Laying The Foundation: Caissons
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2002 - A major highlight in the construction
of long-span bridges is the building of towers, the
tallest and most recognizable components of the structure.
It is the towers, after all, that characterize a bridge
and burn its image into our memories, whether it is
the great stone arches of the Brooklyn Bridge or the
bright red art deco of the Golden Gate.
Equally as fascinating, but far
less visible, is the process of building the massive
underwater foundations on which these towers rest, structures
What exactly is a caisson and how
are they built? Much can be learned by studying the
word itself. Caisson is derived from the French word
caisse, meaning "box". Strictly speaking,
a caisson is defined as a box used for underwater construction.
Originally, these boxes were open-bottom structures;
compressed air was pumped into the box to force out
the water and to keep it from re-entering through the
bottom. Workers labored inside this pressurized box
to remove soil and other unsuitable material by hand.
As this material was removed, the box would sink into
the ground and down to the founding material
-- the bedrock, clay or other earth material on which
the caisson would rest.
The two caissons for the two main towers
of the Greenville Bridge will begin as short, open-topped
boxes of steel and concrete. In their completed form,
they will be columns of concrete 175 and 205 feet (53m
and 62m) tall that will reach from far below the river
bottom to just above the water line. Unlike the pneumatic
(pressurized air) caissons of old, these caissons will
be open dredged, meaning river bottom
material will be removed through the open top of the
long caisson box. As soil and other material is removed,
the caissons will sink into the river bottom to their
To understand how these caissons are being
built, we will first look at the various components
used in their construction.
The cutting edge for Pier 37 was floated
into position on April 10, 2002, with the cutting edge
for Pier 38 following on June 26, 2002. View photos
for both the Pier
37 and Pier
38 cutting edge placements in our Photo Album.
Each caisson will take 12 to 16 months
to complete, with 6 to 8 months of concrete placement
needed for the caissons to reach river bottom and another
6 to 8 months for them to reach their planned elevations
below the bottom of the river.
regarding construction of the Greenville Bridge
are periodically answered on our
Ask The Engineer page. Submit your questions
by e-mail to email@example.com.