Volume Calcs. from Sections

I was creating an as built Civil 3D model of a windfarm job and was asked to calculate a number of quantities. One was the volume of rock fill. This seemed straight forward - compare finished road surface against top of rock and you have your answer. The problem was that my finished road surface slopes down from road edge to top of rock and then batters back from there to meet existing ground. A straight comparison between the two would give incorrect volume, see below:

This includes the portion on the outermost batter. The volume we actually require is this:

To get this we need to create a new surface comprising the road edge featurelines and the featureline where the initial embankment intersects the top of rock. First lets look at my assembly… I am using a combination of generic subassembly (linkwidthandslope) and a conditional subassembly to model different conditions in cut/fill.

If you look at creating a surface from the featurelines generated by the corridor you see that all the individual linkwidthandslope subassemblies have a point code P2 and thus only one featureline I can choose (this would effectively give me the same result as choosing top links to create my surface).

You need to create unique point codes for each subassembly and thus unique featurelines to choose from for corridor surface.
Select the subassembly and change the Point Code  in Properties. Road edge here:

Then slope to rock:

Rebuild the corridor and in now corridor surfaces you should see some extra featurelines:

Now on cross sections you should be able to calculate the correct volume for rock fill by comparing this new surface with top of rock: