Frost heave during construction?

[davidj]

We've finally got our foundation built and backfilled.  At some point this Winter I'll get the photos up now I'm not spending my whole life in a 3ft deep hole!

However, one of the neighbors came up with an issue - is frost heave likely to be an issue this Winter?  The foundation wall is 4ft high fully-grouted block and the top of the 8" deep footing is about 3ft below grade.  Here's a picture:



Once covered and insulated, the crawlspace should stay above freezing (if the next door cabin, which is built the same way, is any guide).  However, do I have to do something this Winter, when it's exposed?  Can it get moved around in a single year or does it usually take several years for frost heave to break stuff?  We're at 5000ft in the Northern Sierra Nevada and I'm guessing the average highs will be in the 40s and lows in the mid to high 20s mid-Winter (with cold days maybe 10F colder).  There should be a few foot of snow over the middle of winter to insulate it.

[rwanders]

Will the interior of the foundation remain open all winter? I would be somewhat concerned to leave it "unbraced" by floor joists all winter----perhaps you might consider placing some bracing across the open area---maybe a 2x6 or 2x8 along the inside of the walls with some good sized 2x  or 4x spanning between them to resist any ground pressure trying to push the walls inwards. The lumber wouldn't be wasted---could be re-cycled when you resume construction in the spring. John or Glenn will, I'm sure, have some good advice for you.

[Redoverfarm]

davidj  if the bottom of your footing is below the frost line then it should be OK.  I am worried that your footing is at the top of your crawlspace elevation.  The frost line is a pre=determined depth that the ciold penetrates the ground and stops freezing.  With the interior not having that same amount of covering it may pose a problem.  If it were mine I would put stone up against the interior wall at least 3-4' from the block the same depth of the outside surface top.  I realize you are probably not ready for gravel in the crawlspace but you do not have to cover entirely just around the inside edge.  This angled amout can be raked level later. 

Yes the exterior is OK but you do not have the same protection on the interior.  If there was a deck built already it would be less likely to occur but out in the open (interior crawlspace) the cold will penetrate the same level as the outside.   

[Bishopknight]

Throw as much hay around the perimeter and inside the crawlspace as possible to insulate the ground from freezing.

[Rover]

Yup, frost can be nasty even in one season.  I agree with using hay along the lower ground elevation and up the wall a bit.  Stack square bales of hay then its easier to remove in Spring.
Maybe even shovel snow against it.   Some winters you get an early snow which would help.  Other winters the cold snaps hit before the snow; that is bad.

[davidj]

Thanks for all the info - it sounds like hay is the way to go for the Winter.  I guess I'll take the truck up and haul in some bails tomorrow.

Officially the frost line is at 18" here (although it's also 18" on the other side of the Sierra Valley, which is considerably colder over Winter).

As for support, the wall is 8" block, has 1/2" rebar at 32" centers horizontally and vertically and is fully-grouted.  Wouldn't that be enough for an unsupported retaining wall holding back 3' of dirt for a few months?  The soil seems to drain reasonably well.

[MountainDon]

Officially the frost line is at 18" here

That means it may not go that deep every year. That'll be the worst likely.

[rwanders]

The rebar and grouting definitely provides a good level of confidence in its resistance to ground pressure.  The straw bales is an excellent idea.

[Redoverfarm]

As for support, the wall is 8" block, has 1/2" rebar at 32" centers horizontally and vertically and is fully-grouted.  Wouldn't that be enough for an unsupported retaining wall holding back 3' of dirt for a few months?  The soil seems to drain reasonably well.

Not real sure of what your definition is of fully grouted. I assume you are talking about the mortar joints.  I might caution you as far as the cavity or the area of the block webbing.  If they are not filled with concrete on the top course water will fall into the bottom course and freeze and could potentially freeze and bust out the block.  If they are not covered then I would use a material to cover. 2X8 , plywood or scrap OSB with a weight applied on top to keep the wind from blowing it off. 

There are plenty of homes built where the foundations are left unfinished during the winter and very few sustain any damage with the exception of water draining into the webbing and freezing,

[JRR]

For years I have seen farmers protect animal shelters, pumps, etc, with hay and other such materials ... dense leaves, branches, sawdust, etc. Especially effective if kept dry by covering over with tarps held down by stones.  Tarps alone would be somewhat effective ... anything to resist air movement from sapping away the ground heat.

[davidj]

It turns out staw is hard to find up our way.  They have some really nice hay, but at almost $20/bale that was getting pretty steep for 100ft of wall.  So in the end we bought a yard of stringy fir bark,  then covered that with fir branches and topped it off with a tarp.  It all looked pretty cozy! 

[glenn kangiser]

Good move.