Composite pressure type uplift pile and construction method

Abstract

The composite pressure type uplift pile comprises a prefabricated solid pile body and a prefabricated hollow pile body, a mounting channel is formed in the prefabricated hollow pile body, the prefabricated solid pile body is located below the prefabricated hollow pile body 1, and the upper end of the prefabricated solid pile body abuts against the lower end of the prefabricated hollow pile body; a plurality of vertically-arranged prestressed tendons and a plurality of vertically-arranged grouting pipes are pre-buried in the prefabricated solid pile, and the upper ends of the prestressed tendons penetrate through the mounting channel and then are connected with a bottom plate foundation under the building; the upper ends of the multiple grouting pipes penetrate through the mounting channel till penetrating out of the bottom plate foundation, and a plurality of guniting holes communicating with the grouting pipes are formed in the bottom and the peripheral wall of the solid pile correspondingly. The composite pressure type uplift pile is simple in structure and stable in performance, pile body concrete can be pressed in the whole process, and crack development of the uplift pile is effectively reduced.

Description

Technical Field

[0001] This invention relates to the field of building pile foundation technology, specifically a composite pressure-type anti-tension pile and its construction method. Background Technology

[0002] An anti-uplift pile is a type of pile foundation that extends deep into the ground. Its main function is to withstand upward tensile forces and prevent buildings or structures from being uplifted. It resists uplift forces through the friction between the pile surface and the surrounding soil, as well as the weight of the pile itself. It is commonly found in structures such as underground parking garages and rail transit hubs that need to overcome uplift forces caused by buoyancy from groundwater.

[0003] Traditional tension piles transfer the uplift load to the entire pile length through the bond between the pile reinforcement and concrete. The concrete in the pile body bears the tensile force, and the pile length and reinforcement amount are mainly determined by the uplift load, the pile-soil friction resistance, and the strength of the pile material to ensure that the uplift bearing capacity meets the requirements and the pile body is not broken. However, this stress mechanism makes pile cracks difficult to avoid, which may affect the durability of the structure. The development and application of composite pressure tension piles provide a safer and more economical solution for structural anti-buoyancy. Composite pressure tension piles effectively suppress crack formation, improve the overall stability and durability of the anti-buoyancy system, and are suitable for anti-buoyancy engineering of underground structures under complex hydrogeological conditions.

[0004] Currently available prestressed pull-out piles consist of prestressing tendons, ordinary steel bars, and a concrete pile body that transfer the load. These three components work together as a whole to bear the load, with the prestressing tendons anchored at both ends of the concrete pile body. Even with this structure, the concrete pile body is still subjected to tensile forces during operation, which means that cracks may still occur in this type of prestressed pull-out pile. Summary of the Invention

[0005] The problem solved by this invention is to overcome at least one defect in the prior art and provide a composite pressure-type tensile pile with simple structure, stable performance, and the ability to achieve full-process compression of the pile body concrete. Applying it to underground structures can effectively reduce the development of cracks in tensile piles.

[0006] To address the above problems, this invention provides a composite pressure-type tension pile, comprising: Precast hollow piles, with an installation channel in the middle; A precast solid pile is located below the precast hollow pile, and its upper end abuts against the lower end of the precast hollow pile shaft. Multiple prestressing tendons, with the lower ends of the multiple prestressing tendons embedded in the precast solid piles and the upper ends passing through the installation channel to connect with the foundation slab under the building; Multiple grouting pipes are arranged circumferentially outside the multiple prestressing tendons. The lower ends of the multiple grouting pipes are embedded in the solid piles. Multiple grouting holes that communicate with each of the grouting pipes are opened on the bottom and outer peripheral wall of the solid piles. The upper ends of each of the grouting pipes protrude from the base slab foundation.

[0007] Furthermore, the system includes multiple pipes, each of which is fitted around the outside of one of the prestressing tendons, with the lower ends of the pipes embedded within the precast solid pile. The pipes primarily serve to prevent the prestressing tendons from rusting, thus ensuring their tensile strength.

[0008] Furthermore, the precast solid pile has multiple perforated fixed-end anchor plates embedded inside, and the lower ends of the multiple prestressing tendons pass through the inner holes of each fixed-end anchor plate and are anchored by fixed-end anchors.

[0009] Furthermore, multiple positioning supports are arranged axially spaced apart on the outside of the multiple pipes.

[0010] Preferably, the positioning bracket is a disc structure, and the disc has multiple connecting holes for the passage of each of the pipes and grouting pipes, and the disc is welded and fixed to the outer wall of each of the grouting pipes.

[0011] Furthermore, the lower ends of the multiple grouting pipes extend to the bottom surface of the precast solid pile to form multiple bottom grouting holes. Each grouting pipe has a horizontally arranged grouting branch pipe connected to its inner cavity on the side wall of its lower end. The outer end of each grouting branch pipe extends to the outer side of the precast solid pile to form multiple side grouting holes.

[0012] Furthermore, a steel reinforcement cage is pre-embedded within the solid portion of the precast hollow pile.

[0013] As an improvement, the precast hollow piles and precast solid piles have the same external structure, both being square main columns, and each of the four sides of the square main column is provided with a concave clearance groove.

[0014] Furthermore, the prestressed tendon is a prestressed steel bar or a prestressed steel strand.

[0015] On the other hand, the present invention also provides a construction method for composite pressure-type tension piles, comprising the following steps: S1: Precast hollow concrete pile: A precast steel cage is placed inside the formwork, concrete is poured to form a hollow concrete pile, it is vibrated to compact, and cured to the design strength. S2: Precast solid concrete pile: The prestressing tendons are inserted into the pipe, the position of the prestressing tendons and the pipe is fixed in the precast mold, and the grouting pipe is fixed outside the pipe. Concrete is poured to form a solid concrete pile body and cured to the design strength. S3: Rotary drilling: Using a rotary drill, a hole with a diameter slightly larger than the outer diameter of the pull-out pile is dug on site to ensure successful pile placement; S4: Lowering the precast solid piles: Using hoisting equipment, the precast solid pile structure is hoisted and lowered as a whole into the rotary-drilled hole, ensuring that the verticality and depth meet the design requirements; S5: Lowering the hollow concrete pile: Use hoisting equipment to slowly lift the precast hollow concrete pile, align it with the top of the precast solid pile, and lower it to the predetermined position at a uniform speed. During the lowering process, use a total station to monitor the verticality of the pile in real time to ensure that the deviation of the coaxiality with the precast solid pile does not exceed the design allowable value. S6: Prestressed tendons protrude from the foundation slab: When setting up the formwork and binding the reinforcing bars for the foundation slab of the basement of the building, a hole with a diameter slightly larger than that of the prestressed tendon is reserved at the corresponding position of the prestressed tendon. The top of the prestressed tendon is passed through the hole, and the length of the protrusion must meet the operating space requirements of the subsequent tensioning construction. The area around the hole is pretreated with waterproof sealing material. S7: Tensioning and anchoring of prestressed tendons: After the foundation concrete reaches the design strength, install the tensioning end anchor plate and tensioning end anchor on the top surface of the foundation. According to the design tensioning sequence, apply tensioning stress in stages using hydraulic tensioning equipment. After reaching the design tensioning control stress, lock the tensioning end anchor to complete the anchoring. Monitor the pile top displacement during the tensioning process to ensure that it does not exceed the specification limit. S8: Grouting through the grouting pipe: Securely connect the special grouting conduit to the grouting pipe port, conduct a pressure sealing test on the grouting pipeline to ensure no leakage; mix cement grout according to the grouting material mix ratio specified in the design, and use a grouting pump to inject grout into the grout outlet area at a uniform speed, controlling the grouting pressure within the design allowable range. After grouting is completed, a bag-shaped enlarged head is formed on the outside of the original precast solid pile.

[0016] In summary, the composite pressure-type tension pile of the present invention has the following advantages compared with the prior art: The composite pressure-type tension pile of this invention has a simple structure, stable performance, and strong tension bearing capacity, effectively controlling the cracking of the pile concrete and the resulting corrosion of the reinforcing steel. Under load, the solid anchorage section of the concrete pile transfers the tensile force to the lower enlarged head section through prestressed tendons. The lower enlarged head section converts the tensile force into pressure on the hollow concrete pile body, ensuring that the hollow concrete pile body is under compression throughout the entire process. Compared with traditional tension-type tension piles, it has stronger tension bearing capacity. This is because, for the same displacement, the load required for the pressure-type tension pile is always greater than that for the tension-type tension pile. This is because the tension pile bearing capacity consists of the pile's self-weight, side skin friction, and vacuum suction at the pile bottom. Under similar conditions, the difference in the pile's stress mode directly affects the side skin friction. Tension piles gradually undergo axial tensile deformation and radial area shrinkage from the moment they are subjected to force, reducing the contact area with the soil. This reduces the surface area of ​​friction between the pile and the soil, making them more prone to upward displacement. In contrast, compression piles are continuously compacted by the lower anchoring end during the upward pulling process, causing the cross-sectional area to expand and increasing the force-bearing area between the pile and the soil. This increases the lateral friction resistance and utilizes the friction between the pile and the soil to improve the pull-out bearing capacity, making the pile difficult to pull out.

[0017] Other improvements and advantages of this application will be set forth in the following detailed description and will be apparent in part from the specification or may be learned by practicing the application. The objectives and other advantages of this application may be realized and obtained through the structures particularly pointed out in the specification and drawings. Attached Figure Description

[0018] Figure 1 This is a sectional view of the composite pressure-type tension pile of the present invention after its connection with the foundation slab; Figure 2 for Figure 1 Enlarged structural diagram at point X; Figure 3 for Figure 1 Enlarged structural diagram at point Y; Figure 4 for Figure 1 Sectional view along line AA in the middle; Figure 5 This is a top view of the precast solid pile in this invention.

[0019] Explanation of reference numerals in the attached figures: 1. Precast hollow piles; 2. Installation channel; 3. Precast solid piles; 4. Prestressed tendons; 5. Base slab foundation; 6. Grouting pipe; 7. Pipeline; 8. Fixed end anchor plate; 9. Positioning bracket; 10. Grouting branch pipe; 11. Reinforcing cage; 1101. Vertical reinforcement; 1102. Horizontal stirrups; 12. Voiding groove; 13. Fixed end anchor; 14. Tensioning end anchor plate; 15. Tensioning end anchor. Detailed Implementation

[0020] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0021] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the term "connection" should be interpreted broadly. For example, it can refer to a fixed connection, a detachable connection, or an integral connection; it can refer to a direct connection or an indirect connection through an intermediate medium; it can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0022] like Figures 1-5 As shown, the present invention provides a composite pressure-type tension pile, comprising a precast solid pile 3 and a precast hollow pile 1 arranged from bottom to top. The hollow cavity of the precast hollow pile 1 forms an installation channel 2. The precast solid pile 3 is located below the precast hollow pile 1, and its upper end abuts against the lower end of the precast hollow pile 1. In addition, multiple vertically arranged prestressing tendons 4 and multiple vertically arranged grouting pipes 6 are pre-embedded in the precast solid pile 3. The upper ends of the multiple prestressing tendons 4 pass through the installation channel 2 in the precast hollow pile 1 and connect to the foundation 5 under the building. Similarly, the upper ends of the multiple grouting pipes 6 pass through the installation channel 2 in the precast hollow pile 1 until they exit the foundation 5 under the building. In this structure, multiple grouting holes communicating with each grouting pipe 6 are opened on the bottom and outer peripheral wall of the solid pile.

[0023] In addition, in order to effectively protect the prestressing tendons 4 and prevent rusting, multiple pipes 7 are also provided in this embodiment. The multiple pipes 7 are respectively fitted on the outside of each prestressing tendon 4 and are coaxial with the prestressing tendon 4. The lower ends of the multiple pipes 7 are pre-embedded in the precast solid pile 3. That is, the multiple pipes 7 and the multiple prestressing tendons 4 are precast together with the precast solid pile 3 to ensure the strength of the connection structure.

[0024] In this embodiment, see Appendix Figure 4 There are four prestressing tendons 4, and they are arranged in a square shape relative to the center of the four sides of the precast solid pile 3. Similarly, there are also four pipes 7 that are fitted outside the prestressing tendons 4. In addition, there are also four grouting pipes 6, and the four grouting pipes 6 are located on the radial outer side of the four pipes 7 respectively.

[0025] More specifically, in order to further enhance the connection strength between the prestressed tendons 4 and the precast solid pile 3, multiple perforated fixed-end anchor plates 8 are pre-embedded in the precast solid pile 3. The lower ends of the multiple prestressed tendons 4 pass through the inner holes of each fixed-end anchor plate 8 and are anchored through the fixed-end anchor 13, thereby increasing the contact area between them and the concrete pile body of the precast solid pile 3 and increasing the tensile strength.

[0026] On the other hand, since multiple pipes 7 are directly exposed at the upper end of the precast solid pile 3 and have a certain length, in order to avoid the upper end of the pipes 7 from being tilted and causing friction between the prestressing tendons 4 and their interior, thus affecting the tensile performance, in this embodiment, multiple positioning brackets 9 are arranged on the outside of the multiple pipes 7 at intervals along the axial direction. Preferably, the positioning brackets 9 here are disc structures, with multiple connecting holes for each pipe 7 to pass through, and the disc is welded and fixed to the outer wall of each pipe 7.

[0027] In this embodiment, the lower ends of multiple grouting pipes 6 extend to the bottom surface of the precast solid pile 3 to form multiple bottom grouting holes. Each grouting pipe 6 has a horizontally arranged grouting branch pipe 10 connected to its inner cavity on its lower side wall. The outer end of each grouting branch pipe 10 extends to the outer surface of the precast solid pile 3 to form multiple side grouting holes. Preferably, each grouting branch pipe 10 and the corresponding grouting pipe 6 are integrally formed, ensuring a stable connection structure that will not detach due to excessive grouting pressure. This ensures that during final grouting, grout is sprayed simultaneously on the bottom and sides, achieving adhesion with the soil at the corresponding location. Bottom grouting consolidates the pile bottom sediment and forms an enlarged head, enhancing the pile end bearing capacity. Side grouting hardens the pile body mud skin and enhances the compaction between the pile body and the soil, increasing friction and improving pull-out bearing capacity.

[0028] On the other hand, in this embodiment, a steel cage 11 is pre-embedded in the solid part of the precast hollow pile 1. The steel cage 11 includes multiple vertical steel bars 1101 that are distributed circumferentially along the pile body of the precast hollow pile 1 and extend axially, and multiple transverse stirrups 1102 that are spaced apart axially on the outer periphery of the multiple vertical steel bars 1101.

[0029] The precast hollow pile 1 and the precast solid pile 3 have the same external structure, both being square main columns. Each of the four sides of the square main column has a recessed clearance groove 12 in the center, forming an irregular structure that increases the contact area between the soil and the pile, thus improving pull-out resistance. In this structure, the side grouting holes on the outer wall of the precast solid pile 3 are located at the bottom of each clearance groove 12. After final grouting, the solidified grout will fill the entire clearance groove 12, thereby increasing the connection strength, i.e., increasing the connection strength with the soil.

[0030] In some other embodiments, a corresponding steel cage 11 can also be installed inside the precast solid pile 3 to improve the overall strength performance of the pile.

[0031] In this embodiment, the prestressing tendon 4 is a prestressed steel bar or a prestressed steel strand.

[0032] On the other hand, embodiments of the present invention also provide a method for fabricating composite pressure-type tension piles, comprising the following steps: S1: Precast hollow concrete pile: Place a precast steel cage 11 inside the formwork, pour concrete to form a hollow concrete pile, vibrate to compact, and cure to the design strength. S2: Precast solid concrete pile: The prestressing tendon 4 is inserted into the pipe 7, the position of the prestressing tendon 4 and the pipe 7 is fixed in the precast mold, and the grouting pipe 6 is fixed outside the pipe 7. Concrete is poured to form a solid concrete pile body and cured to the design strength. S3: Rotary drilling: Using a rotary drill, a hole with a diameter slightly larger than the outer diameter of the pull-out pile is dug on site to ensure successful pile placement; S4: Lowering the precast solid piles 3: Use hoisting equipment to hoist the precast solid pile structure as a whole into the rotary-drilled hole, ensuring that the verticality and depth meet the design requirements; S5: Lowering the hollow concrete pile: Use hoisting equipment to slowly lift the precast hollow concrete pile, align it coaxially with the top of the precast solid pile 3, and lower it to the predetermined position at a uniform speed. During the lowering process, use a total station to monitor the verticality of the pile body in real time to ensure that the coaxiality deviation with the precast solid pile 3 does not exceed the design allowable value. S6: The top of the prestressed tendon 4 passes through the foundation 5 of the basement: the formwork is erected and the reinforcement is tied for the foundation 5 of the basement of the building. A hole with a diameter slightly larger than that of the prestressed tendon 4 is reserved at the corresponding position of the prestressed tendon 4. The top of the prestressed tendon 4 is passed through the hole. The length of the hole must meet the operating space requirements of the subsequent tensioning construction. The area around the hole is pretreated with waterproof sealing material. S7: Tensioning and anchoring of prestressed tendons 4: After the concrete of the foundation 5 reaches the design strength, install the tensioning end anchor plate 14 and tensioning end anchor 15 on the top surface of the foundation 5. According to the design tensioning sequence, apply tensioning stress in stages using hydraulic tensioning equipment. After reaching the design tensioning control stress, lock the tensioning end anchor 15 to complete the anchoring. Monitor the pile top displacement during the tensioning process to ensure that it does not exceed the specification limit. S8: Grouting of 6-hole pipe: Securely connect the special grouting conduit to the port of 6-hole grouting pipe, and conduct a pressure sealing test on 6-hole grouting pipe to ensure no leakage; mix cement grout according to the grouting material mix ratio specified in the design, and use a grouting pump to inject grout into the grout outlet area at a uniform speed, controlling the grouting pressure within the design allowable range. After grouting is completed, a bag-shaped enlarged head is formed on the outside of the original precast solid pile 3, which effectively increases the contact area with the soil, thereby improving the pull-out resistance.

[0033] In the description of the embodiments of the present invention, it should be noted that the terms "front and back", "both sides", "up and down" and other terms indicating the direction or positional relationship are based on the direction or positional relationship shown in the drawings. This is only for the convenience of description and does not indicate or imply that the device or component must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, it should not be construed as a limitation of the present invention.

[0034] In the description of this invention, references to terms such as "this embodiment," "some embodiments," etc., indicate that a specific feature, mechanism, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, mechanisms, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0035] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A composite pressure-type tension pile, characterized in that, include: A precast hollow pile (1) with an installation channel (2) in the middle; A precast solid pile (3) is located below the precast hollow pile (1), and its upper end abuts against the lower end of the shaft of the precast hollow pile (1). Multiple prestressing tendons (4), the lower ends of the multiple prestressing tendons (4) are embedded in the precast solid pile (3), and the upper ends pass through the installation channel (2) and are connected to the foundation slab (5) under the building. Multiple grouting pipes (6) are arranged circumferentially outside the multiple prestressed tendons (4). The lower ends of the multiple grouting pipes (6) are embedded in the solid pile. Multiple grouting holes that communicate with each of the grouting pipes (6) are opened on the bottom and outer peripheral wall of the solid pile. The upper ends of each grouting pipe (6) pass through the base slab foundation (5).

2. The composite pressure-type tension pile according to claim 1, characterized in that: It also includes multiple pipes (7), which are respectively fitted on the outside of each of the prestressed tendons (4), and the lower ends of the multiple pipes (7) are pre-embedded in the precast solid piles (3).

3. The composite pressure-type tension pile according to claim 1, characterized in that: The precast solid pile (3) has multiple perforated anchor plates (8) embedded in it. The lower ends of the multiple prestressed tendons (4) pass through the inner holes of each fixed end anchor plate (8) and are anchored by the fixed end anchor (13).

4. The composite pressure-type tension pile according to claim 1, characterized in that: Multiple positioning brackets (9) are arranged on the outside of the multiple grouting pipes (6) at intervals along the axial direction.

5. The composite pressure-type tension pile according to claim 4, characterized in that: The positioning bracket (9) is a disc structure. The disc has multiple connecting holes for the passage of each of the pipes (7) and the grouting pipes (6). The disc is welded and fixed to the outer wall of each of the grouting pipes (6).

6. The composite pressure-type tension pile according to claim 1, characterized in that: The lower ends of the multiple grouting pipes (6) extend to the bottom surface of the precast solid pile (3) to form multiple bottom grouting holes. Each grouting pipe (6) has a horizontally arranged grouting branch pipe (10) connected to its inner cavity on the side wall of its lower end. The outer end of each grouting branch pipe (10) extends to the outer side of the precast solid pile (3) to form multiple side grouting holes.

7. The composite pressure-type tension pile according to claim 1, characterized in that: The precast hollow pile (1) has a steel cage (11) embedded in its solid part.

8. The composite pressure-type tension pile according to any one of claims 1 to 7, characterized in that: The precast hollow piles (1) and precast solid piles (3) have the same external structure, both being square main columns, and each of the four sides of the square main column is provided with a concave clearance groove (12).

9. The composite pressure-type tension pile according to claim 1, characterized in that: The prestressed tendon (4) is a prestressed steel bar or a prestressed steel strand.

10. A construction method for a composite pressure-type tension pile according to any one of claims 1 to 9, characterized in that, Includes the following steps: S1: Precast hollow concrete pile: Place a precast steel cage (11) inside the formwork, pour concrete to form a hollow concrete pile, vibrate to compact, and cure to the design strength; S2: Precast solid concrete pile: Insert the prestressing tendon (4) into the pipe (7), fix the position of the prestressing tendon (4) and the pipe (7) in the precast mold, fix the grouting pipe (6) outside the pipe (7), pour concrete to form a solid concrete pile body, and cure to the design strength. S3: Rotary drilling: Using a rotary drill, a hole with a diameter slightly larger than the outer diameter of the pull-out pile is dug on site to ensure successful pile placement; S4: Lowering the precast solid piles (3): Use hoisting equipment to hoist the precast solid pile structure as a whole into the hole formed by rotary drilling, ensuring that the verticality and depth meet the design requirements; S5: Lowering the hollow concrete pile: Use hoisting equipment to slowly lift the precast hollow concrete pile, align it with the top of the precast solid pile (3) on the same axis, and lower it to the predetermined position at a uniform speed. During the lowering process, monitor the verticality of the pile body in real time with a total station to ensure that the deviation of the coaxiality with the precast solid pile (3) does not exceed the design allowable value. S6: The top of the prestressed tendon (4) passes through the foundation slab (5): The formwork is erected and the reinforcement is tied for the foundation slab (5) of the basement of the building. A hole with a diameter slightly larger than that of the prestressed tendon (4) is reserved at the corresponding position of the prestressed tendon (4). The top of the prestressed tendon (4) is passed through the hole. The length of the hole must meet the operating space requirements of the subsequent tensioning construction. The area around the hole is pretreated with waterproof sealing material. S7: Tensioning and anchoring of prestressed tendons (4): After the concrete of the foundation slab (5) reaches the design strength, the tensioning end anchor plate (14) and tensioning end anchor (15) are installed on the top surface of the foundation slab (5). According to the design tensioning sequence, the tensioning stress is applied in stages using hydraulic tensioning equipment. After reaching the design tensioning control stress, the tensioning end anchor (15) is locked to complete the anchoring. During the tensioning process, the displacement of the pile top is monitored to ensure that it does not exceed the specification limit. S8: Grouting pipe (6) Grouting: Connect the special grouting conduit to the port of the grouting pipe (6) firmly, and conduct a pressure sealing test on the grouting pipe (6) to ensure no leakage; mix cement grout according to the grouting material mix ratio specified in the design, and use a grouting pump to inject grout into the grout outlet area at a uniform speed. The grouting pressure is controlled within the design allowable range. After the grouting is completed, a bag-shaped enlarged head is formed on the outside of the original precast solid pile (3).

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