retaining wall - frost depth

[redun]

Looking for opinions on soil cover for toe of reinforced concrete cantilever retaining wall. Some will put bottom of footing at local frost depth which is about 3 ft for my area.
For wall say over 3' high,the heel of the footing has plenty of soil cover. For higher walls there of course is alot of soil above the footing from the wall stem back.
I am wondering if the relatively small toe of the wall really needs to be that deep. I would use at least a ft of cover over the top of the footing toe and if the footing was say 10" thick,then I'd have 1'-10" instead of 3' cover.
Has there been any studies on how frost acts on retaining walls?
Given the relatively deep soil right behind the wall, wouldn'that
mitigate the formation of frost lenses to some degree directly under the footing toe?

 

[JAE]

We tend to ignore frozen soils when considering cohesion and passive pressure resistance at the toe of the walls. So to resist the lateral pressures via passive pressure, or via soil cohesion, you need to sink it deeper.

 

[redun]

JAE, Thanks for your response.
I dont count of the first foot or so of soil for passive pressure.
Because of some seasonal shrink and swelling of the soil. I dont usually count on it in front of the footing either.
But, I'm not concerned about any passive resistance, i'm just wondering if frost heave occurs or has much effect on the toe of the footing.With all the mass of soil especially with walls over 4 ft tall, would the ground freeze that close to the adjacent deep wall of soil.
If the wall is over 4 ft tall, then the soil being retained shouldn't be frozen down at the level of the footing. Now right adjacent to that deep soil backfill is the footing toe sticking out beyond say 2 ft max (for my wall). Im wondering if the deep soil backfill (in my case 6 ft.), would insulate to some degree the soil directly adjacent to it under the toe of the footing.

 

[CarlB]

A simplistic approach to the frost penetration through a retaining is that it penetrates the same horizontal distance through vertical walls as it does below the ground surface. So for this approach, no the soil depth at the back of the wall does not protect the toe. In reality the frost depth is a little less directly below the wall, but I wouldn't count on it.
If you're trying to avoid sinking the wall deeper, consider laying insulation board over the toe of footing, to out beyond the toe edge a few feet. 2" thick rigid insulation (extruded polystyrene) is thermally quivalent to more than 2 feet of soil cover.

 

[redun]

CARLB Thanks for your input. Good points on the cold transfer through the wall face and the sytrofoam protection. I actually used the styrofoam for some shallow light pole spread footings where we couldnt go deeper because of other utilities.
The poles have been up for several years and they havent tilted yet.
I probably will just put this retaining wall footing another foot down and forget about it. I am curious about the heat transfer around the wall and what effect the warmer soil back in a few feet from the stem,has on the formation of frost lenses as you get closer to the front edge of the footing. I will have to do some more research.

 

[JAE]

Keep in mind that traditional frost depths recommended for buildings is based upon heated structures (generally) and not exterior, non-heated situations.

I my neck of the woods, the typical building footing frost depth is set at 3'-6" below grade, with the actual frost depth each year around 26 inches.

For an outdoor retaining wall, the actual frost depth might actually get near the 3'-0" to 3'-6" depth.

Adding styrofoam to the toe would worry me about the lateral resistance of the soil at the toe to resist sliding forces. Seems like a bad idea for a retaining wall - even if the use of foam for a building footing is a good idea.

Just my 2 cents.