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 Crews Shore Damaged Subways (10/8/01)

By Aileen Cho and Nadine M. Post


New York city expects to have the N and R subway lines, which run along the east side of the World Trade Center site, back in service in about six months. But the news isn't as good for the devastated 800-ft-long stretch of the 1 and 9 IRT subway tunnel, several feet under ground zero. It could take years to rebuild, say officials, and may even include a new alignment and station, depending on the future plan for the site above. Restoring service on the flooded PATH tubes to New Jersey largely depends on rebuilding the collapsed station in the middle of the basement of the WTC.


OUT OF SERVICE Stretch of IRT tunnel will take years to rebuild, say officials. (Photo courtesy of NYC Transit Authority)

Since the Sept. 11 terrorist attack, engineers have been drawing up emergency shoring, bracing and tunnel cut-off plans for the two city transit lines and the PATH tubes, and contractors have been implementing them. The emergency New York City transit work alone could cost an estimated $2 million to $4 million, says Mysore Nagaraja, the New York City Transit Authority's chief engineer.

Nagaraja expects the rebuilding of the IRT tunnel to be funded through the city's $20-billion federal relief package but he won't hazard a guess as to the price tag. "It would be very difficult right now to come up with any meaningful cost estimate," he says. "Engineers are looking at all the options, and when we get the funding, we can proceed. The alignment of the future cut-and-cover tunnels will likely change," he adds.

ARROWS. The collapses of the twin towers projected arrows of steel and debris onto IRT tracks, breaking signal lines and 3-ft-dia pipes, says Bill McGuinness, vice president for Slattery Skanska, Whitestone, N.Y. Slattery is finishing an emergency contract for the transit authority.

Slattery workers have braced sections of the collapsed tunnel, which runs north-south through the WTC site under the Greenwich Street right of way. Crews have also built 3-ft-thick watertight steel-and-concrete bulkheads at northern and southern ends of the affected section, where the tunnel meets the WTC's slurry wall foundation.

In the first days after the attack, crews pumped out water flowing from broken water mains, rain and fire hoses, and determined strategic areas to install wood beams temporarily to replace buckled steel, says McGuinness. Workers chopped hatches through sidewalks into the stations to allow work to proceed. Investigators tested the air below for gas fumes and utility company workers made sure power lines were defused. A few subway emergency exits were dug out and tied open. "At all times we're trying to stay as far as possible from [interfering with] the street relief effort," says McGuinness.

To secure the area, Slattery had 48 workers on extended shifts six days a week. "We were able to stage some equipment, such as cranes and trucks, concrete pumps, compressors and generators, up on the streets several blocks away," McGuinness says. Slattery also installed electronic tiltmeters along the tunnel's wall and ceiling frames to measure movement as minute as less than 0.001 in.


STATION SUPPORTS N and R Lines get shoring and bracing in station and tunnel.

Much of the emergency work is now focused on the N and R subway tunnel under Church Street. The goal is to shore the street and fill any voids to give enough support for heavy equipment working to remove debris.

The 50-ft-wide subway tunnel has suffered some structural damage on the west side of the Cortlandt Street Station but it is "relatively minor," says David M. Cacoilo, a partner with Mueser Rutledge Consulting Engineers, the local geo-technical engineer hired by the city to assess damage and draw up support and shoring plans for WTC subgrade structures.

Specifically, at the southeast corner of the site, outside the perimeter of the burned-out and now partially demolished Four WTC, one to two levels of basement collapsed against the tunnel wall, says Cacoilo. Additionally, WTC columns pierced the passageways that connect the WTC concourse, one level below grade, to the train platform level, some 15 to 20-ft below grade. And a twin-tower steel column plunged into a new 20-ft-square communications equipment room on the west side of the platform. Fortunately it was empty of equipment, says Nagaraja. Additionally, columns pierced the roof of the escalators for the station's 25-ft-wide pedestrian tunnel underpass.

"We are working on details to be able to support the street and excavate soil" over the portion of the damaged subway station, says Cacoilo.

The transit authority wants to be sure that future construction activity won't interrupt subway service once it is restored, he adds. Though service will be restored, the Cortlandt Street Station will remain closed indefinitely.

Crews have already installed temporary posts in the platform level along the entire length of the station–about 600 ft. The posts provide additional support for the subway tunnel roof under Church Street as a precautionary measure. Work will begin soon on supports for the damaged equipment-room roof.

Crews will soon install lateral support for the remaining sections of the roof of the escalators to the underpass (see drawing), so workers can clear out debris that is up against the tunnel wall. Crews also will install a concrete retaining wall under the plaza level directly above the shored subway platform roof. The shoring and supports will allow workers to excavate to the west of the tunnel. The goal is to clear a north-south strip along the west side of the tunnel to enable workers to gain access to debris from the wrecked buildings. Once the area is cleared out, the transit authority will reopen subway service.


PIERCING IRT Tunnel took direct hits from tower columns and more. (Photo courtesy of NYC Transit Authority)

GOOD SHAPE. On the west side of the WTC site, surveys indicate the north and south PATH tube connections to the WTC slurry-wall foundation projections are sound and the slurry walls in those areas are in "good shape," according to George J. Tamaro, a geotechnical engineer with Mueser Rutledge.

There's still some water from rain and fire hoses that goes via the north tube to Jersey City, N.J. Tamaro says the south tube plug near Jersey City was completed on Sept. 30. The north tube plug should before Oct. 8. The water has not compromised the structural integrity of the tubes, he says.

The big issue in getting PATH up and running is the reconstruction of the WTC station platforms and service areas. The north end of the tube is probably in good enough shape to bring trains in, but the station needs to be rebuilt, says Tamaro.

For the New York City subways, Nagaraja says that, in addition to the costs for emergency work, the transit authority will soon be assessing delay claims that are associated with slowdown of its $3-billion capital program. "It will run into millions," he says.

From: http://www.enr.com/news/wtc_enrindex.asp
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