Site Response and Geohazard Analysis in New York City Area

The 2014 New York City Building Code (NYCBC) requires an evaluation of liquefaction for soil at depths shallower than 15 m if standard penetration test (SPT) N₆₀ values plot in the “liquefaction evaluation required” domain. Typically, the liquefaction assessment is performed using the maximum considered geometric mean peak ground acceleration adjusted for site class effect, which is based on SPT N₆₀ values. A project site in New York City area was assigned Seismic Site Class F based on subsurface conditions (i.e., SPT N₆₀ values) encountered in the borings and liquefaction analysis showing extensive potentially liquefiable layers from 2.7 to 15.5 m below ground surface on site. Therefore, in accordance with the NYCBC, a site-specific response analysis is required to determine seismic parameters for the site. To develop design seismic soil profiles, Seismic Cone Penetration Tests (SCPT) were performed to collect shear-wave velocity (V_S) data. The average V_S profile based on SCPT indicated a classification of Seismic Site Class D. In addition to the improvement of the Site Class, a site-specific ground motion analysis was conducted to develop a design acceleration response spectrum in accordance with the applicable provisions of ASCE 7-10 and NYCBC, using dynamic soil properties. Based on the results of the site-specific analysis, the liquefaction hazard was reevaluated. Liquefaction risk and liquefaction-induced settlements were reduced significantly utilizing site-specific analysis results. The case study demonstrates using a site-specific ground-motion analysis to obtain a refined estimate of potential earthquake-induced shaking at a site, which reduced dynamic loads, and thus reduced design and construction costs significantly.