Another building on wet soils question
I want to replace an outbuilding with a 2 car garage. Current outbuilding is poles on concrete footers. But it does not have a concrete floor and I want a concrete floor in the new building.
I am thinking that I would have footers poured and then a concrete slab for the floor. Then conventional framing on top of block stem walls that rest on the footers. But will the wet soils cause problems with this?
Or should I somehow have a slab poured and put up a new and nicer pole building?
Just for reference, this soil is squishy on the surface now due to spring rains and its low lying nature. Soil is saturated. By late july it dries out pretty well. then in fall /winter, saturates again depending on rainfall and snows. Basically when I dug a 36″ deep hole to pour a footer for the pole barn, it filled with water in 2 hours as soil was so wet. (that was april of 1999)
Are there any ways to deal with building on soils like this? BTW- this will be contracted out, I’m not DIYing, but would like to know best course of action before I start dealing with bids and contractors, etc.
Replies
foot of coarse sand, six inches of base pour concrete on top
I see from your profile that you live in central MD, which will have a winter max frost line of ?? 24-30 inches??
There has been a suggestion posted about the gravel and sand base followed by a concrete pad which would "float" on that base. Frost heave is a consideration/concern with a floating slab in places that experience freezing winter conditions.
Differential frost heaving due to mixed soil types and unequal saturation densites may cause these problems. Soil types and characteristics must be known in order to arrive at a competent decision or at least a good SWAG!
I live in a winter frost zone to a max of 24" and own floated slabs for utility/tractor sheds and shop and of the four floaters I have only one has shown an ugly face due to frost heave. My soils are heavy clays.
The other choice is to place footings down to frost depth, pour walls/ lay block foundation walls, add fill to level and pour a cap slab. If that structure does not trap water and frost heave it will be okay...but it may be as unlucky as my one in 4 floating slabs and crack.
My one cracked slab is still quite usable and the structure is stable...been there since 1987.
Go to the local U.S. dept of agriculture and ask for a soil specialist and ask him about your soils at site. He will have a soils survey that will tell him the characteristics of the soil you own and can quite possibly point you toward a learned decision or a competent foundation contractor.
Brownbaggs most excellent suggestion will be more economical....probably more practical for a garage, too!
.................Iron Helix
No need for footings. The purpose of footings is to support the load of the structure. You already have that supported on the piers. Just a sand/gravel mix ( we call it inch minus here) for drainage and a slip joint at the posts to let them slide or move separately...
But you do have a lot of ground water. I would dig a sump hole and lead a perimeter drain to it, with an electric sump pump top discharge to drain away.
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Check out your post footings...our area pole barn builders drill a hole, drop in a concrete biscuit set the post on top and fill.
Not a true concrete piling/foundation.
Pole barn longevity is 15-30 years and it is usally the posts that rot off at or below ground level.
Professional appraisals usually give a 20 year lifespan to a polebarn in this mixed wet climate.
.................Iron Helix
Do a Google search on "frost protected slab". It may be a more economical but still structurally sound route for you to go...
The connection between wet and unstable soils is implied here...
An important concept to know about building on unstable soils: If your having trouble finding good firm soil to place your foundation on you have two choices: 1) go deeper - which only can go so far (pun intended) or 2) go wider. The wider the concrete is that is placed for the footing, the more support - therefor, theoretically, 2 square feet of concrete placed on soil capable of supporting only 1000 PSF is equivalent to a square foot of concrete placed on soil capable of supporting supporting 2000 PSF. Placing aggregate under the concrete goes even further toward distributing the weight and providing a uniform bearing surface.
A monolithically poured slab foundation lends itself perfectly to the above concept #2 - a pole barn type structure is to the other extreme. You need relatively flat ground for a monolithic pour though.
Perimeter drainage is often not installed with slab on grade construction but as as Piff said it may likely be warranted in your situation. Daylighting the drain system is best - if possible.
A few brief Q's: Would you characterize the soils at your building site as sandy, silty, clay or what? How flat is it?
BTW - when you say "Central Maryland" what does that mean? East of the bay bridge? I used to live up that way and have fond childhood memories of spending summers crabbing in the bay (western side though)...
Thanks everyone, I will do some more research to learn more about this. To answer a few questions:
1) Frost line is 36" per code. But I've got a water line at 28" that has never frozen.
2) Soils are typical central maryland clay/ muck. You know the kind that sticks to your shovel when you try to dig in it . . . .
3) Spot is Level, no fall for a daylight drain.
4) In the past nine winters, I have not seen any evidence of frost heaving.
5) I live in Carroll County, near M/D line. Born and raised here. Spent 30 or my 32 years on the same 2-mile piece of country road. I have fond memories of the bay also, but my memories involve striped bass as opposed to crabs. I prefer to buy my crabs, but fresh caught rockfish simply cannot be beat.
Th obvious answer is to choose a place on the property that does not pose such challenges, but I'm only dealing with 1.75 acres. The house is in the middle, the west side borders the road (setbacks) and contains the drainfield, the north side won't work since it's in front of the house, the south side has no room and the east side (the side in question) is currently underwater. The garden is uphill from the shed. But I am not willing to give up fresh organic produce, so I need to figure out some compromises I guess.
Iron H wrote:
The other choice is to place footings down to frost depth, pour walls/ lay block foundation walls, add fill to level and pour a cap slab. If that structure does not trap water and frost heave it will be okay...but it may be as unlucky as my one in 4 floating slabs and crack.
This is probably most like what I had in mind. Can a typical foundation contractor ascertain whether or not this will likely work? or is this more a specialist scenario? I am not opposed to raising the building out of the ground with block foundation walls and then building the grade slightly around the building.
Edited 4/8/2005 8:05 am ET by frontiercc
Yea - stripers and rock. Blues are good just for the fun of it too. Not much for the table although smoked aint too bad... Unfortinately, I left most of that behind when I moved to centeral NC.
Here is the first hit I got when I googled for frost protected slab. The .pdf has some good pics.
http://www.toolbase.org/tertiaryT.asp?TrackID=&CategoryID=1844&DocumentID=4496
>> Can a typical foundation contractor ascertain whether or not this will likely work? << - yea, probably, but they may have other agendas... like get in, get out, collect check. Around here a soils scientist cost $300 and they give specific recomdations with regard to foundation consturction. Matt
Edited 4/8/2005 8:13 am ET by DIRISHINME
I am considerably North of you and have a deeper frost penetration elevation. 4ft.
Our Buildings Officials have requested P. Eng Design and drawings for all slab on grade construction for the last 10yrs or so
In your situation we would typically remove all topsoil and blackearth for the building footprint 3 to 4 ft beyond on all sides.
We would replace these materials with "B" gravel or "Pit Run" and compact with a large diesel plate achieving a Proctor Test Density of 95% or greater.
We would then top the "B" layer with "A" gravel, (5/8 in minus crushed stone" to allow for precise leveling and raising the gravel bed above the surrounding grade.Compact also. With a level and well compacted subgrade we can control uniformity of slab thickness and thickened edges.
Rebar at the perimeter and wire mesh throughout, is a given.
In "Wet " conditions, our P.Eng typically specs Dow SM underneath the slab, and beyond the footprint a minimum of 2 ft, on all sides. Dow SM literature has tables for frost prevention coverage. 1 in SM equals 1 ft of frost protection. 2 inches is a typical install, in our area.
If the soils are high and dry, with low groundwater content, no SM may be speced at all, but given our penchant for energy efficiency, we prefer to insulate any time we can convince the customer. It does pay back in comfort, and reduced heat loss.
Rather than dewater a site, raising the grade would be our call.
Cheers.