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 Production Hits Full Speed On Site
With the busiest year of construction ahead for the Chimney Hollow Reservoir Project, crews will be in full production on all components to ensure the project meets the rigorous four-year construction timeline.
After a year of construction, there is a lot of energy on site as we are now building vertically. With necessary foundational work for the project now complete, being able to see the visual changes today is exciting. In fact, the main dam has recently reached 75-feet-tall, which is more than a quarter of the way to the final height of 350 feet.
To remain on schedule, crews began working two 12-hour shifts, with 350 employees working during the day and 150 employees at night. The 150-employee increase from a year ago drastically ramps up activity on site. Safety is our top priority, which is why, among other things, crews routinely refuel and check equipment in between shifts.
Full production on site is a team effort put forth by routine tests for environmental compliance from inspectors and safety officers. We are committed to being good neighbors for our community, which is why systems were implemented to monitor noise to stay within a safe decibel and limit light pollution for the ease of our nearby residents. A project of this size requires significant staff time to carefully monitor and actively manage visual disturbances and mitigate noise.
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 Concrete Production Ramps Up for Busiest Year
With this being the busiest year of construction, it is no surprise that, in 2023, crews will place the largest amount of concrete in all four years of construction. In total, the project requires 35,000 cubic yards of concrete. From August 2021 through December 2022, crews placed about 10,000 cubic yards. In 2023, crews will place 20,000 cubic yards. Concrete is placed daily at locations including the saddle dam grout cap, Chimney Hollow conduit thrust blocks, spillway, valve house foundation, tunnel lining and the Carter Lake interconnect.
Concrete is made of coarse aggregates (gravel), fine aggregates (sand), fly ash, water and Portland cement, which is the most common cement. The only materials that are imported to the site are Portland cement and fly ash. Everything else is sourced from the on-site quarry. The quarry is first mapped to find the best hard granite, which is then blasted and hauled to the aggregate processing plant. Both the coarse and fine aggregate go through three crushers, separately, followed by a wash plant to remove fine dust. After the materials go through the aggregate processing plant, they are hauled to the concrete batch plant where they are weighed into batches and mixed.
Next, each batch is tested for air content and consistency. The air content is tested to look for microscopic air bubbles so that when the concrete freezes and thaws, it can expand and contract. The consistency is tested to make sure it’s not too thin or too thick, both of which could cause issues during placement. Once a batch passes the tests, it is ready for placement. The mix then goes into concrete trucks, like you’d see on city roads, and is transported to various areas for placement on site.
Prior to the concrete being mixed, crews prep for placement by completing the form work, which includes rebar, forms and water stops. Inspectors must approve and sign off on the form work before any concrete is placed.
When it’s time to place the concrete at its appropriate location on site, a concrete pump truck pumps it into place. It is vibrated to consolidate and densify the concrete into the form before a roller smooths the top to finish it. Depending on the outside temperature, it is then either cured with insulated blankets or with shade and cold water.
Batching the concrete on site not only minimizes disturbances to the community, but it also keeps trucks off the road. Additionally, the Portland cement is more environmentally friendly due to a minimal amount of processing that results in a 10 percent reduction in the overall carbon footprint.
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Chimney Hollow Construction Provides Economic Benefit
Last month, Chimney Hollow Reservoir construction hit a major milestone: 1 million labor hours recorded at the site.
That milestone reflects the continuing progress being made at Chimney Hollow Reservoir to bring the project to completion in 2025, but it also highlights another indirect benefit of construction: the economic impact the project will have on the region during construction and beyond.
The 12 participants in the project are paying the contractor, Barnard Construction Co. Inc. of Bozeman, Montana, more than $500 million for construction of the dam. In addition, construction managers, dam designers and independent engineers are fulfilling many tasks at the site to ensure the dam is built to state-approved specifications.
For each of those tasks, the time and wages have added up. According to research conducted as part of a similar dam construction project in the region, the salaries of those who work on the project are multiplied several times across the community. For those workers who have temporarily moved to Northern Colorado for the project, salaries have gone toward lodging, meals and other necessities for temporary stays. For the many subcontractors who already live in the area, salaries will be part of their daily living expenses for them and their families.
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Managing Water On Site During Construction Makes a Big Impact
With Chimney Hollow Reservoir construction nearly half complete, managing the site’s water is vital to the construction process and other on-site water needs. Construction crews work hard to maintain a reliable construction water supply, control stormwater runoff and to make sure we only discharge clean water downstream from the site.
A stormwater management plan approved by the Colorado Department of Public Health and Environment (CDPHE) lays the groundwork for the discharge and control of all stormwater associated with construction activities. A temporary cofferdam helps to keep stormwater out of the active construction areas. Crews also operate an on-site industrial wastewater plant to treat water that has come in contact with active construction.
“We utilize a number of measures and best management practices to ensure that any water that runs off our site is clear of sediment and other contaminants,” said Joe Donnelly, Northern Water’s Principal Project Manager for the Chimney Hollow Reservoir Project. “Ultimately, we only discharge water from the site that has gone through a treatment plant, going above and beyond typical construction practices.”
On site, the largest use of water is seasonal dust suppression throughout the warm months of the year. Water trucks spray roads throughout the site to keep dust at a minimum. In addition, crews use non-toxic polymers to help hold dust particles together to cut down further on dust. Another large use of water is wetting embankment materials before they are compacted for the main dam embankment construction. According to Donnelly, this process is now in full production with round-the-clock work to raise the dam to its ultimate 350-foot height. A water line was also built from the local drinking water provider to supply potable water to the on-site offices for approximately 500 people working on site.
Spill prevention control measures, which are regulated through an environmental permit, ensure crews have at least two levels of containment around fueling stations and other areas that work with common construction fluids like motor oil and hydraulic fluid so that no contaminates enter water sources.
“Managing water on site is a small part of the project, but it is hugely important to our long-term project goals of providing a safe and reliable drinking water supply for the region,” Donnelly said.
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 Saddle Dam Construction Underway
Two-and-a-half miles south of the main dam at Chimney Hollow sits the saddle dam, which is often a forgotten, but major, component of the project. Completion of the saddle dam allows 30 percent more capacity in the reservoir.
Unlike the main dam, the saddle dam will be a clay-core rockfill dam, similar to those at Carter Lake and Horsetooth Reservoir. It will be 40 feet tall and contain approximately 100,000 cubic yards of rockfill and 30,000 cubic yards of clay. It will also include a double-row grout curtain extending approximately 70-feet below the dam to reduce seepage through the foundation.
Crews have been working over the last few weeks to excavate the area and begin pouring dental concrete. Soon, crews will begin to place the concrete grout cap, which will be the stable base for the dam to begin rising. A grout cap and a plinth are essentially the same; however, a plinth is the term used with asphalt-core dams, while a grout cap is used in reference to clay-core dams.
Construction is expected to be complete in summer of 2024, one year before Chimney Hollow construction wraps up.
 Asphalt Core Width Decreases as Dam Rises
Last month, the height of the main dam hit 60 feet and with that, the width of the asphalt core decreased from 3.3 feet to 2.95 feet wide. As the dam rises, the water pressure decreases, and therefore, the core does not need to be as thick.
At the base of the dam, where the asphalt core touches the concrete plinth, the asphalt is 6.6-feet wide for two 9-inch lifts, followed by two lifts placed at 5-feet wide and then continued at 3.3-feet wide. The width will decrease three more times and will be 2-feet wide for the top approximately 160 feet of the 350-foot-tall dam.
By The Numbers: 1,000,000 Cubic Yards of Zone 4 Embankment Placed
As of 8 a.m. on April 27, crews placed 1,000,000 cubic yards of zone 4 embankment on the main dam. Zone 4 embankment is the majority of the rockfill that is placed on the main dam and consists of rocks 2-feet in diameter or less. Only 10 million more cubic yards to go!
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Chimney Hollow Tours
We understand there is a great deal of interest in seeing Chimney Hollow construction. Priority for tours this season will be given to the project participants’ staff and leadership. A participant is defined as one of the 12 organizations funding the construction of Chimney Hollow Reservoir. One week before a tour, any remaining spots will be offered to outside groups and the public.
To be notified when tour spots are available, sign up to receive notifications. Please note that spots will be filled on a first-come, first-served basis when the email notification is sent out, and spaces unfortunately cannot be held. Additionally, due to high demand, individuals are limited to one tour per season.
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Stay Connected
Visit the Chimney Hollow Reservoir Project website for the latest news and project highlights.
Also, connect with us on Facebook and Twitter!
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About Chimney Hollow Reservoir
The Chimney Hollow Reservoir Project is a collaboration between 12 Northeastern Colorado water providers to improve the reliability of, or make firm, water supplies from the Windy Gap Project. Chimney Hollow Reservoir will be located just west of Carter Lake in Larimer County to provide dedicated storage to supply a reliable 30,000 acre-feet of water each year for future generations.
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