For those who plan to pursue further study of drainage and sewer pipes,
I learned today that there is/was a "Clay Sewer Pipe Association" while
surfing at ebay.  There is an issue of their journal title "Sewer Facts"
for sale.

ebay page:
http://cgi.ebay.com/aw-cgi/eBayISAPI.dll?ViewItem&item=99744319

1941 journal cover photo:
http://www.crownantiques.com/crownpics/429sewer.jpg

Cheers,

Bill Gordon