substructures

/S AH1 B S T R AH0 K CH ER0 Z/
noun
  1. 1

    The supporting part of a structure (either physical or organizational; the foundation).

  2. 2

    The earth or gravel that the railway tracks are embedded in.

Source: https://en.wiktionary.org/wiki/substructure, https://en.wiktionary.org/wiki/substructures