For my Kinyongia I bought a bag of organic dirt from Lowes (can't remember which one it was, but all organic) and mixed it about half and half with play sand, it holds tunnels really well, and she happily laid her eggs in it (about 10 inches down) once she decided she wanted to. Just remember that it needs to be moist. I found it easier to mix the water in as I was filling the digging bucket, so I always had the chance (except for the very end) to add more sand/soil if I over did it on the water.
Good luck!