Prestressed Double-Casing Prefabricated Foundation Pit Retaining Structure and its Construction Method

Abstract

This invention discloses a prestressed double-casing prefabricated foundation pit retaining structure and its construction method. The structure includes precast piles, a pile-top casing, and an outer casing. The pile-top casing is located on top of the precast piles, and the outer casing is located inside the foundation pit soil layer and outside the pile-top casing. Unbonded prestressed tendons are provided between the pile-top casing and the outer casing, and a pit top slope protection layer is laid on top of both. A composite slab with end-bearing beams and a combined steel support beam are provided between adjacent precast piles, and the two are staggered from top to bottom. Fine stone concrete is poured between the composite slab with end-bearing beams and the outer soil layer of the foundation pit. Anchor rods driven into the outer soil layer of the foundation pit are provided on the composite slab with end-bearing beams. This invention achieves the technical effects of "closed cast-in-place reinforced concrete pile-top casing controlling top deformation, support-type outer casing combined with prestressed tendons strengthening the system's stress, combined steel support beams assisting the overall synergistic bearing of each component while also providing water-stopping function, and wave-shaped staggered anchor rod group strengthening the bearing capacity of the retaining structure".

Description

Technical Field

[0001] This invention relates to the field of prefabricated foundation pit support technology, specifically to a prestressed double-casing prefabricated foundation pit support structure and its construction method. Background Technology

[0002] This invention relates to the field of prefabricated foundation pit support technology. Traditional deep foundation pit support often uses cast-in-place concrete structures, which suffer from problems such as long construction cycles, large amounts of wet work, and poor quality stability, making them prone to safety hazards such as leakage and instability. Although existing prefabricated support structures can achieve factory prefabrication and rapid on-site assembly, they still have defects such as insufficient reliability of node connections, poor force transmission, and difficulty in controlling assembly accuracy, resulting in poor overall coordinated stress performance.

[0003] The top of the foundation pit is susceptible to external loads, leading to significant deformation and water leakage. This can cause increased overall deformation of the retaining structure, reduced load-bearing capacity, and threaten construction safety. It may also cause disturbance to the surrounding strata and damage to buildings and structures. Furthermore, poor coordination between excavation and support procedures, insufficient control over key processes, and inadequate groundwater management can further exacerbate the risks of deformation and leakage in the retaining structure, making it difficult to meet the requirements for safe, efficient, and green construction in deep foundation pit projects. Summary of the Invention

[0004] The purpose of this invention is to provide a prestressed double-casing prefabricated foundation pit retaining structure and its construction method to solve the problems raised in the background art.

[0005] To achieve the above objectives, the present invention provides the following technical solution: a prestressed double-casing prefabricated foundation pit retaining structure, comprising multiple precast piles, a pile-top casing, and an outer casing. The multiple precast piles are driven below the top elevation of the foundation pit. The pile-top casing is located at the top of the multiple precast piles. The outer casing is located inside the foundation pit soil layer and outside the pile-top casing. Both the pile-top casing and the outer casing are continuous and closed structures. Multiple unbonded prestressed tendons are provided between the pile-top casing and the outer casing. The top of both is covered with a pit top slope protection layer; between two adjacent precast piles, there are staggered composite slabs with end bearing beams and composite steel support beams from top to bottom. The two ends of the composite steel support beams are fixed to the precast piles, and the composite slabs with end bearing beams are fixed to the composite steel support beams by high-strength bolts through holes; the cavity between the composite slabs with end bearing beams and the soil layer outside the pit is filled with fine stone concrete, and each composite slab with end bearing beams is provided with no less than 6 anchor rods, all of which are driven into the soil layer outside the pit.

[0006] Preferably, the composite steel support beam includes a ribbed cross steel beam in the middle and end bearing plates at both ends, and the composite steel support beam is fixed to the precast pile through the end bearing plates.

[0007] Preferably, the ribbed cross steel beam includes a cross steel beam and steel ribs, wherein the steel ribs are arranged perpendicular to the flanges of the cross steel beam in the horizontal direction.

[0008] Preferably, the steel rib plate is provided with multiple mounting holes, and the composite plate with end bearing beam is fixed to the steel rib plate by high-strength bolts through the holes, and the positions of the mounting holes correspond one-to-one with the positions of the high-strength bolts.

[0009] Preferably, the composite slab with end bearing beams includes a composite slab with truss reinforcement in the middle and end bearing beams with enlarged cross-sections at the top and bottom. The composite slab with truss reinforcement has pre-reserved anchor holes on its surface. The anchors are driven into the soil layer outside the foundation pit through the anchor holes. Each composite slab with truss reinforcement has at least two rows of anchors, and each row has no less than 3 anchors. The two adjacent rows of anchors are staggered in vertical position.

[0010] This invention also applies for a construction method for a prestressed double-casing prefabricated foundation pit retaining structure, wherein the foundation pit retaining structure is the aforementioned foundation pit retaining structure, and the construction method includes the following steps: Step S1: Measurement, layout, positioning of double protective sleeves, and acceptance of prefabricated components upon arrival at the site. The total station was used to accurately measure the planar position of the pile top casing and the outer casing, and the precast pile driving points were marked within the pile top casing position line to complete the preparation for pile driving and trench excavation. The precast piles are pre-embedded with internal threads at the joint position with the composite steel support beam. The thread size meets the shear bearing capacity requirements, and the thread position corresponds to the bolt hole position of the end plate of the composite steel support beam. When the precast piles arrive at the site, the internal thread hole is sealed to prevent blockage during the pile driving process. The composite steel support beam is formed by welding a central ribbed cross steel beam and end bearing plates together. The steel ribs of the ribbed cross steel beam have several mounting holes, the positions of which correspond to the positions of the high-strength bolts at the ends of the composite plate with end bearing beams. The composite steel support beam is used to construct the frame load-bearing system, assist in the positioning and fixing of the composite plate with end bearing beams, and ensure the integrity of the layered assembly of the foundation pit retaining structure. The cross steel beam and steel ribs are connected across layers to take into account the comprehensive design of the water-stopping device. The composite slab with end bearing beams is integrally formed by the composite slab with truss reinforcement in the middle and the end bearing beams with enlarged cross sections at the top and bottom ends. Anchor holes are reserved on the surface of the composite slab according to the distribution of anchor rods, and the anchor rods are driven into the soil layer outside the foundation pit through the anchor holes. Precast piles, composite steel support beams, composite slabs with end bearing beams and embedded ducts shall be inspected upon arrival at the site. Components with quality defects that may affect hoisting and construction shall not be allowed to enter the site, and inspection records shall be kept. Step S2: Precast pile driving and double-casing trench excavation: After the prefabricated components pass the acceptance test, the prefabricated piles are driven, and the piles are accurately positioned according to the measured positions of the pile top casing and the outer casing. The construction method adopted is "first driving the precast piles in the middle of each side of the pit, then driving the L-shaped precast piles in the corners, and using the guide piles to statically press them to the design elevation below the top of the pit, with the top of the piles at a uniform elevation": using fixed elevation control points as a reference, piles are driven sequentially from the middle of each side of the pit to the corners, controlling the plane position error to be no more than 50mm and the verticality deviation to be no more than 0.3%; L-shaped enlarged cross-section precast piles are used in the corners of the pit, and reusable precast short piles of the same cross-section are used as guide piles to statically press the precast piles to the design elevation below the top of the pit, with the top elevation error controlled within -30mm; Excavate double-casing trenches according to the control lines set by total station. The distance between the pile top casing and the outer casing should not be less than 2200mm, and the slip surface of the foundation pit should be located between the two. The thickness of the casing section should not be less than 1.5 times the side length of the precast pile in the same direction and not less than 800mm. The length of each casing should not be less than 3000mm. The length of the outer casing should be at least 500mm longer than that of the pile top casing and should serve as the anchor support for the prestressing tendon tensioning. Excavate the pile top casing and outer casing trenches separately. The pile top casing trench should be excavated to the top surface of the precast pile that has been placed. Step S3: Installation of the composite steel support beam at the pile head at the bottom of the trench, hoisting of the reinforcing cage for the pile top casing, and installation of the steel sleeve: After the double-casing trench is excavated, the bottom of the trench and the top of the precast piles are cleaned of the soil. The soil at the pile head is manually excavated until the internal thread of the pile body is exposed. The thread is cleaned and the internal sealing parts are removed. The combined steel support beam at the pile head at the bottom of the trench is hoisted and supported by the original soil. It is installed and fixed to the pile head with high-strength bolts so that it can vertically support the joint of the upper and lower end-bearing beam composite plate. Wait for the concrete of the pile top casing to be poured. The double-casing steel cage is hoisted into the trench in sections; during the hoisting of the steel cage at the top of the pile, steel sleeves are pre-embedded according to the location of the unbonded prestressed tendons. The inner diameter of the steel sleeves is more than twice the outer diameter of the unbonded prestressed tendons, and both ends are inserted into the soil layer of the trench wall to complete the concealed acceptance. Step S4: Concrete pouring for the pile top casing, and probing and threading of unbonded prestressed tendons: The process involves "first pouring the concrete for the pile top casing, excavating the foundation pit to form a working surface, then drilling and threading the prestressed tendons, and finally pouring the outer casing concrete." After the pile top casing steel cage and steel sleeve are installed, pour no less than C30 concrete up to 100mm below the top surface of the steel cage and cure it. Once the concrete strength of the pile top casing reaches more than 80% of the design strength, the soil layer of the foundation pit within its range is excavated, and the concrete surface and steel casing opening are cleaned. Drilling equipment is used to explore the hole from the steel casing opening towards the outer casing trench. One end of the unbonded prestressed tendon is fixed to the drill head and pulled out with the exploration equipment to complete the traction-type tendon insertion. The process acceptance record is kept. Step S5: Concrete pouring for the outer casing, construction of the pit top slope protection layer: After the unbonded prestressed tendons are installed, the debris at the bottom of the outer casing trench is cleaned up. The unbonded prestressed tendons are pre-fixed to the outside of the outer casing steel cage using the method of "anchoring one end first and tensioning the other end later". Fine stone concrete with a strength of not less than C30 is poured up to 100mm below the top surface of the casing steel cage and cured. Tie the reinforcement of the top slope protection layer of the pit, with the longitudinal reinforcement extending into the top casing of the pile and the exposed reinforcement of the top of the outer casing respectively, using a double-layer, two-way arrangement; use fine stone concrete with a strength of not less than C30 to cast the top of the pile casing, the top slope protection layer of the pit, and the top of the outer casing into a whole in one go, without leaving construction joints, strengthen curing and acceptance. Step S6: Tensioning and anchoring of unbonded prestressed tendons, excavation of soil below the pile top elevation: After the concrete strength of the pile top casing, the pit top slope protection layer and the outer casing reaches 100% of the design strength, the unbonded prestressed tendons are tensioned and anchored. The 1.05σcon over-tensioning and single-sided tensioning method is adopted. The tensioning is symmetrical from the middle to both ends, and the anchoring is done as the tensioning is carried out. All prestressed tendons are anchored to the inner end face of the pile top casing near the pit. After the prestressed tendons are tensioned and anchored, the soil below the pile top elevation is excavated in layers and sections. The soil between piles, the surface of the composite steel support beam at the bottom of the pile top casing, and the miscellaneous soil in the pile body are cleaned to avoid disturbing the pit wall and reduce soil collapse. Process acceptance is carried out. Step S7: Clean the reserved holes of the composite steel support beam, and hoist the lower composite steel support beam: Clean the reserved installation holes on the steel ribs of the composite steel support beam at the bottom of the pile top casing, pass the high-strength bolts through the reserved holes from the inside to the outside, and temporarily fix them with nuts on the outside to prepare for the hoisting of the composite slab with end bearing beam; After the excavation of this layer of earthwork is completed, hoist the next layer of composite steel support beam at the bottom of the earthwork, clean the reserved holes and temporarily fix the high-strength bolts according to the same process, complete the preparatory work and record the acceptance. Step S8: Hoisting of the composite slab with end bearing beams and installation of anchor bolt sleeves: After verifying that the position of the high-strength bolts on the composite steel support beam is correct, the composite plate with end bearing beam is hoisted into place using hoisting equipment, so that the mounting holes at the upper and lower ends are aligned with the high-strength bolts and inserted, and then tightened with nuts to complete the installation. The joints between the precast piles and the composite slabs with end bearing beams, and between adjacent composite slabs with end bearing beams, are sealed by squeezing with elastic sealing strips. Before installing the anchor bolts, the anchor bolt sleeves are driven into the soil layer outside the foundation pit through the anchor bolt holes of the composite slab with end bearing beams; at least one anchor bolt hole is reserved without a sleeve as a grouting hole to meet the subsequent grouting requirements and complete the process acceptance. Step S9: High-pressure grouting and anchor bolt installation: After the anchor sleeve is installed, a grouting machine is used to perform high-pressure grouting through the pre-reserved grouting holes. A secondary pressurization grouting process is adopted to ensure that the grouting material in the cavity of the composite slab with end bearing beam is filled. The grouting holes can be additional holes or set by combining the anchor holes. When the anchor holes are used as grouting holes, the plugs are pulled out and the anchor sleeves are forcibly installed before the grouting material sets. After the grout test blocks reach the design strength, anchor bolts are installed in each anchor bolt sleeve to complete the anchor bolt group construction and make process acceptance records. Step S10: Waterproofing treatment of vertical joints in the pile wall; repeat to complete the overall assembly construction. In response to high groundwater levels and complex geological conditions, for the foundation pit retaining structure constructed in layers and sections, three layers of asphalt adhesive are used to seal the vertical joints of the pile wall and the joints of the composite slabs with adjacent end-bearing beams, and an additional layer of hot-melt waterproof membrane is added to improve the seepage prevention effect. Repeat the above steps in a layered, segmented, and continuous manner until the prefabricated construction of the entire enclosure structure is completed, and the overall project is accepted and the data is archived.

[0011] Compared with the prior art, the beneficial effects of this invention are as follows: 1. This invention adopts a prestressed double casing system. The closed cast-in-place reinforced concrete pile top casing forms a continuous constraint at the top, directly controlling the deformation of the top of the foundation pit retaining structure. The support-type outer casing provides a reliable anchor support for the unbonded prestressed tendons. Together with the application of prestress, it forms an active prestressing system, which significantly improves the overall stiffness and lateral displacement resistance of the retaining structure.

[0012] 2. This invention uses composite steel support beams to connect adjacent precast piles, constructing a layered assembly frame to achieve coordinated load-bearing of each component. This facilitates the rapid placement of the composite slab with end-bearing beams and also serves as a water-stopping function. The integrated molded composite slab with end-bearing beams is connected with high-strength bolts, resulting in high assembly precision and construction efficiency. The wave-shaped staggered anchor group and modular high-pressure grouting further enhance the structural load-bearing capacity and water-stopping reliability, achieving full-process deformation control from top constraint and mid-section force transmission to prestressing application. This effectively improves construction precision, shortens the construction period, and enhances the safety and durability of the retaining system. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of the foundation pit retaining structure of the present invention; Figure 2 This is a schematic diagram of the planar position of the top component of the foundation pit retaining structure of the present invention; Figure 3 yes Figure 2 Schematic diagram of the AA section; Figure 4 This is a three-dimensional structural schematic diagram of the combined steel support beam of the present invention; Figure 5This is a schematic diagram of the installation node of the composite plate with end bearing beam and the combined steel support beam of the present invention; Figure 6 This is a schematic diagram of the construction method of the present invention. Detailed Implementation

[0014] This invention patent application protects a prestressed double-casing prefabricated foundation pit retaining structure. The overall structure consists of precast piles 1, pile top casing 2, outer casing 3, pit top slope protection layer 4, composite steel support beam 5, composite slab with end bearing beam 6, anchor bolts 7, and unbonded prestressed tendons 9. (See reference...) Figures 1 to 3 The overall technical solution follows the design concept of "closed cast-in-place reinforced concrete pile top casing 2 to control top deformation, support-type outer casing 3 combined with prestressed tendons to strengthen the system's stress, combined steel support beam 5 to assist the overall synergistic bearing of various components and also to prevent water leakage, integrated molded composite slab with end bearing beam 6 using high-strength bolts 8 for efficient positioning, wave-shaped staggered anchor rods 7 group to strengthen the bearing capacity of the retaining structure, and segmented and modular high-pressure grouting to complete the overall structural construction". The specific structure is as follows: The precast pile 1 is driven at least 1500mm below the top elevation of the foundation pit. A continuous and closed pile top casing 2 is provided at the top of the precast pile 1. A continuous and closed outer casing 3 is provided outside the pile top casing 2. The outer casing 3 is located inside the foundation pit soil layer, and the distance between the pile top casing 2 and the outer casing 3 is not less than 2200mm. Several unbonded prestressed tendons 9 are provided between the pile top casing 2 and the outer casing 3. A pit top slope protection layer 4 is also provided at the top of the pile top casing 2 and the outer casing 3.

[0015] Between two adjacent precast piles 1, a composite slab 6 with end bearing beams and a combined steel support beam 5 are staggered from top to bottom, as shown below. Figure 1 As shown.

[0016] See Figure 4 The composite steel support beam 5 consists of a ribbed cross steel beam 51 in the middle and end bearing plates 52 at both ends. The ribbed cross steel beam 51 is integrally formed from a cross steel beam 511 and two steel rib plates 512. The two steel rib plates 512 are respectively arranged perpendicular to the two flange plates on both sides of the cross steel beam 511 in the horizontal direction. The distance between the steel rib plates and the corresponding flange plate ends is not less than 90mm, which can meet the installation requirements of the composite plate 6 with end bearing beam. The end bearing plates 52 are provided with bolt holes 521. The position of the bolt holes 521 corresponds to the internal thread pre-embedded on the precast pile body of the precast pile 1, so that the composite steel support beam 5 can be fixed to two adjacent precast piles 1 through the end bearing plates 52 at both ends.

[0017] See Figure 5The composite slab 6 with end-bearing beams is integrally formed from the central composite slab 61 with truss reinforcement and the end-bearing beams 62 with enlarged cross-sections at both ends. The composite slab surface with truss reinforcement is provided with pre-reserved anchor bolt holes 63. The composite slab 6 with end-bearing beams is fixed to the steel rib plate 512 of the composite steel support beam 5 by high-strength bolts 8 through holes, and the steel rib plate 512 is the steel rib plate closer to the inner side of the foundation pit in the same composite steel support beam 5.

[0018] To meet the grouting requirements of the internal cavities of each module formed by the precast piles 1, the composite steel support beams 5, and the composite slabs with end bearing beams 6, the lowest layer of the composite slabs with end bearing beams 6 is supported on the bottom foundation pit soil layer. The grouting operation can be carried out together with the foundation pit cushion concrete, or the grouting operation inside the lowest layer retaining structure can be carried out after the cushion concrete is poured. This technical method aims to achieve the effect of cushion construction combined with the bottom grouting and sealing of the retaining structure.

[0019] Fine aggregate concrete is poured into the cavity between the composite slab 6 with end bearing beams and the soil layer outside the foundation pit. Each composite slab 6 with end bearing beams is equipped with no less than 6 anchor rods 7, and each anchor rod 7 is driven into the soil layer outside the foundation pit through the anchor rod hole 63 reserved on the composite slab 6 with end bearing beams. Each composite slab 6 with truss reinforcement is arranged with at least two rows of anchor rods, and each row has no less than 3 rods. The two adjacent rows of anchor rods are staggered by at least 450mm in vertical position, so as to form a wave-like arrangement of anchor rods with staggered arrangement in each module section.

[0020] Through the above structure, this invention ultimately forms a composite prefabricated foundation pit retaining structure consisting of a casing, pile, beam, wall, prestressed tendons, and anchor bolts. To achieve this structure, this invention provides an actual construction method for this prestressed double-casing prefabricated foundation pit retaining structure to control deformation at the top of the foundation pit. (See reference...) Figure 6 The construction method includes the following steps: Step S1: Measure and lay out the double-casing position, and inspect all prefabricated components upon arrival at the site; Step S2: Precast pile driving and double-casing trench excavation; Step S3: Install the combined steel support beam at the pile head at the bottom of the trench, and hoist the steel cage inside the pile top casing and embed the steel sleeve; Step S4: Concrete pouring inside the pile top casing, drilling and threading of unbonded prestressed tendons; Step S5: Concrete pouring for the outer casing, reinforcement binding for the top slope protection layer, concrete pouring and curing; Step S6: Tensioning and anchoring of unbonded prestressed tendons, and excavation of soil below the pile top elevation; Step S7: Clean the reserved holes of the composite steel support beam at the bottom of the pile top casing, and hoist the composite steel support beam at the bottom of the earthwork. Step S8: Hoist the composite slab with end bearing beams and install anchor bolt sleeves; Step S9: High-pressure grouting and anchor bolt installation; Step S10: Vertical joint treatment and waterproofing of pile walls. Repeat the above steps until the prefabricated construction of the entire retaining structure is completed.

[0021] The key technical solutions for the above construction steps are as follows: Step S1: Measure and lay out the double-casing position; inspect all prefabricated components upon arrival. First, a total station was used to accurately measure the planar positions of the pile top casing and the outer casing, and the driving points of the precast piles were marked within the position line of the pile top casing, thus completing the preparatory work before pile driving and trench excavation. To meet the on-site installation requirements of the composite steel support beam, internal threads were pre-embedded at the designed connection points between the precast piles and the composite steel support beam during the precast pile production process. The design dimensions of the internal threads should meet the shear bearing capacity under actual working conditions. The positional relationship of the pre-embedded internal threads at each connection node between the precast pile and the composite steel support beam should correspond to the positional relationship of the bolt holes on the end bearing plate of the composite steel support beam. Furthermore, it was ensured that the internal thread holes of the precast piles were sealed upon arrival at the site to prevent blockage of the internal threads during subsequent pile driving, which would affect the hoisting and positioning of the composite steel support beam.

[0022] The composite steel support beam is designed to consist of a ribbed cross steel beam in the middle and end bearing plates at both ends. The ribbed cross steel beam and the end bearing plates are integrally formed by welding. To facilitate the positioning and fixing of the composite slab with end bearing beams, several mounting holes are provided on the steel ribs of the ribbed cross steel beam. The positions of the mounting holes on the steel ribs correspond one-to-one with the positions of the high-strength bolts at the ends of the composite slab with end bearing beams. The composite steel support beam serves two purposes: it constructs the load-bearing system of the frame structure, assists in the positioning and fixing of the composite slab with end bearing beams, and ensures the integrity of the layered assembly of the foundation pit retaining structure. The cross steel beam and steel rib plate cross-layer connection also incorporates the comprehensive design of the water-stopping device.

[0023] The middle part of the composite slab with end bearing beams is a composite slab with truss reinforcement, and the upper and lower ends are end bearing beams with enlarged cross sections. The composite slab with truss reinforcement and the end bearing beams are integrally formed. The surface of the composite slab has pre-reserved anchor holes according to the design anchor distribution. The anchors are driven into the soil layer outside the foundation pit from the anchor holes on the composite slab with end bearing beams.

[0024] For the acceptance of all prefabricated components upon arrival at the site, special attention should be paid to the acceptance of prefabricated piles, composite steel support beams, composite slabs with end bearing beams, and their internal embedded ducts. A comprehensive acceptance should be conducted, and any quality defects that may affect on-site hoisting and construction should not be allowed to enter the site. Strict quality control should be maintained, and acceptance records should be kept.

[0025] Step S2: Precast pile driving and double-casing trench excavation: After all prefabricated components have passed inspection and arrived on site, the prefabricated pile driving operation can begin. On-site prefabricated pile driving is strictly carried out according to the position lines determined by the plan of the pile top casing and the outer casing. The prefabricated pile driving adopts the technical scheme of "first driving the prefabricated piles in the middle of each side of the pit, then driving the L-shaped prefabricated piles at the corners, with auxiliary guide piles used to statically press the prefabricated piles to the design elevation below the pit top, and then unifying the elevation of all prefabricated pile tops after positioning." This means that fixed elevation control points are set on-site, and the prefabricated piles are driven sequentially from the middle of each side of the pit towards the corners. The driving quality of each prefabricated pile is strictly controlled, with the planar position error of each prefabricated pile not exceeding 50mm and the verticality deviation not exceeding 0.3%. L-shaped enlarged prefabricated piles are used at the pit corners. The design of the precast pile section involves driving each precast pile to the top elevation of the foundation pit. Then, reusable precast short piles with the same cross-sectional dimensions as each precast pile are used as static pressure guide piles. The guide piles are used to press each precast pile from the top of the foundation pit to the design elevation below the top elevation of the foundation pit, ensuring that the top elevation of each precast pile after final driving is uniformly the design elevation. The error between the top elevation of each precast pile after placement and the design elevation is controlled within -30mm. The error is checked and adjusted by setting fixed elevation control points on site.

[0026] After each precast pile is driven to the design elevation below the top of the foundation pit, the double-casing trench is excavated according to the control lines accurately measured by the total station in the aforementioned process. The distance between the pile top casing and the outer casing is designed to be no less than 2200mm. The positional relationship between the pile top casing and the outer casing is determined comprehensively based on the specific working conditions of each project to ensure that the slip surface of the foundation pit is between the pile top casing and the outer casing. The design cross-sectional thickness of the pile top casing and the outer casing is no less than 1.5 times the side length of the precast pile in the same direction and no less than 800mm. The design length of both the pile top casing and the outer casing is no less than 3000mm, and the outer casing is at least 500mm longer than the pile top casing. The outer casing is designed as a fixed support for the tensioning and anchoring of the prestressed tendons of the pile top casing. The outer casing and the pile top casing are excavated separately on site, with the pile top casing being excavated to the top surface of each precast pile that has been placed. Process acceptance and recording are carried out.

[0027] Step S3: Installation of the composite steel support beam at the pile head at the bottom of the trench, hoisting of the reinforcing cage inside the pile top casing, and embedding of the steel sleeve: After the double-casing trench is excavated, the soil at the bottom of the trench and the top of the precast piles is cleared. The soil at the pile head of the precast piles is then manually excavated until the soil at the pile head between adjacent precast piles is exposed to the internal threads of the pile body. The soil at the internal threads of the pile body is then cleared, and the bolt heads inside each internal thread are unscrewed. Then, the combined steel support beam at the pile head at the bottom of the trench is hoisted. Using the original soil layer for support, the combined steel support beam at the pile head is installed and fixed with high-strength bolts. After installation, the combined steel support beam vertically supports the joint of the upper and lower composite plates with end bearing beams, awaiting the concrete pouring of the pile top casing.

[0028] After the composite steel support beam at the pile head at the bottom of the trench is installed, the double-casing reinforcement cage, which has been tied, is hoisted into the trench in sections. Simultaneously, to ensure the placement of unbonded prestressed tendons between the pile top casing and the outer casing, during the hoisting of the reinforcement cage inside the pile top casing, steel sleeves with a diameter twice that of the unbonded prestressed tendons are pre-embedded inside the reinforcement cage according to the placement positions of the unbonded prestressed tendons. Both ends of the steel sleeves are inserted into two layers of soil in the trench wall. The concealed works acceptance work is then completed.

[0029] Step S4: Concrete pouring inside the pile top casing; probing and threading of unbonded prestressed tendons. For double casings, the technical solution is to "first pour the concrete for the top casing of the pile, excavate the soil layer of the foundation pit within the scope of the top casing of the pile, use the working surface provided by the concrete for the top casing of the pile to carry out the unbonded prestressed tendon location exploration and tendon threading process, and finally pour the outer casing concrete". That is, after the steel cage inside the top casing of the pile is hoisted and the steel sleeve is buried, the concrete for the top casing of the pile is poured first. The concrete strength is not lower than C30. It is poured in one go to 100mm below the top surface of the steel cage of the top casing of the pile and then cured.

[0030] After the concrete pouring strength of the pile top casing reaches more than 80% of the design strength, an excavator is used to remove all the soil layers in the foundation pit within the pile top casing area. The surface of the pile top casing concrete and the openings of the exposed steel casing are cleaned. Utilizing the excavated working space, drilling machinery is used to drill exploratory holes for the unbonded prestressed tendons from the openings of the steel casing on the pile top casing. The exploratory drilling process extends into the outer casing trench. Then, one end of the unbonded prestressed tendon is fixed to the drill head, and the unbonded prestressed tendon is slowly pulled out with the exploratory drilling equipment to complete the tendon insertion work. Process acceptance and recording are carried out.

[0031] Step S5: Concrete pouring for the outer casing, reinforcement binding for the top slope protection layer, concrete pouring and curing: After the unbonded prestressing tendons are drilled and threaded, the debris at the bottom of the outer casing trench should be cleaned up in a timely manner. The unbonded prestressing tendons adopt the technical solution of "anchoring one end inside the outer casing first and then tensioning the other end". That is, before the concrete of the outer casing is poured, each unbonded prestressing tendon at the end of the outer casing is first fitted with an anchor and each anchor is fixed to the outside of the outer casing reinforcement cage. Then, the outer casing is poured with fine stone concrete with a strength of C30 or above, and poured in one go to 100mm below the top surface of the pile top casing reinforcement cage. Then, curing is strengthened.

[0032] After the outer casing concrete is poured, the reinforcing steel bars of the pit top slope protection layer are tied and laid. The two ends of the longitudinal reinforcing steel bars of the pit top slope protection layer extend into the exposed reinforcing steel bars at the top of the pile top casing and the outer casing, respectively, and are arranged in two layers and two directions. Finally, the top of the pile top casing, the pit top slope protection layer and the top of the outer casing are integrally cast with fine stone concrete with a strength of not less than C30, without leaving construction joints. Strengthen curing and do a good job of process acceptance and recording.

[0033] Step S6: Tensioning and anchoring of unbonded prestressed tendons, excavation of soil below the pile top elevation: Once the concrete strength of the pile top casing, the pit top slope protection layer, and the outer casing reaches 100% of the design strength, the unbonded prestressing tendons can be tensioned and anchored. During on-site construction, the unbonded prestressing tendons are tensioned using 1.05σcon over-tensioning, with single-sided tensioning, symmetrically tensioned from the middle to both ends, and anchored immediately after tensioning. All unbonded prestressing tendons are anchored to the end face of the pile top casing near the inner side of the pit after tensioning.

[0034] After all unbonded prestressed tendons are tensioned and anchored, mechanical equipment is used to excavate the soil below the pile top elevation. The soil between piles is excavated in layers and cleaned in sections. Special attention is paid to cleaning the surface of the composite steel support beam at the bottom of the pile top casing and the miscellaneous soil in the pile body. During the process, the excavated soil wall should not be disturbed, and the collapse of the pit wall should be minimized. Process acceptance and recording should be done well.

[0035] Step S7: Clean the reserved holes for the composite steel support beam at the bottom of the pile top casing; hoist the composite steel support beam at the bottom of the earthwork. To facilitate the subsequent hoisting of the composite slab with end bearing beam, during on-site construction, the reserved holes on the steel rib plate of the composite steel support beam at the bottom of the pile top casing need to be cleaned, and the high-strength bolts need to be passed out from the inside of the reserved holes on the steel rib plate of the composite steel support beam at the bottom of the pile top casing in a timely manner. Nuts are added to the outside of the reserved holes on the steel rib plate to fix the high-strength bolts.

[0036] After the excavation of this layer of earthwork is completed, the next layer of composite steel support beams at the bottom of the earthwork will be hoisted. The same process as above will be used to clean the reserved holes on the steel ribs and temporarily fix the high-strength bolts to prepare for the subsequent hoisting and placement of the composite slab with end bearing beams. Process acceptance and recording will be carried out.

[0037] Step S8: Hoisting of the composite slab with end bearing beams and installation of anchor bolt sleeves: After all the high-strength bolts on the composite steel support beam have been temporarily fixed, the positional relationship of the high-strength bolts between two adjacent precast piles is checked and confirmed to be correct. The end-bearing beam composite slab is then slowly hoisted to the installation location using hoisting machinery. All the mounting holes at the upper and lower ends of the end-bearing beam composite slab are aligned with the high-strength bolts, and all the high-strength bolts are inserted one by one into the mounting holes at the upper and lower ends of the end-bearing beam composite slab. The high-strength bolts are then immediately fixed with nuts to complete the on-site hoisting of the end-bearing beam composite slab.

[0038] After each composite slab with end-bearing beams is hoisted into place, all joints between the precast piles and the composite slabs with end-bearing beams, as well as between adjacent composite slabs with end-bearing beams, are sealed using elastic sealing strips inserted by compression. Before installing the anchor bolts on site, the anchor bolt sleeves should be driven into the soil layer outside the foundation pit through the anchor bolt holes on the already positioned composite slabs with end-bearing beams, following the anchor bolt alignment. Simultaneously, considering the grouting requirements inside the composite slabs with end-bearing beams, at least one anchor bolt hole should be left uninstalled during the installation of the anchor bolt sleeves; this reserved hole will be designed as a grouting hole, and proper process acceptance and recording should be conducted.

[0039] Step S9: High-pressure grouting and anchor bolt installation: After all anchor bolt sleeves are installed, high-pressure grouting is performed using a grouting machine through the anchor bolt holes left on the composite slab with end bearing beams. The high-pressure grouting employs a secondary pressurization and replenishment process to ensure the full filling of the high-strength grout within the internal cavity of the composite slab with end bearing beams. The anchor bolt holes left on the composite slab with end bearing beams can be created using either an additional hole placement method or a method that combines anchor bolt holes with grouting. When using the additional hole placement method, the holes can be sealed directly after high-pressure grouting in the internal cavity of the composite slab with end bearing beams. When using the method that combines anchor bolt holes with grouting, the timing of grout strength enhancement after grouting must be carefully controlled during on-site operation. Specifically, the sealing plug in the grouting hole must be removed before the high-pressure grout finally sets before the anchor bolt sleeves are forcibly installed. After the grout test blocks cured under the same conditions on site reach the design strength, anchor bolts are then installed in all anchor bolt sleeves to complete the on-site operation of the anchor bolt group. Process acceptance and recording are then carried out.

[0040] Step S10: Vertical joint treatment and waterproofing of pile walls. Repeat the above steps until the entire prefabricated retaining structure is completed. For complex working conditions such as high groundwater levels and harsh geological environments, after the foundation pit retaining structure is assembled and constructed in layers and sections, three layers of asphalt adhesive are used to seal all joints at the vertical joints of the pile walls and between adjacent end-bearing beam composite slabs. Finally, a hot-melt waterproofing membrane is added to ensure the seepage prevention of the foundation pit retaining structure. The above steps are repeated layer by layer and section by section in a continuous flow until the assembly construction of the entire retaining structure is completed. The project is then fully inspected and the data is archived.

[0041] This invention relates to a foundation pit retaining structure and its construction method, which is mainly applicable to deep foundation pit engineering, offshore and onshore wind power equipment foundation construction, large equipment foundation pit support, municipal integrated pipe gallery, and high-rise building deep foundation pit retaining. It is particularly suitable for wind power engineering fields with stringent requirements for top deformation control, overall rigidity, water-stopping performance, and construction efficiency, such as onshore wind turbine pile foundations, offshore wind power booster station foundations, and wind turbine tower foundation pits. It effectively solves problems such as large top deformation, insufficient node reliability, and long construction cycles associated with traditional retaining structures, significantly improving the safety, stability, and economy of wind power equipment foundation construction. Furthermore, it can be widely applied to deep foundation pit retaining projects under complex geological conditions and high groundwater levels, such as port terminals, offshore projects, and large industrial plants.

[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 prestressed double-casing prefabricated foundation pit retaining structure, characterized in that: The structure includes multiple precast piles, a pile top casing, and an outer casing. The multiple precast piles are driven below the top elevation of the foundation pit. The pile top casing is located on top of the multiple precast piles. The outer casing is located inside the foundation pit soil layer and outside the pile top casing. Both the pile top casing and the outer casing are continuous and closed structures. Multiple unbonded prestressed tendons are provided between the pile top casing and the outer casing. A pit top slope protection layer is laid on top of both. Between two adjacent precast piles, a composite slab with end bearing beams and a combined steel support beam are staggered from top to bottom. The two ends of the combined steel support beam are fixed to the precast piles, and the composite slab with end bearing beams is fixed to the combined steel support beams by high-strength bolts through holes. The cavity between the composite slab with end bearing beams and the soil layer outside the foundation pit is filled with fine stone concrete. Each composite slab with end bearing beams is provided with no less than 6 anchor rods, and all the anchor rods are driven into the soil layer outside the foundation pit. The composite slab with end bearing beams includes a composite slab with truss reinforcement in the middle and end bearing beams with enlarged cross sections at the top and bottom. The composite slab with truss reinforcement has pre-reserved anchor holes on its surface. The anchors are driven into the soil layer outside the foundation pit through the anchor holes. Each composite slab with truss reinforcement has at least two rows of anchors, and each row has no less than 3 anchors. The two adjacent rows of anchors are staggered in vertical position.

2. The foundation pit retaining structure according to claim 1, characterized in that: The composite steel support beam includes a ribbed cross steel beam in the middle and end bearing plates at both ends. The composite steel support beam is fixed to the precast pile through the end bearing plates.

3. The foundation pit retaining structure according to claim 2, characterized in that: The ribbed cross steel beam includes a cross steel beam and steel ribs, with the steel ribs arranged perpendicular to the flanges of the cross steel beam in the horizontal direction.

4. The foundation pit retaining structure according to claim 3, characterized in that: The steel rib plate is provided with multiple mounting holes, and the composite plate with end bearing beam is fixed to the steel rib plate by high-strength bolts through the holes. The positions of the mounting holes correspond one-to-one with the positions of the high-strength bolts.

5. A construction method for a prestressed double-casing prefabricated foundation pit retaining structure, characterized in that, The foundation pit retaining structure is the foundation pit retaining structure as described in claim 4, and the construction method includes the following steps: Step S1: Measurement, layout, positioning of double protective sleeves, and acceptance of prefabricated components upon arrival at the site. The total station was used to accurately measure the planar position of the pile top casing and the outer casing, and the precast pile driving points were marked within the pile top casing position line to complete the preparation for pile driving and trench excavation. The precast piles are pre-embedded with internal threads at the joint position with the composite steel support beam. The thread size meets the shear bearing capacity requirements, and the thread position corresponds to the bolt hole position of the end plate of the composite steel support beam. When the precast piles arrive at the site, the internal thread hole is sealed to prevent blockage during the pile driving process. The composite steel support beam is formed by welding a central ribbed cross steel beam and end bearing plates together. The steel ribs of the ribbed cross steel beam have several mounting holes, the positions of which correspond to the positions of the high-strength bolts at the ends of the composite plate with end bearing beams. The composite steel support beam is used to construct the frame load-bearing system, assist in the positioning and fixing of the composite plate with end bearing beams, and ensure the integrity of the layered assembly of the foundation pit retaining structure. The cross steel beam and steel ribs are connected across layers to take into account the comprehensive design of the water-stopping device. The composite slab with end bearing beams is integrally formed by the composite slab with truss reinforcement in the middle and the end bearing beams with enlarged cross sections at the top and bottom ends. Anchor holes are reserved on the surface of the composite slab according to the distribution of anchor rods, and the anchor rods are driven into the soil layer outside the foundation pit through the anchor holes. Precast piles, composite steel support beams, composite slabs with end bearing beams and embedded ducts shall be inspected upon arrival at the site. Components with quality defects that may affect hoisting and construction shall not be allowed to enter the site, and inspection records shall be kept. Step S2: Precast pile driving and double-casing trench excavation: After the prefabricated components pass the acceptance test, the prefabricated piles are driven, and the piles are accurately positioned according to the measured positions of the pile top casing and the outer casing. The construction method adopted is "first driving the precast piles in the middle of each side of the pit, then driving the L-shaped precast piles in the corners, and using the guide piles to statically press them to the design elevation below the top of the pit, with the top of the piles at a uniform elevation": using fixed elevation control points as a reference, piles are driven sequentially from the middle of each side of the pit to the corners, controlling the plane position error to be no more than 50mm and the verticality deviation to be no more than 0.3%; L-shaped enlarged cross-section precast piles are used at the corners of the pit, and reusable precast short piles of the same cross-section are used as guide piles to statically press the precast piles to the design elevation below the top of the pit, with the top elevation error controlled within -30mm; Excavate double-casing trenches according to the control lines set by total station. The distance between the top casing and the outer casing should not be less than 2200mm, and the slip surface of the foundation pit should be located between the two. The thickness of the casing section should not be less than 1.5 times the side length of the precast pile in the same direction and not less than 800mm. The length of each casing should not be less than 3000mm. The length of the outer casing should be at least 500mm longer than that of the top casing and should be used as a prestressing tendon tensioning anchor support. Excavate the pile top casing and the outer casing trench separately, and excavate the pile top casing trench to the top surface of the precast pile that has been placed in place; Step S3: Installation of the composite steel support beam at the pile head at the bottom of the trench, hoisting of the reinforcing cage for the pile top casing, and installation of the steel sleeve: After the double-casing trench is excavated, the bottom of the trench and the top of the precast piles are cleaned of the soil. The soil at the pile head is manually excavated until the internal thread of the pile body is exposed. The thread is cleaned and the internal sealing parts are removed. The combined steel support beam at the pile head at the bottom of the trench is hoisted and supported by the original soil. It is installed and fixed to the pile head with high-strength bolts so that it can vertically support the joint of the upper and lower end-bearing beam composite plate. Wait for the concrete of the pile top casing to be poured. The double-casing steel cage is hoisted into the trench in sections; during the hoisting of the steel cage at the top of the pile, steel sleeves are pre-embedded according to the location of the unbonded prestressed tendons. The inner diameter of the steel sleeves is more than twice the outer diameter of the unbonded prestressed tendons, and both ends are inserted into the soil layer of the trench wall to complete the concealed acceptance. Step S4: Concrete pouring for the pile top casing, and probing and threading of unbonded prestressed tendons: The process involves "first pouring the concrete for the pile top casing, excavating the foundation pit to form a working surface, then drilling and threading the prestressed tendons, and finally pouring the outer casing concrete." After the pile top casing steel cage and steel sleeve are installed, pour no less than C30 concrete up to 100mm below the top surface of the steel cage and cure it. Once the concrete strength of the pile top casing reaches more than 80% of the design strength, the soil layer of the foundation pit within its range is excavated, and the concrete surface and steel casing opening are cleaned. Drilling equipment is used to explore the hole from the steel casing opening towards the outer casing trench. One end of the unbonded prestressed tendon is fixed to the drill head and pulled out with the exploration equipment to complete the traction-type tendon insertion. The process acceptance record is kept. Step S5: Concrete pouring for the outer casing, construction of the pit top slope protection layer: After the unbonded prestressed tendons are installed, the debris at the bottom of the outer casing trench is cleaned up. The unbonded prestressed tendons are pre-fixed to the outside of the outer casing steel cage using the method of "anchoring one end first and tensioning the other end later". Fine stone concrete with a strength of not less than C30 is poured up to 100mm below the top surface of the casing steel cage and cured. Tie the reinforcement of the top slope protection layer of the pit, with the longitudinal reinforcement extending into the top casing of the pile and the exposed reinforcement of the top of the outer casing respectively, using a double-layer, two-way arrangement; use fine stone concrete with a strength of not less than C30 to cast the top of the pile casing, the top slope protection layer of the pit, and the top of the outer casing into a whole in one go, without leaving construction joints, strengthen curing and acceptance. Step S6: Tensioning and anchoring of unbonded prestressed tendons, excavation of soil below the pile top elevation: After the concrete strength of the pile top casing, the pit top slope protection layer and the outer casing reaches 100% of the design strength, the unbonded prestressed tendons are tensioned and anchored. The 1.05σcon over-tensioning and single-sided tensioning method is adopted. The tensioning is symmetrical from the middle to both ends, and the anchoring is done as the tensioning is carried out. All prestressed tendons are anchored to the inner end face of the pile top casing near the pit. After the prestressed tendons are tensioned and anchored, the soil below the pile top elevation is excavated in layers and sections. The soil between piles, the surface of the composite steel support beam at the bottom of the pile top casing, and the miscellaneous soil in the pile body are cleaned to avoid disturbing the pit wall and reduce soil collapse. Process acceptance is carried out. Step S7: Clean the reserved holes of the composite steel support beam, and hoist the lower composite steel support beam: Clean the reserved installation holes on the steel ribs of the composite steel support beam at the bottom of the pile top casing, pass the high-strength bolts through the reserved holes from the inside to the outside, and temporarily fix them with nuts on the outside to prepare for the hoisting of the composite slab with end bearing beam; After the excavation of this layer of earthwork is completed, hoist the next layer of composite steel support beam at the bottom of the earthwork, clean the reserved holes and temporarily fix the high-strength bolts according to the same process, complete the preparatory work and record the acceptance. Step S8: Hoisting of the composite slab with end bearing beams and installation of anchor bolt sleeves: After verifying that the position of the high-strength bolts on the composite steel support beam is correct, the composite plate with end bearing beam is hoisted into place using hoisting equipment, so that the mounting holes at the upper and lower ends are aligned with the high-strength bolts and inserted, and then tightened with nuts to complete the installation. The joints between the precast piles and the composite slabs with end bearing beams, and between adjacent composite slabs with end bearing beams, are sealed by squeezing with elastic sealing strips. Before installing the anchor bolts, the anchor bolt sleeves are driven into the soil layer outside the foundation pit through the anchor bolt holes of the composite slab with end bearing beams; at least one anchor bolt hole is reserved without a sleeve as a grouting hole to meet the subsequent grouting requirements and complete the process acceptance. Step S9: High-pressure grouting and anchor bolt installation: After the anchor sleeve is installed, a grouting machine is used to perform high-pressure grouting through the pre-reserved grouting holes. A secondary pressurization grouting process is adopted to ensure that the grouting material in the cavity of the composite slab with end bearing beam is filled. The grouting holes can be additional holes or set by combining the anchor holes. When the anchor holes are used as grouting holes, the plugs are pulled out and the anchor sleeves are forcibly installed before the grouting material sets. After the grout test blocks reach the design strength, anchor bolts are installed in each anchor bolt sleeve to complete the anchor bolt group construction and make process acceptance records. Step S10: Waterproofing treatment of vertical joints in the pile wall; repeat to complete the overall assembly construction. In response to high groundwater levels and complex geological conditions, for the foundation pit retaining structure constructed in layers and sections, three layers of asphalt adhesive are used to seal the vertical joints of the pile wall and the joints of the composite slabs with adjacent end-bearing beams, and an additional layer of hot-melt waterproof membrane is added to improve the seepage prevention effect. Repeat the above steps in a layered, segmented, and continuous manner until the prefabricated construction of the entire enclosure structure is completed, and the overall project is accepted and the data is archived.

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