Geologic Jarring May Indicate Marine Clay

Geologic Jarring May Indicate Marine Clay

On the pottery wheel, clay mixed with water forms "slip," a gooey mixture that makes the pot-to-be glide under the potter’s hands. In the real world, the formation of natural slip causes problems when a townhouse community is sitting on a bed of marine clay and is hit by a year of record precipitation.

In part, that's what happened to the Landsdowne community in the southern end of Kingstowne. On June 22, the hillside behind a row of townhouses ended up on Telegraph Road, wiping out the bike trail as well as some utilities for that community. "Slope failure" is how the homeowners association defined the localized disaster. Now residents must deal with condemned back yards and no solution to the problem in sight.

"The slope was apparently stable for about 10 years," said homeowners association president Lou Talbot at a homeowners association meeting.

A large portion of Kingstowne is on a bank of marine clay, according to John Halle, of Halle Construction, which is building the community of Kingstowne.

"A majority of Kingstowne is marine clay," Halle said. "It's not a problem. If you build correctly, it's fine."

Richmond-American homes built the Landsdowne community, which is on the southern end of Beulah Road just outside Kingstowne.

At the time of construction, Landsdowne was built correctly. However, residents report that problems began with the widening of Telegraph Road. Jeff McKay, chief of staff for Supervisor Dana Kauffman (D-Lee), is familiar with the situation.

"Everyone knew that marine clay was there," McKay said. "It was not caused by marine clay alone."

One way to build correctly is to go deeper with foundations. That's what Halle has resorted to.

"We have to drop the footings two feet," Halle said.

Other Springfield-area communities in the area that have had problems with marine clay in the past include Cedar Knolls and High Grove Estates, said Wally Ayodeji, Fairfax County chief geotechnical engineer.

"Anytime you have a lot of rain, the slopes tend to slide. Drainage has to be taken care of," Ayodeji said.

On the Northern Virginia Soil and Water Conservation visit the Fairfax County Web site, foundations sliding down inclines were addressed.

"Damage to structures from landslides in marine clay can be dramatic," the Web site said. "Slope failures occur most commonly during wet periods of the year when soil is at a maximum."

Marine clay is in other areas of the county as well, according to Virginia Department of Transportation (VDOT) materials engineer David Shiells.

It's classified by Fairfax County as a problem soil," said Shiells.

NORA FOLEY AYUSO, PH.D., of the Eastern Minerals Resource Team, wrote the article "Environmental Characteristics of Clay and Clay Mineral Deposits" for the U.S. Geological Survey. Foley Ayuso describes the clay-water mixture as "slurry."

The instability of the clay is another problem. Due to its chemical makeup, it swells and contracts so it's unstable. Ayuso responded by e-mail to specific questions but was unaware of the Kingstowne situation.

"The group of minerals is commonly referred to as ‘swelling clays’ and bentonites. (There are other clay minerals which do not expand.) A single clay particle of this type can expand almost 100 percent. For example, a 10-angstrom clay mineral may expand to 19.5 angstroms. Swelling clays expand or contract in response to changes in environmental factors such as wet and dry conditions and temperature. These special properties are exploited for certain industrial applications; however, houses, offices, schools, and factories built on swelling clay may be subject to structural damage caused by seasonal swelling of the clay portion of the soil. (It is a serious problem that is probably best addressed by a qualified engineer before construction starts.) "Montmorillonite is apparently the swelling clay that is found in Fairfax County soils," Ayuso wrote. "One thing to keep in mind is that the mineral may not be uniformly distributed within a rock units (shales in this case), so some areas may have more or less of a problem than other areas," she added.

This swelling and contracting isn't good for roads either, said Shiells. When building a roadbed, VDOT digs up the clay and puts down a mixture of crushed rock or gravel as a foundation.

"If marine clay is under a roadway, it will shrink and swell, which will lead to cracking," Shiells said. "We'll remove a layer and replace it with good material."

ACCORDING TO information on the Fairfax County Web site, potential problems associated with marine clay include land slippage and slope instability, shrinking and swelling of clays, poor foundation support and poor drainage. Marine clay occurs in an area of Fairfax County known as the "Coastal Plain," an area located primarily east of Interstate 95.

The shrinking and swelling can crack and damage below-ground walls and ground-floor slabs as well as foundation walls. Land slippage and slope movement can also undermine the foundation support and, in some cases, can cause walls or sections of the house to separate or break apart.

Richard Diecchio, George Mason University professor, said the slick surface isn't limited to marine clay.

"They shrink and swell. It doesn't have to be limited to marine clay to slip," Diecchio said.

Back in Landsdowne, Talbot addressed the residents at their Nov. 10 homeowners association meeting, at Lane Elementary School. So far, homeowners have paid $552,244.32 from the association account.

"Our insurance does not cover land movement," Talbot said.

McKay said the final resolution on the issue has not been determined yet.

"The slope has been stabilized temporarily. We're looking for a long-term fix," McKay said.

Fairfax County does assist with determining if marine clay is a problem, and "in certain cases, low-interest or deferred loans may be available for repair work," according to the county's Web site.

The neighborhoods of Virginia Hills, Rose Hill and Wilton Woods, all built on marine clay, are candidates for the low-cost loans, according to McKay.

A representative of F&R Engineering was on site in Landsdowne surveying the damage on the afternoon of Thursday, Nov. 13. Landsdowne hired F&R to assess the situation. He would not comment on anything about Landsdowne. Landsdowne association president Lou Talbot did not return phone calls.

Although marine clay has a special section on the Fairfax County Web site as well as the Northern Virginia Soil and Water Conservation (NVSWC) Web site, the term "marine clay" is incorrect, according to Dave Harper, project manager for a NVSWC program to update the county soil map. Marine clay is really a combination of soils and sediments deposited by rivers flowing into the Atlantic Ocean millions of years ago in the Cretaceous Age. Technically, "It's a geologic formation, not a soil," said Harper. In 1963, marine clay was not on the legend of the Fairfax County soil survey.