Geotechnical Engineering

DHARM

CAISSONS AND WELL FOUNDATIONS 767

sinking at the desired location. Sand or gravel is invariably used as the ballast inside the
caisson to aid the sinking process. Concrete is seldom used to fill a box caisson.

Unlike open and Pneumatic Caissons, a floating caisson does not penetrate the soil. It
simply rests on a hard, level surface; thus, the load-carrying capacity depends solely on the
resistance at the base as there is no frictional resistance at the sides. A concrete cap is cast on
its top to receive the loads from the superstructure. To prevent scour, rip rap is placed around
the base. (Fig. 19.4). It may also be constructed to contain a number of cells formed by diaphragm
walls. If the caisson is to be floated in rough waters, it is designed as a ship and suitable
internal strutting is provided.

Concrete cap

Sand and

gravel

Box

caisson

Rip rap

Fig. 19.4 Component parts of a floating caisson

19.5.1Stability Aspect of Floating Caissons

The caisson must be stable during flotation. When a body is immersed in water, a buoyancy
force equal to the weight of water displaced acts on the body, according to the principle of
Archimedes. For equilibrium, the weight of the body and the force of buoyancy should be equal.
While the weight, W, acts through the centre of gravity, G, of the body, the force of
buoyancy, U, acts through the centre of gravity of the displaced water, known as the centre of
buoyancy, B (Fig. 19.5). If the caisson is tilted through a small angle θ, the centre of gravity of
the body, G, remains at the same location with respect to the caisson itself, while the centre of
buoyancy, B, changes its position as shown in Fig. 19.5 (b). The point of intersection of the
vertical line passing through B and the centre line of the caisson is known as the metacentre,
M. The caisson would be stable if the metacentre M is above G, when the metacentric height
MG is considered positive.

The metacentric height can be determined analytically (See any standard book on Fluid
Mechanics):
The distance BM is given by

BM =

I

V

...(Eq. 19.8)