ENGINEERS HAVE begun installing rock anchor tie-backs to stabilise the basement walls surrounding the foundations of the World Trade Center in New York.
'We have installed three anchors at Liberty Street at the south end of the site and will be proceeding along as construction teams remove the debris, ' said George Tamaro, a partner at geotechnical consultant Mueser Rutledge.
The WTC basement consisted of a vast 6.5ha concrete box with diaphragm wall sides up to 21m deep keeping out the nearby Hudson River. During construction the walls were restrained with a series of temporary tieback anchors.
Since the terrorist attack on 11 September the walls have been supported only by a random combination of rubble from the buildings which collapsed into the 'bathtub', and the remaining propping action from basement slab sections which survived the disaster. Failure of the wall could lead to disastrous flooding.
Tamaro said his team estimated that around 80% of the number of tieback anchors used during the original construction would be needed, generally installed at around 3.5m centres at up to six levels depending on the basin depth.
Installation involves inclined drilling into the wall and socketing up to 10m into bedrock. Strands are stressed to provide a 300t anchorage force.