Rotary drilling machines have become one of the most popular pieces of equipment for pile foundation construction, thanks to their advanced technology, rapid drilling, and stability. However, geological conditions vary significantly from place to place, making it difficult to perfectly match project designs with actual subsurface conditions. In fact, sometimes the differences between design and reality can be considerable. As a result, geotechnical contractors often face uncertainties when undertaking pile foundation drilling projects. Improper handling of certain technical challenges during drilling can not only lead to significant economic losses but also pose risks to both equipment and personnel safety. Drawing from years of experience in the practical use of rotary drilling machines in different strata, this article summarizes common technical problems encountered and their solutions when drilling in more challenging geological conditions. The aim is to provide valuable insights for those dealing with similar issues.
1. Drilling in Soft or Collapsible Soil Layers
Characteristics and Risks of Soft and Collapsible Soil Layers
Soft soils, recent backfill, and loose fine sand layers are prone to collapse, especially when the groundwater table is high. Even when using drilling mud to support the borehole wall, it is often difficult to maintain stability, leading to collapses or narrowing of the hole. These layers are typically found near the surface, and as the collapse increases, ground settlement around the borehole can occur, causing the casing to sink. In extreme cases, the safety of both the drilling equipment and personnel may be at risk. If the casing is found to be sinking or unsupported, drilling should be halted immediately, and the hole should be backfilled. Once the casing is securely fixed, drilling can resume.
Drilling Method in Soft and Collapsible Soils
In these conditions, using a long steel casing provides the most effective and reliable method. For each rotary drilling machine, approximately three steel casings, one vibrator, and one crane with adequate lifting capacity should be used. The length of the casing should extend to the bottom of the soft layer, and its diameter should be roughly 100mm larger than the pile diameter. Too large a casing leads to concrete waste, while too small can cause the drill head to get stuck. Key points to keep in mind when using long casings include:
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Ensure no large stones (over 100mm) are present in the casing depth range.
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For smaller diameter piles, ensure the drill rod diameter is at least 200mm smaller than the casing.
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Ensure the steel casing is lowered vertically, especially for smaller diameter piles.
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After concrete is poured, the casing should be promptly removed.
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The casing must be synchronized with the drilling process for continuous operations, ensuring higher efficiency.
2. Drilling in Sandy and Silty Layers
Characteristics and Risks of Sandy and Silty Layers
When drilling in fine sand or silt layers, especially with high groundwater levels, using mud for wall support becomes essential. Due to the lack of cohesion between the soil particles, these layers are easily washed away by water currents. As the rotary drilling machine moves the auger up and down, the surrounding water flow can cause significant erosion of the borehole walls, leading to issues such as necking and even collapse.
Drilling Method in Sandy and Silty Layers
To mitigate these risks, the following methods should be employed:
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Lower the drilling head speed and lifting speed to reduce water flow velocity and minimize erosion.
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Increase the angle of the drill teeth to improve the clearance between the auger and borehole walls.
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Increase the number and size of water holes in the auger to reduce negative pressure at the top and bottom, thus slowing the flow of mud.
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Use high-quality drilling mud and monitor the mud’s sand content, making adjustments as necessary.
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Inspect the seal on the auger bottom cover, and repair any gaps to prevent sand leakage.
3. Drilling in Boulder or Isolated Stone Layers
Characteristics and Risks of Boulder or Isolated Stone Layers
During the construction of large bridges, drilling through boulders or isolated stone layers is a common challenge. These stones often have a large diameter and are interlocked, making them difficult to penetrate using conventional auger heads. If drilled improperly, this can result in broken teeth or even damage to the drill rods. Furthermore, isolated stones within the borehole can cause the drill head to deviate, leading to potential issues with subsequent steel cage placement and concrete pouring.
Drilling Method in Boulder or Isolated Stone Layers
When encountering large boulders or stones:
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If boulders are near the surface, use an excavator to remove them and backfill with clay before continuing.
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If boulders are deep and larger than the auger’s opening, use a cylindrical drill bit to remove the stones, followed by a sand bucket to continue drilling.
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For isolated stones inside the borehole, carefully cut or remove them using a cylindrical auger before continuing drilling.
4. Drilling in Hard Clay or Mudstone Layers
Characteristics and Risks of Hard Clay and Mudstone Layers
Hard clay and mudstone are particularly challenging for drilling due to the difficulty in removing cuttings and the tendency for the drill bit to slip, reducing efficiency. In clay layers, the drill bit tends to clog, and in mudstone layers, the material’s high strength and sticky nature cause the drill to slide on the surface, leading to slow progress.
Drilling Method in Hard Clay and Mudstone Layers
To improve efficiency in these tough layers, consider the following methods:
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Use an auger with a wide opening or a bottom-side cutout to improve cutting efficiency.
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For mudstone layers, use a rotary drilling method with a drill rod lock to apply pressure and ensure penetration.
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For highly weathered layers, use V19 cutting teeth to improve penetration.
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In large-diameter holes, consider a staged drilling approach, starting with a larger auger to take a core sample, followed by a smaller one.
5. Drilling in Karst Limestone or Caves
Characteristics and Risks of Karst Limestone Layers
Karst limestone regions are common in mountainous areas and feature uneven bedrock surfaces, with various-sized and shaped caves. Drilling in these regions often leads to problems such as tilted boreholes, mud loss, equipment jamming, and excessive concrete volume during pouring.
Drilling Method in Karst Limestone Layers
When drilling in karst limestone, ensure the following:
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Slow down when nearing the cave top to avoid tilting the borehole.
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Use a straight, guided, extended auger for better directional control.
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If encountering a cave with little or no fill, backfill with clay and use a reverse rotary auger to compact the material.
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Keep adequate mud levels to prevent sudden mud loss and collapse.
6. Drilling in Hard Rock Layers (Granite, Quartz, etc.)
Characteristics and Risks of Hard Rock Layers
Drilling into hard rock layers, such as granite or quartz, requires high-power machinery, typically with a torque rating of over 280 kN.m. Drilling through these materials is tough on the equipment, as wear on the drill teeth is significant.
Drilling Method in Hard Rock Layers
To improve efficiency in hard rock drilling, consider:
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Use high-power equipment capable of generating over 280 kN.m of torque.
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Use durable, high-performance drill teeth designed for abrasive materials.
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Employ staged drilling techniques for large-diameter holes, starting with a larger core bit to create a free surface before drilling with a smaller auger.
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Ensure that any angled rock surfaces are corrected before continuing to drill.
Conclusion
Rotary drilling machines are highly versatile and can be used across various geological conditions. However, to ensure effective drilling, it’s essential to choose the right equipment and drill bits based on the specific geological features of each project. By preparing in advance, optimizing drilling methods, and addressing potential risks, contractors can ensure more efficient and safe pile foundation construction. Properly managing key aspects such as casing, drilling speed, and tool configuration is crucial for a successful project outcome.
