Protecting Washington's Water Supply by Covering Reservoirs

Sponsored by

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.

Tacoma Water has been covering its existing open reservoirs since the 1980s with a systematic replacement program.

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.

Each 33 million gallon tank has an outside diameter of 489 feet at the bottom of the wall. Photo courtesy Syktech Aerial Photo, Fife, WA.

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.

More WaterWorld Current Issue Articles
More WaterWorld Archives Issue Articles

Sponsored by

Did You Like this Article? Get All the Water Industry News Delivered to Your Inbox or Mailbox

Subscribe to one of our magazines or email newsletters today at no cost and receive the latest information.


Local sanitation agency to gain authority for stormwater management under new law

CA Governor Brown has signed into law Senate Bill 485 by Senator Ed Hernandez. When the bill goes into effect on January 1, 2016, SB 485 will allow the Sanitation Districts of Los Angeles County to manage and treat stormwater and dry weather urban runoff in Los Angeles County.

CA water purification project installs online THM instrument for chlorine monitoring

Padre Dam Municipal Water District has installed the THM-100 online trihalomethane monitor from Aqua Metrology Systems to provide real-time data and a various of benefits for the utility.

WEF publishes new textbook geared toward emerging water professionals

The Water Environment Federation recently announced that it has officially published the book, "The Effective Water Professional - Leadership, Communication, Management, Finance, and Governance," geared toward young professionals.

Ovivo, Microdyn-Nadir form strategic partnership to service national MBR market

Ovivo recently signed an unprecedented, multi-year agreement with Microdyn-Nadir to service the U.S. membrane bioreactor market. The agreement secures access to BIO-CEL membrane technology and includes a collaborative venture to build private-label membrane equipment.




© 2015. PennWell Corporation. All Rights Reserved. PRIVACY POLICY | TERMS AND CONDITIONS