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Engineering Properties - Peat

Engineering Properties - Peat

Engineering Properties - Peat

If a situation arises where a deposit of peat cannot be removed and a stability analysis is required for failure of any emabnkment with peat at the toe, what can one do to mimic the behaviour of the peat?

Also, I am curious to know if there are any published values for the undrained shear strength of peat?  Also, if available, what is the common value(s) used in practise?

RE: Engineering Properties - Peat

I just went thru this exercise. Su values can be obtained from "Muskeg Engineering" or a paper by Edil et al "Shear Strength and Ko of Peats and Organic Soils."  I would be happy to fax them to you.  Of course the best way is to drill borings and perform vane shear testing.
Chris Bosy (312 922 1030).

RE: Engineering Properties - Peat

Dear Guest101,

I was involved some in researches on peat long time ago. Unfortunately, I do not have the publications about that with me now. Perhaps you can contact Prof. Budi Susilo Soepandji at the University of Indonesia. He definitely has results from the UU and CU tests on peats.
Just check www.ui.ac.id and contact Prof. Soepandji.


RE: Engineering Properties - Peat

I did My PhD research on geotechnical properties of peat. In the undrained shear strength part I measured a su/sigmavc(consolidation pressure) =0.58 fot peat wit wc=500% and slightly humified. you may reduce this number it you have greater water content or more humified peat.

RE: Engineering Properties - Peat

Undrained shear strengths of peat (as well as consolidation characteristics, etc.) is a function of the type of peat that you are encountering, too.  There are many types ranging from amorphous peats to woody peats (It has been awhile - there is a book - Muskeg handbook - you might find it referenced in the Canadian Foundation Manual) that actually shows some 20 types of peats.

How high is your embankment?  I know that we have left peat in place in a few tailings dams - at the toe, etc. with no problems.  Agree that field vane tests are an appropriate initial method to look into.  Also, you might consider a trial embankment if this is a major project.  Remember, too, a stabilizing berm at the toe will also compress the peat and increase it's shear strength. Use of wick drains with stabilizing berms would surely help to speed up consolidation. Keep in mind though, that as peat consolidates, the coefficient of permeability decreases remarkably.  I remember doing tests on Saskatchewan peats/muskegs that showed two to three orders of magnitude decrease in permeability coefficient under some 1500 psf load.

Hope this helps a bit.  Best regards.

RE: Engineering Properties - Peat

The type of peat is very important and the properties may be region specific.  For woodland peat the deposits are generally shallow and may be more woody and uniform in properties.  Bog and fen deposits have a living mat at the surface that has a high shear strength.  The mat is generally thin of the order of a couple of feet.  Beneath this the peat is older and hence more decomposed.  For peat deposits that are 5 to 10 feet thick the bottom of the deposit tends to be amorphous as a result of (100's of years) of decomposition.  We have dated peat deposits at depths to be over 7300 years old.  The deeper, decomposed, amorphous peat can have undrained shear strengths of 10 kPa (200 psf) or less.  The strength increases with preloading and decrease in moisture content.  Rule of thumb if you don't know the strength well is to fill in stages of 6 feet with about a month between stages and be sure to not disturb the upper mat.  In the zone beneath the mat you can use a vane to determine the shear strength of the peat.

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