FPSF on compacted fill
We are in Minnesota (Zone 6a). Building this summer on 12 acres. 2000 sq/ft ranch, 3 car 900sq/ft garage, slab on grade, L shape footprint. I will be the GC on the project.
Our building site has a high water table. The seasonal high water table has been determined to be at an elevation of 911′. City ordinance requires the lowest floor in any structure needs to be 4′ above the season high water table. So that puts the elevation of the lowest floor at 915′. The highest grade elevation on the property and where our house will go is 914′ down to 912′. So our floor level will be 1′-3′ above current grade. So after the topsoil is stripped and leveled, the footings in some of the lower areas, “may” need to rest on compacted fill instead of undisturbed soil.
I think once a gravel base and insulation are added to the thickness of the footing, I will still be on virgin soil, but I might not be. Does this concern warrant a different foundation type?
My designer/draftsman thinks we should just dig frost footings so the footings we don’t have to worry about the footings on compacted fill. He admittedly doesn’t know much about FPSF and neither does anyone else I’ve talked to in my area. The building department is fine with it, but I’d be the first in our city to have built on a FPSF.
The more I read about FPSF’s the more I like them, but I have some reservations about whether or not it’s appropriate for my situation. I’m fine with hiring an engineer to design it, but I’d like to get as much information as possible before I do.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Scott,
I'm not an engineer, so you should consult one before proceeding. I have built several garages on sites like the one you describe. Here's what I have done: First, strip the topsoil (the layer with roots and vegetable matter), so that you are down to the subsoil. Needless to say, save the topsoil in a pile; it's valuable stuff.
Then, install gravel -- I use crusher-run gravel, which is a mix of small aggregate and sand -- in lifts. Spread each lift and compact it with a walk-behind compactor. (That's what I use, anyway). When you are getting close to the grade you want, finish with 4 or 5 inches of crushed stone (no fines). Compact this layer, too (although many people say that crushed stone is "self-compacting"). Then install your rigid foam, poly, and concrete.
If you have the luxury of time, prepare the site a few months before you build. One or two heavy rainstorms help consolidate fill. You want the fill to consolidate before you install your foundation, not after.
-- Martin Holladay
Thanks Martin. I do plan to hire an engineer. I just want to make sure I'm on the right road, before I start writing checks.
When Martin says "needless to say," that should be translated to "should be needless to say, but you better discuss with your contractor because you can't count on it."