Man’s battle with highway 50 geology |

Man’s battle with highway 50 geology

Sally J. Taylor

In January, California declared war on U.S. Highway 50. Nearly a year later, a truce appears within reach.

Traffic patterns on the highway in the South Fork American River Canyon are expected to return to normal this weekend following 10 months of repairs and construction with detours and delays that frustrated travelers and cut into tourism profits.

Frehner Construction Company, Inc. will continue working on the project but without mid-week traffic interruptions.

Since the flood on New Year’s Day and the Jan. 24 Mill Creek Slide – which completely closed the highway for extended periods – an army of engineers, geologists, strategists, accountants and interpreters has mobilized resources to learn why the slide happened, how to fix it and how to minimize the chance of future slides.

The massive Mill Creek Slide began when sandy soil with almost no clay to hold it together became saturated with rainfall.

“Water pressure reached a point that the strength of the material couldn’t sustain its own weight on the slope,” said Rod Prysock, the California Department of Transportation’s chief north state roadway geotechnical engineer. “Wet, sandy material flows like water and moves rapidly.”

More than 300,000 cubic yards of material slid off the slope, tearing a hole into the mountain 40 to 50 feet deep, 3,000 to 3,500 feet above the highway. It buried Highway 50 under up to 50 feet of debris before tumbling across the canyon and damaging and destroying several vacation homes.

To clear the debris and stabilize the slope, workers carted away 490,000 cubic yards of material in 35,000 truckloads.

Besides the Mill Creek Slide site, Caltrans forces worked at 38 flood- and slide-damaged locations along the 20-mile stretch from Twin Bridges to Riverton.

To stabilize the slide site, workers excavated into the bedrock to form benches, also called keys, to stabilize the slope. The benches were backfilled with drainage material and topped with soil at a more stable angle.

Horizontal drains, pipes 3 inches in diameter with perforations for water to enter, were drilled 100 to 150 feet into the hillside. Pipes and diversion trenches carry water safely down the slope.

Similar stabilization measures have been performed at other trouble sites. Five locations have monitors which keep an electronic watch on the slopes 24 hours a day.

The large amount of heavy equipment moving on the highway required frequent road closures. Remaining work can be accomplished without impacting traffic.

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