Concrete slabs resist lateral soil pressure that acts on foundation walls, which is critical to their stability. Backfilling soil against a foundation causes downward force of the soil, which becomes a lateral force pushing in on the walls. In short, slabs keep foundations walls in place. So, what if there is no slab? How is that pressure handled? That was the problem facing the team building this project in Upstate New York.
The ICCF Perfect Block foundation system they used does not include a conventional slab; foamed glass aggregate was used instead. According to Project Manager Ben Bogie, roughly 250 lb. per sq. ft. of lateral force is pushing inward at the base of the foundation, where the greatest earth pressure occurs (it decreases closer to grade). That pressure needed to be addressed to ensure the foundation’s structural integrity. The engineer’s original solution was to pour concrete beams across the entire foundation, connecting the footings so they would push against each other. However, that idea ignored a primary objective of the project, which is to reduce concrete. “It would have required nearly the same amount of concrete as a slab for this house—without the slab,” Ben explains.
The answer came in the form of StrataGrid geotextile fabric, which Ben describes as similar to chicken wire with its lattice web but made of 1/4-in.-diameter plastic; Tensar Geogrid products would work for this application too. It gets fastened to the top of the foundation footing with a termination bar and a fastener schedule from the engineer—in this case, every 8 in. The downward force of the backfill soil clamps the termination bar and fabric in place. “The more downward pressure there is, the more lateral pressure there is, but also the more clamping pressure there is on the fabric, locking it in place,” Ben says, adding that this technique is commonly used in stacked-block retaining walls. Here, it accomplishes what a 4-in. concrete slab would from inside the foundation—without the embodied carbon load.
The product is 6-ft. wide and is cut to match the length of the footing section to which it is being fastened. It goes down folded to form two layers in an L shape; one leg comes out horizontally from the foundation wall and the other leg travels vertically up the wall. Then an 1/8-in. by 1-in. stainless steel termination bar is installed over the horizontal length; it is drilled and screwed onto the footing. The vertical leg is brought down to lay over the the horizontal lap, and the bar is sandwiched between the two layers to prevent the fabric from pulling out.
This is one of those cases when a common commercial detail makes its way into a residential application. Ben says this solution saved about $25,000 when compared to pouring a slab or the recommended concrete beams—plus it omitted roughly 60 tons of CO2 emissions.
Kiley Jacques is senior editor at Green Building Advisor. Illustration by Patrick Welsh. Photo by Brian McAward.
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