There are several things you can try to prevent your German Shepherd from digging in the yard:

1. Make sure your dog is getting enough exercise. A tired dog is less likely to engage in destructive behaviors like digging.
2. Provide your dog with plenty of toys and activities to keep them entertained. This will help to alleviate boredom, which can be a common cause of digging.
3. Create a designated digging area for your dog. This could be a small sandbox or a specific section of your yard where your dog is allowed to dig. This will give your dog an outlet for their digging instincts while also helping to establish clear boundaries.
4. Supervise your dog when they are in the yard, and intervene if you see them starting to dig. You can try distracting them with a toy or a command, or you can gently redirect them to a different activity.
5. Train your dog to respond to the “leave it” or “stop” command. This can be useful for interrupting undesirable behaviors like digging.
6. If your dog is digging to escape or because they are anxious, consider addressing the underlying cause of the behavior. This may involve providing your dog with more socialization, increasing the amount of exercise they receive, or addressing separation anxiety.
7. You can also try using physical barriers, such as chicken wire or fencing, to block off areas of your yard where you don’t want your dog to dig.

I hope these suggestions help!