Construction principle of bored pile
In hydraulic engineering, the construction principle of bored pile is to drain the water or mud at the bottom of the hole quickly with enough first batch of concrete pouring, and wrap the outlet of the conduit in concrete for a certain depth at one time, so that the concrete poured behind is always isolated from the water or mud in the hole, and then continuously input high-fluidity concrete into the pile hole through the conduit. In the process of concrete input, with the continuous increase of concrete in the hole, the conduit is continuously lifted, but the outlet must be buried in the poured concrete to a certain depth. The subsequent concrete is pressed into the poured concrete by the pressure formed by the height difference in the sealed conduit between the discharge port and the feed port. Part of the first batch of poured concrete is propped on it as an isolation layer, and has been pushed to the pile mouth by the continuously poured concrete.
Pile foundation investigation in the early stage of construction
Reasonable selection of bearing layer at pile tip
The bearing layer at the pile tip refers to the soil (rock) layer that plays a major supporting role for the pile in the foundation. No matter what kind of pile type, there is a problem of selecting the bearing layer at the pile end reasonably. Even for friction piles, there is a problem of pile tip selection in the stratum with relatively large lateral resistance. Choosing the bearing stratum economically and reasonably in the design requires the surveyors not only to stratify the bearing stratum according to the genetic types and geotechnical properties, but also to stratify the bearing stratum carefully.
Correctly provide the standard values of pile side resistance and pile end resistance
The pile side resistance and pile end resistance are the key points of pile foundation design. At present, China mainly determines the table according to the state and density of soil and related specifications. These tables come from a large number of pile load tests and engineering practice experience, and are generally credible and reasonable. However, attention should be paid to avoid the tendency of being too mechanical and simplistic in actual selection. Theory and engineering practice have proved that pile side resistance and pile end resistance are influenced by many factors, and geotechnical engineers should fully understand and consider these factors when providing suggestions on standard values of pile side resistance and pile end resistance.
Correctly estimate the possibility of pile formation
When selecting the bearing stratum at the pile tip according to the geological conditions and stratum distribution characteristics, we should also fully consider whether the pile can reach the selected bearing stratum smoothly. For example, for precast piles, when a suitable bearing layer is selected in the lower part, but a thick sand layer is distributed in the upper part of the rock, the possibility of piling or pressing piles must be fully studied and judged; For bored piles, when there is a good bearing layer on the lower part and a poor silt layer on it, it is necessary to fully estimate and study the possibility of shrinkage and pile breakage during drilling and underwater concrete pouring.
Suggestions on pile type selection and pile foundation design and construction are put forward.
After fully studying the above problems, the geotechnical investigation report is put forward, including geological conditions, stratum distribution, physical and mechanical parameters of each geotechnical layer, recommended values of pile side resistance and pile tip resistance, and relevant suggestions are put forward for pile type selection, design and construction.
Quality control in construction
Pile position measurement
Determine the pile position on the leveled site, accurately mark the center and elevation of each pile position with square piles, and bury the protective piles at the same time. Burying method of pile protection: three piles with a diameter of more than 50cm are evenly distributed in the center of the pile, and the distance is measured. The top of pile protection should be flush with the ground, fixed firmly with mortar and clearly marked. The positioning of deep water pile foundation is fixed by steel casing positioning frame.
Buried casing
Steel liners are used, the steel liners of the main pier on the water are rolled with 12mm thick steel plates, and the steel liners with a diameter of 2.5m are rolled with 14mm thick steel plates, and the rest are rolled with 10mm thick steel plates. The inner diameter of the casing is 20cm~40cm larger than the diameter of the drill bit, and the casing height depends on the soil quality, and the minimum is not less than 2m. During installation, the top of the pile casing is more than 30cm above the ground, which is higher than the highest construction water level or underground water level1.5m ~ 2.0m.. Compacting the clay within 50cm around the pile casing of the dry pier to the bottom of the pile casing, and the water head in the pile casing is stable. The embedding position of the liners must ensure that the deviation between the center and the center of the pile location is not more than 50 mm, and attention should be paid to the connection quality of the two liners. The buried depth of the liner is 2m~4m, and the liner of the main pier on the water should sink below the local scour line not less than1.0m ~1.5m. ..
Drilling and pore-forming
First of all, when installing the drilling rig, the underframe should be stable, the center of the drill bit and drill pipe should be aligned with the center of the casing top surface, and the deviation should not be greater than 50 mm Secondly, the drill pipe (lifting rope) should be kept vertical when drilling, and the drill pipe should be checked with a wire hammer at any time during construction to avoid drilling inclination. Finally, it is suggested to use low-grade slow uniform drilling, especially at the casing outlet, to avoid instability and leakage. Control ROP and WOB at the site of formation change to prevent well deviation. For easy-to-shrink soil layer, cooperate with necessary re-drilling. When drilling in hard plastic clay layer, medium and high penetration rate should be adopted, and when drilling in silt and silt layer, low penetration rate should be adopted. When drilling from hard layer to soft layer, the penetration rate can be appropriately increased, and when drilling from soft layer to hard layer, the penetration rate should be reduced with less pressure.
Deviation of pile position and verticality
The deviation of pile position refers to the deviation between the center of pile hole and the original design center. There are two inspection methods: measuring the center of the pile casing before foundation pit excavation and measuring the center of the pile after foundation pit excavation, which belongs to the master control project. Vertical deviation refers to the angle between the vertical line of the drilling horizontal plane and the drilling axis, which is a general project. If they have errors, the construction accuracy will be inaccurate and the piles will not be docked, which will bring considerable trouble to the later construction. Then the following aspects should be paid attention to when controlling the deviation of pile position and the verticality of pile hole: firstly, according to the design pile position plan and the relevant measurement data on site, calibrate the site datum line and datum line, measure the pile position and the ground elevation of pile position, and supplement the measurement control points as needed. After the pile point is determined, the iron standard pile should be driven to prevent artificial movement or damage. Secondly, before the pile casing is buried, four control points are drawn from the measured pile points by cross positioning method and marked. The material selection and structural design of the casing must have certain strength and rigidity, and be convenient for transportation, installation, burial and hoisting and disassembly. When the rig is installed again, the pile point has been virtualized. Ensure that the foundation is firm, the drilling rig is horizontal, upright and reliable, and the center of traveling block, gyrator and pile point are on the same vertical line. In the process of drilling, you should also check whether the drilling rig is horizontal, geometric and reliable at any time, and make corrections when necessary. At the same time, in the early stage of drilling, it is necessary to gently press and rotate slowly, and the drill bit cannot collide and crush the casing. After the drill bit passes through the bottom of casing, the drilling speed and WOB can be gradually improved. After normal drilling, the drilling technical parameters should be mastered reasonably, and drilling tools should not be lifted at will or WOB should be added blindly to avoid well deviation and man-made accidents in the hole.
Fabrication and hoisting of reinforcing cage in place
According to its length, the reinforcing cage should be prefabricated in whole or in sections, and the skeleton of the reinforcing cage should have sufficient rigidity and stability to avoid looseness and deformation during transportation, hoisting and concrete pouring. During fabrication, a steel bar is added every 2m, and rings are reasonably arranged at the upper end of the skeleton according to actual needs. On the outer side of the main reinforcement of the skeleton, weld positioning steel bars on the main reinforcement of the skeleton, with no less than 4 bars per 1.0m~ 1.5m to control the protective layer. Reinforcing cages shall be lifted in sections in time and accurately, extended to the design depth quickly and fixed, and the fixing facilities of reinforcing cages shall be lifted only after the concrete pouring is completed and the initial setting is completed. Before the reinforcing cage is in place, it is still necessary to check whether there is any hole collapse, so as to take timely measures.
Pouring of underwater concrete
After the pouring pile concrete is transported to the pouring site, its uniformity and slump should be checked. If it does not meet the requirements, it should be resolutely used to prevent pipe plugging. The strength grade of concrete must meet the design requirements, such as sand and gravel, cement, water, etc. Should comply with national standards, and the commercial concrete used must undergo on-site quality inspection. When pouring underwater concrete, the hole depth below the water surface or mud surface and the height of pouring concrete surface should be detected to control the thickness of sediment, the depth of buried pipe and the height of pile top. If the detection is not accurate, the sediment will be too thick, the conduit will leak, and the conduit will be buried too deeply, thus causing accidents such as broken piles, short piles or failure to pull out the conduit. In the process of underwater concrete pouring, the height of concrete surface must be detected, and the corresponding calculation should be made according to the detected height of concrete surface and the poured concrete quantity, so as to check whether there is serious local over-diameter, shrinkage and leakage of the bored pile, and at the same time observe the flooding situation, so as to correctly analyze and judge the situation in the hole and avoid the occurrence of construction accidents. The buried depth of conduit shall be strictly controlled at 2m-6m. It is forbidden for construction personnel to dismantle several sections for the convenience of one-time over-irrigation. It is necessary to check frequently and adjust the buried depth of the conduit in time to prevent the pipeline from being blocked due to too deep buried depth. After the construction, the parameters of underwater concrete pouring should be calculated to provide reference and improvement for the subsequent pile foundation.
label
In water conservancy construction, the construction of bored piles is mostly carried out underground. Before construction, we must be familiar with the design drawings and relevant construction and acceptance specifications, check the geology and relevant data of bored piles, analyze the possible accidents during the construction of bored piles, formulate construction quality standards, acceptance implementation plans and treatment measures to deal with accidents, effectively control the quality of pile foundation construction and ensure the safety and quality of foundation engineering construction.
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