Bored cast-in-place piles are concealed structures buried within the strata, transmitting loads through the friction between the pile shaft and the surrounding soil or the bearing capacity of the pile tip as a bridge foundation form. At present, bored cast-in-place piles are widely used in highway bridges and other engineering fields due to their strong adaptability, moderate cost, and simple construction.

However, since cast-in-place piles are concealed works, many factors affect their construction quality, such as geological conditions, drilling techniques, wall protection, floating of the reinforcement cage, concrete mixing, and pouring. Any slight negligence or improper measure may lead to quality accidents, ranging from minor hole collapse and loosening or necking to major pile breakage and scrapping, which not only affects the construction schedule but also adversely affects the overall project quality.

Therefore, every step in the construction process must be strictly required, all influencing factors must be considered in detail and highly valued, and the construction quality of bored cast-in-place piles must be strictly controlled to avoid accidents and reduce losses caused by accidents as much as possible, so as to facilitate the smooth progress of the project. This article focuses on the causes of pile breakage in bored cast-in-place piles and the preventive measures.

I. Causes and Analysis of Pile Breakage

1.1 Common Forms of Pile Breakage

In the construction of bored cast-in-place piles, due to operational errors, poor equipment, and the influence of hydrogeological conditions, pile breakage accidents are very likely to occur. Generally speaking, there are four common forms of pile breakage:

(1) The space between the concrete pile body and the bedrock is filled with non-setting concrete. The reason is that the tremie pipe is placed too far from the bottom of the hole, so the concrete is diluted by muddy water, increasing the water-cement ratio and causing the concrete not to set.

(2) Non-setting concrete appears in the middle section of the pile shaft. The reason is that due to the influence of groundwater activity or poor sealing of the tremie pipe, muddy water intrudes and increases the water-cement ratio of the concrete.

(3) Rock debris in the pile shaft deposits in layers, separating the concrete pile vertically. The reason is that during concrete pouring, the tremie pipe is lifted and pulled out too much, exposing the concrete surface, or slag inclusion is caused by power outages, waiting for materials, and other reasons.

(4) Cavities appear in the pile shaft. The reason is that the "top-return" pouring method is not adopted; instead, concrete is poured directly from the hole opening, causing segregation of the concrete, resulting in non-dense and non-hard concrete after setting, and loose or cavity phenomena in individual hole sections.

1.2 Factors Affecting Pile Breakage

Pile breakage is formed when mud slurry intrudes into part or all of the end face section of the pile foundation concrete, destroying the integral load-bearing form of the pile. The factors affecting pile breakage are various, but they are mainly caused by improper materials and processes. In summary, these influencing factors mainly include the following:

(1) The initial setting time of cement is too short;

(2) The aggregate particle size is too large, and there are large cobbles in the sand;

(3) The workability and fluidity of cement concrete are poor;

(4) Hole cleaning work is not thorough;

(5) The tremie pipe is buried too deep or too shallow;

(6) Water enters the tremie pipe;

(7) Due to mechanical failures or unreasonable allocation of construction machinery and labor organization, the pouring time is too long.

1.3 Analysis of the Causes of Pile Breakage

As an important cause of serious quality accidents, the factors inducing pile breakage must be thoroughly eliminated at the initial stage of construction, and corresponding countermeasures must be prepared to prevent accidents. Upon analysis, the causes of pile breakage include the following aspects.

(1) During concrete pouring, due to incorrect measurement of the elevation of the poured concrete surface, the tremie pipe is buried too shallow, causing the pipe to be pulled out and leakage to occur, forming interlayer pile breakage. Especially in the later stage of bored cast-in-place piles, when the excess pressure is not large or the detection instrument is not precise, it is easy to mistake the collapsed soil layer mixed in the mud for the concrete surface. Therefore, the concrete surface height in the hole must be measured strictly with the sounding hammer specified in the regulations and carefully checked to ensure that no error occurs when lifting the tremie pipe.

(2) During pouring, the burial depth of the tremie pipe is an important construction indicator. Because the tremie pipe is buried too deep and the pouring time is too long, the fluidity of the poured concrete decreases, thereby increasing the friction between the concrete and the pipe wall. In addition, the tremie pipe adopts flange-connected pipes, which are already very outdated and have great lifting resistance. During lifting, pile breakage occurs because the connecting bolts break or the tremie pipe ruptures.

(3) Pipe blockage occurs: due to the large randomness of manual batching and poor sense of responsibility, there will inevitably be large errors in the implementation of the concrete mix ratio, causing large fluctuations in slump, and the mixed material is sometimes thin and sometimes dry. When the slump is too large, segregation occurs, causing coarse aggregates to squeeze each other and block the tremie pipe; when the slump is too small or the pouring time is too long, the initial setting time of the concrete is shortened, increasing the resistance of concrete falling and blocking the tremie pipe. All of these can lead to pipe blockage and cause pile breakage. Therefore, strictly controlling the concrete mix ratio and shortening the pouring time are important measures to reduce and avoid such pile breakage accidents.

(4) Collapse: due to poor engineering geological conditions and insufficient attention by the construction unit during construction organization, and even subcontracting or transferring contracts in some cases, the construction personnel are inexperienced. During pouring, when the well wall collapses seriously or flowing sand or soft plastic soil appears, mud-sand-like pile breakage is caused. This type of pile breakage accounts for a considerable proportion, is relatively serious, and is deep in location and difficult to handle, which is also an important factor leading to indefinite extension of the construction period and increased costs.

(5) Special reasons: due to water leakage in the tremie pipe, mechanical failures, or power outages, construction cannot proceed continuously, and sudden changes in water level in the well may cause pile breakage. Therefore, taking the preparatory work before pouring seriously is very important for ensuring the quality of the pile foundation.

II. Preventive Measures for Pile Breakage

(1) After the pile hole is formed, it must be carefully cleaned. Generally, washing liquid is used to flush the hole, and the flushing time should be determined according to the sediment conditions in the hole. After flushing, concrete should be poured promptly to avoid sediment at the bottom of the hole. Before pouring concrete, carefully measure the hole diameter and accurately calculate the concrete requirement for the whole hole and the first pouring section. Special attention should be paid to the expansion of the hole diameter in the bedrock to avoid insufficient first pouring volume.

(2) The first concrete pouring volume must meet the requirement of burying the lower end of the tremie pipe by more than 0.8 m. During the first pouring, the first 1–2 hoppers of material should use cement sand, and it should be poured into the hole in one go as much as possible.

(3) The lower limit of the tremie pipe diameter should be controlled at 200 mm or more. The lower end of the tremie pipe should be as smooth as possible, and sealing rings should be added at its connections to prevent muddy water from intruding. The tremie pipe must be cleaned before use to remove dirt and residue. The distance from the lower end of the tremie pipe to the bottom of the hole should preferably be 0.5 m. A rubber plate should be added to the cement water-stop plug inside the tremie pipe to enhance the water-stop effect.

(4) The concrete mix ratio should be reasonable, and its slump should be strictly controlled (generally 16 cm–20 cm is appropriate). When changing the cement grade, variety, and manufacturer, a mix ratio test must be done first, and quality should be controlled according to the mix ratio.

(5) In sections with large groundwater activity, water-stop treatment should be carried out in advance with casing or cement, and concrete can be poured only after successful water stopping. The "top-return" method of pouring from inside the tremie pipe should be adopted.

(6) Various materials required for the concrete to be poured should be prepared in sufficient quantities. Water and power outages should be avoided during pouring.

(7) According to the rising height of concrete inside and outside the tremie pipe, reasonably control the length of pipe dismantling. Under normal circumstances, the lower end of the tremie pipe should be kept buried 2 m–3 m, and it must not be pulled out too much.

(8) During concrete pouring, regularly measure the depth of concrete inside and outside the tremie pipe and draw curves to monitor whether pile breakage occurs. Under normal circumstances, the distance between the concrete interfaces inside and outside the tremie pipe is large at the beginning, then gradually decreases, and finally coincides. If it is found that the distance between the concrete pouring curves inside and outside the pipe increases, and the concrete curve outside the pipe becomes flat while the concrete curve inside the pipe becomes steep, this is a precursor to pile breakage. The cause should be identified and handled as soon as possible.

III. Related Articles on Treatment Methods for Pile Breakage

If a pile breakage accident of bored cast-in-place piles occurs inadvertently, timely remediation should be carried out. The author believes that the general remediation measures mainly include the following:

3.1 Secondary Hole Drilling Method

(1) If the pile breakage position is below 10 m, the reinforcement cage is buried relatively shallowly in the concrete, and the reinforcement cage can be lifted through adjustment, it should be quickly lifted out of the hole as a whole, then re-drilled with a percussion drill, and after hole cleaning, the reinforcement cage should be repositioned and underwater concrete should be poured again. Click here to download construction technical materials for free.

(2) If the pile breakage position is below 10 m from the ground surface and the concrete has already finally set, a percussion drill with a diameter slightly smaller than the inner diameter of the reinforcement cage can be used to perform percussion drilling at the original pile position until 1.0 m below the bottom opening of the reinforcement cage. Then the reinforcement cage should be lifted out of the hole as a whole or one by one, followed by secondary hole drilling to the design diameter. After hole cleaning, the reinforcement cage should be repositioned and underwater concrete should be poured.

3.2 Excavation Method (Open Excavation Method)

If the pile breakage position is within 5.0 m from the ground surface, the geological conditions are good, and the pile location is on dry land, the excavation method can be used to connect the pile. When the concrete strength reaches more than 80% of the design strength, excavate the soil around the pile foundation to the pile breakage position, chisel away the pile head concrete mixed with mud and sediment at the bottom of the hole until the dense concrete layer is reached, roughen the surface concrete, brush off the mud on the surface of the reinforcement cage, then recheck the pile position, erect steel formwork, pour concrete in layers, vibrate the concrete, and after formwork removal, cover with soil for curing.

3.3 Pile Position Adjustment Method

If the pile foundation work volume is not large and the broken pile is the first pile foundation constructed, it can also be reported to the supervision and design units. After approval, the problem can be solved by adjusting the pile position (with the centerline unchanged).

IV. Conclusion

The bored cast-in-place pile project is a highly technical concealed work. From drilling, mud preparation, and reinforcement cage hoisting to concrete pouring, all processes should be closely connected. There are many factors affecting pile quality and many links requiring control, and strengthening pile foundation engineering inspection is one means. To ensure the construction quality of bored cast-in-place piles, the key still lies in people. It is emphasized that on-site management personnel must have a high sense of responsibility, focus on prevention, and fully value and carefully construct each construction link of the pile foundation. Only in this way can the quality control of the pile foundation be guaranteed.