Ray Sandiford presented the March Geotechnical Lecture at the CUNY Graduate Center in Manhattan on March 15, 2007 to a standing- room only audience of over 200 attendees. The lecture focused on geotechnical aspects of the redevelopment of the World Trade Center (WTC) site, both those currently underway and those still in planning.

Mr. Sandiford began the lecture by describing the Port Authority’s facilities in the region, and detailing the history of the WTC site. The site occupies much of the west side of lower Manhattan. About half of the site was once in the Hudson River and was underlain by either fill or deposits of organic silts. The original PATH system tunnels crossed the site and terminated in the old H&M Building. In addition, the 1 and 9 subway lines crossed through the site. The original excavation for the WTC construction provided the fill used to create the Battery Park City development.

Mr. Sandiford then introduced many of the foundation-related challenges to the new WTC development. The site, which will be increased in area to about 1000 ft by 1000 ft by expanding to Church Street and Cedar Street, will contain many new structures in addition to the remnant foundations of the original WTC 1 and 2 buildings. A new east basement will be constructed all the way to Church Street. The new PATH hub, designed by Calatrava, will occupy part of the new space. New tower structures, including the Freedom Tower, and towers 2, 3, and 4, will be constructed and require foundations on rock. A Vehicle Security Center is planned for the south end of the site. The existing 1 and 9 Subway lines will remain beneath the site and must be underpinned to allow for new construction below. A Pedestrian Underpass is also planned for the west side of the site to cross beneath West Street. A final complication is that the existing WTC 1 and WTC 2 foundations cannot be removed from the site. Ray then explained that due to the complexity of the existing site features and proposed new construction, a 4D modeling procedure has been adopted for planning new slurry wall, excavation, and foundation construction.

The staple excavation support system for the WTC site is the slurry wall, which was used for the original construction in the late 1960s and is being used to retain the new portions of the site. Mr. Sandiford explained that slurry walls are currently being installed for the new east basement area, where rock lies at depths of 80 to 100 ft and requires hydromills and drop chisels for construction. The new slurry walls require a 4 ft thickness because of the 20 ft floor to floor distances and discontinuous walls and floor systems, complicating transfer of lateral loads to the below-grade portions of the new structures. Permanent tiebacks are being installed and relining of the existing slurry walls is being performed to protect existing anchor heads and provide drainage.

CAD models are being used to avoid the existing tiebacks. A jet-grouted plug will be used to provide cutoff beneath the tunnel where it crosses through the slurry wall. The Pedestrian Underpass, to be constructed beneath West Street to connect the site with the World Financial Center, will require a number of underground construction techniques. The excavation will be supported by a combination of secant pile walls and ground freezing. The ground freezing will be employed to deal with the areas through which multiple duct banks pass. The Freedom Tower, whose foundations are currently under construction, requires very high capacity foundations on rock. Large uplift and overturning loads are being resisted by permanent tiedown anchors consisting of 3 inch diameter, 150 ksi anchors up to 80 ft long and prestressed on rock. Extensive creep tests have been conducted on the tiedown anchors. The new Vehicle Security Center foundation and belowgrade construction is complicated by the presence of many existing foundations for the adjacent 90 West Cedar building and interference with the Deutsche Bank Building foundations.

Mr. Sandiford concluded the lecture by stating that the entire site is expected to be excavated to subgrade within the next year. Following a number of questions from the audience, Ray Sandiford was presented a Golden Apple from Geotechnical Group in appreciation for the lecture. The slides presented by Mr. Sandiford are available for review below.

 Lecture Slides 3.63 Mb