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Supporting the Elevated Highway

Boston's Central Artery/Tunnel Project promised to keep the city open for business during more than a decade of construction. That meant traffic must flow, and in terms of the number of drivers affected, the biggest piece of that promise was keeping the elevated Central Artery in service and carrying its full load of more than 190,000 vehicles a day while the underground expressway was dug directly underneath. 

The old elevated was six lanes wide. The new underground expressway is eight to ten lanes wide. Every footing that supports the columns holding up the original Artery sat directly in the path of the new highway tunnel. Every one of those footings was removed, but without taking the existing highway out of service. The entire weight of the elevated highway -- it contained enough steel to build five Tobin Bridges -- was shifted onto new supports resting on the walls of the new tunnel. This load shifting process was called "underpinning."

The key element of this process was the slurry wall technique used to build the walls of the underground Central Artery. The walls were dug first, one three-by-ten-foot section at a time, down as much as 120 feet to bedrock. Altogether the project built five miles of slurry walls, more than 26,000 linear feet. After the walls were in the ground, steel beams and concrete decking were placed between them on the surface so that traffic and construction equipment moved above while excavation and highway construction proceeded underground.

Before excavation began under the rusting elevated structure, the old support columns were cut away, which meant that new steel supports were built resting not on footings anchored in soil, but on the new tunnel walls that rest on bedrock. There were 67 rows of three support columns spanning the width of the 1.5-mile elevated highway, plus other supports holding up on- and off-ramps. For the most part, the load was shifted one entire three-column row at a time.

1. First, a steel framework was built next to or around each support column. Two vertical supports rested on beams that in turn rested on the walls. (In some cases the vertical beams rested directly on slurry walls.) A horizontal crossbeam atop the vertical columns was placed under the horizontal beam beneath the roadbed.

2. Next, the horizontal beam in the new support structure was bent, using jacks, to simulate the weight of the highway above. Most of the underpinning structures used a "low pick-up," to bend a huge crossbeam resting slurry walls at the bottom of the support frame. A "high pick-up" bent the crossbeam at the top of the support structure.

3. The jacks pushed against a frame bolted to the support structure, pushing against the full weight of the elevated highway to bend the crossbeams. Once a beam was bent, generally by less than an inch, steel plates or "shims" were inserted to fill the gap at the bottom of the vertical columns and bolted in place. Once the shims were in place, the jacks were released and the old support columns cut away.  

The elevated highway has been demolished, along with the underpinning frames.