When my russians are indoors I keep them on newspaper.

I do this because a well-fed russian is a messy russian.

Lots 'o poop. And it'll spoil any substrate you have. So you might as well make it something you can clean easily.

I put down a couple layers of newspaper, and then use small sections of tile to hold it down in the corners.

Since russians like to have a place to hide, I use a small box with a hole cut in the side of it. I then put in some paper towels that I periodically moisten with water. This provides a humid hide spot. This also eliminates one of the needs for a thick substrate like soil/sand mix. If your tort had the soil/sand mix, she could dig into it and conserve her heat and moisture at night. But soil/sand also hides the fecal matter, and is hard to keep clean.

Now outdoors, the feces aren't such a problem. The sun dries them up and the pen is usually large enough that the feces don't become noticeable between clean-ups.

If you want a deeper substrate, try straw. I know someone who does that and purchases partial bales (flakes?) at a feed store. Throw some in, and spot clean as needed.

Since straw is dry and doesn't last long when wet, you should still make a humid hide box for the torts. Although russians come from dry regions, they still have moisture in the tunnels they dig, and the humid hide simulates that. A perpetually dry tortoise can have kidney and other health problems. So make a spot that is humid (not soaking wet).

Good luck!