encumber vs overburden

encumber

verb
  • To load down something with a burden. 

  • To burden with a legal claim or other obligation. 

  • To restrict or block something with a hindrance or impediment. 

overburden

verb
  • To overload or overtax. 

noun
  • A sterile stratum that lies above the stratum being investigated 

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

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