My biggest concern with working in any enclosed feature (wells, cellars,
etc) is safety.
 
In the USA (and probably most of the world...?) there are health and safety
regulations that cover working in what are called "enclosed spaces." None
of us want to endanger our diggers, but in addition to this, one can get
slapped with a large fine if the state safety inspector drops by and we
don't have an appropriate health and safety plan, an easy way to get out
the hole in an emergency, etc.
 
On the subject of excavation safety... The latest edition (I just got a
review copy last week) of a very well known American archaeological methods
text book has a *beautiful* cover photo of diggers working at the bottom of
a very deep excavation.  How deep? Difficult to say. My guess is 15 feet,
but don't quote me. Yeah they were wearing hard hats, but that wouldn't
save them from a collapsing section or a falling  bucket of soil,
wheelbarrow, or shovel. If you figure $5,000 per violation, the problems in
that one scene would probably eat the entire excavation budget for a year.
Ouch!
 
Adrian "Lemme outta here" Praetzellis
Sonoma State University