Bridging the Big Dig

Imagine the tangle of utility, sewer, and subway lines that lurk underneath one of the nation's oldest cities. Now visualize constructing eight to 10 lanes of underground highway through that mess. The subterranean roadway will dip beneath one subway line, go over another, and veer back down to connect with existing harbor tunnels before emerging onto two bridges that span the Charles River.

Fifteen million cubic yards of dirt will be dug to make room for the highway - hence the moniker "Big Dig".

Such a massive undertaking - touted by the project managers as "the largest, most complex and technologically challenging highway project in American history" - with its huge cost overruns and ongoing delays, is an easy target for criticism. Part of the problem rests in the fact that except for turning Boston's downtown streets into a ragged construction jungle, there's nothing much to see.

The only visible, aesthetically appealing structure that is emerging for dazed drivers, leery pedestrians, and strapped taxpayers after years of construction chaos is the Leonard P. Zakim Bunker Hill Bridge. Literally a bridge at the end of the tunnel, its graceful white stay pipes sheath spider-like cables that gleam in the sun, shimmer in the rain. The effect is downright dainty, a floating structure visible for miles around.

The bridge sets another record for this massive project: It is the widest (10 lane) cable-stayed bridge in the world - and one of the shortest.

"It's gorgeous, but overdesigned for that span. The Charles River is a creek!" says Lacey Prouty, a master's engineering student at Tufts University. "Boston wanted a jewel and they got it." Though, until the dig is complete, it will be without a crown.

Al MacPhail is the resident engineer overseeing the construction of this bridge. "It's really kind of a simple balancing act," says Mr. MacPhail, noting that the bridge's concrete "back spans" on either side of the Charles are heavier than the combined steel-and-concrete decking of the main span over the river. He likens the process to a teeter, where if you add weight to one side, you need to add an equal weight to the other to stay balanced.

Despite a huge number of variables, MacPhail maintains that simplicity underlies the construction: "Figuring out the loads is complex, but it proves the simple balancing act."

(c) Copyright 2001. The Christian Science Monitor