A clay soil found throughout north Amherst may be causing a "dome" effect under some homes, pushing up the centers of basement floors and allowing the walls and edges of foundations to sink, a U.S. Army Corps of Engineers study team has found.
The clay is often found in a layer above a second type of clay that is so soft it has been dubbed "peanut butter."
Together, these two soils have been linked to much of the structural damage plaguing Amherst homes in recent years, the team is expected to report next month in a long-awaited $500,000 federal study of Amherst's sinking homes.
"The findings didn't really surprise me," said Darlene Torbenson, a leader of the North Amherst Residents Coalition.
Although residents did not have the scientific proof, "You could see it happening. You could see the soils shrinking from around the foundation when we have dry weather," she said.
Nearly 1,100 homes have been affected by foundation damage over the past 20 years, or about 3 percent of all Amherst homes, according to a preliminary report.
But damage rates may be 10 times greater in some affected neighborhoods, the study found.
It also suggests many more homeowners ultimately may be affected, but that estimate relies upon "several unpredictable factors."
About 55 percent of the homes in the study had lateral damage, which causes basement walls to bow or buckle inward.
The remaining 45 percent showed signs of sinking.
Four types of pressure on basement walls are believed to be causing the lateral damage, including the pressures exerted by frost, soil weight, water in the soils and soils that swell, according to Bradley E. Guay, technical manager of the study.
"Three of those are not typically accounted for in foundation designs," said Guay, who commented on the soil issues during a meeting Wednesday night of the Buffalo Association of Professional Geologists.
The two problem soils are spread across most of the northern half of Amherst, where builders are developing subdivisions for hundreds of new homes.
The findings could also affect future foundation designs.
A preliminary draft of the report notes that laboratory tests show the stiff clay has a "medium to high potential" to swell or shrink depending upon its moisture content.
Recently, the Amherst building code began requiring builders to obtain soil tests and special engineering reports whenever they encounter expansive or problem soils.
The discovery came as a surprise, Guay said.
"Historic maps did not predict that there would be expansive soils in Amherst," he said.
How does the dome form?
According to the study team, differing moisture conditions below the basement floor and the perimeter footings of homes are suspected as one cause.
Perimeter drainage systems installed near the footings of most houses help to prevent water from settling and seeping into basements.
But they also are believed to cause soils to dry out and consolidate.
And, because most homes do not have drains under their basement floors, water appears to collect below the centers of foundations, the study team indicated.
These differences in soil moisture could cause the ground to swell under the center of basements and to dry out and consolidate near the perimeter, causing foundations to sink, the study team believes.
The finding helps to explain why basement floors are heaving under some Amherst homes while their basement walls are sinking, Guay said. Samples taken from some of these homes confirmed that soils were drier under the foundation walls and more moist under the centers of basement floors in some of the homes studied.
In addition to the dome effect, the shrink-swell tendency may cause some parts of a foundation to fail, or what the study team calls "differential movements" of the house, during soil moisture changes.