IRC Table R404.1.2(8)

[SteelPE]

I have a residential project where the contractor ran into some poor soils conditions when they were excavating for the foundation. The contractor decided to place the footing for the residence on top of virgin ground vs a controlled backfill (which was a wise decision considering who I am dealing with). Now, I will have a foundation wall that is quite tall (upwards of 14'-0") but the unbalanced backfill height is only planned to be 8'-0" (see sketch... for reference, a wood framed floor will be placed at the top of the concrete wall). My question is in relation to IRC table R404.1.2(8) and the "Maximum Wall Height" and "Maximum Unbalanced Backfill Height" as outlined in this table.

As I am looking at this table, it would appear as if the wall in question is outside the purview of the IRC as the wall height is greater than 10'-0". However, if the contractor decided to backfill vs place the footing on virgin soil, then our wall height would fall within the IRC.

1) Should this wall be engineered utilizing normal engineering methods as a cantilever retaining wall(I am figuring a 6'-0" wide footing with a 12" foundation wall reinforced with #6@12" o.c.)?

or

2) Would you twist the code a bit and allow for this wall to fall under the purview of the IRC (utilizing a 2'-0" wide footing and a 10" thick foundation wall)?

 

[8thStreet]

It all about how detail and design the wall. I would rely on engineering design principals and ACI, not the IRC. If the slabs are adequately tied into the wall at the top and bottom, then the wall is pin-pin with at rest soil pressure and you can use a small footing because the wall is braced by the slabs. If the the slab are not tied to the wall, then you will have a cantilever retaining wall which will have a very large footing, and the wall is designing for active pressures since it will be allowed to move. Personally, I would tie the slabs in at the low and high level to brace the wall.

 

[SteelPE]

I do not have plans to tie the slab into the wall. That is not normally done for residential construction in this area. However, there will be a wood floor bearing on top of the wall (which is not shown). I am not sure how well we can tie the floor into the top of the wall. Depending on what you figure the at-rest pressure is of the soil and how you assume the loads will be taken through the wall, the loads at the top of the wall can get quite high quickly.

 

[phamENG]

SteelPE - I'd say your wall still falls under the purview of the IRC. I've always understood the "Wall Height" to be measured from lower grade to top of wall. So a wall height of 8 feet with 4 feet of unbalanced fill, you'd have a 4' crawl space under the house (that's 4' above exterior grade).

Otherwise, how do you have a maximum wall height of 5 feet AND unbalanced fill that's 5 feet high? It won't work.

If you're going to use those tables, I'd encourage you to look into ACI 332: Code Requirements for Residential Concrete. It's the engineering that most of those tables are based on.

 

[SteelPE]

It's definitely nebulous. Given your example, I have always looked at it as having a 8' wall with 4' of grade on the outside of the house and 4' of exposed foundation wall on the outside giving you an 8' basement height (my house is like that as they couldn't put the house any deeper without blasting ledge).

I would love to look at that set of requirements....but this decision is already a day late.