A portion of the slurry w all was deliberately left exposed in the Foundation Hall of the National September 11 Memorial Museum.
By DAVID W. DUNLAP
Published: September 11, 2013

Some of the heroes of Sept. 11, 2001, performed their lifesaving work at the World Trade Center many years before the attack.

Arturo Lamberto Ressi di Cervia, who died last month at 72, supervised the construction of the slurry wall around the trade center foundation in the 1960s. When the towers collapsed in 2001, and the wall began to strain under almost unthinkable pressures from the surrounding water table, his work paid off.

The wall held.

Because the slurry wall held, the 70-foot-deep foundation did not fill with groundwater. And because of that, the PATH tubes were not submerged. And because of that, the subway tunnels below the PATH tubes were not inundated.

How much worse could Sept. 11 have been? Imagine if Hurricane Sandy had followed the terrorist attack by a few hours.

Mr. Ressi’s supervision of the slurry wall construction “may have helped prevent the Hudson River from flooding parts of Lower Manhattan,” said George J. Tamaro, a former staff engineer at the Port Authority of New York and New Jersey who was closely involved with the construction of the trade center.

Deep Foundations Institute Arturo Lamberto Ressi di Cervia supervised the construction of the slurry w all around the World Trade Center foundation in the 1960s. The once-low ly w all became a symbol of resilience in the years after the 2001 attack.

The once-lowly wall became a symbol of resilience in the months and years after the attack. Its importance was so widely acknowledged that a portion was deliberately left exposed in the colossal Foundation Hall of the National September 11 Memorial Museum, which is to open next year.

Of course, that wasn’t what Mr. Ressi set out to build in 1967, when he arrived in New Y ork at age 27, just three years out of the University of Bologna, Italy, from which he held a doctorate in engineering.

He worked for Icanda, an Italian-Canadian contractor that specialized in slurry walls. First, trenches would be dug down to bedrock; about 70 feet below street level, in the case of the trade center.

Slurry ― a muddy soup of water and bentonite clay ― would be pumped into the trench to keep the sides from closing up until a giant cage of steel rebar could be lowered into the trench, after which concrete would be pumped in.

The concrete, heavy enough to displace the slurry, would harden into a wall that no one would actually see until it was completed all around, after which excavation could begin within the 11-acre area enclosed by the wall.

Without such a retaining wall, it would have been impossible to excavate the trade center site, much of which was on spongy landfill dating to the 18th century. And conventional foundation wall construction was ruled out because of the many underground obstacles, from ships’ ballast to the PATH tubes.

Before Icanda could begin, the Port Authority and City Hall had to work out differences over the development of the trade center. (Sound familiar?) That was when Mr. Ressi met a singer named Carol Marie Gerdts, and fell in love. “I basically was twiddling my thumbs for months,” he told my colleague James Glanz in a 2003 interview, “and as a result I got married.”

All else was stress. Icanda had no experience dealing with New Y ork’s construction trade unions. It was employing a technique untried at such a scale, using jury-rigged tools, on a site that brimmed with uncharted obstacles and unhappy surprises, like a sudden 50-foot drop in the bedrock level where a small valley had been carved by retreating glaciers.

Crews worked around the clock. When a crisis developed at 3 a.m., Mr. Ressi thought nothing of telephoning Mr. Tamaro at home. “If I wasn’t sleeping, why should he?” was his philosophy. “We would get a quick decision.”

Mr. Ressi prided himself in saying the main perimeter wall construction was finished in nine months, “pretty much on schedule.”

Thirty-five years later, in 2002, the architect Daniel Libeskind drew the world’s attention to the symbolic significance of the wall, when he proposed its preservation and exposure as part of the redevelopment of the trade center.

“My first experience at ground zero was going down to bedrock and being viscerally transfixed,” he said this week. “I realized this is not only about unimaginable destruction but about the power of resurgence.”

Mr. Libeskind recalled a “very emotional” conversation with Mr. Ressi in which he described his plan to expose the slurry wall. “I think he instantly understood why it was important to do that,” Mr. Libeskind said.

Indeed, he did. In their 2003 conversation, Mr. Glanz mentioned to Mr. Ressi that some engineers had expressed reservations about the idea.

“Oh, no,” Mr. Ressi said. “I was instead so pleased that that was the chosen scheme because I think it is ― well, not only do I like the architecture of it, but I also like the idea of having that space open again. Although I promise you, I never thought I would have seen that wall again in my life.”

A version of this article appears in print on September 12, 2013, on page A29 of the New York edition w ith the headline: On Sept. 11, a Wall Limited the Devastation.