Work continued last week inside a 3.5-million gallon concrete water reservoir under construction in the industrial east side of Redmond.

Within a week, the equipment and lighting inside the tank is expected to be gone or else be under more than 40 feet of water.

Redmond is wrapping up construction on the $5 million project just in time for the peak summer season, when water usage, thanks primarily to residents watering lawns, jumps to 14 million gallons per day from 2 million gallons per day in the winter, said City Engineer Mike Caccavano. Four new booster pumps in a neighboring pumphouse will be able to supply the reservoir with up to 5,000 gallons of groundwater per minute.

“That allows us to fluctuate with the daily demand,” Caccavano said.

The reservoir, which towers over the surrounding area, is the largest tank the city has. Caccavano said it will allow the city to keep up with demand for high-quality drinking water as it grows in population and industry.

“What we’re trying to do is stay ahead of the demand,” he said. “Redmond continues to grow, we’ll need more and more water. We need to build this now to stay ahead.”

Construction on the tank started last summer. After a crane was used to build the round shell and large columns that support the 44-foot-tall structure, a tower that rotates around the tank was installed.

The rotating tower had several purposes during the latter part of construction. Caccavano said it was first connected to a hydroblasting nozzle to roughen up the surface of the concrete tank. It was then used to spray a layer of shotcrete around the reservoir before being used to install prestressed cables in the concrete.

The concrete walls of the tank are one-foot thick. Caccavano said they would have to be more than twice as thick if not for the prestressed construction. The design is expected to be safe in a large earthquake.

“There’d be minor damage, but there shouldn’t be a system failure,” Caccavano said.

The city expects to fill the tank slowly, about eight feet a day, and have the reservoir in service by the end of May, Caccavano said.

Jack Burns of Auburn, Washington-based Skaar Construction, said the Redmond project used about 1,500 cubic yards of concrete. He has built tanks since 1989, with projects ranging in size from 500,000 to 33 million gallons.

The biggest challenge in building large tanks is the height, Burns said.

“As you get taller and taller, everything becomes exponentially more challenging,” he said.

The tank is expected to last 50 to 100 years, Caccavano said. While a concrete tank costs about $150,000 more to build than a steel tank, an advantage of using concrete is that steel tanks require painting and other major repairs every 20 years or so.

“It would probably be a half million dollars every time you do it,” he said.

The city owns enough land next to the reservoir to, eventually, build an identical tank on the site. Caccavano said the city is considering building an eighth well within the next five years and the additional tank could go along with it.

But, for now, he is pleased with the tank.

“Just being around something that is going to be here a long time and really work well,” Caccavano said. “I have a lot of confidence in this project.”

— Reporter: 541-548-2186,