I want to discuss some solutions for constructing homes on expansive soil. There are very effective strategies for building reliable structures in expansive soil zones.
Shaft piers and void
Primary among those is the use of drilled shaft piers. These piers are shafts drilled into bedrock, below the expansive “lens.” The shafts are reinforced with rebar, then filled with concrete. Soils engineers will specify how much penetration into competent bedrock is required. Foundations built on these piers typically have a void space under the wall to allow some expansion at that level.
Another system is using spread footers that are sized to create sufficient load to resist movement. This works in low-swell areas, particularly on over-excavated sites, and must be carefully designed and constructed.
Over-excavation entails removing soil from a site, then replacing it with either good structural fill material or pre-wetted native soil. The dirt is “pre-wetted” and wheel packed in place. Theoretically, pre-wetted soil is also “pre-expanded” so it will not expand further.
Footers are then designed to a minimum load that will resist movement. So far, this approach has worked, although questions remain regarding eventual drying and re-wetting, and regarding performance of slabs on grade.
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Basement floors are also subject to movement on bentonite-laden soils. In fact, slabs, which weigh about 50 pounds per square foot, are less likely to stay put compared to a foundation footer, which might be designed to support 1,200 pounds.
Several solutions have developed to meet basement design challenges. I will list a few, with my observations on the efficacy and challenges of each design. (Disclosure: FloR Systems designs and builds elevated structural concrete floors.) Choosing a system requires a cost and benefit analysis, which will vary from builder to builder and from project to project.
Wood. Early on, many builders used a wood joist and plywood or OSB basement floor, mimicking their main and upper level framing techniques. Eventually, issues with mold arose, and most practitioners went to a less vulnerable galvanized steel joist.
Two-way concrete slabs. A popular floor is a two-way slab, typically 6 inches reinforced with rebar. These floors are formed on a plywood or corrugated steel deck supported by a cardboard void. The void supports the floor while it is built, then disintegrates.
Two-way slabs work well, but can be costly, and must be supported by piers on 9- to 10-foot centers.
Steel and concrete over red iron frame. This floor utilizes either corrugated deck panels or composite steel decking supported by a permanent frame of I-beams. These are good systems, but also are rather costly. Proper installation requires a tall crawl space, increasing the cost of the foundation wall. Correct procedure requires a vapor barrier and active ventilation to avoid moisture build-up and rust.
Stable foundations and basements can be built on expansive soils. Unfortunately, the lawyers are all rooting for failure. Be sure to seek good advice from your design staff, engineers and from practitioners. They can help determine what solution fits your needs, risk profile and budget.
I wanted to compliment you on a good article. My only comment is that maybe the first solution would be better defined as “Deep Foundations” rather than just drilled shafts (one type of a deep foundation). I’ve personally used helical piles, micro-piles, drilled concrete piers, over-excavation, and flat out avoidance as strategies for expansive soil sites.
James A Cherry, PE
JAC Engineering, LLC