Friday nights are usually a time for celebration—the end of the work week and the start of something far better. In Seattle though, this Friday marked the transition into a terrible time, one city officials are calling the period of maximum constraint. At 10 pm, the SR 99 Alaskan Way Viaduct—damaged in a 2001 earthquake and temporarily reinforced—officially closed, for good. Over the next three weeks, traffic-fearing Seattleites will stay home, flee town, or at least carpool. Meanwhile, construction crews will race to open its replacement, the SR 99 tunnel.

Mostly, that work involves connecting each end of that 1.7-mile tunnel, dug by the famously big and beleaguered Bertha, to the rest of the highway. (Then workers will start to demolish the now-empty viaduct.) And while you’d expect this sort of project to involve digging, you might be surprised to learn it involved digging up a perfectly good, carefully buried piece of road infrastructure.

Earlier this month, Washington Department of Transportation workers spent three days unearthing the southern entrance to the tunnel (which is still called an “on-ramp”, even though “in-ramp” would make more sense), which they had built in 2013. Now, they’re dusting it off and preparing it for its new role, ushering drivers into what might be the world’s smartest underground concrete tube.

That burying part is weird, huh? Well, let’s start with why you’d want to bury something you just built and planned on using. To keep things running during the years it took Bertha to dig through all that dirt, crews built temporary roads at both ends of the tube. At the south end, part of that detour road was needed in just the spot where the on-ramp to the tunnel would go. So the engineers decided they’d build the ramp first, before putting in the detour. They filled it up, covered it with a layer of dirt, and laid down the asphalt for the temporary road.

The partially excavated on-ramp to Seattle's new tunnel, before the detour road built over it was shut down and demolished. Those white blocks you see are Geofoam, the packing peanut-like material the support the weight over that "sacrificial" bit of asphalt. Washington State Department of Transportation

“Now that the detour’s coming out, they will take all that material out, open those ramps up, and make those connections into the tunnel,” WDOT Deputy Administrator David Sowers says in a video explaining the project.

That kind of temporary, or “sacrificial” road is quite common in any project where space is at a premium, says Matt Cunningham, a civil engineer and global director of infrastructure for Canadian engineering firm IBI Group. The damaged viaduct runs right through downtown, along the waterfront. “If it were out in the bay, or in a green field, you wouldn’t do it this way,” Cunningham says.

The how of the ramp burial is rather simple. The engineers needed to support the weight of the detour road and the vehicles it would carry, so they filled up the carved out space. Instead of using rocks and soil, though, they used giant blocks made of Geofoam, which is basically the same stuff packing peanuts are made of. It’s inexpensive, easy to put in place and remove, extremely strong, and, most importantly, weighs about a hundred times less than dirt and rocks. That mattered here, because the engineers didn’t want to risk shoving the ramp structure too far into the ground. They covered the geofoam in a thin layer of soil, and built the road atop it.

Once the bit of detour road running over the ramp was shut down, crews knocked away the asphalt, cleared off the dirt, and used cranes to haul out the Geofoam blocks. Boom: On-ramp, reborn. Just in time for the weekend.

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