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How the World Trade Center Slurry Wall Works

World Trade Center Slurry Wall Construction
The slurry wall waterproofed the area beneath the towers.
The slurry wall waterproofed the area beneath the towers.

The slurry trench method seemed like the answer to the problem that the World Trade Center's builders faced, but there was one catch: The technique had been used in the United States only twice before -- and never on a project of this colossal scale. In 1964, while the project's plans were still being finalized, the Port Authority sent an engineer named George J. Tamaro to Italy on a work-study assignment to learn as much as he could about using a slurry trench to build a waterproof wall. Three years later, it was Tamaro himself who headed the construction of the World Trade Center's slurry wall enclosure [source: Tamaro]. An Italian engineer, Arturo Ressi , also was brought in to work on the project [sources: Ressi, Phillips].

The first step in the process was to use drills and digging machines called clamshell buckets to chew through the soil and whatever obstructions were in the way, such as timbers from old wharves and piers and wrecked ships that had been buried beneath the surface long ago. Then, a special powdered clay containing bentonite, a mineral that takes on an especially thick, gooey consistency when saturated with water, was trucked in from Wyoming. The clay was mixed with water to make the thick slurry, and the mixture was applied to the walls of the trench. Eventually, the slurry was replaced with concrete, which hardened to form a solid waterproof wall [source: Phillips, Gutberle]. When the 3-foot-thick (91-centimeter) underground wall was completed, it stretched for 3,500 feet (1,066 meters) around the basement of the towersand reached down in some spots as many as 80 feet (24 meters) [source: Tamaro].

One of the most complicated parts of building the wall was working around the 100-year-old train tunnels. In some places, the bottom of the slurry wall was up against the top of the tunnels, which were carved partly into the bedrock. For example, along the West Street section of the wall, the slurry wall projected out to cover the train tunnels, and was cast against the iron ring that enclosed the tunnel and was socketed to the bedrock on either side of it to create a watertight seal. To provide additional lateral support, 1,500 high-strength steel anchors were hooked into the slurry wall and then driven 35 feet (10.6 meters) into the bedrock and grouted in place. Each anchor could handle a load of between 100 and 300 tons, which enabled the anchors to support that section of the slurry wall against pressure from the groundwater [source: Tamaro].

The slurry wall was built to be tough, and it turned out to be strong enough to survive the collapse of the towers it'd been put in place to protect. How did it wind up becoming a memorial to the Twin Towers and the people who died there tragically on Sept. 11, 2001?