PNEÜMATIC CONSTRUCTION AT ROTTERDAM.
207
working chamber is finally filled with concrète through the vertical shafts
which have previonsly served for purposes of access. The interval of about
18 inches, unavoidably left between adjacent lengtlis of foundation, and the
gap of about 42 inches between the sections of wall, are made good by
cement concrète, the joint being strengthened by vertical grooves in the
connected ends. The wall is continuous above a level of 3 feet above low
water. The height of the working chamber is a little over 5 feet, and it
projects 8 feet in front of the base of the wall, in order to afford a sufficient
area of foundation to support the imposed pressure. The present con-
tractors are Messrs. Hersent & Son, succeeding the original firm of
Couvreux & Hersent, who initiated the system.
In contradistinction to the preceding instances, the use of the pneumatic
chamber has been applied at the port of Rotterdam to the construction of a
portion of the wall considerably above the dock bottoni* (see fig. 143).
The wall is built upon a timber platform, supported by fir piles driven
into the bed of the River Meuse through fascine mattrasses and a layer of
sand previously deposited in a dredged trench. The piles are provisionally
sawn off at low water, and the caisson, 70 feet long by 29 feet wide, is
floated over their heads in such a manner that the ends occupy spaces of
4 feet, specially provided at intervals between the piles, which are otherwise
driven at centres of 3 feet 3 inches. The caisson is then sunk until it takes
its bearing on the landward side, and at one end upon a portion of the plat-
form already placed in position. At this stage, suitable adjustments of
water ballast are made, to maintain equilibrium, and workmen enter the
compressed air chamber, which has previously been occupied by water. The
* Le Port de Rotterdam, by H. A. van Ysselsteyn, sous-director des travaux de la ville.