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Peaking storage tank gets going with concrete

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After years of planning, construction of Bonney Lake's “peaking storage tank” has begun.

The new water reservoir - so named because it is intended to augment the city water supply at peak use times during the summer months - will be about two blocks east of Home Depot on 96th Street East.

When construction is completed, estimated to be in June 2007, the tank will hold 15 million gallons of water.

The diameter of the tank will be 275 feet with 35 foot-high walls.

According to Bruce Gould, the city's building inspector, the floor is made up of a series a concrete slabs and the walls will be concrete with steel seismic cables inside for reinforcement.

“Normally rebar would be used in the walls,” Gould said. “ The cables allow the walls to move independently in case of an earthquake.”

The top will also be concrete supported by 116 columns inside the tank. A membrane cover will be used for roofing material.

The walls will be 14 inches thick and the top 10 inches.

After the walls are poured, steel cables will be rigged around the tanks and tightened to hold the walls secure against the pressure of the water.

A single gallon of water weighs 8.3 pounds, so when the tank is full the water will weigh 124.5 million pounds or 62,250 tons - the weight of one cruise ship, according to Gould.

The floor and walls will be poured in a series of slabs with a heavy plastic seal at all joints to stop any leaking.

The tank will use about 4,000 yards of concrete.

A 12-inch water main will fill the tank and a 16-inch main is used for outgoing water. A third 12-inch main will act as an overflow, emptying into a retention pond near the site.

Gould estimates it will take 15 to 20 days to fill the tank once it is completed.

The contract to build the tank was awarded to Skaar Construction from Auburn as the low bidder at $6.1 million.

Planning for the storage tank began in 2001 when it became clear development in and around Bonney Lake would place too much demand on the city's water supply system.

Plans for a sub-grade tank were considered, but the cost was prohibitive and the above-ground tank was decided upon by the City Council in 2004.

“The council revisited the issue about six months ago to make sure this was the right decision,” Public Works Director Dan Grigsby said.

“Our biggest concern is about two months out of the year,” he added. “During the winter we only use about half of the water we produce, but during a hot spell it will increase 144 percent.”

Grigsby said the tank will “minimize the use of Tacoma Water.”

The city has a contract to us up to 2 million gallons of Tacoma Water per day.

With the Tacoma Water contract and peaking storage tank, Grigsby said, “The city's water needs should be met for about 10 years.”

A booster pump station will be constructed near the site to move water in and out of the tank.

According to Grigsby, a separate contract for construction of the pump station will be sent out for bids this fall.

Dennis Box can be reached at dbox@courierherald.com.

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