Q. I have a ready-mix customer who is supplying concrete for caissons. The mix design is an ordinary caisson mix (similar to CDOT Class BZ) designed for 6- to 8-inch slump placements. The caissons range from 30 to 60 inches in diameter and are 20 to 25 feet deep. They have not encountered any water during drilling. The concrete is being placed with a tremie, with the top 10 feet being vibrated after the tremie is withdrawn.

The problem is that sometime before the concrete hardens, a vertical subsidence of 3 to 4 inches occurs in the middle of the caisson. Anchor bolts placed in the plastic concrete are no longer in the correct position. Why is this occurring?

A. This is simply consolidation subsidence in the lower, unconsolidated portion of the caisson. The tremie-delivered concrete isn’t vibrated, and it settles as the mass of concrete above it forces air from the concrete in the lower portion of the caisson.

To avoid the subsidence, you can either get rid of the tremie, allowing the free fall of the concrete to supply the necessary consolidation to the lower caisson section. Or you can vibrate to the caisson’s full depth. Obviously, it is simpler to opt for allowing the concrete to fall freely into the caisson.

A 1970 study sponsored by the International Association of Foundation Drilling determined that the free fall of concrete to depths of 120 feet was possible without segregation and actually produced superior strengths due to the high degree of consolidation. This was achieved by the energy supplied by the impact of the free falling concrete.

Although your application is not as extreme, you should still observe the primary cautions for this type of placement: a quality concrete mixture, an unobstructed vertical drop, and protection of soil sidewalls to prevent cave-ins.

Regarding the anchor bolts, you should also consider setting them in a plywood template. That would help maintain their correct locations as the concrete sets and would be a good practice even if you weren’t concerned about subsidence.