River should have flooded the site, traveled to New Jersey through the PATH tubes and come back to Manhattan at 34th Street, flooding the whole subway system. A more nightmarish scenario could not be imagined on top of the previous horror. But the wall stood long enough for the first responders to pile dirt against it to support it temporarily while concrete plugs were hastily built on both sides the PATH tubes. Ultimately new tiebacks were installed. A greater tragedy was avoided by the prompt actions of the first responders who benefited from the insight of George Tamaro of Mueser R u t l e d g e C o n s u l t i n g Engineers, who was involved in the original construction of the wall, as well as by the unexpected performance of the slurry wall. I sometimes think that the slurry wall did not fail because “it did not want” to add to the suffering and the misery of wounded city. By all engineering standards, the wall should have collapsed. The successful completion of this project was an eye opener to many engineers who realized the potential of the new technique: slurry walls would act as support of excavation; could be installed in difficult soil conditions, in the presence of high water table and yet be waterproof; could bear vertical loads and be part of the permanent structure. It was not long before this recognition yielded several projects throughout the U.S., where this technique was effectively utilized. As in Europe and the Far East, the technique was particularly advantageous in urban environments. The method’s use in the weaving of transits systems in a built-up city and the possibility of having deep underground parking in the expensive real estate of downtown, prompted slurry wall use in most major U.S. metropolitan areas. tion of the project is accelerated by several months, a clear benefit to the project developer. As slurry wall construction became better understood and appreciated by designers and engineers, it was used in several transit projects, where the cut- and-cover technique known as the “Milan method” allowed construction of subway lines and underpasses in city streets without closing them. By constructing a wall on one side of the street using only one lane of traffic, switching on the other side to do the same, and then constructing the roof of the structure closing half the street at a time, all the remaining work can be done underground, while traffic continues above. This method was used for The World Trade Center “bathtub” fully excavated with the PATH train still crossing the site A particularly effective application of slurry wall technology for constructing deep basements is the “top-down” method, which consists of building the perimeter wall and the foundation elements from the ground level, casting the top slab on grade, leaving some openings, then continuing to mine underneath, casting floors as you go down, while construction of the super- structure proceeds above it. While the excavation is more expensive, the perimeter wall does not need temporary support as the floor systems provide it as the excavation deepens. Furthermore the comple- transit systems in New York, San Francisco and Washington D.C. among others, but the most complex urban transit work in the U.S., and possibly the world, was the Central Artery project in Boston, commonly known as “the Big Dig.” While this project incorporated all the most advanced geotechnical and foundation techniques, including soil mixing, jet grouting and ground freezing, the slurry wall was the star of the show. For most of the alignment, the new underground roadway had to be built below the existing elevated highway, which had to be kept in operation until the new tunnel could be opened to traffic. Besides the usual challenges of working Slurry wall at 63rd street subway site in NYC, circa 1980s in an urban environment including miscellaneous fill, utilities and limited work areas, and the geological ones such as boulders and deep rock penetration, there was the added challenge of installing the wall under the existing structure with only limited head room. Special equipment had to be designed and procured for this purpose both to excavate the wall and to place the heavy beams which constituted 60 • DEEP FOUNDATIONS • NOV/DEC 2012
