Indoor material

[frizzen]

I know SmallHaul uses a dirt/sand mixture over a low pile carpet in his basement. He's mentioned needing to mix in water to keep it from getting too hard to dig.

I think you should just go LittleGiantsConstruction style and dig a bigger basement...

Cat litter is a clumping clay, so it'll probably both powder and turn to hard clay, then snot when watered.

What disadvantage does crumb rubber have?

A water truck would be cool. Could probably do either pressurized tank, windshield pump, fountain pump.