I work in the middle of London, in an area full of large office blocks. Across the road from my office they have started construction of a large building (10 stories plus). Over the last few weeks, diggers have dug a large (and vertical walled) hole. Lorries have taken the resulting dirt and old concrete away, leaving a very neat hole.
In the last day or so, the lorries have returned with new dirt (or the old dirt crushed) and diggers have been putting it back in the hole (and compacting it).
Why put the dirt back? Surely leaving the hole deeper would allow for deeper basement (or digging it shallower would be cheaper)?
I'm not a structural engineer, so this is all lost on me, but I'm fascinated.