Gross Dam construction making steady progress
Denver Water’s Gross Dam in Boulder County continues to rise after a busy summer of construction.
Hundreds of workers are taking part in the Gross Reservoir Expansion Project, which will raise the height of Gross Dam by 131 feet.
As of Sept. 5, crews had raised the dam by 60 feet. The project is designed to increase the water storage capacity of Gross Reservoir, which supplies water to 1.5 million people in the Denver metro area.
“Over the past two years, we’ve been working on the original dam to prepare it for the enlarged height and width,” said Casey Dick, Denver Water’s deputy program manager for the project.
“At the end of June, the concrete work reached the original crest, so now all the concrete placements are above the existing structure.”
Once completed, Gross Dam will be 471 feet tall and around 2,000 feet wide.
As the dam has gone up, it has become easier to see some of the differences between the original dam, which was completed in the 1950s, and the newly renovated structure.
For instance, the original surface of the downstream side of the dam was smooth. Now, the downstream side of the dam is a series of stair steps. The steps were an integral part of the construction process and supported the trucks that deposited layers of concrete onto the original structure of the dam.
The renovated dam will also take on a new shape.
“The original structure was built as a ’curved gravity’ dam,” Dick said. “Now, we’re taking advantage of that curved geometry in the middle portion of the dam to create what’s called a ‘thick arch’ dam in the center of the canyon.”
Arches are used in dam construction because the force of the water in the reservoir pushes up against the arch and into the canyon walls. This gives an arched dam more strength compared to a flat structure.
“We’ve also built what are called ’thrust blocks’ on the sides of the original dam,” Dick said. “These give the dam additional support by essentially extending the canyon walls upward to support the arch.”
As work has risen above the original crest of the dam, workers have built formwork, or temporary molds, on both the upstream and downstream sides of the dam. The temporary structures hold the freshly placed concrete in the proper shape until it hardens and cures.
With the new added concrete added during the project, Gross Dam is now much steeper than the original structure. At the base, the dam is 300 feet thick, but it gets skinnier as it goes up. At the top, the dam will be just 25 feet thick. Crews have had to adjust to the smaller work area to maneuver their equipment as the project progressed.
Work to raise the dam will continue as late as possible into 2025, until weather conditions make it too cold to place concrete.
“We’d like to thank all the men and women out here from Kiewit-Barnard and the other contractors out here,” Dick said. “They are working around the clock and as fast as they can to complete this project."