Piers and caissons are underground cylindrical structural members that serve same purpose as footing or piles. The purpose of these structures is to transmit loads to a stratum capable of supporting it without danger of breaking of the foundation soil or excessive settlement. Usually the ratio of depth to width for piers and caissons is greater than 5.
Distinction between Piers and Caissons
There is no sharp distinction between piers and caissons. In simple terms caissons are large piers. They differ only in the method of installation.
Piers are constructed by making a hole into the ground to the required depth and then concrete is poured. It can be said that piers are large bored piles or piles may be regarded as small piers. If diameter is less than 2 m then they are termed as piles else they are regarded as piers. Piers are often solids.
On the other hand caissons are hollow structures with diameter over 4.5 m. They are constructed at the site by sinking and made to rest on hard stratum.
Functions of Piers
Piers serve same purpose as point bearing piles. It is often difficult to distinguish between the two. The principle difference lies on the:
- Influence of method of construction on the load that can be assigned to the foundation
- Influence of the soil condition on the ease or difficulty in construction
- Integrity of the completed foundation
The following examples illustrate these possibilities.