Abstract
This article analyzes the process characteristics, application scope, and construction techniques of the reverse circulation cleaning method, exploring its application in rotary drilling rig cast-in-place pile construction. The goal is to improve construction efficiency, cost-effectiveness, and quality.
1. Introduction
Rotary drilling rigs are construction machinery widely used in foundation engineering for hole-forming operations. They are extensively applied in municipal construction, highway and bridge projects, railways, water conservancy, and high-rise buildings. The cast-in-place pile technology using rotary drilling rigs is recognized as a “green construction method” due to its high efficiency, superior quality, minimal mud pollution, flexibility, and adaptability to various soil conditions.
Rotary drilling technology represents advanced construction methods and has significant development potential, making it a key direction for future pile foundation construction. These rigs are multifunctional and efficient, capable of automatic mast verticality adjustment and depth measurement. The drilling process involves rotating the drill pipe and bucket, using hydraulic pressure and bucket weight to excavate soil, which is then lifted and discharged.
However, excessive sediment at the bottom of rotary drilling piles often reduces bearing capacity. Ultrasonic testing of underground piles in past projects revealed defects in the concrete edges, primarily caused by localized inclusions. These inclusions consist of sediment from slurry and clay blocks dislodged during reinforcement cage placement. If not addressed, these sediments can lead to structural defects.
Construction standards require the sediment thickness to be less than 10 cm. Given the complex geological conditions in many projects, controlling sediment thickness is a critical challenge. From drilling completion to concrete pouring, deep piles often require over an hour, during which slurry sedimentation occurs. Therefore, reverse circulation cleaning is essential before pouring to ensure quality.
2. Process Characteristics
2.1 Thorough Cleaning
Ensures sediment thickness ≤ 10 cm.
2.2 High Speed
Typically takes about 50 minutes for secondary cleaning after initial circulation.
2.3 Rapid Conversion
Switching from cleaning to concrete pouring takes under 10 minutes.
2.4 Cost-Effective
Requires minimal equipment and materials, offering flexibility and simplicity.
3. Application Scope
3.1 Suitable for:
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Pile holes with depths up to 150 m.
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Projects with strict sediment thickness requirements.
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Land or water-based bored pile construction.
3.2 Applicable Strata:
Clay, sand, gravel, pebble, and rock layers.
4. Construction Process
4.1 Purpose of Cleaning
After reaching the required depth, cleaning removes sediment, a major factor affecting pile bearing capacity. The process uses slurry flow to suspend and extract sediment.
4.2 Theoretical Basis for Cleaning Method Selection
Sediment consists of clay blocks, sand, and rock residues. The critical suspension velocity (V₀) is determined by balancing gravitational, buoyant, and drag forces.
4.3 Drawbacks of Traditional Forward Circulation
Forward circulation relies on slurry flow to lift sediment, but low pump rates and uneven flow distribution often leave large particles unresolved.
4.4 Reverse Circulation Cleaning
4.4.1 Pump-Suction Reverse Circulation
Uses a sand pump to create negative pressure, extracting sediment through the drill pipe. A 4BS pump (200 m³/h) ensures sufficient upward flow (1.58 m/s) to remove sediment effectively.
4.4.2 Air-Lift Reverse Circulation
4.4.2.1 Principle
Compressed air reduces slurry density in the discharge pipe, creating a pressure differential that lifts sediment. Additional slurry is pumped in to maintain circulation.
4.4.2.2 Equipment
Requires only an air compressor (9–20 m³/h), discharge pipes, and connectors.
5. Quality Standards and Control
Post-cleaning sediment must be <10 cm, with slurry density (1.03–1.10), viscosity (17–20 s), and sand content (<2%). Concrete must be poured immediately to prevent re-sedimentation.
Key Operational Notes for Air-Lift Reverse Circulation:
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Discharge pipe depth ≥30 m for optimal pressure.
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Ensure airtight connections.
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Maintain a sufficient slurry supply to prevent collapse.
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Move the pipe laterally for thorough cleaning.
6. Equipment
6.1 Pump-Suction Reverse Circulation:
Sand pump and discharge hose.
6.2 Air-Lift Reverse Circulation:
Air compressor, metal/soft discharge pipes, and fittings.
7. Safety Measures
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Follow crane operation protocols.
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Ensure proper electrical grounding and leakage protection.
8. Environmental Protection
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Prevent oil leakage during machinery maintenance.
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Treat wastewater and waste materials properly.
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Minimize noise pollution.
9. Economic Analysis
9.1 Quality Assurance
Reverse circulation ensures cleaner holes, reducing concrete pouring resistance and improving pile integrity.
9.2 Time Efficiency
Reduces cleaning time from 4 hours (forward circulation) to ~1 hour.
9.3 Cost Savings
Higher efficiency lowers equipment and labor costs.
Conclusion
Reverse circulation cleaning enhances rotary drilling efficiency, pile quality, and economic benefits, making it a superior choice for modern foundation construction.
