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Old 06-27-2017, 02:29 AM   #1
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I think I'm building a doomed slab


I'm not sure what I'm building is even feasible, yet no contractor, building inspector or conservation department has tried to stop me.

My back yard is soaking wet. I have a permit and conservation variance to build a garage there. A tree fell over this winter and in the spring, the hole left by the root ball was filled with water 3 inches from the top. I suppose that puts the water table about 4" below the gravel, 14" below the to-be slab of my garage. I'm in New England, the ground freezes, isn't that asking for some trouble?

I planned on building a frost proof shallow foundation for an unheated space. See here for details, page 17: liteform.com/wp-content/uploads/2017/04/FPSF-Guide-Revised.pdf

I have the hole dug and filled with gravel, insulation and more gravel. I have a bad feeling about pouring concrete in there though. I want to revisit the whole thing before I get more into this.

The footings of the monolithic slab are supposed to be 20" deep and the slab 4" thick. I've thought of upping that to 22" and 6" for added rigidity, but I'm not sure how much weight the gravel and ground below can really even support.

I haven't found a contractor to say it's a bad idea, and I haven't found a good one who knows what a frost proof slab is enough for me to want them to pour it. (I've spoken to about 5, 2 have come out on-site, one a self-proclaimed high-end up-to-date kind of guy)

I'm not sure what kind of research I need to do or who I need to hire to make sure this is done right. What should my next steps be? Should I talk to a geotechnical engineer or something? How much does that even cost? Am I overthinking this and what I'm proposing should be no problem at all? Should I scrap the frost proof idea and dig some proper foundation walls right down to the water line?
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Old 06-27-2017, 06:11 AM   #2
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Re: I think I'm building a doomed slab


Having an engineer go over the plans and analyze the soil is the best way to go but I don't know if it's worth the expense. I don't know what they charge.

I painted a sub division once that was built on a reclaimed swamp. The engineers specified how each house needed to be built. A few had deep pilings poured to support the slab but most were monolithic slabs with a lot of steel added. One of the houses moved a good bit and became unlevel but the integrity was still sound.
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Old 06-29-2017, 05:41 PM   #3
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Re: I think I'm building a doomed slab


Is there a specific branch of engineering that would be best for this, a type of company I should look for? I've been bouncing around websites but can't really find the "right fit," there's environmental, civil and surverying companies.. the word "geotechnical" isn't getting me many great hits
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Old 06-29-2017, 05:50 PM   #4
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Re: I think I'm building a doomed slab


Just because the hole filled water does make the ground water level that high. It probably just filled with surface water.
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Old 06-29-2017, 05:58 PM   #5
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Re: I think I'm building a doomed slab


Hmm I can't edit the original post but you're right, I should have added some more detail on the nature of the water. The variance I got from conservation was permission to build 20 feet from the edge of the wetlands. 60 feet away, and not much of vertical drop, is an 80+ acre swamp. In some areas within 100 feet of the garage, the ground is soft enough that you will sink an inch or 4 just walking on it until mid July.
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Old 06-29-2017, 08:09 PM   #6
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Re: I think I'm building a doomed slab


I'm kinda surprised the conservation authority would grant you that variance but I am often surprised by things. The ground sounds like it is absolutely saturated for at least part of the year. A problem with wetlands is that the soil is often mostly organic material in varying states of decay.

No contractor has tried to stop you: Did you approach them for estimates? Although some might voice their opinion, their role is not attempt to 'police' building activity they are not part of. Some, but not all, building department will offer advice to homeowners, and might even offer a preliminary 'don't even bother', but we don't know what you discussed with them. Once the CA has issued their variance, I doubt they have the expertise or authority to yay or nay your proposal.

I was originally going to say that you should consult a structural engineer but now think think you might want to consult a geotechnical engineer first to see what the ground can handle. You slab runs the risk of disappearing below the horizon.
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Old 06-29-2017, 08:23 PM   #7
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Re: I think I'm building a doomed slab


On land like that we do an engineered reinforced pan slap. the floor and walls are placed as one and it floats and acts as one when the frost heaves it.

If that is your plan the engineering is always the same so the permit people may pass it because they have seen it before.

Ask them for a list of accepted engineers, they would except plans from.
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