Influence of Soil Softening and Liquefaction on Spectral Acceleration

To provide design guidance on the influence of soil softening on ground response, we analyzed ground motions recorded at five instrumented liquefaction sites, including the Wildlife Liquefaction Array, Calif.; the Port Island Downhole Array, Japan, the Treasure Island and Alameda Naval Air Station sites, Calif., and the Kawagiahi-cho apartment buildings, Niigata, Japan. Conclusions from these analyses are (1) where soil softening occurs early in a strong ground motion sequence, soil softening leads to reduced short period $(T<1.0\mathrm{s})$ spectral accelerations compared to those that would have developed in the absence of softening; (2) where soil softening occurs late in a ground motion sequence, soil softening has little influence on short-period $(T<1.0\mathrm{s})$ spectral response; (3) soil softening and liquefaction may lead to amplification of long period $(T>1.0\mathrm{s})$ spectral accelerations, particularly if ground oscillation develops. Comparison of actual spectra calculated from motions measured at liquefaction sites with design spectra incorporated in seismic provisions of the 1998 AASHTO LRFD Bridge Code, the 1997 Uniform Building Code, and the 2003 International Building Code indicate that code spectra conservatively envelope measured motions from liquefied sites for short-period $(T<1.0\mathrm{s})$ structures. For longer period structures, the design spectra are generally conservative if softer site profile types are assigned, but may be unconservative for stiffer site types. Site-specific response analyses are needed for critical long-period structures.