The PC construction method combined steel pipe pile is a new type of retaining pile technology. Its predecessor is the traditional Larsen steel sheet pile. Whether the simple steel sheet pile is U-shaped, Z-shaped or straight web, due to its small section stiffness, it is necessary to need multi-layer supporting and anchor structure in the application of deep foundation pit, which increases the difficulty of construction and makes it difficult to give full play to its advantages of fast and high efficiency. The composite steel pipe makeup of PC construction method is to combine Larsen pile with steel pipe, section steel and other materials to form a composite makeup of various sections for soil retaining and water stop. In the construction process, the combination form of construction method pile suitable for engineering construction can be selected according to the actual situation of foundation pit engineering.

The foundation pit project in the central green axis closed area of Wenzhou, Zhejiang Province, east of Huimin road and south of yueluolong Road, is a parking garage on the first floor underground, with a concrete frame structure, with a total construction area of about 5370m2. The basement is a regular rectangle, with a long side of about 80m and a short side of about 31m. The basement area is about 939m2 and the excavation depth is 4.3 ~ 4.7m. The west side of the project is the main road of Huimin road. Municipal pipelines such as rainwater pipe and sewage pipe are buried underground, and the other three sides are open space. The engineering geological conditions are ① 0 miscellaneous fill, ① clay, ② 1 silt and ② 2 silt from top to bottom.

The engineering retaining row pile adopts PC construction method combined steel pipe pile, which is a new type of combined section steel foundation pit retaining structure. The construction method adopts the combination connection of steel pipe pile and Larsen steel sheet pile to form an integral steel continuous wall composite structure, which is a recyclable steel continuous wall for green construction. According to the construction technical guidance of the imported PC construction method combined with the actual situation of the project, the selection of construction technology and equipment is as follows:

1) The first section of steel pipe pile and Larsen steel sheet pile sinking equipment adopts Hitachi 470h crawler full hydraulic full swing long arm vibrating hammer (hydraulic long arm vibrating hammer). The equipment has large operation radius and flexible loading, can freely adjust the direction and verticality of steel pipe piles, and has high construction efficiency.

2) During the construction of section 2 steel pipe pile, crawler crane and 28rf electric vibrating hammer shall be used for pile sinking. The maximum exciting force of the vibrating hammer is 786kn and the pile pulling force is 400kN, which can fully meet the construction needs of the project.

3) The PC construction method combined steel pipe pile at the corresponding position shall be constructed with ice 28rf resonance free hammer. This type is a high-frequency vibrating hammer with low construction noise and little impact on the surrounding soil, which can realize pollution-free construction.

According to the trial driving situation, the steel pipe pile is slightly better due to its high stiffness. When it is separately buried, the high-power hammer can be used for smooth construction. However, the rigidity of Larsen steel sheet pile is small and it is difficult to penetrate into the soil. That is, if the high-power vibrating hammer is used to squeeze it into the soil, the Larsen steel sheet is also prone to distortion. Due to the uneven hardness of the soil layer, this distortion exists in both horizontal and vertical directions, resulting in the locking phenomenon of the steel pipe pile when the steel pipe pile is inserted into the soil along the latch of Larsen steel sheet pile, which makes it very difficult for the steel pipe pile to sink due to the distortion of the latch, Even the heat generated by friction between the locks is too large to melt. Finally, the lock between the steel pipe pile and Larsen steel plate pile is torn, and the steel pipe pile continues to be buried. In order to avoid this phenomenon, steel pipe piles with appropriate stiffness shall be adopted.

1) Measurement and setting out. According to the survey control network of the project, the center line of the row of piles shall be determined first, then the center position of the first steel pipe pile shall be determined, and the construction shall be carried out along the center line according to the construction sequence. The axis measurement and setting out shall be reported to the supervising engineer for review and confirmation after review.

2) Alignment and verticality calibration of pile position. The key of pile position alignment is to ensure that the pile row construction position is on the positioning center line. Before construction, a theodolite shall be erected on the center line of the row of piles, about 25m away from the construction pile position, and the verticality of the steel pipe pile and the center position of the guide groove shall be observed from the angle of the center line, so as to ensure the verticality and avoid the center position of the row of piles from exceeding the axis position. In addition, another theodolite shall be erected at the position where the first theodolite forms an angle of about 90 ° with the pile position to check the verticality deviation. First lift the section 1 pile and insert it into the underground arranged pile position. First insert it into the soil for 30 ~ 50cm. After the insertion is stable, use two theodolites to carry out vertical correction at the same time until the verticality of the pile meets the specification requirements (the vertical correction shall not be carried out after the pile is driven into the soil for 3M. If there is any deviation, it shall be pulled out and re inserted).

3) Positioning and verticality control of the first pile construction. Because the size adjustment of the guide groove connected between the steel pipe pile and Larsen steel sheet pile is limited in the transverse direction, and the guide groove is consistent with the center line of the row of piles, the positioning and verticality control of the first pile is particularly critical.

4) Section 1 steel pipe pile sinking. Section 1 after the alignment and perpendicularity of steel pipe pile (lower section) are corrected, the crawler type full hydraulic full rotation long arm vibrating hammer (hydraulic long arm vibrating hammer) shall be used for vibration driving. Firstly, the pile weight and hydraulic vibrating hammer weight shall be pressed down. After the pile sinking is stable, the vibrating hammer shall be started, and the working arm shall continue to press down and drive by vibration until the upper end of the steel pipe is about 1m away from the bottom of the groove. During pressing and vibration driving, use two theodolites to observe the perpendicularity.

5) Connection of steel pipe piles. The 2m long inner liner pipe is used as the connecting medium material at the butt joint of steel pipe pile to strengthen the lateral stiffness of the joint and facilitate the butt welding of steel pipe. There are 12 holes at both ends of the inner liner pipe, and there are also 12 holes at the connecting end of the upper and lower steel pipes corresponding to the holes of the inner liner pipe. They are connected with the inner liner pipe with high-strength bolts to temporarily fix each component (inner liner pipe, upper and lower steel pipes) before welding. The welding quality shall be ensured during butt welding, and the weld shall be full. In order to ensure that the upper and lower energy-saving steel pipe piles are pulled out smoothly, four 20cm steel pipes are welded at the connection between the upper and lower steel pipes × 20cm arc steel plate. All four sides of the steel plate shall be welded and the weld shall be full.

6) Section 2 steel pipe pile sinking. Since it cannot be processed at one time, the pipe pile shall be welded first and driven underground after cooling.

7) Larsen steel sheet pile sinking. After the steel pipe pile is sunk to the design elevation, the long arm vibrating hammer lifts the Larsen steel sheet pile, aligns with the connecting lock port of the steel pipe side wall, vibrates and sinks, corrects the verticality and the center line of the Larsen steel sheet pile in the direction of the pile row axis, and vibrates and sinks to the design elevation after meeting the design requirements.

8) Closure of row piles. The combined steel pipe pile row piles of PC construction method are constructed along the axis direction. When they are closed at the end, there are inevitably dimensional deviation and perpendicularity deviation of the closure section, and the regulated steel pipe piles or Larsen steel sheet piles cannot be closed smoothly. At this time, steel plate latch plane connection can be adopted.

epilogue :PC construction method combined steel pipe pile has high strength and good bending and shear resistance, which can better control the risk of foundation pit deformation and collapse and ensure the overall stability of foundation pit. It uses recyclable finalized materials to save the investment of retaining structure, shorten the construction time of foundation pit retaining, reduce the rental cost of foundation pit retaining machinery, and save the occupation of construction funds, so as to save the construction cost.

reference:

[1] Chen Shengxian, Xu quanbiao. Application of PC construction method pile in foundation pit engineering of a project in Deqing [J]. Shanxi architecture, 2016, 42 (33): 8041

[2] yuan Jiao, Zheng Mingxin, Wu Gang. Comprehensive analysis of pit support problems in a deep foundation pit project [1]. Construction technology, 2016, 45 (17): 25-28

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