Lateral Soil Pressure with Foam Based Aggregate
Greetings GBA Community:
I am an owner/builder in Vermont looking to detail a concrete free basement floor. My community does not have code enforcement, so there is no inspector to bounce ideas off of.
My 6’8″ ICF basement will look very similar to the one outlined in this article from GBA:
https://www.greenbuildingadvisor.com/article/foamed-glass-aggregate
The differences will be that I will start with 4″ of 3/4″ crushed stone in the bottom of my excavation to level everything — so there will be crushed stone below my footer — and I will put geotextile on top of the foamed glass aggregate instead of sand to protect my vapor barrier.
Okay, here is my question. My soil is silty-sand, and I will be backfilling with sand, is there an additional concern of lateral soil pressure at the footing and the bottom of the wall since there is no slab? This didn’t even cross my mind until I read this article from GBA:
https://www.greenbuildingadvisor.com/article/geotextile-fabric-for-resisting-soil-pressure
The Section 404.1.1 of the IRC states that concrete walls shall be designed when they do not have permanent lateral support at the top or bottom. I was under the assumption that the first floor deck was enough to resist lateral soil pressures under normal circumstances. Am I way off here? Since the lateral soil pressure increases with depth, you need something to prevent the footer from sliding; I just figured that something was friction.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
This is an interesting question, and made me think of a few different things. Here's an unhelpful collection of my random thoughts:
- Concrete is great in compression, which is what a slab would experience (laterally) from the force of backfill. But, it's pretty narrow along the axis of compression, so wouldn't buckling be a concern, normally? Nonetheless, we don't typically worry about basement floor slabs buckling and don't typically add things like rebar or wire mesh to prevent it.
- Dirt floors used to be common in basements, and those foundations worked (unless other things made them fail).
- On the other hand, the Superior Walls product (which is a prefab foundation wall installed on top of gravel) requires the top side floor system completed and a basement slab poured before backfill, presumably for the reasons being contemplated.
Overall, fun question! I suspect you should have a practicing engineer weigh in. There are things you can do on the outside to minimize the lateral force from the backfill (such as layers of geotextile fabric).
All of this aside, I have to ask: you are building with insulated concrete forms, so why are you hung up about not pouring a slab in the basement?
Matt,
Codes aside, most slabs (and all of whose where the walls are ICFs) are separated from the concrete stem-walls by a layer of foam, so there is no direct contact for the slab to resist lateral forces. Like you, I've always assumed they relied on the soil and fill in between the footings to do that job.
The highest lateral pressure is definitely at the base of the wall, near the footing and foundation wall connection. Some inspectors won’t allow a foam layer between the slab floor and the foundation wall because they want direct contact between the slab and walls since the floor is a brace.
In your situation, you might be able to pour a few footings across your floor, with the cross footings connected with rebar to the footings for your exterior wall. This detail is fairly common in wide free-span commercial buildings where the support columns can “spread”. In your case it would work in reverse and keep your walls from sliding inward.
You may find this helpful:
https://www.greenbuildingadvisor.com/article/do-i-really-need-a-concrete-basement-floor
Okay, after a casual conversation with a geotechnical engineer, I am just going to go with a tried and true slab. After I walk up the learning curve of calculating lateral soil pressures, or pay someone to do it for me, I will quickly equal the increased cost of a slab. If I increase the heel of my footer, or add perpendicular footings, I am pretty quickly approaching the cost of a slab, and I am not decreasing my concrete use by much of a percentage.
I understand the benefit of reducing concrete from a cost and environmental point of view, but I think prescriptive engineering from one of the foam based aggregate companies may be necessary to help make it happen. Who is the lucky PE who gets paid for that . . . who knows! For now, I will try the foam based aggregate as my sub slab insulation; wish me luck!
Thank you for following up. I think you chose the right option. Absent clear cut prior art for details like this, it's often hard to pencil out a positive cost/benefit for the lone experimenter.
I also think that individual builders/homeowners need to be easier on themselves about eeking out every last bit of efficiency while already trying to do a good job. Meanwhile those with much larger hammers continue to swing them in the wrong direction (hat tip to something I read or heard from Dan Kolbert recently about the relative impact of individual vs. collective impact.)