The meeting was opened by Joseph Siano, who gave a brief description of the current demands on the New York City Transit system – 1343.8km of track with 5.7M passengers per weekday. Over the last five years particularly, the authority has increased spending to improve the current infrastructure with new stock, station improvements, installation of ATS to improve timetable reliability and installation of new ticket gates. The current main capital projects for New York include the No.7 Line extension, the East Side Access project, the transit centre at South Ferry and the Second Avenue Subway (see Figure 1).
He then outlined the history of the Second Avenue Subway. Currently, the only north south route east of Central park is the Lexington Avenue Line and is subsequently severely overloaded at peak periods. The new Second Avenue line was therefore conceived to reduce overcrowding and delays, and to improve access for commuters and residents of east Manhattan.
The original scheme was proposed in 1929, but no significant progress was made until the Second Avenue El, an elevated rail route, was demolished in 1942. Plans were prepared in 1946, and in 1951 a US$500M bond issued. However the majority of the funds were transferred to other schemes and no real progress made, even though demolition of the Third Avenue El was completed in 1955.
New plans were published in 1968 and there was a ground breaking ceremony in 1972 with much political fanfare. Three lengths of tunnel were constructed. However, the main scheme stalled again. Most recently the troubled project was revitalised by the Manhattan East Side Alternatives Study in 1995 culminating in the start up of engineering of the current project in 2001.
Project description
Anil Parikh then described the overall project. It is a 13.7km long section of new twin track subway with 16 stations. At the north end, at 125th Street, the line will connect to the Lexington Avenue Line and the Metro-North Railroad. The first section would therefore be constructed by cut and cover. The remainder of the tunnels will be driven using TBMs and drill and blast.
The central section is through rock, and the southern section in soft ground where EPB machines will be required. Most of the stations will be constructed using cut and cover techniques, but four of those in the rock will be at least partially mined (Figure 2). There is a rail connection to the Sixth Avenue Line at 63rd Street (Figures 1 and 5) and passenger interchanges at 53rd Street, 42nd street, 14th Street and at a number of points in the business district.
The current position of the project is that the draft Environmental Impact Statement is complete. Public hearings have been held and work on the final EIS is ongoing. The conceptual design is complete, the site investigation is proceeding and the preliminary design underway together with preparation of Design and Build contracts. Construction is planned to start in September 2004 (see Figure 3).
Every technique?
Some of the engineering challenges of the project were then described by Don Phillips and Dru Desai of DMJM+Harris/Arup. The scheme requires the use of most types of tunnelling technology – Mixshields, Slurry/EPB machines and hard rock TBMs, road headers, drill and blast and cut and cover for the tunnels.
Most of the stations are cut and cover using either diaphragm walls or conventional temporary works, but four stations and a crossover will be mined using drill and blast and road headers.
The alignment has been designed with curves to suit TBM tunnelling methods, maximise speed and minimise the impact of subsurface construction. Track centres are set to suit platform widths but also to stay within the limits of right of way in the street widths.
The southern segment presents its own challenges. Using old maps Phillips showed how the shoreline had changed, and how the current alignment runs along what was the shoreline in the 17th century (Figure 1 inset). This results in a mixture of glacial soils and highly permeable sands which have the potential to liquefy. There is also the potential for boulders at the rock/soil interface together with shallow obstructions from the buried shoreline, such as wharf piles and even old boats.
This has resulted in a relatively deep alignment with the cut and cover stations up to 30m deep. Therefore these have buoyancy issues to be overcome. There are areas of highly permeable clean sand and potential problems at the soil rock interface which has to be penetrated in a number of areas. As with the tunnels there are risks to the stations of obstructions including boulders and bulkheads in the deep fill.
For the southern segment dual mode machines may be required depending on the final evaluation of the soil and rock conditions, and in some areas EPB or slurry machines will be appropriate. Choice of machine will also depend on recent experience in the US and in New York in particular.
Each tunnel will be 5.8m internal diameter with a 300mm thick lining and a 170mm grouted external annulus. Particular challenges include the Grand Street Station, which is to be constructed immediately adjacent to an existing station, and the large number of obstructions that have been identified, particularly soldier piles and sheet piles from previous construction. The tunnel alignment also passes through piles supporting the approach spans to the Brooklyn Bridge.
In the central section there is a relatively shallow undulating bedrock profile dipping up to 10m west to east across Second Avenue. Above the rock is fill of glacial silt with sand and gravel in the valleys. The rock is typically a strong and competent gneiss and schist with faults and shear zones throughout.
The key geotechnical issues in this segment are:
Two of the existing tunnel sections were constructed in the northern segment using soldier piles and lagging and sheet piles. Construction dewatering reportedly damaged adjacent buildings. Protection of existing structures will be critical, particularly at 125th Street and where organic soils are present. The steep rock profile at 92nd street also makes the evaluation of where to change to cut and cover more difficult.
The rock construction in the central segment presents different but no less interesting challenges. In general the schist is a good material, but in areas there is softer pegmatite and major shear zones. Strength is normally in the range 45MPa-65MPa (see Table 1).
One of the key decisions is to choose whether to completely waterproof the excavation or to install drainage. Although the pressure relieved tunnels generally have a lower capital cost, because of the drainage system required they are more expensive to maintain over the life of the scheme.
Station caverns
The caverns for the stations are typically 21m span x 16m high excavated. Initial lining is expected to be steel sets at 1m centres, 5m rock bolts on a 1.5m pattern and between 150mm and 300mm of fibre reinforced shotcrete. At 72nd Street the three track station requires a 31m wide excavation with between 8m and 12m of rock cover. A similar primary support is proposed but with thicker shotcrete.
The cut and cover stations are typically 300m long, 20m wide and between 20m and 30m deep. All the stations have low cover, and for the deeper stations buoyancy is a significant issue. As might be expected there are very large numbers of utilities to be diverted before the station excavation can start.
In the softer ground the stations will be constructed within permanent diaphragm walls, typically 1.1m thick. Where the station is partially in rock temporary diaphragm walls will be taken down to the rock head, and the station box constructed within these.
The intermediate slab in a station is often used to support the walls, but this can produce a feeling of a confined space on the platform, so the Second Avenue Subway have made significant efforts to reduce the impact of this slab to the extent of spanning the wall’s full height, to try to give a more open environment.
The way forward
The work will be procured under a number of forms of contract, but principally Design and Construct. The contracts will include a clear allocation of risk. For example a Geotechnical Base Line Report will be used, which will allow the contractors to assess and price the ground conditions, with the promoter accepting site conditions which are outside this report. There will be an Independent Disputes Board to quickly review and adjudicate on any problems which do occur. New York City Transit will perform the Construction Management.
However, there are still a number of challenges for the contractors, including:
The speakers are looking forward to working with the contractors from next year to complete this impressive project.
Related Files
Figure 5 – Cross section of the double split-level track design adopted for the 63rd Street rail connection
Figure 2 – Subway alignment
Figure 4 – The 86th Street station cavern design
Figure 1 – Map of Manhattan
Figure 3 – Project programme for the Second Avenue Subway scheme
