Comprehensive Guide to Rotary Drilling Pile Construction for Deep Foundation Support

Project Overview: Retaining Structure Design for Qiaoxi Mixed-Use Building

The Qiaoxi commercial-residential high-rise features a 30-story above-ground structure with 2 underground levels. The 8.3 m-deep excavation utilizes a rotary drilling pile (bored pile) + strut support system, with double-row mixing piles forming an external waterproof curtain.

Key Specifications:

• Pile diameter: 1000mm
• Spacing: @150mm
• Concrete grade: C25
• Embedment depth: Minimum 2m into a strongly weathered rock layer

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Geological Conditions

Soil Stratigraphy:

1. Miscellaneous fill
2. Silt clay
3. Silty fine sand
4. Strongly weathered argillaceous siltstone
5. Moderately weathered argillaceous siltstone
6. Slightly weathered argillaceous siltstone

Hydrogeology:

• Primary aquifer: Permeable silty fine sand layer (medium permeability)
• Hydraulic connection with Pearl River, causing tidal-influenced groundwater fluctuations

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Construction Progress

• Total piles: 170
• Planned duration: 24 days (avg. 8 piles/day)
• Actual progress: Initially 8-10 piles/day, reduced to 4-5 piles/day due to equipment issues and holiday delays

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SD20 Rotary Drilling Rig: Technical Breakdown

Core Components

1. Chassis: Specialized expandable crawler base for enhanced stability
2. Drilling Tools:
   • Drill rods: Friction-type or self-locking telescopic options
   • Interchangeable bits: Short auger, standard bucket, sand bailer
3. Power System: Cummins turbocharged engine
4. Control System: Pfaff automatic leveling system
5. Hydraulics: Full hydraulic transmission with load-sensing control

Performance Highlights:

• Maximum depth: 40m
• Automated verticality monitoring (±0.5° accuracy)
• Real-time depth measurement

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Construction Process & Quality Control

7-Step Workflow

1. Stakeout: Laser-guided positioning
2. Casing Installation: Steel casing centered with clay backfill
3. Rig Positioning: Automated alignment systems
4. Drilling:
   • Dry method for stable formations
   • Bentonite slurry (SG 1.05-1.10) for unstable soils
5. Rebar Cage Installation: Crane-lifted with centralizers
6. Concreting:
   • Tremie method with a minimum 1m embedment
   • Continuous pouring (180-220mm slump concrete)

Critical QC Measures

• Pre-construction:
  • Equipment calibration
  • Trial boring for parameter optimization
• During Construction:
  • Real-time verticality monitoring
  • Rock interface identification
  • Post-drilling caliper logging
• Post-construction:
  • Low-strain integrity testing (10% minimum)
  • Core sampling for defect verification (2% minimum)

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Advantages vs Limitations

✅ Key Benefits

• 5x faster than conventional drilling
• Minimal sediment (avg. <5cm vs 30cm in rotary methods)
• Eco-friendly: 80% slurry reuse
• Enhanced bearing capacity from rough bore walls

❌ Challenges

• Rock limitations: Hard rock requires supplemental tools
• High capital cost: $400,000-$600,000 per unit
• Maintenance intensity: 6,000-hour component lifespan

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Testing & Acceptance

Compliance Standards:

• JGJ 94-2008 (Pile Foundation Tech Code)
• DBJ15-60-2008 (Guangdong Foundation Testing Code)

Documentation Requirements:

1. Geotechnical reports + as-built drawings
2. Material test certificates
3. Pile integrity test reports
4. Final as-built survey