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Top of Slide Halts, Bottom Marching On

March 08, 1987|GERALD FARIS | Times Staff Writer

RANCHO PALOS VERDES — The Portuguese Bend landslide has been inching toward the sea for 31 years, and five months after an ambitious and expensive city project began, it still is.

But even though they have not won their battle with the earth, city officials believe they have demonstrated that the slide can be slowed--and maybe even stopped.

"So far, we are very pleased," Mayor Mel Hughes said. "Two years ago, no one was willing to think we could abate the landslide. So far, we have been quite successful."

Widening Crack

Hughes said the upper third of the 260-acre landslide, in the vicinity of Portuguese and Paintbrush canyons, has stopped moving. A widening crack separates the stable upper third from the rest of the slide mass, which is still moving but at a slower rate than before the project began. That land, which includes Palos Verdes Drive South, was moving at a rate of 1 inches a day and has been slowed by 50%when last monitored in December, officials said.

Charles Abbott, city public works consultant who is in charge of the stabilization project, cautioned that the results should be considered preliminary. "There has not been enough time to feel good about them," he said. "We've got to go through a winter with heavy rains." A survey at the end of March will determine whether the stabilization is holding, he said.

Foresees Slowing

Despite Abbott's hesitation to claim victory, he said, "in the long haul you will see this (slide) slow down. In the short run, a 50% slowdown is a dramatic slowdown."

Officials attribute their success to the moving of 600,000 cubic yards of dirt from the top of the landslide--where it was exerting downward pressure on a slick underground slide plane--to an area below, where it acts as a barrier against further movement.

The work made the slide surface, once marked by crevices and ripples that earned it the nickname of "moonscape," smooth and almost flat.

The first phase of the two-year, $2-million project also included measures to prevent water from reaching the slide plane, which is composed of a volcanic material called bentonite. When wet, bentonite provides a slick surface for a land mass to slide on.

Water collection basins were put in the two canyons and connected to a surface culvert that carries water runoff to the ocean. The slide area was seeded to create a ground cover.

Abbott said the cost of the work totaled about $1 million, which was part of a state grant.

Officials said decisions about the next phase of the slide project will not be made until the summer, after new surveys have shown whether the stabilization is holding. Results will be reviewed by an advisory panel of geologists--who earlier gave the city 100% odds of slowing the slide and an 80% chance of stopping it--and they will make recommendations about the next move.

City geologist Perry Ehlig already has called for installation of more wells in the lower portion of the slide to remove a deep accumulation of underground water trapped in clay, which Abbott likened to "a big sponge" above the slide plane. There already are two wells and Ehlig has proposed five more.

Ehlig said that the drier the clay becomes, the more resistance there is to sliding. The water is difficult to remove, however, because it is sealed off by the clay. Ehlig said getting all the water out to stabilize the slide completely "would take a long time."

"We don't know how that bowl of water will act," Abbott said. "That is why dewatering is the most critical phase."

Ehlig said that in the early days of the landslide, it was assumed that it would stabilize itself. "That is not the case," he said. More than 130 homes have been destroyed since the slide began 31 years ago.

Drive to Be Moved

The landslide plan calls for relocation of Palos Verdes Drive to the right-of-way it had in 1956. Abbott said this will not be done until land movement in the area has been slowed to a one-tenth of an inch a day. "We think it will be that slow by fall," he said.

In some areas, the road is 300 feet from its original right-of-way.

Residential properties also have moved and officials said two homes probably will have to be removed because they are in the right-of-way. Abbott said appraisals of the properties have not been completed and negotiations have not started with property owners.

Officials also hope eventually to buttress the point where the slide meets the sea, to prevent erosion and further stabilize the slide.

"We think that with a seawall we can stop the wave action, so . . . that portion of the slide will stabilize itself," Hughes said.

Hughes, council members John McTaggart and Robert Ryan and City Manager Dennis McDuffie were in Washington this week lobbying for money for initial studies by the Army Corps of Engineers on ways to stabilize the coastline. Studies were authorized in an extensive water resources bill passed last year by Congress, but no money was appropriated. A study of the Rolling Hills Flying Triangle Landslide, above the Portuguese Bend slide, also was authorized in the legislation.

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