Workers given the "Shift-A" command are automated but won't alter existing tile improvements.
[Compare to the "A" command which is fully automated -- under the "A" command your workers will freely decide that "this particular mine would look so much better just to the left a little bit -- I think I'll irrigate over this mine and then build the mine to the left . . . on that irrigated tile."]
Always remember the caveat though that automated workers will be less efficient than manually controled workers unless the only thing they're doing is cleaning the occasional pollution.