As it was impossible to drive these piles across the old timber crib
which formed the old dock front, the latter was cut through by a
pneumatic caisson of wooden-stave construction, which formed part of
one side of the cofferdam. At the river end of the cofferdam the rock
was so deep that the concrete could not be carried down to its
surface, and the tunnel section was built on a foundation of piles
driven to the rock and cut off by a steam saw 19-1/2 feet below mean
hightide. This section of the tunnel was built in a 65 x 48-foot
floating caisson 24 feet deep. The concrete was rammed in it around
the moulds and the sides were braced as it sunk. After the tunnel
sections were completed, the caisson was sunk, by water ballast, to a
bearing on the pile foundation.
Adjacent to the condensing water conduits is the 10 x 15-foot
rectangular concrete tunnel, through which the underground coal
conveyor is installed between the shore end of the pier and the power
house.
[Sidenote: _Steel Work_]
The steel structure of the power house is independent of the walls,
the latter being self-supporting and used as bearing walls only for a
few of the beams in the first floor. Although structurally a single
building, in arrangement it is essentially two, lying side by side and
separated by a brick division wall.
There are 58 transverse and 9 longitudinal rows of main columns, the
longitudinal spacing being 18 feet and 36 feet for different rows,
with special bracing in the boiler house to accommodate the
arrangement of boilers.
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