WORK TO stabilise the massive basement walls of the World Trade Center towers in New York began in earnest at the end of October.
Installation of thousands of new tie-back anchors began at Liberty Street at the southern end of the site and will proceed along as construction teams remove debris from the collapsed twin towers. Some areas will require more attention than others. The southern wall has already moved by over 100mm.
The vast 300m by 150m 'bathtub' was extensively damaged when the twin towers collapsed into it, destroying many of the basement floors.
These provided permanent support to the 21m deep diaphragm walls after temporary tie backs used during construction were detensioned (GE October 2001).
Since the September 11 attacks, the walls have been largely supported by a combination of rubble from the destroyed buildings and propping action from those slab sections that survived the disaster.
Any failure caused by hasty removal of rubble could lead to flooding of the basement from the nearby Hudson River, hampering clean-up efforts.
George Tamaro, partner at Manhattan-based geotechnical consultant Meuser Rutledge Consulting Engineers said that about 80% of anchors used in the original construction would be needed, installed at around 3. 5m centres and at up to six levels. Installation involves inclined drilling into the 900mm thick wall and socketing up to 10m into the underlying mica schist bedrock.
Anchors will have working loads of 300t.
Subway tunnels running from New Jersey to the World Trade Center, sealed off as a precaution to flooding, are set to be unplugged to speed up repairs to the line.
According to Bovis Lend Lease LMB senior vice president Paul Ashlin, repairs are expected to be accelerated by moving the tunnel seals to the Manhattan side of the river, enabling contractors to gain early access to repair damage from water ingress. Bovis is one of the construction managers working on the clean-up.