Geotechnical Engineering

DHARM

768 GEOTECHNICAL ENGINEERING

where I = second moment of area of the plan of the caisson at the water surface,

and V = Volume of water displaced.

G

B U

A

W G

WU

A

M

B

(a) Vertical position (b) Tilted position

q

Fig. 19.5 Stability of floating caisson
The metacentric height is computed as
MG=+BM BG ...(Eq. 19.9)
Plus sign is used when G lies below B. When M is above B, the couple caused by W and
U will be such as to rectify the caisson to its original vertical position, thus ensuring its stabil-
ity. If B is above M, the tilt goes on increasing, thus the caisson becoming unstable. In this
latter case, it should be either redesigned, or ballast be added to ensure stability. A minimum
of 1 m of free board is recommended.

19.5.2Merits and Demerits of Floating Caissons

The merits of floating caissons are as follows:

(i) Since floating caissons are precast, good quality can be ensured.

(ii) The installation of a floating caisson is quick and convenient.

(iii) Floating caissons are less expensive than other types; they may also be transported

at a low cost by floating.

The demerits of floating caissons are as follows:

(i) The foundation bed has to be levelled before installing the caisson.

(ii) The base of the caisson must be protected against scour.

(iii) The load carrying capacity is smaller than that of other types of comparable size.

(iv) This type is suitable only if a good supporting stratum is available at shallow eleva-

tion; otherwise, it becomes costly owing to deep excavation, as the saturated soil

tends to flow into the excavation.

19 .6 Construction Aspects of Caissons

Certain salient features of the construction of different types of caissons are considered in the

following subsections.

19.6.1Construction of Open Caissons

The sinking of an open caisson is achieved either in the dry, or from a dewatered construction,

or from an artificial island. The last approach, is popularly known as the ‘Sand Island Method’

of placing an open caisson.