understructure vs groundwork

understructure and groundwork both are nouns.

Word	Noun	Adjective	Verb	Adverb
understructure	Yes	No	No	No
groundwork	Yes	No	No	No

As nouns, understructure and groundwork are synonyms defined as:

Other synonyms of understructure include base, foot, foundation, fundament, substructure.

understructure (noun)	groundwork (noun)
lowest support of a structure	preliminary preparation as a basis or foundation
	lowest support of a structure
	the fundamental assumptions from which something is begun or developed or calculated or explained