Sand works...but not the way they would like it too. They are a burrowing snake. Aspen holds the burrows they make as long as you don't run your hands through it destroying it along the way. It leaves them with a secure spot to hide in. Also when feeding you can tap the rodent right outside of it burrow and it will be more willing to take the item within the security of its burrow. Sand just collapses. In the wild they aren't on play sand its a mix of sand, dust, clay, and dirt. When its all combined they can easily make sturdy burrows. Plus aspen is so much easier to clean and definitely doesn't have a smell to it like soiled sand can. Its all up to your preference. I just wouldn't feed directly on sand for the simple fear of impaction (not too worried about that though).
Would that sand that you can shape burrows out of be good for them? I've been using normal sand but what you said here makes me want to offer something he can build burrows in that won't collapse.