groundwork vs understructure

groundwork and understructure both are nouns.

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

As nouns, groundwork and understructure are synonyms defined as:

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

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