Geotechnical Engineering

DHARM

760 GEOTECHNICAL ENGINEERING

(a) Circular (b) Square (c) Rectangular (d) Octogonal

(e) Twin-circular (f) Twin-rectangular (g) Twin-hexagonal

(h) Twin-octogonal (i) Double-D

Fig. 19.1 Different shapes of cross-section of a caisson

19.2.2Design Loads

A Caisson must be designed to resist all kinds of loads which may act at different times during
service:

(a)Temporary Loads: A Caisson is likely to be subjected to large stresses temporarily

during the construction period. For example, large stresses may occur when the

Caisson gets dropped suddenly during sinking, when it is supported on one side

only at some stage during sinking, or when it is pulled to its correct position to

rectify tilts and shifts; further, a Caisson may be subjected to unbalanced earth

pressure, in which case it may be designed as a vertical beam or a cantilever. In the

case of floating Caissons, Water Pressure during floating, which may cause stresses

due to hogging and sagging, and Torsion, has to be considered in addition to the

towing force. Internal strutting may be needed to take care of towing force as in ship

design.

(b)Permanent Loads: These are the maximum Vertical and Horizontal Loads acting on

the Caisson after it is constructed and sunk into position. Vertical Loads may be

those from the superstructure and the self-weight of the Caisson, less the buoyancy

force at low water level. Horizontal loads may be those due to earth pressure, water

pressure, and wind pressure. In seismic zones, earthquake forces should also be

considered. Wave Pressure, Tractive forces from traffic, ice pressure and forces of

water flow are additional sources of lateral forces. A floating Caisson is subjected to

lateral pressure from inside exerted by the sand and gravel fill at low water stage

(box caissons are seldom filled with concrete).