PNEUMATIC PROCESS AT ANTWERP. 205
hydraulic lime, which is supplied to the contractor by the Administration,
amounts to about 18 shillings a cubic yard.
The system just described is by no means new, having been practised at
the ports of Paimbæuf, St. Malo, La Pallice, and Genoa by the same
contractor (M. Conrad Zschokke), but the work now or recently carried out
at Marseilles represents its full and perfected development.
With this system may be contrasted the pneumatic process adopted at
Antwerp as far back as the year 1877, and still employed for the construc-
tion of additional quays within the last few years. The following is an
account, necessarily succinct, of the process in its most modern form : —*
The wall (fig. 140) is built of brickwork mainly, with a facing of dressed
stone from 2 feet below low-water level up to a coping of ashlar masonry.
It rests upon a foundation of concrete of varying thickness, according to the
depth of excavation required, but ranging generally between 8 and 16 feet.
j<
..._ai^.'. _________,
140. —Quay Wall at
Antwerp.
The batter of the face is 1 in 10 for the lower portion and 1 in 20 above low-
water level. The thickness of the wall at the base is 21 feet 4 inches. The
depth of the base is 24 feet 8 inches below low-water level.
A bottomless metallic caisson, rectangular in plan, is floated out over the
site of the foundation between two barges, connected by overhead framing.
In plan the caisson is 98 feet 4 inches long by 31 feet 2 inches wide. The
structure of the caisson will be readily understood from the cross and longi-
tudinal sections shown in figs. 141 and 142. It is lowered into place and
sunk to a firm clay foundation by excavating inside of it the alluvial bed
of the river. In sinking it is assisted by the weight of the concrete ballast
* Vide “ Anvers, port de Mer, avec appendice,” 1898. Vernon-Harcourt on “ Mari-
time Navigation Works in Belgium,” Min. Proc. Inst. C. E., vol. cxxxvi.