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Re: footing - frost depth

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        You need to protect your foundation from the effects of frost action.  Some considerations are the following.
1.)    Freeze/thaw action can cause concrete to deteriorate.  Solution: use good concrete, relatively impermeable, and use entrained air (4% to 6% for normal concrete, 5% to 7% with "small" course aggregate (say 10 mm or 0.375 inch).
        The problem with frost outside of the foundation is the freezing of water near the foundation.  This results in expansion of the water as it turns into ice (about 10% by volume) which moves or disrupts the foundation.  There are three possible solutions in general.
1.)    Get rid of the frost by insulating or heating the surrounding media.  Earth cover is one form of insulation.  Usually frost lifting the foundation is the main problem, hence, provide frost protection to the UNDERSIDE of the footing; however, sometimes frost can cause problems beside foundation structures such as a retaining walls.
2.)    Get rid of the water by drainage or by the use of bearing material that can not contain water such as many types of bedrock.  After all, it's not the temperature that causes the problem; it's the expansion of the water as it turns into ice.
3.)    Design a structure and foundation where the movements resulting from frost action are irrelevant.  For example, if a mat foundation is strong enough that it will not fracture due to differential movement of the founding soil  the whole structure will simply move up and down a little with frost action.  This may be as irrelevant as a boat moving up and down on the waves.  This can present problems with such things as utility connections.
        Piles have special problems.  Soil freezing around a pile causes an uplifting frictional force which can actually lift the pile.  When the ground thaws out it does not push the pile back down again.  There are two solutions: have enough load that the uplifting friction is not adequate to lift the loaded pile; and/or set the pile deep enough that the working skin friction is greater than the uplifting friction due to the freezing soil.  Normally, in Canada, we set our piles a minimum of THREE TIMES the depth of frost penetration below finished grade.
        Hope this helps.
H. Daryl Richardson
----- Original Message -----
Sent: Wednesday, March 22, 2006 10:32 AM
Subject: footing - frost depth

Can someone give me a brief summary of foundation design (or detailing) with regard to frost depths noted?  Do I basically place the top of spread footing at or below the frost depth?  from finish grade?
A reference would be just fine also.  I rarely design for wind, snow, or frost, but there's a first for everything...
Thanks in advance.

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