DHARM766 GEOTECHNICAL ENGINEERING
(iv)Miscellaneous Equipment: Certain miscellaneous equipment such as motors, pres-
sure pumps, and compressors are usually located outside at bed level. Pressure in
the working chamber is maintained through an air pipe connected to a compressor.
At least one stand-by unit for all equipment should be provided to cope with any
emergency.19.4.2 Safety Precautions
Working in an atmosphere of compressed air poses health hazards for physiological reasons. If
adequate safety precautions are not observed, workmen may be afflicted with what is known
as “caisson disease”. It can range from giddiness in the mild form to death in the ultimate
depending upon the seriousness of decompression. Compared to recompression, decompres-
sion poses more problems and has to be done much more slowly.
Labour laws are stringent in respect of work force employed under compressed air at-
mosphere. After every shift of two hours of work, a rest period of four hours is recommended;
further, not more than four hours of work is permitted per day.
19.4.3Merits and Demerits of Pneumatic Caissons
The following are the merits of Pneumatic Caissons:
(i) Control over the work and foundation preparation are better, since all work is done
in the dry.
(ii) Obstruction from boulders or logs may be readily removed since direct visual in-
spection of the bottom near the cutting edge is possible.
(iii) Concrete placed in the dry is more capable of attaining better quality are reliability.
(vi) Plumbness of Caisson is easier to control than with other types.
(v) Soil can be inspected, samples taken, and bearing capacity ascertained more reli-
ably, if necessary, by in-situ testing.
(vi) No settlement of adjoining structures need be apprehended since no lowering of
ground water table is expected to occur.
(vii) Large depths of foundation can be achieved to bed rock through difficult strata for
major civil engineering works.
The following are the demerits of Pneumatic Caissons:
(i) Pneumatic Caissons are highly expensive and hence should be used only when other
types of caissons are not feasible.
(ii) The depth of penetration is limited to 30 m to 40 m below water table.
(iii) A lot of inconvenience is caused to the workmen while working under compressed
air pressure, and they may be afflicted with caisson disease.
(iv) Extreme care is required for the proper working of the system; even a small degree
of slackness may lead to an accident.19 .5 Floating Caissons
Floating Caissons are also called ‘Box Caissons’ in view of their shape resembling an open box.
They are called Floating Caissons as they are floated to the site after casting on land for