Keywords: static pressure; static pressure prestress; pipe pile construction 

Static pressure prestressed pipe piles have the characteristics of high single pile bearing capacity, fast construction progress, low noise, less pollution, strong ability to penetrate soil layers, convenient on-site construction, good quality control, low material consumption of pile body, and low cost per pile. They are currently widely used. However, the traditional hammering method often emits oil smoke and noise that pollute the environment, seriously affecting the living of surrounding residents. The static pressure high-strength prestressed pipe pile, due to its large single pile bearing capacity, stable quality, low noise, and no vibration, has been widely applied and has broad application prospects. 

I. Control of static pressure pile construction method 

1. Before construction, measurement baselines and benchmarks should be set up, and the baselines should be set in places not affected by construction. 

2. Pile concrete 

The soil must reach 100% of the design strength before transportation to the site. When lifting, bind firmly, and the lifting points should meet mechanical principles. Set lifting points at 0.2 meters from the top of the pile, add padding between the sling and the pile, lift evenly to avoid collision and vibration. When stacking piles, classify by length, stack on solid and flat ground, and set the load-bearing points near the lifting points at 0.2 meters from the end. The stack height should not exceed 2 layers, and the staggered length of piles at both ends should not be less than 10 cm. 

3. Positioning of pile lifting points: use the crane hook attached to the pile frame to lift the pile into position. 

4. Adopt static pressure method for construction. The pile frame mast and pile cap embed the prestressed pipe pile. Between the two slideways of the pile frame, after the pile position and verticality are corrected, start pile sinking. Carefully check the pile body quality when positioning. When driving piles, use a steel pile driver placed on the pile head to drive the pile in. During construction, ensure the pile driver and the engineering pile are aligned, with axis coincidence as the criterion. When the engineering pile is driven to the design depth, the pile driver can be pulled up. Use the hook on the guide pulley wire rope on the pile frame to pull up the pile driver, start the winch, and slowly pull it up. 

5. When the first pile section is pressed to 1 meter above the ground, lift the second pile section, align it with the bottom section, and recheck the verticality. After confirming no error, start welding. After welding meets requirements, press and sink the pile again. When the pile top is 1 meter above the ground, lift the second pile section and continue positioning. Recheck welding verticality and weld, sink the pile until completion. Before welding, check the contact surface of the upper and lower piles. Then recheck the verticality and concentricity of the upper and lower pile sections. Confirm no error or very small error before full welding. The weld seam should be fully welded in two passes, continuous and full. Remove welding slag after welding. Pile splicing should be done quickly to ensure continuous construction as much as possible. 

II. Key points of quality control during construction 

1. Quality control before construction ① Conduct pre-construction review. First, review the construction personnel to understand their technical strength and level, and ensure that each person in the construction team holds a certificate. In addition, strengthen the review of the construction organization design and construction plan progress, then evaluate its feasibility to ensure safety measures are reasonable and in place. ② Strengthen the inspection of pipe piles before entering the site. First, check the test report and specifications of the pipe piles (including outer diameter, wall thickness, pile length, pile bending, etc.) to prevent inferior or non-compliant products from entering the site. Second, according to specifications and drawing design requirements, conduct a comprehensive inspection and measurement of the pile tip. If it does not meet relevant specifications and design requirements, order replacement. In addition, check whether the surface of the pipe pile meets the requirements of the pipe pile specification, including whether there are cracks, fractures, and depressions. ③ Reasonably select and inspect the pile pressing machine. The selection of the pile pressing machine cannot be blind. According to the specific conditions of the project and design requirements, reasonably select the pile pressing machine in conjunction with relevant departments to avoid overload construction. In addition, during construction, if the pile pressing machine fails and stops working, discontinuous pile pressing will occur, and in severe cases, the pipe pile cannot be pressed in, directly affecting the construction quality of the pipe pile. Therefore, great attention must be paid to the reasonable selection of the pile pressing machine. ④ Positioning and setting out and pile position setting. Whether the setting out and pile position are accurate directly affects the position and structure of the building. Therefore, these two processes must be strictly controlled and must not be underestimated. According to the provided measurement control points, place the baseline in a place not affected by construction, then specify the positioning line according to the design drawings, and finally have the project technical personnel review it. Correct any non-compliance in a timely manner, and only proceed with pile pressing construction after confirmation. 

2. Quality control during construction ① Before the pile is positioned and inserted into the soil, use a theodolite to spot-check the deflection of the pile. Pipe piles with unqualified deflection are strictly prohibited from being used in the project. Due to the static pressure method, the mechanical equipment is heavy and easily deforms the soil, causing pile position deviation. Therefore, the positioning and verticality of the pipe pile must be strictly controlled. 

The verticality of the pile is controlled by hanging a plumb line in two vertical directions, and a dedicated person is required. The verticality deviation should be controlled within 0.5%. ② When driving piles, place a steel pile driver on the pile head, with axis coincidence as the criterion. Ensure the pile driver and engineering pile are aligned before driving the pile in. When the design depth is reached, the pile driver can be pulled up. Use the hook on the guide pulley wire rope on the pile frame to pull up the pile driver, start the winch, and slowly pull it up. During pile insertion, if hard soil layers are encountered, do not use excessive force. Pipe piles have weak bending resistance and are often easily broken. Also, lift and place gently, otherwise it may cause pile body cracking or pile frame tilting and collapsing accidents. ③ Before pile splicing, check whether there is significant misalignment between the centers of the two pile sections to ensure the straightness of the two sections. Therefore, it is best to set a pile guide hoop. Since welding is used for pile splicing, before welding, clean the butt joint surfaces and grooves of the two pile sections until metallic luster is exposed. During welding, weld 4-6 points symmetrically around the groove circumference. It is best to have two welders weld symmetrically at the same time. After welding one layer, the slag must be cleaned before welding the outer layer, ensuring at least 2 layers. Since weld seams become brittle when exposed to water and may cause crushing, after welding, allow natural cooling for about 8-10 minutes (strictly prohibit water cooling or pressing immediately after welding), then continue pressing the pile. When pipe piles are densely arranged and there are large cracks at the pile joints, soil heave caused by pile pressing may pull the pile joints apart, causing pile breakage. 

3. Quality inspection after pile formation After the pile foundation is completed, the quality of the pipe piles should be tested and evaluated according to relevant standards and design requirements. Use a pile testing system for low-strain testing to detect pile integrity. That is, install a response sensor on the top of the pile and apply a hammer blow by hand. The pile testing system collects and processes signals to obtain pile integrity test results. For pile body quality inspection, a visual method is generally adopted: sink a low-voltage bulb into the pile cavity. If the pile cavity is intact and dry, it indicates that the pile body quality is basically intact without damage. After pile formation, select representative piles (generally first take more than 20% and not less than 10 piles), then use static load tests to determine the vertical bearing capacity of single piles. Require relevant technical management personnel to supervise on site and cooperate in recording the final settlement and residual settlement. Then select representative piles for static load tests, preferably more than 3 piles and more than 1% of the total number of piles. 

III. Safety control of static pressure pipe pile construction 

Construction personnel should establish safety production awareness on site: ① Before starting the crane, conduct a systematic inspection to check whether components are loose, whether the wire rope is broken or worn, and ensure lubrication of rotating parts. After the crane is started, it is strictly prohibited to stand near the crane to prevent falling objects. ② The lifting range of the crane should be controlled within the specified performance indicators. When lifting pipe piles, the rigging should be hung at more than 1/3 of the length of the pipe pile and maintain a distance of more than 1m from the pile end. Before pile pressing starts, the lifting wire rope must be loosened. During pile pressing, the hook should always be in an unstressed state, and finally the rigger manually unhooks to avoid breaking the wire rope and bending the crane boom, and to avoid accidents such as broken piles falling from heights. ③ During pile splicing, when welding with a welding machine, install a protective cover and provide a dedicated effective ground wire. Various gas cylinders should be labeled. The distance between gas cylinders and open flames should not be less than 10mm, and the distance between gas cylinders should not be less than 5m. In addition, gas cylinders must be equipped with shock-proof rings and protective caps. Gas cylinders are strictly prohibited from being laid flat or upside down during use and storage. ④ During pile pressing, construction personnel must concentrate and not walk around or do other things. Non-workers should stay more than 20m away from the pile machine. If cracks are found in the pile body during pile pressing, stop pressing immediately. Forcibly continuing construction can easily cause pile breakage and injure workers and mechanical equipment. ⑤ After the pile machine is installed, organize relevant personnel for inspection and acceptance. Check whether components vibrate abnormally during operation, whether electrical equipment is installed with leakage protection switches as required, and whether cables are randomly connected or dragged on the ground.