Enlarged Pile Technology (Compacted Expanded Pile)

Enlarged Pile, also known as Compacted Expanded Pile, is a type of cast-in-place pile technique. It was developed to increase the single-pile bearing capacity of cast-in-place piles. The method involves expanding the pile body or pile tip to enhance both the pile tip resistance and the side friction force.

Types of Enlarged Piles

1. New Pile Construction: This method differs from typical drilled piles. A pile hammer is placed inside a casing, driving a stone plug at the bottom of the casing to the design depth. The stone plug, together with the casing’s friction, pushes the stone plug out of the casing, allowing dry and hard concrete to be poured into the pile casing. Further pounding expands the pile head. After reinforcing the cage, a larger slump of concrete is poured, and the casing is removed. The pile is then vibrated using an attached vibrator to densify the concrete.

2. End-Compacted Drilled Pile: This type of pile primarily relies on the enlarged tip for load-bearing, with the side friction of the pile body serving as a secondary factor. It is based on the techniques of impact vibrating and expansion piles. The process involves using a diesel hammer to transmit energy through an inner compaction tube, which directly impacts the in-situ concrete at the pile’s base. This compaction densifies the concrete and forms a broad, flat-bottomed pile tip, increasing the pile’s bearing area and improving soil density.

Construction Process

1. Site Preparation: The pile position is surveyed, and a prefabricated concrete pile tip or dry-mixed concrete is placed at the position on the ground.

2. Pile Machine Setup: The pile machine is positioned, and the internal compaction tube is inserted into the casing.

3. Impact Compaction: The hammer strikes the casing until it reaches the design depth.

4. Concrete Pouring: Once the casing is extracted, a mix of high-strength concrete is poured into the expanded part of the pile.

5. Vibration and Densification: Vibrators are used to ensure the concrete is well-compacted.

6. Completion: Once the pile body concrete is in place, the reinforcement cage is added, followed by the final concrete pouring.

Common Quality Issues in Enlarged Pile Construction

Despite the robust construction techniques, several technical issues may arise during construction, leading to quality problems. The main issues observed during projects include:

1. Pile Diameter Inconsistency: This can result in necking, which reduces the cross-sectional area and bearing capacity of the pile.

2. Poor Concrete Quality: This may cause problems like necking or even pile rupture.

3. Reinforcement Cage Alignment: Misalignment of the cage height can complicate subsequent procedures.

4. Insufficient or Overly Large Expansion Head: An undersized expansion head may not provide enough bearing area, while an oversized head can interfere with adjacent piles.

5. Water or Slurry Infiltration: If water or slurry enters the pile casing, it can disrupt the quality of the concrete and cause construction failures.

Quality Control Measures

To ensure quality during construction, the following measures should be implemented:

1. Control of Expansion Head Size:
   The main causes for improper expansion head size include discrepancies in geological data, improper compaction material distribution, or failure to follow construction guidelines. Measures include pre-expansion tests and constant monitoring of expansion material levels.

2. Pile Diameter and Necking:
   Necking is typically caused by improper casing vibration or poor slurry control. To address this, it’s essential to control the concrete slump (8–250mm) and to adjust the speed of casing extraction to avoid sudden soil displacement.

3. Pile Rupture:
   Pile rupture usually occurs if the extraction speed is too fast or if the concrete has not set properly. Measures to prevent this include controlling the extraction speed, ensuring continuous concrete pouring, and maintaining proper construction sequences.

4. Steel Cage Alignment:
   Steel cage positioning can be challenging due to varying soil conditions. Regular checks and adjustments during construction help ensure the cage is placed at the correct height.

5. Water or Slurry Entry:
   Using appropriate sealing techniques for the pile casing and minimizing the insertion time can reduce the risk of water or slurry infiltration.

Quality Management

To ensure project success, quality management must be rigorously applied. Regular inspections of pile sinking processes and raw materials are necessary. The following construction and material guidelines should be followed:

• Reference Codes and Standards:

  • Building Foundation Design Code (GB50007-2002)

  • Construction Pile Technical Standards (JGJ94-94)

  • Composite Pile Design Guidelines (JGJ/T135-2001)

• Construction Procedures:

  • Ensure the pile machine is leveled before positioning and that the casing remains vertical throughout the process.

  • Maintain strict control over the amount of material used during each compaction stage, ensuring that each layer of soil or concrete is fully compacted.

  • Continuous monitoring of material consistency (such as moisture content in aggregates) and concrete slump is essential.

• Quality Control Standards:

  • Tolerances on pile diameter, verticality, and position should be strictly controlled, ensuring minimal deviation.

  • Reinforcement cages must meet precise spacing, diameter, and length tolerances.

  • Materials must pass quality inspections, with aggregates having a maximum permissible clay content of 3% and sand quality checked for impurities.

Measures for Construction During Rainy Seasons

Special precautions must be taken during wet weather conditions:

• Develop a drainage plan to prevent waterlogging and ensure site accessibility.

• Materials prone to moisture absorption should be stored properly to prevent degradation.

• Weather forecasts should be monitored to prepare for rain, using waterproof materials to protect site equipment and workers.

Conclusion

Enlarged piles offer a reliable foundation solution for challenging soil conditions, providing increased load-bearing capacity through innovative expansion techniques. However, like all construction methods, they require careful management and attention to detail to ensure successful outcomes. By following strict quality control measures and construction standards, contractors can mitigate common problems and deliver durable, high-performance piles suitable for a range of engineering projects.