Skid Platform in Permafrost Environment 2

[KootK]

I’m working on the design of a platform to support some heavy duty batteries. It is intended to be a modular, mass produced solution that will be installed in arctic, permafrost conditions. I haven’t had any experience working on structures in permafrost environments. The superstructure will be sea-can storage containers by others. The foundation will also be others. My scope is really just the platform between the piles and the storage containers. Some questions that I have include:

1) Always piles in permafrost, right? I’m thinking that helical screw piles would make sense here but I suppose that concrete, driven steel, and even timber piles are possibilities.

2) I’ve been asked to develop schemes for the platforms in both steel and concrete. Steel generally makes more sense in remote, cold environments, right?

3) Other than extreme temperature variations, are there any other special considerations that I should be tending to?


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[dhengr]

KootK:
Give us a few hints: size and weight of battery pack, container size and weight? What’s the skid intended to do, and its approx. weight and dimensions? I’d make it out of steel, and truly a skid, so it can be pulled the last three miles with a Cat. How permanent is it, and remote installation site, or otherwise?

One of the biggest things is that you have to separate any heat production associated with the structure from the permafrost soil conditions, or the soils tend to melt, lose strength and settle. Sometimes this involves adding rigid insulation under the bldg. floor; or putting down a mat of gravel some feet thick and larger than the structure in plan, then putting the structure on that mat. This includes radiant heat, from the sun, etc. off of any material, any color, which will absorb heat from the sun. I would think that anything other than drilled piers would be fairly tough to accomplish in permafrost. Then you might actually consider heat extraction units (heat pipes), on piers, etc. to keep the permafrost frozen. And, concrete materials, mixing, and delivery/transport is either in short supply and difficult to do in many remote areas. Take a look at Canadian and Alaskan highway dept. and pipeline specs. on these matters; probably various military construction specs. too.

 

[TLHS]

I've looked into permafrost a little for some proposals up north but never actually done the design work.

Your big goal is to keep any heat from the skid away from the ground either by insulating or by raising the item off the ground. You also want to know what your soils looks like when they thaw. If they're sandy and consolidated you may not have an issue if they thaw a bit.

If this is a skid, and it's got minimal heat coming out of it, can you just throw the thing on a gravel pad with some insulation and have them design any tie ins to external utilities so that they flex? Does anyone care if this thing moves around a bit?

If you can't do that, post/pile foundations are pretty typical so that the structure can be raised off the ground a few feet, giving you a thermal break between the soil and the structure.

This might also be helpful:

http://www.uaf.edu/ces/energy/housing_energy/resources/PermafrostNewFoundations.pdf

I think there's a NAVFAC or other US military guide that talks about permafrost construction but I don't have time to dig for it right now. The CMHC likely has some stuff as well

 

[KootK]

size and weight of battery pack

About 90 kips. There's a PEMB on top too but that's pretty light. Basically just a garden shed.

container size and weight?

It's about 2.5m x 2.5m x 6m and 4000 kg. There may be as many as six on a common platform side by side.

What’s the skid intended to do, and its approx. weight and dimensions?

So far, it's just expected to be a platform to support the batteries and PEMB shelter. The weight and dimensions will depend on my design. "Skid" may not have been a great way to describe this. As far as I know, it's permanent and won't be getting dragged around.

How permanent is it, and remote installation site, or otherwise?

It's permanent and will be installed in a small coastal town in Alaska.

One of the biggest things is that you have to separate any heat production associated with the structure from the permafrost soil conditions, or the soils tend to melt, lose strength and settle. Sometimes this involves adding rigid insulation under the bldg. floor; or putting down a mat of gravel some feet thick and larger than the structure in plan, then putting the structure on that mat. This includes radiant heat, from the sun, etc. off of any material, any color, which will absorb heat from the sun

Excellent point. I had not considered this. The setup will be unheated save the heat generated by the batteries. Would you still expect it to be a concern?

I would think that anything other than drilled piers would be fairly tough to accomplish in permafrost

So no screw piles or driven piles? I was leaning towards steel due to supply issues with concrete.







I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

One concern that I have is that the thermal expansion and contraction of the platform will exert lateral force on the piles. The platform will be a few feet above the ground, in part to introduce some flexibility into the piles.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[BAretired]

Gravel pads were common on the DEW line where I spent one full year just after graduating from engineering school; I was there in 1955 and 1956. The module train (a string of prefabricated buildings joined together in a continuous group) were supported on thick gravel pads. Gravel was plentiful at that site, so that made good sense. Gravel was placed on top of the vegetation without removing it.

In the early days, before I arrived on site, a gravel road had been constructed from camp to the hangar building and airstrip, a distance of over a mile. They made the mistake of removing vegetation before placing gravel for the road, not realizing that the arctic tundra acted as insulation preventing melting of the permafrost; as a result, we had to keep topping up the gravel road in summer because it kept sinking into the soft muck below.

The airstrip was made of a very thick gravel pad placed over the tundra and it was no problem so far as I know. We didn't use the airstrip in winter months because the planes were able to land on the ice much nearer the camp when it was safe to do so.

On another site where gravel was not so plentiful, timber piles were used. They were steamed and frozen in, butt end down; otherwise they would pop out of the ground. When the holes were steamed, the natural soil was mostly liquid muskeg and if thawed would support nothing at all.

Screw piles were not used at that time and I don't know how they would fare but I suspect the installation would damage the frozen subsoil on the way down; I don't know if they could be readily installed in that material and if they were installed, whether they could be relied on for heavy loads. Screw piles would have to be tested before using them in a given permafrost.

Nowadays, with climate change occurring in the arctic, it's difficult to know how long any of the current technology will apply; the permafrost is not so permanent in some parts of the arctic.



BA

 

[KootK]

Thanks so much BA. How thick would the gravel pads be beneath the strings? Would it just be steel platforms bearing directly on the gravel with no concrete in between?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Brad805]

I assume there are no geotech recommendations?

I think you need to make a few inquires as to the trades available in Alaska where the unit will be installed. Much of the foundation design has to do with what is available. Small towns do not always have many options and mob/demob costs can be significant. We have a lot of skids coming into our small Town, and they are always built using steel. The skids in our area are normally founded on screw piles or driven piles, but I do not have to deal with permafrost. Concrete is generally a last resort where a slab is required due to large equipment loads or service requirements. I believe driven piles would be an option, but you might have to pre-drill if the frost is significant. Screw piles may not be an option if the frost is thick, but a quick search of the trades in the area will give you an idea if this is a common practice.

I doubt timber would ever be considered by your client. Most large industrial companies have policies that prevent its use for an industrial site.

I assume Alaska does not have special rules for engineers practicing where there is permafrost? Both the Yukon and NWT have a special process to become registered to practice.

Here is an interesting link from the U of C

http://pubs.aina.ucalgary.ca/cpc/CPC2-58.pdf

I noticed a Permafrost map also:

http://s1109.photobucket.com/user/m4135/media/ps9_zps5c9b1bf0.jpg.html

 

[KootK]

I assume there are no geotech recommendations?

No. At this stage, the idea is to attempt to specify a generic platform and have the foundations be designed by local folks on a case by case basis. It may ultimately evolve into me designing the foundations however.

I doubt timber would ever be considered by your client.

Glad to hear it. I didn't want to have to detail my platform for timber.

Thanks for the links and the point regarding special rules for engineers. That was news to me.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[wadavis]

Will your skid be used for transporting the modules and superstructure, or will the skid be in place when the module arrives?

Some feedback that applies either way:
1) As others have commented, the foundations tend to be designed on what's available in those regions.
2) The shipping cost on wet concrete goes up fast if you are dealing with remote areas, but it may be a popular choice if you are close to a city.

As for the thermal lateral on piles, common workarounds include:
1) elevate the platform a few feet to allow for the pile/soil elasticity to take up the movement.
2) Do not weld your skid to the pile caps, attach guide shoes to the pile caps to restrain excessive movement and walking, but still allows the skid to slide along the pile caps with thermal expansion.

Structural E.I.T., Alberta

 

[KootK]

@WAdavis: the superstructure and platform will arrive together. However, the platform will not be used for transporting the module. The installations will be in high seismic regions. How would you detail the guide shoes such that seismic force could still be transmitted?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[wadavis]

The installations will be in high seismic regions. How would you detail the guide shoes such that seismic force could still be transmitted?

Sorry, this just crossed the limits of my experience. I've only done/seen this in Alberta, not really a seismic zone.

Structural E.I.T., Alberta

 

[KootK]

I suppose one could say that seismic/wind loads are resisted by a friction that would be overcome in a thermal load case.

I turns out that this bad boy will be located in Kotzebue Alaska. If anyone has a contractor relative there, put me in touch.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

Thanks to Brad's map, I can say that this is a "discontinuous permafrost" region. Not sure if that changes anyone's answers.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Brad805]

That site is quite remote, but it is within 550mi of Anchorage. I see that Arctic Foundations has done some work in Kotzebue, Alaska and they have completed projects with this scope. I noticed a lot of driven piles, and screw piles of some form. The drake company refers to slab on grade, but I didn't see where the concrete comes from. I think arctic foundations will get you headed in the right direction. Considering the seismic aspect, this might be a complicated design.

http://www.arcticfoundations.com/index.php/technologies

http://www.arcticfoundations.com/index.php/media-gallery/941-hospital-kotzebue-ak-1

http://drakeconstruction.net/capabilities/

You get some interesting projects.

 

[BAretired]

Thanks so much BA. How thick would the gravel pads be beneath the strings? Would it just be steel platforms bearing directly on the gravel with no concrete in between?

The modules were prefabricated wood which were supported on wood sleepers on a gravel pad approximately four feet thick. An air space was provided between the gravel pad and the floor of the units to allow air circulation. It was sixty years ago and my memory is not infallible, so don't quote me on that.

BA