Introduction
Piling foundation engineering is a critical part of construction projects, involving complex processes and stringent technical requirements. Several factors can affect the quality of piles, such as inadequate geological surveys, design assumptions, and various construction-related issues. Addressing and managing these quality issues is essential for ensuring the structural safety of the building. This article focuses on the most common quality problems encountered during pile driving and pressing, along with their causes and recommended treatment methods.
1. Common Quality Problems in Piling Foundations and Their Causes
Piling foundations may face several quality issues, including low pile bearing capacity, excessive inclination, pile breakage, pile joint disconnection, and excessive pile position deviation. The following sections outline the main causes of these problems.
1.1 Low Pile Bearing Capacity
Common causes of low pile bearing capacity include:
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Insufficient Pile Penetration Depth: If the pile does not reach the required design depth, its bearing capacity will be insufficient.
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Pile End Not Reaching the Load-bearing Stratum: Even if the pile reaches the design depth, if it doesn’t reach the appropriate load-bearing layer, it cannot achieve the expected capacity.
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Excessive Penetration: In some cases, excessive penetration can reduce the pile’s overall strength.
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Pile Damage: Problems such as excessive pile tilt or fracture can significantly reduce the pile’s bearing capacity.
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Geotechnical Survey Inaccuracy: The geotechnical survey data provided, including soil profiles and bearing capacity, may differ from actual site conditions, affecting pile performance.
1.2 Excessive Pile Inclination
Excessive pile inclination is commonly caused by:
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Poor Quality of Precast Piles: Issues such as tilted pile tops or misalignment of the pile tip can cause excessive inclination.
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Misalignment of Pile Driving Equipment: The pile driving equipment may not be vertical, resulting in angled pile installation.
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Off-center Hammer Strikes: If the hammer, pile cap, or pile body is not aligned, it leads to uneven driving and pile inclination.
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Obstacles Encountered During Pile Installation: Encountering stones or hard layers during pile driving can result in tilting.
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Improper Pile Spacing: Insufficient spacing between piles or poor pile installation sequence can create excessive lateral pressure, leading to pile inclination.
1.3 Pile Breakage
Pile breakage can occur due to:
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Incorrect Storage, Lifting, or Transporting: Improper lifting points or storage of piles can cause damage or breakage.
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Excessive Bending During Installation: If the pile experiences excessive bending during installation, especially in hard soil layers, it can break.
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Excessive Hammer Strikes: Using too many hammer blows or a hammer that is too heavy can lead to pile breakage.
1.4 Pile Joint Disconnection
When piles are pre-fabricated in sections, they are usually connected with steel welding joints. Common causes of joint disconnection include:
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Misalignment of Pile Sections: If the centers of adjacent pile sections do not align properly, disconnection can occur.
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Poor Quality of Welding: Insufficient welding or improper welding technique can weaken the joints, leading to failure.
1.5 Excessive Pile Position Deviation
Excessive deviation in pile position may occur due to:
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Surveying Errors: Inaccurate measurements during pile positioning can cause large deviations.
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Pile Installation Issues: Problems such as pile inclination during installation can result in significant position deviation upon completion.
2. Common Solutions for Piling Foundation Quality Issues
When quality issues arise during piling, construction teams mustn’t attempt to resolve them independently. Instead, they should report the issues to the project supervisor and collaborate with design and geotechnical departments to develop a proper solution. Below are the common methods for resolving quality problems in piling foundations.
2.1 Re-Sinking Method
When piles do not penetrate deep enough, or if the soil causes the pile to be lifted during installation, the re-sinking method can be applied to achieve the required depth.
2.2 Supplementary Pile Method
Two types of supplementary pile methods can be used:
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Supplementary Piles Before the Foundation Slab: When piles are closely spaced, supplementary piles can be added by first drilling holes and then driving additional piles.
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Supplementary Piles After the Foundation Slab or Basement: This method involves installing static pressure piles after the foundation or basement is completed, utilizing the existing structure to provide the necessary installation force.
2.3 Connection Reinforcement Method
If pile sections connected by welding joints are disconnected, the connection reinforcement method can be used. This involves re-driving the affected piles to tighten the joints and restore vertical load-bearing capacity.
2.4 Pile Straightening Method
If piles are tilted but not broken, especially in short piles or cases where the tilting occurred due to excavation issues, the pile straightening method using jacks can be applied after local excavation.
2.5 Enlarging the Foundation Slab
In cases where the original pile foundation slab does not meet construction or load-bearing requirements due to issues like excessive pile deviation or insufficient bearing capacity, the foundation slab area can be enlarged. This method also helps in cases where soil-pile interaction needs to be considered for load distribution.
2.6 Composite Foundation Method
To improve the load-bearing capacity of the foundation, the composite foundation method involves using both piles and soil together to distribute the load. This may involve:
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Substituting Soil with Graded Gravel: The soil beneath the pile foundation can be replaced with compacted sand and gravel to improve strength.
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Inserting Cement-Soil Piles Between Main Piles: If the bearing capacity is insufficient, cement-soil piles can be added between the main piles to form a composite foundation.
2.7 Modifying Pile Type or Parameters
If the pile design or installation parameters are found to be problematic, modifications can be made:
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Change of Pile Type: For example, switching from square piles to prestressed pipe piles.
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Adjusting Pile Depth: Shortening pile lengths or increasing the number of piles to avoid encountering difficult soil layers.
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Changing Pile Location: If encountering obstacles during installation causes pile inclination or breakage, the pile position can be adjusted.
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Upgrading Pile Driving Equipment: Using higher-tonnage pile drivers or changing the type of hammer used for installation.
2.8 Other Methods
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Raising the Ground Slab: In certain cases, elevating the ground slab can reduce load on the pile foundation and mitigate weight issues.
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Structural Load Reduction: If major piling issues occur, reducing the number of building floors or using lightweight materials in construction may help decrease the load on the piles.
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Structural Analysis: In cases of low pile strength or inadequate bearing capacity, structural analysis can be used to explore alternative solutions. If analysis confirms that the structure still meets safety standards, no further action may be necessary.
2.9 Comprehensive Treatment
A combination of several of the aforementioned methods may often provide the best results in solving piling foundation issues.
Conclusion
Pile foundation quality directly impacts the overall quality of the building structure. Throughout the piling process, it is crucial to address any issues promptly by consulting with the owner, supervisor, design, and geotechnical teams. By following the proper procedures and using the appropriate methods for resolving problems, construction teams can ensure the stability and safety of the building foundation.
