trench or not?

[cvg]

An 18" SDR11 HDPE waterline (pumped water, ambient temperature, up to about 100 psi) is installed laying on the ground at approximately 20 - 30 percent slope. Thermal expansion and contraction causes it to move around, shortening the design lifetime. Installation in a trench would be expensive and would require blasting or rock trenching. Cover with available waste rock could be done. Approximately how thick of a cover would be required over the pipe to protect it?

 

[MiketheEngineer]

What temperature range are you looking at??

 

[cvg]

water and pipe are both ambient outside temperature, I would assume water range is 60 - 80 degrees. air temperature range from about 30 - 110

 

[btrueblood]

Any cover will add some insulation value, and thermal mass, reducing the daily temperature swing. Short answer would seem to be "as much as possible".

 

[cvg]

well, cost is an issue - if it wasn't I would trench it

 

[jgailla]

How long will the pipe be in service? Is frost a concern? If you are just trying to get the pipe to stop moving too much 2 to 3 feet would probably do it. For non-freezing thermal issues, I would try to use soil instead of rock and bury it under 3 feet of sand or sandy clay.

 

[cvg]

the pipelines will probably remain in service for the foreseeable future. frost is generally not a concern but erosion is.

both soil (silty sand) and rock of all sizes are available but I would prefer to use more erosion resistant material. 3 feet is about what I was thinking

 

[DLite30]

If the goal is to just keep it from moving, you can weight it down with sandbags, kind of like these. I don't think I would put rock directly on a poly pipe, especially if you're concered about longevity.

http://www.pipesak.com/

 

[waterpipe]

What is the length of the pipeline?

I'm thinking about the convection heat transfer that could be calculated by q1=k.delta(T) You can guess the delta(T) between the pipe surface and the ground surface. this should be damped by the pipe cover, so should be equal to the latent heat of the cover which is q2=m.L (m being mass and L the specific latent heat value). assuming q1=q2, inserting your desired delta(T) (considering the expansion factor and your pipeline length, you could have a guess of the max tolerable deltaT before getting to a trouble) and k & L values for rock or soil, then it should give you the required mass of the cover per unit length. Having the pipe diameter, you can get the minimum height of the cover.

As I said, I'm just thinking and not saying this is the solution. But perhaps just an approach.

 

[cvg]

thanks, that's an interesting concept

 

[BigInch]

Put anything you like on top. With that 30 degree slope whatever it is (unless its got a concrete mattress laid over it) will soon be gone. It will be hard enough to keep the trench from washing out as well. Lots of trench breakers will be needed as well as a lot of sand-cement bags. I'd trench it and place backfill inside a geotextile wrap, while using sufficient breakers and maybe you will get it to hold.

If you think trenching it will be costly, how much will replacing the line cost.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[cvg]

1200 feet long, 20% slope
the lines are being replaced after about 10 years in service sitting on top of the ground with no anchors at all. proposed design life is about 5 years and then will only be used sparingly after that. cost to replace is much less than trenching. I hope to increase the life by adding both thermal insulation as well as restraint. My assumption is that trenching will at least double if not triple the installed cost. I'm not even convinced I can (safely) get a rock trencher onto this steep slope.

 

[MiketheEngineer]

I am presuming the water is always moving?? If it is insulated and temp doesn't drop below 30F - I think you should be OK.

Trenching or covering it will also protect it - like from animals or possibly flying debris in a storm. I have had squirrels eat just about anything plastic on my house. They even ate the sewer lead pipe vents and an aluminum floodlight!!

 

[cvg]

I can't guarantee the water is always moving. I am sure that for some part of the year, the temperature occasionally goes below 30 (as low as 17), but not for any extended periods of time. the line is not insulated, the optional earth covering is intended to limit thermal expansion and contraction and weight to keep it from moving.

good luck to any squirrels who think they can get out here and eat this pipe...

 

[btrueblood]

Ohh...now you gone and thrown down on the squirrels...<shiver>

 

[rconner]

While I guess it may be “neither here nor there” at this point, I am aware e.g. “Ten States Standards” prescribe specific “Steep Slope Protection” for pipelines on “20 percent slopes or greater”. While I realize the document e.g. I have seen posted at

http://10statesstandards.com/wastewaterstandards.html

is arguably inapplicable (in that it is obviously for “wastewater” or “sewer” (as per Sec. 33.46) pipelines and perhaps also not in your area, I believe the pipeline design logic employed is probably nevertheless in general rather sound, and incidentally also probably reasonably well-backed by experience.

Periodic transverse anchorage e.g. with concrete pier supports built step-wise or pegged down into the slope probably minimize movement of pipeline, help to stabilize the pipeline (and to some extent also backfill/slope?), and also maybe even beneficially interrupt erosive drainage down same.

I believe ductile iron and steel water pipeline installations, often constructed with such features and on slopes, have typical useful lifetimes far greater than 10 years. While I have not yet seen a “squirrel” that could chew through a 30” plastic pipe wall (as I believe rodents, marine borers, and termites etc. indeed have smaller plastic lines), perhaps stronger ductile iron and steel materials do provide perhaps a little extra security from miscellaneous surface/traffic impacts (and errant bullets from even otherwise well-meaning good ol’ boy hunters and celebrators?)

 

[rconner]

As to also “covering” the (new? 18”, don’t know why I thought 30) plastic line with backfill or mounding of “waste rock”, I do not know exactly what the long-term effects (positive or negative?) might be in so doing. However, I probably should have also mentioned (and also without commenting on the practicality of same in any particular installation, and considering pipe and/or underground rock or pipe movements/migrations) that I am aware the plastic pipe authorities e.g. at

http://plasticpipe.org/pdf/ms-3_model_spec_pe_pipe_tubing_fittings_water.pdf

Sect. 3.02 page 11 now appear to be requiring “Embedment material should be Class I, Class II, or Class III , materials as defined by ASTM D-2321 Section 6…” at greater than 85% Proctor density. Furthermore, page 10 of the document at

http://plasticpipe.org/pdf/chapter-6_installation_construction.pdf

appears to restrict this even further with the statements regarding 6 inches of bedding over-excavation, as well as, “Bedding materials may be Class I, II or III, except that AASHTO Section 30 limits the
maximum particle size for bedding material to 1.25 in. (32 mm).”

 

[cvg]

the first document relates to buried municipal potable waterlines - this project meets none of those conditions. The second document is intended for use on gravity lines for use on highway and airport projects, installed in a trench and then subjected to heavy wheel loads. again, does not apply in this case. Embedment material is the term used for embedment in a trench. there will be no bedding and no trench on this installation and no external loads of any kind after construction is finished.

 

[rconner]

I guess this leaves only one question. If these specifications and this organization are not the appropriate references for what should be put under, around or over plastic pipes, what/who is?

 

[cvg]

the first document is a model specification to be modified by the engineer. guess that would be me. And to the extent possible it will be done to meet the model specs. But it will not be in a trench or in public right of way or in a road, so obviously parts of the spec just do not apply to this application. The second document is an installation guide, not specs at all. A lot of good information that will be taken into consideration. Thanks, I do appreciate the help.