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Safety drive

The New York water tunnel project is one of the largest public works projects in the world as Adrian Greeman discovered.

New York's flamboyant mayor, Rudolf Giuliani, took great delight in opening the first phase of New York's giant Third Water Tunnel project last summer. And well he might, since it has been a long, costly and heroic enterprise, with spending of $1bn, and the sacrifice of 24 lives over 28 years. Finishing the 21km long 7.3m diameter first phase tunnel has been an extended exercise, with years long pauses in work because of financial troubles.

Giuliani liked it so much in fact that he wants to speed up the current phase two to complete its 17km long dogleg route through Queens and Brooklyn by 2001 (see box).

And construction of a tunnel to the south part of Manhattan could also be accelerated. This section was originally due to finish in 2008.

'We are speeding up the current contracts by moving to 24 hour working and weekends,' says spokeswoman Christina Manners at the New York Department of Environmental Protection which has been the client for the project since it merged with the Board of Water Supply in the 1970s.

'And we are using TBMs for the new works which can tunnel faster than the traditional drill and blast used for the first phase' she adds.

The drawn out construction of phase one reflects both the size and complexity of the project and the crisis conditions faced by the city in the 1970s. Though first mooted in the 1950s and started in the late 1960s, financial turmoil paralysed work for nearly a decade. Construction did not get under way until the 1980s.

'Traditional public work procedures slowed it down,' says Manners explaining that 'contracts had to be cleared for Mob affiliation and bidding had to be thoroughly vetted.' Contracts were evenly spaced rather than running several in parallel.

The dangers of the project may also have slowed it. A range of accidents tragically cost the lives of engineers and miners, known as 'groundhogs', mainly caused by tunnel train collisions, falls in shafts, and falling rock and objects. A child playing illegally in the tunnels was killed, and one death was caused by drilling into an unexploded blast charge.

'The increasing awareness of safety on site and changes in technology mean that accident rates have dropped substantially in the last few years,' says Cathy DelliCarpini for the DEP. The switch to the use of a TBM for more recent work is also helping.

Traditional drill and blast technology pushed ahead some 10m a day for the first 7.3m diameter tunnel in the hard New York and Manhattan granites and gneisses, never less than 70m down and sometimes as deep as 245m. The tunnel runs half the length of Manhattan Island, before taking a right angle bend at Central Park to run into Queens.

Three huge underground chambers were needed for valves, with a complex of risers, cross shafts and access points. Just beyond the last and the deepest of these on Roosevelt Island, a dogleg shaped tunnel marks the second phase. Starting in Brooklyn and running westward into Queens, it is well over half complete.

'The first 9km section has already been concreted,' says Amitabha Mukherjee, resident engineer for the DEP's water construction division. 'We are now about 4km into the tunnel drive for the second section.'

The completed tunnel, ending at a single deep access shaft in a backstreet yard in Queens, is 5m finished diameter. The second which runs from the same point, essentially the only major access to the works, is to be 20ft (6.15m)in diameter. But the TBM, a refurbished Atlas Copco Robbins, bores a 7m hole through the hard banded gneisses found deep under the whole area. Boretec, which recently bought Robbins, supplies the TBM train and the 460mm cutters

'They can wear quite fast,' says Mukherjee. 'The rock is around 450M years old and varies in hardness from 130N/mm2 to 275N/mm2'.

'It is very foliated and injected with very stressed and folded types of material such as pegmatite. Foliation doesn't cause too many worries but the rock is often quite blocky with chloride and calcite infillings.'

The machine has driven typicallly about 15m daily, says Mukherjee, though it has hit 32m on occasion. Rock is supported as needed, using bolts and occasionally steel hoops, though falls are less problematical here than in earlier drill and blast sections 'where rock gets fractured'.

Supplies are lowered down the shaft only during the day shift for safety reasons, and this involves a degree of pre-planning to avoid delays. Spare parts and so forth run up to the face on small rail cars. Mucking out is via a complex of conveyors, possibly the longest in the States, supplied by US firm Long-Airdox. A vertical pocket lift at the shaft is from Lakeshore Mining with pockets from Germany's Trellex.

'We have a deal with the Long Island Railroad company which owns the shaft site,' says Mukherjee. 'They take away the spoil in railroad cars to a yard on Long Island where it is crushed for recycling into aggregate and road base.'

The tunnel rail tracks will be used for concreting, running agitator trucks to the formwork. These trucks were bought in from the Channel Tunnel project. Their availability may have been spotted because the contractor, a three firm joint venture, includes Swedish contractor Skanska.

Skanska Tunnelling, a US subsidiary is teamed with Grove Tunnelling owned by Kiewitt and the Perini Corporation, both from the US. The contract was won in early 1995 for a tender of $172M and is due to finish in May 2000.

Concrete will be fed down shaft pipes from a Rotec conveyor for loading and will be pumped at the point of application by a Putzmeister. 'The aggregate is up to 35mm diameter, so it cannot be pumped long distance,' says Mukherjee.

Tunnel lining is important. The tunnel around 200m at this point and in places deeper, and the water has an additional 70m head at the reservoir which means higher enough pressures 'to reach the sixth floor of any building in New York', says Mukherjee. This statement is confirmed by the impromptu but illegal showers taken by Bronx teenagers, who open the fire hydrants during the hot summers.

Tunnel design by the DEP's engineering department calls for no intrusion of groundwater so a balance between the hydrostatic head and the groundwater is required.

'Though we have not hit excessive water,' says Mukherjee, 'grouting was needed for several points where flows of around 80 gallons per minute were hit'. Maximum water so far was one point with 1,600 gallons/minute.

The concrete uses a specially formulated cement supplied by Blue Circle to the DEP

The JV's contract is currently being examined for modification to include extra work on riser shafts as part of the speed up. A separate contract was to be let for the straightforward rock shafts descending from the rock soil interface at 70m depth. From this point a series of steel pipe risers carry water to the surface links into the water distribution system.

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