New York City's Bypass Surgery

The rig in Shaft 5B drills a series of holes for explosive charges. Each blast moves the floor of the shaft down another 10 feet.

*Photo Credit: Courtesy of the N.Y. Dept. of Environmental Protection*

The submarine-like remote operated vehicle was used to identify the leaks in the Delaware Aqueduct near Wawarsing.

*Photo Credit: Courtesy of the N.Y. Dept. of Environmental Protection*

An autonomous underwater vehicle took 360° photographs every eight feet of the leaking section of the Delaware Aqueduct under the Hudson River.

*Photo Credit: Courtesy of the N.Y. Dept. of Environmental Protection*

Historical construction records gave the DEP a good idea why the aqueduct was leaking in those two areas. Both are areas where the aqueduct passes through lime- stone. The other portions of the aqueduct were bored through sandstone, shale, gneiss and granite. The risk of the interaction of the concrete tunnel and the surrounding limestone was known at the time and the contractors used steel pipe to protect the tunnel in those areas, but in the case of the Roseton leak, the steel pipe ended too soon.

“If they had used more steel pipe, we wouldn’t have this problem today,” says Sean McAndrew, program director for the DEP’s Water for the Future program.

Since then, the agency has been able to pinpoint the leaks using advanced technology. Last year, using existing access shafts, crews lowered a remote operated vehicle, essentially a small, remotely controlled sub- marine, into the aqueduct in the Wawarsing area to take pictures of the inside of the pipe. The device found small pinhole leaks in the grouting where sections of the pipe had been joined.

The Roseton leaks required more sophisticated technology, an autonomous underwater vehicle (AUV), that is programmed to operate without remote operation and can take 360° photographs every eight feet, as well as gather sonar, velocity and pressure data.

In 2004 and then again in 2007 and in 2014, the data from the AUV confirmed that the leaks were coming from sections of the tunnel that were not protected by steel pipe. It confirmed that the damage was significant enough to require a much fuller solution but also that the leaks were stable and not noticeably worsening.

Work to address the Roseton leak got under way in 2013 when DEP chose Schiavone to drill two access shafts on either side of the Hudson River under a $101.6-million contract.

The western shaft, 5B, in Newburgh is already about 710 ft deep and will be at 900 ft when completed. At the eastern side of the river in Wappingers Falls, crews have drilled 575 ft of the 700-ft-deep 6B shaft. Both shafts are expected to be completed early next year.

The shafts are being built by what McAndrew calls “drill, shoot and muck.” The contractors drill holes, detonate explosives in them and haul the rubble back up to ground level.

One cycle in that process takes about three days and lowers the shaft about 10 ft, McAndrew says. A wire mesh similar to a chain-link fence is then secured to the walls using 13 -15 in. rock bolts. When the shaft is completed, the walls will be concreted and the mesh will act as reinforcement.

At the very bottom of the shaft a bell-shaped chamber will be carved out of the rock to serve as a staging area in the next step of the project.