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Barrier May Bring End to Cliff's Wandering Ways

Caltrans completes work on a high-tech wall designed to stabilize the slope where Topanga Canyon Boulevard reaches the sea.

January 08, 2004|Bob Pool | Times Staff Writer

It's designed to restrain it. Then drain it. And -- with any luck -- retrain it.

That's why there are high hopes that Malibu's most challenging cliff will be tamed by a new high-tech retaining wall.

Workers last week finished nearly two years of construction on the $5-million slope-stabilization system above the intersection of Pacific Coast Highway and Topanga Canyon Boulevard.

A network of steel-reinforced concrete barriers, anchored to bedrock, zigzags its way up a 100-foot embankment above what has become the coastal community's busiest corner.

For decades, the corner has kept road maintenance crews busy at what is one of the coastal highway's narrowest points.

There is room for only four cramped lanes of traffic between the cliff and the beach, which is beneath a 10-foot drop-off next to the pavement.

And until now, rocks regularly cascaded down the cliff and onto the roadway. Mud and brackish water oozed from the base of the cliff, evidence of geologic pressure from subterranean fault lines and a fickle water table that knocked street lights askew and caused pavement to buckle and bulge.

State transportation engineers are confident that what they call their "tie-back wall" will hold up the slope, catch falling rocks and dirt and divert underground water. In effect, the structure is designed to reprogram the cliff's slip-and-slide nature.

With 2,200 feet of wall, averaging about 10 feet in height, the barrier is a puzzling presence to motorists who study it as they wait for the intersection's lengthy red light to change.

"The top section of the mountain is kind of fractured material that's not real stable. It's seismically active -- with a couple of fault lines going in different directions," said Dan Freeman, a supervising transportation engineer with the California Department of Transportation who is construction manager for the agency in northern and western portions of Los Angeles County.

"We drilled through the fractured material into the bedrock deep in the hillside and put steel strands in to anchor the walls to the rock. On the front of the slope we put in concrete beams. The steel strands go from the wall in front to the bedrock in the mountain."

The Malibu cliff required 380 tieback anchors, each drilled about 200 feet into the mountainside to reach bedrock. Twenty-two pipes inserted a shorter distance into the slope drain groundwater -- the second component of the slope modification effort.

To complete the project, engineers constructed asphalt-paved ledges along the meandering walls to catch falling debris and give cleanup crews access to the slope. The steepest sections of the hillside were wrapped in burlap-covered sections of chain-link fencing installed to prevent falling dirt and debris.

The retaining wall is so eye-catching that it has been filmed for use in an upcoming "Modern Marvels" show on the History Channel, said Judy Gish, a Caltrans spokeswoman.

The project comes at an important time for the PCH-Topanga Canyon intersection.

Canyon residents say daily commuter usage of Topanga Canyon Boulevard by motorists coming from the west San Fernando Valley is at an all-time high. Caltrans statistics indicate that as many as 53,000 cars travel beneath the cliff on PCH each day.

State parks officials, meantime, are seeking to turn the mouth of the canyon, near the base of the cliff, into a park.

The state purchased 1,659 acres north of the intersection for $43 million in 2001 with an eye toward making it an extension of Topanga State Park. Since then, eviction notices have gone to tenants at the site -- primarily those living on the west side of Topanga Canyon Boulevard.

Although some commercial businesses will remain as an amenity to future park visitors, others near the intersection will be shut down. A notable exception is a gas station on the east side of the boulevard directly beneath the cliff -- just outside of the park boundary.

The station, most recently leased to Arco, was shut down several years ago after landslides sent the slope edging close to the rear of the station building.

The station's future is uncertain, even though Caltrans construction barricades near it have been removed and a technician for Thrifty Oil Co., which owns the station site, said it will be usable when new pumps are installed.

But on the hilltop above the station a drilling crew working for Thrifty Oil said the site may be used for a retail development instead of gasoline sales.

Moshe Sassover, a vice president of Thrifty Oil, would not discuss plans for the station, however. "We're a privately held company and we don't comment," he said.

Workers on the hilltop said they were drilling as part of soils testing for the station site. Wet, goopy mud was being pulled by the rotary drill from about 100 feet down and deposited on residential Coastline Drive.

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