Tower Bridge - Pushing the Limits: New Adventures in Engineering - page 47

of a movable span. It was not difficult to foresee the problems associated with controlling a ship
in the narrow confines between the forks of such a river crossing, however, and so other solutions
were sought.
Tower Bridge as it exists was designed in the 1880s by the engineer John Wolfe Barry, who
worked on various dock and pier projects around Britain, including the Southend Pier at the mouth
of the Thames and the Barry Docks near Cardiff, which enabled the Barry Railway Company to
capture some of the export trade in coal from south Wales. Barry was to be knighted and serve as
president of the Institution of Civil Engineers shortly after the completion of Tower Bridge, but
such honors would come only after much criticism.
Having received the requisite parliamentary approval, construction of Tower Bridge began in
1886 with the establishment of midriver foundations. Because it was imperative that no significant
settlement develop when the superstructure was erected and later loaded with traffic, preliminary
work for the structure included determining how much load could be borne by the London clay in
the riverbed. To do this, a test cylinder was sunk and loaded until it began to settle, which started
at 6.5 tons per square foot. A conservative maximum load of four tons per square foot was then
decided on, after which the size of the foundations was fixed according to the design load of the
superstructure, by then established on paper. The size of these foundations generated much discus-
sion in the engineering literature of the time, for only those of the Brooklyn Bridge, completed just
a few years earlier, were larger.
It was next necessary to excavate the riverbed to where the total load could be supported, and
that required the construction of caissons and cofferdams to keep the water out while the excavation
proceeded. The concept of a cofferdam—consisting of a ring of closely fitted piles— dates from
Roman times. Pneumatic caissons, which were used to dig and ultimately contain the foundations
of the Brooklyn Bridge, are essentially large, open boxes sunk upside down in a riverbed, thereby
creating a plenum of air within which workers excavate material as the caisson sinks to the desired
depth. The caissons used for Tower Bridge were open to the atmosphere and thus did not present
the dangers of a compressed-air environment.
For establishing each midstream foundation of Tower Bridge, a group of eight twenty-eight-foot-
square caissons was employed, along with four triangular caissons to give the characteristic pointed
shape to the ends of the piers. To enable the caissons to be leveled individually as they sunk, they
were spaced a couple of feet apart, with piling used between them to form a cofferdam. When the
proper depth was reached, the caissons were filled with concrete, upon which brickwork was erec-
ted, which in turn served as a base for the steel and masonry construction above the waterline. In
reporting on the construction project, the American trade journal Engineering News noted that the
caissons were made of steel. That was necessary because the use of timber caissons was specifically
forbidden by Parliament, presumably because of “the great cost of suitable timber in England.”
The journal further noted that as the river muck was excavated and the caissons descended into
the river, the space filled up with water after a high tide. That necessitated pumping out the water,
leaving only “two to six hours of dry work between successive high tides.” According to Engineer-
ing News, “this awkward arrangement seems to have been the result of an error” in sizing the vari-
ous steel parts of the cofferdam, and the journal expressed surprise that the error was not corrected