Mormon Island Dam Foundation Stabilized Against Liquefaction Failure
Hobbs has overseen repairs to address its underlying instability. The old dam’s foundation is surrounded by mining tailings. 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.
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.
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 ...