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Strip Footing on Crushed Stone 1

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mattradk

mattradk

Civil/Environmental
Jul 23, 2016
22
I'm designing an attached, 24'x30', two car garage for a friend's daughter pro bono in upstate New York. The house it will be attached to is around 15 years old.

I've done a few other residential designs for one and two story homes (foundation and structure), but luckily, the soils have always had enough sand or other stable materials not to have to worry about being expansive or otherwise complicated. So, 8"x16" (or 10"x20") strip footings placed on undisturbed native soil have been sufficient.

The soils at the site for this garage, however, have a high percentage of clay. To deal with this, I had thought to have 8"x16" concrete strip footings placed on 8" of crushed stone in a 24" wide bed, and backfilled with clean fill to spread the loading over a larger area and prevent potential long term settling into the softer soil.

The elevation of the footings are too deep to take any foundation drains "to daylight".

Am I overthinking this (placing footings on crushed stone) for a simple, one story garage? I wouldn't be so concerned if it was unattached, but I'd hate to see the siding buckle as time goes on where the garage joins the existing house. I also don't want to hire a soils engineer for something that's probably simple and straightforward.

Thanks!
 
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For a garage, probably. Clay's behavior can vary quite a bit based on its makeup. Here in the mid-Atlantic swamps it's about the only thing we have, but ours are relatively stable (with a few terrifying exceptions). So your experience may vary. Are there drains under the existing foundation? If not, I'd be careful with the stone, etc. If you inadvertently lower the water table the now "dry" clay under the existing footings could consolidate even more and cause settlement in the existing structure.

The garage isn't going add much weight at all, so I wouldn't be concerned. If you were putting on a 2 story addition, then I might not be so fast to say okay.
 
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If you use a gravel bed for your footing, don't add fill over it without using a filter fabric separator. Otherwise your fill will filter into the gravel voids and cause some settlement.

You can place concrete directly onto the gravel bedding, as long as the concrete has a reasonable slump, say max of 6 inches.

 
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To quantify the problem you would need to run an oedemeter 1d consolidation test on your foundation sample or estimate the consolidation parameters. Then you can perform your settlement calculation for your strip footing for various widths and loads.
 
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Having lived in New York state I question the fear of "clay" for support. First off what is the consistency of the clay? Soft, Firm, Hard? What are the moisture conditions? High water table next to the surface? Just damp,soil? Whether you laugh or not, give it the "thumb test". Full fist penetration to "thumb nail making a dent mark" and in between.
 
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Thanks all for your thoughts. I've been retired for 6 years (but kept up my license) and a request to put some plans together for a simple garage seemed like an easy thing to do..... it gives me something to think about and refresh my CAD skills. I visited the site and dug an exploratory pit. The degree of clay caught my attention and I did a quick "mason jar" test to see how much clay was present. It was about 90% clay that I'd characterize as firm. I did not dig down to the footings of the existing home to see if there were any foundation drains. The owner also had drains installed in her yard to help remove standing water from some low spots. Same with several of her neighbors. The drain lines were shallow and emptied near the street. This, I hadn't seen before but helped me with a gross characterization of the soil. As a note, footings in the area are required to be at 48" min and snow loads are at 45 psf so the footing loads are relatively light for upstate NY. However, it prompted me to consider if crushed stone under the footings would be necessary help stabilize the structure and prevent settling.
 
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Your term "firm" sounds like you don't need any special undercut and gravel fill for support. If there is wet weather and things get a little muddy, maybe enough undercut and fill for providing a firm working surface. I'd think about a slab with thickened edges and no footings. May get some winter heaving, So what? No problem with use. Edit Reduce heaving potential by replacing the outer footing support soil to 2 ft. depth with clean compacted sand. Remember that slab needs this treatment also. Not just footings.
 
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Thanks. I'm thinking more and more that stone isn't necessary. I'd be a bit concerned about heaving, though, considering it's an attached garage. Enough heaving might buckle the siding, even if the foundations are pinned together.

I can't take footing drains to daylight, but I could take french drains to daylight (or at least install gutters) to keep water shed from the roof away from the foundation and reduce the likelihood of soil expansion/movement.
 
 https://files.engineering.com/getfile.aspx?folder=196b1db7-eb2f-4e21-afa5-adb9478088f6&file=Attached_Garage.jpg
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A few inches of sand with foam insulation may help.
 
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retired 13 got my star. Extend any foam out 4 feet since cold takes all paths. Code guys may oK it also. This also makes full slab needed to spread wall loads.
 
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A rare compliment from OG, thanks for confirming the idea :)
 
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