1. Quality Problems in Rotary Drilling Pile Construction
(1) Hole Wall Collapse
Hole wall collapse is a common issue in rotary drilling pile construction, primarily caused by the following factors:
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Improper High Fill Backfilling: When the backfill material consists mainly of moderately weathered siltstone fragments (commonly known as “sandstone blocks”), improper layered compaction during manual backfilling can lead to voids between the fill materials, causing slurry leakage during mud wall protection.
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Poor Hole Wall Protection: The drilling speed of rotary drilling rigs is relatively fast, relying mainly on cutting soil for advancement. As a result, the whole wall protection is inferior to that of bored piles and percussion piles. In loose backfill soil, hole wall collapse is more likely to occur. Additionally, if the mud density is too low or the mud does not circulate, the mud film thickness will be insufficient, leading to collapse.
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Negative Pressure Inside the Hole: When drilling in soft soil or backfill layers, the large contact area between the drill barrel and the soil can create negative pressure inside the hole when lifting the drill bucket. This effect becomes more pronounced with increasing drilling depth.
(2) Casing Bottom Collapse
Casing bottom collapse occurs due to two main reasons:
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Insufficient Water Level: If the stabilizing fluid is not replenished in time when the drill bit or rod is withdrawn, the fluid level may drop near or below the casing bottom, eventually causing the geological layer at the casing bottom to collapse.
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Soil Conditions: When mud flushes through backfill or soft soil, it can erode the soil layer beneath the casing, leading to collapse.
(3) Rebar Cage Floating and Deformation
During construction, if the concrete slump is too small, the concrete sinks too quickly, the rebar cage is too short, or its weight is insufficient, the rebar cage may float upward. Additionally, if the slenderness ratio of the rebar cage is too high, improper lifting point arrangement can cause bending and deformation during hoisting.
(4) Excessive Sediment at Pile Tip
When drilling in sandy geological layers, sand can mix into the stabilizing fluid. Since stabilizing fluid is often reused, sand particles may settle at the bottom of the pile hole, leading to excessive sediment. Poor manual operation during drilling can also allow small sand particles to leak into the stabilizing layer, accumulating over time and increasing sediment thickness.
2. Key Quality Control Measures for Rotary Drilling Pile Construction
(1) Hole Wall Construction Quality Control
To ensure whole wall stability:
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Use Mud Wall Protection: Strictly control mud density (1.1–1.5), viscosity (≥18s), and sand content (≤6%). Add stabilizing fluid directly toward the casing wall.
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Control Drilling Speed: Limit the drilling depth and speed to prevent excessive force. Lift the drill slowly and evenly (lifting speed < 0.6 m/s, lowering speed < 0.8 m/s) to avoid negative pressure.
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Avoid External Disturbances: Prevent material stacking near the hole and minimize rebar cage collisions with the hole wall.
(2) Casing Bottom Construction Quality Control
To prevent casing bottom collapse:
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Set Proper Casing Depth: Typically 5–10 m, adjusted based on geological conditions.
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Ensure Casing Dimensions: The casing diameter should exceed the pile hole diameter by 20 cm, with at least 0.5 m embedded in the clay layer and 0.3 m above ground. Keep the casing inclination rate below 1% and deviation within 3 cm.
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Compact Backfill Properly: Use layered clay compaction.
(3) Rebar Cage Floating and Deformation Control
To mitigate rebar issues:
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Control Concrete Pouring Speed: Prevent excessive impact force from fast-sinking concrete.
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Optimize Concrete Pipe Depth: Ensure the pipe is leak-proof and maintain a burial depth of 1.5–2 m in the concrete.
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Proper Hoisting: Use multiple lifting points to avoid deformation.
(4) Pile Tip Sediment Control
To minimize sediment accumulation:
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Use Specialized Cleaning Tools: Employ a sediment-cleaning drill bit post-drilling.
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Install Sediment Removal Systems: Use pumps or settling tanks to filter out sand and soil from stabilizing fluid.
3. Key Considerations for Rotary Drilling in Backfill Layers
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Review design drawings and geological reports to understand backfill and natural soil conditions.
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Ensure operators are proficient in equipment handling.
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Adjust casing length based on soil structure.
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Select appropriate drill bits for site conditions.
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Monitor geological changes closely during construction.
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Implement emergency measures for loose or collapsing soil.
4. Advantages of Rotary Drilling Rigs in Backfill Layers
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Versatility: Suitable for various backfill soil types.
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High Drilling Capacity: Strong penetration due to dynamic head force and drill rod weight.
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Fast Hole Formation: Up to 1 m/min, significantly faster than traditional methods.
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Eco-Friendly: Minimal slurry use reduces pollution.
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Enhanced Bearing Capacity: Rough hole walls prevent shrinkage.
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Mobility: Easy relocation with crawler mechanisms.
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Precision Alignment: Advanced electronic controls ensure accurate positioning.
5. Conclusion
To ensure rotary drilling pile quality, contractors must address common issues like hole wall collapse, casing failure, rebar cage problems, and sediment buildup through strict quality controls. Proper techniques in drilling, casing installation, rebar handling, and sediment management enhance structural safety and stability.
