Stress Analysis for Pipeline Buried in Peat 1

[mch22112]

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

 

[BigInch]

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

Independent events are seldomly independent.

 

[SNORGY]

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.

 

[racookpe1978]

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<)

 

[dicksewerrat]

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.

http://WWW.amlinereast.com

 

[pmover]

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

 

[BigInch]

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.

 

[SNORGY]

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.pdf

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.

 

[mch22112]

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.

 

[BigInch]

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.

 

[mavman427]

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.

 

[dicksewerrat]

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.

http://WWW.amlinereast.com

 

[rconner]

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!

 

[BigInch]

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.

 

[mch22112]

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!

 

[dicksewerrat]

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.

http://WWW.amlinereast.com

 

[BigInch]

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.

 

[mch22112]

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.

 

[stanier]

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/

 

[rconner]

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")