Let’s say you’ve defined a surface in Civil 3D, including alignments for some roads.
In Civil 3D, a road with an x and y location cannot have more than one z location. In InfraWorks, it can.
So maybe you’d like to add some z locations to your Civil 3D roads, or make other tweaks to them, in InfraWorks, but want everything to end up in Civil 3D.
(If you wish, you can control styling during the import. The default option tries to make the imported corridor look as close to Civil 3D as possible, including any textured material that was applied in Civil 3D. This means that there will not actually be a visible InfraWorks road feature for it, but the corridor will be represented as a series of coverage areas instead. The options in the import dialog let you import the corridor as a centerline + surface or as a centerline only. Either way, you’ll get a road feature that you can style in InfraWorks using native InfraWorks road styles.)
If you prefer to export the data just so it’s ready for later import into InfraWorks, you can do that in Civil 3D. From the ribbon, it’s Output > Export IMX.
Once you bring the .imx version of the roads (or other surfaces) into InfraWorks, you can add z locations to your heart’s content and save the roads in .imx format. Then, in Civil 3D, you can go Insert > Import IMX, import the file…
Upon import into Civil 3D, the .imx contains three surfaces: base, existing, and proposed. The base surface is just the base terrain. The existing surface contains the base terrain plus any roads imported in bulk from GIS data. The proposed surface contains only areas affected by features created using InfraWorks’ drawing tools. The .imx also includes road centerlines which will be mapped to siteless alignments in Civil 3D. Importing the same .imx multiple times will overwrite existing surfaces and alignments with the same names, so you can change your mind, tweak the data in InfraWorks, and reimport it into the same Civil 3D drawing.
If you find that there is some detail that .imx is not handling to your satisfaction, try importing or exporting in LandXML format instead.