For nearly 20 years, residents along the 3200 block of O Street SE watched helplessly as trees, shrubs and out-buildings belonging to the neighbors behind them slid slowly downhill -- moving closer and closer to their own backyards.

The O Street residents complained of muddy water in their basements, lopsided lots and cracked foundation.

The families on the hill behind, who faced Highwood Drive, watched their lawns develop cracks, break off and tumble down the hill,

Between the two streets is one of Washington's largest landslide areas,

Myra Banks, a student who lives with her family on O Street, recalls her first encounter with a neighborhood landslide 15 years ago.

"I was playing in the backyard when a huge boulder came rolling down the hill," she says. "If I hadn't gotten out of the way that boulder might have killed me."

Two decades and $2,2 million later, the residents traded the slides for a 1,100-foot, concrete retaining wall.

In 1975, the D.C. City Council passed legislation authorizing construction of a multimillion-dollar retaining wall designed to prevent further landslides. Originally, the residents were to be assessed for the cost of the structure, but the council eventually designated Economic Development Administration funds for the project. The wall isn't costing the 30 affected homeowners a penny;

Justification for the expenditure was the need to keep O Street free of debris. Before 1975, the city routinely bulldozed the road after every rainfall and consequent landslide.

The underlying cause of the O Street slide is marine clay, an unstable soil commonly found along the Coastal Plain. The clay is considered a poor foundation material by soil scientists. The scientists say that when the clay is disturbed by construction, especially on a steep terrain, the hill may be subject to landslides.

Earl Jones, a civil engineer who works as a planner with the U.S. Department of Housing and Urban Development, explains:

"When these clays are dry, they are as hard as concrete. When they become saturated, they are like toothpaste. . . . When you build on marine clays and the clay becomes wet, the earth will slide as if it was on grease."

Bob Shields, a soil scientist with the Department of Agriculture, said the landslide on O Street is typical of marine clay slippage.

"The O Street is a dramatic example of these landslides," he says. "There is one expensive house out there which had half of its backyard drop off."

Landslides often become active when the toe, or bottom of the slide, is removed and the earth on top has a new place to slide.

Wally Cohen, an engineer with the District's Deparment of Transportation, says the hill may have been undermined when O Street was constructed through the toe.

"We've been having troubles ever since they put O Street through," says Ehtel Cassidy, who lives on Carpenter Road which bisects part of the landslide.

"One day, I think about 20 years ago, the ground just cracked open and I could see the roots of the trees. From that point on, we had troubles."

The retaining wall is 1,100 feet long and reaches a depth of 48 feet in some places. It someday may be cited in engineering texts for the innovative use of slurry walls in landslide protection.

Phil DeWees, project engineer for the O Street slide, said the slurry walls employ an unusual building technique. Soil is used as a mold for the wall, and slurry, a mixture of ash and water, is used to keep the earthen mold from collapsing. This particular wall is reinforced with a series of 130-foot steel beams called tie backs.

"We're kind of proud of slurry walls being used in this manner," says Cohen. "I'm not aware of another slurry wall being used for landslides anywhere. It probably would have cost 10 times that $2 million figure to build a conventional wall."

The only drawback is cosmetic -- many residents say the wall is ugly.

The arching, white retaining wall, which is nearly complete, is set conspicuosly in the center of a barren orange hill. The construction crew plans to lay sod there this month.

"That's the number one complaint we've been receiving from people," says DeWees. "They don't like the way it looks.

"These people are making out like bandits. I never knew the city to come in on private property before and make repairs."

The marine clay in Anacostia can be found throughout the metropolitan area.

"Most of the marine clays are located east of the Anacostia River," says John Redman, of the District's Environmental Services Department. "There is also a pocket of the clay up around Walter Reed on 16th Street."

While the District appears to have the area's largest individual landslide, it also is the local jurisdiction experiencing the fewest landslide emergencies.

Landslide situations exist in parts of Prince George's County, Fairfax County and Alexandria. So far, no other local jurisdiction has undertaken a project of the magnitude of the O Street slurry wall.

City officials say it is unlikely that a project similar to the one on O Street will be repeated anywhere in the District.

"There are small retaining walls throughout the city," says Redman. "Those are all constructed by the homeowners. O Street is a one-shot deal, the only one to be built by the city."

A construction worker on the site, who asked not to be identified, said he thought the O Street slide was just the beginning.

"It looks like we've opened Pandora's box," he said. "We've already had people stopping by from other streets to ask why we aren't building a wall over at their landslide a couple of blocks away."