SPECIAL ISSUE :LANDMARKS While today’s mechanical clamshells Installation of new tie-backs to support the old slurry wall on Liberty Street, NYC Slurry Walls In the U.S. In the last half a century several new foundation techniques have become accepted in the U.S., most if not all of them having originated in Europe or Japan. While some faced initial resistance by local engineers not familiar with how to design them properly, and faced a long gestation period before being widely accepted, one breakthrough technology was used early on in a project of such visibility, difficulty and importance that its introduction in the marketplace was greatly facilitated. I speak of the slurry wall at the World Trade Center (WTC) in New York. Slurry walls had been extensively used in Europe and Japan by the early 1960s, when the New York Port Authority began investigating their use for the WTC foun- dation, but only two projects had been done in the U.S.: a small shaft in Brooklyn done by Soletanche and a cut-off for the Kinzua Dam in Pennsylvania done by ICOS. The Port’s chief engineer Jack Kyle and Chief of Soils Marty Kapp made a courageous decision to hire Icanada, the Canadian subsidiary of the Italian company ICOS, for the job. ICOS had invented and patented the slurry wall process. Formidable Site Challenges The designers of the Twin Towers had to be sure of the competency of their foundations and, given the variability of the Manhattan schist, wanted to expose the rock and properly prepare it. But the site was adjacent to the Hudson River, the rock was more than a 100 ft (30.5 m) deep in places, the soil conditions included urban fill strewn with obstructions, unstable fine silts, boulders and hard glacial till. To top it off, the site was traversed by the PATH train from New Jersey in two cast iron tubes that had to continue in service until the whole foundation was excavated. Alternate foundation methods were explored, such as compressed air caissons, which could have exposed the rock in discrete places, but the use of a slurry wall along the perimeter would have allowed the total excavation of the site under much safer conditions. Kyle and Kapp went to Italy to investigate the technology and visit construction sites where it was used. They returned satisfied that it was the right answer for their needs. The project would present a great challenge today, when equipment has made great advances, but it was indisputably at the edge of what was possible then. AUTHOR: Arturo L. Ressi di Cervia Special Projects Executive Kiewit Infrastructure Group are crane operated and weigh upward to 15 tons (136 tonnes), the ones used then weighed only 1.5 tons (1.36 tonnes) and were manually operated by a winch mounted on a tubular A frame. Workers used light chisels to penetrate the rock; these were mounted on an A frame or on “churn drills,” an automatic chiseling rig based on the action of a walking beam. Progress was slow, at times painfully so, but the work got done in spite of the light tools used. This project reminds us — in today’s world of remote sensors, wireless commun- ications and real time readings of equip- ment performance — that complex tasks could be done with more simple methods. An example of this was installing the wall around the PATH tubes, a delicate operation fraught with potential danger to the tunnel’s structure. To monitor and direct the excavation progress, a telephone line connected the rig operator to a listener perched on the side of the rail inside the tube, who would alert the operator when he heard the scraping of the clamshell on the cast iron! This work could be done only at night, when the trains were not running, and this writer spent a night inside the tube with the Port’s chief engineer on the occasion of the first panel excavation, who wanted to make sure that the operation could be conducted with no risk to the structure! Performance After 9/11 It is not widely known how the slurry wall at the WTC performed after the terrorist attack of 9/11. The wall was originally sup- ported by several rows of tiebacks anchored into the bedrock, but they were all cut when the permanent floor systems of the basement were installed against the wall. As the towers collapsed they demolished most of the basement floors, particularly on the south west corner of the “bathtub.” By all logic, the unsupported wall should have failed and the Hudson DEEP FOUNDATIONS • NOV/DEC 2012 • 59
