Seismic Liquefaction Assessment for the JFK Light Rail System Project

A procedure to evaluate seismic liquefaction potential based on ground response analysis and in-situ characteristics of the soil deposits is developed. This procedure uses the correlation between the Standard Penetration Test (SPT) N60 blow-counts, and the shear wave velocities to establish Shear Wave Velocity (SWV) profiles. The SWV profiles along with soil properties from the subswface investigation, and the design earthquake time history are then used to perform seismic response analysis for the stratified soil deposits. Cyclic Stress Ratio (CSR) profiles are determined from maximum earthquake induced shear stress profiles, based on seismic response analysis, and effective overburden pressures. . The CSR profiles are then used to determine SPT N(1)60 blowcounts profiles, normalized to 1 ton/ft2 effective overburden pressures. Correlations between SPT N(1)60 values and peak ground acceleration at sites, which have and have not liquefied in previous earthquakes are used to develop the SPT N(1)60 profiles. These profiles characterize a boundary between potentially liquefiable and non-liquefiable saturated granular soil layers during a design earthquake. Finally, the normalized SPT N(1)60 are corrected for the actual in-situ effective overburden pressures to determine SPT N60 screening profiles. These screening profiles can be directly compared to the SPT N60 blow-counts per foot measured in the subsurface investigation for each individual boring-log to determine the presence of liquefiable and non-liquefiable saturated granular soil layers. This procedure is used to evaluate the seismic liquefaction potential at the John F. Kennedy (JFK) International Airport Light Rail System (LRS) project. Separate liquefaction analysis for each of the four sections of the project was performed. The average ground surface, ground water level elevations and soil profiles for these areas were determined based on the borings from the subsurface investigation. The drilling and sampling results were conducted to obtain (SPT) N60 values corresponding to a sampling hammer energy equivalent to 60% of that of a free fall. In the early stages of the investigation, each driller and drill rig was calibrated so that N60 values could be determined. To evaluate the procedure, comparisons with seismic liquefaction assessment based on Cone Penetrometer Tests (CPT’s) were performed and compared to that of the SPT’s. Also comparisons between the shear wave velocities predicted from the SPT N60 values, from the subsurface investigation program, and the in-situ shear wave velocities measured during Crosshole Seismic Testing are presented. These comparisons validate the Seismic Liquefaction Assessment procedure based on SPT’s