Mormon Island Dam stabilization in final phase
The bulldozers smoothing out the downstream slope of the Mormon Island Auxiliary Dam mark the final, “permanent fix” in a 25-year effort to reinforce it.
Visible from Green Valley Road, the 900-foot-long, 480-foot-high Mormon Island Auxiliary Dam is one of four dams and eight dikes that make up the larger system of Folsom Dam.
“Most people think the main, concrete structure is Folsom Dam, but it’s the entire facility made up of those 12 structures,” said Bureau of Reclamation Chief of Project Integration Mark Curney.
Between 1948 and 1956 when the Army Corps of Engineers built Folsom Dam they wanted to reach a height of 480.5 feet around the ensuing reservoir (better known as Folsom Lake) so had to create dikes “to connect the high points,” said Curney. Though it’s often mistaken as a levee, MIAD is the fourth and largest dam in the network because it’s a structure that was built to block an ancient path of the American River.
Folsom Dam will count 13 working parts once the Joint Federal Project is complete in 2017. The auxiliary spillway located near the concrete structure of Folsom Dam will improve the ability to manage large flood events by allowing more water to be released earlier in a storm and leave more storage capacity in the reservoir to hold back peak inflow when it arrives.
“Basically in case the mother of all floods happens, the JFP will ensure the dam could still function,” said Curney.
The joint project is currently in its third and fourth of five phases. The final phase will consist of site restoration, including reinstating the land that was used during construction and the Folsom Point recreation bridge that was built to maintain access during construction.
Though the project’s cost to taxpayers hovers around $1 billion, Curney said it could have cost hundreds of millions more had the Army Corps of Engineers and Bureau of Reclamation not taken it on together. “At one point the Army Corps of Engineers was looking at a project for flood control and Reclamation was looking at a project in the same area for dam safety,” he said. “The JFP marks the first time in American history two large federal agencies have come together to create one project to pass flows for flood control and safety in case we had the mother of all storms.”
Folsom Dam was originally built by the Army Corps of Engineers to protect the greater Sacramento area from flooding. Its ownership transferred to Reclamation upon completion for operation and maintenance. By the 1980s an increasing population and new engineering methods had its new guardian taking a new look at the then-40-year-old facility for safety.
“When you look at Folsom Dam as a whole, you can go less than a mile and see houses below it,” said Curney. “If we ever had any kind of event where we had issues with any of the facilities, we have people directly in harm’s way. That’s a big concern whenever you have houses that close. The probability is low, but the loss of life would be great should something happen. We needed to go in and make sure, whether it’s an earthquake or a big storm, that the dam would be solid.”
Maximum Folsom Lake elevation is 466 feet.
“When MIAD was built its core (the narrow pathway at top) went all the way down to bedrock, which means the foundation hit a hard rock,” said Curney. “But the downstream shell (which is the slope that’s visible from Green Valley Road) didn’t go down to the core in the transition zone, but was built on moveable material, which isn’t safe in case of seismic shifts.”
Because MIAD was built partially atop old mine tailings, officials proposed to excavate loose soils, then refill it with more solid material.
In the late 1980s Reclamation tried to create a cement field in between cracks with jet grout, but that was unsuccessful.
In the early 1990s, they built stone columns, which still didn’t create a solid foundation. “At that point we knew we had to go down and open up the downstream portion of the dam itself and put in a solid foundation, which is what we’re doing now,” said Curney.
A comprehensive facility review in 2000 identified potential risks, ranging from hydrologic (potential overtopping of dams and dikes during a major storm event), seismic (potential sliding and/or breaching in the event of an earthquake) and static (potential seepage and piping).
Phase 1, costing $35 million, began in 2010 and consisted of cellular construction down the length of MIAD’s 900-foot, narrow top to make loose soil solid. “We created 60 by 60 foot cells because they didn’t want to open it all up at once,” said Curney. “In a storm you don’t want the dam exposed to all kinds of elements, so this kept a good factor of safety at all times.” One-third of the cell (or 20 feet of it) was made with concrete, then unstable soil was removed. Cross braces were added to each cell as the hole deepened and then each cell was filled with stable soil once bedrock was reached.
Phase 2 of the static seismic modification, or the current overlay phase, began in late 2013. “Phase 2 is designed to work in tandem with Phase 1 to provide a more earthquake-resistant embankment and filter system,” said Curney. “We’re basically building a dam on top of a dam.”
The huge mounds of gray material stockpiled from the spillway and piled up on the far end of the project site are a mixture of sand, gravel and crushed rock and will make a 1.3-million-yard overlay, which will serve as a filter blanket before it’s covered with dirt to again camouflage with the landscape.
“When it rains you want water to hit your structure, but not saturate it,” said Curney. “The blanket will act like a filter and drain any kind of moisture.”
Currently a bulldozer is clearing off the downstream shell across the entire face of the dam to prepare it for the filter blanket. The mass of MIAD will then be increased with overlay material until it extends 900 feet across toward Green Valley Road. It will stretch across, appearing as a plateau instead of the steep embankment seen now.
Suulutaaq, Inc. of Suisun City won the bid for the overlay portion of the project at $45,719,235.
Asked if low water levels have helped or hindered the spillway or MIAD projects, Curney said the dry weather has been a benefit. “It’s enabled the workers to constantly work without rain,” he said. “For both projects the operation of Folsom Dam was key as well as making the littlest impact on the public.”
Once the overlay is in place, the dirt parking lot north of Sophia Parkway will be reopened.
The improvements to MIAD might be complete sooner than the original 2017 target date, maybe as early as spring 2016, said Curney.
“As far as safety this is finally a permanent fix,” he said. “We hope it lasts a lifetime.”