A deep soft soil shallow foundation pit steel pipe pile and steel sheet pile combined cantilever supporting system and a construction method thereof

Abstract

The application discloses a deep soft soil shallow foundation pit steel pipe pile and steel sheet pile combined cantilever supporting system and a construction method thereof, which comprises a plurality of steel pipe piles vertically arranged at the edges of a foundation pit, steel sheet piles inserted into the soil-facing sides of the steel pipe piles and not connected with the steel pipe piles through locking, a combined steel corbel beam sleeved on the top of the steel pipe pile, and cement soil mixing pile reinforced bodies arranged in the form of a grid in the soil in the passive area inside the foundation pit. The steel pipe pile, the steel sheet pile and the mixing pile in the passive area are combined, and a prefabricated assembled corbel beam is used to realize the non-interior-support cantilever supporting, so that the application has the advantages of economical cost, convenient construction, recyclable materials and the like.

Description

Technical Field

[0001] This invention relates to the field of foundation pit engineering support technology, specifically to a steel pipe pile and steel sheet pile combined cantilever support system suitable for shallow foundation pits in deep soft soil and its construction method. Background Technology

[0002] Currently, when constructing shallow foundation pits with an excavation depth of no more than 6 meters in deep soft soil (greater than 20 meters thick), cantilever support structures without internal bracing are typically preferred for ease of construction. However, traditional cantilever support methods, such as cast-in-place piles and SMW piles, suffer from high costs and long construction periods, making it difficult to balance economy and safety. Furthermore, the low soil strength in the passive zone within the pit under deep soft soil conditions makes it difficult to control pile deformation and pit bottom heave in cantilever support systems, further limiting the applicability of cantilever support in such conditions. In addition, the traditional method of using cast-in-place reinforced concrete for pile cap beams presents problems such as inconvenient construction, long construction periods, difficult demolition, and lack of reusability, which are incompatible with the economic and construction convenience requirements of shallow foundation pits in deep soft soil. Summary of the Invention

[0003] To address the shortcomings of existing technologies, this invention provides a combined steel pipe pile and steel sheet pile cantilever support system and its construction method for shallow foundation pits in deep soft soil. The system combines steel pipe piles, interlock-free steel sheet piles, and grid-shaped cement-soil mixing piles in the passive zone of the pit, along with a prefabricated and on-site assembled composite steel cap beam. This system ensures the overall stability and water interception effect of the support system while achieving cantilever support without internal bracing for shallow foundation pits in deep soft soil.

[0004] To achieve the above objectives, the present invention is implemented through the following technical solution: a combined steel pipe pile and steel sheet pile cantilever support system for shallow foundation pits in deep soft soil, comprising steel pipe piles, steel sheet piles, a combined steel cap beam, and geogrid-reinforced soil.

[0005] The steel pipe piles are multiple, vertically arranged, and positioned at a preset interval along the edge of the foundation pit. The preset interval is determined through calculation based on parameters such as the excavation depth, the mechanical properties of the soft soil, and the thickness of the soft soil. This invention does not limit the specific interval value; for example, it can be 1 to 3 meters. The steel pipe piles can be arranged in a pattern of long piles, short piles, and alternating long piles to optimize stress performance and reduce steel consumption. The specific lengths and arrangement patterns of the long and short piles can be adjusted according to the actual stress requirements of the project and are not limited to a single long-short-long cycle.

[0006] The sheet piles are inserted on the soil-facing side of the steel pipe piles, without the need for interlocking joints for connection. Multiple sheet piles are interconnected through their own connecting structures, forming a self-contained unit that serves both water interception and soil blocking between piles. Various models from the Larssen series of sheet piles can be used, such as Larssen II, Larssen III, or Larssen IV, with the specific model selected based on the pit depth and water-stopping requirements. The driving depth of the sheet piles is not less than the excavation depth of the pit to ensure effective water interception.

[0007] The composite steel cap beam includes two steel girders located inside and outside the foundation pit, connected by I-beams to form an integral frame structure. The type of steel girders and I-beams can be determined according to the design load; this invention does not impose specific numerical limitations, for example, I-beams within the range of I25b to I63c can be used. Steel plates can be installed at the connection points between the inner and outer steel girders and the I-beams. These steel plates are welded and fixed to the steel girders and I-beams to enhance connection strength. The thickness and width of the steel plates can be adjusted according to the design load. After prefabrication in the factory, the composite steel cap beam is hoisted and installed as a whole onto the top of the steel pipe piles, eliminating the need for on-site welding. It has a high degree of prefabrication and can be removed and reused after the foundation pit is backfilled.

[0008] The grid-reinforced soil is formed by multiple cement-soil mixing piles overlapping each other, arranged in a grid shape within the passive zone soil inside the foundation pit, to improve the resistance and stability of the passive zone soil. The diameter and reinforcement depth of the cement-soil mixing piles can be determined according to the foundation pit depth, soft soil layer thickness, and passive zone soil strength requirements; this invention does not impose specific numerical limitations. For example, the diameter of the mixing piles can be 400 mm to 600 mm, and the reinforcement depth should not be less than the foundation pit excavation depth. The construction of the grid-reinforced soil is completed before the steel pipe piles are driven, and foundation pit excavation can only proceed after the reinforced soil reaches its design strength.

[0009] The beneficial effects of this invention are as follows: (1) The combination of steel pipe piles and steel sheet piles forms a retaining and water-stopping structure. The steel sheet piles do not need to be connected to the steel pipe piles with interlocking joints, which reduces the construction process, makes the construction speed fast, and reduces the cost. (2) By reinforcing the passive zone of the pit with grid-shaped cement-soil mixing piles, the resistance of the passive zone soil in the pit was improved, the pile deformation and pit bottom heave of the cantilever support system were controlled, and the cantilever support without internal support became an applicable design scheme for deep soft soil shallow foundation pits. (3) The composite steel cap beam is prefabricated in the factory and hoisted and installed on site. It is convenient to construct and saves construction time. It can be removed after the foundation pit is backfilled. The steel sheet piles, steel pipe piles and composite steel cap beams can all be recycled and reused, which reduces the overall cost of the project. (4) The overall structure is simple, and the components work together to share the load and complement each other. It is suitable for shallow foundation pit support under deep soft soil conditions, and has good economy and practicality, making it easy to promote and apply. Attached Figure Description

[0010] Figure 1 This is an elevation view of the combined steel pipe pile and steel sheet pile cantilever support system for deep soft soil shallow foundation pits in an embodiment of the present invention. Figure 2 This is a plan view of the combined steel pipe pile and steel sheet pile cantilever support system for deep soft soil shallow foundation pits in this embodiment of the invention. Figure 3 This is a detailed structural drawing of the combined steel crown beam in an embodiment of the present invention; Figure 4 This is a detailed installation drawing of the combined steel crown beam in an embodiment of the present invention.

[0011] Attached reference numerals: 1—Steel pipe pile; 2—Steel sheet pile; 3—Combined steel cap beam; 4—Glatform reinforced soil; 5—Soil facing side; 6—Inner side of foundation pit; 7—Steel waist beam; 8—Connecting steel plate; 9—I-beam. Detailed Implementation

[0012] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0013] To further illustrate the various embodiments, the present invention provides accompanying drawings. These drawings are part of the disclosure of the present invention, primarily used to illustrate the embodiments and to explain the operating principles of the embodiments in conjunction with the relevant descriptions in the specification. With reference to these drawings, those skilled in the art should be able to understand other possible implementations and the advantages of the present invention. Components in the drawings are not drawn to scale, and similar component symbols are generally used to represent similar components.

[0014] Example 1: like Figure 1 , Figure 2 As shown in the figure, this embodiment takes the application of a steel pipe pile and steel sheet pile combined cantilever support system for foundation pit support in a shallow foundation pit of deep soft soil as an example to further illustrate the present invention.

[0015] A combined steel pipe pile and steel sheet pile cantilever support system for shallow foundation pits in deep soft soil includes steel pipe piles 1, steel sheet piles 2, a combined steel cap beam 3, and grid-reinforced soil 4.

[0016] In one embodiment of the present invention, the steel pipe pile 1 is made of steel pipe with a diameter of 820 mm and a wall thickness of 8 mm, and is vertically driven at the edge of the foundation pit according to the designed spacing. In this embodiment, the steel pipe pile 1 is arranged in a way that alternates between long piles, short piles and long piles, so as to optimize the stress performance and reduce costs. The spacing between adjacent steel pipe piles 1 is determined by calculation based on the depth of the foundation pit, the mechanical properties of the soft soil and the thickness of the soft soil.

[0017] In one embodiment of the present invention, the sheet pile 2 is a Larssen III type sheet pile, inserted into the soil-facing side 5 of the steel pipe pile 1, without the need for interlocking joints for connection with the steel pipe pile 1. The sheet piles 2 are continuously arranged and form a self-contained unit, mainly serving to intercept water and retain soil between piles. Their driving depth is not less than the excavation depth of the foundation pit to ensure the water-stopping effect. The interlocking joint-less connection method involves multiple sheet piles being interconnected through their own connection structure and then attached to one side of the steel pipe pile (hereinafter referred to as "this construction method"). There is no rigid connection between the sheet piles and the steel pipe pile; under the action of soil pressure, the sheet piles will be tightly pressed against the steel pipe pile. From the perspective of seepage prevention, both the method of forming a water-stop curtain by connecting the steel sheet piles themselves (the method used in this project) and the method of forming a water-stop curtain by connecting the steel sheet piles with interlocking PC steel pipe piles (the conventional method) essentially rely on the tight connection between the interlocking joints of the steel components to stop water, and the water-stopping effect is no different. From the perspective of construction period, the method of attaching the steel sheet piles to one side of the steel pipe piles saves the time of customizing steel pipe piles with interlocking joints, thus saving construction time. From an economic perspective, all components of this construction method can be reasonably selected for rental or purchase depending on the length of use of the foundation pit.

[0018] In one embodiment of the present invention, the grid-reinforced soil 4 is formed by overlapping cement-soil mixing piles with a diameter of 500 mm, arranged in a grid shape in the passive zone soil inside the foundation pit 6. In this embodiment, the reinforcement depth of the mixing piles is 6 meters. The construction of the grid-reinforced soil 4 is completed before the steel pipe piles 1 are driven. The foundation pit can only be excavated after the reinforced soil reaches the design strength, so as to improve the passive resistance of the soil in the pit and ensure the stability of the cantilever support system under soft soil conditions.

[0019] Taking the foundation pit support of a sluice gate as an example, the foundation pit is 3.85m deep and is supported by steel pipe piles with a diameter of 820mm and a wall thickness of 8mm. The geological conditions from top to bottom are as follows: 1.57m deep plain fill, 23.50m silt, 18.00m silty soil, 2.00m coarse sand, and 3.60m strongly weathered rock. Using the Lizheng Deep Foundation Pit V7.5 calculation software, keeping other parameters unchanged, calculations were performed on foundation pits with and without passive zone reinforcement. The calculation results show that the pile top displacement of the foundation pit with passive zone reinforcement is only 56.36mm, and the anti-heave stability, overall stability, and anti-overturning stability all meet the specifications. However, the pile top displacement of the foundation pit without passive zone reinforcement reached 128.54mm.

[0020] like Figure 3 , Figure 4As shown, the composite steel cap beam 3 includes inner and outer steel waist beams 7, which are connected by I-beams 9 to form an integral frame structure and are directly fitted onto the top of the steel pipe pile 1. In this embodiment, the cap beam 3 is composed of I50b and I32a I-beams 9, with a 20 mm thick and 360 mm wide connecting steel plate 8 at the connection point, which is welded and reinforced to the steel waist beam 7 and the I-beams 9. The composite steel cap beam 3 is prefabricated in the factory and hoisted to the top of the pile as a whole for installation, without the need for on-site welding, resulting in a high degree of assembly and fast construction speed.

[0021] A comparative analysis of the construction period and cost of precast composite steel cap beams and cast-in-place reinforced concrete cap beams: 1. In terms of construction period comparison, according to relevant engineering practice data, prefabricated steel structure construction can save about 1 / 3 of the construction period compared with traditional cast-in-place construction. In deep soft soil shallow foundation pit projects, capping beam construction is often an important link on the critical path. Shortening the construction period can directly reduce project management costs and equipment rental costs.

[0022] 2. From a cost analysis perspective, the capping beam in this application uses a combination of I50b and I32a I-beams. In terms of overall cost, the precast steel capping beam has significant advantages: ① Recyclable and reusable: The steel capping beam can be dismantled and recycled after the foundation pit is backfilled, allowing for reuse and resulting in high residual material value; while cast-in-place concrete capping beams are a one-time investment, requiring subsequent demolition and incurring additional demolition and waste disposal costs. ② Reduced rental costs: Shorter construction period means shorter rental periods for piling equipment and steel, reducing indirect costs. ③ Reduced auxiliary processes: No formwork, supports, or maintenance measures are required, saving on formwork support and dismantling costs and related labor costs.

[0023] The construction method of the combined steel pipe pile and steel sheet pile cantilever support system for shallow foundation pits in deep soft soil described in this embodiment includes the following steps: Step 1, fabricating a combined steel cap beam 3, welding and assembling it according to the steel type and size required by the design drawings, and simultaneously pre-ordering steel pipe piles 1 and steel sheet piles 2 according to the design drawings; Step 2, after leveling the site and laying out the lines, constructing a grid to reinforce the soil 4 according to the design positioning; Step 3, after the grid-reinforced soil 4 reaches the design strength, driving steel pipe piles 1 at the spacing required by the design drawings; Step 4, driving steel sheet piles 2 at the positions and depths required by the design drawings. Step 5: The sheet pile 2 is inserted on the soil-facing side 5 of the steel pipe pile 1, without needing to be connected to the interlocking joint of the steel pipe pile 1; Step 6: The prefabricated combined steel cap beam 3 is hoisted onto the top of the steel pipe pile 1 and directly fitted onto the top of the pile; Step 7: After the grid-reinforced soil 4 reaches the design strength, the foundation pit soil is excavated in layers to the design foundation pit bottom elevation, and drainage in the pit is kept unobstructed during the excavation process; Step 8: After the underground main structure construction is completed, the foundation pit is backfilled step by step, and the backfill is compacted in layers; Step 9: After the foundation pit is backfilled, the combined steel cap beam 3 is removed, and the sheet pile 2 and steel pipe pile 1 are recycled to achieve material reuse.

[0024] With the increasing development of urban underground space, the demand for shallow foundation pit projects such as municipal utility tunnels, small underground parking garages, and sewage treatment ponds continues to grow, creating an urgent need for economical cantilever support solutions in areas with deep soft soil. This invention presents a combined steel pipe pile and steel sheet pile cantilever support system for shallow foundation pits in deep soft soil, which has the following characteristics: Structural combination method: Existing technologies mostly adopt double-row piles or interlocking connection structures. This application adopts a single-row steel pipe pile + steel sheet pile self-contained combination without interlocking joints, which is simpler in construction. Crown beam type: Existing technologies mostly use cast-in-place reinforced concrete crown beams or welded steel walers. This application adopts prefabricated composite steel crown beams for direct connection, realizing prefabricated construction and material recycling. Piling method: This application adopts a "long-short-long" interval pile layout to optimize stress and reduce steel consumption; Scope of application: This application is specifically for specific working conditions of deep soft soil >20m and shallow foundation pit ≤6m.

[0025] In summary, this invention belongs to the field of foundation pit engineering support technology, and is specifically applicable to shallow foundation pit projects with an excavation depth of no more than 6 meters under the geological conditions of deep soft soil (soft soil thickness > 20m), such as municipal pipe corridors, small underground garages, commercial basements, sewage treatment ponds, pumping stations and other shallow foundation pit support projects without internal support requirements.

[0026] Example 2: The difference between this embodiment and Embodiment 1 lies in the specific parameters of each component. In this embodiment, the steel pipe pile 1 is made of steel pipe with a diameter of 1000 mm and a wall thickness of 10 mm; the steel sheet pile 2 is a Larssen IV type steel sheet pile, with an increased driving depth compared to Embodiment 1; the grid-reinforced soil 4 is formed by overlapping cement-soil mixing piles with a diameter of 600 mm, with a reinforcement depth of 7 meters; the two steel waist beams 7 of the composite steel crown beam 3 are composed of I63c I-beams to accommodate larger design loads. The remaining structural components, connection methods, and construction steps are the same as in Embodiment 1. By adjusting the specifications of the steel pipe piles, steel sheet piles, and grid-reinforced soil, the support system described in this invention can adapt to the support needs of shallow foundation pits under different depths and soft soil conditions, exhibiting good adaptability and scalability.

[0027] The above description is merely an embodiment of the technical content of the present invention. Any modifications or variations made by those skilled in the art using the present invention are within the scope of the patent claims of the present invention, and are not limited to those disclosed in the embodiments.

Claims

1. A combined cantilever support system for shallow foundation pits in deep soft soil using steel pipe piles and steel sheet piles, characterized in that... The system includes steel pipe piles, steel sheet piles, a composite steel cap beam, and a grid-reinforced soil structure. Multiple steel pipe piles are vertically arranged at preset intervals along the edge of the foundation pit. The steel sheet piles are inserted into the soil-facing side of the steel pipe piles without the need for interlocking joints. Multiple sheet piles are interconnected to form a self-contained structure, serving both water interception and soil retention between piles. The composite steel cap beam includes steel girders located inside and outside the foundation pit. These girders are connected by I-beams to form an integrated frame structure, which is fitted onto the top of the steel pipe piles. The grid-reinforced soil structure is formed by multiple cement-soil mixing piles overlapping each other, arranged in a grid pattern within the passive zone soil inside the foundation pit to improve the resistance of the passive zone soil.

2. The combined cantilever support system of steel pipe pile and steel sheet pile according to claim 1, characterized in that, The steel pipe pile is not connected to the steel sheet pile by a locking joint. The steel sheet pile is directly installed on the soil-facing side of the steel pipe pile, and is independent of the steel pipe pile but also shares the load with it.

3. The combined cantilever support system of steel pipe pile and steel sheet pile according to claim 1, characterized in that, The composite steel cap beam connects the tops of each steel pipe pile into one unit, improving the overall stability of the cantilever support system. After being prefabricated in the factory, the composite steel cap beam is hoisted and installed on the pile top as a whole, directly fitted onto the pile top, without the need for on-site welding.

4. The combined cantilever support system of steel pipe pile and steel sheet pile according to claim 1, characterized in that, The steel pipe piles are arranged along the edge of the foundation pit in a way that alternates between long piles, short piles and long piles, in order to optimize the stress performance of the cantilever support system and reduce the amount of steel used.

5. The combined cantilever support system of steel pipe pile and steel sheet pile according to claim 1, characterized in that, The driving depth of the sheet piles shall not be less than the excavation depth of the foundation pit in order to ensure the water-cutting effect of the sheet piles.

6. The combined cantilever support system of steel pipe pile and steel sheet pile according to claim 1, characterized in that, The reinforcement depth of the grid-reinforced soil is not less than the excavation depth of the foundation pit. Adjacent cement-soil mixing piles overlap to form a continuous grid-like reinforcement body, thereby improving the overall resistance of the soil in the passive zone of the pit.

7. The combined cantilever support system of steel pipe pile and steel sheet pile according to claim 3, characterized in that, A steel plate is provided at the connection between the steel wainscoting and the I-beam. The steel plate is welded and fixed to the steel wainscoting and the I-beam to enhance the connection strength of the composite steel crown beam.

8. A construction method for a combined steel pipe pile and steel sheet pile cantilever support system according to any one of claims 1 to 7, characterized in that, Includes the following steps: Step 1: Fabricate the composite steel cap beam. Weld and assemble the steel according to the steel type and size required by the design drawings. At the same time, order the steel pipe piles and steel sheet piles according to the design drawings. Step 2: After leveling the site and laying out the lines, construct the geogrid to reinforce the soil according to the design positioning; Step 3: After the soil reinforced by the grid reaches the design strength, drive steel pipe piles at the spacing required by the design drawings; Step 4: Insert the sheet piles into the soil-facing side of the steel pipe piles according to the location and depth required by the design drawings, without the need for interlocking connections between the sheet piles and the steel pipe piles; Step 5: Hoist the prefabricated composite steel cap beam onto the top of the steel pipe pile and fit it onto the pile top; Step 6: After the geogrid-reinforced soil reaches the design strength, excavate the foundation pit in layers to the design bottom elevation; Step 7: After the underground main structure construction is completed, the foundation pit will be backfilled gradually; Step 8: After the foundation pit is backfilled, the composite steel cap beam is removed and the steel sheet piles and steel pipe piles are recovered.

9. The construction method according to claim 8, characterized in that, The construction of the geogrid-reinforced soil is completed before the steel pipe piles are driven, and the foundation pit excavation can only be carried out after the geogrid-reinforced soil reaches the design strength.

10. The construction method according to claim 8, characterized in that, In step 8, after dismantling the composite steel cap beam, recovering the steel sheet piles and the steel pipe piles, the recovered steel sheet piles and the steel pipe piles are reused.

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