Caissons (Also called drilled piers) are cast-in-place plain or reinforced concrete piers which are placed by augur boring a long shaft in the earth to the bearing elevation or most commonly, down to solid rock depth. Caissons are commonly required when the soil conditions at the surface aren’t conducive to proper bearing pressure of the building.
Smaller/shallower shafts are bored with an excavator or crane with an augur assembly.
More commonly for larger/deeper caissons a crane or dedicated augur machine is utilized.
The shaft is then filled with concrete. The shaft can range from 18” to 8’ but is typically 2’-6” minimum in diameter to facilitate placement and allow for inspection of the bottom.
Sometimes a temporary casing is required to keep water, sand, or loose fill from entering the shaft.
It is common that a bell is required at the bottom of the shaft. This bell increases the bearing area and also resists movement and uplift in case the soil expands. The belled caisson is typically only practical in cohesive soils and is typically formed by an augur attachment that has retractable blades. Sometimes Caissons will include steel piles to add tensile strength. Reinforcing is typically only specified in the top portion of the caisson to resist lateral forces.