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.