build up vs richen

build up

verb
  • To accumulate, to pile up, to increase in stages. 

  • To close up by building. 

  • To strengthen. 

  • In solitaire card games, to place a card over another card of lower value. (e.g., place 5♦ over 4♣) 

  • To erect; to construct. 

richen

verb
  • To gain richness; become heightened or intensified in brilliancy. 

  • To make or render rich or richer. 

  • To become rich or richer; become superior in quality, condition or effectiveness. 

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