Using the correct substrate is very important, juveniles can already dig very long tunnels after five months. As a result of making tunnels, there will be a wider range of temperatures in the enclosure. Therefore care should be taken not to make the front-edge too low because you must provide juveniles with a very high substrate to dig in. Sand alone is not a good choise of substrate because they cannot dig in it without the substrate falling in and sand alone will not hold moisture long enough. Because the substrate must not dry-out immediately, I personally use a sand and soil mixture to keep the humidity levels high and so they can dig a little in it.
It is frequently assumed that an exanthematicus can kept best on an simple solid underground because using substrates can lead to constipations. From findings from D.Bennett show that there is allways a small amount of durt in wild specimens and therefore cannot be seen as harmfull.