By Jarvis Frederick
Placing rock armoring on a 2:1 slope is a fairly straightforward task on land but how about on an underwater slope ranging in depth from 10 feet to 52 feet?
American Construction Company used a clamshell dredge and a three-GPS-system to place filter stone and riprap on a slope of the Blair Waterway in Tacoma Washington to prepare it for a future pier. Lyman Burk configured his WinOps dredge positioning software to replace a gyro with three GPS receivers working together to allow real-time positioning of the crane boom and placement of rock to an accuracy of inches. This is the first application of the three-GPS system for real-time positioning according to Burk.
American used the crane Mukilteo which has a 50-foot beam on a 120-foot long barge to spread the rock. The GPS receivers were mounted as far apart as possible: on the center of rotation of the rig on the deck winch operator’s station near the stern and on the tip of the boom.