ackies like all monitors like to dig. it depends on the enclosure you have them in. a standard aquarium could probably take 8 inches, a trough or custom should be around a foot for a trough and a foot to a couple feet for a custom.
that is more like a text book answer, the true answers will be found with your monitors and what they like and use.
to be honest i have some pretty crappy soil around here. i went to a regular forum participant's place today and picked up a monitor he gave me (thanks again!), and saw he had an awesome substrate in his flavi cross cages. i asked where i could find it, drove and picked up a pick up load of it, awesome. that is called "sandy lome," and his monitors had perfect deep burrows that held well and were easy for them to dig.
cheers, andrew
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Varanus Creations