I believe humidity is important. A substrate of coir (the ground up coconut husk available at any store selling reptile supplies) might work out better than aspen. I mix it with a little calcium sand, and it will hold burrows quite well. Depends on your ambient humidity in the cage if you need a more damp substrate. Where I live, Winter humidity is so low that I have to use a more moisture-laden substrate during that time.
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Mark