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Getting some ‘concrete’ answers about the Gross Reservoir Expansion Project

Filling nooks and crannies key to building Denver Water’s stronger, higher dam.

Raising a dam by adding concrete to it sounds simple. But it’s not. 

Raising Gross Dam in Boulder County involves dozens of workers on site every day doing many different tasks. And the concrete? It’s used in different ways at various stages of the project. 

During summer 2023, the future look of the renovated Gross Dam started to take shape as workers focused on shoring up the sides of the dam with — you guessed it — new concrete.

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The Gross Reservoir Expansion Project, scheduled for completion in 2027, will raise the height of the existing dam by 131 feet to nearly triple the storage capacity of the reservoir. The additional water stored in the expanded reservoir will add balance and resiliency to Denver Water’s collection system, which serves 1.5 million people in the Denver metro area.

Increasing the height of the dam today is just as challenging as building the original dam back in the 1950s, according to Doug Raitt, the project’s construction manager for Denver Water.

Construction workers use a pump truck with a large boom to place concrete on the side of Gross Dam in July 2023. Photo credit: Denver Water.

“It’s a very complex process with multiple steps required before we can move to the next phase of actually building the new dam,” Raitt said. “We’ve got lots of people and equipment and we had some challenging working conditions (at the site) with all the lightning, rain and mud this past spring.”

This illustration shows how new concrete will be placed on top of the existing dam once the foundation is prepared. Image credit: Denver Water.

Concrete solutions

Once the expansion project is completed, Gross will be the tallest dam in Colorado. However, before any work is done to raise the height of the dam, workers have to prepare the foundation to make it longer and wider.

The current dam is 340 feet high, 1,050 feet long across the crest and 267 feet wide at the base. 

The new dam will be 131 feet higher (a total of 471 feet high) and almost twice as long (2,040 feet) across the crest. It will also be up to 90 feet thicker than the existing dam. 

A look at the new footprint of the dam from the north side looking south. The existing dam section is in the middle. The new dam’s crest will be 2,040 feet end to end. Photo credit: Denver Water.

The project started in April 2022, and workers have already spent roughly 15 months excavating rock on the hillsides above the dam to clear the way for the dam’s bigger footprint. Raitt said there are still several months of excavation left before crews are finished with that phase of the project. 

Once the rock was removed, workers were left with two trenches of jagged granite that must be smoothed and strengthened into a foundation sturdy enough to support the new layers of concrete that will be added as the dam is raised. The actual raising of the dam is scheduled to begin in 2024. 

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“When we remove the rock, we can’t just start building on top of it, instead we have to prepare the site,” Raitt said. “This process involves using different types of concrete and grout that serve various functions to make sure we have a secure foundation when we build up.”

The three steps to prepare the foundation include placing two types of concrete, called “dental” and “foundation” concrete, and injecting grout into the rock. 

Dental concrete is the term for a type of concrete that is used to fill in the small nooks and crannies in the excavated areas to smooth them out. It gets its name from its similarity to filling in cavities from tooth decay.

Workers place dental concrete on Gross Dam’s right abutment to fill in shallow irregularities in the rock surface after excavation. Photo credit: Stantec.

Foundation concrete is a thicker mix used to cover the areas where the rock requires additional reinforcement to support the weight of the future, higher dam, or where the rock will not bond properly with the new dam concrete that will be used to construct the new addition.

Workers placed 500 cubic yards of foundation concrete into a section of the right abutment of Gross Dam, on the south side of the dam, in July 2023. The foundation concrete shores up the rock and creates a surface that will bond with the next layer of concrete that will be placed in the future to build the dam itself. Photo credit: Denver Water.

Workers use high-pressured water to roughen the surface of the foundation concrete so it will better adhere to the next layer of concrete that will be placed in the future to form the dam itself. Photo credit: Stantec.

Once the dental and foundation concrete has been placed on the surface, crews take the additional step of injecting grout into the rock. The grout fills tiny gaps in the rock in order to strengthen it to support the new, larger dam.

Construction workers use a drilling rig to bore holes into the rock on both sides of the dam and inject grout. The process shores up the rock under the foundation. Photo credit: Denver Water.

The foundation work will continue through 2023. In 2024, workers will start building the new concrete steps that will form the new dam. 

The raised dam will have a type of concrete called “roller compacted concrete.” (Stay tuned for more on that in future TAP articles and videos.)

“The project is going very well, we’re hitting our objectives for the foundation work,” Raitt said. “The foundation work is critical to provide uniform support and take care of any imperfections in the rock.”