New Mississippi
River Bridge to define St. Louis skyline
This
signature bridge is part of a $1 billion transportation project
that includes improving three major interstates that travel through
and around St. Louis.
A new
Mississippi River bridge has been called a “world-record breaking,
signature structure that will further define the region’s skyline
for generations to come.” And while the bridge is monumental,
it is only part of the $1 billion transportation improvement project
the Illinois and Missouri Departments of Transportation are spearheading.
And Hanson is contributing in some key technical areas.
To
improve traffic flow to and from St. Louis, these state agencies
have undertaken upgrades to the interstate highway network. The
New Mississippi River bridge is the centerpiece, a river-spanning,
cable-stayed bridge featuring three planes of cables and two single-pylon
towers, each inclined nine degrees from the river and soaring 435
feet above Interstate 70. This bridge, north of downtown St. Louis,
will be the world’s widest cable-stayed structure at 222 feet.
The 2,000-foot main span will be the longest clear span across the
river. It will be the longest cable-stayed span in the Western Hemisphere
and the fifth longest in the entire world.
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Major interstate work part of overall project
- In
addition to the new bridge, the project includes:
relocating
I-70 to the north of Interstate 55 to connect with the new bridge;
- providing
a connection between Interstate 64 and the new I-70;
- upgrading
the I-70, I-64, I-55 interchange; and
- relocating
and improving Illinois Route 3.
Construction is scheduled to start in 2004 with completion in
2010.
Geotechnical issues surface early
Hanson
is providing the geotechnical and hydraulic engineering for the
eight-lane bridge, as a consultant to Modjeski
and Masters.
One
of the main geotechnical issues is the existence of thick deposits
of “low-density” sand below the water table. Limestone
bedrock is 120 feet below the surface on the Illinois side and between
30 and 60 feet below the surface on the Missouri side,” says
David Daniels, P.E., senior geotechnical engineer.
The
loose sand, in combination with the project’s high seismic
design requirements, results in the potential for liquefaction during
an earthquake. (Liquefaction occurs when the loose materials below
the water table vibrate into a liquid state as a result of earthquake
shaking.) “Associated lateral spreading within the subsoils,
beneath the sloping ground along the banks of the river, could impose
significant lateral drag forces on the bridge foundations,”
explains Daniels.
To
accommodate these conditions, Daniels says the foundations must
feature 4- to 12-foot diameter, 120-foot-long drilled piers founded
in rock to support the bridge superstructure. “We are also
considering in-situ densification of the loose sands, which is intended
to prevent liquefaction and/or spreading,” he says.
Scour analyses lead to special considerations
The
scour analyses for the New Mississippi River Bridge, however, led
to special considerations regarding the bridge substructure. Scour
is the erosion of inshore sediments and the formation of deep holes
and channels caused by the velocity of the water, particularly during
floods.
Daniels
says that the scour analyses indicated soil removal to a depth of
about 60 feet (bedrock level) around the Missouri bank pylon foundation
and to a depth of about 50 feet around the Illinois bank pylon foundation.
Daniels
also says that the excavation for the Illinois side pylon will be
made using “a large, dredge well caisson.” This method
will incorporate a 112- by 142-foot concrete base containing open
dredge wells with a perimeter steel cutting edge, which is sunk
into the overburden. The soil is removed from the wells by an overhead
clamshell, and the base gradually sinks to rock. On the Missouri
side, where bedrock is much shallower, a large concrete pedestal
and footing foundation on rock will support the pylon.
For
more information, call David Daniels at (217) 747-9266, or
click here to e-mail
him.
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According
to the Illinois and Missouri Departments of Transportation,
the New Mississippi River Bridge’s main span will consist
of 1,000 miles of 0.6-inch diameter stay-cable strand—“enough
for nearly two round trips from St. Louis to Chicago.”
Other elements will include:
-
90,600 cubic yards of concrete for the deck slab, towers and
side spans;
-
8,600 tons of reinforcing steel;
-
1,600 tons of post-tensioning; and
-
14,700 tons of structural steel.
*Information
taken from www.newriverbridge.org
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