Frost Protected Retaining Wall

[bookowski]

I can't find any references/examples for a frost protected footing for cantilever retaining walls. My question is whether insulation can be placed on the low side only, counting the retained soil on the backside as protecting that portion. It seems like this configuation is allowing a thermal bridge through the wall which could possibly negate my logic above, in which case would it make sense to add insulation to the back side of the wall vertically and then horizontal on the low side?

I do not have access on the back side (at property line) to do a horizontal extension which is why I'm looking for an alternate solution.

 

[KootK]

I've struggled with this too.

My question is whether insulation can be placed on the low side only, counting the retained soil on the backside as protecting that portion.

I vote yes. This is exactly how I see things. I imagine a horizontal plane of insulation at the footing level that traps heat beneath the footing. On the high side, the "insulation" is the deep soil.

For what it's worth, this is the dominant detail used, in my very cold region, for retaining walls adjacent to basement entry ramp slabs. Geotechs would seem to be implicitly endorsing it by never objecting to it when they review my foundation drawings.

in which case would it make sense to add insulation to the back side of the wall vertically and then horizontal on the low side?

I question the efficacy of this approach. But, then, that reflects my understanding of how the system works. In my view, the vertical insulation does nothing to help trap heat under the footing.

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.

 

[jayrod12]

What about insulating the exposed face of the wall all the way down to the footing on the low side. and then the horizontal protection on the low side. That would at least aid in combating the thermal bridging.

But typically we just do the same as you and Koot have outlined.

Geotechs would seem to be implicitly endorsing it by never objecting to it when they review my foundation drawings.

You're assuming they actually review your foundation drawings.

 

[bridgebuster]

I tried to get away with that two weeks ago - it's a rather unusual retaining wall project - but I was foiled by the NYC Building Code.

 

[KootK]

You're assuming they actually review your foundation drawings.

Point taken. I do this:

1) Request that service in the Geo TOR.
2) Send them my drawings and request the review.
3) Insist on getting a response of some kind.

That said, I rarely get much of a response. And it seems to come very quickly in many instances. Not much else that I can do except, I suppose, ask very pointed questions about things that concern me.

For the most part, I've found geotechnical engineers pretty unhelpful on the shallow frost protection front. They usually start of by saying that it's my domain which is bull (it depends on the properties of the soil). Then, if I can get them to say anything formally, they give me the standard "1/4" insulation replaces a foot of soil and run it 4'". That's also bull in my opinion. I've read the ASCE doc an have a hard time accepting that it's that simple. And lastly, they'll never chime on the actually insulation product specification because they're liability ninnies. They'll just give me a thermal conductivity target and remain silent with regard to durability.

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.

 

[oldestguy]

The detail does not completely protect. Loss of heat up through the ground surface of the backfill is a main factor to consider. Most tipped walls I have seen are due to backfill pushing wall out horizontally. not due to a foundation support problem. If you can use non-frost susceptible backfill.

 

[DETstru]

Is there a building involved here or just an isolated retaining wall?

 

[bookowski]

No building involved, just retaining wall. It's actually two retaining walls facing each other, both at lot lines and connected by a strap footing so ot is taken through the strap to the opposing wall. It's basically a below grade areaway/patio, three sided box. No ability to overexcavate since it's at the lot line so the wall will go against the support of excavation lagging, i.e. no drainage material behind.

I can't put insulation on the inboard face, it's board formed architectural concrete face. My thought on the vertical backside insulation was that it's better than nothing at limiting the bridge through wall face.

Bridgebuster - Foiled by nyc code or building dept? This is in nyc - are you saying there is some code provision addressing this?

I have a 4ft frost depth requirement and even with this I'll need say 2'-6" to fit the ftg and insulation so not much savings but I'm getting a lot of push to limit the soe so looking for an option to present.

 

[KootK]

On many an occasion, I've scratched my head and wished that there was a comprehensive design guide covering frost effect mitigation for structural engineers. If such a thing existed in earnest, I'd fork over a few hundred bucks for it this afternoon. There's information out there but it's scattered in numerous locations:

- the shallow frost protection system docs.
- things related to piling and ad freeze.
- arctic/permafrost engineering.
- the stuff rolling around in the heads of armies of geotechnical engineers.

And even at that, there still seems to be a lot of gaps between the theories found in the literature and what folk typically do in practice.


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.

 

[bookowski]

Interesting image here:

http://www-personal.umich.edu/~rlmich/research/projects/frost.html

http://www-personal.umich.edu/~rlmich/public/images/frost_heave.jpg

 

[KootK]

That's just what the doctor ordered. It would seem to be a good endorsement of the strategy, at least from a vertical movement standpoint.

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.

 

[bridgebuster]

bookowski - The Building Code. I have a series of retaining walls going up a hill to create a stairway; Bottom of each footing to top of each wall is 18'. I needed to minimize the footing width so I thought about a shallow footing but the geotech said it was not allowed by the BC.

Anyway, I left the walls 18' tall and came up a way to narrow my footings to 5'-6" using flowable backfill.

 

[KootK]

@oldestguy: could one expect vertical, dirt side insulation to push frost lease development back from the wall laterally and help with the tipping issue?

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.

 

[SteelPE]

I'm not sure if this is possible or not, but could you place some high compressive strength rigid insulation on top of a mud slab and then place the wall on top of the insulation?

http://www.foamular.com/assets/0/144/172/174/98cf58e1-c3d2-4b6c-beb5-2063215bea18.pdf

This way you have a level surface to place the rigid insulation. You are not holding up a building and you have another wall opposite this one to counteract the sliding and bearing stresses (which should be low).