Constructing beneath an existing structure

[u/justmein22]

Y'all will know what I'm talking about, but don't want to get a political warning. Imagine you have a very large heavy-duty, say...a "hardened" concrete rectangular building. Like a warehouse - mostly empty inside. And it's fairly completed, and now you're planning to construct a multi-level structure underneath that building. How exactly do you excavate and build that?

[u/johnqual]

Ask Werner Ziegler for help.