The Twin Towers were marvels of structural innovation all the way down to their basement, which covered an area of four city blocks (980 foot) by two city blocks (520 foot), roughly 200 feet from the east shore of the Hudson River. This proximity to water posed a great challenge for how to reach bedrock—nearly 70 feet down—where the steel core and perimeter columns of the towers had to be anchored.
Between 1967 and 1968, the World Trade Center’s basement was constructed using a civil engineering technique developed in Italy in the late 1950s called the slurry wall method. Enclosing nearly the entire original WTC site were 158 slurry wall panels built to form a nearly 3,500-foot-long, 3-foot-thick perimeter wall. Dubbed the “bathtub,” the name reflected its singular purpose: to keep water from the Hudson River out of the construction site and new building basement.
The great triumph of the slurry wall method is that it provided an efficient way to build watertight walls for below-grade construction projects in areas that were either surrounded by water or that had a high groundwater table. Made of concrete and reinforced steel, early slurry walls were cast in place directly against the soil and thus were accentuated by the physical properties of the surrounding earth.
Today, several slurry walls still form the perimeter around the WTC site and keep the Hudson River at bay, but they are mostly modern incarnations. The original bathtub was largely reengineered in the years after 9/11 to accommodate damage to existing structures as well as the new construction dictated by Daniel Libeskind’s site master plan.
As a Museum visitor, you can descend below the Memorial pools and reach bedrock to have an encounter with an approximately 60-by-60-foot section of the bathtub and other historic structures from the original foundation of the WTC site.
The concrete-and-steel slurry wall is a vital element within the Museum because of the many metaphors it invokes, among them the power of resilience and human ingenuity. Because the bathtub survived the devastation on 9/11 mostly intact (thanks to its sophisticated design and construction, but also because of the engineers who worked to stabilize it), an even greater calamity—devastating flooding—was averted.
As an artifact, the segment of the slurry wall is significant because it summons the WTC’s history and links its construction to its destruction. The wall also recalls the WTC’s past and present relationship with the natural landscape. The presence of water, the corrosion of its steel elements and resulting staining on the concrete facade—a result of exposure to moisture and oxidation—are easy to see. Less visible is the formation of salts, which pose a risk to the structural integrity of the concrete. The water and the salts also have the potential to increase the corrosion rate of the metal components.
Thanks to generous support from American Express, the Museum’s conservation team, along with consulting conservators, imaging and preservation specialists, architects and engineers, have been working over the past two years to record the storied history of the wall and scrutinize and document its materials and condition. This information will influence approaches to conservation treatment and the long-term preservation of the wall. The research, innovation and collaboration required to care for such a complex artifact will be documented in this blog over the next few months.
When conservators examine an artifact, we often begin by asking questions: “How was it made?” and “What is its function?” These questions are integral to understanding an artifact’s structure, condition, and potential vulnerabilities, and contribute to establishing its historical significance. Our next post will explore these questions. If you’d like to learn more, keep visiting the MEMO blog as the study of the slurry wall continues.
By 9/11 Memorial Staff