First season of dam-raising construction wraps up at Gross Reservoir

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First season of dam-raising construction wraps up at Gross Reservoir

Workers made steady progress on Gross Dam raise in 2024, but there’s still a long way to go.

Construction workers had a productive year placing concrete at Gross Dam in Boulder County as part of Denver Water’s Gross Reservoir Expansion Project.

The project will raise the height of the existing dam, completed in the 1950s, by 131 feet. When the construction is complete the dam will rise 471 feet above the streambed below. The higher dam will allow the utility to nearly triple the storage capacity of the reservoir.

Gross Reservoir is a critical storage facility that helps Denver Water supply water to 1.5 million people in the Denver metro area.

To raise the dam to its future height of 471 feet, workers are adding a series of steps up the face of the existing structure that will make the dam thicker and wider.

Watch how workers build new steps on the face of Gross Dam.

Workers started building the new steps in May and completed the first 269 feet by the end of November. Work on the steps will resume in spring 2025.

“We had a very productive year here at Gross Dam and it’s impressive to see what we’ve accomplished,” said Doug Raitt, Denver Water’s construction manager for the Gross Reservoir Expansion Project.

Wide shot of a dam under construction with a reservoir behind it — A look at Gross Dam as it’s under construction in October 2024. This photo shows the new concrete, which is the lighter shade of gray, compared to the more beige color of the existing dam. Photo credit: Denver Water.

The project raises the height of Gross Dam while also changing the dam’s engineering design from its original “gravity dam” style to a new “arch dam.”

Gravity dams use their structure’s weight and mass to hold back water in a reservoir.

Arch-style dams use their curvature to transfer the load of the water into the rock walls beside them. The force of the water against the structure actually makes the dam stronger.

“We’re using roller-compacted concrete to build the new dam,” Raitt said. “This type of concrete allows for rapid construction in this unique canyon environment.”

Workers make the roller-compacted concrete on-site using rock from around the reservoir. The concrete then travels on a conveyor system where it falls down a chute to trucks below. The trucks dump the concrete onto the new dam where bulldozers and rolling machines spread and smooth it out.

Heavy equipment work on the concrete dam — Roller-compacted concrete comes down a chute in the upper right. Dump trucks then place the concrete in the work zone where workers spread and smooth it out. Photo credit: Denver Water.

While the existing dam had a flat face, the new dam will feature steps that are 4 feet tall with a 2-foot setback.

Crews are also adding a layer of traditional concrete on the exterior face of the new steps to act as a protective coating against the elements.

Side view of the steps on the concrete dam — A completed section of the dam expansion shows the new steps on the exterior. Each step is 4 feet tall and has a 2-foot setback. Photo credit: Denver Water.

“Traditional concrete has a higher concentration of cement compared to the roller-compacted concrete. The extra cement in the traditional concrete makes it better able to withstand the freeze-thaw temperature cycles that we see up here in the foothills,” Raitt said.

Two workers use heavy equipment to pour concrete — Workers place traditional concrete on the exterior face of the dam. This mix protects the roller-compacted concrete and can withstand the harsh weather conditions in the canyon. Photo credit: Denver Water.

Raitt said the goal is to build the dam up to the new height of 471 feet in 2025, and then finish the top of the dam in 2026. Final work on the project is expected to wrap up in 2027.

“While we’ve accomplished a lot in 2024, we still have only placed about one-third of the concrete needed to complete the dam raise,” Raitt said. “Once we get above the existing dam, the new structure is much wider and longer, so it will take longer to build those steps.”

Side shot of crews working on the concrete dam — The foundation for the new footprint of the dam can be seen in the back, center portion of the photo. Photo credit: Denver Water.

Top and bottom work

Crews will work on the top and bottom of the dam in coming weeks as weather permits.

At the top, workers will be preparing the crest of the existing dam for new concrete that will be placed next year.

At the bottom, workers are starting to build the walls for the dam’s new spillway. The two walls will be 9 feet high on the face of the dam.

A spillway is an important safety feature of a dam that is used when the reservoir reaches capacity and overflows. It works much like an overflow drain on a bathtub or sink that allows water to pass through the dam instead of going over the top.

Learn how researchers at CSU played a role in designing the new spillway.

Photo from the bottom of the dam looking up at construction work — Workers have started construction on the new walls for the dam’s spillway. Two, 9-foot-tall walls are being built and once completed, the walls will channel water down the front of the dam in the event Gross Reservoir reaches capacity. Photo credit: Denver Water.

Team effort

A man smooths out concrete on Gross Dam — A construction worker smooths concrete on one of the dam’s new steps. Photo credit: Denver Water.

Raitt said about 300 people work on the project every day. The team includes men and women from Denver Water and its contractor, Kiewit-Barnard, along with engineering and infrastructure support consultants Black and Veatch, Stantec and AECOM.

“This amazing team worked 24/7 this summer to get where we are today,” Raitt said. “The men and women on this project are a group of dedicated professionals and we are extremely grateful for their hard work.”