The perfect dare

Bridge across Croatan Sound to facilitate hurricane evacuation and open the island to visitors

Randy Midgett, P.E., and Don Barber, P.E., Contributing Authors / May 16, 2003

It took more than four years and a lot of ingenuity to
construct the 5.25-mile crossing from Manns Harbor, N.C., to Manteo, N.C., on
Roanoke Island, but now motorists can traverse the Croatan Sound to the Outer
Banks in less time, and residents and visitors alike can safely and quickly
evacuate the barrier island in the event of a hurricane.

More than 3,000 locals celebrated the grand opening of the
Virginia Dare Memorial Bridge on Aug. 16, 2002, with a speech by North Carolina
Gov. Michael F. Easley and a ceremonial first crossing in a motorcade led by
actor and local resident Andy Griffith, driving his 1935 Packard convertible.

It was good cause for celebration. The Dare Bridge, which is
two miles longer than any other bridge in the state, bypasses downtown Manteo
to cut 20 to 40 minutes off drivers? travel time. It also opens a gateway
from the state?s interior to the beaches, resorts and attractions that
fuel the region?s tourist economy. As part of the North Carolina
Department of Transportation?s (NCDOT) overall highway improvement
program, the $90 million Dare Bridge was built without adverse impact on the
sensitive Outer Banks wetlands.

Everybody out

The Outer Banks is a chain of barrier islands along the
North Carolina coast, surrounded by 900 miles of shoreline and the
world?s largest estuary system. The Dare Bridge replaces the William B.
Umstead Bridge, a 45-year-old two-lane structure on U.S. Highway 64-264, which
connects the county seat of Manteo to the fishing hamlet of Manns Harbor.
Raleigh lies about 197 miles farther inland along this east-west highway.

During rush hours, when vacationers leave the island for the
mainland, the Umstead Bridge caused big-city- style traffic backups in Manteo.
When two hurricanes in August 1996 forced evacuation of the island during peak
tourist season, it was clear that another route was needed to get people off
the Outer Banks in an emergency. Traffic backed up for several miles, from
traffic signals in Manteo to a causeway linking Manteo and Nags Head, N.C.

The NCDOT had planned corridor improvements to make U.S.
64-264 a four-lane facility from Raleigh to Manteo, which would involve widening
about 40 miles of roadway from two to four lanes. The need for a better
hurricane evacuation route accelerated the project. Planning for a new
four-lane bridge began in 1996 and, in January 1997, Wilbur Smith Associates
(WSA) of Raleigh, N.C., was selected for the design. Balfour Beatty
Construction of Atlanta was awarded the prime construction contract in April
1998, and HNTB Corp., Raleigh, N.C., was selected to perform construction
engineering and inspection.

The Dare Bridge is a conventional AASHTO girder bridge
carrying a four-lane highway divided by a center parapet. It rises 66 ft off
the water at its highest point and clears a 200-ft-wide navigation channel. The
concrete structure utilizes precast piles and girders and cast-in-place pier
caps and decks. WSA designed the structure for a 100-year life span, up to
twice as long as its predecessor.

To increase the longevity, NCDOT specified an increase in
the concrete cover on the epoxy-coated reinforcing steel and utilized a
durability-based, high-performance concrete with calcium nitrate corrosion
inhibitors. This was the first full-scale implementation of a durability-based,
high-performance concrete in the state. The addition of silica fume and fly ash
to the concrete increases the resistance to chlorides found in saltwater, which
helps protect the reinforcing steel.

The bridge deck alone has 7,250 tons of reinforcement
steel--enough to stretch 2,633 miles, or a round trip from Manteo, N.C.,
to Wichita, Kan.--and more than 43 acres of concrete riding surface,
enough deck concrete to cover 278 basketball courts to a depth of one foot.
Instead of traditional transverse grooving, the entire deck is diamond ground,
yielding an exceptionally smooth ride.

Sensitive land

At 5.25 miles, the new bridge is twice as long as the
existing bridge, which crosses Croatan Sound at the northern tip of Roanoke
Island and at the narrowest part of the sound. To bypass Manteo, the Dare
Bridge connects at the island?s midsection and crosses the sound on the
diagonal. This alignment minimized the bridge?s impact on the coastal
marshes, but it still crossed about 0.75 miles of wetlands.

Construction of the wetland section proved to be
challenging. The regulatory agencies prohibited NCDOT from placing heavy
equipment in the wetlands, so the contractor built work bridges across the
wetlands at each approach. Instead of a continuous bridge from shore, they
worked off a system of girders that they moved along with them.

To transport material and equipment back and forth from the
mainland, the contractor built an elevated railroad track to accommodate
small-gauge mining trains. This structure had a very small footprint. It was an
ingenious way to minimize the wetland impact. It also was cost-effective,
because the contractor didn?t have to build a temporary bridge. The whole
trestle system could be recycled. Concrete was produced on shore at a plant in
Manns Harbor and carried in buckets by train.

The 30-in. square prestressed concrete
pilings--designed to withstand a ship impact--were transported to the
construction site one at a time. To sink the pilings up to 100 ft into the
sound floor, the contractor had to use high-impact water jets to drive through
a hard sand layer. This churned up tons of sand, posing a challenge for disposal.
Using a clamshell crane would not only tear up the root mat and soil, but also
would tie up a major piece of equipment and slow construction.

After considerable research and a visit to the Newport News,
Va., shipyards, NCDOT found an innovative solution in the vacuum equipment used
by shipbuilders to remove debris after sandblasting inside hulls. A test run on
dry land demonstrated that the 6-in. industrial vacuum hose could suck up and
convey wet sand the 2,000 ft required by site conditions. The NCDOT bought the
equipment and devised a two-step process: Build a 40- x 70-ft containment pen
around each pile site, vacuum up the 1.5-ft layer of sand resulting from pile
driving and convey it as much as 2,000 ft to shore.

Although time-consuming and labor-intensive--it took
three laborers up to a week to clean up each bent--this process saved
about $3 million by shaving 40 days off the construction schedule, compared to
moving a crane to the pile sites. The sand was ultimately recycled to fill the
approach field, and the saw grass and needle rush bounced back, despite being
covered by sand for up to a month. The regulatory agencies were favorably
impressed by the inventive technique and the positive results for wetland

Satellite surveillance

It took two years to get past the wetlands. Crossing the
open water of the sound was significantly less complicated. Materials were
floated out on barges, and a floating concrete plant moved with the
construction. However, coordinating equipment and material for a construction
site that spanned more than five miles was time-consuming. The resulting
paperwork, laid end-to-end, could easily surpass the length of the bridge.

Because there were no existing structures by which to
establish sight lines across the sound, survey crews were unable to use
conventional optical survey instruments. All the horizontal survey work was
done by satellite using GPS, with NCDOT performing verification checks using an
independent GPS and control system. At any one time, there were four sections
of bridge that were not connected to each other. Nevertheless, no layout
adjustments were necessary over the entire five miles.

Construction inspectors were divided into squads to cover
different areas of the site--deck, substructures, pile drivers or roadway.
Each squad had a leader who interacted directly with the contractor?s
foreman/superintendent for that area and who reported to the assistant resident
engineer and resident engineer. This eliminated the position of a field staff
manager who would have been overwhelmed by the number of inspection reports and
physically unable to cover the entire jobsite. Squad leaders shared information
among themselves, and inspection teams cross-trained to ensure everyone was up
to speed on job progress. 

The grand opening

Dare County Board of Transportation officials helped NCDOT
organize a celebration when the bridge officially opened to the public on Aug.
16. The Outer Banks Visitors Bureau orchestrated opening-day activities for the
public, including speeches by Gov. Easley and state Senator Marc Basnight and a
ceremonial crossing by foot, bicycle or motor vehicle. The event was intended
to raise statewide awareness of the new route across Croatan Sound. It is the
Visitors Bureau?s hope that more North Carolina residents will visit the
Outer Banks, which is already a premier vacation spot for Virginia residents.

Named for the first English child born in America in 1587,
the Virginia Dare Bridge is a stunning architectural landmark and an important
link in the overall improvement plan along U.S. 64-264. It will ensure
efficient and safe east-west passage for Dare County residents and visitors to
the Outer Banks region. And it will be a key evacuation route off beaches and
out of the county during hurricane season.

NCDOT has no plans to dismantle the original Umstead Bridge,
which continues to handle local traffic. But it holds great hope for the Dare
Bridge, which is expected to be an economic boon to the region.

About the Author

Midgett was the NCDOT resident engineer on the Virginia Dare Memorial Bridge project. Barber is vice president and director of construction services for HNTB Corp., Prince William, Va.

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