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Soils Expert Recommends Sewer System in Slide Area : Geology: Three-year study of Big Rock Mesa finds that more slides are inevitable unless further steps are taken to reduce ground water levels.


MALIBU — A soils engineer hired by Los Angeles County to suggest ways to shore up slide-prone Big Rock Mesa has recommended that a sewer system be installed there to protect against another catastrophic slide like the one that damaged or destroyed 240 homes in 1983.

In a voluminous report based on three years of study, Bing C. Yen paints a gloomy picture for the Malibu promontory, concluding that more slides are inevitable unless further steps are taken to reduce ground water levels on the mesa.

But while saying a sewage collection system will have beneficial results, he warned that nothing will solve all of the area's stability problems.

Depending on the size and type of system, a remedy could end up costing Big Rock taxpayers millions of dollars in addition to the $13.8 million they are already being charged for work the county is doing to shore up the promontory.

The County Board of Supervisors in 1989 voted to tax Big Rock homeowners more than $100,000 each to help pay for stabilizing the landslide-plagued community above Pacific Coast Highway in eastern Malibu.

Earlier that same year, the county, the California Department of Transportation, dozens of insurance companies and attorneys for many of the homeowners reached a settlement after 4 1/2 years of litigation to pay the homeowners $97 million--or about $200,000 apiece after legal fees--for the damage caused by the 1983 slide.

The Big Rock homeowners had contended that the county government was negligent for not having done enough to prevent a buildup of ground water on the mesa, which contributed to the slide. Caltrans also contributed to the slide, their suit alleged, with excavations for Pacific Coast Highway at the toe of the bluff, weakening the entire mesa.

County taxpayers paid $35 million of the settlement, plus another $13 million in legal fees. Caltrans' share was $40 million.

Besides recommending a sewer, the report suggests adding more dewatering wells and deepening some of the 23 existing wells to either supplement or substitute for a sewer collection system.

Yen said that even though nearly 94 million gallons of ground water has been removed from the mesa since 1988, efforts need to be stepped up to prevent more water from collecting beneath the surface and causing the ground to slide.

Nearly five years of drought have helped the situation, he said, "but it will rain, and increased rainfall and effluent from septic systems will cause the factors of safety to decrease," he said.

Besides offering potential remedies against future landslides, the report--by establishing factors of safety--has big implications for future construction on Big Rock.

A factor of safety is a measure of stability for a given location against a moving force such as a landslide. In California, the usual minimum standard is 1.5, meaning that construction is not allowed in an area that is less than 1 1/2 times as strong as the earth force capable of moving against it.

However, the report found that most areas on Big Rock Mesa where homes exist have safety factors of 1.2 or less and that a few are perilously close to the 1.0 figure, at which the ground can be expected to give way.

Last November, after complaints from some Big Rock homeowners, the City Council voted to relax the standard to 1.25 for homeowners wishing simply to remodel or add to their houses, provided certain other conditions are met.

Under the law, before any work may be done, homeowners are required to sign a waiver excluding the city from liability in the event of future problems.

Some experts view such waivers as controversial both for ethical reasons and because they have sometimes been successfully challenged in court.

"In most cases, they are issued for political expedience because a governmental entity wants to duck the issue of enforcing the standard," said former assistant county counsel Arnold Graham, who has successfully represented clients who have challenged such waivers.

The report by Yen, a former professor at Cal State Long Beach who has a private consulting business, suggests that less than half the properties on the mesa have safety factors of 1.25 or more, and concludes that, even with a sewer system and additional dewatering wells, nowhere on Big Rock will safety factors ever reach 1.5.

"It is a very complicated geologic area, and the results (of the study) do not please the (Big Rock) citizens," Yen told a dinner meeting of geologists who gathered in Montebello last week to hear the results of his research.

Some geologists have expressed concern that a magnitude 7.0 earthquake along the Malibu Coast Fault could cause especially heavy damage on Big Rock Mesa.

The report notes that the risks associated with the landslide are compounded by the fact that the mesa is sandwiched between two large sections of the Malibu fault system, with several smaller sections of the fault actually crossing parts of the mesa.

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