The Animal Doctor Explains Digging by Dogs. He may have explained it, but he didn't offer any real solutions to keeping a dog from digging up your garden. We had the same problem with our dog, who discovered some chipmonks living under a statue in our garden. He was successful at upending the statue and almost demolishing himself and he was successful at digging giant holes but he never caught the chipmonks. We ended up trapping the chipmonks and releasing them in Sligo Creek Park.
As for digging, we tried some commercial products which work by providing an offensive odor to the dog so he won't enter the area. Of course, those didn't work either. No, the simple solution is to use plain old dog poop to stop him. Take some of his fresh doodies and drop them in his digging holes when he isn't looking. Cover them up. (You have to be stealthy, because it's the element of surprise that will get him). Allow him to enter the area on his own and commence digging. When he discovers his own poop in his hole, he'll be very offended. If he's smart, he'll be cured immediately of digging. If not, after one or two more times of finding his own doggie delights, he will be completely cured of digging. This worked for our dog, and he was an ambitious digger. [We got this tip from George Cockrell, a Washington DC area dog trainer.]