Adding to an idea Zep Tepi hit on, if you dig out around a copper vein before you mine it, the ditch will keep trolls and greydwarfs out. They can't jump: if you can walk right up it, so can they, but if you have to jump up, you're safe. That lets you mine the vein in relative safety. (They can still throw rocks, but that's it.) This also helps you see the full extent of the deposit so you don't leave any of it behind.