectors, hoisting of shear wall with end bearing plate and grouting of socket holes; Step S4: High-pressure grouting to seal the joints and installing anchor bolts; Step S5: Tighten the top anchor plate of the shear connector and treat the vertical reinforcing steel bars at the top of the precast column; Step S6: Install the stiffening steel beam and install anchor bolt sleeves; Step S7: Pour the cast-in-place concrete layer at the top of the pit, and install and anchor the anchor bolts.

[0023] The detailed operating steps of the above construction process are as follows: Step S1: Measure and set out the pile positions; construct precast piles and cement-soil mixing piles separately. The on-site retaining pile construction adopts a double-row pile design scheme of "precast piles + cement-soil mixing piles". The design involves driving each anchor rod into the soil layer outside the foundation pit and then anchoring it into the cement-soil mixing pile at the corresponding position.

[0024] First, a high-precision site control network was established based on the design drawings. A total station and level were used for three-level verification to accurately determine the center point of the precast pile location, the control line for the pile top elevation, and the excavation location of the pile top capping beam trench. A cross-shaped positioning marker was set at each pile location, and the planar position deviation had to be controlled within ±10mm. The precast piles were driven using the static pressure method to reduce noise and vibration.

[0025] Before pile driving, accurately verify the quality certificate of the precast piles and check their basic dimensions and appearance quality. Calculated balance points are used for pile lifting, and a double theodolite is used to correct verticality during pile insertion, ensuring a vertical deviation ≤0.3%. During the pile driving process, the pile driving force and penetration depth must be continuously monitored. After reaching the design final driving standard, the pile top elevation is strictly controlled, with an error within 10mm, and the pile position deviation is checked immediately.

[0026] To facilitate the installation of anchor bolts in subsequent processes, cement-soil mixing piles are installed within a range of 1500mm to 2000mm on the outer side of each precast pile. The diameter or side length of the cement-soil mixing piles is not less than twice the design precast pile. The position of each cement-soil mixing pile corresponds to the anchor bolt position on the pile top cap beam and the shear wall with end bearing plate. The entire process of construction of the precast piles and cement-soil mixing piles is inspected and recorded.

[0027] Step S2: Construction of the pile top cap beam and pre-embedding of shear connectors. According to the technical plan, after the precast piles and cement-soil mixing piles are constructed, the top of each precast pile will be chiseled off. The top reinforcement bars after chiseling will be treated according to the anchorage length requirements. Then, the trench for the top cap beam will be excavated, the top cap beam reinforcement cage and the pre-embedded shear connectors will be hoisted and placed, and the adjusted top reinforcement bars will be passed horizontally through the top cap beam reinforcement cage. At the same time, the anchor bolts at the top cap beam will be driven into the anchor sleeves one by one. Finally, the top cap beam will be poured with fine stone concrete with a strength of not less than C30 in one go, without leaving any construction joints.

[0028] During concrete vibration, the anchor sleeves and pre-embedded shear connectors must not be disturbed. Focus on vibrating and compacting the precast piles and the pile top cap beam joints. Before the concrete initially sets, check and adjust the positional relationship of the shear connectors. The positional deviation of the shear connectors must not exceed 5mm, and the elevation error must not exceed 3mm. Conduct acceptance and record-keeping for concealed works.

[0029] Step S3: Hoisting of the shear wall with end bearing plate and grouting of the socket holes. According to the technical plan, after the construction of the pile top cap beam is completed, it will be strengthened for curing. After the concrete strength of the pile top cap beam reaches 100% of the design strength, the anchor rods on the pile top cap beam will be installed one by one according to the design plan, and then the shear wall with end bearing plate will be hoisted.

[0030] The on-site hoisting of the shear wall with end bearing plates adopts the technical solution of "slow placement, end bearing plate supported on the top of the pile cap beam, centering the socket hole, filling the socket hole with high-strength grout, and anchoring the extended steel bars into the cast-in-place concrete layer at the top of the pit". That is, the positional relationship of the shear connector is first checked and corrected again. The shear wall with end bearing plates is first slowly placed on the top of the pile cap beam using hoisting equipment. With manual assistance, the socket hole on the end bearing plate of the shear wall with end bearing plates is accurately aligned with the shear connector pre-embedded in the pile cap beam. Finally, the cavity inside the socket hole is filled with high-strength sulfur mortar grout.

[0031] At the same time, the protruding steel bars on the end bearing plate are straightened, and the anchorage length of the protruding steel bars is checked. If the protruding steel bars cannot meet the design or specification requirements due to the entire process of production, transportation, hoisting, etc., anchor plates can be installed at the ends of the protruding steel bars to meet the design or specification technical requirements. Process acceptance and recording should be done well.

[0032] Step S4: High-pressure grouting for sealing, and installation of anchor bolts. According to the technical plan, once the shear wall with end bearing plates is installed, the grouting and sealing work between the shear wall with end bearing plates and the soil layer outside the foundation pit can be carried out.

[0033] The grouting and sealing method between the shear wall with end bearing plates and the soil layer outside the foundation pit adopts the following technical solution: "The anchor holes of the shear wall with end bearing plates also serve as grouting holes. Before grouting, the anchor sleeves are sealed. One anchor hole is reserved at the bottom as a grouting hole. High-pressure grouting is performed in sections, and the grouting is completed in one go." First, anchor sleeves are installed in the anchor holes of the shear wall with end bearing plates. The anchor sleeves cut through the anchor holes and directly cut out the soil layer outside the foundation pit. The end of the anchor sleeve extends 10mm beyond the surface of the shear wall with end bearing plates. One anchor hole at the bottom corner of the shear wall with end bearing plates is left without an anchor sleeve. Then, high-pressure equipment is used to inject high-strength grout into the anchor hole without an anchor sleeve. After the sealing process is completed, it is left to stand for 30 minutes. Then, a second pressurization and grouting is performed. Finally, the anchor holes at the grouting holes are filled with anchor sleeves. The process is strengthened, disturbance is reduced, and the process acceptance and recording are done well.

[0034] Step S5: Tighten the top anchor plate of the shear connector and treat the vertical reinforcing steel bars at the top of the precast column. According to the technical plan, after the grouting and sealing of the shear wall with end bearing plate and the soil layer outside the foundation pit are completed, and the high-pressure grouting material for sealing is cured to more than 85% of the design strength, the anchor plates are tightened one by one on the top of the shear connectors pre-embedded inside the pile cap beam. The number of anchor plates tightened is based on 100% of the number of shear connectors.

[0035] For the treatment of the vertical reinforcing bars at the top of the precast columns, the anchorage length of the vertical reinforcing bars at the top of the precast piles was readjusted and checked again. The reinforcing cage inside the cast-in-place concrete layer at the top of the pit was hoisted to the design position in sections. The positional relationship between the reinforcing cage and the shear connectors must not conflict. BIM software was used to simulate collisions throughout the entire process of designing, processing, cutting, and binding the reinforcing cage inside the cast-in-place concrete layer at the top of the pit, and the technical requirements were considered in advance. At the same time, all the vertical reinforcing bars of the precast columns were anchored into the interior of the reinforcing cage in the cast-in-place concrete layer at the top of the pit, with an anchorage length of not less than 40d and not less than 800mm, to meet the specifications. Process acceptance and recording were carried out.

[0036] Step S6: Install the stiffening steel beam and install anchor sleeves. According to the technical solution, the top anchor plate of the shear connector that has been tightened and installed, and the treatment of the vertical reinforcing steel bars at the top of the precast column are checked. The installation of the stiffening steel beam is assisted by setting up stirrups or permanent support frames. The stiffening steel beam at the joint between the precast pile and the cast-in-place concrete layer at the top of the pit is connected to the stirrups or permanent support frames by welding. The stirrups or permanent support frames are also connected to the reinforcing cage in the cast-in-place concrete layer at the top of the pit by welding.

[0037] After the rigid steel beams are installed, anchor sleeves are driven obliquely into the soil layer of the foundation pit through anchor holes opened in the web of the rigid steel beams. The direction or angle of the anchor sleeves is consistent with the design direction or angle of the anchor bolts. Simultaneously, it is ensured that the installed rigid steel beams are not disturbed during the installation of the anchor sleeves. Tie bars are added and welded to the anchor sleeves and reinforcing cage to assist in the fixation of the anchor sleeves. After the foundation pit is backfilled, the rigid steel beams can be recycled and reused to reduce construction costs. Proper process acceptance and recording are essential.

[0038] Step S7: Pour the cast-in-place concrete layer at the top of the pit, and install and anchor the anchor bolts. According to the technical plan, after the installation of the rigid steel beams is completed and the hidden works such as the anchor sleeves and the reinforcing cage of the cast-in-place concrete layer at the top of the pit are verified to be correct, the pouring of the cast-in-place concrete layer at the top of the pit can be carried out.

[0039] During on-site construction, the strength of the cast-in-place concrete layer at the top of the pit should not be lower than C30. A technical solution is adopted: "first pour the concrete at the corners of the pit, then at the pile-beam joints, and finally at the remaining areas, pouring symmetrically in one go without leaving construction joints." During the pouring of the cast-in-place concrete layer at the top of the pit, the protection of all internal embedded parts is strengthened. The pouring work is based on controlling the elevation of the top surface of the cast-in-place concrete layer at the top of the pit. The top of the anchor sleeves inserted obliquely inside the stiffening steel beams should be exposed 3mm~5mm above the elevation of the top surface of the cast-in-place concrete layer at the top of the pit.

[0040] After the cast-in-place concrete layer at the top of the pit is poured, it is thoroughly cured. Once the concrete strength reaches 100% of the design strength, the anchor rods are driven one by one into the soil layer outside the pit from inside the anchor rod sleeve. It is particularly important to note that the front end of all anchor rods driven into the soil layer outside the pit should be driven into the cement-soil mixing pile outside the precast column. Process acceptance and recording should be carried out.

[0041] In summary, this invention constructs a mechanical transmission path that coordinates the load-bearing capacity of the soil layer, anchor rod, wall beam, and pile, efficiently transferring the soil pressure outside the foundation pit to the precast piles via anchor rods, shear walls, and capping beams. This effectively controls the deformation of the top of the retaining structure and significantly improves overall stability and construction safety.

[0042] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A prefabricated foundation pit retaining structure for controlling top deformation, characterized in that: The structure includes precast piles, pile cap beams, and shear walls with end bearing plates. The pile cap beams are located on top of the precast piles and have shear connectors embedded inside. The shear walls with end bearing plates are located between two adjacent precast piles, and the top of the shear walls with end bearing plates is fixed to the pile cap beams via shear connectors. A cast-in-place concrete layer is provided above the pile cap beams and the shear walls with end bearing plates. At the nodes of the precast piles, pile cap beams, and shear walls with end bearing plates, stiffening steel beams are installed inside the cast-in-place concrete layer at the top of the pit. The pile cap beams, shear walls with end bearing plates, and cast-in-place concrete layers at the top of the pit are reinforced and connected to the soil outside the pit by anchor bolts.

2. The foundation pit retaining structure according to claim 1, characterized in that: The shear connector consists of a threaded steel bar and anchor plates at both ends of the threaded steel bar. The diameter of the threaded steel bar is not less than 40mm. The thread pattern on the outer surface of the threaded steel bar is consistent with the thread pattern required for tightening the anchor plates at both ends. There are no less than 2 shear connectors on the pile cap beam within the same span, with a spacing of no more than 1000mm. The length of the shear connector embedded in the pile cap beam is not less than 2 / 3 of the height of the pile cap beam. The top of the shear connector extends at least 120mm into the cast-in-place concrete layer at the top of the pit.

3. The foundation pit retaining structure according to claim 1, characterized in that: All vertical reinforcing bars of the precast pile are anchored into the cast-in-place concrete layer at the top of the pit, with an anchorage length of not less than 40d and not less than 800mm, where d is the diameter of the vertical reinforcing bars of the precast pile. After passing through the top cap beam, the vertical reinforcing bars of the precast pile extend into the cast-in-place concrete layer at the top of the pit with a 90-degree hook.

4. The foundation pit retaining structure according to claim 1, characterized in that: The rigid steel beam consists of flanges on both sides and a web in the middle. Each rigid steel beam has no less than 4 anchor holes on its web. The diameter of the anchor holes is 20mm larger than the designed anchor diameter. The drilling direction of the anchor holes in the web is consistent with the designed anchor driving direction. The anchors inside the cast-in-place concrete layer at the top of the pit are driven diagonally through the anchor holes on the web of the rigid steel beam into the soil layer outside the pit. The anchors are anchored on the upper side of the cast-in-place concrete layer at the top of the pit using anchorages.

5. The foundation pit retaining structure according to claim 1, characterized in that: The shear wall with end bearing plate consists of a vertical reinforced concrete shear wall panel and a horizontal reinforced concrete end bearing plate. The horizontal reinforced concrete end bearing plate is provided with socket holes, into which shear connectors are inserted and filled with micro-expansion, low-shrinkage, high-strength grout. The length of the outward-extending steel bars at the end of the horizontal reinforced concrete end bearing plate is not less than 450mm and all of them are anchored into the cast-in-place concrete layer at the top of the pit. The angle between the anchor rod installation direction and the horizontal plane is 45 degrees to 60 degrees.

6. A construction method for a prefabricated foundation pit retaining structure with controlled top deformation, characterized in that, The overall construction process includes the following steps: Step S1: Measure and set out the pile positions, and construct the precast piles and cement-soil mixing piles separately; Step S2: Construction of the pile top cap beam and pre-embedding of shear connectors; Step S3: Adjustment of shear connectors, hoisting of shear wall with end bearing plate and grouting of socket holes; Step S4: High-pressure grouting to seal the joints and installing anchor bolts; Step S5: Tighten the top anchor plate of the shear connector and treat the vertical reinforcing steel bars at the top of the precast column; Step S6: Install the stiffening steel beam and install anchor bolt sleeves; Step S7: Pour the cast-in-place concrete layer at the top of the pit, and install and anchor the anchor bolts.

7. A construction method for a prefabricated foundation pit retaining structure with controlled top deformation, characterized in that, The detailed construction process includes the following steps: Step S1: Measure and set out the pile positions, and construct precast piles and cement-soil mixing piles respectively. The precast piles are driven by static pressure method, and the vertical deviation is controlled to be ≤0.3% and the pile top elevation error is ≤10mm. The cement-soil mixing piles are set 1500-2000mm outside the corresponding precast piles, and their cross-sectional dimensions are not less than twice that of the precast piles, and correspond to the subsequent anchor positions. Step S2: Remove the top of the precast pile and treat the reinforcing steel, excavate the trench for the top cap beam, hoist the cap beam reinforcement cage and pre-embed shear connectors and anchor sleeves, anchor the top reinforcement into the cap beam, pour fine stone concrete with a strength of not less than C30 to form the top cap beam, and control the position deviation of the shear connectors to be ≤5mm and the elevation error to be ≤3mm. Step S3: After the concrete strength of the pile top cap beam reaches 100%, hoist the shear wall with end bearing plate, so that its end bearing plate is supported on the top surface of the pile top cap beam, align the socket hole with the shear connection piece, and fill the socket hole with high-strength grout. Step S4: Use the anchor holes at the bottom corner of the shear wall that do not have anchor sleeves as grouting holes to seal the gap between the shear wall and the soil outside the foundation pit in sections with high pressure grouting. After the grouting is completed, install the anchor sleeves for the holes. Step S5: After the grouting material reaches 85% or more of the design strength, tighten the anchor plate on the top of the shear connector and anchor the vertical reinforcing steel bar at the top of the precast pile into the reinforcing cage of the cast-in-place concrete layer at the top of the pit. The anchoring length shall not be less than 40d and not less than 800mm. Step S6: Install the stiffened steel beam, and drive the anchor sleeve obliquely into the soil layer outside the foundation pit through the anchor holes on its web. The direction of the anchor sleeve is consistent with the design anchor direction, and weld it to the steel cage for fixation. Step S7: Pour a cast-in-place concrete layer at the top of the pit with a strength of not less than C30. Pour the concrete symmetrically in one go without leaving construction joints, and control the top of the anchor sleeve to be exposed by 3-5mm. After the concrete strength reaches 100%, drive the anchor into the soil layer outside the pit through the anchor sleeve, and anchor the front end of the anchor into the corresponding cement-soil mixing pile.

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