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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

I am surprised to see an allowable bearing pressure of 1500 psf for peat. It must be very dry. Considering that, if it gets wet again, which is likely given your water table, I would expect that the allowable bearing pressure would drop considerably.

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

I second Ron. 1500 is not likely there. Replace with suitable material, even low strength concrete is possible under water, but go sufficiently wide to not be affected by adjacent peat.

RE: Retaining wall on 20" of peat

(OP)
Thanks Ron. I extruded the info from (§3285.202 Soil classifications and bearing capacity) that is referenced in https://www.law.cornell.edu/cfr/text/24/3285.202. Admittedly I had to do a lot of searching for this. But anyway, I wanted to hear other opinions. My architect anticipates the design of the wall tomorrow and these 20" are a migraine right now.
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

I'm from Cornell and am embarrassed that such crap come from them. That table is absolutely goofy if I ever saw one.

RE: Retaining wall on 20" of peat

Agree with OG...ludicrous table. I would not rely on generalized tables for such a design. You seem to have a boring log for the site....what bearing pressure did the geotechnical give you?

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

(OP)
I have to claim that I am not surprised by your reaction. I was just a bit taken over by the fact that the geotech did not give me an ASP and I had to extrude it from tables so surely I tried to find answers. In fact he recommended that we do piling actually.
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

Chances are the undercut and replacement will encounter ground water. Not just any fill should be used or compunction will be impossible to achieve. I'd not trust any architect to write a suitable spec for the work. I think your report should be clear about water potential and what needs to be done to get a good job. Of soil type fill, I'd look at clean sand and gravel with no more than say 5 percent passing the No. 200 sieve. Even though the geotech recommended piles, I'd get another comment from them, or another firm, for what bearing to use for toe pressure. 2 t/sf may be high, depending on how it affects he soil below the undercut work.

RE: Retaining wall on 20" of peat

Iasonasx - I see that the ground water you can expect has "tidal influence". How close is the retaining wall site to a tidal body of water? What is the tide range in that body of water, near the site?

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.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Retaining wall on 20" of peat

I'm coming to this late but here is my 2 cents worth:

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

and Oldestguy - by the way, we did beat Columbia 3-0 on Saturday so won't go 0 for . . for the year - - - I see that the table was put out by the law school - do you really think they'd walk across the street to the lads at Hollister Hall to discuss this?

RE: Retaining wall on 20" of peat

SRE....certainly could be pluff mud. Should be obvious when they try to excavate. Since it is near the surface, a probe rod would be helpful to evaluate. If the tidal notation is the top level of tidal influence, then might be able to excavate and backfill with CLSM or #57 stone and sand mix. If the notation is the bottom level of tidal influence, the piles might be the way to go.

RE: Retaining wall on 20" of peat

Ron, BigH, oldestguy - I'm thinking that a driven sheet pile wall is the way to go, if that is acceptable for the use. Perhaps that is what the geotech meant when he mentioned "piling".

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Retaining wall on 20" of peat

Remove the peat.

f-d

ípapß gordo ainÆt no madre flaca!

RE: Retaining wall on 20" of peat

SRE....that would be my choice as well, considering the "crap" he has below and the hesitation to remove and replace. Only issue with sheet pile is proximity to other structures and vibration cause by installation.

RE: Retaining wall on 20" of peat

(OP)
Thank you all for your input! Actually what they were referring by the term "piling" was perceived in different ways. The architect perceived it as use wood piles and set the retaining wall on top, like they intend to do with the large house that will be some 70 ft behind. In that area between there will be a swimming pool. The soils mechanic mentioned that any substantial structure planned for this site to be supported on piles installed through the fill and loose upper natural sand into the lower denser sand below 15 ft. For the drive sheet pile suggestion, which makes perfect sense given these soil conditions, the very "industrial look" of it will definitely not be of interest to the owners of the house which is designed to what would be Colonial/Victorian style. Surely if behind this wall was a power plant, that would be no issue.
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

Sheet piling does not have to have an industrial look. It can be capped with concrete and the sheet pile would only be visible below the cap. A decorative railing can then be installed on the cap.(BTW....I don't know what the height of the wall is above the ground surface at the property line, but if more than 36 inches, you need railing anyway) Sheet piling also comes in plastic, which takes away the rusting/painting issues.

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

A comment on the table referred to by the OP, I think that the conclusion that peat can take 1500 psf bearing pressure is a result of a typo in the table.
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.

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