The structure I am trying to analyze is a 2 cell reinforced concrete box 17 feet below a 6 lane turnpike.  The increase in overburden will overstress the RCB, so we are putting 8.5 feet of EPS Geofoam to reduce the stresses on the top slab and side walls. 

Can I model the RCB with beams and the Geofoam with a very light material?  I am attempting to find the pressures on the walls and top slab using Adonis.  See attached drawing.

Thank you in advance!!!!

unfortunately it's not clear what you really want to do.
you can either use beam or solid elements to simulate RC but please note that beam structural element can only get attached to the boundary edge (edges sharing only one element).