Mormon Island Dam foundation stabilized
An important step in the 20-plus-year effort to reinforce the Mormon Island Auxiliary Dam’s foundation nears completion.
Located just east of the county line, the Mormon Island Dam is a weedy and unnaturally uniform ridge that dominates the horizon north and west of Green Valley Road — a squat, anonymous fraternal twin to Folsom Dam, a troubled brother with deep-rooted instabilities.
The two dams have spent their entire lives standing side by side, but don’t look related and clearly are not made of the same stuff.
Folsom Dam stands as a symbol of power and hospitality. Its muscular concrete superstructure is clearly visible from downstream vantage points, as are the hundreds of boats its reservoir hosts each summer weekend. It was deemed a hero in 1955 when the incomplete concrete superstructure held back the Sacramento floodwaters, preventing thousands of homes from flooding and allegedly offsetting its entire construction cost in the process.
Both dams were constructed between 1948 and 1956 by the U.S. Army Corps of Engineers, along with a system of dikes and levees to contain the ensuing reservoir, known affectionately as Folsom Lake. The whole thing was subsequently transferred to the federal Bureau of Reclamation, Mid-Pacific Region.
Because it’s essentially a 900-foot-long pile of compacted soil, the Mormon Island Dam looks like a levee, but it’s not. And please don’t call it a dike.
“It’s a dam,” insists Bureau of Reclamation Project Manager Larry Hobbs, because it blocks an ancient path of the American River.
Hobbs has spent the last 10 years ensuring that the dam doesn’t suffer a meltdown. He’s overseen several repairs but none fully address its underlying instability. That’s about to change, he said, as a two-year, $35 million first phase of the latest attempt to reinforce the dam’s underpinnings quietly winds down.
Like the Mormon Island Dam itself, the geo-engineering effort to reinforce it has taken place in the shadow of the higher profile Folsom Dam spillway effort — the keystone in the $1.3 billion “Joint Federal Project” scheduled for completion in 2017.
Local agencies sponsored the projects. The Sacramento Area Flood Control Agency and the Central Valley Flood Protection Board took the lead, but two federal agencies did the heavy lifting.
The Bureau of Reclamation and the Army Corp of Engineers worked closely on both, a collaboration that cut the total price by nearly $1 billion, according to former Secretary of the Interior Dirk Kempthorne.
Former Congressman John Doolittle and current Congresswoman Doris Matsui each lobbied for federal funding, arguing that minus the Auburn Dam, Sacramento remained a major flood risk.
Why all the fuss over the Mormon Island Dam?
The answer, in a word, is “liquefaction,” an awkward four-syllable elocution that describes the loss of friction between grains of soil, which can create large-scale slippage, potentially turning portions of a once-solid mass, such as the Mormon Island Dam, into mush.
The old dam’s foundation is surrounded by mining tailings. Dredging operations churned up an estimated billion cubic yards of Folsom, Rancho Cordova and Fair Oaks between 1897 and 1962, leaving the denuded river rock landscape still visible between subdivisions, some of which would be inundated if the dam fails during high water conditions.
Liquefaction first appeared in civil engineering textbooks in the 1930s, according to a 1979 article in Mosaic Science Magazine but wasn’t connected to earthquake damage in dams.
Engineers in the 1940s recognized the potential instability of such an unnatural footing, and cut through it to bedrock for the dam itself, but left the dam’s tapered “shell” resting on an estimated 60 feet of tailings on either side of the core.
Eight years after the dam was complete, seismologists got a harsh lesson in the dangers of liquefaction when temblors wracked Niigata, Japan, and Anchorage, Alaska, in 1964.
Since then, soil liquefaction has been recognized as an ever-present earthquake hazard.
The liquefaction recipe is simple: soil + water + pressure + vibration. Thankfully, the final ingredient is uncommon in the foothills.
A 2010 Bureau of Reclamation environmental study for the Joint Federal Project cites the East Branch of the Bear Mountain Fault, located just eight miles away in El Dorado Hills along Oak Ridge as a potential seismic source of the final ingredient.
The fault is capable of producing a 6.5 magnitude earthquake, the study states, more than enough to induce liquefaction of dredged tailings beneath the dam.
It describes a worst case, high-water dam failure scenario that would dump up to 1 million cubic feet of water per second into Blue Ravine. “The inundation zone would include parts of the south side of the city of Folsom, most of Rancho Cordova and a large part of Sacramento,” according to the EIR.
Seismology studies for the Folsom and Auburn dams concluded that such an event is unlikely.
Hobbs provided a more conservative 6.0 estimated trigger-point during a 2010 interview, enough to instigate slippage, he said, resulting in dam deformation and cracking. Without reinforcement, a breach at high water could expand quickly, he added, describing the results as “An immediate, high force inundation … potentially worse than Katrina.”
Earthquakes strong enough to trigger liquefaction have occurred in the Sierra foothills, Hobbs continued. “It all depends on how strong they are and how close the epicenter is to the dam.”
20-plus years of fixes
The 400 to 600 feet between Green Valley Road and the dam have been a hard hat sandbox for more than 20 years, hosting various attempts at shortcut solutions to stabilize the old earthen structure without knocking it down and starting over.
Efforts to stabilize the foundation began in earnest during a dry summer in the late 1980s, when low water levels exposed the upstream foundation, which was compacted and then overlaid with a thick berm.
Stone columns were installed on the downstream side in 1993 and 1994, but failed to do the trick.
In 2007 bureau engineers injected a high-pressure concrete mixture beneath the dam, in hopes that the mortar-mix would spread into adjacent soils and stabilize the foundation. Testing revealed that the injections helped, but that portions of the foundation remained unstable.
Bureau of Reclamation engineers took a different approach, starting in 2007, opting to mitigate rather than prevent liquefaction, proposing a resistive barrier at the base of the dam would prevent slumping during an earthquake, even if liquefaction occurs, said Hobbs. Similar barriers, thick concrete footings “keyblocked” to bedrock, are common in commercial construction, but were new to the bureau, he added.
The barrier is 60-feet thick and stretches the entire 900-foot length of the dam, at a depth of 35 to 75 feet, through the tailings to bedrock. Early cost estimates ranged from $25 million to $75 million.
The barrier was designed as a row of concrete-lined rectangular cells, the bottom third of each filled with concrete to anchor it to bedrock, then topped off in layers with soils engineered for stability.
To reduce risk to the dam, non-adjacent cells were excavated two at a time.
Shimmick Construction won the job with a $35 million bid in 2010 and broke ground on a proof-of-concept test cell that fall.
The plan called for extensive drains and filters to address anticipated dam seepage, but the old dam proved remarkably water tight. The occasional pools on the downstream side of the dam come from groundwater that collects naturally in the old stream bed, said Hobbs.
Shimmick is currently ahead of the March 2013 targeted completion date, said Hobbs.
For good measure, once the barrier is complete, the material stockpiled from the spillway and currently piled up on the far end of the project site will be used as part of a 1.3-million-yard overlay, covering the barrier and extending all the way to Green Valley Road, creating a 400-foot wide plateau with a steep bank that drops roughly 100 feet to Green Valley Road.
Once the overlay is in place, the dirt parking lot north of Sophia Parkway will be reopened, and the residual environmental work finalized.
A contract for the overlay phase will be awarded in late 2012 or early 2013, with the target for completion in 2017, according to Hobbs.
The bureau acquired a large park site in Kanaka Valley as mitigation for the spillway and the dam reinforcement, and also plans to restore portions of Mississippi Bar on the north shore of Lake Natoma.
Given the scope of the work undertaken, the impact to nearby residents, including a transient driving and boating population, has been minimal, said Hobbs.
Early proposals that called for closing Folsom Point were met with sharp public criticism. The bureau responded with an underpass and staging area that left the popular boat-launch open.
Traffic impacts on Green Valley Road were greatly reduced by putting a cement plant on-site.
“We’ve tried real hard to be sensitive to our impact, and the public’s been very supportive,” said Bureau Program Manager Drew Broussard.
The 900-foot by 400-foot plateau created by the overlay could be a source of much more goodwill in Folsom and El Dorado Hills.
Hobbs described the size of the area as “a couple football fields,” and when asked if such fields could be installed up there, he said the area is federal land under control by the state, “but if the community gets interested, who knows.”
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