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Last edited 11 Sep 2017
It is generally undertaken for safety and/or efficiency reasons, or because of restrictions in the ground, that prevent the use of a pier or pile that is wider along its entire length, and is most common on offshore drilling operations, exploration wells, extended reach drilling, and so on.
Underreaming may also be carried out to create tension piles, where the widened foot resists uplift forces that might otherwise cause the pile to be extracted from the ground. For more information see: Tension pile.
It can also be used in the formation of ground anchors.
Typically, the process involves a drill pipe with cutter blades that can be forced open by pumping a fluid into the pipe. The underreaming tool is kept open by the drilling weight and hydraulics from the pump pressure. The pumps are disengaged when the desired depth has been reached. The underreamer closes and is pulled out of the hole.
There is the risk that if the underreamer is left in the hole for too long, the amount of material left in the hole increases. Also, if penetration rates are too high, this can result in a corkscrewed, under-gauged hole.
It is possible to drill a pilot hole and then underream, or to drill and underream as part of the the same drilling operation.
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