When exporting a StormCAD or SewerCAD model to LandXML, the catch basin structure data is not exported.
This is working as designed. In the LandXML export, the catch basin "struct," or structure, doesn't have any attibutes. Because of this, the structure data is lost on export. You can find documentation on this at the following link:http://www.landxml.org/schema/LandXML-1.2/documentation/LandXML-1.2Doc.html
This is how LandXML is set up, so StormCAD and SewerCAD's LandXML export function must conform to this. There are a couple of things you can try. First, unlike catchbasin, manholes structure type data is expected. If you are exporting a StormCAD/SewerCAD model to LandXML, you can morph the catch basins in your model into manholes. This could work for inlets that are 100% capture. In doing this, structure types should be retained.
Second, Civil3D likely supports shapefile imports. Instead of using LandXML export, you could Convert your model elements to shapefiles and map the data into Civil3D this way. This process will take longer, as each FlexTable must be exported separately.
Alternatively, you would need to adjust the catch basin data after the model has been imported into Civil3D or StormCAD/SewerCAD.
Setting up a parts library in Civil3D for LandXML import
Catchbasins imported as manholes from LandXML