Geotechnical Engineering Award of Excellence: Skyline Tower, New York City
Mueser Rutledge Consulting Engineers PLLC, New York City
A Hybrid Foundation Allows the Skyline Tower to Rise Cost-Effectively
The Skyline Tower project involves the construction of a three-story low-rise podium covering the entire footprint of the site and a 68-story L-shaped tower rising above the central portion of the eastern two-thirds of the site. The first floor is devoted to retail, the second floor to parking, and the third floor to mechanical and amenities. Floors 4 and above will be residential apartments. The entire footprint will have a single basement level, corresponding to a depth of approximately 16 feet (4.9 meters). An existing subway entrance at the southwest corner of the site was demolished, and a new underground direct connection into the adjacent Court Square Station is now in the tower. Both excavation and foundations for such a tall building posed special geologic and other challenges, especially due to the variable top of rock, relatively high groundwater level, presence of a subway tunnel and elevated railroad trestle, and adjacent occupied buildings.
Through site-specific seismic analysis, including liquefaction evaluation, and pre-construction condition surveys of adjacent buildings and New York City Transit facilities, the engineers designed a hybrid foundation system that met the challenges of the variability of the top of rock and the owner’s desire to limit dewatering. The hybrid foundation system consisted of spread footings bearing on soil, spread footings and piers on rock, mat on rock, high capacity mini-caissons and tiedowns socketed in rock. The use of a hybrid foundation system consisting of multiple types of foundations within one site resulted in a cost-effective solution. The engineers also designed and implemented a monitoring program to measure vibrations and movements of existing nearby buildings and subway tunnels and facilities.