I would add another area of dirt - different dirt - nice, moist potting soil (without any fertilizer pre-added) in the area where she is trying to dig. Even if its just a big, loose pile (at least 15-18" deep) you've got nothing to lose at this point, and she might just go for it. If not, you can easily remove it later. She's trying to tell you where she wants to nest, for whatever unfathomable reason, so just go with it...

I'm just guessing here - I've never had tortoise eggs - just turtles and snakes and lizards.

Jay