Construction methods · Bored piles and deep foundations

Continuous flight auger (CFA) piles

A hollow-stem auger is screwed to full depth in one pass and concrete is pumped through the stem as it is withdrawn, then a cage is pushed into the fresh concrete. No casing, no slurry, very fast, moderately quiet; the mass-production pile of European and North American viaducts on soil.

How bridges get built · Bored piles and deep foundations · Continuous flight auger (CFA) piles

Also called: auger cast piles, ACIP, CFA pile

Concept diagram (TheBridgeEng)

How it is done

Drill the auger to depth without removing spoil; monitor depth, torque and speed.
Pump concrete through the stem at a controlled rate while withdrawing the auger; the computer records volume per metre.
Push the cage into the wet concrete with a vibrator; cages are limited to about 12-18 m.
Trim the pile and test integrity.

Where it fits

Soft to firm clays, sands above and below the water table, viaduct approaches, 450-1,200 mm diameter, 10-30 m deep, loads to about 3-5 MN.

Where it does not

Rock sockets, boulders, very loose sands (flighting removes soil and loosens the ground), deep cages.

Plant, pace and money

PlantCFA rig with instrumented head, concrete pump, cage vibrator.

Productivity10-25 piles of 15-20 m per rig per shift.

CostUSD 90-180 per metre for 750 mm.

Risks and controls

What goes wrong

Over-augering in loose sand, necking where the concrete pressure drops, cage not reaching depth.

Quality assurance

Instrumentation record (depth, concrete volume and pressure per metre) for every pile, integrity tests, load tests.

Examples

UK motorway widening, US highway viaducts on alluvium, urban viaducts in Europe.