Burrowing Under Boston, One Section at a Time

The challenge of opening up a passage beneath the infrastructure of a congested city has brought a variety of construction techniques and exotic machinery to Boston. "Slurry walls" - a series of opposing concrete panels - will form the underground side walls of the new Central Artery tunnel. Working from the surface down to bedrock, at depths of up to 120 feet, the Big Dig contractors will construct a total of 26,000 feet of slurry walls, then hollow out the earth between them to create a tunnel.

First, a deep, narrow slot is dug by a special excavator that has a compact head and oppos-ing cutting wheels in straight alignment, which can be dropped into a narrow hold. As it digs, the milling drums of the excavator pulverize earth and stone, which are suctioned off at an astounding rate.

"But you can't dig a huge hole in the middle of the city and expect it not to go anywhere," Maureen McCaffrey '86 points out. The deep, narrow slot is filled with slurry - a gooey "mud" of Bentonite and water. The slurry keeps the hole open until steel beams and concrete are in place. As concrete is pumped in from the bottom, the slurry is sucked off the top, pumped back to a recycling plant, filtered, and used again.

The slurry walls, which are constructed in 24-foot sections, become the permanent walls for the tunnel. Built-up steel struts span the distance from slurry wall to slurry wall, and the tunnel is decked over.

Tunneling under Boston's South Station requires some complex engineering. "We can't just dig our hole and build our tunnel walls like we want to, because there would be nowhere for the trains to run," McCaffrey says. To gain access, Perini excavated two vertical shafts adjacent to the South Station tunnel, then mined two horseshoe-shaped access tunnels, 90 feet apart, under the tunnel. From these parallel tunnels, cross tunnels are mined to insert the post-tensioned roof girders for the new Interstate 93 tunnel. The I-93 tunnel walls will also be constructed from the vertical shafts by mining drifts similar to the access tunnels.

"By the time you're done putting in the roof beams, you've constructed the tunnel - and all the dirt is still inside of it," says McCaffrey. "Then you mine away the dirt between the walls. In the meantime, trains from South Station are running 8 feet above your head! From an engineering viewpoint, that's probably the most intense part of the project."