I agree with the "ground" but also put cypress mulch in the Hermanns house so she can bury herself. She is 9 months and not digging yet.
What kind of tortoise are you keeping?
When I first made my box turtle garden, I put some soil, mixed with peat moss to keep it loose. I added loads of plants, a small pond, and at the end of the garden I made a section with a cover, and filled it with leaf litter and pine hay. They go in there if the weather gets bad, and also to hibernate for the Winter. I've also seen them in there digging for worms and bugs. Just keep it natural, like the forest floor.