Start off by creating your existing ground surface - in my case creating from 3D polylines and AutoCAD blocks representing spot heights. Next create the existing road centreline alignment and profile. (Copy the 3D polyline for road centre and turn it into a 2D polyline - then create alignment from this). Also create existing pavement edge alignments and profiles in the same manner. This is what I have so far...
The subassembly that we are going to use is OverlayCrownBetweenEdges.
Like most subassemblies the layout appearance is not that important but rather the properties of the subassembly.
For a detailed explanation of the behaviour and properties of this subassembly see the help menu. In my case the offset values are going to be controlled by the existing pavement edges - these will be set as targets in the corridor later - and I can ignore the left and right offset properties. I want to achieve a min slope of 2.5% on my finished road surface. The subassembly will examine the existing road surface at each corridor sampling frequency and determine the higher of the control points (pavement edges). It will then apply the min slope from this point back to the centreline and then slope back down to the opposite pavement edge at whatever slope is necessary. Since this slope value will vary and depends on what the existing road surface is like at each sampling frequency you are best to set a relatively high max. slope value. If the slope needed is greater than the max. set in the properties then the corridor will not be created at this point.
I am leaving the crown height value to zero also - setting a value here can override min/max slope values. I want my design parameters based on the slope values only.
When creating the corridor use the existing centreline alignment and profile. In the target mapping dialog box select the alignments and profiles representing existing pavement edge to define the offset and elevation values for the road edges.
The resulting sections look like this...
You can see the result - the existing ground is higher on the LHS, C3D applies the min slope out from this point towards the Cl and then applies whatever slope is necessary to tie back to existing pavement edge. If the slope is too high on the lower side you can easily edit the subassembly properties to reduce the min slope value, or use the corridor cross section editor if you want to modify only a small region.