Turndown Footing Technique, No Frost Depth worries!!

[Atomic25]

Has anyone heard of turndown footing system used in the Northeast that utilizes non-frost susceptible fill? Supposedly this is being used to reduce the turndown depth.

In a nutshell, rock is placed beneath the turndown, and the rationale is that the rock wont frost heave. I'm not so sure I buy into it, especially if there isnt a fully functional drainage system in place.

 

[MarcbSE]

Yes, I've heard of that design and even used it quite a few times. However the only places where I've used it were for some premanufactured buildings that were very small (as in less than 200 sq.ft.). In these cases I designed the slab as a structural slab so if anything moved it would all move together. I wouldn't trust it for anything much larger personally.

 

[JAE]

Read this thread as it is similar to your question....some good responses there.

thread256-188342

 

[civilperson]

I have used non-frost rock subbase under transformer slabs in combination with insulation placed horizontally. In parts of Canada with deep frost, (10-12'), this results in substantial savings.

 

[BigH]

Something to keep in mind with respect to frost depth/penetration and potential heave. First, one must have a frost susceptible material which can heave (clean sand, sand and gravel are not frost susceptible). Secondly, one must have a source of water with which the susceptible soil may suck in to form the ice lenses and that leading to the heave; and (3) one must have cold temperature which lead to freezing. I have had sites in Northern Ontario (frost depth 3 m or so) where we put the footings at 0.75 m or so because (1) the groundwater was more than 4 m down and (2) the upper 8 m was a stratum of clean sand. As civil person indicates, you can negate frost depth by use of insulation. Also in heated buildings only the outside footings need to be placed below the frost zone. See the following link which also has links to other articles:

http://irc.nrc-cnrc.gc.ca/pubs/cbd/cbd182_e.html

 

[Atomic25]

I don't get a warm and fuzzy feeling this ideology is used for anything other than structures of minor nature. Aka, don't build your entire house on it....just your tool shed.

 

[civilperson]

The "minor" structure described was a 280 kip transformer costing three million dollars carrying twenty thousand dollars an hour worth of power. Warm and fuzzy is not applicable to this design, just cost analysis vs reliability.

 

[Atomic25]

Nice, that's what I'm looking for. How would you defend your design? I'm assuming this is something where you stand behind a geotechnical recommendation..correct?

 

[emmgjld]

To Defend the design:
Proper Geotechnical Report to define the site - See BigH above.
Proper definition of shallow subsurface and surface water control - See BigH above.

Million dollar homes in resort areas of Colorado done this way, in total or in part.

 

[miecz]

See thread592-169967. CarlB posted a link to a HUD Design Guide for Shallow Foundations. It essentially uses insulation to reduce the frost depth.

The OP wasn't asking about insulated foundations though. I've used shallow downturned slabs under transformers and outdoor mechanical equipment. Put gravel down to +20% below frost depth. Read here somewhere that placing a gravel fill increases frost beyond normal depths. My thinking is that the gravel may fill with water, freeze, and lift the pad a small amount. But it won't heave inches. I make sure that the equipment has flexible connections so a small amount of lift won't break anything.

 

[civilperson]

A continuous gravel layer from the surface allows the cold to fall through the voids to the base of the layer. A concrete slab with horizontal insulation 4" thick extending 3' beyound the limits of the foundation prevents this intrusion of cold. I do not need to defend this design since it is successful with no failures in many built applications for numerous clients in New York and in Canada.
I will be the EOR for your project for a modest fee if my details are incorporated.

 

[Atomic25]

Thanks for the information guys, I at least have a start.

"because it worked in other places" isn't necessarily a valid argument in court and allows you to get sucked into errors caused by the contractor.

 

[civilperson]

Hire an expert, (someone who has built similar projects).