Seattle tunnel construction avoided costly mistakes of Boston's Big Dig

King 5 News ^ | January 17, 2019 | Amanda Grace

[Tolerance Sucks Rocks]

Seattle and Boston have a lot in common: they are both cities on the water, homes to higher learning institutions and hubs for tech companies. They are connected by Interstate 90 and memories of a Superbowl both cities can’t forget. Now Seattle and Boston are both home to big tunnels that run under the downtown core.

Seattle’s tunnel will now face a new challenge: avoiding the problems that plagued Boston’s Big Dig long after it opened.

The Big Dig Backstory

The Big Dig began as a project to replace the Central Artery, an elevated 6-lane highway that was structurally unsound and horribly congested. The state of Massachusetts decided to build an 8-to-10 lane underground expressway, a system of tunnels and four major highway interchanges. However, costs and the construction schedule ballooned out of control.

When Seattle was planning the Highway 99 tunnel, Boston’s troubled Big Dig represented everything that could go wrong. Opponents rallied against the Seattle tunnel, holding signs reading, “No Big Dig.”

In 2009, then-Washington Governor Christine Gregoire vowed to investigate what went wrong in Boston and make sure that any and every lesson that can be learned would be learned.

Seattle Tunnel vs. Big Dig

Infrastructure experts point out that construction was different on the two tunnels.

"Seattle had the benefit of not having an 8-lane super highway that it had to bury,” says Dan McNichol, a former Big Dig spokesperson who has written books about Boston’s tunnels. He says, in terms of scale, the Big Dig was the equivalent of burying I-5.

McNichol points out that Seattle chose a different construction method: tunnel boring versus cut-and-cover.

"The Big Dig was much shallower; therefore, the whole city was ripped up. They call that kind of construction cut and cover,” he explains . . .

[gogeo]

The hills in and around Boston were pulled down to create the fill. Tough work in the 19th century.

Same thing in Seattle. The tunnel and the viaduct are in fill.

[Tolerance Sucks Rocks]

They used one of those large round visitor ships from the V series to suck all the water out of the harbor so they could then cut and cover.

[21twelve]

LOL. I had my doubts on the tunnel due to the variable geology. Some of the tunnel route is the hydraulic fill of loose sand. Other portions are a wide variety of overburden dumped by the glaciers (gravel, clay, silt, etc.) I think it was something like ten different types of materials they had to dig through. They did have some issues with subsidence due to when it was stuck.

[LanaTurnerOverdrive]

The viaduct was 8-lanes.

The viaduct was SIX narrow lanes....and it had downtown exits, unlike the new tunnel.

[rockrr]

Thanks for the correction. It’s been years since I have ventured into the valley of the shadow...

[moehoward]

Now they are removing buses from the tunnel because they slow down the trains.