Dear busyanybody ,
I do not know the length of concrete encasement but apparently this protection for a road crossing and the total length of crossing may be in the range of 20 meters..That is, the longitudinal moment will govern and the encasement should be modelled as beam supported on elastic foundation. I will suggest to analyze longitudinally as FEM beam elements supported on winkler springs and transversal (cross section) model the system unit width box culvert supported on elastic found.(winkler springs). Another approach, provide EJ at concrete encasement (in the range of 20 mm) at socket-spigot joints, so the each pipe zone will have some freedom to rotate at joints, and analyze the pipe section 6 m , longitudinally rigid beam on foundation and cross section analysis similarly , take unit width of culvert and assume uniform pressure distribution across the bottom . You can calculate in a spreadsheet in case of second approach. Just by experience, the longitudinal reinforcement will govern....