Lake Mary - Phoebe Dam Study

Seventy-three year old dam undergoes seismic study

October 2, 1998

View of Lake Mary-Phoebe dam downstream showing the drilling equipment.
Located at the headwaters of Big Cottonwood Creek about one mile above Brighton, Utah is Lake Mary - Phoebe Dam.  A popular hiking site, Lake Mary is also part of Salt Lake City's water supply.  The dam, built between September 4, 1913 and October 15, 1915 by Owen H. Gray Contractor, holds 742 acre-feet of water collected during spring snowmelt and used during the late summer months to meet Salt Lake City's municipal water needs.
Up-stream view of reservoir from the left abutment.
The concrete-gravity-constructed structure stands 72-feet high and is 330-feet long at the dam crest. The elevation at the top of the dam is 9,529 feet above sea level. It is rated a "High Hazard" dam. As a result of this rating, Salt Lake City Department of Public Utilities is conducting necessary seismic and structure studies to determine the condition of the dam. Until the analysis is completed, the water level behind the dam has been reduced.

On September 8, 1998 the firm of PC Exploration, Inc. of Woods Cross, Utah was awarded a contract to drill test holes in the structure, and the next day a helicopter flew the drilling equipment to the dam site.  Their contract includes coring three test holes through the concrete dam into the supporting bedrock; two vertical and one angle core hole. These holes will be used to measure the water pressure at the base of the dam.

The contractor has collected core samples of the concrete in the dam and the granite bedrock below the dam. These cores will be tested and analyzed. Chuck Call, Chief Engineer for the Department of Public Utilities, in describing the 83-year old structure, said, “We have been very pleased with the quality and condition of the concrete in the dam.”

Water tests were also made in the core holes.  “The dam is in surprisingly good condition considering its age,” according to Mr. Call. During these tests, water is forced under pressure into the dam or bedrock to measure how much water the concrete or granite will take. “The dam is so tight that no water could be forced into one of the holes,” Call reported. Piezometers (plastic tubes) were installed in these holes so the water pressures can be measured in the future. This information will help evaluate the stability of the dam and make sure it is always safe.

Mr. Call related that, “We tried to chip off a small piece of concrete used to make this dam with a granite rock and we found that the concrete is stronger than the granite rock.”

Left to Right: Chuck Call, Chief Engineer, Frank Hamilton, Project Engineer and Jeff Niermeyer, Deputy Director inspect core samples from dam.
One of the first dams built by Salt Lake City, Lake Mary-Phoebe dam remains today as an important water storage facility.  The preliminary results of the current core testing indicate that this structure will continue to provide drinking water supply to Salt Lake City customers well into the future.

Questions regarding this article can be directed to: chuck.call@ci.slc.ut.us

Salt Lake City Deparment of Public Utilities (801)483-6785