top of pipe culvert to bottom of subgrade distance?

[delagina]

i have a 40' private access road going to a small site.
it's a 6" concrete with 8" sub grade. this access road will connect to main road perpendicular to it.
the access road will be crossing an existing ditch of main road so i need to provide a pipe culvert.

our drawings usually only show pipe culvert on plan. no cross section showing the depth of pipe culvert with respect to road.
we usually just have gravel road. in this project i need to show it per client request.

can anyone link me to guide regarding pipe culvert.

 

[cvg]

http://www.ail.ca/site/media/ail/CSPI_Handbook of Steel Drainage & Highway Construction Products.pdf