How the World Trade Center Slurry Wall Works

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.

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?

When plans were laid to create a September 11 museum as part of the reconstruction of Ground Zero, it seemed obvious to many that at least part of the original slurry wall had to be included in the memorial. As Stefan Pryor, president of the Lower Manhattan Development Corporation, explained to The New York Times in 2005: "Now that the slurry wall has been laid bare and infused with meaning, it's our obligation to preserve it and ensure that all who come to the site have the opportunity to view it and pay tribute at it" [source: Dunlap].

The museum's construction budget only allowed the builders to spend $11 million to include a small section of the original slurry wall, consisting of three 20-foot-wide (6-meter-wide) panels. According to a 2008 New York Times article, the process of protecting the slurry wall from damage during the reconstruction project was elaborate. All the digging around the wall was done manually, for extra care. A new concrete liner was poured in front of the original slurry wall to strengthen it from caving in or leaking, and high-strength steel cables were put in place to give the support that the towers' basement once provided. Additionally, to protect the slurry wall's surface from deterioration, it was covered with a protective coating of liquefied concrete known as shotcrete [source: Dunlap].

But museum officials hope the end result will be a moving experience for visitors. Museum president Joseph C. Daniels told The New York Times in 2008 that he envisioned the slurry wall becoming as iconic as Jerusalem's Western Wall, also known as the Wailing Wall, the last remaining section of the city's Second Temple and a sacred spot for members of the Jewish faith. "The idea of being able to get that connection, which will link you to the past, is important," he said [source: Dunlap].