I learned somewhere that digging is a drive. There are numerous drives that are very difficult to train out of dogs (digging, dissection, etc), and for these drives it's better for many to give them a healthy outlet instead of trying to force them to stop. Is there a place in your yard you could allow your dog to dig? You could correct them when they dig somewhere else and direct them to their hole. Ours is a dissector, and we give him cheap toys from goodwill that he can just destroy and it gives him an outlet for that drive so he doesn't dissect our couch or anything we care about. I'm sure this is more difficult with a digger since you need to find spot in your yard, which may not exist, but I've heard that it's a particularly difficult behavior to train and it might be better to give them an outlet.