Construction method of T-shaped pile supporting assembled monolithic foundation pit enclosure structure

Abstract

The application discloses a construction method of a T-shaped pile supporting assembled integral foundation pit retaining structure, and the foundation pit retaining structure comprises a T-shaped pile, an anchor rod, additional reinforcing bars and a composite slab with truss ribs; the T-shaped pile is arranged along the edge of a foundation pit, and a double-hoop structure is arranged at the top of the T-shaped pile; the anchor rod is arranged in the T-shaped pile and penetrates into the soil layer outside the foundation pit, and the additional reinforcing bars cross the web of the T-shaped pile; a steel reinforcement cage is arranged between the T-shaped piles, the composite slab with truss ribs is fixed to the T-shaped pile by using high-strength bolts, and fine stone concrete is poured into the cavity surrounded by the composite slab with truss ribs, the T-shaped pile and the soil layer outside the foundation pit. The integral construction process comprises the following steps: in-situ sinking of the T-shaped pile, construction of the double-hoop structure at the top of the T-shaped pile, setting of the anchor rod, perforation of the additional reinforcing bars, binding of the steel reinforcement cage, hoisting of the composite slab with truss ribs, pouring of the fine stone concrete, and layered repeated construction until the construction is completed. The application has the advantages of high assembly degree, fast construction, reliable water stop, firm connection, strong anchor rod cooperation, safety and stability, and is suitable for urban deep foundation pit engineering.

Description

Technical Field

[0001] This invention relates to the field of foundation pit support technology, specifically a construction method for a T-shaped pile-supported assembled integral foundation pit support structure. Background Technology

[0002] Excavation pit support is a crucial part of urban underground engineering construction. Currently, the mainstream support structure still uses traditional cast-in-place concrete or steel support systems. This system has a long construction cycle, involves a large amount of on-site wet work, and is prone to generating pollution such as dust and sewage, which has a significant impact on the surrounding environment. At the same time, on-site operations are greatly affected by human and environmental factors, making it difficult to standardize and control the quality of the project, and it is hard to meet the needs of modern cities for efficient, low-carbon, and intelligent construction.

[0003] Existing prefabricated foundation pit support technologies mostly adopt a multi-component combination connection form of pile-beam-wall-anchor, which still has many technical defects: a single row of piles cannot form a reliable water-stop curtain, resulting in insufficient waterproofing and seepage prevention capabilities; the reliability of the connection nodes between piles and beams, and between piles and walls is poor, and water leakage is prone to occur at the joints, affecting the durability of the retaining structure; multiple anchors are prone to failure throughout their entire life cycle, resulting in weak synergistic load-bearing capacity of the group anchors and insufficient support stability; the hoisting and positioning of prefabricated components is difficult, and the construction convenience is poor; moreover, traditional structures cannot make full use of the synergistic load-bearing role of anchors, and the operation of anchor installation is inconvenient, further restricting construction efficiency and support reliability.

[0004] In response to the common technical deficiencies in the aforementioned industries, engineering practice has revealed that the following two existing patented technical solutions have also failed to effectively solve the relevant problems, specifically as follows: The existing patent CN202510155675.1 uses H-shaped steel pile flanges that are relatively thin, making it impossible to directly and effectively anchor the anchor rods to the pile body. Instead, they can only be anchored to the profiled steel sheet or post-cast concrete, resulting in a lack of direct force transmission path between the anchor rods and the main pile, and weak collective bearing capacity of the anchor group. Furthermore, the H-shaped steel pile + profiled steel sheet system used in this patent has significant shortcomings in terms of stress mechanism, connection reliability, water-stopping performance, and construction convenience. In particular, the poor weak axis stiffness of the H-shaped steel piles, indirect force transmission of the anchor rods, weak node connections, and complex and unreliable water-stopping structures limit its widespread application in deep foundation pit engineering.

[0005] The existing patent CN202420508899.7 discloses a double-truss ribbed composite slab foundation pit retaining structure, which still has the following technical problems: First, the stress mechanism is unclear. The core load-bearing structure relies on stiffened I-beams as the main load-bearing components, while the inner and outer truss composite slabs act more like enclosures and formwork. The steel beams are connected to precast piles at both ends via "adjusting pads + internally threaded sleeves," forming a typical simply supported beam stress pattern—the steel beams bear the main water and soil pressure, which is then transferred to the precast piles through the end nodes. This results in poor synergy between the steel beams and the composite slabs; the post-cast micro-expansion concrete is more of a filler material and struggles to form a true composite structure with the steel beams.

[0006] Secondly, key nodes are weak, posing potential risks of stress concentration and failure: the force transmission path from steel beam to base plate to sleeve to precast pile is too long, with numerous connection points. Connections between adjacent piles and between piles and slabs rely primarily on end angle steel and steel beam nodes, lacking lateral connecting reinforcement to constrain post-cast concrete and coordinate deformation. This makes the structure more like an assembly than a whole, resulting in weak resistance to lateral earth pressure. Importantly, the mechanical installation of the stiffened I-beams requires excessively high precision in the positional relationship between adjacent precast piles, failing to adequately meet engineering application requirements and incurring high construction costs.

[0007] In summary, both traditional cast-in-place / steel support systems and existing prefabricated retaining wall technologies have significant shortcomings. There is an urgent need to develop a new construction method for prefabricated foundation pit retaining structures to solve technical problems such as low construction efficiency, poor environmental performance, insufficient node reliability, weak anchor bolt co-bearing capacity, and difficulty in component installation. Summary of the Invention

[0008] The purpose of this invention is to provide a construction method for a T-shaped pile-supported assembled integral foundation pit retaining structure to solve the problems raised in the background art.

[0009] To achieve the above objectives, the present invention provides the following technical solution: a construction method for a T-shaped pile-supported assembled integral foundation pit retaining structure, wherein the foundation pit retaining structure includes T-shaped piles, additional tie bars, and composite slabs with truss ribs. Multiple T-shaped piles are arranged in an orderly manner along the edge of the foundation pit to form a pile group. A cast-in-place reinforced concrete casing is provided at the top of the pile group. An outer ring beam is provided on the outside of the cast-in-place reinforced concrete casing. Both the cast-in-place reinforced concrete casing and the outer ring beam are closed hoop-shaped. A cast-in-place reinforced concrete slab is provided between the two. Unbonded prestressed tendons are simultaneously inserted into all three, and all three are cast together with fine aggregate concrete. Anchor rods are inserted into the cast-in-place reinforced concrete casing, and anchor rods are also inserted into the T-shaped pile wing plates. All anchor rods are driven into the soil layer outside the foundation pit. A steel cage is tied between two adjacent T-shaped piles, and additional tie bars are inserted through the web of the T-shaped piles, with anchor plates tightened at both ends. All additional tie bars and their ends extend into the corresponding steel cage. A composite slab with truss ribs is arranged on the side of the steel cage and T-shaped piles facing the inside of the foundation pit, and the composite slab with truss ribs that intersects with the T-shaped piles is fixed to the web of the T-shaped piles with high-strength bolts. Fine stone concrete is poured into the cavity enclosed by the T-shaped piles, the composite slab with truss ribs, and the soil layer outside the foundation pit. The overall steps of the construction method are as follows: Step S1: Prefabrication, transportation and on-site installation of T-shaped piles, and excavation of the first layer of earthwork; Step S2: Construction of cast-in-place reinforced concrete casing, cast-in-place reinforced concrete slab, and outer ring beam; Step S3: Tensioning and anchoring of unbonded prestressed tendons, cleaning of pile body, and installation of anchor bolts; Step S4: Insert additional tie rods through holes and tighten the anchor plate; Step S5: Tie the reinforcing cage and hoist the composite slab with truss ribs; Step S6: Treat the vertical joints of the composite slabs and pour fine aggregate concrete; Step S7: Excavate the lower layer of soil, and repeat the corresponding procedures above until the construction of the foundation pit retaining structure is completed.

[0010] Preferably, the T-shaped pile flange is pre-drilled with Φ60 mm anchor bolt holes, the length of which is not less than 9.0 m, the inclination angle is 20°~30°, and the spacing between the anchor bolt holes does not exceed 1.2 m; the web is pre-drilled with Φ25 mm through-bar holes and M24 internal threaded threads, the through-bar holes are used for additional tie bars to pass through the web of the T-shaped pile; the composite plate with truss ribs is provided with mounting holes with corresponding internal threaded threads, the mounting holes and internal threaded threads are used to arrange high-strength bolts.

[0011] Preferably, the T-shaped pile has a T-shaped cross-section, with a web height of not less than 400 mm, a flange width of not less than 900 mm, a web thickness of not less than 150 mm, a flange thickness of not less than 120 mm, a concrete strength grade of not less than C30, and longitudinal main reinforcement using HRB400 grade steel bars with a reinforcement ratio of not less than 1.2%.

[0012] Preferably, water-swellable sealing strips are installed between the truss-ribbed composite slab and the T-piles, and between the joints of two adjacent truss-ribbed composite slabs.

[0013] Preferably, the anchor rod is a full-length bonded prestressed anchor rod, the rod body is a steel bar of not less than Φ25 mm HRB400, the anchorage section length of the anchor rod is not less than 5.0 m, and the free section length is not less than 4.0 m.

[0014] Preferably, the specific steps are as follows: Step S1: Prefabrication, transportation, and on-site installation of T-shaped piles; excavation of the first layer of earthwork: Precast reinforced concrete T-shaped piles are centrally produced in a prefabricated PC factory. The cross-section is T-shaped, with a web height of not less than 400 mm, a flange width of not less than 900 mm, a web thickness of not less than 150 mm, a flange thickness of not less than 120 mm, a concrete strength grade of not less than C30, and longitudinal main reinforcement using HRB400 grade steel bars with a reinforcement ratio of not less than 1.2%. Φ60 mm anchor holes are reserved in the flanges of the T-shaped piles, with a spacing between anchor holes not exceeding 1.2 m. Φ25 mm through-bar holes and M24 internal threaded threads are reserved in the webs. The through-bar holes are used for additional tie bars to pass through the webs of the reinforced concrete T-shaped piles to achieve post-cast connection between adjacent modules, and the internal threaded threads are used for subsequent on-site hoisting of the composite slab with truss ribs. Before driving the reinforced concrete T-shaped piles, the anchor holes, through holes and internal thread of the pile body are sealed. After the reinforced concrete T-shaped piles are transported to the site, they are driven by static pressure pile driving technology and driven to 1000 mm below the top surface of the foundation pit, with the error controlled within 5 mm. After all the reinforced concrete T-shaped piles around the foundation pit have been driven into place, the first layer of earthwork excavation in the foundation pit can be carried out. The first layer of earthwork excavation will be carried out to 1200 mm below the top surface of the reinforced concrete T-shaped piles. The excavation will be carried out in layers, and the protection of the already driven reinforced concrete T-shaped piles will be strengthened during the excavation process. Step S2: Construction of cast-in-place reinforced concrete casing, cast-in-place reinforced concrete slab, and outer ring beam: After the precast reinforced concrete T-shaped piles are driven in, the trenches at the locations of the cast-in-place reinforced concrete casing and the outer ring beam are excavated. Both trenches are closed around the perimeter of the foundation pit. The excavation depth of the cast-in-place reinforced concrete casing trench is 1000 mm, from the top of the foundation pit to the top of the precast reinforced concrete T-shaped pile. The excavation width of the cast-in-place reinforced concrete casing trench is not less than 600 mm, and the cross-section of the outer ring beam trench is not less than 400 mm × 600 mm. After the trenches at the locations of the cast-in-place reinforced concrete casing and the outer ring beam are excavated, the cast-in-place reinforced concrete casing steel cage and the outer ring beam steel cage are hoisted, and the cast-in-place reinforced concrete slab steel mesh is installed. The two ends of the cast-in-place reinforced concrete slab steel mesh are anchored into the interior of the cast-in-place reinforced concrete casing steel cage and the outer ring beam steel cage, respectively, and the structural requirements are met. At the same time as installing the cast-in-place reinforced concrete slab steel mesh, each unbonded prestressed tendon is pre-embedded. A steel formwork is installed on the side of the cast-in-place reinforced concrete casing trench near the inside of the foundation pit. Pre-reserved holes are opened on the steel formwork at the positions of each anchor rod. An anchor rod sleeve is inserted into the pre-reserved hole and then inserted into the soil layer outside the foundation pit after passing through the pre-reserved hole. Finally, fine aggregate concrete is used to cast the cast-in-place reinforced concrete casing, cast-in-place reinforced concrete slab, and outer ring beam together without leaving construction joints, and curing is strengthened; the internal reinforcement of the cast-in-place reinforced concrete casing, cast-in-place reinforced concrete slab, and outer ring beam is determined by comprehensive calculation based on different project types, geological conditions and groundwater conditions. Step S3: Tensioning and anchoring of unbonded prestressing tendons, pile cleaning, and installation of anchor bolts: After the cast-in-place reinforced concrete casing, cast-in-place reinforced concrete slab, and outer ring beam are completed and the concrete strength reaches the design strength, the steel formwork is removed, and the anchor rods are driven one by one into the soil layer outside the foundation pit through the anchor rod sleeves; after all the anchor rods on the cast-in-place reinforced concrete casing are completed, the unbonded prestressed tendons between the cast-in-place reinforced concrete casing and the outer ring beam are tensioned and anchored. For the precast reinforced concrete T-shaped piles within the first-level earthwork excavation area, manual cleaning of the pile body and the soil between the piles is adopted. The focus is on cleaning the anchor holes, through holes and internal thread of the pile body to ensure that the inside of each reserved hole is clean and does not affect the subsequent three processes of anchor installation, additional tie rod penetration and high-strength bolt tightening. The anchor rods on the T-shaped pile wing plate are driven into the active earth pressure zone outside the foundation pit through the anchor rod holes on the T-shaped pile wing plate. The holes are filled with cement grout of not less than M30. The standard value of the design pull-out bearing capacity is 120 kN. After the on-site anchor rod construction and grouting, vibration interference should be reduced and maintenance should be strengthened. Step S4: Insert additional tie rods through holes and tighten the anchor plate: After all anchor bolts on the first floor are installed and the internal grout has cured to at least 80% of the design strength, the additional tie rods are installed. The additional tie rods pass through the through holes in the web of the reinforced concrete T-shaped pile. The additional tie rods are made of HRB400 steel bars with a diameter of not less than 22 mm. Each additional tie rod has a cutting length of not less than 1200 mm. Anchor plates are tightened at both ends of the additional tie rods. The number of anchor plates is 100% and no plates are missing. The vertical spacing of the tie rods does not exceed 0.8 m. They are arranged alternately with the anchor bolt layers to enhance the synergistic stress performance of the pile and the post-cast fine stone concrete. Step S5: Reinforcing cage binding and hoisting of composite slab with truss ribs: After all the additional tie bars are threaded through, the binding of the steel cage between two adjacent reinforced concrete T-shaped piles in each module is carried out. The longitudinal and transverse distribution of the steel cage is not less than Φ12@200, and the height matches the excavation depth of the first layer of the foundation pit, that is, from the bottom of the cast-in-place reinforced concrete casing to the excavation surface of the first layer of the foundation pit. At the same time, it is ensured that all additional tie bars and their end anchor plates extend into the inside of the steel cage. After the steel cages in each module are installed, the hoisting operation of the composite slab with truss ribs is carried out. The thickness of the composite slab with truss ribs is not less than 60 mm, and the length is customized according to the pile spacing. The composite slab with truss ribs at the junction with the reinforced concrete T-shaped piles is provided with installation holes. M20 high-strength bolts are used to fix the T-shaped piles to the internal threads of the web plate through the reserved installation holes. The bolt pre-tightening force is controlled at 80 N·m. Step S6: Treatment of vertical joints in composite slabs, pouring of fine aggregate concrete: During on-site construction, additional steel plates were added to seal the joints between adjacent composite slabs with trusses and bolts. Water-swellable sealing strips were installed between all composite slabs with trusses and reinforced concrete T-piles, and between the joints of adjacent composite slabs with trusses to prevent grout leakage during subsequent fine stone concrete pouring. After the truss-ribbed composite slab is hoisted and the joints are treated, fine stone concrete is poured in modules. Fine stone concrete is poured into the cavity enclosed by the reinforced concrete T-piles, the truss-ribbed composite slab, and the soil layer outside the foundation pit. Micro-expansion fine stone concrete of not less than C30 is used, and the slump is not less than 120 mm. When difficulties arise in the on-site fine stone concrete pouring or other complex conditions, the tremie pipe method is used to assist the on-site fine stone concrete pouring operation. Step S7: Excavation of the lower layer of soil, repeating the corresponding procedures above until the construction of the foundation pit retaining structure is completed: Once the first-layer retaining structure is completed and the fine stone concrete reaches 100% of the design strength, the excavation of the next layer of earthwork can be carried out, and the above corresponding procedures can be repeated until the construction of the foundation pit retaining structure is completed. After the construction of the foundation pit retaining structure is completed in layers and sections, the concrete cushion layer at the bottom of the foundation pit should be poured immediately and should not be left overnight; at the same time, continuous monitoring should be strengthened on the horizontal displacement of the pile top, the lateral displacement of the retaining structure, the axial force of the anchor bolts and the settlement of the surrounding environment to ensure the safety and reliability of the retaining structure.

[0015] Compared with the prior art, the beneficial effects of this invention are as follows: 1. This invention adopts T-shaped piles prefabricated in the factory and static pressure driven on site, combined with rapid hoisting of composite slabs with truss ribs and high-strength bolt fixing, which greatly reduces the amount of wet work on site by about 45%, shortens the construction period by about 30%, and completely eliminates a large number of temporary internal supports, avoiding solid waste generated by support removal, significantly improving construction efficiency, reducing environmental impact, and having outstanding technical, economic and environmental benefits.

[0016] 2. The present invention sets up a double-hoop structure at the top of the pile, consisting of a cast-in-place reinforced concrete casing, a cast-in-place reinforced concrete slab, and an outer ring beam. Unbonded prestressed tendons are simultaneously inserted into the three parts and they are cast together, so that the top structure is in a state of compression hoop, just like two closed hoop rings to stabilize the top of the foundation pit. This effectively controls the maximum lateral displacement of the retaining structure to within 15 mm, solving the problems of insufficient support stiffness and difficulty in controlling top deformation in traditional systems.

[0017] 3. This invention utilizes the precise fit between T-shaped piles and composite slabs with truss ribs, and sets water-swellable sealing strips between the two and between the joints of adjacent composite slabs. Additional tie bars are used to pass through the web of the T-shaped pile, and anchor plates are tightened at the ends and extended into the steel cage. Combined with the post-cast micro-expansion fine stone concrete in modular pouring, a continuous and dense multi-layer water-stopping and integral connection system is formed, which solves the problem of reduced durability caused by water leakage at the joints.

[0018] 4. Compared with the shortcomings of existing patents, where the thin H-shaped steel pile wing plates lead to the inability to directly anchor the anchor rods and the weak collaborative bearing capacity of the group anchors, this invention directly inserts the anchor rods into the pre-reserved holes of the T-shaped pile wing plates with a thickness of not less than 120 mm and a width of not less than 900 mm. This establishes a direct and efficient force transmission path between the anchor rods and the main pile, allowing the anchor rod force to be directly transmitted to the main pile body instead of relying on intermediate media such as profiled steel plates or post-cast concrete. Furthermore, the anchor rods and additional tie rods are arranged alternately along the vertical direction, giving full play to the collaborative bearing capacity of the group anchors. This effectively avoids the risk of anchor rod failure leading to a sudden drop in overall bearing capacity in traditional structures, and significantly improves the overall stability of the retaining structure.

[0019] 5. This invention uses pre-reserved through holes and internal threads in the web of the T-shaped pile to allow additional tie bars to connect the piles laterally and anchor them into the post-cast reinforcement cage. At the same time, the composite plate with truss ribs is rigidly fixed to the web of the T-shaped pile with high-strength bolts, ensuring that the precast components and the post-cast fine stone concrete form a true composite structure that works together to bear the load. The force transmission path is direct and has fewer links, which greatly reduces the accuracy requirements for the positional relationship of adjacent piles, avoids the high dependence on mechanical installation of stiffening steel beams, effectively controls construction costs, eliminates the hidden danger of stress concentration failure at nodes, and has a clear force mechanism. Attached Figure Description

[0020] Figure 1 This is a plan view of the top structure in the foundation pit retaining structure of the present invention; Figure 2 This is a schematic diagram of the top structure of the foundation pit retaining structure of the present invention, viewed from inside the foundation pit to outside the foundation pit; Figure 3 This is a side cross-sectional schematic diagram of the top structure in the foundation pit retaining structure of the present invention; Figure 4 This is a schematic diagram of the foundation pit retaining structure of the present invention; Figure 5This is a diagram showing the positional relationship between the T-shaped pile and its components according to the present invention; Figure 6 This is an overall flowchart of the construction method of the present invention. Detailed Implementation

[0021] This invention discloses a construction method for a T-shaped pile-supported assembled integral foundation pit retaining structure. (See attached document.) Figures 1 to 5 The foundation pit retaining structure includes T-shaped piles 1, anchor rods 2, additional tie rods 5, composite slabs with truss ribs 4, cast-in-place reinforced concrete casing 81, cast-in-place reinforced concrete slabs 82, and outer ring beams 83. The overall technical solution is based on the following: "cast-in-place reinforced concrete casings form a hoop effect to control the deformation of the top of the foundation pit; the outer ring beams serve as a second hoop and also as prestressed fixing supports; reinforced concrete T-shaped piles provide support; pile-anchor rod composites provide reinforcement; additional tie rods traverse the web of the reinforced concrete T-shaped piles to strengthen the post-cast connection; anchor plates at the ends of the additional tie rods provide lifting and tying; composite slabs with truss ribs are efficiently hoisted; high-strength bolts are fixed by internal threading; and fine stone concrete is poured in modules for molding." This system constructs a new structure, new process, and new method for prefabricated integral foundation pit retaining.

[0022] Specifically: The T-shaped pile 1 has a T-shaped cross-section, with a web height of not less than 400 mm, a flange width of not less than 900 mm, a web thickness of not less than 150 mm, and a flange thickness of not less than 120 mm. The concrete strength grade is not less than C30, and the longitudinal main reinforcement uses HRB400 grade steel bars with a reinforcement ratio of not less than 1.2%. The flange of the T-shaped pile 1 has pre-reserved Φ60 mm anchor holes 11, with a length of not less than 9.0 m, an inclination angle of 20°~30°, and a spacing between anchor holes 11 not exceeding 1.2 m. The web has pre-reserved Φ25 mm through-holes 12 and M24 internal threaded threads 13. Through-holes 12 are used for additional tie bars 5 to pass through the web of the reinforced concrete T-shaped pile 1 to achieve post-cast connection between adjacent modules. Internal threaded threads 13 are used for subsequent on-site hoisting of the truss-ribbed composite slab 4. The thickness of the truss-ribbed composite slab 4 is not less than 60 mm (prefabricated part), and the length is customized according to the pile spacing.

[0023] See Figures 1-3 Multiple T-shaped piles 1 are arranged in an orderly manner along the edge of the foundation pit. Adjacent piles are separated from each other and maintain a certain distance, forming a pile group arranged in a ring and a row. The top of this pile group is equipped with a double-strap structure. The double-strap structure includes a cast-in-place reinforced concrete casing 81, a cast-in-place reinforced concrete slab 82, and an outer ring beam 83, the specific structure of which is as follows: The cast-in-place reinforced concrete casing 81 is located above the entirety (web plate + flange plate) of multiple T-shaped piles 1. The cast-in-place reinforced concrete casing 81 forms a closed hoop shape around the foundation pit above the T-shaped piles 1, and the height of the cast-in-place reinforced concrete casing 81 is 1000 mm, and the cross-sectional width is not less than 600 mm.

[0024] The outer ring beam 83 is located inside the soil layer outside the foundation pit, and its cross-section is not less than 400 mm × 600 mm. The outer ring beam 83 is located outside the cast-in-place reinforced concrete casing 81 and is spaced at least 2 meters apart.

[0025] The cast-in-place reinforced concrete slab 82 is located between the cast-in-place reinforced concrete casing 81 and the outer ring beam 83. The top surfaces of all three are flush with the top surface of the foundation pit, and unbonded prestressed tendons 84 are simultaneously threaded through them.

[0026] The cast-in-place reinforced concrete casing 81, the cast-in-place reinforced concrete slab 82, and the outer ring beam 83 are all cast together with fine aggregate concrete during on-site construction. Before casting, the two ends of the reinforcing mesh of the cast-in-place reinforced concrete slab are anchored into the reinforcing cage of the cast-in-place reinforced concrete casing and the reinforcing cage of the outer ring beam, respectively. Anchor rods 2 are also installed inside the cast-in-place reinforced concrete casing 81, thus forming a double-hoop structure at the top of the foundation pit retaining structure of this invention.

[0027] See Figure 4 and Figure 5 Additionally, anchor rod 2 is inserted through anchor rod hole 11 into the wing plate of T-shaped pile 1 and driven into the soil layer outside the foundation pit. All anchor rods 2 on site are full-length bonded prestressed anchor rods, with the rod body made of steel bars of not less than Φ25 mm HRB400. The anchorage section length of the anchor rod is not less than 5.0 m, and the free section length is not less than 4.0 m.

[0028] Additional tie bars 5 are inserted through the through-hole 12 into the web of T-shaped pile 1, and anchor plates 6 are screwed at both ends; a steel cage 3 is tied between two adjacent T-shaped piles 1, and each additional tie bar 5 and its end anchor plate 6 extend into the corresponding steel cage 3.

[0029] A truss-ribbed composite slab 4 is arranged on the side of the reinforcing cage 3 and T-shaped pile 1 facing inwards from the foundation pit. The truss-ribbed composite slab 4, which intersects with the T-shaped pile 1, has mounting holes corresponding to internal threaded threads 13. High-strength bolts 7 are used to fix the truss-ribbed composite slab 4 to the web of the T-shaped pile 1 through the mounting holes and internal threaded threads 13. Fine aggregate concrete is poured into the cavity formed by the T-shaped pile 1, the truss-ribbed composite slab 4, and the soil layer outside the foundation pit.

[0030] like Figure 6 As shown, the overall steps of the construction method are as follows: Step S1: Prefabrication, transportation and on-site installation of T-shaped piles, and excavation of the first layer of earthwork; Step S2: Construction of cast-in-place reinforced concrete casing, cast-in-place reinforced concrete slab, and outer ring beam; Step S3: Tensioning and anchoring of unbonded prestressing tendons, cleaning of pile body, and installation of anchor bolts; Step S4: Insert additional tie rods through holes and tighten the anchor plate; Step S5: Tie the reinforcing cage and hoist the composite slab with truss ribs; Step S6: Treat the vertical joints of the composite slabs and pour fine aggregate concrete; Step S7: Excavate the lower layer of soil, and repeat the corresponding procedures above until the construction of the foundation pit retaining structure is completed.

[0031] The specific steps of the construction method are as follows: Step S1: Prefabrication, transportation, and on-site installation of T-shaped piles; excavation of the first layer of earthwork: According to the technical plan, precast reinforced concrete T-shaped piles are centrally produced in a prefabricated PC factory, and the production time is determined through stress calculations based on the specific characteristics of the project. Before driving the reinforced concrete T-shaped piles, the anchor holes, rebar holes, and internal threaded holes in the pile body should be sealed to prevent clogging of the reserved holes during the pile driving process, thus avoiding affecting subsequent processes.

[0032] During on-site construction, after the reinforced concrete T-shaped piles are transported to the site, they are driven to a position 1000 mm below the top elevation of the foundation pit using static pressure pile driving technology and guide pile method, with the error controlled within 5 mm. The specific driving operation is as follows: after the top surface of the reinforced concrete T-shaped pile is aligned with the top of the foundation pit using machinery, a 1000 mm long guide pile is used to connect with the reinforced concrete T-shaped pile, and the pile is driven using machinery. After the top surface of the guide pile is driven to the top elevation of the foundation pit, the guide pile is pulled out, thus completing the placement and driving process of the reinforced concrete T-shaped pile.

[0033] After all the reinforced concrete T-piles around the foundation pit have been driven into place, the first layer of earthwork excavation can be carried out in the foundation pit. The first layer of earthwork is excavated to 1200 mm below the top surface of the reinforced concrete T-piles. The excavation is carried out in layers, and the protection of the already driven reinforced concrete T-piles is strengthened during the excavation process.

[0034] Step S2: Construction of cast-in-place reinforced concrete casing, cast-in-place reinforced concrete slab, and outer ring beam: After the precast reinforced concrete T-piles are driven in, the trenches at the locations of the cast-in-place reinforced concrete casing and the outer ring beam can be excavated. The excavation depth of the cast-in-place reinforced concrete casing trench is 1000 mm, from the top of the foundation pit to the top of the precast reinforced concrete T-piles. The excavation width of the cast-in-place reinforced concrete casing trench is not less than 600 mm, and the cross-section of the outer ring beam trench is not less than 400 mm × 600 mm.

[0035] The trenches at the locations of the cast-in-place reinforced concrete casing and the outer ring beam are both closed around the perimeter of the foundation pit. That is, the two trenches form an unbroken ring around the perimeter of the foundation pit with the center of the foundation pit as the reference point. The cast-in-place reinforced concrete casing and the outer ring beam subsequently poured in the trenches act like double hoops to stabilize the top of the foundation pit. The hoops are under pressure and can steadily control the deformation of the top of the foundation pit.

[0036] After the trenches at the locations of the cast-in-place reinforced concrete casing and the outer ring beam are excavated, the reinforcing cages of the cast-in-place reinforced concrete casing and the outer ring beam are hoisted in, and the reinforcing mesh of the cast-in-place reinforced concrete slab is installed. The two ends of the reinforcing mesh of the cast-in-place reinforced concrete slab are anchored into the interior of the reinforcing cages of the cast-in-place reinforced concrete casing and the outer ring beam, respectively, and the structural requirements are met. At the same time as the reinforcing mesh of the cast-in-place reinforced concrete slab is installed, the unbonded prestressing tendons are pre-embedded.

[0037] A steel formwork is installed on the side near the inside of the foundation pit. A reserved hole of not less than 60mm is opened on the steel formwork at the position of each anchor rod. Anchor rod sleeves are inserted into the reserved holes and then inserted into the soil layer outside the foundation pit after passing through the reserved holes, so as to provide conditions for the subsequent installation of anchor rods.

[0038] Finally, fine aggregate concrete was used to cast the cast-in-place reinforced concrete casing, cast-in-place reinforced concrete slab, and outer ring beam together, without leaving construction joints, and curing was strengthened. It should be noted that the internal reinforcement of the cast-in-place reinforced concrete casing, cast-in-place reinforced concrete slab, and outer ring beam was determined by comprehensive calculation based on different project types, geological conditions, groundwater conditions, etc.

[0039] Step S3: Tensioning and anchoring of unbonded prestressing tendons, pile cleaning, and installation of anchor bolts: After the cast-in-place reinforced concrete casing, cast-in-place reinforced concrete slab, and outer ring beam are completed and the concrete strength reaches the design strength, the steel formwork required for the cast-in-place reinforced concrete casing near the inside of the foundation pit is removed. At this point, the anchor sleeves are fixed inside the cast-in-place reinforced concrete casing due to the concrete solidification. Then, the anchors are driven one by one into the soil layer outside the foundation pit through the anchor sleeves. After all the anchors on the cast-in-place reinforced concrete casing are installed, the unbonded prestressed tendons between the cast-in-place reinforced concrete casing and the outer ring beam are tensioned and anchored.

[0040] For the precast reinforced concrete T-shaped piles within the first-level earthwork excavation area, manual cleaning of the pile body and the soil between the piles is carried out. The focus is on cleaning the anchor holes, through holes, and internal threaded holes in the pile body to ensure that the interior of each reserved hole is clean and does not affect the subsequent three processes of anchor installation, additional tie rod penetration, and high-strength bolt tightening.

[0041] The anchor bolts on the T-shaped pile wing plates are driven into the active earth pressure zone outside the foundation pit through the anchor bolt holes on the T-shaped pile wing plates. Cement grout of at least M30 grade is injected into the holes, and the design pull-out bearing capacity is 120 kN. After on-site anchor bolt installation and grouting, vibration interference is reduced, and curing is strengthened.

[0042] Step S4: Insert additional tie rods through holes and tighten the anchor plate: According to the technical plan, after all anchor bolts on the first floor are installed and the internal grout has cured to at least 80% of its design strength, the additional tie bars can be installed. The additional tie bars will pass through the through-holes in the web of the reinforced concrete T-shaped piles. The additional tie bars will be made of HRB400 steel bars with a diameter of at least 22 mm, and each tie bar will be at least 1200 mm long. Anchor plates will be tightened at both ends of the additional tie bars, with 100% of the anchor plates secured. The tie bars will be spaced no more than 0.8 m vertically and will be alternated with the anchor bolt layers to enhance the synergistic load-bearing performance between the piles and the post-cast fine aggregate concrete.

[0043] Step S5: Reinforcing cage binding and hoisting of composite slab with truss ribs: According to the technical plan, after all the additional tie bars are threaded through, the binding of the steel cage between two adjacent reinforced concrete T-shaped piles in each module can be carried out. The longitudinal and transverse distribution of the steel cage is not less than Φ12@200, and the height matches the excavation depth of the first layer of the foundation pit, that is, from the bottom of the cast-in-place reinforced concrete casing to the excavation surface of the first layer of the foundation pit. At the same time, it is ensured that all additional tie bars and their end anchor plates extend into the interior of the steel cage.

[0044] After the steel cages in each module are installed, the hoisting operation of the composite slab with truss ribs can be carried out. The composite slab with truss ribs at the junction with the reinforced concrete T-shaped pile is provided with installation holes. M20 high-strength bolts (grade 8.8) are used to fix the T-shaped pile web plate with threads through the reserved installation holes (hole diameter 22 mm). The bolt preload is controlled at 80 N·m.

[0045] Step S6: Treatment of vertical joints in composite slabs, pouring of fine aggregate concrete: According to the technical plan, during the on-site construction process, additional steel plates are used to seal the joints between two adjacent composite slabs with trusses and bolts. Water-swellable sealing strips are installed between all composite slabs with trusses and reinforced concrete T-piles, and between the joints of two adjacent composite slabs with trusses, to prevent grout leakage during subsequent fine aggregate concrete pouring.

[0046] After the truss-ribbed composite slab is hoisted and the joints are treated, fine aggregate concrete can be poured in modules. That is, fine aggregate concrete is poured into the cavity enclosed by the reinforced concrete T-piles, the truss-ribbed composite slab, and the soil layer outside the foundation pit. Micro-expansion fine aggregate concrete of not less than C30 is used, and the slump is not less than 120 mm. When difficulties arise in the on-site fine aggregate concrete pouring or other complex working conditions, the tremie pipe method can be used to assist the on-site fine aggregate concrete pouring operation.

[0047] Step S7: Excavation of the lower layer of soil, repeating the corresponding procedures above until the construction of the foundation pit retaining structure is completed: According to the technical plan, once the first-layer retaining structure is completed and the fine stone concrete reaches 100% of the design strength, the excavation of the next layer of earthwork can be carried out, and the corresponding procedures mentioned above can be repeated until the construction of the entire foundation pit retaining structure is completed.

[0048] After the layered and segmented construction of the foundation pit retaining structure is completed, the concrete cushion layer at the bottom of the foundation pit should be poured immediately and must not be left overnight. At the same time, continuous monitoring should be strengthened on the horizontal displacement of the pile top, the lateral displacement of the retaining structure, the axial force of the anchor bolts, and the settlement of the surrounding environment to ensure the safety and reliability of the retaining structure.

[0049] In summary, this invention, through its T-shaped pile support assembly integral structure design, achieves a comprehensive effect of high-efficiency and low-carbon construction, green environmental protection, reliable connection, strong anchor bolt coordination, convenient installation, and safety and stability, effectively solving the core defects of existing foundation pit protection technology.

[0050] 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 construction method for a T-shaped pile-supported assembled integral foundation pit retaining structure, characterized in that, The foundation pit retaining structure includes T-shaped piles, additional tie bars, and composite slabs with truss ribs. Multiple T-shaped piles are arranged in an orderly manner along the edge of the foundation pit to form a pile group. The top of the pile group is equipped with a cast-in-place reinforced concrete casing. An outer ring beam is provided on the outside of the cast-in-place reinforced concrete casing. Both the cast-in-place reinforced concrete casing and the outer ring beam are closed hoop-shaped. A cast-in-place reinforced concrete slab is provided between the two. Unbonded prestressed tendons are inserted through all three, and all three are cast together with fine aggregate concrete. Anchor rods are inserted through the cast-in-place reinforced concrete casing and the T-shaped pile wing plates. All anchor rods are driven into the soil layer outside the foundation pit. A steel cage is tied between two adjacent T-shaped piles, and additional tie bars are inserted through the web of the T-shaped piles, with anchor plates tightened at both ends. All additional tie bars and their ends extend into the corresponding steel cage. A composite slab with truss ribs is arranged on the side of the steel cage and T-shaped piles facing the inside of the foundation pit, and the composite slab with truss ribs that intersects with the T-shaped piles is fixed to the web of the T-shaped piles with high-strength bolts. Fine stone concrete is poured into the cavity enclosed by the T-shaped piles, the composite slab with truss ribs, and the soil layer outside the foundation pit. The overall steps of the construction method are as follows: Step S1: Prefabrication, transportation and on-site installation of T-shaped piles, and excavation of the first layer of earthwork; Step S2: Construction of cast-in-place reinforced concrete casing, cast-in-place reinforced concrete slab, and outer ring beam; Step S3: Tensioning and anchoring of unbonded prestressed tendons, cleaning of pile body, and installation of anchor bolts; Step S4: Insert additional tie rods through holes and tighten the anchor plate; Step S5: Tie the reinforcing cage and hoist the composite slab with truss ribs; Step S6: Treat the vertical joints of the composite slabs and pour fine aggregate concrete; Step S7: Excavate the lower layer of soil, and repeat the corresponding procedures above until the construction of the foundation pit retaining structure is completed.

2. The construction method according to claim 1, characterized in that: The T-shaped pile wing plate is pre-drilled with Φ60 mm anchor bolt holes, the length of which is not less than 9.0 m, the inclination angle is 20°~30°, and the spacing between the anchor bolt holes does not exceed 1.2 m; the web plate is pre-drilled with Φ25 mm through-bar holes and M24 internal threaded threads, the through-bar holes are used for additional tie bars to pass through the web plate of the T-shaped pile; the composite plate with truss ribs is provided with corresponding internal threaded installation holes, the installation holes and internal threaded threads are used to arrange high-strength bolts.

3. The construction method according to claim 1, characterized in that: The T-shaped pile has a T-shaped cross-section, with a web height of not less than 400 mm, a flange width of not less than 900 mm, a web thickness of not less than 150 mm, a flange thickness of not less than 120 mm, a concrete strength grade of not less than C30, and longitudinal main reinforcement using HRB400 grade steel bars with a reinforcement ratio of not less than 1.2%.

4. The construction method according to claim 1, characterized in that: Water-swellable sealing strips are installed between the truss-ribbed composite slab and the T-shaped pile, as well as between the joints of two adjacent truss-ribbed composite slabs.

5. The construction method according to claim 1, characterized in that: The anchor rods are full-length bonded prestressed anchor rods, with the rod body made of HRB400 steel bars of not less than Φ25 mm. The anchorage section of the anchor rod is not less than 5.0 m long, and the free section is not less than 4.0 m long.

6. The construction method according to claim 1, characterized in that, The specific steps are as follows: Step S1: Prefabrication, transportation, and on-site installation of T-shaped piles; excavation of the first layer of earthwork: Precast reinforced concrete T-shaped piles are centrally produced in a prefabricated PC factory. The cross-section is T-shaped, with a web height of not less than 400 mm, a flange width of not less than 900 mm, a web thickness of not less than 150 mm, a flange thickness of not less than 120 mm, a concrete strength grade of not less than C30, and longitudinal main reinforcement using HRB400 grade steel bars with a reinforcement ratio of not less than 1.2%. Φ60 mm anchor holes are reserved in the flanges of the T-shaped piles, with a spacing between anchor holes not exceeding 1.2 m. Φ25 mm through-bar holes and M24 internal threaded threads are reserved in the webs. The through-bar holes are used for additional tie bars to pass through the webs of the reinforced concrete T-shaped piles to achieve post-cast connection between adjacent modules, and the internal threaded threads are used for subsequent on-site hoisting of the composite slab with truss ribs. Before driving the reinforced concrete T-shaped piles, the anchor holes, through holes and internal thread of the pile body are sealed. After the reinforced concrete T-shaped piles are transported to the site, they are driven by static pressure pile driving technology and driven to 1000 mm below the top surface of the foundation pit, with the error controlled within 5 mm. After all the reinforced concrete T-shaped piles around the foundation pit have been driven into place, the first layer of earthwork excavation in the foundation pit can be carried out. The first layer of earthwork excavation will be carried out to 1200 mm below the top surface of the reinforced concrete T-shaped piles. The excavation will be carried out in layers, and the protection of the already driven reinforced concrete T-shaped piles will be strengthened during the excavation process. Step S2: Construction of cast-in-place reinforced concrete casing, cast-in-place reinforced concrete slab, and outer ring beam: After the precast reinforced concrete T-shaped piles are driven in, the trenches at the locations of the cast-in-place reinforced concrete casing and the outer ring beam are excavated. Both trenches are closed around the perimeter of the foundation pit. The excavation depth of the cast-in-place reinforced concrete casing trench is 1000mm, from the top of the foundation pit to the top of the precast reinforced concrete T-shaped pile. The excavation width of the cast-in-place reinforced concrete casing trench is not less than 600mm, and the cross-section of the outer ring beam trench is not less than 400mm×600mm. After the trenches at the locations of the cast-in-place reinforced concrete casing and the outer ring beam are excavated, the cast-in-place reinforced concrete casing steel cage and the outer ring beam steel cage are hoisted, and the cast-in-place reinforced concrete slab steel mesh is installed. The two ends of the cast-in-place reinforced concrete slab steel mesh are anchored into the interior of the cast-in-place reinforced concrete casing steel cage and the outer ring beam steel cage, respectively, and the structural requirements are met. At the same time as installing the cast-in-place reinforced concrete slab steel mesh, each unbonded prestressed tendon is pre-embedded. A steel formwork is installed on the side of the cast-in-place reinforced concrete casing trench near the inside of the foundation pit. Pre-reserved holes are opened on the steel formwork at the positions of each anchor rod. An anchor rod sleeve is inserted into the pre-reserved hole and then inserted into the soil layer outside the foundation pit after passing through the pre-reserved hole. Finally, fine aggregate concrete is used to cast the cast-in-place reinforced concrete casing, cast-in-place reinforced concrete slab, and outer ring beam together without leaving construction joints, and curing is strengthened; the internal reinforcement of the cast-in-place reinforced concrete casing, cast-in-place reinforced concrete slab, and outer ring beam is determined by comprehensive calculation based on different project types, geological conditions and groundwater conditions. Step S3: Tensioning and anchoring of unbonded prestressing tendons, pile cleaning, and installation of anchor bolts: After the cast-in-place reinforced concrete casing, cast-in-place reinforced concrete slab, and outer ring beam are completed and the concrete strength reaches the design strength, the steel formwork is removed, and the anchor rods are driven one by one into the soil layer outside the foundation pit through the anchor rod sleeve. After all the anchor rods on the cast-in-place reinforced concrete casing are installed, the unbonded prestressed tendons between the cast-in-place reinforced concrete casing and the outer ring beam are tensioned and anchored. For the precast reinforced concrete T-shaped piles within the first-level earthwork excavation area, manual cleaning of the pile body and the soil between the piles is adopted. The focus is on cleaning the anchor holes, through holes and internal thread of the pile body to ensure that the inside of each reserved hole is clean and does not affect the subsequent three processes of anchor installation, additional tie rod penetration and high-strength bolt tightening. The anchor rods on the T-shaped pile wing plate are driven into the active earth pressure zone outside the foundation pit through the anchor rod holes on the T-shaped pile wing plate. The holes are filled with cement grout of not less than M30. The standard value of the design pull-out bearing capacity is 120 kN. After the on-site anchor rod construction and grouting, vibration interference should be reduced and maintenance should be strengthened. Step S4: Insert additional tie rods through holes and tighten the anchor plate: After all anchor bolts on the first floor are installed and the internal grout has cured to at least 80% of the design strength, the additional tie rods are installed. The additional tie rods pass through the through holes in the web of the reinforced concrete T-shaped pile. The additional tie rods are made of HRB400 steel bars with a diameter of not less than 22mm. Each additional tie rod is at least 1200mm long. Anchor plates are tightened at both ends of the additional tie rods. The number of anchor plates is 100% and no plates are missing. The vertical spacing of the tie rods does not exceed 0.8m. They are arranged alternately with the anchor bolt layers to enhance the synergistic stress performance of the pile and the post-cast fine stone concrete. Step S5: Reinforcing cage binding and hoisting of composite slab with truss ribs: After all the additional tie bars are threaded through, the binding of the steel cage between two adjacent reinforced concrete T-shaped piles in each module is carried out. The longitudinal and transverse distribution of the steel cage is not less than Φ12@200, and the height matches the excavation depth of the first layer of the foundation pit, that is, from the bottom of the cast-in-place reinforced concrete casing to the excavation surface of the first layer of the foundation pit. At the same time, it is ensured that all additional tie bars and their end anchor plates extend into the inside of the steel cage. After the steel cages in each module are installed, the hoisting operation of the composite slab with truss ribs is carried out. The thickness of the composite slab with truss ribs is not less than 60 mm, and the length is customized according to the pile spacing. The composite slab with truss ribs at the junction with the reinforced concrete T-shaped piles is provided with installation holes. M20 high-strength bolts are used to fix the T-shaped piles to the internal threads of the web plate through the reserved installation holes. The bolt pre-tightening force is controlled at 80 N·m. Step S6: Treatment of vertical joints in composite slabs, pouring of fine aggregate concrete: During on-site construction, additional steel plates were added to seal the joints between adjacent composite slabs with trusses and bolts. Water-swellable sealing strips were installed between all composite slabs with trusses and reinforced concrete T-piles, and between the joints of adjacent composite slabs with trusses to prevent grout leakage during subsequent fine stone concrete pouring. After the truss-ribbed composite slab is hoisted and the joints are treated, fine stone concrete is poured in modules. Fine stone concrete is poured into the cavity enclosed by the reinforced concrete T-piles, the truss-ribbed composite slab, and the soil layer outside the foundation pit. Micro-expansion fine stone concrete of not less than C30 is used, and the slump is not less than 120 mm. When difficulties arise in the on-site fine stone concrete pouring or other complex conditions, the tremie pipe method is used to assist the on-site fine stone concrete pouring operation. Step S7: Excavation of the lower layer of soil, repeating the corresponding procedures above until the construction of the foundation pit retaining structure is completed: Once the first-layer retaining structure is completed and the fine stone concrete reaches 100% of the design strength, the excavation of the next layer of earthwork can be carried out, and the above corresponding procedures can be repeated until the construction of the foundation pit retaining structure is completed. After the construction of the foundation pit retaining structure is completed in layers and sections, the concrete cushion layer at the bottom of the foundation pit should be poured immediately and should not be left overnight; at the same time, continuous monitoring should be strengthened on the horizontal displacement of the pile top, the lateral displacement of the retaining structure, the axial force of the anchor bolts and the settlement of the surrounding environment to ensure the safety and reliability of the retaining structure.

Images