Rotary drilling rigs have become one of the mainstream construction machines for bored piles due to their advanced technology, rapid drilling speed, and stable performance. However, engineering geological conditions vary significantly across regions, and design specifications often differ from actual strata, sometimes substantially. This creates numerous uncertainties for geotechnical contractors undertaking bored pile foundation projects. Improper handling of technical issues during drilling can lead to significant economic losses and even pose threats to equipment and personnel safety.
Based on recent practical experience with rotary drilling rigs in various formations, this article summarizes common technical challenges and solutions for drilling in special geological conditions.
1. Rotary Drilling in Soft and Collapsible Strata
Characteristics and Risks
In soft soil, newly backfilled soil, or loose fine sandy layers with high groundwater levels, borehole walls are prone to collapse or necking even with slurry support. Since these strata are typically near the surface, excessive collapse can lead to ground subsidence and casing pipe sinking, endangering equipment and workers.
If the casing pipe becomes suspended or sinks, stop drilling immediately, backfill the hole, and reposition the casing before resuming.
Construction Methods
The most effective approach in water-bearing soft or newly backfilled soil is full-length casing support. Recommended equipment includes:
- 3 steel casings (adjusted based on difficulty)
- 1 vibratory hammer
- 1 crane with sufficient lifting capacity
Key Considerations:
- Ensure no large stones (>100mm) obstruct casing installation.
- Select a vibratory hammer matching the casing diameter and length.
- Maintain a ≥200mm gap between drill rod and casing (larger for deeper casings).
- Ensure verticality, especially for small-diameter piles.
- Remove casings promptly after concrete pouring.
- Coordinate casing installation and drilling for efficient workflow.
2. Drilling in Sandy and Silt Strata
Challenges
In water-saturated fine sand or silt, slurry-supported drilling is necessary. However, these cohesionless soils are easily eroded by fluid flow, leading to borehole enlargement or collapse.
Solutions
- Reduce drill bit lowering/raising speed to minimize slurry turbulence.
- Increase the tooth angle to widen the gap between the bit and the borehole wall.
- Enlarge drainage holes in the drill bucket to reduce suction pressure.
- Use high-quality slurry and monitor sand content.
- Ensure the tight sealing of the bucket to prevent sand leakage.
3. Handling Boulders and Isolated Rocks
Risks
In boulder-rich strata (e.g., mountainous or river-crossing bridge piles):
- Large boulders may jam standard buckets or damage drill teeth.
- “Protruding rocks” (partially embedded in the borehole wall) can cause deviation or rebar cage installation issues.
Techniques
- Shallow boulders: Excavate and backfill with clay before casing installation.
- Deep boulders: Use a coring barrel to extract large rocks before switching to standard buckets.
- Protruding rocks: Cut and remove with a coring barrel—avoid forced drilling.
4. Hard Clay and Mudstone Formations
Challenges
- Hard clay causes bit balling and soil discharge difficulties.
- Mudstone softens when wet, leading to bit slippage and low efficiency.
Solutions
- Hard clay: Use open-side buckets or notch modifications for easier discharge.
- Mudstone:
- Preferably, locking drill rods for direct pressure application.
- Use V19 teeth and asymmetrical bit designs for better penetration.
- For large diameters, employ multi-stage drilling (coring first, then reaming).
- Avoid sparse tooth arrangements and maintain sharp-edged teeth.
5. Limestone Karst Formations
Risks
- Uneven bedrock surfaces cause hole deviation.
- Sudden slurry loss leads to collapse or equipment damage.
- Excessive concrete consumption in large cavities.
Methods
- Slow drilling near bedrock interfaces to prevent deviation.
- Pre-stock slurry and clay for rapid cavity sealing.
- Use straight, elongated buckets for better guidance.
- Backfill and compact large voids before concreting.
6. Hard Rock (Granite/Sandstone) Drilling
Requirements
- High-torque rigs (≥280 kN·m) for large-diameter piles (>1.5m).
- Wear-resistant drill teeth and vibration-resistant equipment.
Techniques
- Add water to dry formations to reduce tooth wear.
- Multi-stage drilling: Start with a 600–800mm coring barrel to create a free face.
- Correct deviation early—hard rock reaming is extremely difficult.
7. Key Takeaways
- Effective Borehole Stabilization:
- Use slurry support where possible; opt for full casing in weak soils.
- Proper Equipment Selection:
- Smaller rigs for shallow, simple strata; heavy-duty rigs for hard rock.
- Optimized Drill Tools:
- Customize tooth layouts based on trial drilling.
- Critical Zone Precautions:
- Slow drilling near cavities, inclined bedrock, or protruding rocks.
By adapting techniques to specific geological challenges, rotary drilling rigs can achieve efficient and safe bored pile construction across diverse conditions.
