Rigid Insulation Beneath Retaining Wall
Rigid Insulation Beneath Retaining Wall
(OP)
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
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






RE: Rigid Insulation Beneath Retaining Wall
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.
RE: Rigid Insulation Beneath Retaining Wall
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Rigid Insulation Beneath Retaining Wall
For economic reasons, I prefer to allow the retaining wall to move up and down with the surrounding soil.
BA
RE: Rigid Insulation Beneath Retaining Wall
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
RE: Rigid Insulation Beneath Retaining Wall
RE: Rigid Insulation Beneath Retaining Wall
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.
RE: Rigid Insulation Beneath Retaining Wall
@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?
RE: Rigid Insulation Beneath Retaining Wall
RE: Rigid Insulation Beneath Retaining Wall
Dik
RE: Rigid Insulation Beneath Retaining Wall
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.
RE: Rigid Insulation Beneath Retaining Wall
You have raised a new problem:This means that your retaining wall footing is bearing on fill. Maybe a pile supported retaining wall is a better option.
BA
RE: Rigid Insulation Beneath Retaining Wall
@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.
RE: Rigid Insulation Beneath Retaining Wall
RE: Rigid Insulation Beneath Retaining Wall
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.
RE: Rigid Insulation Beneath Retaining Wall
RE: Rigid Insulation Beneath Retaining Wall
BA
RE: Rigid Insulation Beneath Retaining Wall
RE: Rigid Insulation Beneath Retaining Wall
RE: Rigid Insulation Beneath Retaining Wall
Dik
RE: Rigid Insulation Beneath Retaining Wall
What is the lentgh of the wall and what is the height retained and length?
As BA said, it may be more practical to allow it to float OR move depending on the temperature if it is a relatively low wall and not attached to a structure. That may eliminate the practical rigid wall design for this application and may be more in the realm of the "geo" types to design.
A flexible wall may serve the purpose since they have a history of performance.
Dick
Engineer and international traveler interested in construction techniques, problems and proper design.
RE: Rigid Insulation Beneath Retaining Wall
@ concrete masonry: the length is about 20 m and the retained height is about 1200 mm. I've been considering it a floating wall all along.
RE: Rigid Insulation Beneath Retaining Wall
RE: Rigid Insulation Beneath Retaining Wall
RE: Rigid Insulation Beneath Retaining Wall
No foam insulation is required even at the north pole.
Dick
Engineer and international traveler interested in construction techniques, problems and proper design.
RE: Rigid Insulation Beneath Retaining Wall
Dik