But fast forward to the present day, and much to the consternation of more than 1,000 Amherst homeowners, it turns out that the clay has still not dried. The result is a town-wide calamity of collapsing foundations, for which repairs will likely run into the millions of dollars.
The biggest culprit for the problem, according to one local soil scientist, is the typical failure of builders to secure soil samples before building.
Expansive clay soils like bentonite and montmorillonite often threaten foundations because builders and homeowners failed to account for their likely behavior when foundations were put in place.
“Our most important recommendation to our clients is to investigate the site and make sure you have accurate soil information,” says Don Owens, soil scientist and president of Amherst-based Earth Dimensions Inc. “In most cases, they don’t do it, and the reason they don’t do it is that it’s an expense. Obviously they don’t want to have any more expenses than are necessary. It might be $1,500, but that $1,500 may be the best investment they ever had.”
The Amherst foundation issues began to appear in 2005, prompting the town to call in the U.S. Army Corps of Engineers. The Corps determined through a $500,000 study that expanding clay was pushing up the center of basement foundations, which make it appear that the walls and edges of foundation walls have sunk.
The problem has rarely occurred in as sudden or as widespread a manner as it did in Amherst, but the phenomenon of structures being threatened by soil problems is hardly unusual.
Kari Sever, president of Tyron, Oklahoma-based Horizon Environmental, says builders need to do a much better job of gathering soil information, especially since the information is readily available on the Internet from the U.S. Department of Agriculture. (http://websoilsurvey.nrcs.usda.gov)
“A lot of builders don’t check it,” Sever says. “We had a guy out here who was going to build a half-million-dollar house, and all the neighbors were saying, ‘He’s sitting on top of a clay hill.’ It was only after the contractor was hammered by the neighbors that he listened. He wasn’t from around here.”
Sever says pH level is an especially important characteristic of soil. “If the pH is really high or really low, it’s going to be corrosive to concrete or metal,” Sever says. “And all of that information—the type of clay and the relative clay content, the pH, all that information—can be found on a soil survey.”
Sever also cautions builders to familiarize themselves with the parent material of a particular clay, as this will have a major impact on how the clay responds to moisture.
“Other than expansive clays—bentonite and montmorillonite—another issue for contractors would be a seasonal subsurface water table,” Sever said. “They should know how to recognize the seasonal high water table – sometimes called a perched water table—if they are putting in basements or storm cellars. And of course, surface drainage and runoff areas would be a problem.”
Mike Trotter, president of Doraville, Georgia-based Trotter Company, has been busier than ever with his waterproofing and foundation repair business—all because failure to compact soil has made it susceptible to expansion during heavy rains. The result has been a phenomenon of foundation failure that has run to epidemic proportions.
“Atlanta has extensive cement-block foundation walls,” Trotter says. “The problem with the blocks is that, typically, even eight-inch block is only rated for seven-foot-high walls, and most foundation walls are nine or 10 feet nowadays.” And of course, builders backfill against these inadequate walls. “They may hold fine at first, because the builders don’t compact that soil, but when you get stormwater and drainwater, the soil settles, and we’re just in an epidemic situation in Atlanta,” says Trotter. “It never fails that when it rains, I get calls that basement walls cave in.”
Unfortunately, with sheetrock covering the walls, it is all but impossible for homeowners to recognize that the walls are about to cave in—until they do. In responses to the problem, Trotter applies one of three different solutions, depending on the severity of the wall’s problem. His first priority is to reinforce the wall once the first sign shows itself—usually in the form of a long horizontal crack. For bows of less than three inches, Trotter uses four-inch-wide epoxied carbon fiber strips, which requires him to remove paint from the wall before running the strips from top to bottom. The strips generally prevent the wall from collapsing because they can’t break the exceptionally strong carbon fiber.
For walls that have bowed between three and five inches, however, the fix requires a five-by-five-foot steel beam on six-foot centers across the face of the wall, anchored below the floor. And beyond five inches?
“We suggest the customers temporarily support the house—we build a stud wall under the first floor of the house about two feet out from the bowed wall—and then we dig the backfill out of the house and build a new wall, and reinforce it,” Trotter says. “And of course, you’ve got to waterproof it and go through the whole nine yards of getting everything back in place.”
Repairs typically cost between $50,000 and $100,000, requiring most unfortunate homeowners to take out second mortgages. Another solution to these block foundation problems would be a reinforced poured-in-place concrete foundation designed to resist expansive soil pressures.
Trotter blames some of the problem on fast residential growth in the Atlanta area, creating an incentive for builders to get houses up as fast as possible—and a disincentive to do their homework on issues like soil content. Another issue, however, is the rolling terrain that requires a great deal of backfilling.
“Our gorgeous red clay is terrific,” Trotter says. “But we have hilly, rolling terrain, and to get a level building site, they have to usually cut one side of a lot and fill the other side. We have what they call cut-and-fill lots, so when they cut one side and build a basement, they’d level the house up so your first floor is level, and that involves backfilling against the foundation of the house. You dig a hole, build a house and backfill around it. In residential construction, typically that backfill dirt is not compacted.”
Stormwater consequently causes the air to drain out of the fill dirt, causing the soil to settle and causing a slope that once ran away from the house to start running toward it.
Mike Turner, president, SureVoid Products Inc., based in Englewood, Colorado, says soil-related building problems have long resulted from people choosing to live in areas having less than ideal soils. “Initial populations usually center around rivers and other water sources that typically have stabile soils, such as rock, sand, etc. As cities expand people are forced to live in areas having less ideal soil conditions.”
SureVoid specializes in void form products, which are designed to create space between concrete structures and expansive soils. They create space into which the soil can expand without damaging the structural integrity of the concrete. Its “void forms” or “carton forms” are made from corrugated paper.
They provide a temporary support platform for concrete placement until the grade beam or structural slab can support itself across drilled piers, pads, intermittent footings or other concrete work. The SureVoid material is designed to absorb ground moisture. Once the concrete has set, the SureVoid material loses its strength, which creates the space into which the soil can expand.
“Our products isolate the concrete that is going to come in contact with the soil,” Turner says. “Under the concrete walls between the piers is where the void material is to be placed.” Sever agrees that piers are a crucial element of prevention, particularly if they are drilled deep enough.
“In our area, we drill piers and make deep foundations to try to get down where the clays are not expansive, because they don’t encounter water at that lower depth. Another thing they do is make French drains around the foundations.”
With soil survey information widely available, and preventive measures on the market ranging from the conventional to the highly innovative, builders should recognize that there is no reason to put their work at risk of an Atlanta epidemic or an Amherst calamity.
Turner urges builders to understand that the key lies in the first few feet of the earth’s crust, and that expansive soil behaves much like flour when exposed to yeast. Moisture will lead to expansion, so room to accommodate that expansion may be the difference between a stable foundation and a disaster.