A common scenario is a pipe network that crosses multiple surfaces. One way to handle this is to divide the network into two separate networks, each with its own associated surface. Note that the network properties have a default surface and alignment that is assigned to each part. These defaults can be changed at any time and for any given part. You can change the default references in bulk by right-clicking the item view or Panorama column header.
Another useful method when creating a pipe network that would cross multiple surfaces, is to create a composite of the two surfaces. For example, if you have an existing road surface and a finished ground surface, you can create a composite volume by which to define the network. For more information, see About Composite Volumes.