Crushable void form under frost suscepyible foundations 1

[canwesteng]

I received a possible recommendation for frost susceptible foundations from Geotech to put crushable void form below the foundation. I wouldn't do this for building foundations, but I'm thinking for fuel tank foundations for example, where I don't care if it stays level, I just need the concrete to act as a raft and maintain containment. Has anyone seen this before? I assume the slab will slowly sink into the ground as the void form creeps, but perhaps not an issue.

 

[dik]

Use crushable under structured slab only (and not often)... if load bearing I've often used a high density extruded polystyrene... manufactured by DOW and others as long as movement can be tolerated.
Some outfits make a foam void form that has low crushing strength... I used to use aerofoam 'skins' way back with cardboard voids under slabs only. I've done a couple of reports where the cardboard ones under beams have 'disappeared' and the void has filled with water causing heaving. Way back on one northern project used 'beer boxes' under the slab with masonite and poly over to create the void. I've used DOW Hi-60 lots of times.

https://www.dupont.com/content/dam/...ighload-40-60-and-100-pis-43-D100079-enNA.pdf

From my project notes:

PROVIDE 6" DYNAVOID BY BEAVER PLASTICS, OR APP'D EQ, UNDER ALL SUSPENDED SLABS ON GROUND SUBJECT TO FREEZING U/N. CARDBOARD VOIDFORM SHALL NOT BE USED

PROVIDE 6" FROST CUSHION BY BEAVER PLASTICS, OR APP'D EQ UNDER ALL GRADE BEAMS U/N. CARDBOARD VOIDFORM SHALL NOT BE USED


Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Enable]

dik and I had a discussion about using them some months back. He used them with great success in Western Canada, and I hate them, and will never use them again. I prefer compressible insulation when building structural slabs-on-grade.

But the slab shouldn't sink into the ground at all, really. The slab should be designed as suspended and as spanning over the void forms. You might not have the DL deflections occur before your put your equipment in-place (if the void forms don't collapse before then) but that's the only additional issue to contend with. And if they collapse before the equipment goes on, then the slab has already deflected under DL and all is well.

 

[dik]

Design it as a SOG with high density insulation under to minimise heave... extending the insulation 6' or so beyond the edge... as long as movement can be tolerated. I would never construct a SOG on collapsible void.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Brad805]

If it is a suspended slab design, then I follow using void form, but as described I do not understand the goal of this. One could use Rigid SM or HI40/60/100 if you want. The Canadian Foundation manual has a chart for the insulation extension Dik describes. but unless there is a heat source I am not sure what one achieves with that. We have used insulation under building floating slabs.

We use Plastispan and I have had good luck with their tech support Link

 

[dik]

the geothermal heat keeps the soil from freezing... soil is not frozen under a stack of hay bales... Even an unheated building over minimises frost penetration under a structure.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dik]

DOW does also...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dik]

If structured, it has to be compressible.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Enable]

I prefer compressible vs void forms (per our previous discussion). I wasn't referring to compressible vs XPS.

If this is a true SOG, I concur that rigid insulation of sufficient thickness (approx. 1" / ft of frost depth) in the field + extended the frost depth distance beyond the edges (or placed vertically at SOG perimeter) is the way to go. Greater load-rated insulation can be purchased to minimize long-term creep if desired as well (e.g. HL100 creeps less than HL40 IIRC).

 

[Brad805]

Dik, the amount of insulation I have seen used in practice is not similar to a hay bale, but if you are using thicker amounts, then I agree.

 

[dik]

Brad... see Enable's comment... and a hay stack provides the same effect... it's only bigger.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[canwesteng]

I'm thinking of something (or the Geotech was thinking of something) quite unique actually. It would be something like the Dynavoid, not cardboard void form, but it would support the slab permanently. If any heave occurs, the void form is much weaker and crushes before the concrete breaks. Relatively weaker insulation would do the same thing, which I suppose say 4" of highload 40 may be able to crush 2" before causing issues with the concrete, as long as the concrete can take localized stress from a 40 psi point load.

 

[dik]

You wouldn't use highload for any type of void form... way too strong... you have to realise that 40psi is a whole bunch of psf...+

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[canwesteng]

Sure, but this isn't really meant to be a void form. It's meant to be a crushable slab support, so that in the case of heave the insulation gives way before the concrete.

 

[geotechguy1]

I'm curious what you mean by 'below the foundation'.

I have recommended void form in the past in Alberta, Canada, but the application is beneath a structural slab to prevent large uplift forces being transferred to the slab / foundation. Alternatively a slab-on-grade would be used which is isolated from the foundation elements and allowed to float somewhat, typically with structural details to allow some movement of the ground floor slab on grade without putting loads on the floor above.

Footings are placed either below the frost depth or, (much more rarely because engineers and owners don't seem to trust insulation), with an insulation system extending out from the building to prevent frost penetration.

 

[canwesteng]

This is for the case where we have containement or other slabs on grade where large cracks or other damage due to frost heave is not acceptable. Visualize an 8" slab, on void form. Not a suspended slab, but you could call it structural. It would behave like a raft ideally. This isn't my idea, it comes from the geotech report. I could ask the geotech about it, but they are pretty clear in the report with their intent.

 

[dik]

that's where some confusion is coming into this tread. To me, a void form is something that is intended in not applying a load to the slab or of providing any support. This has to be a low strength plastic 'foam' product or treated cardboard. To me, rigid high density insulation is intended to transfer load, just like compacted granular base. It's purpose is to provide insulation in much the same manner as several feet of native soil. I've used this many times to reduce the founding depth of footings. Hope this clears up, where I'm coming from.


This would be an example of the second type where the insulation is used to transfer load to the sub-base and to provide insulation. I've done numerous buildings in Ontario using this approach. I refer to these generally as stiffened slab foundations. I've also used insulation for strip and spread footings.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dik]

With details similar to...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[geotechguy1]

I agree with dik.

What you're describing sounds like a misunderstanding by someone either writing or reading the report. For the situation you were describing insulation beneath the slab would probably work (but it wouldn't be a void form, it would be insulation) but would need to be below the slab and also extending out from the building footprint a certain distance. The mechanism would be preventing the ground beneath the building from freezing.

Curious where you are practicing - geotech engineers in some areas (eg. western canada) are frequently alarmingly clueless about how structures work.

 

[dik]

I've been lucky... I've found them to be pretty good.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik