substructure vs fundament

substructure and fundament both are nouns.

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

As nouns, substructure and fundament are synonyms defined as:

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

substructure (noun)	fundament (noun)
lowest support of a structure	lowest support of a structure
the basic structure or features of a system or organization	the fleshy part of the human body that you sit on
	the fundamental assumptions from which something is begun or developed or calculated or explained