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Gross Dam ready to go up

Final preparations underway at reservoir before dam raise begins.

The top of Gross Dam in Boulder County is bustling this spring as workers build the specialized structures needed to raise the dam.

Denver Water is raising the dam 131 feet as part of the Gross Reservoir Expansion Project. The project will nearly triple the storage capacity of the reservoir and add balance and resiliency to Denver Water’s collection system.

Excavation and foundation preparation at Gross Dam wrapped up in April. The far side of the photo shows the new footprint of the dam. Photo credit: Denver Water.

“Over the past two years we’ve excavated 260,000 cubic yards of rock and placed 27,000 cubic yards of concrete to get the existing dam and the rock around it ready for expansion,” said Doug Raitt, Denver Water’s construction project manager for the Gross Reservoir Expansion Project. 

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The next phase of the multiyear project begins in May, when crews will begin the process of building 118 new concrete “steps” that will create the higher dam. Construction on the expansion project began in April 2022 and is scheduled to wrap up in 2027.

Roller-compacted concrete will be placed on top of the existing dam to raise it to a new height of 471 feet. A total of 118 new steps will make up the new dam. Image credit: Denver Water.

The steps will be made of roller-compacted concrete and around 800,000 cubic yards of concrete will be needed to build them. 

So, to prepare for raising the dam, a team from Kiewit Barnard is building a sophisticated concrete batch plant near the top of the dam. At the plant, cement, fly ash, sand and aggregates will be mixed together to make the specific type of concrete mixture used to build the steps.

The batch plant will produce roller-compacted concrete on-site using rock quarried from around Gross Reservoir. Photo credit: Denver Water.

“Producing the roller-compacted concrete on-site really makes for an efficient process so we don’t have to haul it in from off-site,” Raitt said. “We’re also crushing rock that we quarried on-site as well.”

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Crews are also building an elaborate conveyor system that will carry the concrete from the batch plant to the dam. 

Workers are building a conveyor system that will move concrete from the batch plant to the dam. Photo credit: Denver Water.

Once conveyed over the top of the dam, the concrete will slide to the bottom via a chute system, which also will be built this spring. 

At the bottom of the dam, workers are creating a flat surface that will be the base for the new roller-compacted concrete steps.

Workers are building the base of the dam that will serve as a platform for the roller-compacted concrete steps. Photo credit: Denver Water.

“It’s an exciting time as we get ready for the actual dam raise phase of the project,” Raitt said. "Once the roller-compacted concrete process begins, it will take about three years to complete the expansion.”