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Vibratory Stone Column Construction: Methods and Quality Control

23

Site Preparation for Stone Column Construction

Before vibratory sunk pipe stone column construction, proper site preparation is essential:

Site Leveling

  • Level the construction site to predetermined elevations based on topographic maps
  • Follow the “fill rather than cut” principle during grading
  • Remove surface obstructions to allow smooth vibrator penetration
  • Establish onsite access roads, drainage systems (including blind ditches and longitudinal drains), and material storage areas
  • Install lighting for potential night operations

Site Clearing and Excavation

  1. For dry, hard areas:
    • Use bulldozers with manual support and loaders for removal
    • Transport waste materials to designated disposal areas using dump trucks
  2. For low-lying, wet areas:
    • Employ excavators for primary clearing
    • Use manual methods where machinery access is limited
  3. Additional requirements:
    • Remove all organic matter, debris, and topsoil (minimum 200mm depth) within embankment areas
    • Ensure that clearing meets elevation requirements for stone column tops

Survey Layout and Column Arrangement

  • Conduct survey measurements to re-establish centerlines and boundary markers
  • Prepare a properly leveled ground surface with functional drainage
  • Implement column arrangement according to design drawings
  • Create and verify measurement layout diagrams with project supervision
  • Mark all column positions clearly with numbered identifiers
  • Conduct comprehensive technical briefings before commencement

Material Specifications

Vibro-replacement stone columns require:

  • Clean, weathered gravel or crushed stone (natural gradation 1-2-3)
  • Crushing value <32%
  • Maximum mud content ≤10%
  • Maximum particle size: 8cm (with >50% exceeding 5cm)
  • Minimum particle size: 2cm

Construction Sequence for Stone Columns

The vibratory sunk pipe method employs a jumping pattern sequence:

  • Sandy soils: Work from the perimeter inward with a row-skipping pattern
  • Clayey soils: Progress from center outward in row-skipping fashion
  • Minimum stone volume: 0.128m³/m per column

Step-by-Step Process:

  1. Site Preparation: Level and mark column positions
  2. Alignment Survey:
    • Establish control points from the provided benchmarks
    • Protect centerline markers appropriately
  3. Positioning:
    • Mark column locations per specifications (≤2cm tolerance)
    • Prevent marker disturbance during operations
  4. Equipment Setup:
    • Align the pile machine vertically (≤1.5% deviation)
    • Calibrate pipe length and feed openings
    • Place the initial stone layer at the column base
  5. Vibratory Penetration:
    • Lower pipe to design depth with 30-second pauses every 0.5m
    • Slightly raise the pipe to open the bottom flaps
  6. Stone Feeding:
    • Calculate volumes using theoretical values and fill factor (typically 1.02, minimum 0.95)
    • Maintain consistent feed rates to ensure column uniformity
  7. Pipe Extraction:
    • Begin withdrawal when stone fill exceeds 1/3 pipe length
    • Use vibration during extraction (1.2-1.5m/min average speed)
    • Implement 40cm re-penetration at 0.5-1m intervals with 30-60 second pauses
  8. Multi-stage Construction:
    • Complete the first stage based on the design length
    • Open the secondary feed port for additional stone
    • Continue vibration during final extraction
  9. Surface Completion:
    • Feed the remaining stone at ground level
    • Apply pressure until the equipment lifts slightly
  10. Post-construction:
    • Move the equipment to the next position
    • Maintain a clean worksite

Surface Treatment

After completion:

  • Install an 80cm gravel bedding layer over the columns
  • Compact the entire area with a 20t vibratory roller (minimum two passes with overlap)

Quality Control Measures

Stone column quality is verified through:

Electrical Current Method

  • Monitor lifting current or impact current

Comprehensive Parameter Method

  1. Vibration Parameters:
    • Frequency: 24.5Hz standard
    • Voltage: 380V±20V
    • Excitation force: 280- 345kN (site-tested)
  2. Construction Controls:
    • Stone feed height: 1-1.5m
    • Densification current: Site-determined
    • Vibration duration: 30-60 seconds (site-tested)
    • Extraction rate: 1.2-1.5m/min (site-tested)
  3. Densification Techniques:
    • Combine extraction vibration with pauses and re-penetration
    • For upper 1-2m zones (reduced lateral confinement):
      • Use the over-feed method
      • Minimum stone volume: 0.2m³ per depth unit

Quality Assurance

  • Reject columns showing:
    • Discontinuities
    • Insufficient stone volume
    • Position deviations
    • Strength deficiencies
    • Defects (broken, necked, or settled columns)

This comprehensive approach ensures proper installation of vibratory stone columns for ground improvement applications, meeting all technical specifications and quality standards.

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