I know about digging out! Eventually they'll try and it can be just as dangerous as a tie-out.

The rescue/shelter where I volunteer solved the problem by laying chain link on the floor, making sure it was all connected and not sticking up anywhere. Then they wired it to the sides with the wire meant for chain link. Over the wire, they packed dirt so the dogs feet wouldn't hurt. It works for their biggest escape artists and is long term. You could use a thinner wire, but in time it could rust, break and end up being painful to step on.

At home I put cement blocks around the edges of the fence where they dig a lot. When they try to dig under, the block falls into the hole, further annoying them. With this method, you have to keep an eye on the blocks and reposition them occasionally.

Mary