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Seattle has a lot of tunnels, you'd be surprised. The big issue with this one is that they decided to bore it with a single tube rather than a smaller tube for each direction of traffic. So they're using the largest tunnel boring machine ever built, and it's straight from the factory untested. Then it ran into... something. They've never really announced a convincing answer for what happened. So then they had to dig a pit to haul the tbm up to the surface and fix it.
Most of Seattle's tunnels are cut and cover, though. That allows you to bank and stuff. I-90? Cut'n'cover. 99 under downtown? Cut'n'cover. The drilling process they're using is the same one they used for the Brightwater conveyance corridor and that didn't go so smoothly.