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Post 55D – Some other things about foundations.

Like always, there are exceptions to the rules, so any time something is written as an absolute fact, watch out  – it may not be so.  Such is the case in the statement about the traditional method of building footers below the frost line.  There is more to learn and this also shows a very good example of learning about learning.

There is a whole class of foundations called “Frost Protected Shallow Foundations (FPSF)”.  This originally came from the Nordic Countries but is also being done in the northern states in USA and probably in Canada.  There are several parts that must be thought about when implementing this procedure.  To form an “ice lens” under the footer two things must be present, water to be turned into ice, and cold to freeze the water.

The cold under the building is prevented by adding rigid insulation as shown in the picture above.  This allows heat loss through the floor of the building to heat the earth near the building and raise the frost line.  But what happens in an unheated building?   That is where the concern about moisture comes into play.

Obviously, there is major problem if the water table is high.  In this case lots of work must be done with drainage.   This could even involve having to install sump pumps to pump the water away, but usually involves hauling in dirt to raise the elevation to keep water away.   The second, and to me more interesting issue comes from natural wicking of water in dirt.  This capillary action will cause water to rise around small particles to bring water to a higher level from a wet area to a dryer area and frozen water is dry. Capillary action can be seen by sticking the corner of a paper towel into a glass of water and watching the water climb up the towel.

This is the action that occurs when forming the “ice lenses”.   The answer to this is to put a layer of crushed stone or other larger particles so the are fewer tight spaces  under the foundation to act as capillaries to pull the water under the foundation.

There are other interesting stories of foundation failures and successes in fixing these failures.  The two most interesting are: The Leaning Tower of Pisa,  and the Moving of the Cape Hatteras Lighthouse  because the bank was eroding near it.  A video talking about the movement of this lighthouse.

The pictures on this post are from a HUD government document about FPSF.

Gary

 

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