Big Bridge, Little Bridge: the big dig soars across the Charles River - includes related article on the Central Artery/Tunnel project - two new bridges across the Charles River in Boston, Massachusetts - Cover Story

Public Roads, Sept-Oct, 1999 by Sybil Hatch

A composite steel box girders with a cast-in-place concrete deck will form the $26 million Storrow Drive bridge, which is about 30 meters west of the cable-stayed bridge. It has three continuous spans, 69-, 116-, and 69-meters-long, respectively. The 9.4-meter-wide steel box girder, with cantilever floor beams, make up a finished roadway width of 22 meters. They are thought to be the largest of their type in North America.

A Busy Corridor

There are major obstructions all around the project. Some were known before construction and some were not.

The Massachusetts Bay Transit Authority's (MBTA) Orange Line tunnel crosses the river at the same location as the north tower of the mainline bridge. The bridge foundations straddle the Orange Line tunnel, a concrete box about 10.7 meters wide by 9.1 meters deep.

Its roof is about 12 meters below the top of the bridge foundation. Prior to construction,Atkinson-Keiwit probed widely and deeply to verify the Orange Line tunnel's exact location. However, during installation of steel sheeting for cofferdams at the north tower, they found more than they bargained for - the coffer and a 0.91-meter waler, used during original Orange Line installation, at a depth of 12 meters.

The north tower foundation plans originally called for a 6-meter-thick reinforced concrete footing with a 2-meter-thick concrete tremie seal. This would be eight meters deep.

The south tower foundation straddles a 0.91-meter-diameter water main and is two meters away from the Orange Line tunnel. The water main, parallel to the slope down to the water, was not located at the elevations originally thought, causing an extra cofferdam to be installed for protection. Portions of the pipe were encased in concrete for added protection.

Dedicated Lanes

Traffic, itself, caused significant design challenges. A very constricted waterfront area precluded the North End neighborhood on the south end of the bridge from having its own northbound on-ramp. Motorists would have to travel south, then turn around to get onto I-93 northbound.

The ultimate solution was two cantilevered lanes on the east side of the mainline bridge. The cantilevered portion will accommodate northbound traffic from the Sumner Tunnel and the North End, but it creates an asymmetric cross-section.

The asymmetric layout posed significant design challenges. The cantilever loads along the main span are carried by the east side cables at stresses up to 50 percent higher than on the west side. Also, with 10 lanes of traffic, this bridge is the widest cable-stayed bridge ever built. The entire bridge design has undergone intense scrutiny from a panel of experts convened by FHWA to evaluate its viability and integrity.

Little Elbow Room

Space to work and laydown areas are major issues throughout the Central Artery/Tunnel project. Separate contractors are working on four different projects in the south shore area alone. One strip of staging area was accidently promised to two contractors at once. Atkinson-Keiwit is using barges in the river for extra space.