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The Google Street View above shows the I-70 embankment where RTD’s West Corridor light rail will go under the freeway on its alignment to the Jefferson County Government Center in Golden.

It might seem like a reverse way of making a tunnel, because it is.

But the contractor for RTD’s FasTracks West Corridor light rail says “top down” construction will spare major traffic disruptions while building the tunnel under Interstate 70, on the north side of the Sixth Avenue Freeway just east of the intersection of Colfax Avenue.

It’s an innovative way to get the job done, demonstrating that there’s usually a way to deal with any logistical problem a construction job can face. In this case, it’s how to punch a train tunnel under the man-made embankment of one of the most vital travel corridors in the entire state without bringing drivers to a halt.

As it is, travelers using I-70 will encounter overnight lane restrictions for two to three weeks and one full weekend closure of the highway – once for eastbound and once for westbound – while crews build the single-track tunnel under both carriageways of I-70 over Sixth Avenue.

“Top down” means drilling caissons down into the ground from atop I-70, then placing girders across them and replacing the highway paving. That leaves the concrete framing of the tunnel buried, encased in the soil in which it was drilled. But once in place, workers can go under the highway and scoop out the dirt without having to close the highway above to traffic again.

It’s the same method that was used to build the light rail tunnel under Colorado Boulevard between Buchtel Boulevard and Interstate 25 in the T-REX project.

Lawrence Construction Co. of Littleton, a family-owned general contracting company with more than eight decades of experience in heavy highway bridge and roadway construction, is performing the work for the West Corridor general contractor Denver Transit Construction Group.

The window of time to get this done is very narrow. The two tunnel segments must be wrapped up by Nov. 15, under rules imposed by the Colorado Department of Transportation. CDOT wants no closures after that date because of the start of ski season. This throat of I-70 as it approaches the foothills is the major gateway to the principal mountain resorts from the Denver metro area, including out of state tourists arriving at Denver International Airport.

Average traffic count here last year was about 80,000 vehicles a day. But there is a window of time with the seasonal drop-off from the heavy summer tourist season to the start-up of winter ski season.

View West Corridor Light Rail Tunnel in a larger map
Click on the blue box for details.

Preliminary “top down” work will require nightly lane restrictions, for the next two to three weeks starting this weekend. Two lanes will be closed at a time, leaving one lane open to traffic overnight from 7 p.m. to 5:30 a.m. Still, drivers might want to avoid the area by using Sixth Avenue Freeway to Interstate 25, then back to I-70 at the Mousetrap. Local traffic can also use Sixth to Indiana Street, then back to I-70.

While lane restrictions every night for up to three weeks might be disruptive to late night traffic, it’s less so than an around-the-clock full highway closure for the duration of the construction.

Still, one full weekend closure for each direction will be required. For eastbound lanes, that comes either the weekend of Oct. 9-11 or 16-18, assuming weather cooperates.

RTD's Jim Starling is the new project manager for the West Corridor light rail project.

Work on the westbound lanes is supposed to start immediately after the eastbound work is completed. The full closure of westbound I-70 lanes is tentatively scheduled for Nov. 6-8, but that could be moved up.

Here’s how “top down” works:

Starting this weekend, Lawrence’s crews will sawcut openings into the asphalt of two of the three I-70 eastbound lanes. They will first carve out working space underneath the level of the lanes which they can continue to work, Starling said.

These spaces will be covered up with steel plates by the morning re-opening each day, to allow resumption of full three lanes of traffic. Then at 7 p.m. each day, the plates will be lifted and work resume.

From below grade, workers drill down and pour concrete caissons deep into the ground, below the planned level of the light rail track. The caissons will be poured butting up against each other to form a solid wall, one on each side of the eventual tunnel.

Once the two walls of caissons are poured, the full closure weekend allows for quick placement of girders spanning the tunnel top and rebuilding of the I-70 roadway above it.

With this concrete “box” completed but buried, excavation crews move in to pull out the dirt, clean off the caissons and finish the walls and ceiling. Traffic rolling above should see no impact from this work.