A noteworthy instance of the use of pneumatic appliances in cylinder
sinking for foundations is that in progress at the Forth Bridge. The
wrought-iron cylinders are 70 feet in diameter at the cutting-edge,
and have a taper of about 1 in 46. They are, however, at a height of 1
foot above low water (that is, at the commencement of the masonry work
of the pier) reduced to 60 feet in diameter; at their bottoms there is
a roofed chamber, into which the air is pumped, and in which the men
work when excavating, this roof being supported by ample main and
cross lattice girders. Shafts with air-locks and pipes for admitting
water and ejecting silt are provided. The air-locks are fitted with
sliding doors, worked by hydraulic rams, or by hand, the doors being
interlocked in a manner similar to that in which railway points and
signals are interlocked, so that one door cannot be opened until the
other is closed. The hoisting of the excavated material is done by a
steam engine fixed outside the lock, this engine working a shaft on
which there is a drum inside the lock, the shaft passing air-tight
through a stuffing box. A separate air-lock, with doors, ladder, etc.,
complete, is provided to give ingress and egress for the workmen.
I have already adverted to one Scotch bridge; I now have to mention
another, viz., the Tay Bridge, also now in course of construction.
Here the cylinders are sunk, while being guided, through wrought-iron
pontoons, which are floated to their berths, and are then secured at
the desired spot by the protrusion, hydraulically, of four legs, which
bear upon the bottom, and thus, until they are withdrawn, convert the
pontoon from a floating into a fixed structure.