Basically the rough manual work that doesn't require knowledge of a wrench or a screwdriver - save your employed workers some time, and put your inmates to work in a sensible way (Bonus karma if they are forced to fill up their own tunnels

So basically you'd have a long conga line of prisoners daisy chained to each other with a couple of armed guards escorting them around (At least if you're sending them outside the prison confines). Add a tiny chance of escape attemps if they manage to sneak along something to pry open their cuffs with (or use the tools at hand) - The inmate would have to be desperate enough to risk running from an armed guard though.