Material Selection for Gravity Sewer Piping?

[rconnor]

Hi Everyone,

I'm looking to specify a piping material for a 4" underground gravity sewer pipe.

The old material was cast iron but it has corroded and we are now looking to replace with HDPE. My only concern is that I know HDPE is rather flexible and I'm worried about little peaks and valleys causing settlement to build up and eventually clog the line. My research has shown different results; some sources recommend HDPE for gravity sewer lines, others don't for this very reason.

Another thing to note is that this will be going in a climate with extreme temperature differences between summer and winter (+30 C in the summer and -40 C in the winter). The existing pipe is only buried 4 feet deep so I expect there to be some significant ground movement due to freezing and thawing. This is partly why HDPE was thought to be a good alternative, because it will be able to withstand this movement. Also, we are concerned that a bell and spigot PVC pipe line could lose some joint integrity due to the movement. However, will this make the issue with the bumps in HDPE worse?

Is this a valid concern? Are there ways around it (ie. specific cell class of HDPE or installation procedure)? Would PVC be the better alternative?

Thanks in advance for any suggestions!

 

[77JQX]

Is there evidence that the existing CI pipe moved due to frost? How about sewage freezing in the pipe? HDPE is the only pipe material I'm aware of that doesn't burst due to freezing water, so that may influence your choice. I think it comes down to the particulars of your installation - if you have a good slope, then frost heave shouldn't affect you too much. And if you do a good job with installation and bedding, then the movement should be minimized. I tend to think the flexibility of HDPE would be a plus in your situation - I'd rather have valleys than a shear failure.

 

[cvg]

if the old cast iron was not displaced due to frost heave then why not use ductile iron for the replacement?

If you are really concerned that you can't get good enough compaction or you have frost susceptible material, than you can bed the pipe (HDPE or PVC) in flowable fill, in effect forming a cradle. that would prevent the bumps.