residential foundations in peat 13

[3of12]

I have been asked to design several single family home foundations on a lake in southern Michigan. The site is underlain by 8 to 14 feet of fibrous peat. I believe my only options here are to drive timber piles to the substantial strata below the peat OR excavate the peat out of the site and replace with granular material. Water table may allow a crawl space but certainly no basement.

Has anyone out there had experience in this design or know of some good references they could point me to.

Thanks to all.
3of12

 

[UcfSE]

The only other thing I have heard of in this type of a situation is using a mat foundation and a very low bearing stress, say less than 500 psf. Of course, the actual allowable bearing stress, if any, would be given to you by your geotech. Since I have never actually done this myself, that's about as much as I can give you.

 

[dicksewerrat]

Over time the peat will consolidate. Use piling on the house with grade beams. Tie the sewer run to the floor so when the peat shrinks the pipe will stay. Are these going to be connected to a public utility?

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[3of12]

Reply to dicksewerrat. Yes these homes will have public sewer and water hook-ups. Thank you sir for the tip.

3of12

 

[fndn]

If you're not too close to the lake and basements are desired, you could use 3" rock placed 8-10 feet below basement footings. You'll find that the break-even cost is around 10 feet of rock comparable to that of timber or steel pipe piles. You would still need a geotechnical firm to monitor the dewatering and excavation blowout dimensions. Have your geotech co-ordinate foundation design with a structural they have used in the past for similar jobs.

One tip-if you move the house slightly to the lesser peat depth direction, you could save some rock tonnage and save. There is no issue with footing design if one side has 6 feet of rock and the other end 9 feet.

 

[BigH]

(Note this is a copy of the reply I made with 3of12 placed his question in a thread on probe rods in the Geotechnical Thread)

"... but from your description - see the developer, say hello followed by "No thanks." and take a hike. Basements in fibrous peat - meaning that the water table is high?? In New Jersey, we would preload, make sure we had 5 to 6 ft of sand base down above the peat and maybe, a one to two story residential structure would be built. Piles are the other alternative, like you said, but not with a basement. You said substantial soil beneath - not sure what you mean other than your pile wouldn't be on rock. That's no problem if you have a compact sand below - or stiff clay. I would think that piles (timber) would be more economical than removing and replacing 8 to 12 ft of peat - don't know what the cutoff is any more but it used to be about 9 ft. Do remember the concept of dragdown loads when designing any piles. Any fill to the site will cause it to a degree. See SlideRuleEra's web site for his series of Foundation Know-hows. He has extensive references for timber piles - real piles in real situations."

 

[MRM]

With residential structures in mind, I'm not so sure I would look at anything other than piles or peat removal, if for no other reason other than trying not to further confuse a developer that is possibly not up to a super-innovative solution, and to simplify your own life.

For lightweight commercial or industrial buildings, some other options may be available. BigH has alluded to one such solution-preloading.

One paper comes to mind related to preloading on peats. That is a paper called "Use of Preloading for Construction on Peat" by Tuncer B. Edil, P.E. I think this paper was presented at one of the past geotech conferences in Minneapolis. Mr. Edil tells the story of using preloading to help preconsolidate and stabilize a site containing peat. This site preparation was followed by the construction of a large retail store-with success. Is the building still performing well? I don't know. It is an interesting read anyways.

I've use preloading to preconsolidate soft marls on some sites; mostly to mitigate future pile downdrag below residential structures, help stabilize streets against semi-deep seated settlement in fill areas, and to generally help maintain the planned grading on the site for drainage, etc. I haven't considered using a preloading option alone for site improvement on peat soil especially for residential construction yet, however. Perhaps it's done routinely...

Keep in mind that if you select piles for your site, you may want to consider some degree of preloading prior to pile installation to help with those items mentioned above. A preloading program would also help reduce differential utility/structure movements as dicksewerrat mentioned.

 

[GOTEK]

I just wanted to emphasize what others have already told you...peats even if normally consolidated still exhibit significant secondary settlement, therefore, accounting for downdrag is critical. I am working on a roadway project with organic silts and peat ranging from 20 to 30 feet thick underlain by a large layer of soft clay...the top two design alternatives are pile supported slabs and conventional surcharging with wick drains. Based on my experience,I wouldn't be too quick to rule out piles (pile supported mat foundation), but you have a complex site and should hire an engineer who has experience working with these soils. That being said I would prefer to remove the peat if it is at all cost effective.

 

[3of12]

Thanks to all for the good advice on solutions to my design issue. I wanted to specifically ask Big H where to find the SlideRuleEra's web site that shows Foundation Know How items because the web site I did find for SlideRuleEra had foundation topics but none had to do with foundations on peat. Thank you. 3of12

 

[SlideRuleEra]

3of12 - Glad that you found the piling documents on my site. You are correct that they do not address foundations on peat - but this in not important.

When soils are "bad" enough to require piles, a properly designed pile supported foundation is essentially a "bridge at ground level". The soil directly below the foundation (be it peat, silt, etc.) is not carrying any vertical foundation loads - the piles transfer load directly to the lower (suitable) strata that is under the peat.

Designing a pile supported foundation is in the "gray area" between geotechnical and structural - you may want get assistance if you go that way.

http://www.SlideRuleEra.net

 

[dmoler]

Preload is expensive and time consuming compared to extending the piles to probably a few feet more for extra capacity for a lightly loaded, isolated, structure. Not really necessary.

A little secret about preloading for large, residential areas, that I would like to throw out there. When preloading is used for residential developements on a large scale, the whole developement benefits in many ways.

Preloading large areas works very good in that the roadways, utilities, building pads, and overall performance of the development benefits. Staged preloading is the best, since on fill source is moved from area to area as the developement construction proceeds.

 

[BigH]

3of12 - SlideRuleEra gave you the advice you need. Piles bypass poor soils which is why you use them (note that poor soils for one structure is not necessarily poor for all structures). Piling through peat is as old as piling itself. The references in the Foundation Know How will help you in design of the piling and in taking into account downdrag (or dragdown - depending on location) effects.

 

[NHSlide]

Preloading can be a good idea when dealing with peats. There is a good paper "Secondary Compression of Peat with or without Surcharging" (1997) by Professor Mesri at the University of Illinois at Urbana-Champaign, who has done extensive work on peats. I recommend contacting him if you need expert help.

You can find the paper I mentioned from Journal of Geotechnical and Geoenvironmental Engineering 123, v.5, p.411 (1997).

 

[91267]

Since we all know about the large secondary settlement of fibreous peat, I am curious how can we completely resolve the issue by just preloading? If I understand correct, preloading generally induces settlement due to primary consolidation, however the secordary compression will continue for many many years!! Is it correct?

 

[BigH]

Not true 91267 - The idea of preload is to put on a load that will cause sufficient settlement to build out the expected settlement under the working/design load in a reasonable period of time (with or without vertical drains). Say, you expect, under your design dead/long term live load to settle 5 inches and with, say, 2 cycles of secondary another 0.5 inches. Total - 5.5 inches - say 6 inches. You would put on preload and leave it on until the 6 inches (or slightly more) has occurred, then remove the preload and construct (let's not get caught up in this discussion of the rebound and reloading, etc). You have now built out two log cycles of secondary. Of course, you would look at the details of your preload to ensure that the compression ratio chosen for your structure's settlement appears reasonable, etc. Again, too, your foundations would be designed to be somewhat forgiving. I would, say, use concrete mat foundations for any housing built (presuming spread foundations) that is judged sensitive so that we would have the structure settle/tilt as a unit, not as individual footing components.

 

[ecanuck]

Hi 3of12 (Civil/Environme)

You say you want to support single family home foundations. If you are doing a sub-division, you have to consider services, etc. Just to get on the peat is a consideration. You must start with perhaps geofabric using light machinery. I would not even consider excavation and replacement - a messy prospect unless there is some expertise locally in the construction community. This will take a lot of time and tricky maneuvering, For the houses the best option would probably be wooden piles provided the bearing stratum is suitable - one thing remember to imbed the wood piles sufficiently to prevent uplift from buoyancy of the piles. I know of one prominent consultant in Canada who is out a million by forgetting that. The piles lifted a few inches - not obvious and were not discovered until the building settled. Long term settlements, will be a headache for streets, services. And with wooden piles the ground water must always stay up to prevent rot. Lots of times these bogs are drained with good intention and disastrous consequences.

 

[fndn]

Not in residences ecanuck-The homeowners are not interested in engineering marvel. They want a $30 k foundation instead of $40 k. Piles that are less than 25 feet are costly and 8-10 feet of rock is most economical if you have a basement.

If you only have a slab-on-grade house( no basement), then I would lean on pipe piles filled with concrete and a structural slab. This is based on economy, speed of construction and practicality.

 

[jdmm]

We have a lot of peat in the Vancouver BC area so we are familiar witht the problems. Two issues that haven't been mentioned are the differential settlement between a pile supported structure and the surrounding ground. All access points such as driveways and sidewalks will settle on the peat. Settlement on up to 14 feet of peat can be quite large and 9 to 12 inches in 10 years would not be uncommon. Consider transition slabs where one side is supported on the piled building and the other side of the (reinforced slab) on the ground. Alternatively, preload the ground around and beneath the building before piling.

The second issue is that of methane generation. In our area methane dissipation systems beneath buildings founded on peat are required. This is especially critical if the building will have a crawl space below grade or a basement. For slab on grade a passive system may be suitable but for crawl spaces an active (pressure system) may be required.

 

[MRM]

jdmm,
In our area, we've typically just relied on the code recommended passive ventilation systems for peat sites. I'd like to get some more info on active systems for gas removal. Have you got any good references/papers you would recommend for the "interested student?"

 

[BigH]

jdmm - just reviewed your post due to MRM's comments. I did a job on the Burnaby Peat Lands where the peat settlement caused access problems to truck bay. After the clients repeated "top it up" repairs, we used Elastizel to use the top up loadings as a preload. I went back 2 years afterwards - and no movement. Somewhere, I have a copy of the original Burnbaby Peat Study done by Fenco back in the 1960s.