Digging deep

15 April 2008

Five-and-a-half years after terrorists crashed planes into the Twin Towers in New York City, the famed Ground Zero site is fervent with construction activity. On a blustery, gray day in late January, a dozen or so cranes are running full tilt. In preparation for the erection of the new Freedom Tower that will fill the void in the Manhattan skyline, a Tadano rough terrain crane works from the street level lifting a steel bucket full of dirt from a narrow, tunnel-like hole in the street. Several other RTs, large and small, work at the 16-acre site, lifting and moving equipment and supplies to lower level staging areas.

But the more impressive activity is going on down in the so-called “bath tub,” the hole that will become the foundation for the new tower. In the bath tub some four crawler cranes are working to build a new and stronger slurry wall foundation for the new building, while other cranes assist in the erection of steel towers that will form the skeleton of the new structure.

Cranes owned by Bay Crane, Essex, EE Cruz Co. and Kiewit, to name a few, are working in the bath tub, including two Liebherr HS 855 duty-cycle units rigged with slurry wall grabs.

Building the slurry wall foundation is a systematic process that requires strong duty-cycle cranes that hoist a narrow clamshell-type digger that digs trenches that will be filled with the slurry substance (often a mixture of bentonite clay and water). Once the trenches are complete to the desired depth, the slurry substance is displaced with concret.

The repetitive effort of guiding the slurry wall grab down the tunnel to excavate the dirt and then bring it back to the surface level is demanding work for the crawler cranes. For the most part, the reinforced concrete slurry walls are excavated down to the bedrock, and serve as dams to prevent water from seeping in from the Hudson River. Th e concrete wall foundations keep water out and also hold water in, which is why the hole was nicknamed the bath tub. Th e bath tub will eventually include seven basement levels and attach to New York's rapid transit and subway lines.

Construction on the new tower started on April 27, 2006, and almost a year later crews are still completing slurry wall work. Th e first task was to relocate utilities around the site in order to begin the slurry wall construction. By mid-January 2007, the digging for the slurry walls was almost complete and the required concrete pouring was also more than 50 percent finished.

Tower rising

With 2.6 million square feet of office space, the 1,776-foot-tall Freedom Tower will also feature tenant amenity spaces, an observation deck, restaurants, and broadcast and antennae facilities. In addition to the rapid transit system, the basement area will offer access to retail shops and restaurants.

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