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Stone Column Construction Organization Design

1. Project Overview

The project is located in the alluvial plain area of the Yellow River. The unfavorable geological conditions in this section mainly consist of soft soils, including silt, silty clay with a low liquid limit, and muddy silt. The physical and mechanical properties are as follows:

  • Moisture content (W) = 51.0%
  • Void ratio (e) = 1.380
  • Liquid limit (WL) = 59.6%
  • Plastic limit (WP) = 21.0%
  • Compression coefficient (a₀.₁₋₀.₂) = 1.12 MPa⁻¹
  • Compression modulus (ES) = 2.31 MPa

These soft soils exhibit high water content, large void ratio, high compressibility, and low strength.

2. Ground Improvement Principle

The primary objectives of vibro-replacement stone column ground improvement in sandy soil are to enhance bearing capacity, reduce settlement, and improve liquefaction resistance. The anti-liquefaction mechanism of stone columns in sandy soil includes three key aspects:

(1) Compaction Effect

During pile formation, the pile tube exerts significant lateral pressure on the surrounding sand layer, compacting it and reducing the void ratio, thereby increasing density.

(2) Drainage and Pressure Relief Effect

Stone columns filled with gravel, pebbles, or coarse sand act as vertical drainage channels, effectively dissipating excess pore water pressure and preventing liquefaction while accelerating soil consolidation.

(3) Pre-Vibration Effect

The intense vibration from the hammer during pile installation densifies both the backfill material and the surrounding soil, enhancing liquefaction resistance. The composite soil layer acts as a cushion, distributing loads evenly and improving overall bearing capacity while reducing settlement.

3. Construction Schedule

The stone column construction is planned to commence on October 12, 2003, with completion expected by mid-November 2003. (Detailed schedule attached.)

4. Equipment and Personnel Deployment

  • 15 DZ40-60 stone column machines will be deployed.
  • Each machine will be equipped with one generator, one winch, and 5-6 operators.
    (Detailed equipment list attached.)

5. Stone Column Construction Technical Plan

(A) Preparatory Work

  1. Prepare and approve the construction organization design before commencement.
  2. Clear and level the site, removing overhead and ground obstacles.
  3. Conduct surveying, mark centerlines and boundaries, measure elevation, and ensure proper drainage.
  4. Plot the stone column layout based on design spacing (±150 mm tolerance).
  5. Reserve 140 cm clearance around boreholes.
  6. Prepare crushed stone materials.
  7. Implement safety measures and approved operating procedures.

(B) Material Requirements

  • Use clean, uncrushed gravel or crushed stone with 1-2-3 natural gradation.
  • Maximum particle size: ≤4 cm; mud content: ≤5%.

(C) Construction Process (Vibro-Replacement Method)

  1. Level the site and mark pile positions.
  2. Position the machine, ensuring verticality ≤1.5%.
  3. Vibrate the pile tube to design depth (0.5 m intervals with 30 sec vibration).
  4. Slightly lift the tube to open the pile tip.
  5. Fill the tube with crushed stone.
  6. Lift the tube while vibrating (1-2 m/min), with 30 cm re-driving every 1 m.
  7. Repeat backfilling and compaction until the full pile length is achieved.
  8. Conduct final compaction and surface backfilling.

(D) Trial Pile Testing

Before full-scale construction, conduct trial piles to determine:

  • Effective pile length.
  • Optimal parameters (vibration frequency, retention time, re-driving depth, etc.).
  • Stone filling quantity verification.

(E) Quality Control Measures

  • Ensure accurate pile spacing (equilateral triangle arrangement).
  • Use approved crushed stone materials.
  • Follow jump-sequence construction (outer to inner).
  • Monitor and record real-time parameters (current, depth, stone volume).
  • Prevent “broken piles” or “necking” by controlled backfilling.
  • Report anomalies (soil variation, abnormal filling) immediately.

(F) Permissible Construction Tolerances

Item Tolerance Inspection Method
Pile Spacing ±150 mm 2% sampling
Pile Diameter ≥Design 2% sampling
Pile Length ≥Design Records
Verticality ≤1.5% Records
Stone Volume ≥Design Records

Compensatory piles are required if the design volume is unmet.

(G) Quality Inspection

  1. Pre-construction testing: 9-16 trial piles; adjust parameters if needed.
  2. In-situ tests: Standard Penetration Test (SPT), Cone Penetration Test (CPT), Rayleigh Wave Method.
  3. Post-construction testing (after 15 days):
    • SPT: 1% of total piles.
    • CPT: 3 points/5000 m².
    • Rayleigh Wave: 1 point/40 m.
    • Dynamic Probing: 2-3‰ of piles.

Acceptance Criteria:

  • SPT: N₆₃.₅ ≥ Liquefaction threshold.
  • Shear wave velocity: ≥200 m/s (sandy soils), ≥150 m/s (muddy clay).
  • CPT: qc ≥ 9 MPa (fine sand), ≥0.8 MPa (silt).
  • Load Test: Single pile ≥300 kPa; Composite foundation ≥150 kPa.

(H) Site Management

  1. Assign one supervisor per machine to monitor parameters and documentation.
  2. Track daily progress and conduct quality checks.
  3. Maintain real-time records of pile length, stone volume, and machine performance.
  4. Submit daily reports to the supervision team.

This plan ensures compliance with technical standards while optimizing efficiency and quality for stone column ground improvement.

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