Rotary drilling rigs have become one of the mainstream construction machines for bored piles due to their advanced technology, rapid and stable hole formation. However, engineering geological conditions vary significantly across regions, and design specifications often differ from actual stratigraphy, sometimes substantially. Consequently, rotary drilling pile foundation construction faces considerable uncertainties. Improper handling of technical issues during drilling can lead to significant economic losses and even endanger equipment and personnel safety. Based on years of practical experience in various formations, this article summarizes common technical challenges and solutions when using rotary drilling rigs in special strata, providing valuable references for industry peers.
In soft soil, recently backfilled soil, or loose fine sandy layers with high groundwater levels, borehole walls remain unstable even with slurry wall protection, leading to frequent collapses or necking. Since such strata are typically shallow, increasing collapse volumes often cause ground subsidence around the borehole and guard pipe sinking. Severe cases may threaten drilling equipment and personnel. If the guard pipe becomes suspended or sinks during construction, drilling should halt immediately. The hole must be backfilled, and the guard pipe repositioned before resuming work.
For water-bearing soft or recently backfilled soil where slurry wall protection is ineffective, the most reliable method is to use long guard pipe walls. Each rig should be equipped with:
3 steel guard pipes (adjusted based on drilling difficulty)
1 vibratory hammer
1 crane with sufficient lifting capacity
Guard pipe specifications:
Length: Must extend to the bottom of the soft layer
Diameter: ~100mm larger than pile diameter (excessive size wastes concrete; insufficient size risks drill bit jamming)
Key considerations:
Ensure no large stones (>100mm) obstruct guard pipe installation; select a vibratory hammer with appropriate power.
For small-diameter drilling, maintain a minimum 200mm gap between drill rod and guard pipe (larger gaps for longer pipes).
Ensure vertical alignment during guard pipe installation, especially for small-diameter piles.
Remove guard pipes promptly after concrete pouring.
Coordinate guard pipe placement and drilling for efficient workflow.
In fine sand or silt layers with high groundwater, slurry wall protection is essential. These cohesionless strata are highly susceptible to erosion under water flow. During drilling, the drill bucket’s movement generates high-velocity slurry flow, scouring borehole walls and reducing slurry effectiveness, leading to necking or collapses.
Mitigation measures:
Reduce bucket lowering/raising speeds to minimize slurry flow erosion.
Increase the tooth outward angles to widen the gap between bucket and borehole walls.
Enlarge bucket drainage holes to reduce suction-induced slurry flow.
Use high-quality slurry and monitor sand content; take corrective actions if超标.
Inspect bucket seal integrity to prevent sand leakage.
Boulder layers in mountainous or river-crossing bridge projects often contain large, interlocked rocks that standard buckets cannot penetrate, risking tooth/drill rod damage. Isolated rocks (“protruding stones”) may cause hole deviation, rebar cage installation difficulties, or post-pouring collapses.
Shallow boulders: Excavate and backfill with clay; reposition guard pipe before drilling.
Deep boulders: Use core barrels to extract large rocks, then switch to standard buckets.
Isolated rocks:
Fully exposed rocks: Extract directly with core barrels.
Protruding rocks: Cut and remove gradually to avoid deviation.
Hard clay causes bit balling and soil discharge difficulties. Mudstone, with high strength and water-softening properties, often leads to drill slippage and inefficiency, especially with friction drill rods.
Hard clay: Use open-side buckets or notched designs for easier discharge.
Mudstone:
Prefer locked drill rods for direct pressure; add weight to friction rods if necessary.
Use V19 teeth with staggered heights/angles for better penetration.
For large diameters, adopt multi-stage drilling (core barrel first, then full-diameter bucket).
Avoid sparse tooth arrangements and uneven wear.
Common in southern China, karst formations feature uneven bedrock surfaces and variable cave sizes, causing deviated holes, sudden slurry loss, collapses, or equipment damage.
Slow drilling near cave roofs; stockpile slurry and clay for backfilling.
Use straight, elongated buckets for better alignment. Weld guide beams if deviation occurs.
Backfill and compact large empty caves to prevent concrete over-pouring.
Large-diameter piles (>1.5m) in moderately weathered hard rock require high-torque rigs (≥280 kN·m). Rapid tooth wear and equipment vibration are major challenges.
Use high-hardness teeth and water cooling in dry strata.
Multi-stage drilling: Start with 600–800mm core barrels to create free surfaces.
Correct hole deviation immediately; reaming is extremely difficult.
Wall Protection: Prioritize slurry walls; use full guard pipes where slurry fails.
Rig Selection: Match rig power to formation hardness and pile dimensions.
Tool Configuration: Optimize tooth layouts based on exploratory drilling.
Critical Zones: Proceed cautiously near inclined bedrock, protruding rocks, or caves.
By adapting techniques to specific strata and maintaining rigorous operational protocols, rotary drilling rigs can achieve efficient and safe pile construction across diverse geological conditions.
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