There is a show on National Geographic channel called Dog Town, and they had a dog that did the same thing. he never stopped digging. The dog behaviorist did what ginger899 above said to do, keep him busy with something else. He would put the dogs food in a Kong toy, and the dog had to figure out how to get it out, which kept him busy. He also jogged with him, which tired him out alot, which in turn kept his mind off digging.
thanks you she's really out of control now! but I'll try distracting her!
Dogs are like that. They get hold of a fun idea, then lo and behold.....it becomes an obsession!
The dirt could contain worm eggs, especially if there have been other creatures around like mice. Dirt, in itself isn't harmful (in small doses!) but mice and other critters do infest some places with worms.
Obviously the first thing is to go get her a good broad spectrum wormer from the vet. I say that because I used to give my dog an over-the-counter wormer, but it started making her vomit. So when I got one from the vet instead it has no side effects.
The thing that makes a dog start wih an obsession, and keep going back to it is mainly habit, and boredom. She's got used to doing it so she wants to keep doing it.
The thing to do is a quite sharp, firm "NO!" as soon as she bends down to start her digging. She will probably look up at you straight away. That's the point of saying it sharply. To capture her attention instantly. Then when you have her attention, get her immediately away from the digging, call her to come play, or come inside, or come for a walk, etc. The best thing is to substitute the digging with something else which is good fun, and do not let her get bored for a second. But.....(this is the tricky bit) -not making it seem like the play/walk/etc is a REWARD for the digging! So there has to be a few moments between her stopping digging and coming to you, and you replacing the behaviour with something else.
This is going to mean dedicated work on your part for a while. But don't worry, because the more you repeat this method, the more used she will get to going "cold turkey" on her digging addiction!
My dog was digging for moles all the time. I was worried it would put strain on her spine, so did this method with her, and it started to really work after a few days. Now she can walk straight past a mole hill and ignore it.