substructure vs groundwork

substructure and groundwork both are nouns.

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

As nouns, substructure and groundwork are synonyms defined as:

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

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