Dan et al.-
 
That is a load of glass.  Do you have any references of glass being
used to deter rats and insects?  I think your "guess" is right;  I
have an 1873 entry from a diary by William Wallace White, Vance
County, NC, saying  "...brought 3 Bbl's [barrels]  iron shavings, to
put between walls of house to make it rat proof."  White's house
burned and he was rebuilding on the original foundation.
 
The only layers of glass I know of were from a bottle dump at Buffalo
Springs, VA.  The company that bottled the water claimed they did not
reuse bottles for sanitary reasons but I suspect they didn't want
people rebottling water under their name.
 
Richard Kimmel