cave in vs overburden

cave in

noun
  • The location where something has caved in. 

  • The act of relenting. 

  • The act of something collapsing or caving in. 

verb
  • To collapse inward or downward. 

  • To relent; to grant approval against one's initial will. 

overburden

noun
  • The rock and subsoil that lies above a mineral deposit such as a coal seam. 

  • A sterile stratum that lies above the stratum being investigated 

verb
  • To overload or overtax. 

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