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.
Burk modified the WinOps static dig pattern feature to create a placement pattern designed for American’s rock skip Big Red. The pattern is simple to configure and gives the operator a tool to place the rock consistently according to project specifications. American Construction Company computed the optimum digging radius and boom positioning for the Mukilteo and Burk programmed these parameters into the system.
To place the rock the operator views a template with a real time pattern of the 12- by seven-foot rock skip box and of each rock placement target rectangle measuring 24 feet by seven feet. American pre-calculated the volume to be placed in each target rectangle and that amount was loaded into each dump of the rock skip allowing the operator to swing the correct volume and dump it across the target rectangle. WinOps records each rock skip load while the real time display in the cab shows the operator and superintendent when the entire area is covered.
The operator’s monitor shows all the equipment in use including the barges spud locations and a moving red rectangle that represents the rock skip
Once the rock is placed to grade in a certain spot that spot is filled with a solid color on the monitor which removes guesswork from quality control.
American began the project in November 2000 by dredging 50000 cubic yards of material to create a 2:1 slope which is 100 feet from toe to top ranging in depth from –10 feet to –52 feet.
The crane barge worked next to a rock barge where a front-end loader filled the rock slip with 10 tons of rock or seven cubic yards per load. The entire slope area was first covered with a filter blanket of smaller than 2.5-inch material and then with 12-inch riprap.
With careful pre-planning methodical workmanship and an experienced operator the placement was accomplished efficiently to meet project specifications because the static dig pattern feature in WinOps reduced unnecessary passes. For this project each positioning of the barge allowed four boom angles for placement of rock in four tiers before moving ahead.
In the past American tracked the placement of rock with a combination of marks on the deck of the barge and an angle indicator in the cab. When the placement width exceeded 50 feet errors frequently occurred in rock placement due to fixed boom angles and the rotation over the side of the rig. Therefore the derrick was required to make additional passes to stay within the 50-foot beam to achieve the required project specifications. Now with the advantage of a placement template American is able to complete the 80-foot-wide placement in a single pass while maintaining consistent placement of the rock over the side of the derrick.
By the end of the project the Port of Tacoma’s specifications were more than adequately met. After conducting a post rock survey only a handful of areas required further coverage which could be attributed to sloughing rather than misplacement. The project was completed on time and under budget in January 2001.
ABOUT THE AUTHOR: Jarvis Frederick is project superintendent for American Construction Company headquartered in Everett Washington. He holds a degree in mechanical engineering from Georgia Tech and has been with American for three years. Prior to joining the company he worked with the U.S. Forest Service in Alaska engineering log dump facilities in the Tongass National Forest a job that required use of GPS. He observes that since that time GPS technology has improved by orders of magnitude and that one-foot accuracy is a “worst case scenario; we are usually within a few inches.”