Flat slopes on sewer outfall using HDPE
Flat slopes on sewer outfall using HDPE
(OP)
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
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





RE: Flat slopes on sewer outfall using HDPE
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.
RE: Flat slopes on sewer outfall using HDPE
RE: Flat slopes on sewer outfall using HDPE
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
RE: Flat slopes on sewer outfall using HDPE
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.
RE: Flat slopes on sewer outfall using HDPE
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.
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RE: Flat slopes on sewer outfall using HDPE
http://www.publicworks.gov.za/PDFs/consultants_doc...
RE: Flat slopes on sewer outfall using HDPE