Yes, that's a great site for this type of thing. I used EPS and know the plant manager at the EPS factory so I was able to pick his brain. I am also a civil engineer and have used EPS as lightweight fill under roadways above gas mains and at railroad crossings. There is nothing at all wrong with putting EPS underground including those places where moisture may be present. 25 psi EPS is just as strong as the bearing capacity of most subgrades so strength is not an issue. Concrete, 6", only weighs 0.5 psi. The trouble is when you wreck a truck and pour fuel into the foam. Then it shrinks. Probably happens with XPS too but I've never tried it.