Open caissons (“caisson” a French word) are called as wells.

The dredge area for a double-D shape is smaller when compared to others; hence it can be more economical than a singular circular well.

The following components of a well have to be considered in the design of a well foundation: well curb cutting edge, steining, Bottom plug and well cap.

The disadvantage of a circular well is that in the direction parallel to the span of the bridge, the diameter of the well is much more than the minimum size required.

The common types of well shapes are: i) single circular ii) Twin circular iii) Dumb-well iv) Double-D v) Twin-hexagonal vi) Twin-octagonal vii) Rectangular.

As the perimeter of the circle is equidistant at all points from the center of the dredge hole, the sinking is more uniform than other shapes.

A pneumatic caisson has its lower end designed as a working chamber in which compressed air is forced to prevent the entry of water and thus permit excavation in dry.

The caissons are of three types: i) Box caissons ii) open caissons iii) pneumatic caissons.

The choice of a particular shape of well depends upon the dimension of the pier or abutment, the care and cost of sinking, the considerations of the tilt and shift during the sinking.

In India, well foundation forms the most common type of deep foundation for bridges.