One World Trade Center
The Freedom Tower, slated to become the tallest building in the United States when completed, was one of the more challenging projects from the standpoint of excavation and foundation work. The project was divided into three zones, the first of which took place within the main bathtub of the World Trade Center site, while the second and third zones consisted of construction the foundations for the outlying building columns and shear walls for the new tower inside of a PATH tunnel while two rail lines remained active at all times. Work consisted of rock excavation via blasting, mechanical methods, and also via hand chopping and mechanical splitting tools, the drilling of 160, 80′ long, 3.5″ diameter SAS Stress Bars tensioned to a load of 600 kips to anchor the building, and the pour of concrete spread footings and shear whall footings, in some cases as thick as 12′ to support the massive loads of the 1,766′ tall structure. Work in the outside zone was advanced very rapidly, allowing the first, ceremonial building columns to be erected just five months after the commencement of the foundation work.
The logistically challenging nature of the work within the PATH tunnel required Laquila’s vast resources, extensive experience, and a fair bit of “outside thinking” for its successful completion. A proprietary shoring system around each footing location was installed to support the earth and the PATH rail lines, while overburden and rock was removed via conveyors, crane and bucket, and other methods to make way for the new foundation reinforcing steel and concrete. 8,500 psi and 12,000 psi concrete mixes necessitated the use of conveyors and other novel placement methods to place concrete within these logistically challenging areas, in order to assure the successful and timely completion of the work.