I know the difference between an Architect and an Engineer. It depends on what type of engineer as well. But The Architect should have not said a d@mn thing if he wanted me to find my own way of installing it. This is a function of his design.
And like i said its a 2 foot deep with rock filling about 2/3's of it
Tar if its not exposded to any UV light will last longer than the house will.