The brutal winter which recently plagued the upper Midwest, then devastated the nation’s midsection with flooding, continued to make its presence known long after the waters had subsided. In Sartell, MN, near St. Cloud, meltoff from the record snowfall rushed through a dam near a Champion International paper plant with such force that tons of rocks submerged in 11.6 meters (38 ft.) of water at the foot of the dam were displaced resulting in a “scouring” effect on the Mississippi River bottom. The rocks are used to cushion the impact of the water as it plunges downward.
The Right Stuff
Replacing those boulders — some of which weigh in excess of 8,165 kilograms (18,000 lbs.) — was the challenge that faced Environmental & Marine Services (EMS). The Eden Prairie, MN-based firm specializes in underwater construction, inspection and repair work.
According to Ron Westberg, EMS’ marketing manager, careful planning and the right equipment helped the Sartell dam project move along smoothly and without any problems.
“We established a nice system for getting the rock out there and into place,” he said. “The granite boulders were brought in by flatbed trailer from a quarry just south of St. Cloud. By project’s end we brought in about 14,000 yards — nearly 33,000 tons — of rock. They were off loaded and moved into position by front end loader then made ready to be handled by our grapple-equipped excavators.”
EMS used two Cat 345B excavators equipped with Genesis GSD 90 grapples at the Sartell dam. One unit was situated onshore to take boulders that were off loaded, then placed them onto a “staging” barge located next to shore. Another identical excavator/grapple — located on a second barge used to transport boulders for placement — transferred the rocks from the staging area to that placing barge. During the loading process, special attention was given to ensure ideal weight distribution and access to the appropriate boulder when needed. Westberg said this was just another area in which the value of the grapples became apparent.
“The Genesis grapples were excellent throughout this project. Trying to do this type of movement and placement with just a bucket would have been next to impossible. The grapples handled virtually anything put in front of them including some boulders weighing at least 18,000 pounds and measuring in excess of five feet in diameter. They really performed well for us.”
Put Down Those Rocks
Once the placing barge was loaded to what the engineering staff felt was a safe limit — it is rated to safely float 25,401 kilograms (56,000 lbs.) — it was moved into position by cables and winches located on both shorelines and on the dam itself. EMS had initially planned to use a marine propulsion system to move the barge into place. However, when they first arrived on site, the volumes of water coming through the dam were much higher than anticipated. So the company was forced to seek the alternative method for positioning the barge for placement.
Westberg said, based on historical data, they were expecting volumes in the 85-cubic-meters (3,000 cu. ft.) per second range.
“When we got on site, that rate was in the area of 11,500 cubic feet per second and it rarely went below 9,000 cubic feet per second, thereby dictating the need for the change,” he said. To fully appreciate the force of the water which initially moved the boulders, estimates for peak flow during that spring’s meltoff put it at 1,104 cubic meters (39,000 cu. ft.) per second.
Actual placement of the boulders was based upon floor profiles compiled using a sonar device on an ongoing basis by EMS. An EMS engineer directed the placement of the barge, and, when it was determined to be in position, signaled for the dropping process to begin. At this point the grapple-equipped excavator grabbed each of the boulders it had on board and dropped them at the appropriate point at the foot of the dam. The area below the water’s surface which was affected by the meltoff measured approximately 61 by 76 meters (200 by 250 feet).
Wrapping Up the
Dam Project
Though the company was not actively involved in any type of follow-up monitoring, Westberg said the river bottom at the base of the dam would be looked at during subsequent inspections conducted by the Federal Energy Regulatory Commission. He added that, aside from the unforeseen problem with the higher-than-anticipated water volumes at the outset, everything else went very smoothly.
“Right from the beginning this was an interesting project. While we’ve done rip-rap projects in the past, the size of the material being placed at Sartell made this job truly unique. Having the right equipment to do the job really made the difference in us getting it done right and in a timely manner,” he said.
(This article appears courtesy of Genesis Equipment & Manufacturing.)
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