The Hampshire County Council planned to resurface and upgrade the M27, the only major road to and from the naval city of Portsmouth, Hampshire, Eng-land. A CCTV inspection of a 2,000-foot stormwater pipe under the southbound emergency lane revealed offset joints and two lengths of protruding rebar snagging leaves and debris.
The 26-inch corrugated metal pipe was too small to handle downpours, and the accumulated debris in it elevated the risk of flooding. The council considered excavating to replace the affected portion of pipe, but that required working at night with the freeway closed. Preferring a trenchless approach, the council gave the general contractor one week to remove the rebar before resorting to excavation.
Three commercial cutters had enough power to cut the rebar and the ability to travel the required distance, but all were too wide for the 24-inch manholes. The contractor asked Jason Burgoyne of Cobra Technologies LTD in Luton, Bedfordshire, to help solve the problem. “Of course I said yes, then panicked over how to do it,” he says.
By joining a camera crawler with a specialized grinder, Cobra engineers produced a powerful cutting machine that severed the rebar in less than half an hour and saved the council millions of pounds sterling.
Making ready
The lengths of rebar were 65 and 260 feet from the entry manhole. “The biggest problem we saw was the power necessary to travel 265 feet dragging a cable and 1.5-inch pneumatic hose while carrying an 11-pound cutter,” says Burgoyne. “The transporter also had to be wide enough to sit solidly in the pipe while cutting, yet fit down the manhole.”
According to Burgoyne, Euro-pean transporters are lighter and lower-powered than those used in the United States. “Cobra’s 55-pound 8-inch mainline crawler is known as ‘big American muscle’ in the U.K.,” he says. “Our engineers chose it to transport the cutter to the rebar.”
They added 12-inch-wide balloon wheels on the crawler for additional height and support in the larger pipe. They inserted a Cobra PTZ (pan-tilt-zoom) camera into the nose of the crawler and vertically in-line with the cutting head for a clear view of the work. They also added spring steel bumpers on each side of the crawler’s manually adjustable elevator to prevent the unit from turning over in the pipe.
The cutter was a 4- to 9-inch pneumatic grinder with right-angled, fully rotatable cutting arm from Dancutter in Stokholm, Denmark. Burgoyne’s team used it to reopen ferules in potable water pipes. With a motor speed of 13,000 rpm and pressure of 100 psi, Burgoyne knew the grinder could cut rebar.
To hold the grinder, the engineers fabricated and bolted a crescent cradle assembly to the top of the elevator frame. Then they bolted a second assembly between the bumpers. From these two positions, and depending on how the grinding head was positioned, the tool could cut 360 degrees inside a pipe.
The hardest part was working out the vertical and horizontal angles to position the metal-cutting disc in line with the rebar, which protruded 15 inches into the pipe at 2 o’clock. Burgoyne added a copper carbide reinstatement head to the top of the disc to gauge distance. “If I saw sparks coming from it, I’d be too close to the pipe,” he says.
Where there’s smoke
It took four hours for Burgoyne to pass U.K. Health and Safety requirements before gaining access to the freeway. Other contractors working on the freeway had diverted traffic from the work site. Burgoyne’s five men were joined by the contractor and a consultant from the council to witness the machine’s performance.
Following confined-space entry procedures, a worker assembled the components as they were lowered 5 feet down the manhole. The floor of the pipe was free of debris and water. The mounting of the cable drum below the pneumatic hose drum enabled the lines to enter the manhole side by side and kept them from fouling each other. Setup took 20 minutes.
“We were fortunate that the rebar entered through a recessed part of the corrugation and not through a rib,” says Burgoyne. “The recessions were 3/4-inch deep, and the contractor said that we could leave 1.5 inches of rebar.”
Once in the pipe, the crawler had no problem traveling the required distance. Uncertain how much kickback the cutting action would produce, Burgoyne used the crawler’s variable speed control to apply a little forward motion as he engaged the disc. The angles on it to the rebar proved perfect.
Smooth cutting
“I didn’t need much power,” says Burgoyne. “Once the disc bit into metal at 100 psi, nothing was going to push it back. I’d cut a little, then reverse the crawler to let the disc cool down. The disc was aligned 20 inches above the camera, but the zoom function enabled Burgoyne to see the cut almost as it happened. Then he saw smoke. He stopped cutting and more smoke appeared.
“The contractor and consultant are watching, and I’m thinking the motor is on fire,” says Burgoyne. “It’s my worst nightmare.” When he reversed the crawler, he saw that leaves entangled on the rebar were smoldering, ignited by sparks from the cutting.
Fears allayed, he advanced the crawler one last time, and the first rebar snapped off, leaving a 1.5-inch-long stub. The contractor was overjoyed. “Brilliant. Lovely. Leave it at that,” he told Burgoyne, who repeated the maneuvers on the second length of rebar, cutting it flush with the ribs. The contractor later removed the pieces and stormwater once again flowed through the pipe unimpeded.
