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Stress Analysis for Pipeline Buried in Peat

Stress Analysis for Pipeline Buried in Peat

Stress Analysis for Pipeline Buried in Peat

(OP)
Hi all,
Has anyone performed stress analysis for a pipeline buried in peat? I'm attempting a preliminary stress analysis using Caesar II and an American Lifelines Alliance soil model. Actual soil data is not yet available. Does anyone have typical soil properties for peat? Although an imported / engineered backfill may be used I would like to start by analysing pipeline behaviour in the native soil.
Thanks
Mark

RE: Stress Analysis for Pipeline Buried in Peat

My best advice is don't waste your time. Peat is not suitable for anything except excavation and replacement.

Independent events are seldomly independent.

RE: Stress Analysis for Pipeline Buried in Peat

I looked into this and came up with, in absence of better definition, an undrained shear strength of 5 kPa, a friction angle between 40 and 50 degrees, and a density between 1050 and 1100 kg/m^3. Once you enter that and find that it will give rise to virtual anchor lengths of one or more kilometers, the ensuing problems that you will have with designing restraints at riser sites and preventing "pipeline walking" will lead you to BigInch's suggestion. Your riser sites will probably need engineered backfill with material from suitable borrow pits.

RE: Stress Analysis for Pipeline Buried in Peat

Can you treat it like a suspended pipe across water: like a lake where it doesn't touch bottom?

or is that even easier than suspending it across peat? 8<)

RE: Stress Analysis for Pipeline Buried in Peat

How deep is the peat? You may be looking at piling to support the pipe. Do pH testing on the peat and any water you find. When you dig out the peat to lay the pipe, sell it to a landscaping supplier.

Richard A. Cornelius, P.E.
WWW.amlinereast.com

RE: Stress Analysis for Pipeline Buried in Peat

contact Aleyska Pipeline Co., BP, ConocoPhillips, or University of Alaska (Anchorage or Fairbanks) as they have "may" have reference material for you. perhaps an online search using UAA or UAF library will generate results.

hope this helps. Good luck!
-pmover

RE: Stress Analysis for Pipeline Buried in Peat

Snorgy, that surely must be a wet density, putting dry density around 100/M3
If the pipe is anything bigger than 6", or 6" with std wall or less, it will float without concrete coating.
The shear strength hardly has enough strength to support any negatively buoyant load.
The friction angle must be in the dry condition as, wet it is nearly fully fluidized and has a 0 friction angle.
No matter what the friction angle is, there isn't much point to try to use any material with such a low, low density.

Excavate the peat and sell it to a whiskey distilling company. Hopefully you're in Scotland.

Independent events are seldomly independent.

RE: Stress Analysis for Pipeline Buried in Peat

BigInch,

Here is one of the articles I read, or shall I say "scanned briefly":

http://www.sgem.pwr.wroc.pl/n34-05/art04_n34_2005....

I was pretty tired and perhaps I should have spent some more time correctly interpreting the data. Hopefully, the OP can make use of the data from this and similar links.

RE: Stress Analysis for Pipeline Buried in Peat

(OP)
Thanks everyone for the information. It's very helpful. For those interested it's an 8" line in Scotland (good guess BigInch). Buoyancy is not an issue.

RE: Stress Analysis for Pipeline Buried in Peat

No kidding. Lucky U!
My fav is Dalwhinnie.

Carefull. If it's 8" and there's water around, it wants to float.

Independent events are seldomly independent.

RE: Stress Analysis for Pipeline Buried in Peat

I would advise you to re-route. Peat is terrible and settles/degrades like crazy over time. We have high pressure natural gas transmission pipelines running through areas with up to 30' of peat. Pipes are settling, elbows are yielding...it's an ugly situation.

RE: Stress Analysis for Pipeline Buried in Peat

Advertise "Free peat" and draw out the lines you want excavated. Have some one on site to direct the people that show up. Excavation cost one guys time

Richard A. Cornelius, P.E.
WWW.amlinereast.com

RE: Stress Analysis for Pipeline Buried in Peat

While I hate to admit it (as I actually rode across Scotland many years ago!) and I am probably the only slow one who may not have gotten the reference, I had to look up http://www.whisky.de/archiv/experts/peat.htm. I conclude I have led a too sheltered existence!

RE: Stress Analysis for Pipeline Buried in Peat

mavman,
elbows always get bent in Scotland, but perhaps for different reasons than you're thinking.

rconner, OK.
Well it wasn't TOTALLY obvious.
I was there a couple of years ago and can't wait to go back.
Not for the peat.. for the REpeat.

Independent events are seldomly independent.

RE: Stress Analysis for Pipeline Buried in Peat

(OP)
Thanks again for the help guys. It has a decent wall thickness and we have done buoyancy calcs to show there will be no upward buoyancy force. Actually, we are a little worried about sinking / settling. Mavman's comments seem to justify this concern. We are considering lining the trench with a geotextile fabric to minimise movement due to settling. Unfortunatley re-routing is not an option! On a positive note, at least I'm learning more about whisky!

RE: Stress Analysis for Pipeline Buried in Peat

Look at roads and retaining walls that use geo webs. The idea is to include a larger area of soil to increase the stablilty of the road/wall. The web should be much wider than the bottom of your average trench. And yes, as peat rots, the land will sink.

Richard A. Cornelius, P.E.
WWW.amlinereast.com

RE: Stress Analysis for Pipeline Buried in Peat

If in contact with water, free or water pore pressure in the peat, there will be an upward buoyancy force. Whether that is more than the empty weight of pipe and coating is the question. It is a bit dangerous to depend on peat above contributing anything to the downward force. Pipe and coating should have a ratio of around 1.2 downward force to the upward buoyant force. You can accept a slightly negative buoyancy to keep it low out of the freeze zones above to avoid liftup jacking by freezing of surrounding water in the soil during the winter.

Independent events are seldomly independent.

RE: Stress Analysis for Pipeline Buried in Peat

(OP)
Thanks Dicksewerrat and BigInch. I wasn't very clear. If using geotextile we will not cover just the trench bottom. We would extend it beyond the edges of the trench. This may not be effective, but we are currently digging trial pits to see exactly what we are dealing with. Yes Biginch, that is the approach we used for the determination of buoyancy. My wording wasn't correct as you picked up. That's a good tip regarding freeze zones.

RE: Stress Analysis for Pipeline Buried in Peat

From another era and discipline George Stephenson crossed boggy areas

on the Liverpool to Manchester railway. His novel solution was to use a mattress to distribute the load. Perhaps your could do the same in the trench bottom? Not sure how much Glenfiddich he had to consume to come up with that gem.

“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King
http://waterhammer.hopout.com.au/

RE: Stress Analysis for Pipeline Buried in Peat

You don't mention the length of pipeline you are dealing with, and specifically the extent required in whatever the depth is of this bog/material. I mention this as of course there is some long term precedence for rather strict elevation control of pipelines strapped down to piles or piers, that are in turn driven down to firm strata, and even underground (as long as pipes and piers etc. are structurally adequate for the concentrated loads/reactions involved etc. I guess I would have mentioned this earlier, but BigInch got me much off point with the mere mention of Scottish whiskey. Luckily I had already penned this one befoe stanier hit me with the reference to excellent, but probably now unaffordable, "Glenfiddich")

RE: Stress Analysis for Pipeline Buried in Peat

If you have an open mind, screw anchors are a great way to cross swamp and peat bogs. Many people don't have any experience with them and shy away, but I've used a lot of them in Minnesota, ("Land of 10,000 Lakes") and they work great. No concrete coating needed.

Independent events are seldomly independent.

RE: Stress Analysis for Pipeline Buried in Peat

Probably stating the obvious but we use saddle weights to cross huge areas of muskeg/swampy areas where I am.

The Floodwood swamp in Minnesota brings back a lot of memories if you have ever been there BigInch...

RE: Stress Analysis for Pipeline Buried in Peat

I remembered a lengthy thread getting pretty wild on another forum some time ago at http://cr4.globalspec.com/thread/80027/Excavating-..., that while really not the same subject sort of touched on related issues. If you are working much around such stuff or the like with heavy equipment this might make for at least entertaining read -- one of most amazing posts to me read,

"In 1830 Robert Stephenson (of "Rocket" fame to all of you who know about the history of early railways) was building a rail line between Liverpool and Manchester. The route had go over several miles of of a peat bog known as "Chat Moss". His solution was to put bails of cotton onto the moss, covered by bracken with a top layer of shale. The rail line, without substantial structural modification, is still in use today carrying regular passenger trains and some 6000 ton coal trains."

Anyone who has been around Northern Minnesota much knows that many of the at least older roads are sort of like riding on a roller coaster. I guess this is perhaps some due to differential support of the subgrade, but I've also thought may have had something also to do with sort of ice lenses formed periodically in the near always very high water content native soil (as from personal experience I know it's just so darn cold to boot!))

RE: Stress Analysis for Pipeline Buried in Peat

brimmer, I don't remember Floodwood by name, but I did work on a lot of the Northern Natural Gas system from Owatonna to Hibbing. Actually the thing I remember the most were the mosquitos ... Oh.. and the fresh northern pike filets they sold in a lot of the local restauraunts ... and the northern lights. Took me a while to figure out what those were. Thought they were some kind of flood lights pointed upwards. After driving for some 50 miles, I realized I was never seeming to get any closer to them. Then they started waving around and changing colors.

rc, yes the freezing ice lenses jacks up the roads wherever they form. In near permafrost, after a few thousand years, they eventually build into small hills 20 to 50 feet tall. They freeze, thaw, but not completely, then freeze again.

Independent events are seldomly independent.

RE: Stress Analysis for Pipeline Buried in Peat

I seem to remember a large pipeline failure in a bog here a few years a go that required some larger scale engineering for a solution to continue to allow the company to operate the pipeline through the bog. They used geotextile fabric, brought in fill, used other methods to stablize the pipe in that area. Attached is a breif article on the methods they used.

RE: Stress Analysis for Pipeline Buried in Peat

Check out the screw anchor possibility. Goes in much easier than all of that. As long as you can get to good soil down below somewhere, they'll work.

Independent events are seldomly independent.

RE: Stress Analysis for Pipeline Buried in Peat

Some of those old roller coaster roads were built on top of logs laid across the right of way. Called corduroy. The road floats a little. But this was 70-80 years ago. They probably didn't remove the logs for the newer road. And yes it gets cold there. that's why I moved to south Carolina for the winters. Walleye is better than northern.

Richard A. Cornelius, P.E.
WWW.amlinereast.com

RE: Stress Analysis for Pipeline Buried in Peat

(OP)
Some very good / entertaining ideas there! Thanks! We are still waiting on trial pit test results.

RE: Stress Analysis for Pipeline Buried in Peat

"Copy" on the mosquitos and walleyes, in that order (I have also wondered with regard to the former how it is anatomically possible those big coldblooded bloodsuckers manage to emerge from the swamps about the same time "the ice goes out")!!

RE: Stress Analysis for Pipeline Buried in Peat

Don't they migrate in like ducks.

Independent events are seldomly independent.

RE: Stress Analysis for Pipeline Buried in Peat

I don't know whether its allowed on this site, but me and the company I used to work for have spent years, yes years, designing a pipeline accross a peat bog in Ireland and more recently shetlands up to 5 m deep. if you really have no option then the only real way of doing it is to dig it out and lay a stone road to lay the pipeline in. Trust me, nothing else works as your pipeline will either flex all over the place as the pipe to soil friction is virtually zero and hence as said above will have a virtual anchor length of hundreds of metres or the pipe will float or sink. Apart from any other consideration it is very diffcult to safely construct and peat, once youve worked it a few times turns to liquid mush. Peat only exist where it rains over 250 days a year so it will rain a lot when you're triny to drag your digger / sideboom out of the bog it has just fallen into. Let me know if you want to know more....

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