Autodesk CIVIL3D - CIVILDESIGNCOMP 2007 SLM CD Tutorials Manual page 677

21 Change your view to a Front view. The stack of work planes and the
transitions created in step 17 are displayed.
22 Following the same methodology for the first batch of transitions created
in steps 16 and 17, add transitions between the Top of Riser 2 Profile,
and the smaller Top of Barrel profile, then the larger Top of Barrel profile
and the Bottom of Structure profile. (You may have to use Draw Order
to send certain profiles to the back, in order to choose the correct one
for each transition.) The model you created should now look similar to
the following illustration:
Exercise 2: Defining the Manhole Geometry | 665