Pile foundations are widely used due to their high bearing capacity, good stability, small settlement and differential deformation, fast and stable settlement, strong seismic performance, and adaptability to various complex geological conditions. Let’s examine the following key points of pile foundation construction.
1, Pile Making
Steel formwork should be used for pile-making formwork, which has sufficient rigidity, and should be flat and accurate in size. There are many methods for making square piles, such as the parallel method, interval method, and overlapping method. Due to site limitations, the overlapping method is more commonly used for on-site production. The number of overlapping layers is determined according to the allowable load of the ground and the construction conditions, but it should not exceed 4 layers. An isolation layer (such as linoleum, kraft paper, plastic paper, paper reinforcement ash, etc.) should be made between piles. The pouring of the upper pile or adjacent pile should be carried out after the concrete of the lower pile or adjacent pile reaches 30% of the design strength. Since the overlapping method construction requires the entire pile to be hoisted and used after the concrete of the upper pile reaches age, the pile can also be made into a stepped shape.
The coarse aggregate of the precast pile concrete should be crushed stone or open pebbles, and the particle size should be 5-40mm. The concrete strength grade is usually C30-C40, and it is advisable to use mechanical mixing and mechanical vibration, pour and compact continuously from the top of the pile to the tip of the pile, and complete it at one time. After production, it should be watered and cured for no less than 7d.
2, Lifting, Transport, and Stacking of Piles
Lifting of piles
Reinforced concrete precast piles should be lifted only when the concrete reaches 70% of the design strength, and can only be transported and piled when it reaches 100% of the design strength. If hoisting is carried out in advance, measures must be taken and the calculation must be qualified before it can be done. When lifting, the lifting point must be reasonably selected to prevent damage during bending during the lifting process. If there is no lifting ring and the design does not specify it, the number and position of the binding points shall be determined according to the length of the pile shall comply with the principle of minimum lifting bending moment, and can be tied according to the position.
Transportation of piles
The concrete strength of the pile body should reach 100% of the design strength during transportation. It can be transported by flatbed trucks, and it should be kept stable during transportation. The pile stacking site should be flat and solid, without uneven subsidence. The position of the pads should be the same as the position of the lifting point and kept on the same plane. The pads of each layer should be aligned up and down.
Stacking of piles
The stacking of piles should be carried out in order according to the requirements of pile driving, and the number of stacking layers should not exceed 4 layers. The pile body should be transported to the piling position and stacked. It should be arranged within the working radius of the lifting hook attached to the piling frame, and the lifting direction should be taken into consideration to avoid turning.
3, Pile Positioning
Technical requirements for pile foundation construction measurement
(1) Technical requirements for building axis measurement. The positioning measurement of the building pile foundation is generally based on the measurement control points or baselines provided by the architectural design or design unit and the relevant data of the new building. First, the building positioning rectangular control network is measured and the building positioning measurement is carried out. Then, according to the building positioning rectangular control network, the building pile axis is measured and set. Finally, the pile position of the foundation is measured and set according to the pile axis.
(2) Technical requirements for elevation measurement. Leveling points are set before elevation measurement. Their positions should not be affected by construction and should be easy to use and preserve. The number is generally not less than 2~3. Elevation measurement can be carried out according to the fourth-class leveling measurement method and requirements. The round trip is poor, and the fit or loop closure error should not be greater than ±20L, where L is the length of the leveling route in km. The elevation measurement of the pile point is generally carried out using the ordinary leveling instrument scattered point method, and the elevation measurement error should not be greater than ±1cm.
Quality control of pile foundation construction measurement
(1) Large wooden piles with a diameter of 8 cm and a length of 35 cm need to be buried in the rectangular grid points of building positioning. The pile positions should be convenient for both operation and preservation, and small iron nails should be nailed on the wooden piles as center marks. The wooden piles should be reinforced with cement for protection. Attention should be paid to protection during construction and inspection should be carried out before use. For large or complex projects with a long construction period, cement piles with a top size of 10 cm × 10 cm, a bottom size of 12 cm × 12 cm, and a length of 80 cm should be buried as long-term control points.
(2) Inspection work must be strengthened, including all calculated data of the pile position measurement layout diagram. A second person must conduct a 100% inspection and confirm that there are no errors before on-site measurement and setting can be carried out. Only after the building positioning measurement results are checked and meet the requirements can the building pile position axis be measured and set for building positioning measurement.
Quality control of pile axis survey
The survey of the building pile axis is carried out after the survey of the building positioning rectangular network is completed. It is based on the building positioning rectangular network, and the internal division method is used to measure the pile axis with the theodolite line precision distance method. The survey of the center point of complex buildings is generally carried out by the polar coordinate method. Small wooden piles should be driven into the guide piles of the measured pile axis, and small iron nails should be nailed on the top of the wooden piles as the center point of the pile axis guide piles. For the convenience of storage and use, the top of the pile is required to be flush with the ground, and white lime should be sprinkled around the guide piles.
After the survey of the pile axis is completed, the length between the pile axis and the length of the pile axis should be tested in time. The difference between the actual distance and the design length should not exceed ±1cm for single-row piles and ±2cm for group piles. The survey of the pile position of the foundation can only be carried out after the pile axis detection meets the design requirements.
Quality control of pile axis measurement
The measurement of the building pile axis is carried out after the measurement of the building positioning rectangular network is completed. It is based on the building positioning rectangular network, and the internal division method is used to measure the pile axis guide piles with theodolite line determination and precision distance measurement. The polar coordinate method is generally used to measure the center point of the circle of complex buildings. Small wooden piles should be driven into the guide piles of the measured pile axis, and small iron nails should be nailed on the top of the wooden piles as the center point of the pile axis guide piles. For the convenience of storage and use, the top of the pile is required to be flush with the ground, and white lime is sprinkled around the guide piles.
After the measurement of the pile axis is completed, the length between the pile axis and the length of the pile axis should be detected in time. The difference between the actual distance and the design length should not exceed ±1cm for single-row piles and ±2cm for group piles. The measurement of the pile position of the foundation can only be carried out after the pile axis detection meets the design requirements.
Quality control of pile position measurement
The measurement of pile positions of building piles is based on the guide piles of the pile axis. The pile foundation design is divided into group piles and single-row piles according to the needs of the ground building. When measuring and setting, the rectangular coordinate method, line intersection method, polar coordinate method, etc. can be used for measurement and setting according to the relationship between the pile position and the axis given by the design. For the measurement and setting of pile positions of complex building piles, the data provided by the design cannot be used directly and must be converted before measurement and setting. After the pile position of the pile is measured, small wooden piles should be driven in as pile position marks and sprinkled with white lime to facilitate pile foundation construction.
After the pile position of the pile is measured and set, it should be tested in time. The difference between the actual distance between the pile positions of the current pile and the designed length should not be greater than ±2cm, and the difference between the actual distance between the pile positions of adjacent piles should not be greater than ±3cm. After the pile point is tested and meets the design requirements, it can be handed over to the pile foundation construction unit for pile foundation construction.
4, Site Preparation
(1) Obstacle handling. Before sinking piles, requirements should be made to relevant units such as urban management, water supply, power supply, gas, telecommunications, and housing management to carefully handle obstacles in the air, on the ground, and underground. A comprehensive inspection should be conducted on the buildings and underground pipelines around the site. If there are dangerous buildings or dangerous structures, they must be reinforced vibration isolation measures must be taken or they must be demolished.
(2) Site leveling. The piling site must be flat and solid. If necessary, roads should be paved and compacted by rollers. Drainage ditches should be dug around the site for drainage. The route of the pile-making materials entering the site and the route of the piles transported to the piling site should not be interfered with.
(3) Leveling and laying out. Leveling points should be set at the piling site. Their positions should not be affected by the piling. There should be at least two leveling points for leveling the site and checking the depth of the piles. The pile positions of each pile foundation should be determined according to the axis of the building.
(4) Before construction, the technical person in charge and the construction workers at the construction site should conduct a comprehensive inspection of the construction preparations for each hole, conduct technical safety briefings and safety education at each level, and ensure that safety and technical management are implemented in terms of ideology, organization, and measures.
(5) A dedicated person is responsible for numbering the pile positions, making records of the vertical center line, axis, pile diameter, pile length, and bedrock soil of the pile holes; hidden acceptance records of the steel cage and pile body concrete, etc., and after completion, compiling and sending them to relevant units and to the technical department for archiving.
(6) The height of the wall template sections depends on the soil conditions, and generally 50~100cm can be used.
5. Key Points of Pile Foundation Testing
(1) During low-strain testing, for rock-embedded piles, the time domain reflection signal at the bottom of the pile is a single reflection wave and is in the same direction as the hammer signal; the measured signal is complex and irregular, and it is impossible to accurately evaluate it; the pile body cross-section is gradually changing or changing, and the change range is large. For concrete cast-in-place piles, the static load method or core drilling method should be used for verification.
(2) During high-strain testing, there are defects in the pile body, and the vertical bearing capacity of the pile cannot be determined, or the pile body defects have an impact on the horizontal bearing capacity; the single-shot penetration is large, the same-direction reflection at the bottom of the pile is strong and the reflection peak is wide, and the side resistance wave and end resistance wave reflection are weak, that is, the waveform shows that the vertical bearing characteristics are inconsistent with the geological conditions in the survey report, then the static load method can be used for further verification.
(3) The same-direction reflection at the bottom of the rock-embedded pile is strong, and there is no obvious end resistance reflection after time 2L/C, and the core drilling method can be used for verification.
(4) Shallow defects in the pile body can be verified by excavation.
(5) Precast piles with cracks in the pile body or joints can be verified by the high-strain method.
(6) When a single-hole core drilling test finds quality problems with the pile body concrete, it is advisable to add a hole for verification on the same foundation pile.
(7) For piles or Class III piles whose integrity category cannot be determined by the low strain method test, the static load method, core drilling method, high strain method, excavation, and other appropriate methods can be used for verification and testing according to the actual situation.
(8) When the results of the single pile bearing capacity or core drilling method sampling test do not meet the design requirements, the reasons should be analyzed and the sampling test should be expanded after confirmation.
(9) When the sum of Class III and IV piles found by the low strain method, high strain method, and acoustic wave transmission method for sampling pile body integrity is greater than 20% of the number of sampled piles, the original test method should be used (the acoustic wave transmission method can be replaced by the core drilling method) to continue to expand the sampling test among the piles that have not been inspected.
6. Common Pile Foundation Problem Treatment Measures
Supplementary sinking method
This method can be used when the prefabricated pile is not deep enough into the soil, or when the pile is lifted due to soil uplift.
Supplementary pile method
The supplementary pile method involves constructing the pile according to the supplementary pile plan issued by the design unit in conjunction with the opinions of the designer, supervisor, and owner. However, this method requires a large investment and a long construction period, and it is difficult to recognize by all parties.
Supplementary delivery combined method
When the pile is driven into the foundation using a segmented connection, if the quality of the pile body does not meet the standard, then during the sinking process, the connection node may be disconnected. At this time, the treatment of the pile foundation can be combined with supplementary delivery. For the piles with doubts, they should be re-driven to make the pile sink, which can make the disconnected pile connection tighten again and make the joint have vertical bearing capacity; supplementary piles can be carried out, and some complete piles can be appropriately added so that the foundation can not only meet the bearing capacity requirements but also increase the seismic load of the building foundation.
Correction method
If the pile body is found to be tilted during the piling process and the pile is not long and intact, or the pile body is tilted due to the excavation of the foundation pit but the pile body is still intact, the pile body can be partially excavated and then the pile body can be corrected using a jack.
Expansion of the pedestal
(1) If the pile position has a large deviation, the original pedestal design size cannot meet the structural normative requirements. In this case, the pile position deviation can be corrected by appropriately expanding the pedestal area.
(2) Pile-soil interaction. If a single pile cannot meet the design-bearing capacity requirements, it is necessary to expand the pedestal and take into account the upper load borne by the natural foundation and the pile.
(3) Pile foundation quality issues. Uneven pile foundation quality is prone to occur during construction. In order to prevent the pedestal layout cloud settlement caused by pile foundation quality issues in the later stage and to improve the seismic resistance of the building, an integral pile foundation pedestal can be used to improve the integrity of the foundation.
Composite foundation method This method
(1) is to use the principle of pile-soil interaction to properly treat the foundation, improve the foundation bearing capacity, and more effectively share the load of the pile foundation. Commonly used methods are as follows.
(2) Make a soil replacement foundation under the pedestal. Before the construction of the pile foundation pedestal, dig out a certain depth of soil, replace it with sand and gravel filling layers, and then construct the pedestal on the artificial foundation and pile foundation.
(3) Add cement soil piles between piles. When the pile-bearing capacity does not meet the design requirements, the method of dry spraying cement in the soil between piles to form cement soil piles can be used to form a composite foundation.
Modify pile type or pile sinking parameter method
(1) Change pile type. For example, change prefabricated square piles to prestressed pipe piles.
(2) Change the depth of piles in the soil. For example, when prefabricated piles encounter thick dense silt sand or silt soil layers during penetration, the piles are difficult to sink or even break. In this case, the pile length can be shortened, the number of piles can be increased, and the dense silt sand layer (expansive soil layer) can be used as the bearing layer.
(3) Change pile position. If a hard, small underground obstacle is encountered during pile sinking, causing the pile to tilt or even break, the pile position can be changed and the pile can be re-sunk.
(4) Change pile sinking equipment. When the pile sinking depth does not meet the design requirements, a large-tonnage pile frame can be used to sink the pile using a heavy hammer low-hit method.
7. Safety Measures
Safety technical measures to prevent collapse
(1) Make good hole mouth protection planning to prevent surface water from entering the hole.
(2) Before excavation, make a good wall protection scheme design according to different geological conditions. During excavation, the geological conditions must be carefully reviewed and the hole wall protection work must be strictly carried out according to different geological conditions.
(3) For hole walls that require concrete protection, excavation must be carried out strictly according to the specified advance, and the wall protection concrete can be removed only after it reaches the design strength.
Safety measures for ventilation in the hole
(1) Manual bored piles should be well-ventilated. When the hole depth is greater than 5m, ventilation pipes should be used to supply air to the hole. Operators should rest outside the hole after working for about 2 hours.
(2) For areas with special geology, harmful gas detection should be carried out during the hole-digging process.
Measures to prevent falling objects in the hole
(1) There should be a safety reserve of no less than 10 times for the wire rope of the lifting frame, and its wear should be checked regularly.
(2) The bucket should not be too full to prevent the slag from scattering.
(3) The operator of the hole should wear a safety helmet and, for special hole positions, a life rope should be tied.
Measures to prevent electric shock
(1) The installation and removal of all power supplies and circuits at the construction site must be performed by certified electricians.
(2) Electrical equipment must be strictly grounded or zero-protected and installed with leakage protectors. The power supply of each pile hole must be separated. It is strictly forbidden to use one switch for multiple purposes.
(3) The cables above the hole must be suspended for more than 2m and buried in the soil. The cables and wires in the hole must be protected by sheaths and other protective measures to prevent wear, moisture, and breakage.
(4) The lighting in the hole should use safe mining lamps or a safe voltage below 12V. Operators in the hole should wear work gloves and insulated rubber shoes.
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