When you dig near foundation piers or footings, you can not dig anywhere under the 45 degrees of the lowest part of the footing. This area needs undisturbed soil for compression underneath to support what it is holding up.
I think I may have even confused myself..
Basically from a 45 from the bottom of the piers, do not excavate..
And be safe, if your dirt starts to cave in or a pier is starting to move a little , stop and brace it in any way you can.
My first stop would be at the library...for a book with pictures.
I did this to my first home where I installed the water heater, boiler, well pump and tank in this area. It was the best thing I could have done.
Be ready for lots of asprin.