King Street underpass at the transit hub

[GtwoK]

That's exactly how it's being constructed. Foundations were dug out and poured first. Some early photos show the foundations poured - they look like a bunch of pillars overlapping slightly. I believe it was the same process used on the catenary pole foundations, albeit on a larger scale. The bridge deck was then poured on top of that. Finally, they're now excavating the entire thing. It's sort of like how some thing are casted out of metal, I suppose; an underground mold.

The one thing I'm not sure of is how they went from those overlapping pillars to the flat-faced walls we've got now.