520 Park Avenue (Courtesy Zeckendorf Development) FEATURE ARTICLE Complex Deep Foundation System for Tall Tower in New York City When completed, 520 Park Avenue will amend the Manhattan skyline and be one of the most prestigious high-rise towers in New York City. The 52-story condominium building is located on a 60 ft by 100 ft lot (18.3 m by 30.5 m) on East 60th Street between Park Avenue and Madison Avenue. The geographically complex site is bounded by two fragile five-story masonry buildings on the east and west, a mid-rise building on the north, and a New York City Transit (NYCT) subway tunnel on the south. To add to this complexi ty, the ambi t ious development required an excavation between 70 and 80 ft (21.3 and 24.4 m) deep, mostly below groundwater level in highly-permeable rock, to accommodate four levels of cellars. The new building also cantilevers 24 ft (7.3 m) over a five-story historic building on the east. The limited footprint of the building coupled with large compressive and tensile axial loads, considerable base shear and over-turning moments, and the required deep excavation, presented unique foundation challenges. This article describes how the challenges were solved with an innovative combination of permanent secant pile walls and drilled shafts. Site plan and existing adjacent structures Site Geology The 1865 Viele Map provides the site topographic condi t ions pr ior to development in the area, and indicates that the site was at the eastern edge of a historic Engineers, and Alfredas Daugiala of Underpinning & Foundations Skanska marshland, which was approximately west of Park Avenue and north of 58th Street. A historic stream, which ran alongside the eastern edge of the site, was part of a larger stream network that traversed the surrounding area. This stream network is also visible in the 1865 Viele map. In addition, the bedrock beneath the site lies in a highly weathered zone due to a couple of ancient faulted zones that transect the site. These indicate that a zone of highly weathered rock is expected. Subsurface Conditions Two subsurface investigations were performed at the site, one in 2012 and another in 2013. The initial investigation, performed by another firm, consisted of seven borings. At that time, the site was occupied by a five-story masonry structure with one basement level, and five out of the seven borings were made within the basement using a low headroom, portable electric rig, which is not efficient. The remaining two borings were located outside the property line within the sidewalk; these borings were performed AUTHORS Sitotaw Y. Fantaye, P.E., T.C. Michael Law, Ph.D., P.E., G.E., and Renzo D. Verastegui, P.E., of Mueser Rutledge Consulting DEEP FOUNDATIONS • MAR/APR 2017 • 73
