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.
Pile driven through soft soil into dense sand – In such case the pile tip displaces the soil and compact it. On the other hand pier loosens the soil during digging or drilling and makes the soil to expand. Hence resistance of pile is many times more than the resistance of pier.
Dense sand located beneath a sequence of soft clay and thick layer of sand – In such deposit if pile is used most of the energy of driving is used in overcoming the skin friction and driving up to the level of the bearing stratum of the dense sand would not be possible. The required resistance at the base will not be obtained. Piers, on the other hand, would be much safer and more economical than point bearing piles.
Sound rock overlain by decomposed rock – Decomposed rock may contain fragments of less decomposed material. These materials prevent driving piles into sound rocks. For piers they can easily be removed by excavation.
Medium clay over a deep deposit of stiff clay – In such stratum entire load from a column can be supported on the surface of the stiff clay through a single machine drilled pier with an enlarged base. This type of foundation is far economical than a friction piles in stiff clay.
Stiff clay with seam water bearing seams of sand and silt – Such soil condition would not permit forming of the bell at base. The soil at base will collapse or there will be inflow of loose wet soil at bottom and consequently the pier would deprive of the firm support at base. In such situation driven piles are better solution.