Introduction
In the rotary drilling rig construction specifications, the allowable verticality deviation is 1%. For instance, a 100-meter pile length permits a maximum deviation of approximately 1.7 meters, while a 50-meter pile allows around 0.85 meters. Nowadays, many pile foundation projects use ultrasonic testing equipment to inspect pile length, diameter, and verticality with high precision. Exceeding the allowable deviation range can negatively impact hole quality and delay construction progress. Today, we will discuss the issue of deviated holes in rotary drilling in detail.
Analysis of Deviated Hole Phenomena
1. Sudden Deviation
Sudden deviation is typically caused by geological conditions. Due to uneven weathering of rock layers or irregular rock distribution, the drill bucket may suddenly get stuck at a certain depth, or the drill pipe may abruptly tilt to one side, indicating an immediate deviation at a specific position in the hole.
2. Gradual Deviation
When lowering the main hoist, the first section of the drill pipe gradually moves closer to one side of the casing as the hole deepens. If this trend continues, the hole slowly deviates in one direction—the closer the drill pipe gets to the casing, the more severe the deviation becomes.
3. Irregular Deviation
As the drill pipe is lowered, the first section may sway left and right. In dry hole drilling, an irregular spiral deviation may be observed. This type of deviation often occurs in fully or highly weathered rock layers, where uneven force distribution during drilling leads to longitudinal deviation.
Causes of Deviated Holes
1. Geological Conditions
Geology is the primary cause of deviation. If the soil consists entirely of loess or clay layers, deviation is unlikely. However, geological variations, such as weathered rock or uneven rock distribution, can cause uneven force on the drill bucket during operation.
2. Drill Bucket Design
While the main function of a drill bucket is cutting and drilling, it also plays a role in guiding and preventing deviation. The central pilot tip helps with positioning and alters the drilling sequence. Consistency in the number and angle of drill teeth, as well as the height of side cutters or protective strips on both sides of the bucket, ensures balanced force distribution during drilling, reducing deviation risks.
3. Operational Factors
When encountering rock, boulders, pebbles, or weathered layers, improper control of flotation, pressure application, or excessive drilling speed can lead to sudden deviation.
How to Prevent Deviated Holes
1. Understand Geological Conditions
Before construction, study geological reports, core samples, and the surrounding environment to gain a thorough understanding of the ground conditions.
2. Choose the Right Drill Bucket
Empirical evidence shows that spiral drill buckets are more effective in preventing deviation compared to double-bottom sand buckets. The spiral design reduces drilling resistance, and the pilot drilling method further minimizes deviation risks.
3. Optimize Drill Bucket Design
Enhance the drill bucket to reduce drilling resistance. The central pilot tip plays a crucial role in positioning and helps prevent deviation.
4. Use Guided Drill Buckets
Weld a 2-meter-long casing (matching the hole diameter) above the spiral or double-bottom drill bucket. This extended guide drilling method effectively prevents deviation.
5. Proper Operation Techniques
Control flotation, pressure, rotation speed, and drilling speed to minimize deviation.
6. Adjust Drilling Methods
Monitor vibration and load to assess geological conditions and adapt drilling techniques accordingly. Methods include:
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Cutting drilling
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Crushing drilling
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Stirring drilling
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Grinding drilling
Adjusting the drilling method can significantly reduce deviation risks.
Remedial Measures for Deviated Holes
A. Minor Deviation: Adjust the drilling rig and continue. For severe cases, backfill with clay, allow it to settle, and then resume drilling. Avoid using impact drill heads for correction to prevent jamming.
B. Moderate Deviation (Soft Rock): Use a flat-bottom drill bit to level the hole before resuming, or drill a pilot hole with a smaller bit before reaming.
C. Severe Deviation (Hard Rock): Stop drilling immediately, fill the hole with concrete of the same grade as the pile, and wait until the concrete reaches 70% strength. Then, drill a pilot hole with a smaller bit before reaming.
D. Shallow Deviation (Low Water Table): Correct the inclined rock surface manually using an artificial digging method.
Conclusion
Deviated holes in rotary drilling can significantly impact construction efficiency and pile quality. By understanding geological conditions, optimizing equipment, and employing proper techniques, operators can minimize deviation risks and implement effective corrective measures when necessary. Following these best practices ensures smoother operations and higher-quality outcomes in rotary drilling projects.
