tortoise shell..and another question

[Bartman]

I built a cave kinda thing for my toroise..its a large piece of very flat rock than is held up on 3 rocks. Every time he goes under it he pushes off rocks and raises upward and grindes his shell on the top of the "cave". Than he gets white spots where he scraped his shell. Will this harm him? and is there anything i can buy to rub on his shell to protect it? And another question my russian tort is about 4-5 inches long at about 3 years old..is there a possibility of him getting impacted by sand?

thanks in advance