Crews Shore Damaged Subways (10/8/01)From: http://www.enr.com/news/wtc_enrindex.asp
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.
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
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.
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
"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.
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
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
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
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.