Yes , the safety advise is excellent since there could be subsurface electrical conduit. There are some very experienced privy diggers
on this site who can give you major advise - perhaps if you search for post's regarding privy digs - you can PM. some of these
folks , they'll get you headed in the right direction regarding safe techniques and the right equipment for the task .There is a
Bottle and Glass subsection if you scroll way down the forum - though it may be more about identification and values .
( I just remembered the username of one of the very experienced privy diggers it's - jgas - good luck !)