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Construction Method and Technical Measures for Tri-Axis Cement Soil Mixing Piles and SMW Method Piles

1. Tri-Axis Cement Soil Mixing Pile Construction Method and Key Technical Measures

1.1 Equipment Selection and Construction Method

This project utilizes the JB-160 tri-axis drilling machine for constructing Ф850@600 tri-axis cement soil mixing piles, with a total length of approximately 350,000 meters. The pile lengths are as follows:

  • K7+726–K7+755 (22m)

  • K7+755–K7+815 (22m)

  • K7+815–K7+965 (21m)

  • K7+965–K8+020 (10m)

Refer to the project’s enclosure drawings for detailed specifications.

Construction Sequence:

  • One tri-axis mixing pile machine will be deployed, starting from the south side of K7+726 and moving eastward.

  • Before construction, verify pile positions through measurement.

Construction Technique:

  • Ф850@600 tri-axis mixing piles adopt a method where the side shafts rotate forward for grout mixing while the central shaft rotates backward for air-jet mixing.

  • The “four-mixing, two-spraying” process (two up-down cycles with grout injection during descent) is applied.

  • After completion, conduct a waterproof curtain test before excavation to verify sealing effectiveness.

1.2 Construction Process Flow

1.3 Technical Requirements and Measures

1.3.1 Underground Obstruction Removal & Trench Excavation

  • Remove obstructions larger than 150mm (stones, concrete blocks).

  • Fill with plain soil; for river sections, construct cofferdams, dewater, dredge, and backfill.

  • Excavate a 1200mm wide × 1200-1500mm deep guide trench.

  • Ensure the construction platform is level (≤50mm deviation) and compacted before laying roadbed plates.

1.3.2 Surveying and Layout

  • Use a total station for control point measurement (four-round directional observation method).

  • Accurately mark pile centerlines and depths based on design drawings.

1.3.3 Construction Sequence

  • Skip-slot double-hole full re-mixing connection ensures overlapping quality.

1.3.4 Pile Machine Positioning

  • Adjust verticality (≤1/250) using a plumb bob.

  • Positioning deviation must be <20mm.

1.3.5 Mixing Speed and Grout Control

Key Parameters:

  • Cement: P.O 42.5, 20% cement ratio, water-cement ratio 1.5–2.0.

  • Lifting speed: ≤0.8m/min.

  • Cold joint treatment: Overlap with an extra pile or high-pressure jet grouting (HPJG) if intervals exceed 2 hours.

  • Deviation limits: Center offset ≤50mm, verticality ≤1/200.

Grout Preparation:

  • Mix on-site with a strict 2-hour usage limit.

  • Filter cement to prevent clogging; discard stagnant slurry after 2 hours.

Construction Monitoring:

  • Record下沉 sinking (sinking) and lifting speeds, depths, and grout volumes.

  • Ensure continuous operation to avoid pile breaks.

1.3.6 Joint Treatment

  • 12–24h delay: Add 3 overlapping mixing piles externally.

  • >24h delay: Reinforce with 3 Ф800@500 HPJG piles.

1.4 Quality Inspection and Pile Testing

  • After 7 days, inspect pile integrity via excavation.

  • Core drilling (after 28 days): Test uniaxial compressive strength, continuity, and depth (≥2% of piles, min. 3 piles).

2. SMW Method Pile Construction and Key Technical Measures

2.1 Equipment and Construction Method

  • JB-160 tri-axis mixer constructs Ф850@600 (or Ф1000@750) SMW piles with embedded H700×300×13×24 (or H800×300×14×26) steel beams.

  • Arrangement:

    • Dense insertion at K7+559–K7+592.

    • Alternate insertion at K7+755–K7+965.

    • HPJG piles (Ф800@600) at corners/joints.

2.2 SMW Pile Construction Process

2.3 Construction Methodology

2.3.1 Site Preparation

  • Level and compact the ground to support 50-ton cranes and pile machines.

2.3.2 Surveying

  • Set control points using a total station; offset the enclosure pile centerlines by 10cm to account for deformation.

2.3.3 Guide Trench Construction

  • Dimensions: 1.0m (bottom) × 1.2m (top) × 0.6m (depth).

  • Align trenches precisely with pile axes (deviation ≤10mm).

2.3.4 Construction Sequence

  • Full-circle overlapping method ensures continuity.

2.3.5 Steel Beam Installation

  • H-beams are welded (E43XX electrodes), tested via UT, and coated with friction reducer (15% oxidized wax + additives).

  • Insert beams vertically (deviation ≤1/200) using a 50-ton crane.

2.3.6 Extraction of Beams

  • Post-construction: Extract beams after backfilling, using jacks and pure cement grout (W/C=1.15–1.18) to fill voids.

2.3.7 Quality Control

  • Core testing: ≥2% of piles (min. 3) for strength and integrity.

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