Prevention and Treatment of Pile Breakage in Bored Cast-in-Place Pile Construction
2013-09-29
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Bored cast-in-place pile foundations are widely used in bridge construction due to their simple construction equipment and ease of operation. However, due to various factors, bored piles are prone to breakage during construction, so the prevention of pile breakage is an important issue that we need to consider. Based on work practice, the author summarizes and analyzes this problem from several aspects: the causes of pile breakage, how to prevent it, and how to deal with it after it occurs, in order to exchange ideas with peers.
Causes of pile breakage
Poor aggregate gradation and poor concrete workability cause segregation and pipe blockage. Due to small concrete slump, large stone particle size, and small tremie diameter, the tremie is blocked during pouring and cannot be cleared before the initial setting of concrete, so the tremie has to be lifted, resulting in pile breakage. Due to long transportation or waiting time, concrete segregates and is not re-mixed; during pouring, a large amount of aggregate is stuck in the tremie, which has to be lifted for cleaning, causing pile breakage. The interval between concrete placements exceeds the initial setting time of concrete. During concrete pouring, hole collapse occurs and cannot be cleaned, or the cleaning with a mud suction machine is incomplete, causing interruption of pouring and pile breakage. Due to detection and calculation errors, the tremie length is insufficient, making the distance between the bottom opening and the hole bottom too large; the first batch of concrete cannot bury the bottom of the tremie, thus forming a broken pile. Improper use of the tremie can also cause pile breakage. In addition, other unexpected reasons (such as mechanical failure, power outage, insufficient material supply, etc.) cause concrete to be unable to be poured continuously, and the interruption time exceeds the initial setting time of concrete, making it impossible to lift the tremie and forming a broken pile.
Measures to prevent pile breakage
Concrete should have good workability, and the slump should be controlled at 18cm~22cm. If the pouring time is long, a retarder can be added to the concrete to prevent the initially poured concrete from setting and blocking the tremie.
The diameter of the tremie should be determined according to the pile diameter and the maximum particle size of the stone. Large-diameter tremies should be used as much as possible. Each section of the tremie should be assembled and numbered. After assembly, a review and inspection system should be established. Before use, the tremie should be subjected to leak and tensile tests to prevent leakage.
When lifting the tremie, the lifting length should be carefully calculated by measuring the concrete pouring depth and the length of the removed tremie sections. Blind lifting without measurement and calculation is strictly prohibited.
When lowering the tremie, the depth of its bottom opening from the hole bottom should be controlled between 25cm and 40cm (the tremie opening must not be buried in the settled silt), and at the same time, it must be ensured that after the first batch of concrete is poured, the tremie is buried at least 1m. During subsequent pouring, the embedment depth of the tremie is generally controlled within the range of 2m to 6m.
When fabricating the reinforcement cage, butt welding should generally be used to ensure smooth welds. When lap welding is used due to conditions, it must be ensured that the weld seam does not form a misalignment inside the reinforcement cage. In addition, stiffening bars should be placed on the outside of the main reinforcement to prevent the reinforcement cage from catching the tremie. If the reinforcement cage catches the tremie, the tremie can be rotated to free it from the cage.
Before pouring concrete, leak and pressure tests should be conducted on the tremie to prevent leakage. At the same time, excessively old tremies should not be used to avoid breaking during lifting.
Key equipment (such as concrete mixing equipment, generators, transport vehicles, etc.) should have backups, and materials (sand, stone, cement, etc.) should be prepared sufficiently to ensure continuous concrete pouring.
When concrete blocks the tremie, the tremie can be lifted and inserted with shaking (note that the tremie opening must not be pulled out of the concrete surface). When the remaining tremie length is short, a thick steel bar can be inserted into the tremie to impact and clear it, or a flat-plate vibrator can be fixed on the tremie to vibrate and clear the concrete in the tremie.
Treatment plans after pile breakage
If the reinforcement cage can be extracted from the borehole after pile breakage, the cage can be quickly pulled out, then re-drilled with a percussion drill, the cage lowered, the hole cleaned, and concrete poured again.
If the breakage is located more than 10m below the ground surface, a percussion drill with a diameter slightly smaller than the inner diameter of the reinforcement cage can be used to drill at the original pile position to 1m below the reinforcement, then the reinforcement can be removed (the cage can be lifted by a pulley or crane, and an appropriate amount of explosive can be placed at the bottom of the hole to loosen the cage, then lifted out as a whole or piece by piece), and then reamed to the design diameter, cleaned, and concrete poured again.
If the breakage is within 5m below the ground surface and the geological conditions are good, the excavation method can be used. After excavating to the breakage location, remove the mud or mud-mixed concrete, expose the good concrete and roughen it, clean the mud on the reinforcement, then set up formwork and pour. After formwork removal, backfill and compact in time.
If the breakage is located between 5m and 10m below the ground surface, or within 5m but with poor geological conditions, the concrete pipe method can be used (connect concrete pipes slightly larger than the pile diameter section by section until they sink to 0.5m below the breakage location), remove the mud and mud-mixed concrete, expose the good concrete surface and roughen it, clean the mud on the reinforcement, and then pour concrete using the concrete pipe as formwork.
If the pile breakage caused by hole collapse, inability to pull out the tremie, etc. cannot be treated, supplementary piles can be installed on both sides of the original pile.
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