Discussion on foundation design of punching cast-in-place pile in high-rise building?

With the rapid development of China's economy, there is less and less urban construction land in China. The development and use of high-rise buildings have greatly improved the utilization rate of urban land, and at the same time put forward higher requirements for construction enterprises. Punched cast-in-place pile is an important part of high-rise building. Reasonable and effective design and construction according to different geological conditions is the key to ensure the quality of high-rise buildings. This paper briefly discusses the design and construction points of punching cast-in-place pile in high-rise building.

In recent years, the rapid development of China city makes the available urban land less and less, and the increasing urban population makes the per capita land less and less. In order to alleviate the per capita land reduction caused by the increase of urban population, municipal departments, on the one hand, expand the urban area, develop the original suburbs and increase the urban land area. On the other hand, actively carry out the transformation of the old city and improve the land utilization rate by building high-rise buildings. Punched cast-in-place pile is a widely used pile type in pile foundation engineering of high-rise buildings. In high-rise buildings, large-diameter, deep-long or rock-socketed cast-in-place piles often become the main choice for foundation treatment of high-rise buildings because of the high requirements on bearing capacity and deformation (vertical settlement and horizontal displacement) of the foundation.

1. Analysis of the application status of punching cast-in-place pile in high-rise buildings in China

In the pile foundation of high-rise buildings and old city reconstruction, punched cast-in-place piles are widely used because of their low noise, little impact on the surrounding environment and no soil squeezing effect. In the traditional construction technology of bored pile, in order to avoid hole collapse, mud must be used to protect the wall, but the mud skin formed by mud wall protection affects the play of pile side friction; At the same time, due to the construction technology, the sediment at the bottom of the hole is not easy to remove, which also affects the play of pile tip resistance. Therefore, the common method at present is to adopt post-grouting technology in punching cast-in-place piles, the main purpose of which is to reduce or eliminate the influence of mud wall protection and sediment at the bottom of the hole on the ultimate bearing capacity of single pile and improve the bearing capacity of single pile. Bored cast-in-place pile can pass through all soil layers, but the bearing capacity of the pile will be affected by mud wall protection when it is bored, and the sediment at the bottom of the pile is not easy to remove. If the pile tip enters the weathered rock, the cost will be higher. If the bearing layer at the end of the pile is selected on the gravel layer and post-grouting technology is adopted at the same time, the pile length can be greatly shortened. Grouting can eliminate or compact the mud skin on the side of the pile and the sediment at the bottom of the pile, and improve the bearing capacity of the end of the gravel layer, thus greatly improving the bearing capacity of the single pile and reducing the project cost.

Second, the foundation design of punching cast-in-place pile in high-rise building

Before the design of punching cast-in-place pile in high-rise building, the foundation form should be selected and the engineering requirements should be designed according to the local construction experience and the on-site geological conditions.

(1) foundation design of punching cast-in-place pile in high-rise building

When designing the foundation of punching cast-in-place pile in high-rise building, the geological conditions of the building should be determined by exploring the geology where the project is located. Then, determine the choice of pile foundation. In the case of heavy building load, strict deformation control and poor geology of the proposed project, the construction purpose should be achieved through the comprehensive application of various cast-in-place piles. Through analysis and investigation, the position and quantity of punching cast-in-place piles are determined and calculated effectively. Firstly, the pile length is determined, and then the ultimate bearing capacity of single pile is estimated. The standard value of vertical ultimate bearing capacity of piles is calculated according to the standard formula quk = qskqpk = u ∑ qsikliqpkap (d < 800 mm) and (d ≥ 800 mm). In addition, more attention should be paid to the consideration of each sub-coefficient. With the increase of the variation coefficient of live load, the partial coefficients of pile side resistance, end resistance and soil reaction at the bottom of pile cap decrease, and the partial coefficient of dead load decreases correspondingly, while the partial coefficient of live load increases correspondingly. With the increase of the variability of the friction angle or viscous force of the bottom soil of the pile cap, the partial coefficients of pile side resistance, end resistance, dead load effect and live load effect decrease, and the partial coefficient of the bottom soil reaction of the pile cap increases accordingly.

After the design of punching cast-in-place pile in high-rise building is completed, in order to ensure the quality of pile foundation, engineering piles should be selected as test piles according to the ratio of 1% and not less than 3 piles (not less than 2 when the total number of engineering piles is less than 50), and the pile foundation can be fully constructed only after the static load test of the test piles is qualified.

(2) Matters needing attention in quality control of punching cast-in-place pile design in high-rise buildings.

Matters needing attention in quality control of punching cast-in-place pile in high-rise building should first control the quality of hole formation. The quality of hole formation includes: hole position deviation, pile hole verticality deviation, hole diameter deviation, hole depth, hole bottom sediment thickness, etc. According to the allowable values in the relevant codes, the design requirements are specified, and the actual engineering construction process is strictly controlled according to the regulations. Another factor affecting the quality of punching cast-in-place pile is the quality of hole cleaning. After the drilling reaches the design depth, before pouring concrete, the thickness of sediment at the bottom of the hole should meet the following requirements: end-bearing pile ≤ 50 mm; Friction end bearing and end bearing friction pile ≤100 mm; Friction pile ≤300mm. Therefore, in the process of hole cleaning, the mud should be constantly changed until underwater concrete is poured, and the mud proportion within 500mm of the hole bottom should be maintained.

(3) Analysis of the perfusion design of punching cast-in-place pile in high-rise building.

Because the punching cast-in-place pile in high-rise building is the key to the quality of the whole project, excellent design should be carried out in advance for concrete pouring. According to the relevant requirements of relevant codes, design the concrete mixture ratio, pouring quantity and concrete mixing time suitable for the project, and make reasonable design according to different geological conditions and engineering conditions. Under the designed mixing amount, the interval time of grouting should be controlled at 65438 05 minutes, and the longest time should not exceed 30 minutes. The whole grouting process of each pile should be controlled within 4 ~ 6 hours as far as possible to ensure the uniformity of concrete. This requires the calculation of single pile and concrete used before construction, and the concrete should be mixed well through preliminary design planning and calculation before construction. Moreover, the air retention time of concrete has a certain influence on the strength of concrete. Therefore, according to the requirement that concrete should be poured within 1.5h, and should be poured within 1.0h in summer, concrete should be mixed according to the requirements of cast-in-place piles, otherwise retarder should be added. Concrete shall be poured to the specified height above the designed pile top elevation to ensure the concrete quality below the designed pile top elevation.

Third, the test process of punching cast-in-place pile in high-rise building-the test of design effect

In the construction of punching cast-in-place piles in high-rise buildings, steel pipes are generally used in grouting pipe. Grouting pipe is fixed and welded on the reinforcing cage, and the upper end stops at 0.2-0.3m from the ground to prevent the drill pipe from being damaged when moving or replacing the machine. The grouting pipe at the pile end is 50 mm lower than that at the pile end, and it is connected with grouting pipe through special grouting valves at the pile side and at the pile end. In order to prevent pipe plugging, the grouting pipe at the pile end can be changed to grouting pipe with large diameter at the main pile end and grouting pipe with small diameter at the auxiliary pile end, so as to prevent pipe plugging from affecting the grouting quality. In order to ensure the smooth grouting channel, check whether the grouting pipes are connected, and push fine particles such as mud residue and mud skin to the periphery. Before grouting, water pressure test must be carried out. At the same time, the stable pressure of water pressure test is recorded, and its stable water injection pressure can be used as the initial grouting pressure of grouting construction. The water pressure is subject to the pressure channel. In grouting pipe, the water pressure should reach 10MPa in some places that are not unobstructed, and it should be repeated for many times until the pressure passes, so as to ensure the smooth progress and quality of grouting. Grouting will be carried out after 1 week, and the grouting time of each pile is 1-2 hours. Pressure grouting takes the stable pressure of water pressure test 1.5MPa as the initial grouting pressure, and the final grouting pressure is greater than 2.5MPa and the grouting quantity is 1000kg cement as the construction control index. When the grouting pressure of single pile is lower than 2.5MPa and the grouting amount is large, the grouting amount of cement can be appropriately increased without ground, which is generally controlled at around 2000kg; For piles with grouting pressure higher than 7MPa and poor groutability, thick slurry, slow grouting and intermittent grouting are adopted on site, and the termination time is controlled by grouting pressure, the maximum grouting pressure is not higher than 10MPa, and the cement dosage is not less than 800kg. After the grouting of each pile is completed, screw the grouting pipe on the plug immediately to prevent back grouting and affect the grouting effect. Finally, through inspection, it is verified that the design of punching cast-in-place pile in high-rise building meets the engineering requirements.

conclusion

Punching cast-in-place pile in high-rise building is the focus of the whole project design and construction, which has an important influence on the whole project quality. Therefore, in the design of punching cast-in-place piles for high-rise buildings, the geological conditions of the proposed site and the climate characteristics of the construction area should be fully considered, and all links of grouting should be fully considered to ensure the construction quality of the project.

For more information about project/service/procurement bidding, and to improve the winning rate, please click on the bottom of official website Customer Service for free consultation:/#/? source=bdzd