Auger Cast Piles or Continuous Flight Auger (CFA)

Some contractors working on the Miami Signature Bridge bragged about pioneering the use of "auger cast piles" for building bridges in the US. A comment by mkeit on those notes taught me that the technique is also called Continuous Flight Auger (CFA). Using many piles under a foundation has been the solution for many decades as to how to hold up heavy structures. Either the piles go down to bedrock or they are long enough to create enough friction to hold the structure. Initially, the piles were wood poles driven into the ground. Then H-beams and precast concrete columns were driven. But when the piles are really thick, they can't be driven. For thick piles, a hole is bored into the ground and then filled up with reinforced concrete. The drills I have seen have an auger with just a few turns on the end of a long shaft.

Screenshot

The auger is lowered into the bore and the shaft is turned until the auger fills up with material. Then the auger is brought up out of the bore, the unit turned to its side and the auger is spun to fling the material out of the auger. Then the cycle is repeated by placing the auger back into the bore. If the soil is not competent, then a steel casing has to first be driven into the ground to keep the soil from collapsing back into the bore being dug. Once the bore is dug, a rebar cage is lowered into it, and it is filled with concrete.

Continuous Flight Auger uses an auger that is a long as the desired depth of the bore and that has a hollow shaft. It allows drilling bores in incompetent soil without the need to first drive a casing into the soil. It also removes the dead time of raising the auger out of the hole every few feet to discard the material. Since a picture is worth a 1000 words, this video explains the process.

(new window, 1:57)

This video explains the process using real equipment.

(new window, 4:30)  (It is a shame that they play the torture music during the narration. In one case, it actually drowned out some of the words!)