Causes and Countermeasures of Overbreak in Pile Foundation Hole Formation by Rotary Drilling Rigs
2013-12-10
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Rotary drilling rigs often encounter overbreak problems when drilling in soft flow-plastic strata and loose strata. Borehole overbreak not only wastes materials and affects pile quality but can also cause in-hole accidents in severe cases. Therefore, overbreak must be effectively controlled during hole formation to complete pile foundation construction with high quality and efficiency.
1 Causes of overbreak in hole formation
1.1 Mud-related causes
1.1.1 The mud quality is substandard; the mud density, viscosity or sand content fails to meet technical requirements, so the mud cannot effectively stabilize the hole wall, causing collapse of loose strata.
1.1.2 The mud level in the hole is lower than the groundwater level. In areas with high groundwater, the mud level in the hole is lower than the groundwater level, and groundwater seeps into the hole, causing flow failure of the soft flow-plastic layer and collapse of loose strata.
1.2 Operational causes
In soft flow-plastic strata, because the drilling feel is not obvious, the drill bit is often pressed into rather than drilled into the stratum, causing the bit to adhere to the surrounding soil and forming a bit balling phenomenon. When the drill is lifted, a piston effect occurs, drawing part of the soil around the bit to the lower part of the bit and causing overbreak.
1.3 Stratum-related causes
In soft flow-plastic strata, disturbance by the drilling tools tends to liquefy the layer, causing the stratum to lose its original stable state, and part of the soil from the hole wall flows into the hole, causing overbreak.
1.4 Connection problems between drill bit and drill pipe
Due to long-term wear of the square drive connection between the drill bit and drill pipe, the male square of the drill pipe wears smaller while the female square on the drill bit wears larger, increasing the connection clearance. During rotary drilling, the drill bit and drill pipe form a certain angle, making the rotation diameter of the drill bit larger than the bit diameter, so the formed hole is larger than the designed hole diameter, causing overbreak.
2 Measures to prevent overbreak in hole formation
2.1 Maintain the mud level in the hole and increase pressure on the hole wall.
In soft flow-plastic and loose areas with high groundwater levels, first ensure mud quality so that all indicators meet construction requirements and the mud can fully stabilize the hole wall. To maintain the mud level in the hole, build a cofferdam at the hole mouth connected to the mud in the hole. When the drilling tools are lifted out, the mud in the cofferdam replenishes the hole, keeping the mud in the hole always higher than the groundwater level. The higher the mud level, the greater the pressure on the hole wall and the more stable the hole wall.
2.2 Control drilling to avoid bit balling
When drilling in soft flow-plastic strata, to prevent the bit from being pressed into the stratum, record the hole depth after each drilling pass and start drilling from that depth on the next run; control the drilling speed to reduce excessive disturbance to the layer.
2.3 Control penetration
In soft flow-plastic strata, the penetration should be less than that in normal strata to avoid or reduce the resistance caused by the piston effect during drill lifting.
2.4 Keep the drill bit surface smooth and avoid or reduce bit balling. In soft flow-plastic strata, cohesive soil often adheres around the bit during each drill lifting; it should be removed promptly to reduce adhesion between the bit and the surrounding cohesive soil and prevent bit balling.
2.5 Reduce the connection clearance between drill bit and drill pipe. Frequently inspect the square drive connection between the drill bit and drill pipe, and repair wear promptly to reduce the connection clearance.
3 Conclusion
Overbreak is a common problem for rotary drilling rigs in soft flow-plastic and loose strata, and prevention should come first. This requires rig managers and operators to recognize the hazards of overbreak and to formulate preventive measures in advance for various strata to avoid greater losses during construction.
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