Normally, cavies won't dig in the dirt or grass. But, if you are keeping your cavies outdoors to live with grass as being the "floor," then make sure the grass hasn't been treated with pesticides or fertilizers. Also, make sure the cage is strong enough to not let any other animals brake in and still your cavies. Eventually, the grass under the cage will be eaten, and you'll need to rotate the cage to another spot. If you don't want to end up with a bare yard, I suggest you using some other flooring.