By using the IMX format from AutoCAD Civil 3D, you can easily export surfaces from Civil 3D to Autodesk Infraworks, here is how:

 

From AutoCAD Civil 3D:

1.From Civil 3D, export the current model to IMX format using the “Export IMX” button from the “Output” ribbon menu (The IMX file will be located in the same location as the C3D model)

 

From Infraworks:

 

2.From the “Create and manage your model” command collection in Infraworks, choose “Data Sources”

3.From the “Data Sources” palette choose “Autodesk IMX” from the “Import Source” dropdown menu

4.Right-Click on the newly added data source, and choose “Configure…”

5.Verify that the desired objects available for import has been selected, and review the current import coordinate system settings (“NZTM” in this sample)

6.Click on “Close & Refresh” to add the new information to the current Infraworks model

 

...and that's it!

 

Regards,

Chris