The story of the slurry wall starts in the mid-1960s, when the Port of New York Authority decided to build a pair of 110-story office towers with more than 10 million square feet of office space -- more than all of the office space in the city of Houston -- in lower Manhattan [source: Glanz and Lipton]. The site chosen by the Authority, a Lower West Side warehouse district built upon an old landfill, presented a daunting technical challenge for builders: They would have to dig a huge hole, six stories into the soft soil, to reach bedrock that could support the heavy foundation of the towers. But before that, they had to figure out a way to make the hole totally waterproof, so that the powerful tide of the Hudson River wouldn't seep through the porous ground and flood the hole during construction. "If the big hole were dug at the site without some protective measures, it would quickly become a reservoir," a circa 1966 New York Times article explained [source:Phillips].
The only way to keep that from happening was to put a waterproof barrier in place to protect the hole that would eventually become the World Trade Center basement. But digging a 60- to 80-foot-deep (18 to 24-meter-deep), narrow trench and then pouring a concrete wall also was a tricky proposition because the soft, moist soil would collapse on the excavation. Fortunately, the builders discovered there was a way around that problem. Back in the late 1940s, Italian builders, inspired by oil well drillers, had developed a technique called slurry trenching. As they dug a deep, narrow trench, they coated the sides of the trench with a mixture of powdered clay and water that had the thickness of buttermilk. The gooey coating plugged any leaks that developed in the soil; it prevented water from seeping through the soil, causing the soil to collapse into the trench. Once the digging was completed, the builders would then push pipes down 60 or more feet (18 meters) into the slurry-filled trench and pump concrete into it, starting at the bottom. As the concrete filled the trench, it would displace the slurry solution -- pushing it up and out. Eventually the concrete would harden to form a wall [source: Phillips].
The World Trade Center's builders figured that once the slurry wall was in place, they could then go ahead and build the rest of the towers' six-story underground basement, the so-called "bathtub" [source: Phillips]. On the next page, we'll look at how the builders actually constructed the wall.