Foundation options for expansive clay soils??
I am planning to build a super insulated home in the next year or so and would like to avoid using a basement. Our soils here in southern Saskatchewan (9000 HDD) are quite variable when it comes to clay content, but almost all of the homes in the local area (regular concrete basements on wide footers) eventually end up having some problems with foundation movement due to expansive soils. I was really hoping to use a monolithic slab (FPSF frost protected shallow foundation). Any of the home builders who get an engineer involved end up building very costly foundations involving deep concrete piers and void form.
Your thoughts are appreciated.
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Garth,
Of course I'm interested to hear the opinions of other GBA readers. Here's my own opinion: don't be tempted to cut corners with foundation details in a region with expansive clay soils.
The cost to repair foundation problems will bankrupt most builders (and some small insurance companies).
If foundation engineering is ever required, it's in areas with expansive clay soils. If local engineers with experience in your region are telling you that X, Y, and Z are required, I'd listen.
You wrote, "Any of the home builders who get an engineer involved end up building very costly foundations involving deep concrete piers and void forms." So what makes you think that it would be advisable to ignore the recommendations of an engineer?
Is it possible that the design requirements of FPSF foundation technology are just not very well understood by the engineering community in this part of the world? Or maybe there is reluctance to move away from proven (though more expensive) methodology? From what I've read, Northern Europeans seem to have had alot of success using the FPSF approach. Perhaps this is a marketing issue?
Martin
Thanks for weighing in. You are right to advise listening to the engineers, but it is my experience that engineers tend to over design to "cover their butts". However, my main reason for ignoring their recommendations is of course, due to limited funds. I am hoping to hear of some less expensive options to explore.
Lucas
The main concern with the FPSF technology is that it is susceptible to damge from soil movement. From what I have been able to research, the only way to prevent such damage is to ensure that the moisture content of the soils below the foundation remain unchanged over the life of the structure...hard to do but not impossible. With the use of a large "apron" area around the structure, and the avoidance of plantings and especially trees to close by, maybe it is possible to avoid problems???
Here is a design guide for FPSF foundations:
http://www.toolbase.org/PDF/DesignGuides/revisedFPSFguide.pdf
Maybe it can help. I live in NW Ontario and am considering using this type of foundation over clay as well.
Garth,
I'm an advocate of FPSFs, but I've had the luxury of building on well-drained soils with enough slope to run the perimeter drain to daylight.
I think it should be possible to prepare a clay site for a FPSF, but I would suggest a 2-part pour, particularly if you're going to install a radiant slab (which I also advocate). I use a reinforced gradebeam with only 10" bearing on the soil for a two-storey house. I form it myself with RS 2x10s stacked two high for a 20" x 10" perimeter beam (12" below grade with 2" XPS and 2" XPS wing insulation, and 8" above grade for clearance). I build the forms 12" wide to pre-install the exterior 2" foam insulation with galvanized deck screws sticking out on the interior to bind to the concrete, and a 2" thick by 6" deep slab edge insulation attached to the top inside edge of the forms. This way, a 12" thick double wall (modified Larsen Truss) frame will cover both exterior and slab edge foam. Interior bearing pads or beams are poured 6" or 8" below the grade beam elevation to allow for continuous vapor barrier and 2"-4" of subslab foam under a 4" slab.
Then I install radon vent and subslab plumbing (and woodstove combustion air intake), backfill to 6" or 8" below grade beam with granular aggregate (sand or gravel) compacted in 6" lifts. This creates a well-drained capillary break of non-expansive soil under the slab, 12"-14" thick.
The narrow grade beam bearing surface will increase the per square foot gravity pressure of the house on the soil to resist expansion. But I would also excavate the site at least a foot deeper and fill with compacted sand/gravel mixed aggregate over road fabric. If there is enough grade (doesn't take much with a shallow foundation) to daylight a perimeter drain bedded in crushed stone all wrapped in a burrito of filter fabric, then the subslab fill should stay high and dry.
Also, careful site grading, wide roof overhangs and complete rain guttering and downspouts to sub-grade drain pipes are necessary to keep the foundation dry. One inch of rain on a 1000 sf roof will drop almost 625 gallons of water at the eaves.
Contact me at HouseWright at Ponds-Edge dot net if you'd like more information.
Robert
I was hoping you would weigh in here, I always look forward to all your comments. I really like your system and I think that I could adapt your grade beam to be supported by concrete piers. I would have to use void form between the piers to be sure that all the weight of the structure is borne by the piers.
I also like your floor system a lot. Don't know if the expansive soil effects can be prevented here, but with good drainage, site grading etc. (I do have enough slope for drainage to daylight) I am hoping it will work. If necessary, I can construct the interior non bearing walls so that the floor can move a little and have no effect on the structure. Interior bearing points could be provided by concrete piers as well, as long as they are sleeved wear they penetrate the floor.
Thanks again and thanks for the offer to contact you.
Lucas
Thanks for the link to the FPSF document. I have seen the previous edition and was not aware of this one. Sounds like you and I are dealing with similar soils as well as climates....and maybe similar budgets too...
Here is another perspective, without sounding like a preacher...
As a deigner I'm always intrigued why a client wants to "have it all" on a limited budget vs. reducing the size of the house or "lower" some of the decorative or secondary specs.
If having a basement is that important to you and/or your market, why not reduce the size of the home, hire "the right" engineer and be able to afford the necessary specs?
We have very expansive soils in Dallas and almost all foundations are designed by engineers but builders must learn to hire the right subcontractor no matter what the trade is.
Armando
Thanks for your comments. Actually, I am trying to avoid a basement and am planning a very modestly sized, modestly decorative, super insulated home. I also plan to do most of the construction myself. But, I do take your advice to consider engineering seriously.
Garth,
What kind of expansion damage are your neighbors experiencing? Uplift or lateral displacement? Wall or slab movement and cracking?
Using wide footings is not a solution to heaving ground, as the bearing is more dispersed and there is less building weight per square foot to resist uplift. One of the reasons that raft foundations are used on clay soils is to reduce the soil contact area and concentrate the building loads on smaller areas of soil.
If the soil expansion is mostly at the surface where wetting occurs and the foundation walls are displacing laterally, that's a great argument for a shallow foundation with less unbalanced loading.
Expansive soils are not problematic unless they go through significant wetting/drying cycles. Does your climate include alternating wet and dry periods or is it mostly wet? If you can keep the soil under your foundation either consistently wet or consistently dry, you can avoid problems and the drastic and expensive measures to mitigate them.
Even subgrade piers aren't foolproof if they're not deep enough to get below the expansion layer. Have you looked into a shallow mat or raft type foundation?
Robert
The most severe damage seems to occur on basement slabs (uplift in the center mostly). This seems to only show up several years after being built...about as long as it takes for the surrounding trees to get their roots under the walls. Our climate here is semi-arid and yes we do experience the extremes of wet and dry from time to time. There is also lots of evidence of lateral movement...shows up on PWF (permanent wood foundations) which seem to be very popular for some reason and code approved to boot.
Sub grade piers do seem to be doing the trick as most of the wetting and drying occurs in the top six to eight feet of soil, but yes they are usually made deeper to get below the active layer. (Engineered depth up to fifty feet some places!!) Deep is also necessary as these pilings rely on soil friction for support. Most local builders just dig as deep as their skidsteer rigs will go.(if they even bother to go this route...most don't bother)
I am somewhat familiar with raft foundations ( we use them under our grain bins) but no builders seem to use them under houses. I will look into this option further. Thanks again.
Garth,
Here's a link to a good overview (although from a TX perspective) of expansive soil foundation options:
http://foundationperformance.org/projects/FPA-SC-01-0.pdf
And there's a decent entry in Wikipedia on waffle mat foundations:
http://en.wikipedia.org/wiki/On-Grade_Mat_Foundation_for_Expansive_Soils
Thanks Robert!
Garth,
It sounds like you and I are working on parallel tracks towards the same goal. If you have the time I'd love to share/trade some ideas. My email is hamptond76 at hotmail dot com.
Lucas
Email sent...
Garth: Is building on pilings an option in your area? I've seen it done over permafrost soils, but have never explored it. Too much hassle?
JKLINGEL
I would think that building on pilings is an option just about everywhere. I can't speak to your situation though....14000HDD is much colder than where I live. The only reason that I would consider it is to try to avoid structural damage caused by expansive soils....probably not a concern for you...
Garth,
I've built several hundred homes in the Oklahoma City area (quite a bit of elastic soils) and I've always used a pier and grade beam foundation. I guess, due to poor designs in the past, many builders here are leary of this kind of foundation and many subdivisions prohibit it. This perplexes me because I would never put in a standard footing foundation here unless absolutely forced to. Out of the hundreds of homes we've built over the last 14 years, we've only had to go back and shore up the foundations of 2 homes at a cost of only a few thousand dollars. I'll put that record against anyone's design. The reason I like the pier and grade beam approach is because I think that the less contact your foundation has with expansive soils, the better. A standard footing foundation has more shallow surface area subject to the ground movement than our design. We also save a lot of money and use less resources with our design. It's a win-win.
Our footing is 12" wide by approx. 16" tall with 4 horizontal pieces of #5 rebar. Piers are 10" diameter and are typically 3' deep or to undisturbed soil. The piers are spaced 6' apart and/or on every corner. We insulate the bottom 8" of the foundation and backfill it to slope the water away from the house. The insulation helps protect the foundation from frost heaving and has energy saving benefits as well. In your climate, you would probably also have to add some horizontal insulation outside the foundation for frost protection. We haven't had to, but it's probably a good idea to add a layer of gravel underneath the grade beam and have a way to drain it.
If your soils are particularly unstable, you would probably require deeper, wider or more piers. I know that in our design, the beam does bear some load. Also, even though it's not necessarily designed as such, the friction on the sides of the piers also bears a load; not just the bottom of the piers.
With all this being said, I'll bet a variation of this design or Robert's would get you not only the best option structurally, but the lowest cost as well. Good luck.
Garth: "The only reason that I would consider it is to try to avoid structural damage caused by expansive soils...." •• That is why I asked if you had considered it for yourself. Sounds like you've already looked at it. I have no interest in it for me. The pier and grade beam system sounds like a similar setup, only in the ground; seems to be the same concept, if I read it correctly. I gather that the piers' function is to get below the risky soil.