<P> Groundbreaking was on August 5, 1966, marking the beginning of construction of the World Trade Center's foundations . The site of the World Trade Center was located on landfill, with the bedrock located 65 feet (20 m) below grade . In order to construct the World Trade Center, it was necessary to build "The Bathtub", with the slurry wall along the West Street side of the site, to keep water from the Hudson River out . This method was used in place of conventional dewatering methods because lowering the groundwater table would cause large settlements of nearby buildings not built on deep foundations . </P> <P> The slurry method involved digging a trench, and as excavation proceeded, filling the space with a "slurry" mixture . The mixture, composed of bentonite, plugged holes and kept water out . When the trench was dug out, a steel cage was inserted and concrete was poured in, forcing the "slurry" out . The "slurry" method was devised by Port Authority chief engineer John M. Kyle Jr . Towards the end of 1966, work began on building the slurry wall, led by Montreal - based Icanda, a subsidiary of an Italian engineering firm, Impresa Costruzioni Opere Specializzate (I.C.O.S.). It took fourteen months for the slurry wall to be completed, which was necessary before excavation of material from the interior of the site could begin . The original Hudson Tubes, which carried PATH trains into Hudson Terminal, remained in service as elevated tunnels until 1971 when a new PATH station was built . </P> <P> Construction work began on the North Tower in August 1968 with construction beginning on the South Tower by January 1969 . In January 1967, $74 million in contracts were awarded to the Pacific Car and Foundry Company, Laclede Steel Company, Granite City Steel Company, and Karl Koch Erecting Company to supply steel for the project . The Port Authority chose to use many different steel suppliers, bidding on smaller portions of steel, rather than buy larger amounts from a single source such as Bethlehem Steel or U.S. Steel as a cost - saving measure . Karl Koch was also hired to do all the work of erecting the steel, and a contract for work on the aluminum facade was awarded to the Aluminum Company of America . Tishman Realty & Construction was hired in February 1967 to oversee construction of the project . </P> <P> Extensive use of prefabricated parts for the perimeter framing and floor truss systems helped speed up the construction process and reduce costs, while providing greater quality control . Steel components were freighted into a Penn Central yard in Jersey City . From there, they were brought in the early morning hours through the Holland Tunnel to the construction site, then lifted into place by a crane . Larger pieces were brought to the construction site by tugboats . A special type of crane, suitable for constructing such tall buildings, that used hydraulics to lift components and provided its own power was used in construction of the World Trade Center . The Favco Standard 2700 Crane, manufactured by Favelle Mort Ltd. of New South Wales, Australia was informally called a "kangaroo crane ." </P>

When did construction start on the twin towers
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