Shani Wallis, TunnelTalk

Exchange Place station on the Port Authority Trans-Hudson (PATH) commuter rail service between New York and New Jersey, lies beneath high rise office buildings on the west side of the Hudson River and is the last station on the New Jersey side before trains head into the 100-year old rail tunnels for the trip under the river into lower Manhattan. The station was added to the PATH system in 1910 and was never intended as a terminus facility. Trains had to travel under the river and through the loop configuration of the PATH station built into the lower basement levels of the World Trade Center to make the return trip.

With the collapse of the World Trade Center (WIC), Exchange Place station and the river crossing running tunnels had to close as well. Two 16ft long concrete plugs were installed also at the Exchange Place end of the under river tunnels to prevent flooding of the subway system as a result of water flowing from broken water mains and sewer lines and generated by extensive fire fighting efforts at Ground Zero.

Also destroyed in the disaster was the WIC station on the N ew York City Transit Authorities No 1 and 9 Subway Lines.

In the months since clearing the WTC site, reconstruction has concentrated on getting these vital rail services operating again and in the shortest time possible. Subway services on the NYCTA's No 1 and 9 Subway Lines, linking upper Manhattan with the South Ferry Terminal and beyond, are scheduled to resume by December 2002. Trains will pass through a reconstructed cut-and-cover tunnel across Ground Zero and without making a stop at the World Trade Center.

At a deeper level the Port Authority (PA) of New York and New Jersey is reconstructing temporarily its World Trade Center PATH terminus station on the same configuration as prior to September 11. Permanent works for both temporary reconstructions will be incorporated into final design of the WIC site's redevelopment. The PA developed and owned the WIC complex and used part of it as its main headquarters. The Authority lost 75 employees in the disaster and is now directly involved in the redevelopment program.

For the PA, PATH service restoration centers on three distinct components - resumption of services to Exchange Place station, rehabilitation of the severely flood-damaged running tunnels under the Hudson to Manhattan, and construction of the temporary PATH station at Ground Zero. Estimated at a total $544 million, the restoration program was approved in December 2001 with the goal of having PATH services to Exchange Place restored by June 2003 and services to lower Manhattan resumed by December 2003.

Sections of the existing concrete lining are being retained in new tunnel profiles

"This is a very tight program," admitted Lou Menno, Restoration Program Director for the PA, when TunnelTalk visited the PA project offices in early August 2002, "but we are committed to this schedule. In February 2002 we awarded a $300 million contract for the civil works on all three sections of our Downtown PATH Restoration Program Phase 1 to a joint venture of Yonkers Contracting Co, Tully Construction Co of New York, and AJ Pegno Construction Corp of New Jersey. We are now seven months into the contract period and despite more time taken than expected to mobilize specialized plant and machinery at short notice, the program is on schedule. We have pre-purchased long lead supply items such as new track and electrical and mechanical requirements for the tunnels and station works and have engaged specialist design engineers to prepare construction drawings and design documents under tight schedules to get construction started as soon as possible. There are attractive bonuses for the contractor to finish early but in recognition of the tight deadline there will be no penalty for being late. Everyone involved realizes the urgency of the situation and all are pulling together to keep our promised opening dates to the travelling public."

Reconfiguring Exchange Place
To reopen Exchange Place by June 2003 and ahead of the lower Manhattan services, the existing facilities must be modified significantly to provide turnouts and crossovers to allow the underground station to operate as a terminus.

In addition, the PA is taking advantage of the forced closure to extend the station platforms to accommodate longer 10-car commuter trains. Plans to extend the existing 7-car platforms were already in development, with proposals to extend and enlarge the running tunnels at the river end of the station and complete the work at night to avoid disrupting services.

"This would have been difficult and expensive work," said Paul Pietropaolo, Project Manager for the PA engineering department. "Forced closure of the station gave us the opportunity to rethink the project completely. Platform extension work is now incorporated into the restoration program and is moved to the landside of the station where modifications for the terminus turnouts and crossovers are also located. This has reduced considerably the risk of breaking out and enlarging sections of the old running tunnels at the start of the river crossing, and is being undertaken as part of the modification works contract."

Changing the shape of the existing underground facilities over a total of about 1,000ft (300m) at Exchange Place is a great deal more complicated than might be expected. Excavated in the mica schist of the New York/New Jersey region, the station complex comprises a set of five,almost parallel, tunnels. Turning the existing elements into a terminus and extending the platform tunnels a further 150ft involves excavating new crossovers, modifying the profile of existing tunnels, raising the invert and crown in others by up to 9ft, and back filling unneeded tunnel spaces to create the ground in which new spaces can be excavated (Fig 1).

To assist its engineering department with the intricacies of the project, the PA has engaged a team of design specialists headed by consultants Golder Associates of New York.

Existing condition
At about 75ft (23m) below surface at track level, the structural condition of the existing tunnels in the dry mica schist environment is very good, according to Ray Sandiford, Chief Geological Engineer with the PA's engineering/architectural design division. "That includes the quality of the 100 year old unreinforced concrete lining of the original tunnels."

Roadheaders replaced drill-blast excavation oin the hard mica schist and quartz

Surveys carried out for the PA, including digital scanning undertaken by the Dibit Company using its digital image observation technology system, reveal that the rock excavations are basically self-supporting with the 6-8,000psi compressive strength concrete lining providing a non-load bearing finish. There are no steel arches or rockbolt supports in the older excavations and there is no contact grouting behind the older, in-situ concrete linings. As a result there are voids above the lining in the crown. "Where concrete lining in the walls is 3-4ft thick, thicknesses in the crown can be as little as 8-12in," explained Sandiford. These voids are being filled with a program of contact grouting to secure the lining before demolition starts.

Given the fine quality of the existing concrete lining and the time constraints on the project, the modification design uses as much of the existing structures as possible. In some instances, the original lining on one side of a tunnel is maintained, breaking out only the crown lining and the opposite wall to accommodate the new profile. The Dibit surveys were used to maintain the excavation profile within allowable tolerances of the gradually enlarging cross-sections.

In preparing the design specifications, the designers, with the PA and the contracting JV, have introduced several new techniques to the New York/New Jersey tunneling scene. To start, roadheaders are being used for the first time in the hard rock of the area. Drill+blast was intended initially, but the works lie directly beneath highrise office and residential apartment blocks and while third parties did agree that blasting was necessary, blast times and vibration limits were restrictive, with blasting not allowed at all during the day.

"This was too restrictive on this very tight project program," said Sandiford, "and although we started off with drill+blast - setting up two or three faces to blast each evening - trials with a relatively small Dosco machine proved that roadheaders could tackle the rock." The predominant mica schist ranges from 3-7,000psi with up to 12,000psi maximum. Seams of quartz rising to 20-30,000psi in strength presented problems for the Dosco and larger machines were required. "It took time to source available used machines," said Sam Leifer, Assistant Chief Geological Engineer for the PA engineering department, "but the JV has procured two further roadheaders - a 48 ton Alpine machine and a 56 ton Voest Alpine - both of which are now engaged. There was no time to give these machines a thorough overhaul before they were introduced and there have been maintenance delays, but production when they are attacking the rock is impressive and production volume is within the program expectations."

Another first to the local tunneling market is the use of steel fiber reinforced, wet-mix shotcrete as the primary and long-term final lining. Despite the favorable quality of the host rock and the minimal support used in the original excavations, support of the new excavations complies with current design codes and follows methods of best practice. New facilities lie very closely adjacent to existing structures and the span of new excavations' particularly at the widest part of the crossover chambers, range up to 60ft maximum (18m).

Excavation of new tunnels for the station

Design therefore comprises a comprehensive regime of immediate and long term support starting with patterns of 8, 12, and 15ft long resin anchored and galvanized rockbolts that are prestressed and torqued to 40kps. In addition, lattice girders are installed on 5ft centers to further reinforce the steel-fiber shotcrete and to provide a guide for the minimum thickness and profile of the final shotcrete lining. "Wet mix steel fiber reinforced shotcrete was specified for the project principally to achieve high early strength support properties and long-term durability," said Sandiford. "Quality control samples confirm that the shotcrete is meeting the minimum 5,000psi 28-day compressive strength specifications. In addition, exposed and unsupported excavation of any advancing face is limited to 10ft before rockbolts must be installed."

Given the complex geometry of the new and modified excavations, the design and fabrication of each lattice girder is an individual challenge. Very few girders are of the same shape or size. Each must conform to the changing height, span and profile of the new excavations with some spanning from the top of the retained section of the original concrete wall.

Procuring the design and fabrication of a uniform set of girders for one or two different profiles is complex enough, but managing the diversity and individual design requirements of the Exchange Place project required specialist attention. For this, specialist design firm, the Dr G Sauer Company (DSC) of Virginia, was engaged by the Port Authority to provide the JV and its lattice girder suppliers, American Commercial Inc, with the drawings and design specifications for each individual girder at 5ft centers. "This has been an intense design exercise," explained Gerhard Urschitz, Manager of the US DSC office, "particularly as each set of drawings is required at very short notice, and some within a matter of days."

The Yonkers/Tully/AJ Pegno JV mobilized on site immediately on being awarded the estimated $70 million Exchange Place civil contract in February 2002. Work started with stripping out the existing ballast and track and all the existing electrical and service equipment. Initial drill+blast excavation of new tunnel headings and profiles is now almost totally replaced with roadheader excavation and since the TunnelTalk visit in early August 2002, some 30% of the demolition and new excavation work is complete. Some of the concrete rubble and rock excavation is being used to raise tunnel inverts while the remainder is mucked out via the PATH system tunnels to the New Jersey portal about 4-5 miles west of the underground station. Since this route uses underground sections that are still in operation running PATH services from Newark and Hoboken stations to mid-Manhattan's 33rd Street station, muck trains out of Exchange Place can only operate at night between 9pm and 5am. A specially equipped works train carries the muck out and also transports materials in from the surface staging areas. Only man access is available through the elevator facilities at the closed Exchange Place station.

Wet mix shotcrete, with its 100lb/yd3 steel fiber included, is being delivered ready-mixed from a local concrete batching plant and is transferred into the underground works via a drop pipe from street level. It is being applied from the concrete pumps using hand held nozzles.

Old tunnel refurbishment
As work progresses at Exchange Place other crews have started rehabilitation of Tunnels E and F under the Hudson. Flooding of the two 5,000ft long x 16ft diameter (1.5km x 4.8m) tunnels displaced and contaminated the track ballast and corroded the electrical systems.

Rail tunnels stripped back to their initial cast iron lining

Work on the estimated $90 million tunnel rehabilitation phase of the contract started with stripping the tunnels back to their primary cast iron lining, removing all furnishings, fittings, ballast and track, leaving in place only the permanent invert infill. Flood water was pumped from both ends of the tunnels by November 2001 and the concrete plugs at the New Jersey/Exchange Place site were removed in February 2002. When TunnelTalk visited, a thorough survey of the old cast iron lining was under way before the business of refitting the tunnels could start.

"Surprisingly, the old tunnels are in remarkably good condition," said Pietropaolo. "Little by way of renovating the primary lining is needed, except to treat local patches of corrosion and replace missing caulking in some joints. Once completed the tunnels will be refitted with state-of-the-art systems. These will include new welded rail track fixed directly to new 5,000psi reinforced concrete plinths using insulated fasteners and epoxycoated concrete inserts. A walkway will be installed to one side of both tunnels and new cabling will be laid for new electrical systems."

Station work at Ground Zero
In the reclaimed basement of the WTC, the Yonkers/Tully/Pegno JV has started reconstruction of the PATH station. To get this facility opened as soon as possible, the station will have a functional open air design using part of the original passageways, and passing under the higher elevation of the No 1 and No 9 Subway Lines, to reach the street surface. The temporary station reconstruction is estimated at $140 million and services are scheduled to resume by December 31, 2003 - if not before.

To date, the Port Authority has funded all expenditure on the PATH Phase 1 Restoration Program although negotiations are underway with the Federal Emergence Management Agency (FEMA) and with various insurance underwriters for contributing and reimbursement funds. "Until its destruction in September 2001, some 70,000 commuters a day used the PATH World Trade Center station," explained Pietropaolo. "Commuters to lower Manhattan and to Exchange Place in New Jersey will be relieved when these vital elements of public infrastructure are reinstated and opened again for business."

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