groundwork vs underpinning

groundwork

noun
  • The foundation; the basic or fundamental parts that support or allow for the rest. 

underpinning

noun
  • A support or foundation, especially as a structure of masonry that supports a wall. 

  • The lower body or legs. 

  • A basis for something. 

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