I had this question today (this is for you Pablo!)
Customer states that they have an Existing Roadway (I assume a Survey of an existing Road) that I assume they are going to strip out the existing Road Bed Materials from and recycle them and they want to know the quantities of the different materials in the Roadbed for the existing as a result (Pablo if this is wrong let me know).
The enclosed Video Response shows what I think you wanted to see.
The existing Road has been provided as a set of Surveyed lines that follow the Centerline, Edge Of Pave, Edge of Tapers, Edge of Shoulder etc. They are in the Project as CAD Lines (either field measured or created by Data Prep from the Plans. If they are CAD Lines then you need to convert the CAD Lines to Linestrings - you can use the "trick" to convert CAD lines to Linestrings (using a piece of Text that says 100.000 a decent way from the lines you want to convert, you then run Elevate Lines using Text and select the Lines and the piece of text, but make your offset tolerance very small e.g. 0.01 so that no lines are elevated - however the process converts CAD lines to Linestrings in this process).
Now you have Linestrings - you need to change the Sharpness Property of all the linestrings to Sharp and Texture Boundary - this is what will limit the Site Improvements to the areas you care about.
Now you will create the Materials you need in the Road Bed eg Asphalt, Gravel, Sand etc. using the MSI Manager
Now you will use the MSI Manager to create a Site Improvement for the Road Bed e.g. called Road Bed and have a Area based Site Improvement that has 3 Layers - Asphalt, Gravel, Sand and set the layer thicknesses based on the Core samples you have in the plans.
Now you will apply the Site Improvement to the Road Surface Model created from the strings you started with. You basically place a dot on a layer e.g. SITE - Site Improvement Markers and that carries the Site Improvement that you want to apply e.g. Road Bed.
As you do the above the surface should change color to whatever you specified as the color for your Site Improvement
Now you can use the Create Subgrade Surface to build a surface model for the Bottom of Asphalt, Bottom of Gravel and Bottom of Sand / Engineered Materials.
If you just want to run Surface to Surface volume calcs on the surfaces - you can do that now.
If you want you can build a Corridor Model using the Centerline Linestring as an alignment and then build a template using Surface Instructions for the Finished Grade, Bottom of Asphalt (Top of Gravel), Bottom of Gravel (Top of Sand) and Formation (Bottom of Sand).
Now you have the Corridor Model you can run the Corridor Earthworks report to compute the volumes and report them. You will need to have an existing ground model in order to run these Quantities. If you don't have one - draw a rectangle at an appropriate Elevation (relative to the road) and create an Existing Surface from it to act as your existing ground. If all you care about are the Road Bed quantities then this doesn't really matter - if you want the Cut and Fills also then you will need a proper Existing surface to get those.
The enclosed video shows the process that is outlined above.
The alternative to this is to use these Road Bed Surfaces below "Existing" as Strata Layers for the Corridor Model - that way the Corridor for the New Road will cause those layers to be stripped out and their quantities will be reported as Cut quantities - I will try to record a second video on that tomorrow to show you how to do that.
This is a very quick and dirty video as I was running out of time tonight - hope that it gives you what you need though