Foundation pit enclosure structure and construction method based on symmetric double-bevel precast pile

Abstract

The application discloses a foundation pit retaining structure based on symmetric double-tongue precast piles and a construction method thereof, and the overall structure comprises symmetric double-tongue precast piles, cast-in-place hidden piles, cast-in-place shear walls and composite slabs with truss ribs; the symmetric double-tongue precast piles are arranged at intervals along the edges of the foundation pit, cast-in-place hidden piles are arranged at both sides of each precast pile, cast-in-place shear walls are arranged between the two cast-in-place hidden piles between two adjacent precast piles, and the cast-in-place hidden piles and the cast-in-place shear walls are formed by fine stone concrete pouring; additional reinforcing bars are arranged in the precast piles and extend out of the symmetric double-tongue precast piles and into the cast-in-place shear walls; the composite slabs with truss ribs are fixed to one side of the two adjacent precast piles which faces the inside of the foundation pit through high-strength bolts, anchor rods are arranged on the slabs, and the anchor rods are driven into the soil layer outside the foundation pit. The application significantly improves the overall lateral stiffness and impermeability of the retaining structure, reduces the construction risk of the foundation pit, and is suitable for deep foundation pit engineering in soft soil and complex environmental conditions.

Description

Technical Field

[0001] This invention relates to the field of foundation pit support technology, specifically to a foundation pit support structure based on symmetrical double tongue-and-groove precast piles and its construction method. Background Technology

[0002] With the continuous expansion of urban underground space development, the number of deep foundation pit projects in soft soil and complex surrounding environments is increasing, leading to the widespread application of foundation pit retaining systems combining precast piles and cast-in-place shear walls. Traditional foundation pit retaining structures often employ a combination of ordinary precast piles and post-cast shear walls, which has revealed numerous technical shortcomings in engineering practice. Firstly, the connection between precast piles and cast-in-place shear walls is simple in construction, and the shear bearing capacity at the pile-wall connection is insufficient. Stress concentration is prone to occur at the joint between precast piles and cast-in-place walls, resulting in frequent local cracking and structural damage. In severe cases, it can directly cause the overall load-bearing capacity of the retaining structure to fail. Secondly, the precast piles, hidden piles, shear walls, and support anchors lack reliable tie-and-transfer structures, resulting in poor overall synergistic bearing capacity. Lateral earth pressure and water pressure cannot be evenly distributed and transmitted, and the overall lateral stiffness of the retaining structure is low. Third, the sealing and seepage prevention structure at the joint of the pile wall is weak, and there are prominent water leakage phenomena at multiple points during the operation of the foundation pit. In the soft soil layer, it is very easy to cause linear quicksand disease, and the quicksand danger is difficult to deal with, which greatly increases the construction safety risks of foundation pit collapse and settlement and deformation of surrounding buildings. Fourth, traditional retaining structures have a low degree of prefabrication, involve numerous on-site steel reinforcement binding and formwork erection procedures, have long construction cycles, involve a large amount of on-site wet work, and have high labor costs and construction period control pressures, making them difficult to adapt to the rapid construction needs of complex deep foundation pits in high-density urban areas. Summary of the Invention

[0003] The purpose of this invention is to provide a foundation pit retaining structure based on symmetrical double tongue-and-groove precast piles and its construction method, so as to solve the problems raised in the background art.

[0004] To achieve the above objectives, the present invention provides the following technical solution: a foundation pit retaining structure based on symmetrical double tongue-and-groove precast piles, including symmetrical double tongue-and-groove precast piles, cast-in-place hidden piles, cast-in-place shear walls, and composite slabs with truss ribs; the pile bodies of the symmetrical double tongue-and-groove precast piles are driven at intervals along the edge of the foundation pit, and symmetrical tongue-and-groove joints are opened at their left and right ends. Each of the symmetrical double-tongue-and-groove precast piles is provided with cast-in-place hidden piles at both sides of the tongue and groove. The cast-in-place hidden piles are formed by the steel reinforcement cage after the fine stone concrete is poured. The cast-in-place shear wall is provided between the two cast-in-place hidden piles between two adjacent symmetrical double-tongue-and-groove precast piles. The cast-in-place shear wall is formed by the steel reinforcement cage after the fine stone concrete is poured. Additional tie bars are inserted inside the symmetrical double tongue-and-groove precast piles. Both ends of the additional tie bars extend out of the symmetrical double tongue-and-groove precast piles and are anchored inside the cast-in-place shear wall. The composite slab with truss ribs is fixed to the side of the adjacent two symmetrical double tongue-and-groove precast piles facing the inside of the foundation pit by high-strength bolts. Anchor rods are inserted on the slab and driven into the soil layer outside the foundation pit.

[0005] Preferably, the tongue and groove depth is not less than 60mm, the outer width of the tongue and groove is not less than 1 / 2 and not more than 2 / 3 of the side length of the symmetrical double tongue and groove precast pile in the same direction, and the angle between the tongue and groove slope and the horizontal plane is 30°~45°.

[0006] Preferably, both ends of the horizontal reinforcing bars in the steel cage extend into the steel reinforcement skeleton, and the cast-in-place hidden pile and the cast-in-place shear wall are cast together.

[0007] Preferably, the vertical reinforcing bars of both the steel reinforcement cage and the steel reinforcement skeleton are installed using the insertion method.

[0008] Preferably, depending on the location of the cast-in-place hidden piles, the cast-in-place hidden piles are divided into ordinary cast-in-place hidden piles and right-angle cast-in-place hidden piles, and the right-angle cast-in-place hidden piles are only set at the corners of the foundation pit.

[0009] Preferably, both ends of the additional tie rod extend at least 500mm beyond the symmetrical double-tongue precast pile, and the end of the additional tie rod near the corner of the foundation pit is anchored inside the right-angle cast-in-place hidden pile.

[0010] Preferably, the symmetrical double tongue-and-groove precast pile has a reserved through hole and an internal threaded thread inside, both of which are pre-embedded during the production process of the symmetrical double tongue-and-groove precast pile PC in the factory; the reserved through hole has a diameter of 30mm and is used for the additional tie rod to pass through; the internal threaded thread is located on the side of the symmetrical double tongue-and-groove precast pile facing the pit and is used to tighten the high-strength bolt; the two ends of the composite plate with truss ribs have mounting holes for the high-strength bolt to pass through.

[0011] This invention also applies for protection of a construction method for a foundation pit retaining structure based on symmetrical double tongue-and-groove precast piles, comprising the following steps: Step S1: Measurement and layout for pile positioning, production, transportation, and on-site driving of symmetrical double tongue-and-groove precast piles: Based on the comprehensive geological conditions, the excavation depth of the foundation pit and the earth pressure, the symmetrical double tongue and groove precast piles are designed and precast. At the same time, in accordance with the design requirements for the additional tie rods and the installation requirements for the composite slab with truss ribs, the reserved perforations and internal threads inside the symmetrical double tongue and groove precast piles are pre-embedded. First, a total station was used to accurately measure the locations of the symmetrical double tongue-and-groove precast piles arranged around the foundation pit. After the concrete strength reached 100% of the design strength, it was transported to the construction site. Rubber plugs were used to seal the openings of each pre-reserved hole, and temporary short bolt heads were tightened into the internal threaded sleeve to prevent the pre-reserved holes from being blocked during the driving process of the symmetrical double tongue and groove precast piles. When carrying out pile driving operations, symmetrical double-tongue precast piles are symmetrically driven from the middle of the foundation pit to both sides, and symmetrical double-tongue precast piles are not installed at the corners of the foundation pit. Step S2: First layer excavation, additional tie rod perforation: After all the symmetrical double-tongue precast piles around the foundation pit have been driven, the first layer of earthwork will be excavated using the central island excavation method. Sufficient working face will be left near the foundation pit retaining structure. The soil layer between the piles will be cleaned manually. The excavation depth of the first layer of earthwork will be strictly controlled according to the design height of the composite slab with truss ribs. After the first layer of earthwork is excavated, the symmetrical double tongue and groove precast piles within the depth range of the first layer of earthwork are cleaned, and the rubber plugs and short bolt heads in the reserved holes and internal threaded threads of the symmetrical double tongue and groove precast piles are pulled out. All contact surfaces of the symmetrical double tongue-and-groove precast piles and cast-in-place underground piles are roughened using a hydraulic splitter, with a roughness rate ≥80%. The additional tie bars are passed through the reserved holes inside the symmetrical double tongue and groove precast piles one by one, ensuring that the left and right extension lengths of the additional tie bars after drilling are consistent. Step S3: Anchor plate installation, rebar cage installation: After the additional tie bars within the first-layer excavation area are drilled, anchor plates are tightened at both ends of each additional tie bar to achieve 100% quantity. The steel reinforcement cage is connected to the upper and lower layers using the insertion process, that is, the group of vertical steel bars is inserted to a depth of at least 500mm below the bottom elevation of the first-layer excavation, and then the inserted vertical steel bars are fixed with stirrups to form the steel reinforcement cage. The overall planar structure of the steel reinforcement cage at the corner of the foundation pit is right-angled. The additional tie bars at the two nearest symmetrical double tongue-and-groove precast piles at the corner of the foundation pit extend into the right-angled steel reinforcement cage, and each of the remaining additional tie bars crosses the steel reinforcement cage. Step S4: Install the reinforcing cage in sections and intervals, and hoist the composite slab with truss ribs: After the steel reinforcement cage is installed, the steel reinforcement cage is hoisted between the two adjacent steel reinforcement cages between the two adjacent symmetrical double tongue and groove precast piles. The steel reinforcement cage also adopts the insertion process, that is, the group of vertical steel bars are inserted to a depth of at least 300mm below the bottom elevation of the first layer of earthwork excavation. Then, the horizontal steel bars and tie bars are tied one by one or laid out by electric welding. The installation is completed in sections and intervals. Finally, the truss-ribbed composite slab is hoisted to the side of a set of two adjacent symmetrical double tongue-and-groove precast piles facing the inside of the foundation pit. High-strength bolts are then passed through the mounting holes at both ends of the truss-ribbed composite slab and tightened into the internal threaded sleeve to fix the truss-ribbed composite slab. The joints between each truss-ribbed composite slab are treated by adding sealing strips and applying waterproof membrane strips by hot-melt method. Step S5: Drive in anchor bolt sleeves and pour fine aggregate concrete to form cast-in-place hidden piles and cast-in-place shear walls: After all the truss-ribbed composite slabs within the first-floor excavation area are hoisted, each anchor sleeve is driven into the soil outside the foundation pit through the pre-reserved anchor holes in the truss-ribbed composite slab. The anchor sleeves must not collide with the steel cage already installed between the soil outside the foundation pit and the truss-ribbed composite slab. The inner diameter of the anchor sleeve is 20mm~30mm larger than the designed anchor diameter. The cutting length is the prefabricated thickness of the truss-ribbed composite slab + the side length of the symmetrical double tongue-and-groove prefabricated pile in the anchor length direction + 450mm. The lowest anchor hole in each section of the first floor is reserved and no anchor sleeve is installed for the time being. Finally, using mechanical equipment, high-pressure grouting is carried out by inserting grouting pipes into the lowest anchor holes reserved in each section of the first floor to form cast-in-place shear walls and cast-in-place hidden piles. The strength of fine stone concrete is not less than C35. After grouting is completed, anchor sleeves are immediately installed in the anchor holes used for grouting, and curing is strengthened to reduce vibration. Fine aggregate concrete was poured into the right-angled steel reinforcement cage at the corner of the foundation pit in advance, and after it was cured to 90% of the design strength, fine aggregate concrete grouting was carried out in the middle of the perimeter of the foundation pit. Step S6: Install anchor bolts and excavate the lower layer of soil: After the first layer of fine stone concrete is poured and cured to 100% of the design strength, the anchor bolt installation work will be carried out. First, the anchor rods are driven one by one into the soil layer outside the foundation pit through the pre-embedded anchor rod sleeves; the anchor rods are full-length bonded steel anchor rods, and positioning brackets are set at the ends of the anchor rods to ensure that the anchor rods are centered. After the anchor bolts are installed, pressure grouting is immediately carried out. M30 pure cement grout is used as the grouting material, and the grouting pressure is controlled at 0.5~1.0MPa until grout returns to the borehole. After the grout reaches the design strength, the anchor bolts are tensioned and locked. The tension force is controlled at 70%~80% of the design axial force, and the anchor plate is installed and locked. After the anchor bolts are installed, the next layer of earthwork is excavated. The excavation method is a central island excavation, carried out in layers and sections, with each layer not exceeding 2.0m in depth. A working platform of not less than 1.0m wide is left on the side near the symmetrical double-tongue precast piles. Excess soil between piles is cleaned manually with the help of small machinery. During the excavation process, collisions with the installed anchor bolts and fine stone concrete shear walls are strictly prohibited, and the deformation of the foundation pit is monitored in real time. After the next layer of earthwork is excavated to the design elevation, the pile body is cleaned in time to provide a working surface for the next additional tie rod perforation and steel reinforcement cage installation. Step S7: Repeat the above process until the entire enclosure structure construction is completed. After the lower layer of earthwork is excavated, the exposed vertical steel bars below the bottom elevation of the first layer of earthwork excavation are treated. The surface soil of each exposed steel bar and the soil and loose stones at the bottom of the fine stone concrete that has been poured in the first layer are cleaned. Then, the lower part of the vertical steel bars of each cast-in-place hidden pile and the vertical steel bars of the cast-in-place shear wall steel cage within the excavation range of the lower layer of earthwork are also connected to the exposed vertical steel bars below the bottom elevation of the first layer of earthwork excavation using the same dowel bar process. Repeat all the above processes until the construction of the entire retaining structure is completed. Finally, carry out the full excavation of the soil in the foundation pit. After the soil is excavated to the bottom elevation of the foundation pit, immediately pour the foundation concrete.

[0012] Compared with the prior art, the beneficial effects of this invention are as follows: 1. This invention uses symmetrical double-tongue precast piles as the foundation load-bearing components. The piles are symmetrically equipped with 30°~45° sloped tongues at both ends, which interlock and are embedded with the cast-in-place underground piles. With the additional tie bars penetrating the piles, the precast piles, cast-in-place underground piles, and cast-in-place shear walls form a continuous load-bearing whole. The two ends of the additional tie bars extend into the interior of the shear wall and are equipped with anchor plates. At the same time, the steel reinforcement skeleton and steel reinforcement cage achieve reliable lap splicing of the vertical steel reinforcement between the upper and lower layers through the dowel bar process, which completely solves the problem of weak shear resistance at the traditional pile-wall joint. Soil pressure and water pressure can be evenly transmitted between the precast piles, cast-in-place underground piles, shear walls, and anchors. The overall lateral stiffness of the retaining structure is significantly improved, and the collaborative performance of each component is greatly optimized, effectively avoiding joint cracking, local damage, and even overall load-bearing failure.

[0013] 2. On the one hand, this invention features a precast pile tongue and groove joint that interlocks with the cast-in-place concealed pile, with the pile body contact surface roughened by hydraulic splitting, significantly improving the bonding density between the precast components and the cast-in-place fine stone concrete interface, thus blocking seepage channels. On the other hand, the truss-ribbed composite slab joints adopt a double waterproof structure of sealing strips and hot-melt waterproof membrane, and the grouting is formed by high-pressure injection of C35 fine stone concrete, resulting in no through gaps at the pile-wall joints. The anchor sleeves are precisely matched with the pre-embedded holes in the wall, reducing seepage gaps and solving the problems of multi-point leakage and linear quicksand in the foundation pit from the source, thereby reducing the construction risks caused by water damage in soft soil foundation pits.

[0014] 3. In this invention, right-angle cast-in-place hidden piles and right-angle steel reinforcement cages are set separately at the corners of the foundation pit. The additional tie bars of the precast piles on both sides of the corner extend into the inside of the right-angle steel reinforcement cage, which specifically strengthens the stress concentration area at the corner of the foundation pit. During the construction stage, the corner concrete is poured first and cured to 90% of the design strength before the middle wall is constructed. The step-by-step forming reduces the risk of corner cracking, comprehensively improves the overall stability of the retaining structure, and reduces the risk of foundation pit deformation and instability. Attached Figure Description

[0015] Figure 1 This is a plan view of the foundation pit retaining structure of the present invention; Figure 2 This is a flowchart illustrating the construction method of the foundation pit retaining structure according to the present invention. Detailed Implementation

[0016] To address the shortcomings of conventional precast pile foundation pit retaining structures, such as weak lateral stiffness and poor synergistic bearing capacity of the pile-soil-support system, this invention discloses a foundation pit retaining structure based on symmetrical double tongue-and-groove precast piles. (See attached document.) Figure 1 The overall structure mainly includes symmetrical double tongue-and-groove precast piles 11, cast-in-place hidden piles 21, cast-in-place shear walls 31, additional tie bars 12, composite slabs with truss ribs 51, and anchor rods 62.

[0017] The pile body of the symmetrical double tongue and groove precast pile 11 is driven at intervals along the edge of the foundation pit. The left and right ends are symmetrically distributed tongue and groove. The tongue and groove depth is not less than 60mm, the outer width of the tongue and groove is not less than 1 / 2 and not more than 2 / 3 of the side length of the symmetrical double tongue and groove precast pile 11 in the same direction, and the angle between the tongue and groove slope and the horizontal plane is 30°~45°.

[0018] Each symmetrical double-tongue precast pile 11 has a corresponding cast-in-place pile 21 at both sides of the tongue and groove. The cast-in-place pile 21 is formed by a steel reinforcement cage 22 after fine stone concrete is poured. The steel reinforcement cage 22 includes vertical steel bars and stirrups. According to its setting position, the cast-in-place pile 21 is divided into ordinary cast-in-place pile 2a and right-angle cast-in-place pile 2b. The right-angle cast-in-place pile 2b is only set at the corner of the foundation pit. Its horizontal cross-sectional structure is adapted to the corner of the foundation pit, and both ends are symmetrical double-tongue precast piles 11. The ordinary cast-in-place pile 2a is set at the non-corner positions around the foundation pit.

[0019] A cast-in-place shear wall 31 is provided between two cast-in-place underground piles 21 between two adjacent symmetrical double-tongue precast piles 11. The cast-in-place shear wall 31 is formed by pouring fine aggregate concrete from a steel cage 32, and the cast-in-place underground piles 21 and the cast-in-place shear wall 31 are cast together. Before pouring, both ends of all horizontal steel bars in the steel cage 32 extend into the steel reinforcement skeleton 22. The symmetrical double tongue and groove precast pile 11 has a reserved through hole and an internal thread. Both are pre-embedded in the PC factory production process of the symmetrical double tongue and groove precast pile 11. The reserved through hole has a diameter of 30mm, and the internal thread is located on the side of the symmetrical double tongue and groove precast pile 11 facing the inside of the foundation pit.

[0020] Additional tie bars 12 pass through the reserved holes inside the symmetrical double-tongue precast piles 11. Both ends of the additional tie bars 12 extend at least 500mm beyond the symmetrical double-tongue precast piles 11 after passing through the holes, and simultaneously pass through the steel reinforcement cage 22 not located at the corner of the foundation pit. Each additional tie bar 12 is provided with an anchor plate at its end. The anchor plate is located either in the cast-in-place shear wall 31 or in the right-angle cast-in-place hidden pile 2b at the corner of the foundation pit.

[0021] Between two adjacent symmetrical double-tongue precast piles 11, a composite plate 51 with truss ribs is provided. The composite plate 51 with truss ribs has mounting holes at both ends. High-strength bolts 52 are passed through the mounting holes and tightened into the internal threads of two adjacent symmetrical double-tongue precast piles 11 for fixation. Anchor bolt holes are also provided on the composite plate 51, with anchor bolt sleeves 61 inserted into the anchor bolt holes. Anchor bolts 62 pass through the anchor bolt sleeves 61 and are driven into the soil layer outside the foundation pit, thus forming a symmetrical double-tongue precast pile-cast-in-place hidden pile-cast-in-place shear wall-anchor bolt integrated collaborative bearing technology system.

[0022] It should be noted that the horizontal cross-sectional structure of the composite slab 51 with truss ribs connected to a set of adjacent symmetrical double tongue-and-groove precast piles 11 near the corner of the foundation pit is also adapted to the corner structure of the foundation pit.

[0023] The aforementioned foundation pit retaining structure is achieved through the following construction method; please refer to the overall workflow diagram. Figure 2 : Step S1: Measurement and layout for pile positioning, production, transportation, and on-site driving of symmetrical double tongue-and-groove precast piles: The symmetrical double tongue-and-groove precast piles are designed and prefabricated based on factors such as geological conditions, excavation depth, and earth pressure. Each symmetrical double tongue-and-groove precast pile is prefabricated in a prefabricated PC factory. The reserved perforations and internal threads inside the symmetrical double tongue-and-groove precast piles are pre-embedded according to the design requirements for the additional tie rods and the installation requirements for the composite plate with truss ribs. The positional error of the pre-embedded perforations is controlled within 6mm, and the positional error of the pre-embedded internal threads is controlled within 3mm.

[0024] During on-site construction, a total station was first used to accurately measure the locations of the symmetrical double-tongue precast piles arranged around the foundation pit. Once the concrete strength of the symmetrical double-tongue precast piles reached 100% of the design strength, they could be transported to the site for pile driving.

[0025] Before driving the symmetrical double tongue and groove precast piles, rubber plugs are used to seal the openings of each reserved perforation hole, and temporary short bolt heads are tightened into the internal thread to prevent the reserved holes from being blocked during the driving process of the symmetrical double tongue and groove precast piles.

[0026] The symmetrical double-tongue precast piles are driven using a technical method that involves driving them symmetrically from the center of the foundation pit to both sides, without using symmetrical double-tongue precast piles at the corners of the foundation pit, but instead using right-angle cast-in-place hidden piles.

[0027] Step S2: First layer excavation, additional tie rod perforation: Once all the symmetrical double-tongue precast piles around the foundation pit have been driven in, the excavation of the first layer of earthwork can begin.

[0028] The earthwork excavation adopts the central island excavation method, leaving sufficient working face near the foundation pit retaining structure. The soil layer between piles is cleaned manually, and the excavation depth of the first layer is strictly controlled according to the design height of the composite slab with truss ribs.

[0029] After the first layer of earthwork is excavated, the symmetrical double tongue and groove precast piles within the depth range of the first layer of earthwork are cleaned, and the rubber plugs and short bolt heads in the reserved holes and internal threads of the symmetrical double tongue and groove precast piles are pulled out to prepare for the subsequent reinforcement threading process.

[0030] To enhance the connection between the symmetrical double tongue-and-groove precast piles and the cast-in-place hidden piles on both sides, as well as the cast-in-place shear walls, after the excavation of the first floor, all contact surfaces of the symmetrical double tongue-and-groove precast piles and the cast-in-place hidden piles were roughened using a hydraulic splitter, with a roughness rate ≥80%.

[0031] Then, each additional tie rod is passed through the reserved hole inside the symmetrical double tongue and groove precast pile, ensuring that the left and right extension lengths of the additional tie rods after drilling are consistent.

[0032] Step S3: Anchor plate installation, rebar cage installation: After the additional tie bars within the first-layer excavation area are drilled, anchor plates are tightened at both ends of each additional tie bar to enhance the anchoring force of the additional tie bars after the subsequent shear wall concrete is poured, thereby improving the overall cohesive bearing capacity of the retaining structure.

[0033] During on-site construction, the steel reinforcement cage is connected to the upper and lower layers using the insertion process. This involves inserting groups of vertical steel bars at least 500mm below the bottom elevation of the first-layer excavation, and then fixing the inserted vertical steel bars with stirrups to form the steel reinforcement cage.

[0034] The overall planar structure of the steel reinforcement cage at the corner of the foundation pit is right-angled. The additional tie bars at the two nearest symmetrical double tongue-and-groove precast piles at the corner of the foundation pit extend into the right-angled steel reinforcement cage to improve the structural load-bearing capacity at the corner of the foundation pit; each of the remaining additional tie bars traverses the steel reinforcement cage.

[0035] Step S4: Install the reinforcing cage in sections and intervals, and hoist the composite slab with truss ribs: After the steel reinforcement cage is installed, the steel reinforcement cage is hoisted between the two adjacent steel reinforcement cages between the two adjacent symmetrical double tongue and groove precast piles. During the on-site construction, the vertical steel reinforcement in the steel reinforcement cage is also inserted using the insertion process. Each vertical steel reinforcement is inserted to a depth of at least 300mm below the bottom elevation of the first layer of earthwork excavation. Then, the horizontal steel reinforcement and tie bars are tied or laid out by electric welding one by one. The installation is completed in sections and intervals.

[0036] Finally, the truss-ribbed composite slab is hoisted to the side of a set of two adjacent symmetrical double tongue-and-groove precast piles facing inward into the foundation pit. High-strength bolts are then passed through the mounting holes at both ends of the truss-ribbed composite slab and tightened into the internal threads to fix the truss-ribbed composite slab. The joints between the truss-ribbed composite slabs are treated by adding sealing strips and applying waterproof membrane strips using the hot-melt method.

[0037] Step S5: Drive in anchor bolt sleeves and pour fine aggregate concrete to form cast-in-place hidden piles and cast-in-place shear walls: After all the truss-ribbed composite slabs within the first-floor excavation area have been hoisted, each anchor sleeve will be driven into the soil outside the foundation pit through the pre-drilled anchor holes in the truss-ribbed composite slabs. The anchor sleeves must not collide with the steel reinforcement cage already installed between the soil outside the foundation pit and the truss-ribbed composite slabs. The inner diameter of the anchor sleeves should be 20mm to 30mm larger than the designed anchor diameter. The cutting length should be the precast thickness of the truss-ribbed composite slab + the side length of the symmetrical double-tongue precast pile in the anchor length direction + 450mm. One anchor hole at the bottom of each section of the first floor (near the symmetrical double-tongue precast pile) will not be installed with an anchor sleeve for the time being and will be used as a subsequent grouting hole.

[0038] Finally, the fine aggregate concrete was poured using mechanical equipment and the duct method. This involved inserting a grouting duct into the lowest anchor holes reserved in each section of the first floor to perform high-pressure grouting to form cast-in-place shear walls and cast-in-place hidden piles. The strength of the fine aggregate concrete was not lower than C35. After grouting was completed, anchor sleeves were immediately installed in the anchor holes used for grouting to strengthen curing and reduce vibration.

[0039] It is particularly important to note that for the right-angled steel reinforcement cage at the corners of the foundation pit, fine aggregate concrete should be poured in advance. After it has cured to 90% of its design strength, the fine aggregate concrete grouting work in the center of the foundation pit perimeter should then be carried out. The reason for adopting the approach of "strengthening the corners of the foundation pit retaining structure first, and then improving other parts of the retaining structure" is as follows: Because the corners of the foundation pit are subjected to greater stress, deformation control at the corners is particularly important. Furthermore, during the construction of the internal underground structure after the completion of the entire foundation pit retaining structure, deformation monitoring at the corners is a key construction focus. Therefore, timely pouring of concrete at the corners also greatly helps in the stable construction of other parts of the foundation pit retaining structure.

[0040] Step S6: Install anchor bolts and excavate the lower layer of soil: After the first layer of fine aggregate concrete is poured and cured to 100% of the design strength, the anchor bolt installation work will be carried out.

[0041] First, according to the design drawings, the anchor bolts are driven one by one into the soil layer outside the foundation pit, passing through the pre-embedded anchor bolt sleeves. Full-length bonded steel anchor bolts are used, and positioning brackets are installed at the ends of the anchor bolts to ensure they are centered.

[0042] After the anchor bolts are installed, pressure grouting is immediately carried out using M30 pure cement grout. The grouting pressure is controlled at 0.5~1.0MPa until grout returns to the borehole. After the grout reaches the design strength, the anchor bolts are tensioned and locked. The tension force is controlled at 70%~80% of the design axial force, and the anchor plate is installed for locking.

[0043] After the anchor bolts are installed, the lower layer of earthwork is excavated using a central island excavation method, carried out in layers and sections, with each layer not exceeding 2.0m in depth. A working platform at least 1.0m wide is provided near the symmetrical double-tongue precast piles. Excess soil between piles is cleared manually in conjunction with small machinery. During the excavation process, collisions with the installed anchor bolts and fine aggregate concrete shear walls are strictly prohibited, and the deformation of the foundation pit is monitored in real time. After the lower layer of earthwork is excavated to the design elevation, the pile body is promptly cleared to provide a working surface for the next additional tie rod perforation and steel reinforcement cage installation.

[0044] Step S7: Repeat the above process until the entire enclosure structure construction is completed. After the lower layer of earthwork is excavated, the exposed vertical steel bars below the bottom elevation of the first layer of earthwork excavation are treated. The surface soil of each exposed steel bar and the soil and loose stones at the bottom of the fine stone concrete that has been poured in the first layer are cleaned. Then, the lower part of the vertical steel bars of each cast-in-place hidden pile and the vertical steel bars of the cast-in-place shear wall steel cage within the excavation range of the lower layer of earthwork are connected to the exposed vertical steel bars below the bottom elevation of the first layer of earthwork excavation one by one using the dowel bar process.

[0045] Repeat all the above processes until the construction of the entire retaining structure is completed. Finally, carry out the full excavation of the soil in the foundation pit. After the soil is excavated to the bottom elevation of the foundation pit, immediately pour the foundation concrete.

[0046] In summary, this invention addresses the pain points of traditional foundation pit retaining pile walls, such as insufficient shear resistance, poor synergistic stress, and seepage and quicksand, through symmetrical double tongue-and-groove precast piles, cast-in-place hidden piles, cast-in-place shear walls, additional tie bars and composite slabs with truss ribs, and integrated anchor bolt construction and supporting layered construction technology. It solves these problems from multiple dimensions, including node stress, seepage prevention structure, assembly construction, and corner reinforcement. It also has the comprehensive advantages of high assembly, high efficiency, high rigidity, and high seepage resistance, and has broad application value in complex deep foundation pit engineering in soft soil.

[0047] 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 foundation pit retaining structure based on symmetrical double tongue-and-groove precast piles, characterized in that: It includes symmetrical double tongue-and-groove precast piles, cast-in-place hidden piles, cast-in-place shear walls, and composite slabs with truss ribs; the symmetrical double tongue-and-groove precast piles are driven at intervals along the edge of the foundation pit, and symmetrical tongue-and-groove joints are opened at both ends. Each of the symmetrical double-tongue-and-groove precast piles is provided with cast-in-place hidden piles at both sides of the tongue and groove. The cast-in-place hidden piles are formed by the steel reinforcement cage after the fine stone concrete is poured. The cast-in-place shear wall is provided between the two cast-in-place hidden piles between two adjacent symmetrical double-tongue-and-groove precast piles. The cast-in-place shear wall is formed by the steel reinforcement cage after the fine stone concrete is poured. Additional tie bars are inserted inside the symmetrical double tongue-and-groove precast piles. Both ends of the additional tie bars extend out of the symmetrical double tongue-and-groove precast piles and are anchored inside the cast-in-place shear wall. The composite slab with truss ribs is fixed to the side of the adjacent two symmetrical double tongue-and-groove precast piles facing the inside of the foundation pit by high-strength bolts. Anchor rods are inserted on the slab and the anchor rods are driven into the soil layer outside the foundation pit. The symmetrical double tongue-and-groove precast pile has a reserved through hole and an internal threaded thread inside, both of which are pre-embedded during the production process of the symmetrical double tongue-and-groove precast pile PC in the factory; the reserved through hole has a diameter of 30mm and is used for the additional tie rod to pass through; the internal threaded thread is located on the side of the symmetrical double tongue-and-groove precast pile facing the inside of the foundation pit and is used to tighten the high-strength bolt; the two ends of the composite plate with truss ribs have mounting holes for the high-strength bolt to pass through.

2. The foundation pit retaining structure according to claim 1, characterized in that: The tongue and groove depth shall not be less than 60mm, the outer width of the tongue and groove shall not be less than 1 / 2 and not more than 2 / 3 of the side length of the symmetrical double tongue and groove precast pile in the same direction, and the angle between the tongue and groove slope and the horizontal plane shall be 30°~45°.

3. The foundation pit retaining structure according to claim 1, characterized in that: Both ends of the horizontal reinforcing bars in the steel cage extend into the steel reinforcement skeleton, and the cast-in-place hidden pile and the cast-in-place shear wall are cast together.

4. The foundation pit retaining structure according to claim 1 or 3, characterized in that: The vertical reinforcing bars of both the steel reinforcement frame and the steel reinforcement cage are installed using the insert reinforcement process.

5. The foundation pit retaining structure according to claim 1, characterized in that: Based on the location of the cast-in-place hidden piles, the cast-in-place hidden piles are divided into ordinary cast-in-place hidden piles and right-angle cast-in-place hidden piles. The right-angle cast-in-place hidden piles are only set at the corners of the foundation pit.

6. The foundation pit retaining structure according to claim 5, characterized in that: Both ends of the additional tie rod extend at least 500mm beyond the symmetrical double-tongue precast pile, and the end of the additional tie rod near the corner of the foundation pit is anchored inside the right-angle cast-in-place hidden pile.

7. A construction method for a foundation pit retaining structure based on symmetrical double tongue-and-groove precast piles, characterized in that, Includes the following steps: Step S1: Measurement and layout for pile positioning, production, transportation, and on-site driving of symmetrical double tongue-and-groove precast piles: Based on the comprehensive geological conditions, the excavation depth of the foundation pit and the earth pressure, the symmetrical double tongue and groove precast piles are designed and precast. At the same time, in accordance with the design requirements for the additional tie rods and the installation requirements for the composite slab with truss ribs, the reserved perforations and internal threads inside the symmetrical double tongue and groove precast piles are pre-embedded. First, a total station was used to accurately measure the locations of the symmetrical double tongue-and-groove precast piles arranged around the foundation pit. After the concrete strength reached 100% of the design strength, it was transported to the construction site. Rubber plugs were used to seal the openings of each pre-reserved hole, and temporary short bolt heads were tightened into the internal threaded sleeve to prevent the pre-reserved holes from being blocked during the driving process of the symmetrical double tongue and groove precast piles. When carrying out pile driving operations, symmetrical double-tongue precast piles are symmetrically driven from the middle of the foundation pit to both sides, and symmetrical double-tongue precast piles are not installed at the corners of the foundation pit. Step S2: First layer excavation, additional tie rod perforation: After all the symmetrical double-tongue precast piles around the foundation pit have been driven, the first layer of earthwork will be excavated using the central island excavation method. Sufficient working face will be left near the foundation pit retaining structure. The soil layer between the piles will be cleaned manually. The excavation depth of the first layer of earthwork will be strictly controlled according to the design height of the composite slab with truss ribs. After the first layer of earthwork is excavated, the symmetrical double tongue and groove precast piles within the depth range of the first layer of earthwork are cleaned, and the rubber plugs and short bolt heads in the reserved holes and internal threaded threads of the symmetrical double tongue and groove precast piles are pulled out. All contact surfaces of the symmetrical double tongue-and-groove precast piles and cast-in-place underground piles are roughened using a hydraulic splitter, with a roughness rate ≥80%. The additional tie bars are passed through the reserved holes inside the symmetrical double tongue and groove precast piles one by one, ensuring that the left and right extension lengths of the additional tie bars after drilling are consistent. Step S3: Anchor plate installation, rebar cage installation: After the additional tie bars within the first-layer excavation area are drilled, anchor plates are tightened at both ends of each additional tie bar to achieve 100% quantity. The steel reinforcement cage is connected to the upper and lower layers using the insertion process, that is, the group of vertical steel bars is inserted to a depth of at least 500mm below the bottom elevation of the first-layer excavation, and then the inserted vertical steel bars are fixed with stirrups to form the steel reinforcement cage. The overall planar structure of the steel reinforcement cage at the corner of the foundation pit is right-angled. The additional tie bars at the two nearest symmetrical double tongue-and-groove precast piles at the corner of the foundation pit extend into the right-angled steel reinforcement cage, and each of the remaining additional tie bars crosses the steel reinforcement cage. Step S4: Install the reinforcing cage in sections and intervals, and hoist the composite slab with truss ribs: After the steel reinforcement cage is installed, the steel reinforcement cage is hoisted between the two adjacent steel reinforcement cages between the two adjacent symmetrical double tongue and groove precast piles. The steel reinforcement cage also adopts the insertion process, that is, the group of vertical steel bars are inserted to a depth of at least 300mm below the bottom elevation of the first layer of earthwork excavation. Then, the horizontal steel bars and tie bars are tied one by one or laid out by electric welding. The installation is completed in sections and intervals. Finally, the truss-ribbed composite slab is hoisted to the side of a set of two adjacent symmetrical double tongue-and-groove precast piles facing the inside of the foundation pit. High-strength bolts are then passed through the mounting holes at both ends of the truss-ribbed composite slab and tightened into the internal threaded sleeve to fix the truss-ribbed composite slab. The joints between each truss-ribbed composite slab are treated by adding sealing strips and applying waterproof membrane strips by hot-melt method. Step S5: Drive in anchor bolt sleeves and pour fine aggregate concrete to form cast-in-place hidden piles and cast-in-place shear walls: After all the truss-ribbed composite slabs within the first-floor excavation area are hoisted, each anchor sleeve is driven into the soil outside the foundation pit through the pre-reserved anchor holes in the truss-ribbed composite slab. The anchor sleeves must not collide with the steel cage already installed between the soil outside the foundation pit and the truss-ribbed composite slab. The inner diameter of the anchor sleeve is 20mm~30mm larger than the designed anchor diameter. The cutting length is the prefabricated thickness of the truss-ribbed composite slab + the side length of the symmetrical double tongue-and-groove prefabricated pile in the anchor length direction + 450mm. The lowest anchor hole in each section of the first floor is reserved and no anchor sleeve is installed for the time being. Finally, using mechanical equipment, high-pressure grouting is carried out by inserting grouting pipes into the lowest anchor holes reserved in each section of the first floor to form cast-in-place shear walls and cast-in-place hidden piles. The strength of fine stone concrete is not less than C35. After grouting is completed, anchor sleeves are immediately installed in the anchor holes used for grouting, and curing is strengthened to reduce vibration. Fine aggregate concrete was poured into the right-angled steel reinforcement cage at the corner of the foundation pit in advance, and after it was cured to 90% of the design strength, fine aggregate concrete grouting was carried out in the middle of the perimeter of the foundation pit. Step S6: Install anchor bolts and excavate the lower layer of soil: After the first layer of fine stone concrete is poured and cured to 100% of the design strength, the anchor bolt installation work will be carried out. First, the anchor rods are driven one by one into the soil layer outside the foundation pit through the pre-embedded anchor rod sleeves; the anchor rods are full-length bonded steel anchor rods, and positioning brackets are set at the ends of the anchor rods to ensure that the anchor rods are centered. After the anchor bolts are installed, pressure grouting is immediately carried out. M30 pure cement grout is used as the grouting material, and the grouting pressure is controlled at 0.5~1.0MPa until grout returns to the borehole. After the grout reaches the design strength, the anchor bolts are tensioned and locked. The tension force is controlled at 70%~80% of the design axial force, and the anchor plate is installed and locked. After the anchor bolts are installed, the next layer of earthwork is excavated. The excavation method is a central island excavation, carried out in layers and sections, with each layer not exceeding 2.0m in depth. A working platform of not less than 1.0m wide is left on the side near the symmetrical double-tongue precast piles. Excess soil between piles is cleaned manually with the help of small machinery. During the excavation process, collisions with the installed anchor bolts and fine stone concrete shear walls are strictly prohibited, and the deformation of the foundation pit is monitored in real time. After the next layer of earthwork is excavated to the design elevation, the pile body is cleaned in time to provide a working surface for the next additional tie rod perforation and steel reinforcement cage installation. Step S7: Repeat the above process until the entire enclosure structure construction is completed. After the lower layer of earthwork is excavated, the exposed vertical steel bars below the bottom elevation of the first layer of earthwork excavation are treated. The surface soil of each exposed steel bar and the soil and loose stones at the bottom of the fine stone concrete that has been poured in the first layer are cleaned. Then, the lower part of the vertical steel bars of each cast-in-place hidden pile and the vertical steel bars of the cast-in-place shear wall steel cage within the excavation range of the lower layer of earthwork are also connected to the exposed vertical steel bars below the bottom elevation of the first layer of earthwork excavation using the same dowel bar process. Repeat all the above processes until the construction of the entire retaining structure is completed. Finally, carry out the full excavation of the soil in the foundation pit. After the soil is excavated to the bottom elevation of the foundation pit, immediately pour the foundation concrete.

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