By the time divers get inside a pressurized chamber in Bertha, North America’s largest tunnel-boring machine, to investigate an obstruction that has the 57.5-ft-dia giant sitting idle since Dec. 6, finding a boulder in the way could prove the most welcome sight.
While the next issue of Engineering News-Record will showcase in detailhow crews will move inside Bertha later this week, what types of soils they have expected to deal with so far and just how far along Bertha is on the 1.7-mile project to build a highway tunnel under downtown Seattle, my article will also explain how specialized divers will go at removing a boulder, if that is what ails Bertha’s progress.
Stopped just 1,019.5 ft into the project, Washington State Dept of Transportation officials and joint-venture contractor Seattle Tunnel Partners leaders didn’t expect an “intervention” would be needed so early into the dig. But that is where they’re at. Still on a downgrade from the start of tunneling and only 55 ft from the surface—Bertha will get as deep as 200 ft at the deepest point—finding a boulder in the path wouldn’t come as a surprise. Almost anything else would be.
Boulders, though, come with contingency plans aplenty.
If a boulder is stuck in place, Bertha is designed to rip it apart, using the hundreds of different types of tools on her cutter head to chunk it down to piece 3 ft in diameter or less. If the boulder shifts in loose soil when Bertha engages it, divers will take hand tools with them outside the cutter head to chip away at the rock. That remains the same plan if the boulder proves just too large for Bertha to handle on her own. Seattle Tunnel Partners has all kinds of plans for dealing with boulders. Anything else—including a clogged Bertha—will require a fresh strategy. And that is why a boulder may prove the best-case scenario.
Tim Newcomb is Engineering News-Record’s Pacific Northwest contributor. He also writes for TIME, Popular Mechanics, Sports Illustrated and more. You can follow him on Twitter at @tdnewcomb or visit his website here.