I hope someone has the answer for you because my dog is a digger as well. We had to put cement blocks along the fence to keep him from visiting the neighbor (luckily she does not complain). He also found seams to our sod that we put down in October and tore up big holes. It seems to help when I get him out eachday for a good walk but with the weather lately we been holed up in the house alot more.
All I can say is excercise, excercise, excercise. I bought an attachment to my bike for when the weather is better so he can run along side and not get caught in the wheels. A tired dog is a happy dog (esp w/ these breeds).