Protecting Washington's Water Supply by Covering Reservoirs

By Judy Horning

Located at the foot of Mount Rainier in Washington State, the City of Tacoma boasts being the state's third largest city with a population of more than 203,000 residents. Tacoma Water also serves an additional 100,000 people in the cities of University Place, plus portions of the City of Puyallup and Pierce and South King counties.

In order to comply with Washington State Department of Health and U.S. Environmental Protection Agency regulations, Tacoma Public Utilities - Water Division plans to cover approximately 100 million gallons of stored drinking water at their existing McMillin Reservoir Complex in Puyallup. Tacoma Water has been covering its existing open reservoirs since the 1980s with a systematic replacement program. The McMillin complex includes three uncovered reservoirs with a total capacity of 210 million gallons. Construction is currently underway to build two 33-million gallon tanks.

Tacoma Water investigated several alternatives for covered water storage capacity and determined that prestressed concrete tanks offered the highest quality, longest life, and lowest maintenance option for this project. Constructed concurrently alongside its sibling tank, each 33 million-gallon tank has a huge outside diameter of 489 feet at the bottom of the wall. This makes them the largest diameter circular, prestressed concrete tanks of their kind.

The 66 wall sections taper from the thickness of 14 ½" at the top to 34 ¼" at the bottom. The height of the walls are 26'-9" with a 9" thick, cast-in-place roof slab. The cast-in-place floor slabs are 6" thick. Both the floor and the roof slabs were poured in 16 sections. Each roof is supported by 332 - 18" diameter columns inside each tank, totaling 664 columns.

The tank corewall will be both circumferentially and vertically prestressed by DYK Incorporated. The tank corewall will be post-tensioned vertically with 1,188 high-strength threadbars that are subsequently grouted with epoxy. The tank wall will be circumferentially prestressed by DYK's unique strandwrapping machine which will apply the desired force to the almost 300 total miles of 3/8" diameter galvanized strand. (That total length of strand, if placed across Interstate 5, would span the entire state of Oregon, from the Washington border to the California border!)

The strandwrapping machine will continuously and electronically monitor the applied stressing force as it is applied. By keeping the corewall in compression along with the independent base and top designed connections, a long-life, water-tight structure is ensured. Once prestressed, the strand will be encapsulated with several coats of machine-applied concrete. The machine used to apply the strandwrapping and shotcreting is considered to be the world's most technologically advanced, fully automated machine and will ensure that the necessary quality control is consistently maintained.

The project team includes Tacoma Water, owner, RH2 Engineering, Inc., Bothell, WA, project engineer, Peterson Structural Engineers, Portland, OR, structural engineer, Skaar Construction Inc., Auburn, WA, general contractor, and DYK Incorporated, El Cajon, CA, tank prestressor.

Completion of the twin water storage tanks will reflect Tacoma Water's mission to make the best use of available resources in order to provide high-quality and reliable water service to both its residential and business customers. Replacing the open basin reservoirs with covered prestressed concrete tanks will result in a safer water supply by protecting the drinking water from natural contamination and possible harm.

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