Steel Floor on Permafrost

[aayvat]

Hi everyone,

I'm working on a project in a remote region of continuous permafrost in northern Canada. The average permafrost temperature is -8deg C, active layer is approx. 3ft. My client has an existing structure in place; a steel structure on concrete foundation walls, columns on pilasters and footings. The building size is about 30ft x 75ft. The foundation walls are 5ft high, with about 3ft in the earth. The exterior of the foundation walls are covered with 4in rigid insulation. There is also 4in of horizontal insulation underneath the building (T.O. insulation is B.O. foundation wall). Above this layer of insulation is 1.5ft crushed stone, followed by 8in sand w/ PVC membrance, and the top layer is 8in crushed stone. The floor finish is currently crushed stone.

My client has asked for the design of a permanent steel floor and proposed to have the floors supported on piles. A concrete slab is not preferred. My understanding is that the building has been designed to retain heat inside the building and the insulation prevents the heat from transferring to the frozen earth below the building, and so, the permafrost remains frozen and settlement is minimized.

By installing piles, as proposed by the client, the permafrost below the structure will be disturbed during installation. I also believe the pile will act as a break in the thermal gap formed by the insulation. The heat transferred from the inside of the building through the pile can thaw the surrounding permafrost, resulting in annual settlement. Is it correct to assume this?

There are options to keep the piles from transferring heat to the surrounding soil, like artificial refrigiration and insulating the piles. These may be costly solutions for such a remote site.

Another idea is to provide intermediate pilaster/footings at 15ft and support the floor beams on the existing and new intermediate pilasters, but I'd like to keep the floor seperate from the existing structure. Is building a floor directly on the crushed stone a reasonable solution?

Are there any permafrost specialists out there that can inspire me with their two cents? I've attached a sketch showing a section of the current building and my concern with installing piles.

 

[miningman]

There tends to be an awefull lot of theoretical BS spouted by so called permafrost experts who have never ventured North of 60. From personal experience, I can assure you that permafrost can be an asset rather than a problem to be solved but you have to have a good knowledge of local geology. In many areas, rock outcrop is ubiquitous and settlement is non existent. Of course if there happens to be ice lenses within the rock, then yes you most certainly get settlement with doors that no longer close etc etc.

Minus 8 is actually quite extreme as a virgin rock temperature... I have worked with temperatures between minus 4 and minus 18. But again is this trully what you mean or is it some other jargon. Tell me more about the local geology and / or apprx location of your project and I might be able to be more specific.

 

[aayvat]

The site is located near the 62 parallel in northern Quebec. Near the village of Salluit. The minus 8 is the avg temperature of the permafrost year round. As for the local geology: mainly consisting of gravels, ice rich silts and sand. The rock level is fairly high, varying from 10ft to 20ft. I've been told that the preferred method for foundations on site is to drill hole to 4ft into the bedrock. The pile is fit into the hole and filled with grout.

 

[miningman]

OK, the 62nd parallel is quite South in terms of permafrost. The concept of "average permafrost temperature" is a bit of an oxy moron, as perhaps used by non technical people. Salluit seems to be quite close to a large body of water, which tends to mitigate the effects of permafrost...... by definition you cannot have permafrost at a location where there is open water. This might seem a bit anal but it helps understand the phenomena. I would expect true virgin rock temperatures around minus 4... and on southern facing slopes, it might actually be discontinous permafrost rather than true permafrost.

With 10-20 feet of till with ice lenses, I would support the local usage of drilling into solid rock for installation of piles. Regardless of heat transfer into the surface materials, this at least gives a solid footing.

Personally I would have thought that a concrete floor, laid on top of insulation might be the best way to go, but if local demands really require a steel floor, then I would suggest that the steel be say say 2 feet above grade so that there is constant flow of cold air under the floor during the 9- 10 months of the year that are below freezing.

One would think that insulation on top (or under) the steel floor would then be necessary for economic reasons, but that is a separate discussion.

 

[fattdad]

by definition you cannot have permafrost at a location where there is open water.

sand and gravel exploration in Barrow, Kaktovic (Barter Island) and below the Arctic Ocean there's an interval of thawed sediment and then it's back to frozen earth. I can't tell you how many times I've seen permafrost below open water all along the North Slope of Alaska.

We always used foam insulation below ground supported buildings along with layers of gravel. We had no rock for bearing so it was always adfreeze piling (popcicle piles). For ground supported water tanks cryoanchors.

f-d

¡papá gordo ain’t no madre flaca!

 

[aayvat]

Maybe my sketch wasn't clear enough, but the elevated steel floor would actually be on the interior of the heated building. The building has 4" rigid insulation on the outside of its foundation walls as well as a 4" horizontal layer of insulation about 3ft below the grade level of the interior of the building. They have proposed to install the piles to support a steel floor system, but the grade above insulation in the building will not freeze during the year and there will be no flow of cold air below the floor.

 

[miningman]

Fatdad, I wouldnt want to get into an argument with you , especially since I have zero expertise on the North Slope area of Alaska. But in the interests of initiating a possible discusion of mutual interest, when I suggested that permafrost cannot exist below open water, I was referring to my own experiences in inland conditions where a lake containing fish, cannot have permafrost under it unless the fish have developed the ability to enter into deep hibernation during the winter. In one case in my career we were relatively close to a very large lake, and as we sank a mining shaft, we went thru the permafrost layer at about 1650 feet below surface. On the 2600 level it was a pleasant 2 degrees C. The local geothermal contours were quite interesting to study.

I have no trouble with the concept that permafrost might exist below saline water, but I wonder if this would not be closer to what I call " discontuous permafrost" rather than true permafrost.