The following is the relevant content about the static pressure construction technology and quality control of human PHC pipe piles brought to you by Zhongda Consulting for your reference.
Based on specific engineering examples, the construction technology and method of static pressure method for PHC prestressed pipe piles are introduced in detail, and prevention and control measures for common problems in pipe pile pressure quality are proposed. The pile foundation inspection shows that the PHC prestressed pipe piles of this project are The pipe piles meet the design requirements. It is pointed out that high-strength concrete prestressed pipe piles have the advantages of reliable quality and single pile vertical bearing capacity, and are worthy of promotion.
Introduction
Prestressed high-strength concrete pipe piles (hereinafter referred to as PHC pipe piles) are suitable for geological conditions with deep bedrock burial, strongly weathered rock formations or thick weathered residual soil layers. It has the advantages of high single pile bearing capacity, low project cost, stable pile quality, easy control of construction quality, and short construction period. It has been widely used in various engineering foundations, while the construction of static pressure PHC pipe piles has no vibration or noise. The characteristics are particularly suitable for the construction of new buildings that are close to adjacent buildings or adjacent to residential areas and schools.
1 Project Overview
The Tengfei New Village expansion project consists of two 12-story small high-rise residential buildings with frame shear wall structures, with a total construction area of ??19087m2. The foundation is designed as a pipe pile, with two pile types: PHC-AB400 (95)-9, 9 and PHC-AB400 (95)-10, 10. The concrete strength of the pile body is C80; the pile tip bearing layer is the 10th and 11th layers. Sandy ginger powdery clay. The design characteristic values ??of the vertical bearing capacity of a single pile are 800kN and 900KN respectively, and the limit values ??of the vertical bearing capacity of a single pile are 1600KN and 1800KN respectively. The static pile pressing method is used for construction, with a total of 537 PHC pipe piles. After the completion of the project, 4 piles were selected for static load testing. The ultimate bearing capacity of the 20m pile length static load test was 1980kN. The ultimate bearing capacity of the 18m pile length static load test was 1980kN. 114 piles were randomly selected for low strain variation testing. As a result, there were 100 type I piles and 14 type II piles. The test results show that the quality of the pile foundation project all meets the design and specification requirements.
2 Engineering geological conditions
According to engineering survey data, the soil characteristics related to the construction of static pressure PHC pipe piles are as follows:
(1) Miscellaneous fill soil : Grey-brown to brown, layer thickness 0.73m;
(2) Silt soil: light brown-yellow, slightly wet and dense, layer thickness about 0.97m;
(3) Clay : Light gray-yellow brown, mainly soft plastic, high dry strength and toughness, layer thickness 1.14m;
(4) Silt soil: light gray-brown, soft plastic-plastic, slightly wet- Dense, layer thickness is 1.5m;
(5) Clay: earthy yellow-brown-brown, soft plastic-plastic, medium dry strength and toughness, layer thickness is generally 1.15m;
(6) Fine silty sand: light gray-brown-yellow, wet, loose, layer thickness is 1.78m;
(7) Clay: earthy yellow-dark gray-gray green, mainly plastic , with medium dry strength and toughness, and a layer thickness of 3.34m;
(8) Ginger sand silty clay: gray-green-brown, plastic-hard plastic shape, with medium dry strength and toughness, and a thick layer 3.88m;
(9) Fine silty sand: mainly yellow, wet, medium dense, mainly composed of quartz and feldspar, with a layer thickness of 1.24m;
(10) Contains Ginger silty clay: earthy yellow, plastic-hard plastic, medium dry strength and toughness, layer thickness 6.77m;
(11) Ginger silty clay: brown-yellow, plastic-hard plastic Shape, dry strength and toughness are medium, not exposed.
3 Construction Technology
3.1 Machinery Selection
Since the construction site is close to residential areas and middle schools, in order to reduce noise pollution during construction, static pipe pile construction is used. For the pressure pile method, the YZY-500 pressure-holding static pressure pile driver produced by Wuhan Pile Driver Factory is selected. This model uses double clamps. Because the clamps are longer, it can more effectively ensure the verticality of the pile body and can be used in The friction force is increased without increasing the pile clamping force, and the lateral pressure on the pile body is reduced.
3.2 Process Flow
Pile position measurement → Pile driver in place → Pile lifting and pile insertion → Pile body centering and straightening → Static pressure of the first pile → Pile welding → Press the second pile → Send the pile to the top of the pile to the design elevation → Move the pile after completion.
3.3 Construction preparation
3.3.1 Site preparation. The construction site should be as flat as possible. When there is a slope, the slope should not be greater than 1/100, and the site's ground endurance should be no less than 140kN/m2.
Pile driver working distance requirements: The distance between the center of the pile position and surrounding buildings (structures) should be greater than half the width of the pile driver.
3.3.2 Preparation for pile position measurement. Stake out according to the datum point, lead the axis control point 6m to 8m outside the site, establish a measurement control network, and then release the pile position of each pile according to the control line and the pile position size of the design drawing. During the construction process, Frequently review pile positions to prevent pile displacement due to construction and other reasons.
3.4 Pile lifting and pile insertion
3.4.1 The pile driver is in place. Move the pile driver to the pile position, level the pile driver, use the on-board crane to lift the pipe pile and slowly insert it into the holder of the pile driver. When the lower end of the pipe pile is 20cm away from the ground, gradually tighten the holder to clamp the pile. The pressure should not be greater than 5MPa and should be pressurized successively.
3.4.2 Pile insertion. Align the center of the pipe pile with the center of the pile position, then lift the pipe pile a little and perform pile tip welding. Depending on the actual conditions of the project, pile pressing without pile tips can also be used. When the pile is inserted, the machine platform should be leveled according to the level gauge on the machine, and the theodolite should be used for calibration in two vertical directions to monitor the verticality of the pile body, and the deviation should not be larger than 0.15.
Pressing the first section of the pile well is the key to ensuring the quality of the entire pile. If the verticality of the pile body is too large, the pile must be pulled out and re-inserted, and the level of the pile body must be adjusted according to the theodolite instructions to make the pile body hang down; on-site technicians need to control the entire pile pressing process to ensure the verticality of the pile.
When pressing piles, you should always pay attention to the changes in the pile body and the pressure gauge. When there is a sudden drop in the pressure value, a sudden increase in settlement, a sudden tilt of the pile body, broken concrete in the pile body, obvious uplift of the ground and When adjacent piles float or move excessively, pile pressing should be suspended, and the reasons should be analyzed in a timely manner with the design, supervision and other relevant personnel, and effective measures should be taken before continuing the construction. Each pile should be constructed continuously at one time without any interruption in the process.
3.4.3 Connect piles.
(1) When connecting piles, the pile head of the lower pile section should be 0.8m to 1.1m above the ground to facilitate pile connecting and welding operations. Guide hoops should be installed at the joints of the lower piles to facilitate the placement of the upper piles, and the docking deviation of the upper and lower piles should not be greater than 2mm.
(2) Before pile docking, the surfaces of the upper and lower end plates should be cleaned with a wire brush, and the grooves should be brushed until the metallic luster is exposed. When the gap at the pile end is large, it can be filled with iron sheets and then welded.
(3) When welding piles, it is recommended to symmetrically spot weld 4 to 6 points on the circumference of the groove. After the upper and lower pile sections are fixed, remove the guide hoop, and then weld in layers. There should be two welding points. The welders proceed symmetrically.
(4) The number of welding layers is generally three, and shall not be less than two. The first layer must be made with <312mm welding rod to ensure root penetration. For the second layer, thick welding rod (<4mm or <5mm) can be used. The welding rod should be E43XX type, and the inner layer of welding slag must be cleaned. When welding the outer layer, the weld seam should be full and continuous.
(5) Shorten the pile connection time as much as possible. The welded pile joints should be naturally cooled before pile pressing can be continued. The natural cooling time should not be less than 8 minutes. It is strictly forbidden to use water cooling or apply pressure immediately after welding. .
3.4.4 Final pressure. This project is a friction pile, and the pile pressure control is controlled according to the designed pile length. However, before construction, three piles are tested according to the designed pile length. After being parked for 24 hours, the final pressure equal to the designed ultimate bearing capacity of the piles is restored. Pressure, the result of the re-pressure pile body is stable, can be controlled accordingly; for end-bearing piles or end-bearing friction piles, control is based on the final pressure.
4 Common problems with static pressure pipe piles and their prevention and treatment methods
4.1 Pile body fracture
4.1.1 Phenomenon: During the pile sinking process, the pile body Sudden tilt dislocation, when there is no special change in soil conditions at the pile end, but the penetration gradually increases or suddenly increases, then the pile body may break.
4.1.2 Reasons:
(1) The slenderness ratio of the pile section is too large, and the pile sinking encounters a harder soil layer;
( 2) When making piles, the bending of the pile body exceeds the prescribed limit, the pile tip deviates greatly from the longitudinal axis of the pile, and the pile body tilts or bends when sinking;
(3) After the pile is inserted into the soil, it encounters a large hard surface Obstacles push the pile tip to one side;
(4) The pile is not vertical when stabilizing it. After it is pressed into the ground to a certain depth, it is corrected by moving the pile machine frame to make the pile bend. ;
(5) When connecting piles, the two connected piles are not on the same axis, causing bending;
(6) The concrete used to make the piles is not strong enough. Cracks or fractures in piles during stacking and lifting were not discovered.
4.1.3 Preventive measures:
(1) Before construction, the obstacles under the pile positions should be cleared, and if necessary, each pile position should be drilled to understand;
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(2) During the process of stabilizing the pile, if it is found that the pile is not vertical, it should be corrected in time. When the pile is seriously tilted after being pressed into a certain depth, the method of moving the frame shall not be used to correct it;
(3) During the stacking and lifting of piles, relevant regulations should be strictly followed. Piles that are found to be cracked beyond the relevant regulations must not be used.
4.2 Pile sinking fails to meet the design requirements
4.2.1 Phenomenon: When designing piles, the final penetration and final pile length are the final controls for construction. Generally, one control standard is used as the main control standard and another control standard is used as a reference. Sometimes the pile driving fails to meet the final control requirements of the design.
4.2.2 Reason analysis:
(1) There are not enough exploration points or rough exploration data, and the engineering geology is unclear, especially the undulating elevation of the bearing layer, which leads to design considerations The bearing layer or pile selection is wrong;
(2) The exploration work is based on points instead of areas. It is impossible to fully understand the local hard interlayers and soft interlayers, especially under complex engineering geological conditions. Underground Obstacles. When pile pressing construction encounters this situation, it will not meet the construction control standards required by the design;
(3) The pile body breaks, causing the pile to be unable to continue to be pressed in.
4.2.3 Preventive measures:
(1) Exploring the engineering geology in detail, conducting additional surveys when necessary, and rationally selecting construction machinery, construction methods and pile pressing sequences;
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(2) Prevent the pile body from breaking.
4.3 Pile top displacement
4.3.1 Phenomenon: During the pile sinking process, adjacent piles undergo lateral displacement or the pile body floats.
4.3.2 Reasons:
(1) After the pile was inserted into the soil, it encountered a large hard obstacle and squeezed the pile end to one side;
(2) During the construction of two-section piles or multi-section piles, the two connected piles are not on the same axis, resulting in twists and turns;
(3) The soil is saturated and dense, and when the pile is driven, the soil is squeezed to the ultimate density and Uplift, causing adjacent piles to be floated;
(4) When constructing dense pile groups on soft soil foundations, the pore water pressure caused by pile sinking pushes adjacent piles to one side or float.
4.3.3 Preventive measures:
(1) Take precipitation or drainage measures;
(2) Take measures to excavate the foundation pit first and then press the piles .
4.4 Looseness and cracking at the pile connection
4.4.1 Phenomenon: Looseness and cracking at the pile connection occur during pile sinking.
4.4.2 Reasons:
(1) The surface of the connection is not cleaned, leaving impurities, rainwater, oil, etc.;
(2) Connectors Uneven, with large gaps, resulting in weak welding;
(3) The welding quality is poor, the weld is discontinuous, not full, or contains slag;
(4) Two The joint piles are not in the same straight line, and twists and turns occur at the pile joints. During pile sinking, concentrated stress occurs at the pile joints and the connection is destroyed.
4.4.3 Preventive measures:
(1) Before connecting the piles, the impurities, oil stains, etc. on the connecting parts must be cleaned to ensure that the connecting parts are clean;
(2) Check whether the connecting parts are firm, flat and meet the design requirements. If there are any problems, they must be corrected;
(3) When connecting piles, the two piles should be on the same axis and the embedded parts should be welded It should be smooth and conformable. After welding, check it carefully and continue to press it if it meets the requirements.
Conclusion
After the construction of this pile foundation project was completed, the pile foundation inspection showed that the PHC prestressed pipe piles of the project met the design requirements. Therefore, the high-strength concrete prestressed pipe piles have reliable quality and The advantages of single pile vertical bearing capacity are worthy of promotion.
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