Vital subway links running beneath New York's World Trade Center complex were severed by the 11 September attack. Diarmaid Fleming reports on the remarkable co-operative effort to restore them.
Many engineering heroes emerged amid the ruins of Ground Zero but the rebuilding of the stretch of subway destroyed in the attacks on the Twin Towers is an equally remarkable story.
Last month trains rolled again along the 1 and 9 lines, vital arteries to Lower Manhattan that were severed on 11 September.
Some predicted that their reinstatement would take years, but the job has been completed in months, and for little over half the cost estimated by the city's subway engineers.
It is also an example of what can be achieved by engineers and contractors working in a spirit of complete co-operation.
When the towers fell, thousands of tonnes of debris smashed into the subway line running alongside the WTC site.
Steel beams speared the mere 2m of soil that protected the shallow tunnels, penetrated the roof and lodged in the trackbed below. About 200m of track collapsed completely, while another 200m of structural steelwork failed, leaving sections of roof sagging under the vast load of wreckage.
Thankfully, in one of the positive stories from New York's bleakest day, all subway passengers and trains were evacuated safely before the collapse. And as engineers probed the wreckage, it became clear that the damage was not as catastrophic as originally feared.
Completed in 1918, the twin track tunnels were built by the cut and cover method and featured a novel form of composite construction. Steel box frames were set up at 1.5m centres and a mass concrete trackbed poured over the bottom steelwork.
Formwork was set up, and concrete replicas of traditional brick jack arches constructed between both the vertical legs and the roof beams.
In 1918 reinforced concrete was still a novelty - by replicating the jack arch design engineers prevented tension developing in the mass concrete.
In the event, this form of construction helped to limit the scope of damage, says New York City Transit Authority (NYCTA) chief engineer Mysore Nagaraja.
'Even with all the destruction above, the damage was limited to 1,400 feet (430m). The tunnel is in effect made up of 1.5m panels and if there is damage to one panel, then you lose that panel, not the whole tunnel, ' Nagaraja explains.
While others were preoccupied with the recovery effort at Ground Zero, Nagaraja and his team set about looking at ways of reinstating the line.
The exercise in fast tracking that followed was unprecedented on a job of this size.
Nagaraja and his deputy chief engineer Connie Crawford assembled an in-house team of almost 100 top engineers and technicians to perform a Herculean design task.
'We prepared all the design, about 400 drawings and the specification, in under five weeks, working over Christmas and New Year, ' says Crawford.
People worked seven days a week, close to 24 hours a day to get the work out. We had a highly motivated crew.'
The NYCTA team's decision to order track and signalling equipment early in the design process helped speed the project and brought about real partnering.
'After we put the documents together, we picked five contractors we had confidence in and we had experience with, ' Nagaraja says.
'We called them in, gave them the drawings of what we intended to do and asked them to comment. For two weeks, we had almost continuous meetings with them, getting their comments on whether it was buildable or whether we should make changes. The designers and contractors almost finalised the design together.'
Bidders had only two weeks to price the job, but Nagaraja says their close involvement in the design helped compensate for this amazingly brief tender period. The five contractors split into two joint venture groups, one comprising three firms, the other two.
'Because of all the risks involved we put many contingencies into our estimate. We did not know how access to the site, which was then a crime scene, would be controlled, for example, ' says Nagaraja.
'Then there were contingencies for labour - the contractor would have to get a lot of people in a short period of time, with up to 350 working 24 hours a day, seven days a week. Plus there was a schedule contingency because the whole thing is compressed so much.
'Our estimate was $150M, so we were surprised when the bids came in at $92M and $112M, ' he says.
'Part of the reason was that everyone wanted to be involved and to contribute to the work.
Plus, because they were involved with us so closely in the design, they were not going to give us a hard time, and decided to work with us to solve problems in a spirit of co-operation and partnership. So they took away a lot of the contingencies in their bid, ' he adds.
Winning bidder Tully Pegno was already working at Ground Zero, which made access easier when work began in April. Industrial practices that could have slowed the job, such as union demarcation or unionised workers leaving site early in high temperatures, were suspended.
Crawford says: 'There was tremendous motivation on the job. The two presidents of the firm lived on the site. Many of the people on the project worked on the original recovery at Ground Zero, so this was almost like therapy for everybody.'
On site the feeling is echoed:
'A lot of these men were involved in removing the debris from the World Trade Center, and in the recovery of human remains, ' says NYCTA construction programme manager Joe Trainor. 'So it is not just the money, it is the sheer emotion of seeing something being rebuilt here.'
But emotional commitment was not the only motivation relied on: the lump sum contract with milestone payments contained significant incentives, another strategy introduced by Nagaraja's team.
Liquidated damages of $100,000 a day for finishing after 30 September this year were written into the contract, with a corresponding reward for finishing earlier, up to a maximum of $3M. The contractor finished on 1 September.
Work has included relaying 8km of track from South Ferry Loop to Chamber Street, new lighting and signalling compatible with the existing systems. A station 'box' to replace the destroyed Cortland Street station will form the shell for a new station to open once the decisions on what is to be built above ground are made: the nearby Rector Street station is being upgraded to take passengers instead.
The cost of the work is being funded from insurance of the subway provided by Lloyd's of London, a policy unlikely to be renewed in these changed times.
While Nagaraja and his team can look back with pride as the first trains roll, other positive experiences have flowed from the project.
'We had engineers and consultants making decisions in one day that might before have taken a month, ' he says.
'There are things we can learn from this about the way we do business.'