Albert and Delores Klein's backyard is sliding down Alexandria's Seminary Ridge. Each day, especially when it rains, the Kleins are fearful that their $165,000 house will start slipping down the hill, too.

The deck almost went. After the brick piers sank a couple of feet, Klein saved the deck, left suspended in mid-air, by propping it up with jacks.

The Kleins, like some other residents of their expensive subdivision on Seminary Ridge, have discovered that their house was built on unstable marine clay.

When it gets wet, marine clay, usually hidden under other layers of earth, turns into an ooze. The ordinary soil above it collapses and starts to break away.

A group from the Seminary Ridge Civic Association, concerned that many other houses may also have been built on the clay, took a plea for help yesterday to City Manager Douglas Harman and other city officials.

Harman said the city followed all the appropriate building regulations when the subdivision was begun in 1969 and has no legal responsibility for what has happened.

"This is principally an issue between the home-owners and the builder," Harman said.

For the last year and half, according to Charles Everly, head of the building and inspection department, the city has been requiring soil test borings before permitting construction in areas where marine clay is suspected.

Those precautions are too late for the Kleins, who moved into their house in 1973. Their yard first began to fall away in 1975, they said. The latest slide, which has pulled their yard down the hill as much as a foot a day, began after the recent heavy rains.

"You work all your life for something," said Klein, a research and engineering specialist at Fort Belvoir, "and you find you'll be up to your ears in debt to get the repairs done."

Even if their house survives, the Kleins estimate they will have to spend from $25,000 to $40,000 to stabilize the ground with pilings.

None of the damage is covered by insurance, Klein said.

His wife, Delores, says: "We're concerned not only by our own situation - and it is terrifying - but also about our community."

Next door to the Kleins, the slide has ripped away a smaller part of the backyard of the Allen Andersons. "It's had a big impact, there's no question about it," Anderson said. "We don't really know if the slide has stopped."

City inspectors come to the Kleins' house every day to monitor the extent of the damage. If the slide continues, Klein said, the city may have to condemn the house as uninhabitable.

To slow the slide, Klein has laid sheets of plastic over the yard, which is split by several deep crevices. But he said he wonders if yesterday's rain will accelerate the slippage.

The Kleins' son, Mark, 13, has a bedroom on the ground floor in back. "He'd be the first to go if the house started sliding," Mrs. Klein said. So Mark has been moved to another room upstairs.

The Kleins' house and other residences in the subdivision were built by the Pulte Homes Corp. According to Klein and other residents, the lots in the susdivision - which sit on top of a steep hillside - were built up with fill.

Everly said Seminary Ridge subdivision's problems are not unique. "We've got active slides in four other areas," he said. Two of the areas - the western edge of Newport Village and the rear of Battery Hill - have dwellings, he said.

Everly said marine clay runs through the southern edge of the city. "I can't begin to count the number of slides that have occurred," he said.