Maybe this is what we call a "bottom-less culvert" or a "Conspan". If that's the case, what is your specific geotechnical recommendation? If it's a flexible-type culvert, soil modulus may be a factor along with an understanding of the "soil-structure" interaction. If it's rigid, then simple earth pressures may be all that's needed. Either way, you'll need a geotechnical engineering evaluation to properly size the foundations for the extent of the soil loading above the culvert - don't forget the weight of the structure also. Get some borings and figure out whether you're working with a concrete or flexible system. Here's where a structural engineer can be your friend.
