Wednesday, July 11, 2007

Dam Retrofit

Nice look at a retrofitting job at the Stony Gorge Dam by Willows.

A new face: Dam retrofit project under way
Article Launched: 07/08/2007 12:14:01 AM PDT

ELK CREEK -- It may not be apparent to those lazily enjoying the serene waters and golden hills of Stony Gorge Reservoir about 21 miles northwest of Willows, but just over the dam, there's a whole lot going on.

Stony Gorge Dam is getting a facelift.

It's in the early stages of a federal modification project the U.S. Bureau of Reclamation designed to strengthen it in the event of a strong earthquake.

The dam was built between 1926 and 1928 — long before an earthquake fault was discovered nearby, and when earthquake standards for construction weren't as stringent as today. It was last modified in 1986.

The problem is, in a study conducted in 2002, the Bureau realized that the dam's structure could fail in a lateral, or sideways-moving earthquake, measuring magnitude-6.0 or greater.

So after years of conducting environmental and engineering studies, plans and designs, the project was awarded to a construction company and got under way.

Reclamation awarded Shimmick Construction Co. Inc. the project late last year. The company is also retrofitting the Golden Gate Bridge in San Francisco…

… From the walkway that spans the width, Stony Gorge Dam offers a fantastic view downstream toward Elk Creek. On its opposite side is the reservoir, a sparkling blue jewel dotted with boats and surrounded by campers.

The dam itself is an imposing structure. At its lowest point, it rises 139 feet upward to a crest elevation of 847 feet above sea level. It's roughly like standing at the foot of a 10-story building and looking up.

It isn't until you get close to the structure, at the foot of the dam, that you see the depth of the bays between the vertical buttresses, and the steep 45-degree slope of the concrete face that sits against those structures.

Some horizontal concrete struts lend crosswise support between the thick buttresses, but in a lateral quake the buttresses could move back and forth or flex. The movement could cause the dam face to crack or collapse, which could pose risks to the folks downstream.

The plan is to strengthen the buttresses by filling the space between them with concrete walls rising toward the top of the dam to within about 36 feet of the top; or as Milani said, up to about the 811-foot elevation mark.