Crushed Stone Footing For Residential Concrete Foundation Wall

[ForestStructural]

I am a structural engineer in Boston and I have a client who didn't follow my Shallow Frost Protected Foundation Wall detail closely and now only has about 6" along the perimeter to embed the downturned slab at the perimeter. They did, however, add approx 18" of compacted crushed stone which is acceptable as a footing for precast foundation walls and wood foundation walls in IRC. The use of compacted crushed stone is not specifically added as a footing type for concrete wall which leads me to conclude that it's not an allowable footing for concrete. Having said that, I have found a lot of anecdotal stories where people have poured walls on gravel footings and been fine. We have a single family 1.5 story home so I'm not overly concerned about the weight along the wall but the inspector (correctly) flagged the insufficient soil cover. If I consider the gravel as the footing then the soil outside of the gravel can be considered as coverage and there's no issue. I should also note that there is ledge below the soil which is the reason why the contractor didn't go 12" below grade everywhere...some areas were higher than others. Can compacted crushed stone be considered a footing for a CONCRETE foundation wall? If you have a code or material guideline reference in support it would be greatly appreciated!

 

[Enable]

I'm a tad confused since there is no footing in insulated slab-on-grade construction (regardless of sub-base or surrounding soil materials). But if you mean to ask if granular materials may be used as a sub-base for an insulated slab-on-grade home the answer is most assuredly yes. We do so all the time for both homes as well as for concrete pavements.

Canada Mortgage and Housing Corp (CMHC) SOG Foundation Guide (link)

Ontario Provincial Standard Drawing for Concrete Pavements (link)

 

[ForestStructural]

IRC R403.4.1 specifically states that crushed stone may be used as a FOOTING, not just a sub grade. I found the same reference in regards to wood foundation walls. I’m wondering why they would allow a crushed stone footing for precast walls but not poured walls.

 

[Enable]

IRC is not my code but a footing in the context of insulated slab-on-grade construction is a new one for me. Even if your entire base was concrete it wouldn't be a footing, but we use concrete for footings all the time, right? It's not about the material.

But I see what you're trying to do now. You're trying to make definitions of things fit the IRC mold so you can satisfy the building inspector. Since that's a local code issue I got nothing for you. But as the engineer of record you should be able to deem it acceptable and move on. Worst case add some additional perimeter insulation to make up the depth (1" / foot in XPS roughly).

 

[emmgjld]

As one who deals with frost heave, as well as insulation issues, I ask:
What are the shallow ground water issues?
What are the site drainage conditions?
Can shallow or deeper soil moisture continually migrate beneath the slabs or foundation elements and freeze?
Will the "ledge below the soil" trap/hinder water drainage, creating a perched water which contributes to frost heave?

If the issue is insulation & insulation only, then the "compacted crushed stone" may satisfy the issue.
If true frost heave is part of the issue, then the above questions need to be answered. If the 'ledge', combined with poor surface drainage creates a real potential for frost heave, then the construction may not be proper.

In the mountains of Colorado, Assuming the above questions have been properly answered, I have used free draining 'non-wicking gravels to provide both support, insulation and protection from frost heaving for over 45 years.

 

[ForestStructural]

Thank you emmgjld, I think the inspectors concern is just satisfying the requirement for 12” of soil cover.

The lot is raised up about 1’ higher than the surrounding soil so it’s almost like the slab on grade is on a perch. With 18” of compacted stone I can’t imagine water accumulating and freezing due to its elevation.