Atkinson Construction used two Target (TGT)
to aid in the placement more than 3,000 marine piles and 100 caisson structures. Both systems utilize Trimble's RTK receivers to position the barges with centimeter level accuracy.
The first stage of the project, at Bath Iron Works Land-Level Transfer Facility Project, Bath, Maine, required the accurate placment of metal templates, which will be used to position five caissons at a time. The template was welded onto the side of the barge, and the center of the first caisson within the template was loaded into the Target:Pile system. Using the position of the first caisson and the required heading of the template, the barge could be moved into location.
Once on location the template was spudded onto the seabed and detached from the barge. Two GPS antennas were then attached to the template and a final position and heading of the template determined.
The second phase of the project required the accurate positioning of the barge for the placement of vertical marine piles. Up to 12 piles could be placed from a single barge location. Once the barge was on location, Target:Pile provided station and offset information for each pile position from a baseline on the edge of the barge.
The station and offset information was then used to set the template system used to hold the piles in place. The template could be used to place up to 12 piles at a time, and each pile could be individually set or adjusted within the template structure. The Target:Pile system provides real-time station and offset for each pile location by accurately determining the position and heading of the barge using two Trimble 7400 RTK GPS receivers
(now replaced with the company's new MS750 receiver). Each receiver is capable of determining a centimeter level position and height in real-time.
Atkinson Constructions Survey Manager Don Nicholson, installed the first system for the caisson template operations. "The Target:Pile system is ideally suited to our project as it provides us with the capability of working 24 hours a day," he said. "In our harsh environment we use the system to monitor the position of the barge in real-time, especially when moving onto location. This can be a tricky operation as we can experience currents in the river up to eight knots. We also monitor the position of the barge while on location when the tide changes direction. The system provides us with the station and offset for each pile placement in real-time."