Well before I read your thread I was going to suggest adding a second dog, but I see you already have two! I had a horrible digging problem with my Bella which totally stopped when I got Ripley. In my case it was boredom on Bella's part and having Ripley out there with her stopped it immediately. I had to smile because you also mentioned boredom in your thread.

I guess until your younger dog matures a little and gets through this phase you will just have to watch them when they are outside and correct the behavior when you see it. I know that is easier said than done! Good luck!