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Pile Construction Guidelines: Best Practices for Prestressed Concrete Piles

1. Basic Regulations

  • Dense Pile Groups: Construction should proceed from the center outward symmetrically.
  • Pile Groups Adjacent to Buildings: Construction should move from the building side to the opposite side.
  • Based on Foundation Design Elevation: Deeper piles should be constructed before shallower ones.
  • Based on Pile Specifications (Diameter & Length): Larger-diameter and longer piles should be constructed first.
  • Prestressed Concrete Pile Connections:
    • Methods: End-plate welding, flange connection, mechanical interlocking, threaded connection.
    • Maximum of 3 joints per pile.
  • Pile Ends in Water-Sensitive Strata:
    • If embedded in easily softened weathered rock or unsaturated soil, apply anti-seepage measures (e.g., micro-expansion concrete fill or flexible waterproof coating) within 2m of the pile tip.
  • Test Piling:
    • Conduct trial piles before official construction (1% of total piles, minimum 5).

2. Test Piling Requirements

Basic Guidelines:

  • Use actual pile locations for test piles (later used as working piles).
  • Test piles should represent site geology, size, and length.
  • Place near key boreholes.
  • Follow the same construction method as production piles.
  • Use high-strain dynamic testing (PDA) during trial driving.

High-Strain Testing Procedure:

  1. Estimate pile length based on geotechnical data + 3–4m.
  2. Drive pile using standard methods, recording details.
  3. Pause near bearing layer; install sensors and resume hammering.
  4. Stop when resistance reaches 2Ra/a (where Ra = design capacity, a = soil recovery factor: 1.2–1.4 for clay, 0.9–1.1 for sand).
  5. Measure depth, final penetration, and blows per last meter.
  6. Re-test after 24h. If resistance ≥ 2Ra, adopt the results; otherwise, adjust a and repeat.

Required Test Data:

  • Ultimate vertical bearing capacity.
  • Side friction and end resistance.
  • Maximum hammer energy & pile stresses.
  • Pile integrity.
  • Embedment depth, blow counts, and penetration rate.

3. Construction Process Flow

  1. Survey & Layout
  2. Pile Rig Setup
  3. Pile Alignment
  4. Driving
  5. Splicing (if needed)
  6. Final Driving (Termination Criteria)
  7. Cutting Excess Pile Length
  8. Inspection & Testing

4. Operational Methods

Survey & Layout

  • Control points should avoid pile interference.
  • Mark pile positions with stakes/lime.

Pile Rig Setup

  • Ensure “three accessibilities” (water, power, road) and level ground (bearing capacity ≥140 kPa/m²).
  • Soft Ground Improvement:
    • Fill with 0.5–1m brick slag for soft clay.
    • 0.3–0.5m brick slag for loose fill.
  • Adjust rig for vertical alignment (±0.5% tolerance).

Pile Handling

  • Precast piles must reach 70% strength for lifting; 100% for driving.
  • Attach pile tip before lifting (no post-lift welding).
  • Use pile-specific helmets and cushioning (120–150mm compressed thickness).
  • Hammer cushions: 150–200mm hardwood or steel rope.

Pile Driving

  • Initial Penetration: Use low energy in soft layers.
  • Hard Strata: Drive continuously to minimize delays.
  • Stop Conditions: Abnormal resistance, pile damage, or excessive heave.
  • Blow Count Limits:
    • PC piles: ≤2000 total blows, ≤250 blows/last meter.
    • PHC piles: ≤2500 total blows, ≤300 blows/last meter.
  • Pre-Drilling (If Needed):
    • For hard layers/thick sand.
    • Drill diameter: 50–100mm smaller than pile.
    • Max depth: 12m; complete within same shift.

Splicing

  • Weld at 50–100cm above ground (2 layers, 3 passes).
  • Align sections (<2mm misalignment); weld symmetrically.
  • Cool naturally (≥5min for manual welding; ≥3min for CO₂).

Final Driving (Termination Criteria)

  • Primary Controls: Bearing layer + final penetration/blow count.
  • Penetration Standards:
    • Typically ≥20mm/10 blows (average of 3 readings).
    • For thin weathered rock: ≥15mm/10 blows.
  • Ensure load capacity overrides strict penetration rules.

Cutting & Inspection

  • Use pile cutters (no manual breaking).
  • Check: Elevation, length, position, and capacity.

Testing & Waiting Periods

  • High-Strain/Static Load Test Delays:
    • Sand: 7 days.
    • Silt: 10 days.
    • Clay: 15–25 days (saturated).
    • Water-sensitive rock: ≥25 days.
  • Test Quantities:
    • 1% static tests (min 3 piles); 20% integrity tests for risky sites.
    • 8% high-strain tests (min 10 piles) for others.

Post-Construction Protection

  • No excavation until pile work finishes (+15 days in clay/silt).
  • Deep excavations require staged plans and monitoring.
  • Limit soil pile height difference to 1m; avoid machinery contact.
  • Soft soil sites: Use manual excavation or ground improvement.

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