Subsidence of Peaty Soils 6

[jkan]

I am currently working on a site with peaty soils. The site is capped with ~5 feet of alluvial fill and then underlain by 5 to 10 feet of soft to medium stiff peat. Beneath the peat is soft clay of approximately 5 feet thick. Medium stiff to stiff sandy clay, clayey sand and clayey silt deposits were encountered underneath the clay and extends to the termination depth of our borings (~30 to 40 feet).

The groundwater is at ~8 feet below ground at time of drilling, so portions of the peat layer are above groundwater. The groundwater level fluctuates greatly at this site (range from 5 to 17 feet).

I was wondering if there will be a ground subsidence concern due to decomposition of the peat layer. Will consolidation settlement be significantly higher than subsidence due to decomposition? If not, what will be the best method to analyze settlement due to decomposition? Should I suggest removal of the peat layer?

Furthermore, will methane gas release be of concern too if the peaty soil decompose? I have found literature about the methane gas and it appears that a passive venting system could resolve the gas release problem. Does any of you have experience on this? Is there any case history/article about this?

Any suggestion on this will help.
Thanks all!

 

[BigH]

jkan - peat soils can be quite tricky. First a few questions - then perhaps after your response, we can make some better suggestions.
1. How long has the alluvial fill been placed above the peat?
2. How do you know the groundwater fluctuation? Have you monitored the site for a year or more?
3. What is the nature of the peat? Is it amorphous or fibrous?
4. Do you have any samples of the peat to determine the past consolidation pressure - we used to get 6 or 8 inch tube samples and then test them in a special machine we had built.
5. Are you planning to place additional fill?
6. What is the nature of the development you are planning? Is it for a residential sub-division? Is it for a dam or tailings dam?
7. Assuming you are talking about development with light to moderate structures, what are you planning for foundations? Taking them through the peat and soft clay to underlying firmer clay? Or, are you going to try to put your foundations on floating raft foundation in the alluvial fill (or in new fill placed above)?
8. Are there roadworks involved?

In peaty soils secondary consolidation is quite important - much more so than for most clayey soils. So, you have consolidation and secondary compression to worry about. I've not ever worried about the methane gas problem. If you know the groundwater regime so well, surely there must be some evidence or not of potential problems at your site.

I suggest that you find the "old" book by Brown on Peat Soils - it was issued by the Canadian Research Foundation many many moons ago. Sorry I don't have the exact name. It is a classic on the basics or working on and with peat.

I've had a couple of projects that the peat played extensively - one in rehab or the site and the other for major tailings dam in northern Saskatchewan. Interesting stuff.

Looking forward to your response to my queries.

 

[jkan]

This is the first time for me to investigate a peat site. Let me first answer your questions:

1) Based on historical maps and aerial photos, the alluvial fill is present for at least 40 years.

2) The groundwater measured in Borings performed in Winter 1991 is ~ 5 below ground surface. Recent borings performed in Summer 2004 revealed that groundwater level ranges from 5 to 17 feet. These are water level measured during drilling and therefore might not be very representative. Since the site is right next to a slough, tidal action can affect site ground water level.

3) I would consider the peat as amorphous, although some roots are still visible.

4)We have two consol samples of the peat soil, one of them appeared to be very distubed. Anyhow, the OCR is approx. 1.2; with insitu overburnden being around 1 ksf.

5)Final plans are not provided to me, but I will anticiapte the owner to pick grade up to as high as around 4 feet.

6) Single family residences with a total of 120 lots.

7) I am thinking about a surcharge program and PT slab foundation. The structural ask about pile, but if considered the thickness of soft soil and downdrag forces, the piles might be around 30 feet deep, which could make the development too expensive. I haven't been thinking about floating foundation.

8)There will be subdivion roads and utilties within the residential complex. Therefore, long term secondary differential settlement could be an issue.

I think we might suggest installing piezometers within the site to monitor the groundwater levels. I didn't know about the methane gas problem until a co-worker with environmental background informed me about that. And since this will be a residential development, even minor gas release from soils could be a big concern.

I am a fairly young engineer and felt overwhelmed by this project. So, any suggestion will surely help.

 

[eric1037]

Check out the Muskeg Engineering Handbook by Ivan C. McFarlin.

You may want to consider wood piles for foundations, however, they are difficult to splice and 30 feet is getting close to the maximum length. Another option is helical piers.

The peat probably has not gone through "tertiary consolidation" (degradation of the organic material), which can be significant. Sometimes it can exceed the traditional secondary consolidation.

If this were my project, I would use deep foundations for the homes. I would give several options to the owner for the pavements. I would consider not raising grades. In fact, I would consider cutting grades a bit and installing some kind of lightweight fill. A geotextile may also help.

Surcharging can be effective, but it may take some time to work.

Whatever method you use, you should stress to the owner that there are risks of poor pavement performance over time and maintenance will be required. The best way to reduce those risks is to remove the organic materials.

 

[BigH]

Eric - good points - and you are right - it is McFarlin (Brown was the frost guy) - it's been years since I've seen my copies of the books. I agree that piles would be the most positive solution even for single homes. In NJ we used piles through organic silts mainly for multi-story townhomes. For single story homes, we did preload, though and things seem to be okay. He might consider preloading the home areas with a couple of metres of fill which he could then use for the roads later on. Then, he could put the homes on stiffened raft foundations - perhaps with a crawl space below grade to "remove" the long term alluvial weight. One thing of importance is if the peat has "layers" of sand within that could speed up settlements - our NJ organic silts did. Client needs to know the ups and downs of various options - the choice is really his.

There is no question that standpipes or piezometers would be beneficial to gain an appreciation of the real fluctuation of the groundwater regime. I would be surprised in peaty areas with OCRs only slightly more than 1 that the water level is really down 17 ft. This is a big problem with a lot of geotechnical investigations - thinking that you can see the water at the time of drilling (or even after 24 hrs in an open hole) and believe that you really have a handle on the groundwater level. In my jobs, standpipes were always required (well, say 90% of the time). We would make it a habit to go back to get the water level. Water levels are a critical bit of information.

Eric also gives good information on the need to inform the client as to the potential behaviour of the roads over time (we are finding a lot of undulations in our very soft alluvial clay plain soils with moderate (3 to 4m) embankments. Peat would potentially be much worse. jkan may wish to consider using a surface dressed road rather than asphalt - they are good for residential subdivisions - although most laymen would want it black. The surface dressed road might be easier to repair as needed. You regrade the road if it becomes too wavy and then re-surface dress it.

Finally, jkan - if you are a young engineer, insist that an experienced engineer in your firm be strongly involved. If you don't have an experienced engineer in your firm (someone with peat experience), then, I would say - "BE CAREFUL !" You may actually wish to suggest that an outside consultant be retained as part of your team - one that is knowledgeable in peat deposits. As indicated before, peat can be tricky. FYI, there was a major study done for Burnaby British Columbia on developing light industrial zones in the Burnaby Lake Peat plain - about 1960. The costs in the report are woefully outdated, but the techniques in it are good. Where to get? Burnaby - my copy is in deep deep storage.

 

[BigH]

I was doing some searches tonight on undulations in roads constructed on soft clays - ran across a document from Florida DOT (FDOT). I'm sorry that I don't have the complete URL, etc. but the project is called "Sander's Creek SR20" - the file name (from the FDOT web site, I am sure) is: FDOT_BC354_17_rpt.pdf . I skimmed it and it is a good report on building embankments on peat/organic soils. I'd suggest that it be obtained and reviewed. In it, though, they have reference to a Chuck Brawner paper on the Burnaby Freeway I noted earlier:
Lea, N.D. and C.O. Brawner, "Highway Design and Construction over Peat Deposits in Lower British Columbia", Highway Research Record No. 7, pp 1-32.

 

[BigH]

URL:

http://www.dot.state.fl.us/research-center/Completed_Proj/summary_SMO/FDOT_BC354_17_rpt.pdf