Frost-Protected Foundation at Landscape Wall

[Bisbee_Structural]

I'm designing a small retaining wall in the backyard of an attached rowhouse (neighbors on both sides, shared party walls)

I've attached a sketch of the detail.

The retaining wall surrounds a lowered patio and also supports two columns for the deck above.

The frost depth is 4 ft., and I want to keep the total depth of excavation 4 ft. or less to avoid issues at the property lines.

The patio side of the retaining wall is 26 in. below the other, and therefore on this side of the wall the footing is too shallow to meet the frost depth requirement.

In order to avoid digging the footing an additional 4 ft. below the patio, I am proposing to use frost-protection per ASCE-32 to protect the footing foundation, based on the design of an unheated foundation in ASCE-32.

My questions are:

1) Has anyone used this approach for a footing like this, that isn't part of a building foundation? It's not explicitly discussed in ASCE-32 but I think the application is reasonable.

2) I have 2" of XPS insulation running up the back of the wall. The intent of this is to address the cold bridge that happens through the bottom of the stem, because without this insulation heat would have a path to escape from the soil through the stem to the outside air. Does this seem logical?

3) p. 9 of ASCE-32 says "Outside the foundation perimeter, the insulation shall have a minimum of 10 inches of soil cover". Based on this I have provided 10" of soil between the slab and the rigid insulation, but there is no soil between the slab and the top of the concrete footing. Do I need to have 10" of soil between the slab and footing at this location? The slab and footing have a combined thickness of 14 in. so I think there is some protection provided from this concrete thickness, plus the insulation is thicker than required (2 in. provided vs. 1.5" required). So I am thinking this is OK but I would appreciate any input on this.

Thanks in advance!

 

[CarlB]

Your scheme looks reasonable to me.

In my locale (10' max frost depth, deck footings don't need to go below frost line; it's recognized to be not practical and decks can tolerate some movement.
Responses:
1-Yes, for retaining walls, pipe supports, planters
2-Is logical. Suggest 6" gravel behind it as well, since frost will penetrate insulation some, better that it's non-frost susceptible
3-I agree the concrete thickness of >10" plus 1/2" extra insulation exceeds 10" soil thermal resistance.

 

[Bisbee_Structural]

Thanks for the quick reply.

In my experience the authorities in my city are careful to ensure deck foundations are frost protected, plus this is a steel deck structure (for fire reasons), and I don't think it's going to be as flexible/forgiving as a wood deck if there's foundation movement. Plus, if the frost depth is only 4 ft. so I guess there's less of an issue digging down that far vs. 10 ft. where you are!