| Road Science
Complexity Makes Major Projects Challenging
From Boston’s Big Dig to California’s need to add
earthquake-resistant features to bridges, agencies and contractors meet
those challenges.
by Ruth W.
Stidger, Editor-in-Chief
 What makes a challenging project? Some engineers say it is complexity.
If so, today’s bridge and road projects will become ever-more
challenging.
Many of these big and tough projects are built in urban areas, where
buildings and other infrastructure already occupy every square foot of
land. Others call for new technologies to help solve the problems that
make them necessary.
Agencies, engineering companies, and contractors work together as a
team on these projects more than on any other type of work. It’s
absolutely necessary because the work is just too daunting to go it alone.
New materials, designs, and techniques all find their place in today’s
most challenging projects.
Boston Continues Its Big Dig
Boston’s $15-billion Central Artery/Tunnel project, fondly called the
Big Dig, stands as one of the most daring projects in history.
To deal with its massive traffic problems, the Metropolitan Transit
Authority started the project that, when finished, will include an eight-
to-10-lane underground expressway, a two-bridge crossing of the Charles
River, and a tunnel under South Boston and Boston Harbor to Logan Airport,
including the four-lane Ted Williams Tunnel.
The elevated six-lane highway called the Central Artery that it
replaces was built in 1959 and could carry about 75,000 vehicles
comfortably. With 200,000 vehicles crowding the road daily, traffic
congestion problems were rampant.
“Traffic crawls for more than 20 hours each day,” says the Web site
covering the Big Dig. “The accident rate on the deteriorating elevated
highway is four times the national average for urban Interstates. The same
problem has plagued the two tunnels under Boston Harbor between downtown
Boston and East Boston/Logan Airport. Without major improvements to the
Central Artery and the harbor crossings, Boston could expect a stop-and-go
traffic jam for up to 16 hours a day — every waking hour — by 2010.
“The annual cost to motorists from this congestion — in terms of an
elevated accident rate, wasted fuel from idling in stalled traffic, and
late delivery charges — is estimated to be $500 million.
The solution, the Central Artery/Tunnel Project, scheduled for
completion in 2004, includes two major components:
1. The six-lane elevated highway will be replaced with an eight- to
10-lane underground expressway directly beneath the existing road. At its
northern limit, there will be a 14-lane, two-bridge crossing of the
Charles River.
After the underground road is finished, the crumbling elevated highway
will be demolished and replaced by development and open space.
2. I-90, the Massachusetts Turnpike, will be extended from south of
downtown Boston through a tunnel under South Boston and Boston Harbor to
Logan Airport. The first link, the four-lane Ted Williams Tunnel under the
harbor, opened to limited traffic in 1995.
Project Details
To put these highway improvements in the ground in a city like Boston
amounts to one of the largest, most technically difficult and
environmentally challenging infrastructure projects ever undertaken in the
United States, project engineers report.
The project covers 7.8 miles of highway, or 161 lane miles. About half
of this is in tunnels.
The project’s contractors will place 3.8 million cubic yards of
concrete and excavate over 16 million cubic yards of soil.
The larger of the two bridges, the Leonard P. Zakim Bunker Hill Bridge,
is a 10-lane, cable-stayed hybrid bridge. It is the widest ever built and
the first to use an asymmetrical design.
Four major highway interchanges connect the new roadways with existing
highways.
At Logan, a new interchange will carry traffic between I-90 and Route
1A as well as onto the airport road system.
The underground interchange in South Boston moves traffic between I-90
and the waterfront/convention center area.
A new interchange will also connect I-93 north of the Charles River to
the Tobin Bridge, Storrow Drive, and the new underground highway.
At the south end of the underground road, an interchange between I-90
and I-93 is being completely rebuilt on six levels. Two of these are
subterranean to connect with the underground Central Artery.
An interchange at Massachusetts Avenue on I-93 has been rebuilt.
A Tough Site
 The project’s unique challenge is the fact that it is being built in
the middle of Boston. Work of this magnitude has never been tried in the
midst of an urban area. And, the contractors must maintain traffic
capacity and access to residents and businesses throughout the work.
Mitigation efforts account for a quarter of the total budget. This
includes old road demolition, creation of open land, and the new Spectacle
Island, where project dirt is capping an abandoned dump in Boston Harbor.
Work started 11 years ago. About 80% of the construction has been
completed.
 A bridge across the Charles River
(pictured) connecting I-93 in Charlestown with
Leverett Circle and Storrow Drive opened in 1999.
The I-90 Logan Airport extension through South Boston will open this
month.
Northbound lanes of the underground highway will open in November,
while the southbound lanes are slated for use in late 2003.
Project completion will be in late 2004, with the demolition of the
elevated highway and restoration of the surface.
The Central Artery/Tunnel Project is owned and managed by the
Massachusetts Turnpike Authority and is part of the Metropolitan Highway
System. Design and construction management consulting is provided by
Bechtel/Parsons Brinckerhoff, a joint venture of Bechtel Corporation of
San Francisco and Parsons Brinckerhoff Quade & Douglas, Inc. of New
York.
Construction Timeline
1982: Planning for the Central Artery Tunnel Project began.
1991: Construction of the Ted Williams Tunnel and a bypass road through
South Boston began.
1995: Ted Williams Tunnel was completed.
1999: Leverett Circle Connector Bridge over the Charles River opened.
2002: I-90/Massachusetts Turnpike extension opens in three phases;
northbound Central Artery I-93 underground road and Leonard P. Zakim
Bunker Hill Bridge completed by the end of the year.
2003: Southbound Central Artery I-93 underground road open at the end
of the year.
2004: Project completed, including demolition of elevated Central
Artery and restoration of the surface.
Facts About Carrying Traffic
1. Downtown Central Artery I-93 will carry 245,000 or more vehicles a
day with ease.
2. I-93 will be up to 10-lanes wide and safely underground. It will
have fewer on- and off-ramps than the elevated road and a new network of
surface streets.
3. The Ted Williams Tunnel will carry more than 90,000 vehicles a day.
4. Boston’s carbon monoxide levels will drop about 12% from the
decrease in stalled traffic.
Storrow Drive Bridge
The Big Dig’s Storrow Drive Connector Bridge was designed with steel
for cost and constructability purposes, according to Sena T. Kumarasena,
P.E. and Raymond J. McCabe, P.E., with the HNTB Corporation.
When considering materials, Tampa Steel suggested fabricating the
entire 830-foot-long box girder in sections and delivering them by
ocean-going barge.
Key design challenges, according to Kumarasena and McCabe were:
1. The exceptional size and weight of the box-girder sections.
2. Weak and variable rock conditions adding to needed foundation design
measures.
3. Potentially liquefiable soil, especially on the south bank.
4. Visual harmony with abutting structures and with the mainline
cable-stayed bridge, also crossing the Charles River.
The use of the single box girder with floor-beam cantilevers was chosen
for aesthetic reasons, Kumarasena and McCabe say. Tampa Steel provided
nine field sections, which were spliced on site.
A curved steel multi-box girder viaduct at the south interface and a
concrete segmental viaduct at the north interface abut the bridge. Modular
expansion joints were used between the bridge and both interfaces.
Main piers are founded on 8-foot-diameter shape-drilled rock sockets.
The drilled shafts were designed to hold against forces developed by
lateral spreading of post-liquefied soils. At the south-end pier, a
permanent steel outer casing adds even more reinforcement to the drilled
shafts.
A 1-inch-thick, 4,500-psi concrete deck slab acts compositely with the
main box girder, the transverse floor beams, and the longitudinal fascia
girders.
The contractor for the $22.3-million bridge was Daniel O’Connell,
Holyoke, Massachusetts. HNTB designed and oversaw construction.
Early Opener
The Ted Williams Tunnel under Boston Harbor opened early in the
construction project.
The 0.75-mile underwater section of the $1.3-billion, 1.6-mile tunnel
was built with steel tube sections that were sunk into a trench on the
harbor floor and connected.
Traffic has been limited until the tunnel is fully connected to the
rest of the project.
Reprinted from Better Roads Magazine
September 2002 |
