Retaining wall on 20" of peat
Retaining wall on 20" of peat
(OP)
I have a project of designing a retaining wall, but there is a 20" thick layer of peat in the soil right underneath. Below that there is water table. I see that peat can take an allowable pressure of 1,500 psf. The rest of the soil is a good solid 3 tons per square foot capacity. I wonder if 20 inches is something worth worrying about or if I should just recommend that we dig out all that peat, replace it with infill and build the retaining wall. I fear that anything else is going to make this much more expensive. Any ideas on this matter?





RE: Retaining wall on 20" of peat
20 inches of thickness is certainly something to be concerned about. I've seen peat materials that, when loaded, reduced in thickness by half! You do not want your wall to settle even a few inches, much less 10 inches!
Has a consolidation test been run on the peat material? I would expect its primary consolidation to occur quickly but secondary compression could go on for a very long time.
I would remove the material and replace with select fill, properly compacted.
RE: Retaining wall on 20" of peat
RE: Retaining wall on 20" of peat
Thank you for your reply.
P.S. The specific of 1500psf comes from this: http://www.ecfr.gov/cgi-bin/text-idx?c=ecfr&si...
RE: Retaining wall on 20" of peat
RE: Retaining wall on 20" of peat
Also, the allowable bearing pressure does not tell the whole story....you still have significant settlement to deal with!!
RE: Retaining wall on 20" of peat
In my letter to the architect I am recommending the replacement of the peat with compacted infill and in the wall analysis I used 2tons per ft^2 for ASP which translates to approximately 28psi, and I am setting the design to reach a maximum of 17psi. I think that this will be somewhat safe enough considering that I added an extra 2.5 ft of surcharge as a safety cushion in my calculations. Please correct me if I am wrong or you think that there are other precautions I need to take. Thanks again.
Here's my first project with a retaining wall! I should get some exedrin now.
RE: Retaining wall on 20" of peat
RE: Retaining wall on 20" of peat
That "tidal influence" note could indicate that a LOT of water may be expected during excavation. Perhaps enough for a shallow cofferdam. If so, piling may be a more cost effective option. Depends in part on the length of the wall.
Ron, oldestguy - I wonder if what is called "Dark Brown Peat" is actually Pluff Mud, with fill on top of it.
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RE: Retaining wall on 20" of peat
1. How high is your retaining wall? What kind of retaining wall - typical cantilever, MSE ?? A sketch would help to see how things 'fit'.
2. Peat - I wouldn't even think of leaving the peat in place. Not worth the headache. You have not mentioned if this is fibrous or amorphous peat - and that would/could make a difference but given that the peat is only down to some 3.5 ft or so, dig it out. Don't you have to provide some embedment for your wall? Also, wouldn't the peat "behind" the wall create a bit of a potential for slide zone? This would have to be checked out as well and, if the geometrics permit, I'd dig out the peat for a distance behind your wall of something like half the wall height or more - if you can. and if you can't the peat layer can affect the global stability. (after I wrote this I saw that you are taking out the peat).
3. The soils you have under the peat are not all that good in themselves. SPT N values of 6 indicate a loose relative density - and you wouldn't want to put much more than 75 kPa (about 1500 psf) on them - and are they subject to liquefaction - are you in a seismic zone?
4. You deal in psi for geotechnical foundations? Mmmm
5. You might be better to use a flexible retaining wall system such as a gabion faced MSE wall - are you building the wall so that you can raise grade behind? If so, then I would also suggest that the site be stage loaded (depending on height of wall) - again, as a precaution of your relatively loose soils below the peat layer.
Again - my 2 cents worth - a sketch would have/would be very helpful in visualizing what you are really attempting to do.
RE: Retaining wall on 20" of peat
RE: Retaining wall on 20" of peat
RE: Retaining wall on 20" of peat
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RE: Retaining wall on 20" of peat
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ípapß gordo ainÆt no madre flaca!
RE: Retaining wall on 20" of peat
RE: Retaining wall on 20" of peat
I am attaching here a preliminary design. I am still wondering if I should do some kind of combination of concrete piles with a shorter footing and leave the soil as is, of keep this rather large footing and invest in removing the peat and replacing it with sand and gravel infill.
RE: Retaining wall on 20" of peat
You can also drive wide flange sections or H-pile in and drop hollow core panels in the "slot" between the flanges for a nice looking wall
RE: Retaining wall on 20" of peat
For peat, the table says to Refer to 3285.202(e), which states "In lieu of determining the soil bearing capacity by use of the methods shown in the table, an allowable pressure of 1,500 psf may be used, unless the site-specific information requires the use of lower values based on soil classification and type." However, I believe the (e) is a typo and should be (f), which states "If the soil appears to be composed of peat, organic clays, or uncompacted fill, or appears to have unusual conditions, a registered professional geologist, registered professional engineer, or registered architect must determine the soil classification and maximum allowable soil bearing capacity."
This would make a whole lot more sense.