Backfilling new foundation in frozen conditions 7

[paul1177]

My contractor just completed the foundation for my new home extension and installed french drain w/ 3/4" gravel cover. It's -15 Celsius and I am anxious to backfill but the piles of earth are too frozen for the small dozer to handle it (no room for a bigger one). Should I consider a geotextile membrane over the gravel cover and add a few feet of compact 0" to 3/4" and cover with an insulating blanket, then backfill with the soil in the spring? Also, is there possible damage after 2 weeks of this cold?

 

[DRC1]

Backfilling in cold weather can really only be accomplished with crushed stone. The gravel will freeze and you will not be able to compact it. This will lead to problems with your basement slab. The other option is to wait for the thaw. Note that when the thaw comes the first week or so it can be very muddy. This is because the top has thawed, melting the ice trapped in the soil, but the deeper frost has not melted yet, preventing the melt water from draining, resulting in mud. This will disapear in a week or so.
If you want to backfill now use 3/4 in stone. Get a price first, because it will be a little pricey.
As for the concrete, as long as you had 2-3 days of +5d C curing, the concrete should be okay. The strength gain will slow or stop in freezing weather, but will resume in warm weather.

Good Luck.

 

[fndn]

Exactly as DRC1 put it. I would add to have a swiss hammer test readings on the walls-you should get over 3000 psi. If the weather has not dropped by more than 10 Deg.C from pour date to 48 hrs thereafter, then you'll likely be fine with the strength gain and will avoid cracks.

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[paul1177]

Thanks, DRC1 and fndn for your helpful replies. Now if I choose to backfill after the thaw (likely 3 months from now), is there potential for frost heave and therefore damage to my footings or even walls due to extended exposure to very low temps? Is filling the ditch with straw a viable option to ensure there is no freezing at that level, or is it too late to consider? If I go with 3/4" gravel, is there potential for scour/scraping of my waterproofing membrane? As for the issue of concrete cure, I was fortunate enough to have better weather.

 

[fndn]

paul;

Frost heave is not a problem, because you poured before the ground froze, and you already have the walls up, thus having some load on the soils. You can throw double blankets over the footings or loosened straw , until you backfill. Keep us posted on any developments.

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[BigH]

fndn - careful; better safe than sorry and straw is cheap; besides, paul1177 can feed it to 'is cows in the spring - oops, he's not in India so may not have 'em. I had a thick concrete slab (weighing more than the footer and wall I am quite sure as it was industrial grade full depth slab on grade) and we had 50mm heave on a shale base during the course of the winter. After, the slab returned to normal level and all was fine - this was critical in that the structure was a computerized warehouse and even little movements could throw off the forks from sliding properly into place.

 

[eric1037]

If it's just backfill around your basement, what does it matter if it settles in the spring? The most you will have to do is add more soil.

You should protect the foundations because frost heave is a definite problem.

At a minumum, I would put in 3 to 4 feet of clean sand.

Straw is next to useless if it is cold for a long period of time.

 

[paul1177]

Thanks for the additional responses. My concern with heave is that my new foundation wall is connected into existing, so there could be cracking due to differential movement. Regarding protecting it over the winter, I am a bit confused now. Straw cover I thought was the universal "way', now I am hearing it won't work in the long run. I am planning on adding some insulation blanket as well if that makes a diffference. Sand fill I would think would eventually wash through the gravel beneath and probably plug the french drain, no? Or maybe I could put a geotextile membrane before I pour the sand?

 

[gravel]

I think what Paul1177 is getting at, is potential frost heave of the exposed installed foundations. Since the footer is not covered up, the frost depth is really from that exposed footer base downward; and yes, it will freeze & heave within that region. I suggest getting some fabric & temporary cover/insulation or whatever placed, as thick as frost depth would be... say 30" or so. You don't have to compact it.

 

[fndn]

Paul-Don't be confued. Blankets work well, but if you get high winds with drifting snow, the large frozen chunks of soil holding down the blankets will move about, and you will get partial cold elements into the soil areas. However, with loosened straw, if thick enough, say 24", would work best. I have worked in Subdivisions for over 15 yrs in the upper midwest and know this from first hand experience.

As for the heave issue-in order to get frost heave, there has to be frozen soil that is progressing in depth and little weight to suppress it. Sine you poured your footings in the warmer weather, there was no frost in the soil to begin with. Then you've protected from the element I am assuming and have already had the wall poured giving you gravity loads.

If it is my home, I would use 24" loosened straw over the footings. Also as an independent opinion talk to Residential Superintendents and see what has worked well for them. In the end you'll have to decipher all this info and make your own recommendations.



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[1gr8opr8r]

Too tight for a dozer? Have you considered bringing in an excavator with a frost tooth to rip the frost. Once the crust of frost has been broken from the backfill material the core will undoubtedly be free of frost. Your operator can then utilize his dozer to backfill. If you can monitor the operation be carefull to have the operator segregate the frost clear of the backfill area so as not to run the risk of having a large chunk of frost enter the overdig. A large piece of frost entering the overdig can be uncontrolable, and could easly fall into a wall with enough force to blow out the wall. Also, be carefull with straw, remember that all of that material will need to be removed, all of it. Unlike flatwork where straw has its applications, underground you must be cautious. Should you not get all of that straw out of the overdig prior to backfill, be prepared to endure an indefinite length of time during which the gasses produced by the decomposition of the organic material will migrate through the draintile and into the dwelling or structure.