Caisson (engineering)

Hollow concrete structures are generally less dense than water so a box caisson must be secured to prevent it from moving offsite until it can be filled with ballast.

Adjustable anchoring systems combined with a GPS survey enable engineers to position a box caisson with pinpoint accuracy.

[citation needed] A caisson is sunk by self-weight, concrete or water ballast placed on top, or by hydraulic jacks.

The leading edge (or cutting shoe) of the caisson is sloped out at a sharp angle to aid sinking in a vertical manner; it is usually made of steel.

The shoe is generally wider than the caisson to reduce friction, and the leading edge may be supplied with pressurised bentonite slurry, which swells in water, stabilizing settlement by filling depressions and voids.

To combat this problem, piles may be driven from the surface to act as: H-beam sections (typical column sections, due to resistance to bending in all axis) may be driven at angles "raked" to rock or other firmer soils; the H-beams are left extended above the base.

When the caisson hits bedrock, the sandhogs exit through the airlock and fill the box with concrete, forming a solid foundation pier.

[6] A pneumatic (compressed-air) caisson has the advantage of providing dry working conditions, which is better for placing concrete.