DHARM770 GEOTECHNICAL ENGINEERING
19.6.2Construction of Pneumatic Caissons
Pneumatic caissons may be constructed at the site or floated to the site and lowered from
barges. The sand island approach may also be used. The cutting edge is carefully positioned.
Compressed air is introduced into the working chamber to keep off mud and water. After
dewatering the working chamber and keeping it dry, workmen descend into the working cham-
ber through the air-shafts via airlocks. As workmen carry out the excavation in the dry, the
Caisson gradually sinks. The air pressure is increased to equalise the pressure due to the head
of water as the sinking goes on. The excavated material is removed by buckets through the air
shafts. In granular soils, the excavated material can be removed by the blow-out method through
the blow-out pipe. When the valve in the blow-out pipe is opened, the granular material is
blown out by high value of air pressure in the working chamber.
After the caisson has reached the desired depth, the working chamber is filled with
concrete. The air pressure in the chamber is kept constant till the concrete has hardened. A
stiff mix of concrete is then packed into the working chamber upto the ‘roof level’. Cement
grout is used to pack any space left between the concrete and the underside of the roof of the
working chamber. No space is to be left since it may lead to settlement when the caisson is
loaded. The shaft tubes are then dismantled, and finally, the shaft itself is filled with lean
concrete.
19.6.3Construction of Floating Caissons
As indicated earlier, floating caissons are constructed or cast on land, floated to the desired
location, and sunk to the desired depth in an already excavated space. The sinking will be
aided by ballast such as sand or gravel. Unlike other types, a floating caisson does not pen-
etrate the soil, but is made to rest on a levelled surface. This means more of work with regard
to the preparation of the base. Further, this places limitations on the depth to which it can be
taken, as also the load-carrying capacity, which is much less compared to that of an open or a
pneumatic caisson of comparable size.
A concrete cap is cast at the top to receive the loads from the superstructure. Floating
caissons are invariably constructed with reinforced concrete, and sometimes with steel. Inter-
nal strutting and diaphragm walls may be required, especially if it is to be floated in rough
waters.
19 .7 Illustrative Examples on Caissons
Example 19.1: Determine the cross-sectional dimensions of a cylindrical open caisson to be
sunk through 33 m of sand and water to bed rock if the allowable bearing pressure is 1800 kN/m^2.
The caisson has to support a load of 55 MN from the superstructure. Test the feasibility of
sinking if the skin friction is 30 kN/m^2. Also calculate the necessary thickness of the seal.
Solution:
Let De m be the external diameter of the caisson (Fig. 19.7) and Di m its internal diam-eter. Di may be taken as De
2
nearly.