Midwest Mobile Waterjet developed unique waterjet cutter that was dropped between 20ft and 40ft inside the piles to cut them off 3ft below the riverbed.
When record spring floods made it impossible for the Audubon Bridge’s contractors to remove their construction pier piles using conventional methods, they turned to Midwest Mobile Waterjet, a Minnesota-based waterjet contractor whose motto is ‘if it is possible, we can do it’.
The bridge contractors had just finished constructing the longest cablestayed bridge in the Western Hemisphere, the John James Audubon Bridge, north of Baton Rouge, LA, and they faced hefty penalties if they did not meet their deadline for removing the temporary piles from the Mississippi River. Normally most of the steel piles would have been on dry land and easily dug up, cut with a torch and removed with a crane, but the flood waters left them deep in the water, inaccessible with standard equipment.
The bridge contractors challenged Midwest Mobile Waterjet to rapidly develop a first-of-its-kind abrasive waterjet cutting system that could be dropped from 20ft to 40ft inside of the piles and cut them off 3ft below the riverbed. The tool needed to be light, compact, and adjustable for different pipe diameters, from 24in to 48in, and it needed to maintain balance to keep the cut line at the same height all the way around. It also had to be operable in muddy water 25ft below the waterline with little visibility.
"To my knowledge, this had never been done before," says Brian Gleeson, Midwest Mobile Waterjet vice-president.
"We’ve done a lot of cutting, but this was definitely one of the more challenging operations."
Working off a barge, the waterjet contractor pumped the river water out of each pile and then dropped their waterjet cutting device inside the pile. Their ingenious system utilized two high-flow cutting heads that were powered hydraulically from a motion device positioned slightly above the cutting heads to allow for better visibility in case the pile began filling with water during the cutting process.
The cutting-head speed, direction, high-pressure water and abrasive feed were controlled from a remote console on the barge. They used a 36,000psi Jet Edge iP36-250DX diesel-powered waterjet intensifier pump to create the ultra-high pressure water, hydraulically power the cutting tool, and pneumatically power the abrasive feed.
Midwest Mobile Waterjet has nine Jet Edge waterjet pumps, including five diesel pumps and four electric pumps.
"We pumped the water out of the pile prior to the cutting operation," Gleeson recalls.
"It would leak in slightly during the cut, but the mud kept it from gushing in, otherwise it would have filled all the way to the top of the pile. I had to get it done quickly. The Jet Edge pump was ideal for the project because it has hydraulic and air auxiliary power on the pump, making set up much faster."
Visibility was a huge challenge, Gleeson noted. His crew used reflectors, spotlights, and air movers to help them with visibility during the process.
"We were cutting from 50ft away and could not see what we were cutting," he recalls. "You’re looking down the pipe and all you see is steam."
The crew cut each pile in about 20min, cutting through the 5in-8in thick steel using 21.2gpm of 35,000psi water and two pounds of abrasive per minute. After the cuts were complete, a special cement mix was pumped into the pile to act as a plug, then the top portions of the piles were lifted out and hauled away. Plugging the holes quickly and completely was critical because the piles were driven into a man-made Mississippi River levy and the integrity of the levy could not be jeopardized.
"We cut the piles that were in the deepest water and would never be accessible with standard excavating equipment," Gleeson adds.
"We ended up cutting out ten of the piles. It was very fast and easy to set up and much safer and more efficient than using divers. The cut quality and steel separation was also much better with the waterjet, as divers were hand cutting with no visibility and were trying to cut with soft mud behind the steel, which can be a challenge."
For more information, Midwest Mobile Waterjet.