AS IS typical for construction of tall buildings in Manhattan, the now destroyed World Trade Center (WTC) buildings 1, 2, 3, 4 and 6 were founded on bedrock.
Up to 21m below ground level, all foundations and thus all building basements - seven storeys deep in the case of the towers - were in a 6.5ha area enclosed by the 'bathtub', a 1km long 21m deep diaphragm wall.
As well as supporting the doomed buildings, the wall had the vital job of keeping out the Hudson River which flows alongside the site.
But after the buildings collapsed one after the other, vast quantities of rubble tore through the basements and into the foundations. Whereas previously, the propped cantilever wall was restrained by the buildings, today it is kept standing largely due to the remains of those buildings.
But threatening an even greater disaster are the damaged rail tunnels running beneath the fallen buildings.
What is now clear is that should the wall now fail, these tunnels could act as conduits for river water. With the Port Authority Trans-Hudson (PATH) lines connecting to New Jersey, around 6.5m below the level of the site, and the subway lines to the remainder of the network, inundation would cripple much of New York's subway system and cause massive flooding in New Jersey.
But despite all the misery now evident in New York, one of the rare strokes of good fortune has been that the man who designed these foundations, George Tamaro, is still one of his native city's top engineers.
Now a partner with New York consultant Mueser Rutledge, he could scarcely have imagined the importance of his knowledge when working as an engineer for the New York and New Jersey Port Authority on the original WTC construction in 1967.
Since the disaster, Tamaro says, his work has been 'seat of the pants' stuff 'Finite element analysis doesn't count on this one - this is real engineering, ' he says.
Only two months ago, the new owners of the towers asked him to inspect the foundations and he confirmed that they were in good condition.
When disaster struck, he joined the batallions of engineers who offered assistance.
'Other engineers were looking up from the ground - I thought I would be of better use looking down, ' he says.
His detailed personal knowledge was put to immediate use.
Working from memory, Tamaro was able to direct rescue teams around the site. 'Basically it was down to making judgment calls. I could remember where weaker areas such as voids were, and then could tell fire chiefs where it was or wasn't safe to put down heavy machinery, ' he says.
While others were preoccupied with events above ground, Tamaro was immediately aware of consequences underneath the site. Engaged by New York City's department of design and construction, Tamaro and his Mueser Rutledge team were forced to make a painful decision. Fire, police and rescue chiefs all wanted to remove as much of the debris as possible in the foundations believing some colleagues could still be alive.
But Tamaro knew that removing debris could risk collapse and possible flooding and had to order that the rubble remain in place while a restraining solution is found for the wall.
It was Tamaro who nicknamed the basement area the 'bathtub'. The Hudson River once ran through the site, but in stages since around 1600 has been filled with silt. 'It's very random fill, made up of riverbed material, oyster shells, and sand, ' he says, describing it as variable poor quality material compared to the underlying bedrock.
The bathtub walls, which vary in depth from three to seven storeys, also housed a number of sub-basment floors which provided lateral restraint. A pressure head of around 21m is also contained, with the water table varying with the 1.5m tidal range in the Hudson.
The walls were built using a slurry wall technique in a series of 7m long sections excavated and keyed to the bedrock. A clay and water slurry prevented the excavation from collapsing before reinforcement was installed and concrete pumped in to replace the slurry.
A series of ground anchors or 'tiebacks' supported the wall during excavation. These were spaced at around 3.5m on plan, at up to six levels vertically depending on the basement depth. High-yield steel strands were inclined downwards and anchored into rock. Those at the bottom levels were prestressed up to 300t, and those higher up around 150t.
These anchors were removed once the structures inside gave sufficient restraint to the top of the wall.
'Our calculations show that the wall will not stand as a cantilever, so it needs to be restrained, ' says Tamaro.
Salvaging the tiebacks is considered possible but would involve stretching the now corroded strands to the required stress and fixing the ends with steel plates. Installation of new anchors is therefore one of the solutions being examined.
Tamaro's colleague and senior associate Daniel Hahn also has a wealth of knowledge of the site as the former deputy Port Authority chief engineer.
'The tiebacks would have to be around 30m long, inclined at 45infinity. We are looking at using a Freyssinet anchor with the ends tied with a nut or a plate. But we are going to need quite a few and we'll need them quick, ' says Hahn.
The major difficulty is access.
Reinstalling the tiebacks with machines inside the wall may not be possible because of the rubble, and removing large sections could threaten the wall.
Installing with machines from above and behind above the wall may also prove difficult because of the loading this will place on the structure.
Another option being examined is to install floor bracing to restrain the wall, but again access could prove a problem.
Hahn says that a mixed solution may be adopted - with basement slabs providing restraint where they remain, and ground anchors elsewhere.
There also remains the agonising possibility that the rubble could contain many bodies, so work will need to proceed alongside recovery teams. Pressure to complete this work means that a solution must be found fast.
So far the wall has not shown signs of structural distress.
Some workers reported seeing water leak, but Tamaro is not unduly worried.
'There was always a flow of water into the building. We have been measuring the quantities of flowing through to Jersey and they are manageable, ' he says.
Were the worst to happen, tunnels would be sealed with 4.5m thick concrete plugs at the New Jersey side a soon as possible to prevent inundation.
'We don't know the condition of the train tubes, but if there were any problems, they would have manifested themselves by now. At the moment, my judgment is that the wall is secure, but we must be careful to keep it that way, ' he says.