Port Authority of New York and New Jersey engineering programme manager Peter Rinaldi says that most of the mountain of debris from the World Trade Center has already been taken away to the Fresh Kills landfill site on Staten Island. 'Of the estimated 1.5Mt of debris, about 1.1Mt has been removed, ' he says. Rinaldi is seconded to the New York City Department of Design and Construction (DDC), which is supervising recovery work.
'At first, removing debris at and above ground level was OK - there were big pieces which could be handled relatively easily. But the remaining material is the most difficult. It is at the lowest levels and in the most unstable areas.'
It was clear from day one that debris searching and removal and wall stabilisation were inextricably linked. The Bathtub contained a plaza level and six basement levels including parking and mechanical areas, a Port Authority PATH train station and tracks and WTC substructures and foundations for the towers, WTC 3 (a hotel) and WTC 6 (the Customs House).
Built between 1967 and 1968, the 1,070m long, 20m high and 900mm thick slurry wall of the Bathtub is made up of 6.7m wide panels of C28 concrete reinforced with massive cages of 44mm diameter rebar. It was built in filled land beyond the original shoreline of Manhattan Island, explains George Tamaro of Mueser Rutledge Consulting Engineers (MRCE), which is acting as geotechnical consultant.
Ground conditions are 4.5m of fill overlying up to 9m of organic clay, 3m to 12m of glacial sands and silts, a thin layer of glacial till and bedrock, mainly mica schist typically lying at 21m below ground level.
Tiebacks into the bedrock were used to support the wall during basement excavations and detensioned after the basement floors and columns were built. Much of the internal structures was destroyed in the collapse, removing permanent support for the wall and replacing it with debris.
'We knew from the start that we would have problems with the stability of the slurry wall, ' Rinaldi says. 'Debris was bridging gaps in collapsed material below, but once we reached street level, we started to lose that bridging.
And when we removed more debris, we began to take away support for the wall.'
MRCE's first task was to ascertain the state of the wall and the damage to the basement structures and to determine the make up of the debris. Underground inspections and mapping gave engineers more of an idea of what they were dealing with.
Once on site, the team quickly came up with a 'three pronged programme', says Rinaldi. This was in place by the third week of September and comprises dewatering, monitoring and stabilisation of the slurry wall once debris is removed by installing replacement tiebacks.
Dewatering started at the beginning of October and was designed to relieve hydrostatic pressure and lower groundwater levels along the west (West Street) and south walls (Liberty Street) where there was most concern about wall movements.
Installation of the replacement tiebacks began at Liberty Street.
It was here that one of the most worrying moments occurred.
On 7 October, Rinaldi and Tamaro were alerted to massive movement of a section of the slurry wall at Liberty Street. 'We were about to start tieback installation here when I got a call at 7.30am to tell me that a crack had opened up behind the wall during the night, ' Rinaldi recalls.
The 60m long tension crack lay 8m behind the wall and was up to 100mm wide at its centre.
After visual inspection it was clear that it was a 'classic' earth pressure wedge failure, Rinaldi says, with about 50mm of settlement behind the wall.
The main problem, he adds, was that debris lying against the wall was of unknown composition and density. Visual inspection revealed the collapsing south tower had ripped out four of the basement slabs at the Liberty Street end. This, combined with a large void in the debris, left a 46m long, up to 14m high section of the wall unsupported.
Tamaro took the immediate decision to start emergency backfilling. Four days of backfilling slowed the movement and a further three days saw temporary stabilisation, but by then, the wall had moved more than 300mm.
Emergency tieback installation then began in earnest.
Although in relatively good condition, it was not possible to retension the original anchors. The original plan was to install a pair of tiebacks either side of a panel joint. 'We were worried that the very large loadings would cause differential movement between the panels, ' Rinaldi explains.
But after the first two pairs were put in, it was found that the differential movement was small - 'so we decided that we could install them anywhere'.
The new tiebacks typically comprise 22, 15mm diameter steel strands, installed by contractor Nicholson Construction Company at 3.7m centres along the wall at 45infinity from horizontal.
Basement slab remnants are removed before a 300mm diameter hole is cored through the wall.
A tieback trumpet is then installed and the steel cased 180mm diameter borehole for the tieback drilled down to bedrock, with a minimum 0.6m long, 150mm diameter rock socket.
After the strands are inserted the hole is grouted over its full length, with the casing left in.
The tiebacks are proof tested to 800kips (3559kN), 133% of the design load, and locked off at 600kips. At any one time, there are between 20 and 30 tiebacks awaiting testing while the grout is curing, which takes about three days. A maximum of 14 tiebacks have been tested in one eight hour shift.
Tamaro estimates that about 1,150 anchors will be needed to stabilise all the wall sections, although this is 'a moving target - we won't be sure until the slabs are removed'. Some 500 (more than half of the planned number) have been installed.
Liberty Street moved another 25mm while the tiebacks were installed. Tensioning actually pulled back the top 6m of the wall by some 60mm.
Work then moved to West Street. 'While the wall didn't move here, there was the greatest potential liability from failure because of the amount of groundwater behind and the ability to recharge, ' Rinaldi says.
By early February, excavation was down about 80% to subgrade level at the Liberty Street end and down to 50% in the centre.
The next challenge is in the northern part of the site where most of the basement slabs are intact. Tieback installation will have to work in and around these structures - they are not being removed just yet, Rinaldi says.