By Susan M. Carter
The unpredictable, rushing water of the Des Moines River is being harnessed to benefit a network of 60 municipalities. In August 2014, construction began on the 36.4-MW Red Rock Hydro Project, about 3 miles southwest of Pella, Iowa.
The project is a retrofit of an existing dam, and construction work includes building a powerhouse to contain two Kaplan turbine-generator units on the downstream side of the dam and an intake structure on the upstream side of the dam. When complete in 2018, the hydro facility will produce enough electricity to meet the needs of about 18,000 homes each year.
Missouri River Energy (MRES) is developing this, its first hydroelectric facility, to add to the renewable resource portfolio of its 60 member municipalities in Iowa, Minnesota, North Dakota and South Dakota. “Hydroelectric power is a dependable source of green energy,” said Tom Heller, chief executive officer of MRES. “When the wind isn’t blowing, wind turbines aren’t producing electricity. This project is going to produce electricity for us during the peak period.”
Red Rock Dam is owned by the U.S. government and operated by the U.S. Army Corps of Engineers. It was constructed in the 1960s to impound Lake Red Rock for flood control. Consisting of a rolled earthfill embankment and a gravity concrete control section, the dam is 5,200 feet long and 95 feet high above the flood plain, with a crest elevation of 797 feet above sea level. MRES chose this dam to diversify both its portfolio and location of resources for its members. The Red Rock hydro project will be operated by MRES and is owner-financed by Western Minnesota Municipal Power Agency, which owns the power supply and transmission facilities that serve MRES members. Red Rock is one of few hydroelectric projects under construction in the U.S. where a non-federal owner is building a powerhouse at a dam owned by the federal government.
Fit to a “T”
Ames Construction, a heavy civil and industrial general contractor headquartered in Burnsville, Minn., is the general contractor for the project. Ames began performing hydropower work in the 1980s, with expertise in constructing complex water-related projects throughout the nation. Although Ames is performing more than 75% of the work, key subcontractors are working on the project with Ames to provide specialty services. Case Foundation is performing the deep foundations work, the Boldt Company will install the turbine-generator and penstocks, Woody’s Rebar is installing rebar, Winger Mechanical is providing mechanical services and Sachs Electric is supplying electricity services.
Red Rock Dam will retain most of its original structure and Ames will add the components necessary to operate a hydroelectric plant. Since crews arrived at the site in August 2014, massive amounts of work have been performed underground to prepare for construction of the powerhouse and intake structure, while ensuring that construction activities do not interfere with the dam’s operation. (See the sidebar on page 45 for more details.)
For example, a secant pile retaining wall was built in 2014 and 2015 on the downstream side of the dam to hold back the earthen dam and to ensure stability of the dam during excavation for the powerhouse. More than 200 anchors were drilled diagonally into rock, grouted, stressed to more than 400,000 pounds of tension (400 kip) and locked down. A downstream cellular cofferdam was also installed to serve as a water barrier between the river and powerhouse excavation/construction. With completion of the wall and installation of the rock anchors and cofferdam, excavation for the 150-foot-by-185-foot powerhouse hole began in May 2015, with contractors digging down nearly 100 feet from the existing ground surface.
On the upstream side of the dam, a secant pile cofferdam has been installed to facilitate construction of the intake structure.
|This aerial photo provides an overview of the work under way to add a two-unit, 36.4-MW powerhouse at the existing Red Rock Dam in Iowa. (photo courtesy MRES)|
To support the earthen dam on the upstream side, crews also constructed a permanent 100-foot-long by 85-foot-deep concrete diaphragm cutoff wall along the axis of the dam to ensure stability during construction, with T-wall elements that made up an array of 21 T-shaped elements and five flat-panel elements. Some elements contain nearly 90 tons of reinforcing steel. Excavation work for placement of these elements was accomplished using a unique hydromill designed solely for drilling diaphragm walls. Supplied and used by Case Foundation, the hydromill has built-in instrumentation that enables operators to monitor inclination and angular rotation while descending into the excavated trench. The operator can perform real-time adjustments during installation to maintain vertical alignment to its original centerline. This equipment can achieve vertical tolerances of <0.3%.
A significant milestone at the project in 2015 was completing the construction and overlapping placement of the 26 diaphragm wall elements. The steel rebar cages for the diaphragm elements were built in a staging area and transported nearly 10 miles to the construction area. Cages weighed up to 92 tons, with the largest measuring 132 feet long.
At the site, two massive cranes worked in unison to pick up each cage from its horizontal position on the custom-built transport truck (see sidebar on page 47 for more information) and maneuver it to a vertical position for placement. Critical to the repositioning was keeping the cage straight while being lifted from the transport and then lowered into its excavated position.
Perhaps the biggest challenge to date was testing the first element of the diaphragm wall using one of the largest reinforcing assemblies Ames crews had built. Complexity arose from the need to keep the excavation from caving in while placing the reinforcement steel and concrete in the diaphragm wall elements to form the wall.
For this testing, the first cage was lowered into the ground and more than 800 cubic yards of tremie concrete mix was poured into the frame, followed by up to 45 days of curing time. “This is very challenging, specialized work,” remarked David Gatto, business development manager for Ames Construction. “Successfully accomplishing this placement allowed us to move forward with that portion of the work.”
The entire 240-foot-long diaphragm wall used more than 2.5 million pounds of rebar. It was completed in May 2015.
For 18 months, the majority of civil work has been performed underground, which included earthwork, rock excavation and combined pours of more than 35,000 cubic yards of concrete. In February 2016, crews started construction of the powerhouse.
A high degree of precision is required for components to align with the eventual installation of the turbines, starting with the draft tube forms. The two custom wood draft tube forms being used are unique to the vertical Kaplan turbine powerhouse being built. The Ames team must position the centerline of the units vertically and horizontally to align within thousandths of an inch. The wood forms must also be very precise - to within an eighth of an inch.
Water passing by the turbine’s wicket gates will flow vertically and spin the turbine-generator unit. After passing through the draft tube, the water will make a 90-degree shift to travel horizontally, then exit through two square water conduits into the new tailrace.
The complex concrete work for the powerhouse will continue until early summer 2017. Once the draft tube portion of the concrete is complete, crews will begin assembling the Kaplan turbines by placing the large embedded components from Voith Hydro that will hold and support the non-embedded, rotating parts. Voith Hydro was awarded a $46 million contract to supply turbines and generators for the plant in April 2012.
With construction of the powerhouse under way, the project schedule has crossed the halfway point, with completion scheduled for mid-2018.
Protecting people and natural resources
More than 150 to 200 workers are anticipated to remain on the project during peak construction, which started in the summer of 2016 and will continue throughout 2017.
The small footprint of available space requires extra safety precautions for construction activities. During excavation and blasting, all work crews, engineers and managers on site were on alert to identify any signs of structural issues. Instrumentation - including inclinometers, pressure cells, piezometers and survey monuments - has been placed in multiple locations within the dam, permanent diaphragm retaining wall, and temporary secant retaining wall downstream to monitor the dam for any unanticipated movement during construction. And, because the work is being performed in the adjacent river and reservoir, rescue boats are ready.
|A downstream cellular cofferdam was installed to serve as a water barrier between the river and powerhouse excavation/construction.|
On-site safety lunches are also scheduled with all personnel, members of the community and visiting managers to emphasize how important safety is to the project, the community and their personal well-being. “Safety is always the priority on our projects,” said Jim Schaefer, project manager for Ames on the site. “We want everyone to leave at the end of their shift in the same condition that they came in at the beginning.”
Crews were honored for their safety on the job when MRES received the 2015 Silver Safety Award from Liberty Mutual for having an incident rate at the Red Rock hydro project at least 60% lower than the industry average.
Protecting the environment is also a priority. Before work began, the project underwent a rigorous environmental review process that included the Federal Energy Regulatory Commission, Corps, U.S. Fish and Wildlife Service, Iowa Department of Natural Resources, and numerous other agencies and stakeholders. This intensive process ensured that the project would have minimal or no impacts on fish, wildlife and water quality.
A project the size and scope of Red Rock involves numerous stakeholders affected by its development, from owners and engineers to subcontractors and vendors to regulatory agencies. Yet, the project’s construction not only makes an imprint on the terrain, it makes an impression on the community. Owners of neighboring properties are inconvenienced by the increased traffic, staging locations that occupy adjacent property, and the noise from construction activities. Some popular recreation facilities around the dam have been closed during construction. Building rapport with the community becomes just as important as building the project.
To help compensate for some of the facility closures, MRES added new recreation facilities before construction began that residents and visitors can enjoy during and after construction. In addition, a project website at www.redrockhydroproject.com has been established to provide progress updates and information on potential impacts on traffic.
Ames has assisted the local Pella Chamber of Commerce by providing aerial photography of its annual Tulip Time scenery, and the company worked with Central College on a building demolition to make way for additional college facilities. The safety staff also holds routine ambulance and rescue drills with the local authorities.
At the project’s start, Ames needed 15 work trucks and called for local bids. Pella Motors won the bid and outfitted the trucks with specific toolboxes. “Ames Construction and its employees have been great ambassadors for the community of Pella,” said Craig Ford, Pella Motors owner. “They bought houses, bought vehicles, and bought materials locally whenever they could. They are truly trying to be part of the community.”
In the project’s first year, Ames used more than 25,000 yards of locally purchased concrete, with another 100,000 yards anticipated by project end. In fact, economic analysis performed by the Department of Economics at Iowa State University indicates that development of the Red Rock project will provide economic benefits of more than $250 million to the four-county region in Iowa during construction, creating more than 300 direct and indirect jobs in the area over the four-year construction period.
The intake structure will become the next major undertaking for the project. The intake will direct water from the reservoir to the two 21-foot-diameter penstocks, then to the turbine’s spiral case. This major renovation will rely on all the previously constructed temporary and permanent supports being able to withstand the constant movement and flow of the water.
|Construction of the 36.4-MW Red Rock powerhouse is well under way, with the custom draft tube forms being built here.|
Once the intake structure is complete and water can be impounded behind the two new intake gates, Ames will bore two 28-foot-round holes through the 30-foot-thick concrete dam to make the final connection to the penstocks that will allow water to flow to the powerhouse.
The final steps include the equipment and electrical work to be installed, followed by testing the project and its equipment.
Today’s success, tomorrow’s future
Construction of the Red Rock hydro project reflects renewed interest in hydropower nationwide, which could bring changes to scores of American dams. Hydroelectricity provides about 7% of the nation’s power using about 2,500 dams - a fraction of the 80,000 in the U.S. Many entities are keeping close watch on the progress at Red Rock. The success of this project could spark a surge in tapping the potential hydroelectric power from more of those existing resources.
Considered an investment in Iowa’s future, the design capacity of the Red Rock project will be about 36.4 MW, but the project will be capable of providing capacity of up to 55 MW at times of the year when the river is running full. When operational in 2018, Red Rock will be the second largest hydropower generating facility in the state of Iowa.
Susan Carter is senior writer for Ames Construction.
At a glance
Construction of the Red Rock Hydroelectric Project, to be accomplished over four years, includes:
Installation of a 240-foot-long by up to 133-foot-deep diaphragm wall, which includes 26 elements of varying sizes
- Installation of a 100-foot-long by 85-foot-deep concrete diaphragm cut-off wall along the axis of the earthen dam to ensure dam stability during construction
- Construction of a secant pile cofferdam to facilitate building of the intake structure
- Construction of an intake structure to draw water into the penstocks
- Installation of a cellular cofferdam and earthen dike to facilitate powerhouse construction
- Installation of a secant pile retaining wall to hold back the earth embankment of the dam during powerhouse construction
- Construction of the powerhouse, which is 185 feet long by 112 feet wide by 143.5 feet tall)
- Construction of two penstocks
- Installation of equipment, including turbines, generators, controls and transformers
- Construction of a 69-kV substation and transmission line and 13.9-kV backup distribution line and utilities
Ames equipment specialists deliver pivotal performance
When Ames Construction needed a specialized transport at the 36.4-MW Red Rock Hydroelectric Project in Iowa, equipment suppliers didn’t have one and fabricators wouldn’t engineer one - so they built it themselves.
The transport was needed to move 21 T-shaped diaphragm walls reinforced with steel rebar cages, with some of them weighing more than 90 tons, nearly 10 miles to the upstream construction area with several tight-radius turns along the route.
Ames’ Utah-based team of mechanics spent three weeks building, testing and perfecting prototypes in the equipment yard, simulating expected route challenges. The resulting custom-made transport consisted of three flatbed trailers and two transport trailers with trucks, one in front and one in the center. With a two-point pivot system, each truck steered independently to make the difficult turns while maintaining stability. The custom transport also disassembled into two pieces for transport from Utah to Iowa.
The night of Nov. 13, 2014, three cranes worked in unison to set the cage, and the new transport carried its first load, measuring 157 feet long by 20 feet wide and weighing 320,000 pounds (transport and load). Averaging between 4 and 4 miles per hour, crews completed the move in less than two hours. The remaining cages were transported over the next several months.