Engineers used excavators to reclaim as much washed-out material as they could from adjacent fields, and they added dredged and hydraulically pumped sand from the Mississippi, says Jim Pogue, a spokesman for the Memphis District.
Great Lakes Dredge & Docks (GLLD), Oakbrook, Ill., began work on Oct. 6 on a $2.9-million contract to pump fill. A 24-in. cutter-head dredge pumped 15,000 to 20,000 cu yd per day. "The Corps did some real quick footwork to put our dredge on-site and begin pumping in a matter of days," says Jon Nieman, vice president of GLDD's rivers and lakes division.
But shielding the blue hole from the dredge spoil was another matter. The first idea was to build a rock containment dike to separate the new hole from the old one. But with the 50-ft depth, estimates were that a rock dike would cost $2.5 million to $3 million and might not even do the job.
Then Clay, drawing on a pre-Corps stint in wastewater treatment plant design, suggested a silt curtain: a barricade of thick tarps with floats on the top and weights on the bottom to control sediment migration. Some wastewater treatment plants use them as baffles to partition lagoons and isolate sediment.
Clay says he expected the idea to get shot down, but "Cory Williams, our geotechnical branch chief, and Shane Callahan, civil design section chief, said to look into it."
Clay knew the curtains would have to be custom-made to fit the deep bowl of the hole. He didn't know if he could get them made and installed quickly enough, but he ran with it. He made a survey, prepared construction drawings and searched for manufacturers online. Ultimately, he commissioned fabrication of the curtains for $50,000. "Me and three other guys put it in," Clay says. The system was in place by Sept. 26.
Elliott Davis, speaking for the curtain supplier, Granite Environmental Inc., Sebastian, Fla., says silt curtains manage suspended solids by speeding settlement and dropping solids in defined areas. He says the curtains often are used incorrectly, "but, in this particular situation, it was the perfect solution."
The curtain panels are in 6-ft widths of varying lengths to match the hole's bottom contour. Lengths are adjusted by reefer lines so the lower edge hovers one foot above the bottom to allow for some water flow.
The panels arrived on an 18-wheeler. "We rolled the curtain off, borrowed a farmer's tractor and brought it to the landing," Clay says. "We did final assembly—zip-tying adjacent panels—pulled it out in the water with a boat, attached the anchors, loosened the reefer lines and unfurled the panels. They work just like mini-blinds."
With Frank "Tater" Mills, a Corps design engineer, and two men from Granite, Clay installed two parallel rows of curtain with a 20- to 40-ft gap between. The first curtain acts as a baffle, slowing currents and dropping sediment. "[Sediment] barely goes under the primary curtain and only has a vertical foot before it impacts the second curtain, which also has a one-foot gap at the bottom," Clay says. "We calculated an angle of repose and figured out how steep of a slope was required—if you just dump sand out there—to hold its own slope."
Clay says if the only objective were to control sediment transfer and the cost were the same as a rock dike, he still would prefer the curtain over a dike "because it was more effective."
