Foundation designs may require caissons instead of piles. A caisson is a cast-in-place pile formed by drilling a hole, inserting reinforcement, and filling the hole with concrete Caissons are larger in diameter and extend to greater depths than piles. Caissons are used where the building design and/ or soil conditions make pile driving difficult or inadequate. The larger diameter of the caisson allows for greater load-bearing capacity, which means that caissons can be spaced further apart than piles, yet are able to carry more load. Columns, rather than walls, carry the main vertical loads transmitted to the foundation and soil below.The depth of soil penetration needed to support the column loads may not be possible using grouped pile system.Caissons may be used because these large surface area supports the loads.
Caissons are drilled with a crane and drill rig attachment that drills a hole of a specified diameter into the soil. A casing is a metal cylindrical shell that is driven into the ground to restrain compacted soil near the surface, After the casing is inserted into the soil, the remainder of the caisson is drilled through compacted soil to specified depth of the caisson. Once the caisson is drilled to proper depth of the caisson, a rebar cage is set in the hole and concrete is placed to the proper elevation. Casings for deeper holes are constructed of sections that are added as the drilling proceeds.
A belled caisson features a greater bearing area at the base of the caisson. After the caisson hole have been bored to desired depth, a belling tools is attached to the drilling head. The bottom of the hole is then dug out to the bell shape.