Staying A Step Ahead: Water Security Is Vital to City Building and Population Growth

As the main constituent of the Earth’s hydrosphere and the fundamental fluid of life, water is everything. Since the dawn of civilization, humans have worked hard to leverage the power of water; learning to channel it, protect it, direct it and harness it in many ways to continual benefit.

Aside from accounting for roughly 70 percent of the human body’s mass, water is ubiquitous in daily life. From cooking and cleaning to transportation and energy generation, it would be hard to think of many meaningful human objectives that don’t involve water in some way. However, with the effects of climate change encroaching on life across the globe, the need to enhance resource security has never been more pressing. Denver Water Construction Manager Doug Raitt, P.E., PMP, gets it.

“Of all the forms of infrastructure I have been involved with, which is most of them, water is the most fascinating,” begins Raitt. His career trajectory of nearly 40 years in civil engineering has covered roads, bridges, heavy and light rail, wastewater, and water resources for large-scale engineering and construction firms before joining Denver Water in 2009. “There is nothing more important when you consider must-haves in civilization. The supply side of water resources poses a series of very complicated civil engineering challenges involved in collecting it, storing it, treating it and distributing it municipally for consumption.”

While occupying the same central Denver campus since 1878, Denver Water’s existence as a public utility was formalized in 1918 when city officials recognized the need for well-orchestrated stewardship of this most-precious resource. Today, Denver Water serves the daily water needs of approximately 1.5 million people in metro Denver by collecting snow and rain from nearly 4,000 miles of surface area stretching from Denver west to the Continental Divide and beyond.

“My interest is focused on the collection system,” says Raitt of the big-picture objective. “Back in the late 1880s, the focus was on developing the South Platte River and other local tributaries as the primary source of water for residents. However, the South Platte is very seasonal. It was obvious to the founders that the collection system needed to extend into the mountains.”

Working with Water Scarcity

As Denver’s population swelled, growing from roughly 106,000 in 1890 to 250,000 by 1920, Denver Water’s need for ever-greater reach also grew. As The Dust Bowl began to parch the Midwest and Great Plains in 1930, Denver Water wisely began investing in water retention. Construction of the Eleven Mile Canyon Dam was completed in 1932. Although 80 percent of Colorado’s precipitation is on the western side of the state, 80 percent of the population resides east of the Rockies. So, Raitt and his predecessors at Denver Water have been working on staying a step ahead since the start.

“Even back in the beginning, Denver Water understood that the western slope offered a lot more water than the eastern slope,” continues Raitt. In 1935, construction began on the Moffat Water Tunnel Diversion project, which involved enlarging and partially lining the pilot bore of the famous Moffat Railroad Tunnel to carry water from west to east across the continental divide. “Of course, once you collect all that water you must be able to store it, so you have it all year long. In the U.S., the completion of the Hoover Dam in 1936 began the heyday of dam building that followed in the 1940s, ’50s and ’60s.”

Among the many dams built, one of Colorado’s most-impressive pieces of infrastructure is the Gross Reservoir Dam in Boulder County. Completed in 1954 and named for Dwight D. Gross, Denver Water’s former chief engineer, Gross Reservoir receives much of the water collected from the western slope. Sitting at 7,282 feet above sea level, Gross Reservoir has a surface area of 440 acres and a water volume of 41,811 acre-feet. However, back in the early 1950s when the dam’s construction was just underway, Denver Water’s planners were still thinking ahead to the time when they’d need to store even more water. Now their forethought is about to come to fruition, once again.

 

 

Raising the Dam

“The original designers anticipated the eventual need to expand the capacity of Gross Reservoir, so they designed the dam to be raised when the time came,” shares Raitt. “In 2002, there was a drought that highlighted the importance of our water-supply system and how little we had in reserve. With so little room for error, the plan to expand Gross Reservoir by raising the height of the dam began in 2003.”

Objectively, the goal was to raise the dam by 131 feet, which would increase Gross Reservoir’s existing capacity by 77,000 acre-feet, nearly tripling its size. Although well intended, Denver Water’s plan was met by stiff resistance from the Boulder County Board of County Commissioners, the governing body where the reservoir resides.

 

 

“There was a very long and thorough examination of the need for this expansion and the environmental impacts of doing so,” continues Raitt. “There was a lot of public interest and a lot of good questions asked. Ultimately, the shape of the project was crafted to account for various environmental protections like stream restoration and improving fish habitats in Grand County, among many more.”

Following a decades-long legal process, in November 2021 Denver Water and Boulder County Board of County Commissioners reached a settlement that allowed the plan to move forward. The more than $12.5 million settlement includes $5 million of funding intended to mitigate resident disruption and $5.1 million for open-space funding to replace lands that will be inundated by water. Another $1 million is earmarked for the restoration of a critical species habitat downstream on South Saint Vrain Creek.

 

 

The Team and Plan

In preparing for the dam’s expansion, Denver Water worked with a multi-faceted engineering team headed by Stantec and AECOM while concurrently leveraging the expertise of Kiewit Barnard, a joint venture on the construction side under a construction manager/general contractor (CM/GC) procurement model.

Todd Orbus, deputy project director, is a 25-year Kiewit employee who has made dam building his life’s work. He has been working on the Gross Reservoir expansion plan since Kiewit joined the preconstruction effort in 2019. With a degree in civil engineering from the California Polytechnical State University, San Luis Obispo, Orbus spends his days crunching schedules, calculating costs and managing the logistics of this massive building program. First, he explains the rationale behind the Kiewit Barnard Joint Venture.

“Either Kiewit or Barnard could have constructed this project individually,” explains Orbus. “Instead, we choose to form a fully integrated partnership with people from both firms involved in every aspect of the project.”

The project calls for adding 750,000 cubic yards of roller-compacted concrete (RCC) to the existing structure. When construction is complete in July 2027, Gross Dam will be the tallest in Colorado and the tallest RCC dam in the United States. “Kiewit has a great deal of experience with roller-compacted concrete under more-ideal weather conditions along the West Coast. Barnard has roller-compacted concrete experience in cold-weather climates, like what we will face at Gross Reservoir.”

Why RCC?

Despite its obvious durability and worldwide use, concrete can be fickle. The compound’s structural integrity is particularly vulnerable during the mix, pour, set and cure period. Concrete’s specificity has to do with the exothermic reaction that occurs between cement and water during the cure. If the concrete gets too hot, it may gain strength quickly, but the final strength will be less than ideal. If the concrete stays cold, it will not set, stalling placement progress. Curing concrete between ideal temperatures of 50 to 90 degrees results in the best combination of final strength vs. time to cure. For the expansion of Gross Reservoir, the use of RCC allowed Kiewit Barnard to build the expansion significantly faster than would be possible with traditional cast-in-place concrete techniques.

“Roller-compacted concrete, or RCC, is the state-of-the-art for building dams today,” says Orbus. “RCC has a much lower cementitious content than standard concrete. It comes out at about 340 to 360 pounds per cubic yard compared to 600 to 700 pounds per cubic yard for typical conventional concrete. In terms of consistency, it lays out like a damp base rock rather than flowable concrete.”

The technique takes its name from the construction method used to build it, which is more akin to putting down an asphalt road than pouring a concrete slab. Although RCC has the same basic ingredients as conventional concrete—cement, water and aggregates—it’s produced in a much drier mix than typical concrete; it arrives at the formwork in a gravel-like condition instead of a slurry. This stiffness allows it to be spread out by track bulldozers and compacted by vibratory rollers.

“We will place this in 1-foot vertical lifts, with widths ranging from 35 feet to 75 feet, and lengths up to 2,000 feet. We will go up 471 feet, one foot at a time for two building seasons,” continues Orbus. Although the Gross Reservoir project is only 40 miles west of Denver, the drive over mountainous terrain above Boulder, Colo., isn’t easy or quick. At the site, the cold, rugged conditions offer a very limited annual window when the concrete can be placed. “The cold makes the construction season very short. We will place the RCC from May 1 to November 1 in both 2024 and 2025. Due to the large surface area of fresh RCC during placement, there is no way to protect this volume of concrete from the cold efficiently.”

While a cold cure is detrimental to productivity, a hot cure will result in ineffective bonding, which could have greater consequences. For Denver Water and the Kiewit Barnard Joint Venture, the heavy lifting involved in countering unwanted heat gains occurs while batching the concrete. The batch plant developed for expanding Gross Reservoir is the most-advanced concrete production system Orbus and Raitt have ever seen.

“The RCC for this long, arch-shaped dam has to be placed at 50 degrees or less, which is very unusual,” notes Orbus. “To do that, we have to bring it out of the batch plant at about 40 degrees. This involves a variety of techniques to cool the material down as it’s processed.”

 

 

Beginning with a wet-belt conveyor, the locally quarried course and intermediate aggregates are sprayed with chilled water to drop their core temperature as the aggregate goes into the plant. Likewise, the sand for the mix is cooled on a conveyor that operates a bit like an air hockey table, dropping the material’s pre-mix temperature. In the concrete mix, flake ice will replace water wherever possible to decrease heat gains.

A Special Dam

“From a construction perspective, this dam raise is unique in converting an existing gravity dam into an arch dam,” explains Orbus. “Usually, RCC is used on massive structures widthwise. This is a very narrow, arch-shaped overlay, so RCC placement rates are a lot lower than normal. From an engineering perspective, I believe this is the first time RCC has been used as an overlay in an arched formation in the U.S.”

With a full complement of dam builders, engineers and other experts of many types at their disposal—between Denver Water, Stantec, AECOM, Kiewit and Barnard—there was no lack of mental acuity available to sort through the logistics of building a dam against a dam.

“Using the CM/GC model, Kiewit Barnard was brought in early to consider cost, schedule, logistics and constructability in concert with the Stantec/AECOM team,” continues Orbus. “The biggest impact we had on design was to move from a concept that involved three structural elements to just one. Originally, two thrust blocks would have acted like natural canyon abutments built on either end of the overlay with a center arch between the two. In terms of the engineering, that would have worked well. From a constructability perspective, that would have meant another building season. We helped convert that to a single-element dam that starts at the bottom and builds up against the face of the existing dam, which allows us to build this in two seasons rather than three.”

As Orbus and his team prepare the site and situation for the first building season, everything is as planned.

“We have prepped the existing surface for the overlay,” shares Orbus of the two years of work put in by many since the April 2022 construction start. “That involved using hydro-demolition machines that spray water at 20,000 psi to remove the outer 3 inches of the existing concrete, so there is a rough bondable surface. There was extensive foundation grouting of several types. We also built a full-scale trial placement of more than 5,000 cubic yards to test the mix design, replicate the means and methods, and train the crews. And now, finally, we are ready for the fun part, which will be rolling out the concrete and increasing the height of this 340-foot-high dam by another 131 feet.”

Like many who have come before them, Raitt and Orbus are proud of the teamwork and tenacity demonstrated by their cohorts in pursuing the future of resource security for Denver.

“There are people who question if Denver Water will ever be able to fill this reservoir,” finishes Raitt of the long-range objective. “It will take quite a while, likely close to the end of the decade. When it is full, Gross Reservoir will give Denver Water the assurance we need to face even greater fluctuations in precipitation.” 

The post Staying A Step Ahead: Water Security Is Vital to City Building and Population Growth first appeared on Informed Infrastructure.