If your dog considers the backyard (or wherever he digs) to be a place of banishment, it is more likely digging will occur there. I am suggesting that your dog should think of the backyard as more than just a place of banishment.
Are you like millions of people who put your dog out in the backyard every day you go to work? Many dogs will feel a certain level of anxiety if the backyard is the place where theyre always left alone. For many owners, this cant be helped. It is important that you give your dog other, more positive experiences in the yard, as well.
For example, play with the dog in the backyard regularly. You can also feed the dog in the backyard and obedience train your dog in the backyard. Help the dog to associate positive things with the yard and you will be eliminating one of the major causes of digging.