I just found a similar topic regarding HDPE here:
http://www.eng-tips.com/viewthread.cfm?qid=179991&page=5
We are installing 3200 ft of 8-in SDR11 HDPE pipe. It will connect into a 10-in ductile iron pipe. Fluid is water (groundwater). Pipe will be buried minimum of 24-in below surface.
I'm...