underbuilding

词干

后缀

-ing: Used to form nouns or noun-like words (or elements of noun phrases) from verbs, denoting the act of doing something, an action, or the embodiment of an action. · Used to form nouns denoting materials or systems of objects which are used or employed in an action, or considered collectively.

From underbuild (“to build beneath, lay a foundation for”), equivalent to underbuild +‎ -ing.