Bored Pile
Bored cast-in-place piles have the advantages of low construction noise, and low vibration, pile length and diameter can be freely changed according to design requirements, pile tips can reliably enter the bearing layer, and single pile bearing capacity is large. However, from the beginning of drilling to the end of pile formation, quality problems or even quality accidents are easily caused due to the influence of many factors. Therefore, quality control has become a difficult point in construction.
Part 1 Rebar Cage Floats
During the concrete pouring process, the steel skeleton that has been sunk to the designed depth position is higher than the original designed position, which is commonly known as a “floating cage.”
1.1 Cause Analysis
1) The distance between the inner diameter of the steel cage skeleton and the conduit is small, the particle size of the coarse aggregate is too large, the lap welding head of the main bar is not welded flat, and the flange is hung with the steel cage during the lifting and sinking process of the conduit.
2) During the installation process of the steel bars, the skeleton is twisted, the stirrups are deformed, desoldered or the pipe is tilted, causing the steel bars to be in close contact with the outer wall of the pipe.
3) Sometimes due to machine failure, the pouring of concrete is stopped, and the concrete between the conduit and the steel bars has solidified, and the steel bars are taken out when the conduit is lifted.
4) If the concrete is poured too fast, the concrete surface will rise to the bottom of the steel cage, causing upward “buoyancy”, causing the steel cage to float up.
1.2 Solutions
1) If a “floating cage” appears just after pouring concrete, it is mainly because there is a hanging belt between the conduit and the cage; stop pouring concrete immediately and repeatedly shake the conduit up and down or rotate it in one direction.
2) During the pouring of concrete, as the conduit is pulled out, the cage floats up, but the concrete surface does not move. This is also because there is a hanging tape between the conduit and the cage. The conduit should be repeatedly shaken and moved up and down to cut off the two. connect.
3) During the pouring process, as the concrete surface rises, the cage floats up, that is, the amount and speed of concrete pouring should be controlled.
Part 2 Sinking Cage
The steel bars had been sunk to the designed depth position. During the pouring process, the steel cage fell and the steel frame was lower than the original designed position. This is commonly known as the “sunken cage”.
2.1 Cause Analysis
1) The welding between the hanging bars and the main bars or the segmented steel bars is not strong or the hanging rings fall off.
2) When vibrating the conduit up and down, the conduit is hung with steel bars, exerting a large external force on the steel bars, and the lifting rings are loose.
Once the conduit is detached from the steel cage, the cage sinks into the hole.
2.2 Solutions
1) If the cage sinks into the concrete to a depth that is not deep (less than 2 meters), you can leave it alone and continue pouring concrete. After the foundation pit is excavated, manually or mechanically excavate the original pile position, and dig out the pile head steel bars to connect the height. Come up, the pile head concrete must be chiseled, and then concrete with a strength level higher than the original grade.
2) When the floating slurry on the pile head is removed after excavation of the foundation pit, a sinking cage is found, but the depth of the sinking is unknown. At this time, the pile must be replenished or the designer must approve reinforcement measures on the basic structure.
Part 3 Catheter Removal
During the pouring of concrete, the conduit breaks away from the concrete surface and muddy water enters the conduit, causing the pile body to become smaller or break.
3.1 Cause Analysis
The construction crew made a mistake and pulled out the conduit too quickly.
3.2 Precautions
Strictly control the lifting speed of the conduit. Before lifting, the height of the concrete surface should be measured, and the length of the conduit embedded in the concrete and the height that can be lifted this time should be calculated.
3.3 Solutions
1) When the conduit is pulled out, the conduit should be quickly inserted into the concrete, use a small water pump or small-diameter pumping equipment to pump out the water in the conduit, and continue pouring concrete.
2) Quickly pull out the conduit and reset the water barrier. After flushing out the conduit, the conduit should continue to be lowered until the conduit cannot be inserted, then slightly lift the conduit and continue pouring concrete.
Part 4 Buried Pipe
During the pouring of concrete, the conduit was buried in the concrete and was not pulled out in time. After the concrete hardened, waste piles were formed.
4.1 Cause Analysis
1) The initial setting time of the concrete is short or the intermittent time is long due to construction machinery, power supply, etc. When the concrete is re-poured, the lower part of the concrete has hardened and the conduit cannot be pulled out.
2) The conduit is caught by the steel bar.
3) During the pouring of concrete, the hole wall collapsed and a large amount of mud and sand buried the conduit.
4.2 Precautions
1) The initial setting time of concrete must be twice the normal pouring time. A retarder should be added during summer construction to ensure continuous supply and pouring of concrete.
2) Avoid conduits hanging with steel cages.
3) Measures to prevent hole wall collapse: The original soil depth of the casing should be at least 1m. According to the soil characteristics of the site, the wall protection mud should be correctly selected. The mud concentration should not be too low, and it is strictly prohibited to put water into the hole. When constructing holes next to adjacent piles that have just been poured, the distance shall not be less than 4 times the pile diameter, or the minimum interval shall be greater than 36 hours.
4.3 Solutions
1) When the conduit is hung with a steel cage, if you find that the steel cage is not buried deep in the concrete, you can lift the steel cage and rotate the conduit to separate the conduit from the steel cage, otherwise, you have to abandon the conduit.
2) If the conduit is buried in concrete and cannot be pulled out, it is generally discarded and the piles will be refilled after approval with the designer.
Part 5 Pile Position Deviation Is Large
After the foundation pit was excavated, the pile position was checked against the axis. The pile position deviation exceeded the allowable range of the specification.
5.1 Cause Analysis
1) There is a deviation in the setting out by the construction personnel or the positioning of the mechanical drilling is inaccurate.
2) When excavating the foundation pit, the depth of soil excavation is too large at one time, and the pressure on the soil side causes the piles to shift.
5.2 Precautions
1) Improve the professional level of construction personnel, enhance their dedication to work, and ensure accurate drilling positioning.
2) The excavation should be carried out in layers, and the depth of each excavation should be controlled at about 4m.
5.3 Solutions
If the pile position exceeds the allowable deviation range of the specification, the designer should be asked to verify and take necessary reinforcement measures; if it is a single pile foundation, the piles should generally be replenished.
Part 6 Water Coming out of the Pile Head
After the pouring of the foundation pit cushion concrete was completed, water seepage occurred at the pile head.
6.1 Cause Analysis
1) The concrete is not poured densely, there are cracks or mud in the pile body (especially the pile head), the particle size of the stones in the concrete is too large, and the gradation is uneven.
2) When pouring concrete, the relative density of the mud is too high, there is mud between the concrete and the main reinforcement, and groundwater emerges along the mud.
3) During foundation pit excavation, the excavating machinery collided with the pile body, causing cracks at the pile head.
4) When cutting piles, machinery such as pneumatic picks excessively impacts the concrete at the pile head, causing cracks in the concrete at the pile head and water seepage.
6.2 Precautions
1) Strictly control the quality of concrete. The coarse aggregate particle size should be 5~25mm. The pipe should be vibrated up and down frequently during the pouring process.
2) Strictly control the relative density and consistency of the mud to keep it within the allowable range of specifications.
3) During earth excavation, it is strictly prohibited for the excavating machinery to collide with the pile body.
4) When cutting piles, the distance between the pile cutting site and the designed pile head is generally controlled to be about 1m. The concrete in this 1m section must be manually chiseled, and it is strictly forbidden to use mechanical tools such as pneumatic picks for chiseling.
1) If water is found to emerge from the pile head, the inspection unit should be asked to measure the concrete strength of the pile body and whether there are any defects in the pile body. If cracks are limited to a certain length of the pile head, the pile head concrete should be chiseled away until the solid surface is exposed. After concrete is poured, concrete with a strength level higher than the original level is poured.
2) If there are serious defects in the quality of the pile body, the design and testing center should be asked to determine whether the pile needs to be replenished.
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