|
Foundations Supporting the Towers
The massive concrete and steel foundations that support the bridge towers of
the new suspension bridge are called caissons. Construction of the foundations
began on land and ended on the Narrows seabed, some 200 feet below the surface
of Puget Sound.
In early 2003, Todd Pacific Shipyards in Seattle built the first part
of the caisson called the “cutting edge.” After the caissons
made their journey from Seattle to the Port of Tacoma, construction continued
on the outer steel skin. By summer of that year, and after reaching 78
feet in height, the monoliths made yet another trip. Four Foss Maritime
tugboats hauled what looked like floating cities through Commencement
Bay, around Point Defiance and to the Narrows.
Upon their arrival in the Narrows, each rectangular box of waterproof
concrete and reinforced steel measured 130-feet long, 80-feet wide and
78-feet high and weighed 14,000 tons. When completed, each caisson weighed
in at 85,000 tons and contained six millions pounds of steel along with
40,500 and 37,000 cubic yards of concrete in the Tacoma and Gig Harbor
structures, respectively.
After six months of placing concrete within the foundations’ walls,
the Gig Harbor and Tacoma caisson touched down on the Narrows seabed in
December 2003 and January 2004, respectively. The Gig Harbor caisson touched
down first (133 feet below water) as the west Narrows seabed is shallower.
And, the Tacoma foundation reached bottom at 154 feet. In the case of
both, a global positioning system and laser surveying (beams) positioned
these giants with precision.
During the first half of 2004, workers continued building the caisson
walls. Shortly thereafter, the caissons were embedded another 62 feet
below the Narrows mud line. Workers dredged ton after ton of seabed soil,
and as a result of dredging, the massive weight of the caisson pushed
it deep below the seabed.
By mid-summer 2004, caisson construction had wrapped up: bridge workers
poured a 15-foot concrete seal or distribution cap over the top. That
cap became the base for the new bridge towers.
| GIG HARBOR CAISSON |
|
| West Narrows Water Depth |
133 feet |
| Depth Below Seabed |
57 feet |
| Total Height of Finished Caisson |
190 feet |
| |
|
| TACOMA CAISSON |
|
| East Narrows Water Depth |
154 feet |
| Depth Below Seabed |
62 feet |
| Total Height of Finished Caisson |
216 feet |
|
 |
| • |
Caisson is the engineering term
for a suspension bridge's foundation. It's from the French word caisse,
meaning box. |
|
| • |
A caisson is made of steel-reinforced and waterproof
concrete with an open central core. |
|
| • |
The Gig Harbor caisson touched down on the Narrows
seabed in December 2003, the Tacoma caisson reached bottom in January
2004. |
|
| • |
Each completed caisson has six
million pounds of steel overall and roughly 40,000 cubic yards of
concrete. |
|
|
