Need help with outdoor enclosure in the UK

[HorseTort]

I'm planning on turning an old wooden raised garden bed into an enclosure for my horsefield tortoise now that the weathers starting to get a bit warmer.
Its about 9 x 4.5 feet and 1 foot in depth.
I've looked over some other threads about outdoor enclosures and am planning to start buying cold frames, blue slate, etc.
My main problem right now is im not sure what to do for substrate. The beds currently filled with about 6 inches high of topsoil and i'd prefer to not remove it, but its full of small rocks (my tortoise loves to try eat them), and has plenty of worms still living in there. It was also used to grow potatoes last year, I'm sure all have been removed but some roots may still be in the soil.
What do you guys think would be the best option for me, thanks

 

[wellington]

You will have to remove the dirt or your risk your tortoises life.
You can cover it with wood,so he can't get to any of it, but that's the only options.
Also unless you are putting a chicken wire or hardware wire top on it, you will need to make the sides much taller.