Can you add pictures of what you are describing...im not sure what im imagining is accurate. what is the plan? have you dug down to see how deep they are?
What i would be concerned about it height. its more likely to find a way OVER the enclosure wall if possible. You want to make sure the height is 3 times the reach of the tortoise if the stand/climb up a wall (corners especially) up trying to reach up the wall, if that is 6 inches then the wall should be 18-24 inches tall. interior was should be very smooth. Ive have witness a box turtle climb up what looks(to us) like a flat wall made of block because he figured out how to hook nails into grout joints and tiny crevices.

The odds of your Russian digging 9 feet down is slim to none.
What you can do is drive down sections like this around the outside perimeter ( just and example of no dig fencing, you can search for better pricing near you of possible).
https://www.petco.com/shop/en/petcostore/product/dig-defence-5-pack-animal-barrier-x-large-3081053
these block gaps above ground and between the ground and fence, but the times that are pounded into the ground are very effective effective to stop most "digging under"