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CFG Pile: Construction Techniques, Quality Control, and Applications in Composite Foundations

Introduction to CFG Piles

Cement Fly-ash Gravel (CFG) piles are a type of composite foundation pile made from a mixture of gravel, stone chips, fly ash, cement, and water. These piles are formed using specialized piling equipment and exhibit a certain degree of bonding strength. Together with the surrounding soil and a cushion layer, CFG piles form a composite foundation system—a highly effective ground improvement method.

CFG Pile Construction Methods

CFG piles can be constructed using four primary techniques:

  1. Vibrating Steel Tube Cast-in-Place Method

  2. Long Auger Drilling Cast-in-Place Method

  3. Long Auger Drilling with Internal Pumping of Mixed Materials

  4. Slurry Wall Drilling Cast-in-Place Method

CFG pile composite foundations are widely recognized for their cost-effectiveness, quality performance, and relatively short construction timelines. However, factors such as geological conditions, foundation type, structural requirements, and load-bearing capacity can significantly impact pile integrity, leading to issues like voided pile tips, necking, or broken piles.

1. Selection of CFG Pile Construction Techniques

Choosing the appropriate construction method is crucial for ensuring pile quality. The decision should consider:

  • Ground conditions

  • Reinforcement requirements

  • Environmental constraints

  • Project budget

Primary CFG Pile Construction Methods

Vibrating Steel Tube Cast-in-Place Method

  • Advantages: Simple process, low construction cost

  • Disadvantages: Noise pollution, highly dependent on soil conditions

  • Best for: Soft soil layers (e.g., silt, clay) in areas with minimal noise restrictions

Long Auger Drilling with Internal Pumping Method

  • Advantages: Minimal noise, reliable pile quality

  • Disadvantages: Complex process, higher costs

  • Best for: Dense clay or sandy soils where noise and slurry pollution must be minimized

For sites with hard intermediate layers or sensitive soils, a combination of long auger drilling followed by vibrating tube pile forming is recommended to prevent soil structure damage.

2. CFG Pile Construction Process

The construction sequence includes:

  1. Site preparation & pile positioning

  2. Drilling & pile formation

  3. Concrete pumping (for auger methods)

  4. Pile head trimming & capping

3. Pile Cap Construction

When CFG piles are used to reinforce weak foundations, pile caps are essential for:

  • Enhancing load distribution

  • Improving bearing capacity

  • Reducing settlement

Pile Cap Construction Steps:

  1. Excavate around the pile head manually

  2. Remove excess soil above the design pile cut-off level

  3. Trim pile head using cutting/chipping (no dynamic methods)

  4. Install formwork & cast a concrete cap

4. Key Construction Considerations (Long Auger Pumping Method)

  1. Site Investigation: Understand soil & hydrogeological conditions.

  2. Equipment Parameters: Verify pump capacity, pipe size, and lifting speed.

  3. Pre-Construction Prep: Ensure proper staffing, site coordination, and leveling.

  4. Depth Control: Mark pile lengths clearly; account for elevation differences.

  5. Concrete Quality: Maintain proper slump (160-200mm) and workability.

  6. Pumping-Drilling Coordination: Avoid air gaps; ensure continuous concrete flow.

  7. Lifting Timing: Begin lifting only after full concrete discharge.

5. CFG Pile Composite Foundation Requirements

  • Mix Design: Follow specified ratios; control slump (30-50mm for vibrating tube; 160-200mm for auger pumping).

  • Lifting Speed: 1.2-1.5 m/min (slower in soft soils).

  • Pile Length Tolerance: ±100mm.

  • Testing: Conduct 28-day cube tests (3 samples/machine/day).

  • Excavation: Use combined mechanical/manual methods to prevent pile damage.

6. Common CFG Pile Defects & Solutions

Issue Causes Solutions
Pile Misalignment Poor monitoring, incorrect marking Use dual plumb bobs; verify positioning
Short Piles Insufficient concrete, poor marking Mark pile lengths; increase supervision
Shallow Breakage Improper excavation/cutting Use small excavators; cut piles via sawing
Pile Head Porosity Fast lifting, poor mix Control lifting speed; ensure proper overpour
Segregation Poor workability, pumping stops Monitor mix quality; maintain continuous flow

7. Quality Control Measures

  1. Soil-Specific Method Selection: Match technique to ground conditions.

  2. Piling Sequence: Use center-outward or linear progression in soft soils.

  3. Lifting Rate Control: Sync with pumping speed (1.2-1.5 m/min ideal).

  4. Slump Management: Maintain 3-5cm for vibrating tube; 16-20cm for auger pumping.

  5. Protective Length: Cast extra 0.5m; trim post-curing.

8. Post-Construction Testing

  • Timing: Low-strain integrity tests after 7 days; load tests at 14-28 days.

  • Tolerances:

    • Length: ±10cm

    • Diameter: ±2cm

    • Verticality: ≤1%

    • Position: ≤0.5×diameter (rafts); ≤0.25×diameter (strips)

9. Conclusion

CFG pile composite foundations—evolved from stone column techniques—optimize side friction and end-bearing capacity while utilizing industrial waste (fly ash). Strict adherence to:

  • Construction sequencing

  • Precision positioning

  • Controlled lifting/pumping

  • Proper curing & testing
    ensures reliable performance, minimizing foundation risks in civil engineering projects.

 

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