Big construction projects are rarely straightforward. The new San Francisco–Oakland Bay Bridge is a prime example, thanks to unexpected problems with the steel. But there, at least, nature was not to blame. This week a long-running dam project made the news when a geological discovery forced the San Francisco Public Utilities Commission to move millions of dollars around and push back its schedule by three years. Big excavations are like exploratory surgery, no matter how carefully planned, because mapping the underground is never certain.

The Calaveras Reservoir dam, east of Milpitas, is a massive earthen structure built across Calaveras Creek in 1925. These traditional dam designs are generally robust—the Crystal Springs Reservoir on the Peninsula has one that easily weathered the 1906 earthquake while the fault moved beneath it. However, the Calaveras Dam needed more work. The reservoir needs to be a reliable six-month backup water supply in case the Hetch Hetchy Aqueduct is out of action. And what could damage the aqueduct? Among other things, a major earthquake on the Calaveras fault, which happens to run right under the Calaveras Reservoir next to the dam.

The plan for meeting this dire scenario was to build a new dam just downstream of the old one. It will use the existing hills on either side of the creek as buttresses and be largely made of the rock excavated from the hills. The work was proceeding without incident until June of last year when excavators on the western buttress found signs of a huge, ancient landslide in the Temblor Formation.

Cutting into the hillslope under those circumstances would risk reactivating the slide, which could endanger the workers and threaten the new dam ever afterward. So now they will cut deeper to create a shallower slope, producing about a million and a half cubic yards of extra rubble and pushing the construction schedule back three years.

Here’s a view of the project as seen looking south from Sunol Regional Wilderness in July 2012. The reservoir, its water level drawn down for safety reasons, is just visible between the two cuts making up the buttresses for the new dam. The cut on the left (the right bank of Calaveras Creek) is in bluish metamorphic rock of the Franciscan Complex, and the cut on the right is in the much younger brown sandstone of the Temblor Formation. (The chopped-up hill in the middle is downstream from the dam site; the creek runs around it on the left side.)

Calaveras Dam project
The hill on the right, where the landslide was discovered, is Observation Hill. Both it and the smaller hill at center are composed of Temblor Formation sandstone. Photo by Andrew Alden

The landslide was discovered in the Temblor Formation. Here’s the part of the Alameda County geologic map that covers the area. The photo above was taken from roughly the “25” mark at the top of the map. The solid black lines running north-south down the middle are major strands of the Calaveras fault.

Geologic map of the Calaveras Dam area.
From oldest to youngest: KJfm, Franciscan melange; Ks, sandstone of Cretaceous age; Ttem, Temblor Formation (Miocene age); Tcs, Claremont Shale (cousin of the Monterey Formation); To, Oursan Sandstone; Tbr, Briones Formation.

The amended dam plan, filed with the San Francisco Public Utilities Commission last December, shows an east-west cross section of the dam site. The extra digging will happen on the left side to create a slope that is less steep and more stable.

From the Addendum to Environmental Impact Report (Dec. 13, 2012), San Francisco Public Utilities Commission
From the Addendum to Environmental Impact Report (Dec. 13, 2012), San Francisco Public Utilities Commission

I have to say a little about the Temblor Formation, which occurs mostly in the southern Central Valley beneath the Monterey Formation. It gets its name from the Temblor Range, where its type section was described. And the Temblor Range got its name after the great 1857 earthquake ripped through the area. It’s a coarse-grained marine sandstone full of fossils, and this spot appears to be its northernmost outcrop—right next to another earthquake fault.

The Temblor is strong stone, but landslides can affect any kind of rock if it’s fractured enough. And around major faults like the Calaveras, everything can be assumed to be fractured. The mountains of the Bay Area look beautiful and strong, but watch them for a few thousand years and you’ll find them rather crumbly. Alternatively, scientists with the right equipment can survey the ground with millimeter precision. In the Bay Area’s steeper hills they have detected many large, old landslides that are creeping very slowly, as they have for centuries. Maybe the landslide at the dam site was just pausing for breath. Better for the Calaveras Reservoir project to get this one out of the way.

Landslide at the Calaveras Reservoir 1 May,2013Andrew Alden

  • Jim Crowley

    This is a great story and it’s always a pleasure to read about how geology and the natural environment affects big civil engineering projects like this.

  • Michael J. Lehner

    What is also really refreshing about this development regarding the uncovering of the ancient landslide is that this is one of those rare instances where a budget gets blown out of the water and everyone just shrugs and moves on. Although it’s a bit of bad news for California, I love how this issue was handled. Great article!


Andrew Alden

Andrew Alden earned his geology degree at the University of New Hampshire and moved back to the Bay Area to work at the U.S. Geological Survey for six years. He has written on geology for since its founding in 1997. In 2007, he started the Oakland Geology blog, which won recognition as "Best of the East Bay" from the East Bay Express in 2010. In writing about geology in the Bay Area and surroundings, he hopes to share some of the useful and pleasurable insights that geologists give us—not just facts about the deep past, but an attitude that might be called the deep present.

Read his previous contributions to QUEST, a project dedicated to exploring the Science of Sustainability.

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