Frost depth

[COEngineeer]

What happen if you cannot achieve frost depth on retaining wall. Can you put helical piers underneath the wall to counteract lifting from the frost? How do I know how much I need?

 

[UcfSE]

Is this the same as your thread in the Earth Rentention Engineering forum? thread255-167968. If it is you don't need to ask again. Please explain.

 

[COEngineeer]

I just want to ask general question if you can go around the frost depth for concrete structure.

 

[MotorCity]

Unless prohibited by local regulations, adding sufficient rigid insulation can decrease the foundation depth due to frost effects.

 

[civilperson]

Retaining walls that are free standing can survive and function after experiencing frost heave and settlement. Just like the top is expected to rotate for active pressure, the slight movement of the wall up and then back down can be predicted and tolerated with sufficient batter and integral strength along the length of the wall.

 

[COEngineeer]

So if I have a wall retaining 4 ft of soil with walkway on the top of it, its ok to use cantilever retaining wall with only 1 ft burried at the toe? (frost depth is 4')

 

[cooperDBM]

Frost heave won't occur evenly along the length of the wall and the wall won't necessary return to it's starting position. After a couple of years, or more, your wall may be a bit bent out of shape and cracked. If you're trying to support a walkway, which you probably don't want to heave, then you'd better provide proper frost protection.

DBM

 

[youngstructural]

I would humbly suggest that you follow Motorcity's advice and use rigid insulation. It does not suffice to use piles to penetrate down below the frost layer, unless you specifically design your wall to act as a beam being forced up by the frost heave on its underside, with the piles (and pile to retaining wall connections) designed to support the uplift applied by having to hold the wall down.

Also, bear in mind that when specifying insulation it must be a bearing type insulation, AND that you must insulate in order to acheive a "thermal path" not less than the depth of the frost line. And a good rule of thumb for the thickness of insulation required is 1" = 1' of soil cover. Thus, since in your case you have a frost depth of 4', with your footing having 1' of soil cover, you need to provide min 3" of SM rigid structural load bearing insulation that extends out 3' from the face of your footing. Also remember than your retaining wall should not be considered capable of protecting the founding soii from heave. That means that you need to extend the insulation back into the soil being retained or up the inside face (which is more typical) of the retaining wall in order to ensure a 4' thermal path.

Two products come to mind immediately: DOW Chemical's Styrofoam Highload series and Owens Corning's Celfort series (I believe this is the Canadian trade name, and that OC has another trade name for the product in the US). Remember that all of these products must be protected from any medium and long term exposure to sunlight, since UV rays degrade them considerably. Typical details of how to go about using them are available both from DOW and Owens Corning.

I can draw a sketch and post it if you like, as I have done designs like this numerous times.

Hope that helps,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[CarlB]

I agree with youngstructural's approach - assuming soils are such that frot heaving is possible. In addition to insulation, some thickness of non-frost susceptible (NFS) material (such as clean sandy gravel) should be provided below the base, and behind the wall. Even with insulation the freezing front will penetrate a small distance beyond insulation. As a practical matter insulation can be placed over the toe rather than under it. Placing insulation on the wall (usually against the soil) is also recommended, since frost penetration though the wall can freeze the backfill and cause lateral forces/movement.

 

[StructuralEd]

If it's only 4' tall, or so, why not take it out and do it according to code and protect yourself from liability claims that may ensue?

Don't underestimate the effects of frost. I have seen the framing of a 2 story building raise about 3" when the basement area collected water during winter construction.

 

[COEngineeer]

Cant go to frost depth because it is too close to the neighbor. I ended up doing funky design with helical piers.

 

[StructuralEd]

In the interest of mutual improvement, we'd generally approach the neighbor for access.

 

[youngstructural]

Morning all.... (It's 7am here in sunny Christchurch New Zealand)...

I can understand the COEngineer's client not wanting to approach the neightbour. Not everyone has people adjacent to them that you can approach, unfortunate as that may be. I've seen one (admittedly extreeme) situation where approaching the neighbour caused unbelievable head-aches. The neighbours employed a surveyor to monitor and guarentee that no "transgretions" (sp?) occured... Unbelievably difficult people.

I agree with CarlB regarding placing the foam above the footing... That typically saved you from having to use one of the higher load bearing grades. I would, however, point out that the need to insulate the back end is very sensitive in this case, as you need to ensure that there is a sufficiently long thermal path. All while bearing in mind that you can't assume the concrete to be providing any protection. What I'm getting at is that penetration around your insulation and through the footing to wall interface is an issue.

Regards,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...