Engineering feats connected with Big Dig

Some engineering feats associated with Boston's Big Dig project:

_ Tunnel Jacking.

Part of the project called for a tunnel extension under an active Amtrak railroad. Project managers realized the soil was so unstable that the rail lines could collapse. Engineers built a gigantic concrete box open on both ends, froze the soil using hundreds of rods and nudged — or jacked — the box under the railroad a few feet at a time. The top of the box supported the rail lines, the inside became part of the tunnel.

_ Slurry Wall Construction.

The Big Dig featured the most extensive use of slurry wall construction in North America. To create the tunnel walls in downtown Boston, excavators dug a narrow trench, sometimes more than 100 feet down. To keep the trench walls from collapsing, a thick slurry mixture was pumped into it. The slurry was then displaced as the trench was filled with concrete.

_ Supporting the Old Elevated Highway.

To keep the old six-lane elevated highway running as they built an eight to 10 lane tunnel directly underneath, project officials had to devise a way to remove the old support columns without the elevated highway collapsing. The solution? Build wider horizontal supports, then cut away the bottom parts of the original struts and lower them onto the new horizontal braces, shifting the entire weight of the overhead highway onto the new horizontal braces.

_ Casting Basin.

Building a tunnel under Fort Point Channel, an extension of Boston Harbor into South Boston, proved to be another challenge. The channel was too narrow to float in tunnel sections, so engineers built a massive casting basin or dry dock to build the concrete sections on site. The basin was then flooded and the section floated and sunk exactly into place — no second attempts possible.

Source: Massachusetts Turnpike Authority and Associated Press news reports