The Barton Tower on the south side of the
bridge is located in the water 500m from
the shore line. The pier below the tower is
a concrete structure about 16m deep
supported on twin hollow circular caissons,
each 24m in diameter, that penetrate the
Kimmeridge clay to a depth of 8m.
To build the south tower and its foundations,
the contractor needed to erect a temporary
jetty from the shore to the tower site where a
figure of eight steel cofferdam was driven
around the site and filled with sand to form a
temporary island. Considerable problems had
to be overcome before the foundation was
successfully completed. The steel shortage of
1973 caused delay in building the jetty and in
putting down the cofferdam. Scouring around
the cofferdam proved to be worse than had
been envisaged and some 12,000 tonnes of
chalk had to be dumped outside the
cofferdam to overcome the problem. When
the sand island was complete, it provided a
base for building the first sections of the
caissons.