This trick is handy to know
If you build a corridor and from that a corridor surface model and then apply Site Improvements to it using the Apply Surface Site Improvements command and then create a Subgrade Surface using the Create Subgrade Surface command, and then you want to reference that Subgrade surface back into a Corridor Model you can do the following. Note that you should complete the surface before doing this so that you do not have to edit it again later on (because you will not be able to).
Open the properties pane for the subgrade surface model
Set the Rebuild method to “By User” this locks the surface so it doesn’t update
Change the Reference Surface to “None” - this breaks the link to the corridor model from which it is derived
Now you have a surface derived from the corridor surface itself that you can add into the original corridor as a surface instruction for the purposes of adding subgrade models to determine subgrade quantities.
Note that once you have added this back into the corridor you will not be allowed to reset the reference surface or the rebuild method because that will create a circular reference that would break the modeling processes.