Frost protection - void form thickness design 1

[monopoly]

Hi all, this may be old to someone but i need your input on frost heave design for my foundation.
I'm designing an unheated structural slab supported on piles. Design depth of frost penetration is 2700mm from soil report. I'm going to use Dow ethaform void form but i dont know how to calculate the thickness of void form to reduce uplift on my foundation. Let say, for frost heave uplift of 100 kPa, from soil report, if using 100mm thickness void form, what is the pressure underneath of foundation and if using 200mm thickness void form, how better i will have ???
Anyone has design guide or manual or any detail reference, it would be appreciate.
Thank you

 

[CarlB]

Don't know of any manual offhand; does the manufacturer provide any guidance?

Uplift presures, and total movement, are difficult to predict with any degree of accuracy. Ask the geotech what total amount of frost heave to expect (It's unlikely they will stick their neck out, or may say 0" to 12"). If you use a 100mm thick form and heave is more than that, structure may be subject to the same pressure as if no form was used. You might consider the use of non-frost susceptible (NFS) material under the slab, and board insulation in or below that NFS material. By reducing the frost penetration into the underling frost-susceptible soil, the frost heave amount will be reduced.

If NFS soil & insulation is prohibitive, I'd suggest using the 200mm void form, it will decrease the risk of damaging uplift.

 

[kslee1000]

Instead of design for the heaving, it may be better to simply reduces the potential of ice forming beneath the unheated slab by improving subgrade to avoid capillary water raising to the ground level. Have a look on the link below to gain some ideas.

 

[kslee1000]

Sorry, here is the link.

http://www.pavement.com/Concrete_Pavement/Technical/Fundamentals/Frost.asp

 

[JAE]

Frost heave is about 10% of total frozen soil depth, as a maximum.

kslee1000, capillary water raising to the narrow strip of soil at the ground level isn't the big issue. If soil is wet, it will freeze, it will expand across its full depth of freezing. With a fully designed structural slab, I wouldn't count on eliminating the water from beneath the slab. A void is the proper thing to do.

 

[kslee1000]

JAE:

I said: " Instead of design for the heaving, it may be better to simply reduces the potential of ice forming beneath the unheated slab by improving subgrade to avoid capillary water raising to the ground level."

Yours: "capillary water raising to the narrow strip of soil at the ground level [red]isn't the big issue[/red]. If soil is wet, it will freeze, it will expand across its full depth of freezing."

What is the issue? The issue is volume expansion due to H2o cgange state from fluid to solid BY FREEZING TEMPERATURE penetration downward. The expansion causes heave that pushes the structure above.

I don't know where is your point different from mine. The reason to avoid ground water rasising to the ground - so the soil won't contain water required for ice forming. It is done by proper subgrade preparation. Once this (freezing) source is eliminated, then the structure wouldn't experience heave force.

I don't want to argue which method (void form or soil improvent) is more effective or desirable. I have been dealing with more drastical measures - design/specify theomo blanket for out door equipment, however, the properties were well tested/documented, thus design was a piece of cake. For the OP's situation - lack of design information, I think it is proper to point out there is other way to handle this type of situation, especially the temperature influence zone is quite deep (8'-9'). He is better off the bring the perimeter foundation directly below frost line though. Then improve and properly prepare the subgrade under slab (are you suggesting to skip this step in constructing any sob?). If there is extra budget, sure, add void form as insurance.

 

[JAE]

kslee1000,

My point was that spending lots of money on a structural slab, then suggesting that using a granular capillary break would be a safe way to deal with frost wasn't a good idea. Yes you can minimize ground water in the top few inches with granular material. You could also put in 6 feet of granular material but that would be very costly.

A simple void under the slab is the best, and most sure, way to avoid frost heave on a structural slab.

You state that "Once this (freezing) source is eliminated, then the structure wouldn't experience heave force." isn't accurate in that you can never be sure that the water is eliminated. A capillary layer of granular material, or 6 feet of it, won't do a dang thing for you if the water table is high.

Your own link even states: [blue]Eliminating the supply of water to the soil below the pavement is virtually impossible.[/blue].

I'm not arguing with your whole point - just suggesting that eliminating water isn't a very sure way to go. Especially when the original post stated that it had almost 9 feet of frost depth to contend with.

 

[oldestguy]

Things for Monoply to think about. As implied above, but not too clearly, is that using a plastic foam material to absorb some soil heave also insulates and that alone reduces the frost formation effect.

Nothing has been said about how the "depth of freezing" was figured. Too often these are cookbook standards locally with no thought about the soil type or presence of ground water.

Also remember that sandy soil has low frost heave potential per foot depth, due to low moisture content, Yet, depth of freezing in sand goes much deeper in sand than with clay soil. Matter of fact due to the lack of a "frost susceptible fraction" heaving or production of frost lenses is nil regardless of the water table location.

Also, so the slab heaves some, what is the reason for being so fussy about that? What is the use of the building? An unheated building usually is not that fussy a use.

I'd just underlay the slab with 3 inches of the pink stuff (closed cell foam) and be done with it.

 

[kslee1000]

In my previous post:

"I don't want to argue which method (void form or soil improvent) is more effective or desirable."

I the end, I was to point out the fact that subgrade preparation is one of the major step/consideration in the design/construction of the slab-on-grade. With proper design, freezing of the immediate soil layer can be avoid.

In case of the void form, on top of placing under the slab, I one time was asked to provide it vertically against footing to the bottom of footing stem, which was located at the frost line. The reason was - the concrete will pass the cold temperature from out door to indoor, thus causes adjacent soil to freeze if left unprotect. Well, I didn't argue then.

 

[JAE]

kslee1000, I don't disagree with you that taking steps to minimize soil heave isn't a possible thing to do. I agree. It helps.

Your first post said, "instead" of designing for heaving, one should improve the soil. Instead means "do this INSTEAD of that". I took it as either/or.

You also said that you can eliminate water from beneath the slab and you implied that doing so would be a better way to go.

All of your posts are telling Monopoly to NOT design a void under the slab and INSTEAD minimize water. Your own link states that you CANNOT for sure get rid of the water.

I was just pointing that out.

 

[kslee1000]

"Dot not design for heave" - Isn't this true? What is the heave force to consider? Mind you, I have no clue, and never done that.

Instead of losing hairs in chasing some material without clear explanation and instruction/documentation, I was pointing out more estabilished way to handle this situation. There are many studies on improving subgrade to avoid/eliminate potential for freezing and heaving, I wasn't the only one believed that, I think his geotech would be able to provide some insight and direction on this.

Maybe I am ignorant by not having the material (void form)sheet in front of me, then, spent that much time in arguing, couldn't you simple point out how to figure out the thickness the OP's requires to satisfy his situation?

 

[kslee1000]

Or, easiest way, agree to oldestguy - "I'd just underlay the slab with 3 inches of the pink stuff (closed cell foam) and be done with it."

I wouldn't argue with it, since I DON't KNOW.

 

[kslee1000]

JAE:

Please do not take above as attack. I knew you are a well rounded person/engineer, not to argue that.

I admit I wasn't a great writer, sometimes fingers faster than my thought, and laze in correcting myself (for minor things) after the fact is out (submit & posted).

I answer a lot of questions, not say I am an expert on everthing I was talking about, but rather have similar experience on them with some personal thinkins (right or wrong). In any way, I enjoy the environment of this forum, except name calling and focused attack. Let's keep the sprit of this forum - free to express one's opinion and discuss the matters (not person) in a civilized manner.

If you simply stated "I don't think soil improvement would achieve..", I would have stopped there without those back and forth afterward.

We won't befriend, but why not leave some space even there is no mutual respect.

 

[aeoliantexan]

Predicting the magnitude of frost heave is complicated and uncertain. Your geotech may be able to make a conservative estimate. Some additional issues to consider include:
1. Be sure the void space is well drained so it won't be full of ice when the subgrade heaves.
2. The freezing ground may grip the piles and try to heave them. The adhesion could be pretty strong. Special coatings or sleeves may be needed.
3. Insulation underlain by nonfrost susceptible fill can be designed to eliminate frost heave. The approach is to keep the frost penetration within the nonfrost susceptible material. I can't find the design reference I have used, but a Google book search indicates it can be found in "Frozen Ground Engineering" by Orlando Andersland. You will need to extend the insulation beyond the building or run it vertically to some depth around the perimeter to prevent heave of the perimeter. This approach can also resolve the pile heave concern.

 

[monopoly]

Thank you for all input. I have used Geospan void form by Plasti-fab. You can find the product specification and design manual in their web site.

Void form can be used but piles have to be designed to take all loads plus uplift forces which is transfered from void form to foundation due to frost heave.

Insulation + frost stable fill can be used to eliminate frost heave (like aeoliantexan said). Piles are designed just take only loads, no uplift forces on foudantion.

 

[bobbybeaver]

Determining the necessary thickness of any foam plastic "solid void" material (yes, I know, that's contradictory)to absorb subgrade uplift is a difficult calculation. Stress climbs rapidly with strain, so the thickness required for even a small amoount of freezing or clay swelling can be surprising, depending on the net retraint of the slab and pile structure. Using the calculators at beaver plastics/DYNAFLEX can provide some answers.

 

[bobbybeaver]

I was going to add to the above: Yes I work for Beaver Plastics. No this is not necessarily a commercial message. I like this stuff, and that's the reason why I worked with geotechnical engineers to develop the calculators. Once you have determined the maximum stress that your slab/piles can tolerate, then you can pick and choose what kind of "void" material would serve best.

 

[dik]

My $.02…

Coming from an area where we have the occasional cold day in winter, coupled with a highly plastic clay having occasional silt pockets, we use void forming quite extensively.

We typically use 6” thick voidform under gradebeams; the gradebeams, in turn, are generally supported on friction piles. I typically spec a product called Frostcushion manufactured by Beaver Plastics. I also use a product called GeoSpan and GeoVoid, the former providing greater support. I have not used the cardboard voidform for gradebeams for over 40 years; I’ve done reports where the cardboard voidform has collapsed and the void filled with water and frozen and lifted the gradebeam from the pile. Care must be selected in the choice of voidform used. Geospan, for example, has a higher compression strength than Geovoid. The latter is used for structured slabs. Occasionally structured slabs are founded on loose compacted fill with the intent that the clay beneath the slab desiccates and creates the void.

As noted frost action comes from freezing of water within voids in the soil. Of a bigger concern is heaving that occurs from the creation of ‘ice lenses’ caused by the occasional inclusion of silt. For this latter mechanism, freezing causes a negative pore pressure that attracts water to the site of freezing. Silts are sufficiently fine grained that the flow through them is possible. The clays are sufficiently fine grained that little or no water passes through them. Freezing is less of a problem with these; it is possible to have water in a liquid state that is super-cooled at -40 degrees. They can, however, still freeze.

Liquid can freeze adjacent to the piles, and these can be lifted. We refer to the action as adfreezing or ‘frost jacking’.

Depth of frost penetration varies and in our local, it is common to use 6’ as the frost penetration. Frost was found in a parking lot at one of the projects at a depth of 13’. Frost in cleared areas can penetrate to greater depths because the consolidation of the soil minimises the air voids which fill with water as well as the areas being cleared of snow cover. In another area where I worked, the typical frost penetration was 4’, but, ice was found at the 7’ level.

Dik

 

[bobbybeaver]

Re Dik's 2 cent message
I just have to know - what city did you discover the 13 feet of Frost???

 

[dik]

It was at Polo Park Shopping Centre in Winnipeg... deepest I've ever encountered and not common... but, had all the right elements. Even in Lindsay, Ontario the 7' was excessive... they usually consider 4'...

Dik