I have been a bridge engineer for fifteen years and have just a few comments/questions.
1. Bridges (and other structures subjected to vehicle loading such as retaining walls, culverts, etc.) are designed per the AASHTO LRFD code. This code (reference chapter 3) requires the design of such structures to withstand loadings from "HS-20" truck types. For design purposes, these trucks have a total weight of 72 kips, or 8 kips for the front axle and 32 kips each for the two rear axles. Are you sure of your wheel loads? That is one hell of a big vehicle you are designing for. Such a truck would need a special permit just to be on the road in the state I live in. Such a truck could collapse a typical freeway bridge or elevated interchange. Is this a special military vehicle you are designing for on a military facility? Such a truck is nearly 7 times what a freeway bridge is designed for and would never receive a special permit to pass on a typical freeway structure.
2. AASHTO requires 250 psf surcharge (approx. 2 foot soil surcharge) for loadings where highway traffic is close to a retaining wall.
3. Tire contact area, whether one or two wheels, is taken to be 20 inches wide by 10 inches in length.