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
Geological Conditions
Soil Stratigraphy:
- Miscellaneous fill
- Silt clay
- Silty fine sand
- Strongly weathered argillaceous siltstone
- Moderately weathered argillaceous siltstone
- Slightly weathered argillaceous siltstone
Hydrogeology:
- Primary aquifer: Permeable silty fine sand layer (medium permeability)
- Hydraulic connection with Pearl River, causing tidal-influenced groundwater fluctuations
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
SD20 Rotary Drilling Rig: Technical Breakdown
Core Components
- Chassis: Specialized expandable crawler base for enhanced stability
- Drilling Tools:
- Drill rods: Friction-type or self-locking telescopic options
- Interchangeable bits: Short auger, standard bucket, sand bailer
- Power System: Cummins turbocharged engine
- Control System: Pfaff automatic leveling system
- Hydraulics: Full hydraulic transmission with load-sensing control
Performance Highlights:
- Maximum depth: 40m
- Automated verticality monitoring (±0.5° accuracy)
- Real-time depth measurement
Construction Process & Quality Control
7-Step Workflow
- Stakeout: Laser-guided positioning
- Casing Installation: Steel casing centered with clay backfill
- Rig Positioning: Automated alignment systems
- Drilling:
- Dry method for stable formations
- Bentonite slurry (SG 1.05-1.10) for unstable soils
- Rebar Cage Installation: Crane-lifted with centralizers
- 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)
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
Testing & Acceptance
Compliance Standards:
- JGJ 94-2008 (Pile Foundation Tech Code)
- DBJ15-60-2008 (Guangdong Foundation Testing Code)
Documentation Requirements:
- Geotechnical reports + as-built drawings
- Material test certificates
- Pile integrity test reports
- Final as-built survey
