residential foundations in peat
residential foundations in peat
(OP)
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
Has anyone out there had experience in this design or know of some good references they could point me to.
Thanks to all.
3of12





RE: residential foundations in peat
RE: residential foundations in peat
Richard A. Cornelius, P.E.
WWW.amlinereast.com
RE: residential foundations in peat
3of12
RE: residential foundations in peat
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.
RE: residential foundations in peat
"... 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."
RE: residential foundations in peat
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.
RE: residential foundations in peat
RE: residential foundations in peat
RE: residential foundations in peat
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.
www.SlideRuleEra.net
RE: residential foundations in peat
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.
RE: residential foundations in peat
RE: residential foundations in peat
You can find the paper I mentioned from Journal of Geotechnical and Geoenvironmental Engineering 123, v.5, p.411 (1997).
RE: residential foundations in peat
RE: residential foundations in peat
RE: residential foundations in peat
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.
RE: residential foundations in peat
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.
RE: residential foundations in peat
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.
RE: residential foundations in peat
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?"
RE: residential foundations in peat