Modelling Underground Pipe in Box Culvert

Dear all,
What is the right way to modeling Underground pipe in Box Culvert, i think i don't have to use soil modeler in this case, am i right? but i face a problem to modeling this pipe when it enter the box culvert and when its out of it, should i modeling the part when the pipe go inside and go outside with an anchor? but the culvert have its own thickness, or do i have to modeling it as a rigid without a weight and then place the restraint before and after it?
please an enlightenments for me....

Regards

LNG

Thanks a££, after read the technical manual, i model the pipe when it down to the ground as underground pipe with soil modeler n in the part when the pipe goes into the culvert back as aboveground. I hope i doing it right. If any input please suggest me. Thank you.