Those loosely deposited soils could turn from solid to liquid, and once they do, they lose their strength. —Curt Pledger, area manager of the BOR's Provo office
ECHO DAM, Summit County — A seismic overhaul of an aging dam is the largest federal project of its kind ever embraced in Utah, representing a four-year, $50 million effort.
When complete in late 2014, the earthquake safety modifications will meet or exceed federal standards, and the U.S. Bureau of Reclamation will be able to say one less dam in the state is at risk of catastrophic failure.
Excavation at the base of the dam south of I-84 in Weber Canyon began in June and is six weeks ahead of schedule because of a warm spring and an early draw on the waters from the dam, which was built in the late 1920s and early 1930s as a single-purpose irrigation project.
Crews will remove 665,000 cubic yards of dirt down to bedrock at the downstream slope of the dam, using heavy equipment to gouge out a massive hole. A safety analysis for Echo Dam found that dirt at its foundation and underneath the spillway controls could liquefy in an earthquake.
"Those loosely deposited soils could turn from solid to liquid, and once they do, they lose their strength," said Curt Pledger, area manager of the BOR's Provo office. "The top of the dam might drop down, and it would be a catastrophic failure."
A seismic analysis shows a fault plane between Henefer and East Canyon Dam to the west that is capable of a 6.5-magnitude earthquake.
Pledger said a collapse of the dam would imperil all the communities downstream — Henefer, Morgan, Peterson, Stoddard, Uintah and South Weber. Flood waters would reach the flatlands of Plain City — more than 50 miles away in Weber County, Pledger said.
"What we are doing is removing all that liquefiable material all the way down to bedrock and replacing it with stronger, denser material," he said. An estimated 1.2 million cubic yards of that new dirt will be used.
The dirt that has been removed has been piled in a 80-foot high mounn that is growing bigger by the day. Once the hole has been replaced with the denser material, contractors will construct an earthen stability berm designed to buttress the dam, Pledger said. Another upstream berm will be constructed and compacted to further minimize the risk of any catastrophic failure.
That project will begin in the fall as the reservoir levels continue to drop.
Ivan Ray, manager of the Weber River Water Users Association, said the goal is to drop the water level to 4,770 acre feet of water by Aug. 15. Capacity is nearly 74,000 acre feet.
"It's a third full now and dropping a foot a day," Ray said. While that upstream berm is under construction from October to early December, Ray said the reservoir will be at its lowest level since it was built.
Ray said he has 12,500 acre-feet of water stored in Rockport Reservoir to help against any water crunch, and he's hopeful the winter will bring much-needed precipitation to bolster water supplies going into the spring. Work on the spillway to modify it for earthquakes won't begin until late summer of 2013, in time to have it functioning for the spring runoff in 2014.
The Echo Dam seismic retrofit follows similar, but smaller-scaled projects the bureau has done at Pineview Reservoir in Weber County and at Deer Creek Reservoir outside of Heber City. All dams are inspected annually, and every six years a more comprehensive analysis probes engineering, design and other aspects of a particular dam's operation and maintenance.