Dredging units built on site
(...continued)...pontoons, derrick boats and other units? The decision was to build them at the damsite. Shipbuilders from all over the country flocked to Fort Peck, and thus was born the biggest shipyard in Montana, which begat the "Fort Peck Navy': Four dredging units had to be built, each with more than 12,500 horsepower, and each could dig more than 50 feet below the water surface. With the aid of booster pumps, material could be pumped through 20,000 feet of pipeline with a maximum 250-foot lift.
The complex hydraulic fill method also necessitated that huge wooden platforms be built to support miles and miles of 28-inch-diameter pipeline, through which the fill material would be pumped. The mixture pumped consisted mostly of sand and water, with just enough clay and silt to form an impervious core in the middle of the fill. The material was pumped just inside the slopes of the dam and was deposited between that line and the "core pool," which was a large, still body of water in the center zone which was maintained for settling out the finer material. This material then formed the most impervious part of the dam-the core. Extreme caution and care went into selecting the "borrow pits" from which the material was dredged, so that the materials forming the core and the shell of the dam met the design requirements.