Flat slopes on sewer outfall using HDPE

[willemeulen]

Hi,

Read some of the other thread's and found the consensus was that for bulk outfall sewer on can go up to slopes of 0.1 - 0.08%. The area I'm working in has dolomite; dolomite areas have the renowned sink holes. Most sink holes are cavities in the soil which can collapse, the collapse is mostly triggered due to water related activity and research has proven 96% of sinkholes are caused by human activity (water leaks, diverting of stormwater, lowering of water table etc.). Due to this dolomite the pipe materials have to conform to certain standards, the most important being that the pipe should be free of joints, pipe should be able to handle vertical stress/pull without joints being dislodged.

As you can guess for wet services either HDPE or Steel pipes are the options available.

First option would be HDPE (steel is costly and also not preferred with sewage).

My questions are the following:
Is HDPE a suitable material for the construction of flat slopes of 1:1000 (0.1%)?
In my experience HDPE can be nice and straight aligned in a trench in the morning until temperatures rise and expansion of the HDPE material causes it to look like a “snake” in a trench

How would one determine the allowance in alignment (dips) and how would this be quality checked?
My best guess would be laser and CCTV inspection.

Thanks in advance.

W

 

[bimr]

It would seem that other materials would be preferred over the HDPE. You should consider ductile iron and FRP pipe.

Note sure about the reasoning behind the restrained joints. If the restrained joints fail because of ground movement, you would be faced with the same situation that you are trying to avoid.

 

[rconner]

Am curious what size is the sewer (and maybe also how long without access etc)?

 

[LittleInch]

You might be better with a more solid mechanically jointed or push fit outer pipe (concrete say) and then line it with PE internally with a thinner PE liner to provide your leak free interior which is resistant to pull outs of the main pipe. You might also need to remove the inner weld bead to avoid any interference to the sewage flow or provide anything for fouling to occur at the joints

PE is great material but is inherently flexible and any small deviation it will slowly conform to the ground profile beneath it

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[willemeulen]

The sewer size would start of with a Ø630mm and become larger towards the end, all in all the sewer will be approximately 15km long.
It't the dolomite specifications which state HDPE or Steel needs to be used. My feel is that HDPE is suitable for the smaller pipe sizes as it come in rolls of 50 to a 100 meters eliminating a lot of joints (potential leaks)On the larger diameters steel will have structural advantages over HDPE in my opinion, however if it proves to be very expensive HDPE might be on the cards again. In case of the sewer the steel would need to be lined with HDPE or something similar.

Concrete lined with PE as suggested by LittleInch is not accepted for wet services in dolomite, it's regarded that concrete will not accommodate any settlement (due to dolomite/sinkholes soil movement can locally occur). My professional view is that no matter what material is used a pipe of that size will fail in case of a sinkhole.

This part of the world (South Africa) there is a different set of rules. Skill of the contractors varies a lot and authorities will be cautious in using new or unusual pipe materials. To give you an idea, on some of the stormwater structural wall HDPE was installed. After some time we found pipes, particular the larger ones, were burned out......some of the locals struggling to survive made fires in them to burn off the HDPE in order to get hold of the steel reinforcing.....which they than sell to a scrapyard for cash.

 

[SlideRuleEra]

In my experience HDPE can be nice and straight aligned in a trench in the morning until temperatures rise and expansion of the HDPE material causes it to look like a “snake” in a trench.
This problem can be solved by having backfilling of the trench following directly behind, and without delay, the HDPE pipe laying. Since PE is a relative weak material, soil friction with the pipe is normally sufficient to keep the PE from expanding / contracting due to (reasonable) temperature changes.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[bimr]

Here is a link to a development guideline from New Zealand that covers infrastructure design in dolomite areas. HDPE pipe is specified.

http://www.publicworks.gov.za/PDFs/consultants_docs/PW_344_DOLOMITE_MANUAL_September_2010.pdf

 

[rconner]

It sounds like this proposed installation of gravity sewer outfall is kind of on the ragged edge at least as far as appropriate minimum slope for ~24 inch sized sewer pipe practice in the USA is concerned. Incidentally, I would be quite cautious on buying any hype that hdpe is going to be any better than any other pipe with regard to flow, unless they can supply you some actual proof with real working conditions in comparable field. At least traditionally in the USA, some quite reputable standards/Engineers have assumed e.g. the same range flow coefficients for most sewer pipe materials, with either the direct knowledge or assumptions that all will develop a slime layer, and results of clean water in a lab mean nothing as far as that slimed condition is concerned (and hdpe often has a smaller inside diameter/flow area to begin with, in that thicker pipe walls of the weaker material are required to handle structural loads etc.- also, if you are not familiar with the internal bead of fused plastic pipes LittleInch referred to, same have been discussed on different threads in the past including

http://eng-tips.com/viewthread.cfm?qid=311645).

I am curious also what exactly are "It't the dolomite specifications" that direct you to steel or hdpe (do you have a link to same we could examine)? That being said, it seems to me that if a gravity line of any piping material, and maybe particularly one designed to a flat profile develops a "sag" e.g. as a result of subsidence, there might then really be nothing hydraulically there (other than generally undesirable surcharge??) to drive the in effect siphon created.