Point Reyes Light - September 6, 2001

The innovative repair of White’s Hill slide

By Patrik Jorgensen

Sir Francis Drake Boulevard (then called Drake’s Highway) over White’s Hill was built in the late 1930s, and "the day after the road first opened in 1938," noted Kevin McGowan of the county Public Works Department, "it already started sliding."

Ever since then, the county has continually applied more and more asphalt to raise the roadway every time it slid. By now, there is 30 feet of asphalt over the slide just east of the crest.

Finally the road is getting the repairs it has needed all along. Why now? "The reason we’re building the bridge now," McGowan said, "is because the funding is finally there."

He said the county has saved large portions of federal road improvement grants until there was nough for this huge project.

The project, however, is no simple matter. The county isn’t trying to fix the slide in a traditional way, which would be to "scrape" off the top layers of dirt and asphalt, including slide material, and then put back the top dirt.

That, said McGowan, would simply be too expensive. "The slide is 75 feet deep [in some places], and we would have to actually store all the dirt that we took off in order to put it back after the slide was stabilized."

Instead, the county is crossing the slide with one of the longest single-span bridges in the state, McGowan said, noting that a new type of steel will allow for this innovative way to build.

Regular steel, he explained, would be too heavy and not strong enough to support its own weight as it spanned a gap 380 feet long.

While the bridge is being built, motorists who use Sir Francis Drake Boulevard to get back and forth between East and West Marin can expect traffic delays during non-commuter hours. (Completion of the new bridge is scheduled for April if winter weather is not too severe.)

One lane of traffic will be closed occasionally to allow for trucks and other heavy machinery to operate safely, project inspector Jason Jurrens told The Light.

Paul Chang, project engineer, said that there will be times when both lanes of traffic will be closed. "We are trying to minimize traffic problems, but when we receive and start erecting the steel girders, we will have to close both lanes, but only during the night," Chang said.

Road to close at night

The girders are the support structures that actually hold up the bridge, McGowan of the Public Works told The Light. He added that people can expect the road to be closed at night around the second or third week of November.

The bridge will cross a ravine rather than follow the existing roadway, and this will improve driving conditions because the sharp turns across the slide will be replaced by softer turns leading onto the bridge, Chang said.

"This will certainly improve the geometrical line for drivers," Jurrens added. "The road will be straighter, and the shoulder will be wider."

The innovative nature of the bridge is one reason it has been able to receive so much federal grant money. "We’re using Core 10 steel, which is both lighter and stronger than regular steel," McGowan said. He predicted the bridge would be a more permanent solution to the slide as well as the cheapest and most efficient way to proceed.

Slide to slip under bridge

The slide itself, McGowan said, will probably keep moving for a while before it stops, but the slide will be able to pass underneath the bridge, leaving drivers out of harm’s way. He added that the support structure for the bridge will be strong enough to handle the pressure from moving dirt.

Of interest to the environmentally sensitive population of West Marin is the fact that the county hired a landscape artist to help make the bridge blend into the surroundings. McGowan said the county plans to stain the railings and support structures so that the colors will blend into the hillside although these plans are not yet final.

In addition, workers will move traffic lanes closer to the uphill side of the road during construction. To prepare for this, workers are building a concrete retaining wall to hold back the hillside. "There are little tubes in the concrete where they will be able to put native plants," noted Jurrens. The plants, he added, will grow to cover the hillside, making it look the same as it previously did.

Support for project

In the San Geronimo Valley, there has been clear support for the project, Chang said. "A few local residents stop by from time to time," he said, "and they all have positive comments."

The only real problem that the workers are now facing is that people driving by are not paying attention to the warning signs and are driving too fast, Jurrens said. The project inspector said he has almost been hit, and he urged drivers to slow down since members of the construction crew have to cross Sir Francis Drake Boulevard on foot from time to time.

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