foothold vs substructure

foothold

noun
  • A secure position from which it is difficult to be dislodged. 

  • A solid grip with the feet. 

  • Airhead, beachhead, bridgehead, lodgement. 

substructure

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

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

  • Synonym of base (“the forces and relations of production that produce the necessities and amenities of life”) 

How often have the words foothold and substructure occurred in a corpus of books? (source: Google Ngram Viewer )