Geotechnical Engineering

DHARM

CAISSONS AND WELL FOUNDATIONS 765

Air

lock

Air

release

Compressed

air

Blow-out

pipe

Ladder

Air shaft

for men

and

material

Working chamber

Fig. 19.3 Component parts of a pneumatic caisson

(ii)Air Shaft: This is a vertical passage which connects the working chamber with an

airlock. It is meant to provide access to the working chamber for workmen. It is also

used to transport the excavated material to the ground surface. In large caissons,

separate shafts may be provided for men and materials. Air-shafts are made of Steel,

the joints being provided with rubber gaskets to make them leak proof. Each Shaft

is provided with its own air lock at its top. As the caisson sinks, the air shaft is

extended to keep the airlock always above the water level.

(iii)Air Lock: This is a steel chamber provided at the upper end of the air shaft above the

water level. Its function is to permit the workmen to go in or come out of the Caisson

without releasing the air pressure in the working chamber.

The chamber of the airlock is provided with two air-tight doors, one of which

opens to the shaft and the other to the atmosphere outside. When a workman enters

the airlock through the outside door the pressure in the chamber is kept at atmos-

pheric value. This is gradually raised till it becomes equal to that in the working

chamber, and the workman allowed to enter the airshaft through the door to it, and

to descend into the working chamber. The procedure is just reversed when one has

to come out. However, the decompression must be effected much more slowly to

prevent caisson disease. A minimum of half-an-hour is necessary for the pressure to

be reduced from 0.3 N/mm^2 to atmospheric pressure. Fresh air is circulated into the

working chamber by opening a valve in the airlock in order to prevent the air inside

becoming stale. The workmen should not be made to work inside the working cham-

ber under compressed air pressure for more than two hours at a stretch.