Thank you everyone for setting my mind at rest and patiently (mostly) pointing out my error.Also, the apparent perimeter of the stadium in photos of the site is a bit deceptive. It's actually the perimeter of the sheet-piling that encloses the excavation and prevents the surrounding soil from collapsing. I'd expect that to be a metre or two outside the eventual line of the stadium itself, so they have room to work. So it makes the gap look a bit narrower than it will be .
This is a wonderful forum with great contributors and we are all going to get the stadium we deserve.
Can't wait.