26 W. H. WARREN. 
The piers of the Hawkesbury River Bridge, founded 170 
feet below high water and 126 feet below the bed of the 
river, are at present the most remarkable example of the 
open dredging process; but equally difficult conditions may 
be encountered in the Sydney Harbour Bridge in one of 
the piers. The pneumatic process is by no means new, 
but it has been considerably improved in detail during the 
last few years, mainly in the design of the air-locks and 
caissons; but the effect on the workman under great 
pressures producing what is known as Caisson disease, is 
not entirely under control, although much has been 
accomplished in this direction by careful regulation of 
the changes in pressure, more especially in leaving, by the 
use of elevators, and by general attention to the health 
of the workmen employed. Probably 150 feet is about 
the limit attainable by this process, although so far there 
are no examples of so great a depth having been attained. 
In the new East River Bridge the depth reached was 
115 feet from high water, but the air pressure was not 
equal to this head of water, in consequence of the retentive 
character of the material passed through. 
The pneumatic process has been used in a few cases in 
Australia, but in other parts of the world it has been 
employed to a considerable extent, both in the foundation 
of bridge piers, quay walls, and in connection with shield 
tunnelling under rivers. In America it has also been used 
recently in the foundations of the high buildings for which 
that country is famous, and many interesting examples 
have been described in American technical journals, such 
as the Engineering Record and Engineering News. 
A considerable amount of work has been accomplished 
in depositing concrete in sub-aqueous foundations, forming 
a monolithic mass capable of carrying the heaviest loads. 
