Novel foundation pit support system construction method

Abstract

The invention discloses a novel foundation pit support system construction method which is applied to the field of foundation pit excavation and support of traffic civil engineering. According to the method, Larsen IV type steel sheet piles, reinforced concrete structure rigid frame beams and phi 630 * 8 steel pipe piles are adopted to form an enclosure system, and the enclosure system is jointly stressed through connection of the rigid frame beams and anchor pulling steel bars. The construction method comprises the construction steps of inserting and driving the steel sheet piles and the steel pipe piles, constructing reinforced concrete of the top beams and the rigid frame beams and constructing anchor pulling steel bars. The method solves the problems that traditional construction is limited by factors such as sites and traffic, the construction space in the foundation pit is limited, and the construction period is long, the advantages of all components are played, excavation safety and stability are improved, use of supports in the foundation pit is reduced, construction efficiency is improved, the construction period is shortened, and remarkable economic benefits are achieved.

Description

Technical Field

[0001] The invention relates to the technical field of traffic civil engineering, in particular to a novel foundation pit retaining system construction method. Background Art

[0002] In the field of transportation civil engineering, foundation pit excavation and support are key pre-construction links before the construction of excavated underground structures. With the continuous advancement of urban construction, construction sites often face many complex restrictions.

[0003] On the one hand, the existence of surrounding projects makes the construction site space extremely limited, and the parking of construction equipment and the stacking of materials are greatly restricted. On the other hand, the task of traffic diversion is arduous. When constructing in busy areas of the city or near major traffic arteries, it is necessary to ensure that the construction process does not affect the normal operation of surrounding traffic, which puts extremely high demands on the construction methods and construction schedule of the foundation pit retaining system. In addition, river protection is also an important factor that cannot be ignored. When constructing in areas near rivers and streams, it is necessary to prevent the construction from causing damage to the river water body, embankment structure, etc., and avoid causing environmental problems such as soil erosion and water pollution.

[0004] Traditional methods of foundation pit retaining system construction, such as setting up a bracing structure in the foundation pit, have obvious drawbacks. In terms of space utilization, the bracing structure occupies a large amount of space inside the foundation pit, making the construction operation space of the main structure of the underground structure extremely narrow, making it difficult for construction machinery to operate, and limiting the range of activities of construction personnel, which seriously affects the convenience of construction. In terms of construction period, the installation and removal of the bracing structure are cumbersome, which increases the construction links and construction time, resulting in an extension of the overall construction period, which in turn increases the construction cost, including labor costs and equipment rental costs.

[0005] Therefore, there is an urgent need for a new foundation pit retaining system construction method that can effectively cope with complex conditions such as limited site conditions, difficult traffic diversion, and river protection requirements, so as to solve the shortcomings of traditional construction methods in terms of space and construction period, improve construction efficiency, and reduce construction costs. Summary of the invention

[0006] The purpose of the present invention is to provide a new type of foundation pit retaining system construction method, which can effectively deal with the complex and difficult problems faced by foundation pit excavation and support construction in transportation civil engineering projects, and comprehensively improve the safety, convenience and economy of construction.

[0007] In order to achieve the above object, the present invention is realized by the following technical scheme: a new foundation pit retaining system construction method, using a new retaining system composed of Larsen IV steel sheet piles, reinforced concrete structure rigid frame beams, and φ630×8 steel pipe piles, and connecting the steel sheet pile retaining structure and the steel pipe piles through the rigid frame beams and anchor steel bars to be a whole and bear force together, and the specific construction steps are as follows:

[0008] Step S1, driving steel sheet piles and steel pipe piles: two-point lifting and unloading are adopted, and a vibrating hammer is used for construction by using an excavator, and a pile driving purlin bracket consisting of a purlin and purlin piles is set up, and the position, direction and verticality are controlled during the driving of the piles;

[0009] Step S2, reinforced concrete construction of crown beam and rigid frame beam: excavate the soil layer to 100mm above the bottom of the crown beam and manually level it, cast the bottom formwork of the crown beam with 50mm thick C20 concrete, backfill after the soft base section is treated, make and install the bottom formwork and side formwork, and carry out reinforcement construction and concrete pouring;

[0010] Step S3, anchor steel bar construction: completed before the concrete pouring of the crown beam and the rigid frame beam. When the φ25 anchor steel bar is connected to the crown beam, it is anchored into the steel sheet pile through a hole and bent with a length of 43d. When connected to the steel pipe pile, it is wrapped around once and welded to the steel pipe at the rear end 10d. The anchor steel bar in the rigid frame beam is protected by a φ50PVC casing and cement slurry injected into the pipe.

[0011] As a further improvement of the technical solution of the present invention, the length of the Larsen IV steel sheet pile is 15m.

[0012] As a further improvement of the technical solution of the present invention, the size of the reinforced concrete structure rigid frame beam is 800×800mm. As a further improvement of the technical solution of the present invention,

[0013] As a further improvement of the technical solution of the present invention, the length of the φ630×8 steel pipe pile is 12m.

[0014] As a further improvement of the technical solution of the present invention, during the construction of the crown beam bottom formwork, the soft base section is mechanically excavated to remove the 500 mm thick soil layer and then backfilled with stone slag or brick slag, and the foundation treatment width is widened by 100 mm on both sides according to the designed size.

[0015] As a further improvement of the technical solution of the present invention, the bottom formwork of the crown beam and the rigid frame beam is made of pressed plywood, and the bottom formwork is fixed by hammering the threaded steel bar into the soil layer close to the 20mm thick surface of the template edge, and the side formwork is roughly fixed by cross-tie rods and slightly tightened by external tension wire ropes.

[0016] As a further improvement of the technical solution of the present invention, during the construction of the crown beam reinforcement, when the stirrups are disconnected by the steel sheet piles, open stirrups are welded to the steel sheet piles, and the welding length is 10d.

[0017] As a further improvement of the technical solution of the present invention, the crown beam concrete is poured using a direct current method, using a movable small chute to enter the mold, and is poured and vibrated in layers. It is strictly forbidden to touch the formwork and steel frame on both sides during vibration.

[0018] As a further improvement of the technical solution of the present invention, a real-time monitoring system for the deformation, internal force and surrounding environmental changes of the foundation pit retaining structure is established during the construction process. The monitoring contents include the horizontal displacement, vertical displacement of steel sheet piles and steel pipe piles, internal force changes of crown beams and rigid frame beams, and settlement and inclination of surrounding buildings.

[0019] As a further improvement of the technical solution of the present invention, the distance between the steel sheet piles and the steel pipe piles is 11m at the unearthed location; and the distance between the steel sheet piles and the steel pipe piles is 9m at the non-unearthed location.

[0020] It should be noted that the new foundation pit retaining system adopted by the present invention is composed of Larsen IV steel sheet piles (15m long), reinforced concrete structure rigid frame beams (size 800×800mm), and φ630×8 steel pipe piles (12m long). With the help of rigid frame beams and anchor steel bars, the steel sheet pile retaining structure is firmly connected to the steel pipe piles to form an overall force system, giving full play to the advantages of each component and synergistically resisting the soil pressure on the foundation pit side.

[0021] Construction steps:

[0022] Precise driving of steel sheet piles and steel pipe piles: Use the two-point lifting method to safely load and unload steel sheet piles to the designated location, and carefully protect the lock mouth during the whole process to avoid damage. Use an excavator equipped with a vibrating hammer as a construction machine. In order to ensure the verticality of the steel sheet pile sinking and the flatness of the wall surface of the driven board wall, a pile driving purlin bracket consisting of purlins and purlin piles is specially set up. When driving the piles, strictly align the lock mouth, put on the pile cap immediately after each steel sheet pile is inserted, and perform a light hammering operation. During the piling process, the position, direction and verticality of the steel sheet piles are strictly controlled in real time. The driving requirements of steel pipe piles are consistent with those of steel sheet piles to ensure the uniformity and stability of the construction quality of the two.

[0023] Fine construction of reinforced concrete for crown beam and rigid frame beam: When the first soil layer is excavated to 100mm above the bottom of the crown beam, manual excavation is used for leveling to ensure the flatness and accuracy of the foundation surface. The bottom formwork of the crown beam is cast-in-place with 50mm thick C20 concrete. For the soft base section, mechanical excavation is first used to remove the 500mm thick soil layer, and then stone slag or brick slag is used for backfilling. The foundation treatment width is implemented according to the standard of widening 100mm on both sides of the design size. After the foundation of the bottom surface of the beam body is processed as required, the production and installation of the bottom formwork and side formwork are carried out in an orderly manner. Both the bottom formwork and the side formwork are made of pressed plywood. The bottom formwork is fixed by directly hammering the threaded steel bar into the soil layer by pressing it against the 20mm thick surface of the template edge line; the side formwork uses a cross-pull rod to roughly pull the template to fix it, and combines the external pull wire rope to slightly tighten the pull to further ensure the stability of the side formwork. During the construction of the crown beam reinforcement, the stirrups were disconnected due to the presence of steel sheet piles. Open stirrups were used and firmly welded to the steel sheet piles with a welding length of 10d. 10 stirrups were selected from the end (tail) of the main reinforcement and spot welded to the main reinforcement into a fixed section according to the design spacing requirements to initially form a simple reinforcement skeleton. Subsequently, all stirrups were tied or spot welded to the main reinforcement in batches. The crown beam concrete was poured using the direct current method. In view of the crown beam height of 0.6m, it was planned to use a movable small chute to enter the mold, and implement layered pouring and vibration operations. During the vibration process, it is strictly forbidden for the vibration equipment to touch the formwork and steel skeleton on both sides. The operating principle of fast insertion and slow lifting should be followed. When there are no bubbles on the concrete surface, the vibration can be stopped to ensure the quality of concrete pouring. The reinforced concrete construction and precautions of the rigid frame beam are the same as those of the crown beam to ensure the consistency and reliability of the entire enclosure system structure.

[0024] Reliable construction of anchor steel bars: The construction of anchor steel bars must be completed before the concrete pouring of the crown beam and the rigid frame beam. When connecting the φ25 anchor steel bars to the crown beam, the steel sheet pile is perforated and anchored into the crown beam, and the bending and anchoring length is strictly implemented according to 43d. When connecting the φ25 anchor steel bars to the steel pipe pile, the steel bar is wrapped and firmly fixed on the steel pipe pile. After wrapping the steel pipe pile once, the end is welded to the steel pipe with a length of 10d. For the anchor steel bars in the rigid frame beam, φ50PVC casing is used for protection, and the pipe is filled with cement slurry to effectively enhance the durability and stability of the anchor steel bars.

[0025] The present invention has the following beneficial effects:

[0026] Innovative support system, improved stability: An innovative support system of rigid frame beams combined with steel pipe piles is used outside the foundation pit to replace the traditional pit support structure, which completely changes the traditional support mode that only relies on a single support structure to withstand the soil pressure on the side of the foundation pit. Through this combined system, the force characteristics of the new retaining system are fully utilized to achieve the extrusion and solidification of the soil between the piles, successfully solving the problems of limited construction area and the inapplicability of traditional construction methods, significantly improving the safety and stability during the foundation pit excavation process, and providing a solid and reliable foundation guarantee for subsequent construction.

[0027] Optimize space utilization and improve construction efficiency: The application of the new combined enclosure system has greatly reduced the use of bracing in the foundation pit. This optimization measure has created spacious and convenient space conditions for the subsequent main structure construction of underground structures, and effectively solved the problem of limited space in the foundation pit for the main structure construction. On the premise of ensuring the structural quality of underground structures, construction personnel and equipment can carry out operations more efficiently, thereby effectively improving construction efficiency and significantly shortening the construction period. At the same time, reducing the use of bracing structures also reduces the investment cost of materials and equipment, which has good economic benefits.

[0028] Taking advantage of components and enhancing safety performance: The new retaining system gives full play to the good water-proofing property of Larsen steel sheet piles and the good bending resistance of steel pipe piles. The Larsen steel sheet piles and steel pipe piles are tightly connected through reinforced concrete structure rigid frame beams to form a double-row pile retaining structure. The rigid connection of the rigid frame beam provides a strong lateral tension for the steel sheet piles, effectively making up for the shortcomings of relatively small lateral stiffness and large deformation of the steel sheet piles. This innovative design changes the situation where the existing technology can only rely on the setting of steel supports to ensure the stability of the foundation pit, so that the new retaining structure can give full play to the advantages of each part and effectively improve the overall safety performance of the retaining structure. Since the overall system has sufficient lateral tension to resist soil pressure, no support structure is required inside the foundation pit during construction, which not only meets the needs of the construction work surface, but also makes the setting of the excavation channel more flexible and convenient, further improves the construction efficiency, and achieves a significant improvement in comprehensive economic benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Other features, objects and advantages of the present invention will become more apparent from the detailed description of non-limiting embodiments made with reference to the following drawings:

[0030] Figure 1 It is the cross-section diagram of the enclosure system;

[0031] Figure 2 This is a schematic diagram of the construction of the steel sheet pile retaining structure;

[0032] Figure 3 This is a schematic diagram of steel pipe pile construction;

[0033] Figure 4 Schematic diagram of connecting steel sheet piles and steel pipe piles for rigid frame beams;

[0034] Figure 5 This is a schematic diagram of the connection between the anchor reinforcement and the crown beam;

[0035] Figure 6 This is a schematic diagram of the connection between anchor steel bars and steel pipe piles. DETAILED DESCRIPTION

[0036] The present invention will be described in detail below in conjunction with the accompanying drawings and specific embodiments. The exemplary embodiments and descriptions of the present invention are used to explain the present invention, but are not intended to limit the present invention.

[0037] It should be noted that all directional indications in the embodiments of the present invention (such as up, down, left, right, front, back, upper end, lower end, top, bottom...) are only used to explain the relative position relationship, movement status, etc. between the components in a certain specific posture (as shown in the accompanying drawings). If the specific posture changes, the directional indication will also change accordingly.

[0038] In the present invention, unless otherwise clearly defined and limited, the term "connection" should be understood in a broad sense, for example, "connection" can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements, unless otherwise clearly defined. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0039] In addition, in the present invention, the descriptions of "first", "second", etc. are only used for descriptive purposes and cannot be understood as indicating or implying their relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include at least one of the features; in addition, the technical solutions between the various embodiments can be combined with each other, but they must be based on the ability of ordinary technicians in the field to implement them. When the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such a combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0040] The present invention is further described in detail below in conjunction with the accompanying drawings.

[0041] Reference Figure 1-Figure 6 A new construction method for foundation pit retaining system: a new retaining system consisting of Larsen IV steel sheet piles, reinforced concrete structure rigid frame beams, and φ630×8 steel pipe piles is adopted. The steel sheet pile retaining structure and the steel pipe piles are connected through rigid frame beams and anchor steel bars to bear the force as a whole.

[0042] The construction method of the new foundation pit retaining system is now introduced in detail. The specific steps are:

[0043] (a) Driving of steel sheet piles and steel pipe piles. Use a two-point lifting method to load and unload steel sheet piles to the specified position, and pay attention to protecting the locking mouth from damage. The construction machinery uses an excavator equipped with a vibrating hammer. In order to ensure the verticality of the steel sheet pile sinking and the flatness of the wall surface, a pile driving purlin bracket should be set when the steel sheet pile is driven in. The purlin bracket consists of a purlin and purlin piles. Use an excavator to lift the steel sheet pile to the insertion point for pile driving. When inserting the pile, the locking mouth must be aligned. After each piece is inserted, put on the pile cap and hammer it lightly. The position, direction and verticality of the steel sheet pile must be controlled during the piling process. The driving and precautions for steel pipe piles are the same as those for steel sheet piles. Figure 1 , Figure 3 shown.

[0044] (b) Construction of reinforced concrete for crown beam and rigid frame beam. When the first soil layer is excavated to 100mm above the bottom of the crown beam, manual excavation and leveling are adopted. The bottom formwork of the crown beam is cast in 50mm thick C20 concrete. After mechanical excavation and removal of 500.0mm thickness in the soft base section, stone slag or brick slag is used for backfilling. The foundation treatment width is widened by 100.0mm on both sides according to the design size. After the bottom surface foundation of the beam body is treated as required, the bottom formwork and side formwork are made and installed. The bottom formwork and side formwork are both made of pressed plywood. The bottom formwork is made of threaded steel and hammered directly into the soil layer at the 20.0mm thick surface of the template edge. The side formwork is roughly pulled and fixed by a cross-drawn tie rod, and the side formwork is slightly tightened and fixed by an external pull wire rope. The crown beam reinforcement includes main reinforcement and stirrups. The stirrups are disconnected by steel sheet piles and welded to the steel sheet piles with open hoops. The welding length is 10d. At the end (tail) of the main reinforcement, some stirrups (take 10) are spot welded to the main reinforcement to form a fixed section according to the design spacing requirements to form a preliminary simple reinforcement skeleton. Then all the stirrups are tied or spot welded to the main reinforcement in batches. The crown beam adopts the direct current method. Considering the crown beam height of 0.6m, it is planned to adopt a movable small slide into the mold, layered pouring and vibration. It is strictly forbidden to touch the formwork and steel skeleton on both sides during concrete vibration. Vibration requires fast insertion and slow lifting, and vibration should be stopped when there are no bubbles on the concrete surface. The construction and precautions of reinforced concrete for rigid frame beams are the same as those for crown beams. Figure 2 , Figure 4 shown.

[0045] (c) Anchor steel bar construction. The anchor steel bar construction is completed before the concrete pouring of the crown beam and the rigid frame beam. The φ25 anchor steel bar is connected to the crown beam by piercing holes in the steel sheet pile and anchoring it into the crown beam. The bending and anchoring length is 43d. The φ25 anchor steel bar is connected to the steel pipe pile and fixed to the steel pipe pile by winding the steel bar. After winding the steel pipe pile once, the end is welded to the steel pipe with 10d. The anchor steel bar in the rigid frame beam is protected by φ50PVC casing and cement slurry injected into the pipe. Figure 5 , Figure 6 shown.

[0046] It should be noted that before the construction of excavated underground structures, foundation pit excavation and support construction must be carried out first. The form of the foundation pit retaining structure is generally set according to the requirements of the underground structure. The retaining structure is mainly used to withstand the soil pressure on the side of the foundation pit after excavation, and at the same time has a certain water-stopping performance. The cross-sectional diagram of the retaining system is shown in the figure. Figure 1 .

[0047] Since the construction site is affected by surrounding projects, traffic diversion, and river protection, in order to ensure the smooth construction of underground structures and provide spatial feasibility, a new type of enclosure system steel sheet piles + reinforced concrete structure rigid frame beams + steel pipe piles are used. The steel sheet piles are Larsen IV type (15m), the rigid frame beams are reinforced concrete structures (size 800x800mm), and φ630*8 steel pipe piles (12m). Rigid frame beams and anchor steel bars are used to connect the steel sheet pile enclosure structure and the steel pipe piles to bear the force as a whole. The construction of the steel sheet pile enclosure structure includes the insertion of steel sheet piles and the construction of the crown beam reinforced concrete structure (size 600x600mm). Figure 2 , steel pipe pile construction Figure 3 The steel frame beam adopts reinforced concrete structure, connecting steel sheet piles and steel pipe piles respectively. The distance between steel sheet piles and steel pipe piles is 11m (excavated part) and 9m (non-excavated part). Figure 4 φ25 anchor steel bar is connected to the crown beam by drilling holes in the steel sheet pile and anchoring it into the crown beam. The bending anchor length is 43d. Figure 5 The φ25 anchor steel bar is connected to the steel pipe pile and fixed on the steel pipe pile by winding the steel bar. After winding the steel pipe pile once, the rear end is welded to the steel pipe with 10d. The anchor steel bar in the rigid frame beam is protected by φ50PVC casing and cement slurry is injected into the pipe. Figure 6 .

[0048] In order to solve the problems that the construction of the existing foundation pit retaining system is limited by factors such as site and traffic diversion, and the use of traditional construction methods (setting support in the foundation pit) leads to inconvenient construction of underground structures in the foundation pit and increased construction period, the present invention innovatively proposes to use a new retaining system of steel sheet piles + reinforced concrete structure rigid frame beam + steel pipe piles. First, a support system of rigid frame beams combined with steel pipe piles is used outside the foundation pit to replace the support structure in the pit, which changes the traditional support mode of using a single support structure to bear the soil pressure on the side of the foundation pit. The combined system is used to give full play to the force characteristics of the new retaining system, realize the extrusion and solidification of the soil between the piles, solve the problem of limited construction area and inapplicability of traditional construction methods, and improve the safety and stability of excavation. Secondly, the use of a new combined retaining system reduces the use of support in the foundation pit, provides convenience for the subsequent main structure construction of underground structures, solves the problem of limited space in the foundation pit for the main structure construction, and effectively improves the construction efficiency, shortens the construction period and has certain economic benefits on the premise of ensuring the structural quality of the underground structure.

[0049] 3. Beneficial Effects

[0050] Innovative support system, improved stability: An innovative support system of rigid frame beams combined with steel pipe piles is used outside the foundation pit to replace the traditional pit support structure, which completely changes the traditional support mode that only relies on a single support structure to withstand the soil pressure on the side of the foundation pit. Through this combined system, the force characteristics of the new retaining system are fully utilized to achieve the extrusion and solidification of the soil between the piles, successfully solving the problems of limited construction area and the inapplicability of traditional construction methods, significantly improving the safety and stability during the foundation pit excavation process, and providing a solid and reliable foundation guarantee for subsequent construction.

[0051] Optimize space utilization and improve construction efficiency: The application of the new combined enclosure system has greatly reduced the use of bracing in the foundation pit. This optimization measure has created spacious and convenient space conditions for the subsequent main structure construction of underground structures, and effectively solved the problem of limited space in the foundation pit for the main structure construction. On the premise of ensuring the structural quality of underground structures, construction personnel and equipment can carry out operations more efficiently, thereby effectively improving construction efficiency and significantly shortening the construction period. At the same time, reducing the use of bracing structures also reduces the investment cost of materials and equipment, which has good economic benefits.

[0052] Taking advantage of components and enhancing safety performance: The new retaining system gives full play to the good water-proofing property of Larsen steel sheet piles and the good bending resistance of steel pipe piles. The Larsen steel sheet piles and steel pipe piles are tightly connected through reinforced concrete structure rigid frame beams to form a double-row pile retaining structure. The rigid connection of the rigid frame beam provides a strong lateral tension for the steel sheet piles, effectively making up for the shortcomings of relatively small lateral stiffness and large deformation of the steel sheet piles. This innovative design changes the situation where the existing technology can only rely on the setting of steel supports to ensure the stability of the foundation pit, so that the new retaining structure can give full play to the advantages of each part and effectively improve the overall safety performance of the retaining structure. Since the overall system has sufficient lateral tension to resist soil pressure, no support structure is required inside the foundation pit during construction, which not only meets the needs of the construction work surface, but also makes the setting of the excavation channel more flexible and convenient, further improves the construction efficiency, and achieves a significant improvement in comprehensive economic benefits.

[0053] Implementation Cases:

[0054] Project Overview

[0055] Assume that a certain city underground transportation hub project has a foundation pit excavation depth of 8m, the site is adjacent to existing buildings and traffic arteries, the construction space is narrow and the traffic diversion requirements are extremely high. At the same time, one side of the site is close to a river, and the river ecological environment needs to be strictly protected. In view of this complex working condition, it is decided to adopt the new foundation pit retaining system construction method of the present invention.

[0056] Construction preparation

[0057] Material preparation: In accordance with the design requirements, purchase qualified Larsen IV steel sheet piles (15m long), φ630×8 steel pipe piles (12m long), steel bars (including various specifications of steel bars for cap beams, rigid frame beams and anchor bars), cement, sand and gravel and other materials, ensure that the material quality certification documents are complete, and conduct sampling inspection as required.

[0058] Preparation of mechanical equipment: Equipped with excavators, vibratory hammers, steel bar processing equipment (such as bending machines, cutting machines, etc.), concrete pouring equipment (such as concrete delivery pumps, vibrators, etc.), and measuring instruments (such as total stations, levels, etc.) with good performance, and debug and maintain the equipment to ensure its normal operation during the construction process.

[0059] Site preparation: remove obstacles in the construction site, level the site, set up a temporary drainage system, and ensure smooth drainage of the site. Determine the locations for inserting and driving steel sheet piles and steel pipe piles based on the survey and layout results, and set up obvious signs.

[0060] Construction process:

[0061] Steel sheet pile and steel pipe pile driving

[0062] A two-point lifting method is used to smoothly load and unload Larsen IV type steel sheet piles to the designated location. During the loading and unloading process, a special person is arranged to protect the locking mouth of the steel sheet pile to prevent deformation or damage.

[0063] Use an excavator equipped with a vibrating hammer as a piling device, and drive the steel sheet piles according to the pre-set pile driving purlin bracket (composed of purlins and purlin piles). When driving the piles, accurately align the lock, and immediately put on the pile cap after each steel sheet pile is inserted, and use a light hammer to initially drive it into the soil. During the piling process, use a total station and theodolite to monitor the position, direction and verticality of the steel sheet piles in real time to ensure that their deviations are controlled within the design allowable range. If it is found that the verticality deviation exceeds the specification requirements, it should be corrected in time by adjusting the pile frame or pulling out the pile and re-driving it.

[0064] The driving method of steel pipe piles is the same as that of steel sheet piles, and the driving quality is also strictly controlled to ensure that the steel pipe piles and steel sheet piles are in the same plane and the depth of penetration into the soil meets the design requirements.

[0065] Reinforced concrete construction of crown beam and rigid frame beam

[0066] When the first layer of earthwork is excavated to 100mm above the bottom of the crown beam, stop mechanical excavation and use manual excavation to level the surface to ensure that the surface of the crown beam foundation is flat and solid.

[0067] For the construction of the crown beam bottom formwork, in the soft base section, first use mechanical excavation to remove the 500mm thick soft soil layer, then use stone ballast or brick ballast for layered backfilling and compaction, and the backfill width is widened by 100mm on both sides according to the design size. After the foundation treatment is completed, pour 50mm thick C20 concrete as the crown beam bottom formwork to ensure that the flatness and strength of the bottom formwork meet the requirements.

[0068] The bottom formwork and side formwork are both made of pressed plywood. The bottom formwork is fixed by hammering a threaded steel bar of appropriate diameter against the 20mm thick surface of the template edge to ensure that the bottom formwork is stable and does not move. When installing the side formwork, the template is roughly pulled and fixed by a cross-drawn tie rod, and then slightly tightened by an external pull wire rope to further adjust the verticality and stability of the side formwork to ensure that the side formwork is firm and reliable.

[0069] In the construction of the crown beam reinforcement, since the stirrups are disconnected by the steel sheet piles, open stirrups are used and firmly welded to the steel sheet piles with a welding length of 10d. 10 stirrups are selected from the end (tail) of the main reinforcement and spot welded with the main reinforcement to form a fixed section according to the design spacing requirements to form a preliminary simple reinforcement skeleton. Then, the remaining stirrups are tied or spot welded with the main reinforcement in batches to ensure the integrity and stability of the reinforcement skeleton.

[0070] The crown beam concrete pouring adopts the direct current method. Considering the crown beam height of 0.6m, a movable small chute is used to enter the mold for layered pouring, and the pouring thickness of each layer is controlled at about 300mm. During the concrete vibration process, an inserted vibrator is used, following the principle of fast insertion and slow lifting. It is strictly forbidden for the vibrator to touch the formwork and steel skeleton on both sides. When the concrete surface no longer sinks, no bubbles are generated, and the surface is slurry, stop vibrating to ensure the quality of concrete pouring.

[0071] The reinforced concrete construction process of the rigid frame beam is similar to that of the crown beam. The steel bar binding, formwork installation and concrete pouring are carried out strictly in accordance with the design requirements and construction specifications to ensure that the strength and size of the rigid frame beam meet the design requirements.

[0072] Anchor steel bar construction

[0073] Before pouring concrete for the crown beam and the rigid frame beam, the construction of the anchor steel bar is completed. When connecting the φ25 anchor steel bar to the crown beam, holes are accurately punched on the steel sheet piles according to the designed position, and then the anchor steel bar is anchored into the crown beam. The bending and anchoring length is strictly implemented according to 43d to ensure that the connection between the anchor steel bar and the crown beam is firm and reliable.

[0074] When the φ25 anchor steel bar is connected to the steel pipe pile, it is tightly fixed on the steel pipe pile by winding the steel bar. After winding the steel pipe pile once, the end is welded to the steel pipe with a length of 10d to ensure effective force transmission between the anchor steel bar and the steel pipe pile.

[0075] For the anchor steel bars in the rigid frame beam, φ50 PVC casing is used for wrapping and protection, and the pipe is filled with cement slurry to prevent the anchor steel bars from corrosion and enhance their durability.

[0076] Construction monitoring

[0077] During the entire construction process, a complete construction monitoring system is established to monitor the deformation, internal force and changes in the surrounding environment of the foundation pit retaining structure in real time. The monitoring content includes the horizontal displacement and vertical displacement of steel sheet piles and steel pipe piles, the internal force changes of the crown beam and rigid frame beam, the settlement and inclination of surrounding buildings, etc. The construction parameters are adjusted in time according to the monitoring data to ensure that the construction process is safe and controllable.

[0078] Through the above specific implementation process, the new foundation pit retaining system construction method of the present invention has been successfully applied in the city's underground transportation hub project, effectively solving problems such as limited site, traffic diversion difficulties and river protection, while improving construction efficiency, ensuring project quality, and achieving good economic and social benefits.

[0079] The technical solutions provided by the embodiments of the present invention are introduced in detail above. Specific examples are used herein to illustrate the principles and implementation methods of the embodiments of the present invention. The description of the above embodiments is only applicable to help understand the principles of the embodiments of the present invention. At the same time, for those skilled in the art, according to the embodiments of the present invention, there will be changes in the specific implementation methods and application scopes. In summary, the content of this specification should not be understood as limiting the present invention.

Claims

1. A new construction method for foundation pit retaining system, characterized in that: A new type of enclosure system consisting of Larsen IV steel sheet piles, reinforced concrete structure rigid frame beams, and φ630×8 steel pipe piles is used. The steel sheet pile enclosure structure and steel pipe piles are connected as a whole through rigid frame beams and anchor steel bars. The specific construction steps are as follows: Step S1, driving steel sheet piles and steel pipe piles: two-point lifting and unloading are adopted, and a vibrating hammer is used for construction by using an excavator, and a pile driving purlin bracket consisting of a purlin and purlin piles is set up, and the position, direction and verticality are controlled during the driving of the piles; Step S2, reinforced concrete construction of crown beam and rigid frame beam: excavate the soil layer to 100mm above the bottom of the crown beam and manually level it, cast the bottom formwork of the crown beam with 50mm thick C20 concrete, backfill after the soft base section is treated, make and install the bottom formwork and side formwork, and carry out reinforcement construction and concrete pouring; Step S3, anchor steel bar construction: completed before the concrete pouring of the crown beam and the rigid frame beam. When the φ25 anchor steel bar is connected to the crown beam, it is anchored into the steel sheet pile through a hole and bent with a length of 43d. When connected to the steel pipe pile, it is wrapped around once and welded to the steel pipe at the rear end 10d. The anchor steel bar in the rigid frame beam is protected by a φ50PVC casing and cement slurry injected into the pipe.

2. A new foundation pit retaining system construction method according to claim 1, characterized in that: The length of the Larsen IV steel sheet pile is 15m.

3. A new foundation pit retaining system construction method according to claim 1, characterized in that: The size of the reinforced concrete structure rigid frame beam is 800×800mm.

4. A novel foundation pit retaining system construction method according to claim 1, characterized in that: The φ630×8 steel pipe pile is 12m long.

5. A novel foundation pit retaining system construction method according to claim 1, characterized in that: During the construction of the crown beam bottom formwork, the soft base section is mechanically excavated to remove the 500mm thick soil layer and then backfilled with stone slag or brick slag. The foundation treatment width is widened by 100mm on both sides according to the designed size.

6. A novel foundation pit retaining system construction method according to claim 1, characterized in that: The bottom formwork of the crown beam and the rigid frame beam is made of pressed plywood, and the bottom formwork is fixed by hammering the threaded steel bar into the soil layer close to the 20mm thick surface of the template edge, and the side formwork is roughly fixed by cross-pull rods and slightly tightened by external pull wire ropes.

7. A novel foundation pit retaining system construction method according to claim 1, characterized in that: During the construction of the crown beam reinforcement, when the stirrups are disconnected by the steel sheet piles, open stirrups are welded to the steel sheet piles, and the welding length is 10d.

8. A novel foundation pit retaining system construction method according to claim 1, characterized in that: The crown beam concrete is poured using the direct current method, using a movable small chute to enter the mold, and is poured and vibrated in layers. It is strictly forbidden to touch the formwork and steel frame on both sides during vibration.

9. A novel foundation pit retaining system construction method according to claim 1, characterized in that: During the construction process, a real-time monitoring system for the deformation, internal force and surrounding environmental changes of the foundation pit retaining structure is established. The monitoring contents include the horizontal displacement of steel sheet piles and steel pipe piles, vertical displacement, internal force changes of cap beams and rigid frame beams, and settlement and inclination of surrounding buildings.

10. A novel foundation pit retaining system construction method according to any one of claims 1 to 9, characterized in that: The distance between the steel sheet piles and the steel pipe piles is 11m at the unearthed part; the distance between the steel sheet piles and the steel pipe piles is 9m at the non-unearthed part.

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