Synonyms for Underpin:

n.

development, fortification, assemblage, ground, building, assembly, masonry, build, construction, lay, establish, predicate, over, found, rest, root, base, develop. lie at the heart of something, figure, emphasize, loom large, come before, predominate, count, signify, dominate, matter. underpin (noun)
corroborate, support, bear out.

v.

base (verb)
found, rest, ground, predicate, root, build, establish, prop, hinge.
support (verb)
buttress, uphold, prop, back, hinge, bolster, support.

Other synonyms:

predicate. found. base, establish. build. Other relevant words:
establish, bear out, base, predicate, build, ground, corroborate, found, emphasize, root, rest.