On import, Autodesk Civil 3D reads the LandXML.Units.diameterUnits, and if they are defined, those units are used for both pipes and structures. If units are not defined, the following unit settings are applied. Either the Imperial or Metric diameter settings are applied, based on the defined LandXML.Units.linearUnit in the file.

public sealed class SettingsDiameterUnits : TreeOidWrapper

Public NotInheritable Class SettingsDiameterUnits _
	Inherits TreeOidWrapper

public ref class SettingsDiameterUnits sealed : public TreeOidWrapper