The Collaborative International Dictionary of English v.0.48:

Substructure \Sub*struc"ture\, n. [Pref. sub- + structure.]
   1. (Arch.) Same as Substruction.
      [1913 Webster]

   2. An under structure; a foundation; groundwork.
      [1913 Webster]

WordNet (r) 3.0 (2006):

substructure
    n 1: the basic structure or features of a system or organization
         [syn: infrastructure, substructure]
    2: lowest support of a structure; "it was built on a base of
       solid rock"; "he stood at the foot of the tower" [syn:
       foundation, base, fundament, foot, groundwork,
       substructure, understructure]

Moby Thesaurus II by Grady Ward, 1.0:

41 Moby Thesaurus words for "substructure":
   base, basement, basis, bearing wall, bed, bedding, bedrock, bottom,
   floor, flooring, fond, footing, foundation, fundament, fundamental,
   ground, grounds, groundwork, hardpan, infrastructure, pavement,
   principle, radical, riprap, rock bottom, rudiment, seat, sill,
   solid ground, solid rock, stereobate, stylobate, substratum,
   substruction, terra firma, underbuilding, undercarriage,
   undergirding, underpinning, understruction, understructure