You know, its funny but I just found one of my delhezis doing the same exact thing this weekend! Mine wasn't burried as deeply as yours though and unfortunately I did not have a camera handy to snap a few shots
my del buries itself in the middle all the time. just sits around peering with his head and tail poking out. and my congicus does it when i do a water change.
i wanna bury one of mine to teach them. I think I have the exact same sand as you too, actually you know what, my sens are probably too fat to get in there.