In the Upper Mississippi River: Dredge and Dewatering Wheel in Use for Grand Encampment Island Unloading
The Michael is pumping about 1,000 feet into the shaker screen on the sand plant. Transport water is returned to the island via two 14-inch pipes, visible next to the dredge. The sand falling into the hopper is dry enough for handling.
This view of the sand wheel shows the ascending buckets with the orange screen on the inside. The water drains out through the screen as the wheel slowly revolves. At the top of the wheel, the sand is dry enough for transport by barge and truck, and drops onto a conveyor, visible on the left, which feeds it into the barge.
Anyone driving along the Upper Mississippi River must be familiar with the mountains of sand visible on islands and shore sites. These are temporary dredged material placement sites for maintenance material from the river’s nine-foot navigation channel. These sites are periodically unloaded to provide more storage capacity.
The Grand Encampment Island unloading project, at Upper Mississippi River Mile 756.5, in Pool 4, is underway by Black River Constructors – a joint venture that J.F. Brennan and Hoffman Construction formed to bid on the project.
The contract is an experiment by the St. Paul Engineer District of a new way of contracting unloading projects. Historically, the district has dictated the details of removal and transport of sand for island unloading projects, doing all the research, and procuring and designing the fill sites. Finding permanent fill sites for the large amount of material involved was becoming increasingly more difficult. Putting the responsibility on the contractor for finding fill sites or uses for the sand, and allowing the contractor to determine how the material is transported to the end use, has taken some of the pressure off the district.
The J.F. Brennan/Hoffman Construction joint venture created a relationship that maximizes the expertise of both contractors. The dredging, dewatering and transport best uses the expertise and resources of dredging contractor J.F. Brennan. Finding appropriate fill sites, trucking the material and large scale fill site activities best use the expertise and resources of Hoffman Construction, a heavy earth-moving company headquartered in Black River Falls, Wisconsin. The two companies developed a plan for moving and handling 500,000 cubic yards of sand that won them the contract with the low bid of $7,299,000 on November 22, 2013. Work began on May 19, 2014.
At the Alma Marina, Brennan’s SENNEBOGEN excavator unloads the barges into a gravity feed hopper for stockpiling. Hoffman then loads the material onto dump trucks and hauls it to the fill sites it developed.
Black River Constructors’ plan combines a hydraulic dredge, a sand wheel dewatering system, hopper barge transport, high-production barge unloading, trucking and locating beneficial use placement sites.
To excavate the island, Brennan is using the 12-inch articulating ladder cutter suction dredge Michael, which digs into the island to excavate the 50-foot-tall tall bank. When needed, an excavator cuts down the banks to reduce the amount of material caving to the dredge. The dredge pumps the sand through a short pipeline to the dewatering system just outside the island and adjacent to the main channel west of the island. The dredge discharge feeds into a velocity box, which dumps onto a shaking screen to remove oversize rocks and debris. Everything passing the screen goes through a chute and into the tank in which the bucket wheel is working. Excess water spills over the sides and into two 14-inch lines that feed back to the island.
Brennan’s tugboat James Brennan moves a filled barge along the Minnesota shoreline from the Grand Encampment Island to Alma, Wisconsin, 1.5 miles south. A deckhand watches for obstructions in the river from the deck of the barge.
Brennan’s sand wheel is constructed of 27 three-cubic foot buckets. As the buckets move upward, water drains through screens at the back of the buckets, and by the time they crest the top of the wheel, the material is dewatered to the point of being transportable by both barge and truck. On the downward rotation, the sand falls onto a conveyor, which loads a hopper barge.
When setting up the system, Brennan slightly angled the dewatering system barge against the current, which holds the material barges against the dewatering barge with minimal use of tie lines.
When a barge is full, a Brennan tugboat takes the barge three miles south to the marina at Alma, Wisconsin. There, Brennan’s Sennebogan excavator offloads the barge onto conveyors, which stockpile the sand. The system uses a total of three barges to minimize any wait time on the dewatering or offloading end.
From the stockpile, Hoffman Construction uses a large loader to fill dump trucks with the sand, and then coordinates trucking operations with many small and disadvantaged businesses to satisfy the small and disadvantaged business requirements of the contract. The dump trucks haul the material to Hoffman’s fill sites.
Sand falls from the conveyor into a hopper barge after dewatering by the sand wheel.
To accommodate the large volume of material in this project, Hoffman developed three fill sites for the material. The first site, north of Alma, was able to hold half of the total sand. The second site, south of Alma, is a privately owned sand borrow pit. Hoffman entered into an agreement with the owner and permitting agencies to design and fill this site back to pre-mining conditions, accommodating the remainder of the 500,000 cubic yard volume.
In the course of the project, the Corps added an additional 28,000 cubic yards to the contract, for which Hoffman contracted the coalfired power plant in town, which had a need for fill sand for an ongoing project, as well as possible additional sand for coal ash landfills.
The project is scheduled for completion in early August 2015.
The sand wheel feeds dewatered sand into the hopper, which feeds onto the conveyor. The overflow water feeds into tanks on the side (where company name is painted) then back flows back to the island via 14-inch pipelines.
Going forward, Brennan will use the sand wheel dewatering system on the McMillan Island Unloading project in Pool 10 at Upper Mississippi River Mile 618.6 – 619.0. J. F. Brennan was awarded the $1.7 million project in the fall of 2014, and will begin work upon completion of the Grand Encampment project. In the Mc-Millan project, the dredge will pump sand from the island two miles to the sand wheel dewatering system. The sand will then be stockpiled at the Buck Creek Road site for beneficial use. The sand wheel system will benefit this project by allowing beneficial re-users access to the site during filling operations, which wasn’t possible using the old system of multiple dikes and weirs to dewater the sand. The McMillan project will be complete by the end of October 2015.Edit Module