A borrow pit on the western side of the Coon Rapids Dam was non-stop action early on in the second phase of the rehabilitation work with this Cat 325L backhoe filling trucks with sand to dump into the cofferdam.
A two-year, $16 million rehabilitation project on a 100-year-old dam that marks the end of the navigable portion of the Mississippi River in Minnesota will soon be completed. Wide ranging concerns from structural deterioration to the health of the state’s fishing industry sparked the decision to make the improvements, said Jason Boyle, state dam safety engineer of the Minnesota Department of Natural Resources, (DNR).
Edward Kraemer and Sons Inc., based out of Plain, Wis., took on the project. Besides two offices in Wisconsin, the company has an office in Minnesota and one in Colorado. With more than 100 years of experience, its team of employees and equipment are a full-service construction company in heavy civil and industrial projects across the country, according to the company’s Web site.
The Minnesota DNR is overseeing the construction work and managing the dam during the two year project, which began in May, 2013.
General details of the project include replacing the 20 year old pneumatic rubber gate system with steel crest gates, replacing an existing scour ridden dam apron with a stilling basin, repairing cracks on the original stilling basin running along the west side of the dam and making minor repairs on the existing steel control gate.
Advancing scour damage led the push for the rehabilitation work on the dam, located 16 mi. (25.6 km) north of Minneapolis, according to the Minnesota DNR web site. At the same time, the rapid migration of four species of an invasive carp upriver from the south alarmed DNR staff.
According to National Park Service documents, the aquaculture industry in the southern United States imported the carp from Southeast Asia to help combat algae and plant growth problems as an alternative to using chemical treatments.
The fish, made up of the Silver, Bighead and Grass carp species, escaped from the ponds into regional rivers. A fourth non-native species also present in Minnesota, the black carp were an incidental part of the shipment but propagated in the south to control snails.
All four species eventually found their way into the Mississippi River, which snakes its way almost due north for 2,320 mi (3,734 km) from the Louisiana gulf coast to northern Minnesota.
Migrating quickly into the lakes and rivers of the upper Midwest, they are now in Minnesota. With a very healthy appetite, they are conquering aquatic ecosystems in their wake to starve out native fish. They also are a serious hazard to boaters.
Easily startled by engine noise, spectacular news videos show dozens of carp bursting from the water surface and boaters ducking and dodging a collision with the fish that can jump as much as 10 ft. (3 m) and weigh on average 20 to 40 lb. (4.5 to 18 kg).
Along with the threat to boaters and the potential harm to waterfront properties, at stake here is the state’s $4 billion recreational tourism industry of which sports fishing plays a major role, according to the DNR Web site.
Though fighting the snow and cold last winter, phase one of the project was completed on schedule in the early spring of 2014. Then, the lingering snow and cold changed over to a cool, wet spring.
Heavy rains during May and June raised the river to flood levels and caused record breaking flow rates over the dam, forcing a later start to the second year of the project.
At one point, “the Mississippi River peaked at 47,000 cubic feet/second. There were 13 days in June where flow in the river at Coon Rapids was the most ever recorded for that particular day,” Boyle said.
Despite gearing up for construction in late June last year, “the contractor has done a good job in meeting the challenges to this project and we’re about on the same schedule as 2013 for installing the gates and getting them operating.”
One hundred years old now, the regional power company completed the Coon Rapids Dam in 1914 to generate electric power. According to archival information, more than 600 men worked around the clock using hand tools, horses and coal powered shovels to build the original structure.
After 50 years of generating electricity, the power company ceased its hydroelectric operation in 1966. Several years later, the power company donated the dam and land on both sides of the river to the regional county park system now known as the Three Rivers Park District.
Since the mid-1990s, the park district has operated the dam to maintain the lake pool behind it, built a new walkway across the dam and installed the pneumatic rubber gate system across the length of the 1,000 ft. (1,600 km) dam.
The on-going scour threat to the original dam apron and an aging and outdated gate system prompted park management and DNR staff to review all structural concerns related to the dam, including the threat posed by the invasive carp.
With the goal to stop the invasive carp migrating farther north to Minnesota’s rich lake country, DNR staff agreed with an independent consultant’s recommendation to install a steel crest gate system.
According to this study, the steel gates could be lowered in increments and provide a “consistent and higher barrier along the length of the dam to manage the lake pool behind it” and at the same challenge the leaping abilities of the carp.
The dam crosses the Mississippi River in the city of Coon Rapids. Originally constructed in sections, the dam’s original operating components included the hydroelectric plant on the eastern shoreline, one main steel control gate, a stilling basin along the western section of the dam and an apron along the eastern section.
An island forming a bay separates the main span of the dam from the former hydroelectric plant on the eastern shoreline of the river.
Construction began on the eastern half, or phase one of the project, after high waters in the spring of 2013 dropped, said Aaron Rosenbery, Edward Kraemer and Sons senior project manager. Despite hitting some obstructions below the river bed early on that forced the cofferdam alignment to be moved back a bit and fighting the snow and cold of last winter, “we wrapped up phase one successfully in the spring of 2014.”
That year, crews installed five of the nine new steel crest gates and replaced the original apron pocked with scour with a 450 ft. (136 m) long stilling basin on the downstream side and eastern section of the dam.
The original starting date for the second phase of the project called for work to begin in May of last year “but the high water and record flow rates held up our starting date until late June with the coffer dam installation,” Rosenbery said.
“Since then, though, we have stayed on schedule and anticipate wrapping up gate installation by mid-January of this year and will begin removing the cofferdam in late January.”
When construction began in late spring of 2013, truckers dumped up to an estimated 15,000 cu. yd. (11,900 cu m) of fill to build a causeway to access the eastern half of the dam. Since a new stilling basin would be constructed on this part of the dam, crews drove sheet piling for both the upstream and downstream portions of the dam to dewater the work zones, Rosenbery said.
Estimated quantities for phase one of the project included 2,000 cu. yd. (1,520 cu m) of concrete poured and strengthened by 200,000 lb. (90,909 kg) of rebar for the new basin. It sits on 1 ft. (.3 m) deep layers of sand and rock and is anchored by more than 230 H-beams pounded into the river bed.
Crews also drove an estimated 25,000 sq. ft. (2,250 sq m) of sheet piling and trucked in 20,000 cu. yd. (15,000 cu m) of sand to reinforce the sheet piling, poured 550 cu. yd. (418 cu m) of concrete for the pier abutments and placed five new gates in 2013.
“The demolition both years went very well,” Rosenbery said. Along with the concrete chipping, “we relied on wire sawing concrete removal techniques that were simplified by changing the alignment of the wire saw locations and using different sized backhoes with the mechanical breakers.”
Workers removed the existing pneumatic air bag system and its piping in July 2014. Crews then began chipping away at the pier abutments and base of the dam to reconfigure the existing pier geometry to fit the new, steel crest gates.
The new gates required that the existing piers be widened.
“We cut out around the piers and the bottom of the dam and then poured back concrete and beefed up the new piers with additional concrete,” Rosenbery said.
By mid-summer last year, the cofferdam area over the western section of the dam crawled with workers and equipment. Kraemer crews and their sub-contractors relied on more than a dozen pieces of heavy equipment to dig, import fill and break up concrete over the length of the two year project.
Included in this mix last summer was a Cat 325 backhoe for digging and removal operations and a Cat 345D backhoe with a concrete breaker to chip away pier abutments to make room for the new gate system. A Cat mini backhoe working with a John Deere backhoe supported the larger backhoe in concrete removals.
A Volvo L110F wheel loader roamed the site hauling fill and excavated material back and forth.
Stationed just inside the cofferdam was an American 150 ton (135 t) crane and an American 100 ton (90 t) crane. Crews also relied on a LinkBelt 338 100 ton (90 t) crane for lifting operations and operated smaller, JLG lifts.
The design specified that the original four rubber gates be replaced by nine steel crest gates. Manufactured by Rodney Hunt, Orange, Mass., they were trucked in 1,300 mi. (2,200 km) to the dam.
Relying on the American 150 ton crane, crews lifted into place nine new steel crest gates along the dam. Six measured 97 ft. (29.1 m) long, two at 67 ft. (20.1 m) long and one gate at 69 ft. (20.7 m) long.
Crews installed the gates “by placing steel false work on the sloped spillways, flying the gates over the access bridge and lowering them onto false work channel beams and then rolling the gates in on rollers,” Rosenbery said. “The system worked very well and we were able set two gates per week.”
The new gates rotate on a bottom hinge and are raised or lowered by hydraulic cylinders. Sensors on the gates, which can be controlled manually or be programmed, will read the river flow rates and make adjustments to the positions of the gates when necessary.
The contract also called for some minor repair work on the existing stilling basin along the western section of the dam. Working under water, divers saw-cut approximately 200 ft. (60 m) of linear cracks and injected them with epoxy to make these repairs, Rosenbery said.
Crews also inspected and repaired minor damage to the original, 103 ft. (31 m) steel control gate located on the western end of the dam.
Major construction is now winding down and the new gate system is expected to be in operation in February.
This spring, a new lighting and railing system will be installed with landscaping work to wrap up the remaining work, Boyle said.
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