DHARM
764 GEOTECHNICAL ENGINEERING
caisson has reached the desired location, a fairly thick concrete seal is provided. The thickness
of the seal may range from 1.5 to 4.5 m.
A special type of open Caissons used in India are known as “WELL FOUNDATIONS”,
dealt with in detail in later sections.
Advantages of Open Caissons are:
(i) It is feasible to extend to large depths.
(ii) The cost of construction is relatively low.
Disadvantages are:
(i) The bottom of the Caissons cannot be inspected and thoroughly cleaned.
(ii) The concrete seal is necessarily placed in water, and may not be satisfactory.
(iii) The help of divers may be necessary for excavation near the haunches at the cutting
edge.
(iv) If obstructions of boulders or logs are encountered, the work is slowed down signifi-
cantly.
19 .4 Pneumatic Caissons
Since a Pneumatic Caisson is closed at the top and open at the bottom, with a working cham-
ber under compressed air pressure at the bottom, the work can proceed in the dry by excluding
water and mud from the working chamber. Pneumatic Caissons are suitable in soft soils with
danger of scour and erosion. When the Caisson is sunk to the final position, the working cham-
ber is filled with concrete.
19.4.1Component Parts
A pneumatic caisson consists of the following component parts:
(i) Working Chamber
(ii) Air Shaft
(iii) Air Lock
(iv) Miscellaneous Equipment
These are shown in Fig. 19.3.
(i)Working Chamber: This is made of structural steel, about 3 m high, with a strong
roof, and is absolutely air tight. The air pressure in the Chamber is raised above
atmospheric and is kept at a certain specified value to prevent entry of water and
soil into it. This pressure varies with the depth at which excavation is proceeding at
any time. The outside surface is made smooth to reduce friction. A cutting edge is
provided at the bottom to facilitate the sinking process. The air pressure must be
sufficient to balance the full hydrostatic pressure due to water outside. However,
there is a maximum limit to the air pressure in view of the physiological character-
istics of human beings; a pressure greater than 0.4 N/mm^2 (400 kN/m^2 ) is beyond
the endurance limit of human beings. Therefore, the maximum depth of water
through which a pneumatic caisson can be sunk successfully is about 40 m. Working
under a pressure greater than 0.4 N/mm^2 may cause a special sickness called ‘Cais-
son disease’.