Article source: Autodesk Labs
The Corridor Solids Technology Preview for AutoCAD® Civil 3D® 2012 software provides functionality to create AutoCAD® solids from AutoCAD Civil 3D Subassembly shapes that are contained in an AutoCAD Civil 3D Corridor Model. By creating solids from subassembly shapes, representative real-world objects such as tunnels, retaining walls, and pavement layers can be more easily used for visual analysis, design verification, and construction coordination through enhanced model sharing between various project team members who are using applications such as Autodesk® Navisworks® 2012 products, Autodesk® Revit® Structure 2012, Autodesk® 3ds® Max Design 2012, and the new Autodesk® Infrastructure Modeler 2012 software.
The two videos below give a good overview of the Corridor Solids technology.