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.

 

[geotechguy1]

All depends on the engineer and the company I suppose

 

[Liam Nesson]

I would definitely be questioning the geotech. What if a large section of soil heaves and voidform starts collapsing, will slab weight redistribute and collapse all the void form, then you have a slab sitting on colapsed voidform. Or maybe a small section would heave, collapse void form locally, then soil might thaw and settle to create a void under the slab.

A 5 minute call with the geotech should give you an idea if he knows what he is talking about.

 

[Brad805]

I do not quite follow the intent of this dynamic void product. If you have localized regions where you are concerned about heaving it would seem to work, but if you have non-uniform heaving of the complete slab it seems I end up with odd bearing for my slab that is going to shift vertically unpredictably. I would use the traditional approach of insulation or provide piles with a normal void form that will collapse before heaving occurs. We work in areas where the degree days is greater than 1600, so we find many times piles are more cost effective if you have any areas around the structure where you will have heavy equipment.

We have seen two scenarios where we end up with problematic geotechs. One is enviro guys that do geotech reports as a sideline, and the second are the clients that go solely with the low bid. In the case of lowest cost reports we have seen some geotechs from the lower mainland that are not very aware of how to deal with clays and frost. The low cost guys have a stock report they use. Send out whatever tech they can to log the holes (some seem to allow the driller). Do some lab work, and crank out the same old report. Its all about getting the field work for many as that is where you make the $$.

 

[dik]

Void form purpose is to provide temporary support until the concrete slab hardens... after that it has no purpose. The concrete reinforcing supports the load and spans to whatever support you have. You do not want the void form to be able to transfer loads from heaving soil to the slab and other than supporting the plastic concrete, you do not want it to support loads. I've used this product for years and find it excellent.

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

-Dik