I've read that dogs dig because they are bored. Can yours spend more time with you, or be taken places sometimes? To keep our "diggers" from tunneling under the chain-link fence we got some field fence from the local feed store (chicken wire fence also worked). We dug down under our chain link fence about 6 inches & laid down a section of field fence flat on the ground & covered it up with dirt. Our dogs dug at the edge of the chain link fence a couple more times but finally stopped when they kept hitting the field fence. It kept them from getting out of the yard, which was the most important thing. This was a couple of years ago. Now grass has grown back in those places & you would never know there is a section of fence buried there. This also worked for a friend of mine. It's not expensive either. Good luck.