Rigid Insulation Beneath Retaining Wall 7

[KootK]

My client is adamant about having rigid insulation beneath the footing of an exterior retaining wall that I'm designing. The footing of the retaining wall will be built at an elevation above frost depth. I have two questions about this system:

1) My client believes that the rigid insulation will provide protection against frost heave, both wholesale and differential. Can anyone explain to me why rigid insulation so placed would be useful as protection against frost heave? I'm sceptical.

2) I'm worried about the insulations ability to transfer shear from the footing into the soil. Anybody know where I could find friction coefficients for concrete in contact with insulation or insulation in contact with soil?

Thanks,

KootK

 

[flight7]

I take it this is a wall that is not attached to a building. If so, then then the insulation is doing nothing except separating the footing from the heat conducting up from the center of the earth. I would guess that, theoretically, this would make the frost depth a little more shallow, and would be detrimental. (In reality, the effect is probably miniscule as there is not much heat and it migrates around the sides of the footing.)

I can't help with the friction coefficients. I would be more concerned about the elastic modulus of the insulation. If the higher bearing pressure side of the footing deflects, you'll get some rotation of the wall.

 

[msquared48]

Other than the cost of extra excavation and more concrete, is there a problem with just taking the footing to frost level?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[BAretired]

In my locale, frost can penetrate six to eight feet below grade in an open area. Insulation below the footings can prevent frost penetration, but it must extend laterally in all directions so that the frost cannot enter from the sides.

For economic reasons, I prefer to allow the retaining wall to move up and down with the surrounding soil.

BA

 

[dik]

There are some high compressive strength extruded polystyrene insulations that can work. Problem is 'creep' due to the sustained loading. If you can live with that, then put a bit of a batter on the wall towards the soil side so that if or when the wall rotates, it approaches vertical and doesn't appear to be tipping over.

Can you place the insulation on top of the footing? If frost penetration is 8', then you may be looking at 4" or more of insulation... compresses a bit more, also a better floatation device <G>... Also, is that the max depth of penetration... in Winnipeg, we typically use 6'... when we did the alterations to a shopping centre here about 20 years ago, we found frost at 13'... and in Lindsay, Ontario, the frost is typically 4', and we found frost on one project at nearly 9'... these were in roadway areas that are cleared of snow...

Dik

 

[KootK]

@Flight: yes, this wall is not attached to a building. Excellent point regarding the deflection. Fortunately, the wall is fairly small. Bearing and deflection ought not be a problem.

 

[KootK]

@MS^2: the wall is to be installed above an area dense with shallow, underground utilities. Going deep inevitably gets us into micropiles and other more costly solutions.

In this situation, what is the mechanism by which frost action would be prevented by insulation extending laterally? Are we trapping underground heat and preventing frost development in the vicinity of the footing? That's the only thing that I can think of.

If trapping underground heat IS the goal, then the area over which insulation will be required must be pretty extensive, right? I've had some experience with shallow frost protected footings which were part of heated buildings. That I get. I'm just having difficulty grasping the concept as it applies to unheated structures.

 

[KootK]

@BA: the last two paragraphs of my response to MS^2 were actually intended for you. I'm in your neck of the woods.

@Dik: Placing the insulation on top of the footing would be a big improvement and would be consistent with my understanding of how things work. My client sees things differently however. They feel that the rigid insulation is sacrificial. The argument is that frost lenses will form locally and thus heaving will occur locally. The hope is that the insulation will crush in these local areas and wall heaving will be prevented. I have two problems with this notion:

1) If the rigid insulation has enough capacity to serve as a bearing substrate for the retaining wall, it's hard for me to imagine it crushing without lifting the wall and;

2) If the insulation does crush locally, doesn't that mean that the wall will be essentially unsupported for significant lengths once the frost dissipates?

@All: thanks for the input so far. I take it no one knows where to get mu values for insulation on concrete / soil?

 

[KootK]

@dik: you mentioned that the insulation might have to be 4" thick. How does one go about calculating the thickness?

 

[dik]

Explain that the entire footing is cold with the insulation on the bottom, where if the insulation is placed on top, only the cold 'wall fin' transmits cold to the footing, a much smaller area and greatly reduced effect.

Dik

 

[SkiisAndBikes]

For a retaining wall, I would agree with dik and place the insulation on top of the footing. For insulation thickness, we generally use the rule of thumb that 1" of rigid insulation is equivalent to 1'-0" of soil, however, I am in an area of only 4' to 5' of frost depth.

The insulation needs to extend beyond the edge of the footings. I was taught to draw a line along the path heat must escape, i.e. draw a line from the bottom of your footing horizontally to the edge of the insulation, then vertically up to finished grade. This line, at a minimum, should be equivalent to the local depth of frost penetration.

 

[BAretired]

KootK,

You have raised a new problem:

the wall is to be installed above an area dense with shallow, underground utilities. Going deep inevitably gets us into micropiles and other more costly solutions.

This means that your retaining wall footing is bearing on fill. Maybe a pile supported retaining wall is a better option.


BA

 

[KootK]

@skis: thanks for the info on thickness calculation.

@BA: our wall loads will be very light in this situation. Our geotechnical recommendations account for the fact that we'll likely be bearing on fill and/or underground utilities. Good point though.

 

[KootK]

Another concern that I have is the durability of the rigid insulation. If it serves as the bearing stratum, do we not worry about it potentially being washed away / eroded?

 

[SkiisAndBikes]

From memory, I believe the standard for insulation in Canada is CAN CGSB 51.20 - Type 4. There are many types of rigid insulation that are completely unsuitable for below grade insulation (white bead board for example).

There are several companies that manufacture suitable products, like Dow or Celfort, that can provide technical support related to their products and provide answers to your questions with test results and case histories.

 

[KootK]

Does anybody feel comfortable with transfering shear through a rigid insulation substrate?

 

[BAretired]

Only if the coefficient of friction between concrete and wet insulation is adequate and the shear resistance of the insulation is adequate to provide the required resistance. These properties must be available to make an informed decision.

BA

 

[dik]

FYI...

 

[dik]

Ditto...

 

[dik]

Ditto... ditto....

Dik