M29 Transmission Feeder Line

Cost: $432 million

Adding power capacity to the New York City area is usually a mammoth task, with major efforts to build new generating stations or cable transmission networks attracting intense scrutiny and protracted struggles for approvals. But sometimes, an opportunity comes along for a project like Consolidated Edison’s M29 Transmission Feeder Line – a combination of smaller pieces that can fill a power gap and prepare for future demand.

It’s not that the $432-million M29 project isn’t complicated, geographically sprawling or challenging. It requires digging a 660-ft, subriver tunnel, running high-voltage lines for 10 mi and repurposing an out-of-date facility with a new switching station. But the low-profile project also clears some hurdles by largely using existing Con Edison land and rights of way, and it will only take about three years from start to finish.

The project’s big aim is to offer a stopgap power source to an Upper Manhattan district that has made headlines several times in recent years because of localized blackouts. It will connect to Upstate power sources in Westchester County through a new feeder line running south through the Bronx, down through the new tunnel under the Harlem River and over to the new Academy Switching Station in Manhattan’s Inwood neighborhood. “There are a lot of pieces to the project,” says Jim Shannon, project manager for Con Edison.

The 350 MW of electricity the lines could supply is close to the production of a midsized power plant, and the project will fill other goals in the region, such as laying groundwork for backup capacity that can reach as far south as 49th Street in Manhattan.

“But the main goal is to improve reliability of [power service] for Washington Heights and a big portion of the Bronx,” Shannon says.

Work began in November 2007 at the Academy site, and now crews are active all along the project path. The whole effort should be complete in about December 2010, Shannon adds.

The heart of the project is at the Harlem River, where crews on either side are digging the 165-ft-deep shafts and the new tube. The tunnel work began in August, starting on the Manhattan side with crews building one of the two shafts by driving a secant pile concrete wall system into the ground in order to excavate soil and rock, says Tim Stewart, Con Edison’s construction manager for the project.

advertisement

Over the winter, the team drilled and blasted through white marble bedrock to get down to the tunnel level and soon after began horizontal mining under the river. The team is now driving from the Manhattan side to the Bronx, drilling 10- to 12-ft-deep holes; loading them with explosives to blast and pulverize the rock; and then removing the muck with an excavator, which fills up buckets lifted to the surface by crane.
 
On the Bronx side, the crews have installed the secant wall for the other shaft and were set to start excavation downward in June, Stewart says. That will allow them to roughly line up with the arrival of the tunnel dig, which in late April was about 450 ft from the Manhattan shaft.

The team, which includes Kiewit Constructors of WHERE? as general contractor on an $85.2-million contract, will later add waterproof membranes, reinforcing steel and concrete liners to cap the effort.

When completed in April 2010, the 12-ft-wide, 13-ft-high horseshoe-shaped tunnel will be ready for installation of the 345 KV feeder cable, other utilities and a 6-in-diameter return line circulating oil back and forth in order to control temperature levels.

Con Edison has a 36-in. cast iron gas main that was built in 1912 crossing under the river, and the new tunnel will reinforce that supply with a new 16-in. high-pressure main, bringing gas from the Bronx to the tip of Manhattan. And several electric distribution feeders will connect Manhattan power sources to reinforce the Riverdale neighborhood of the Bronx, backing up some of the cables in use since 1913 to 1918, Shannon says.

The switching station portion of the project is also well under way. After demolition of the prior power station, the team used piles and other portions of the old foundation. It finished installing the new foundation in November, Shannon says. Then the team began to assemble a precast concrete building that will house the switch facility.

Stewart says project crews finished erecting the 51,000-sq-ft building, which came in 764 pieces, in early April. Con Edison was set to award another contract to install the equipment and HVAC units for the structure. The gas-insulated facility will house switching equipment to transform incoming 345 KV power into 138 KV so that it can go to the nearby Sherman Creek substation for distribution to customers.

The other leg of the project entails running the 9.5 mi of dedicated 345 KV feeder line from the Sprain Brook substation in Yonkers to the Academy site. Most of the line construction – handled through five separate contracts – involves digging open-cut trenches to install new conduit and an oil-filled pipe cable to keep the line temperature stable. The line will cross several bridges along the route as well. The job also entails construction at the Yonkers facility.

Team Box

Owner: Consolidated Edison, New York, New York
General Contractor, Tunnel: Kiewit Contractors, Omaha, Nebraska
Engineer, Tunnel: Meuser Rutledge, New York, New York
Secant Wall, Tunnel: Intercoastal Foundations and Shoring, Rockville Centre, New York
Dewatering, Tunnel: Moore Trench, Hackensack, New Jersey
Waterproof Membrane: Wisko America, Chantilly, Virginia
General Contractor, Switch Station: Universal Electric, Canonsburg, Pennsylvania
Precast Concrete, Switch Station: Universal Concrete, Stowe Pennsylvania
Concrete, Switch Station: Villa Concrete, Wappingers Falls, New York
Precast Concrete Erection: A.J. McNulty, Maspeth, New York