The Collaborative International Dictionary of English v.0.48:

Substruction \Sub*struc"tion\, n. [L. substructio, fr.
   substruere, substructum, to build beneath; sub under +
   struere to build.] (Arch.)
   Underbuilding; the foundation, or any preliminary structure
   intended to raise the lower floor or basement of a building
   above the natural level of the ground.
   [1913 Webster]

         It is a magnificent strong building, with a
         substruction very remarkable.            --Evelyn.
   [1913 Webster]