Unanticipated Seismic Vulnerability of Fine-Grained Plastic Soils

Significant earthquake-induced settlements occurred in saturated fine-grained soils at the Carrefour Shopping Center in Turkey during the 1999 Kocaeli Earthquake (M = 7.4). Construction was ongoing when the earthquake struck. Extensometers had been installed to monitor settlements beneath a 3.3-m surcharge fill. These devices made possible the measurement of earthquake-induced settlements beneath the fill at six elevations within the profile. Most of the settlement was due to undrained cyclic failure of silt/clay (ML/CL) and high-plasticity clay (CH) strata. Each suffered about 1% vertical strain. Because the soils were plastic and did not meet the Chinese criteria, they were classified as "non-liquefiable" by site engineers and unanticipated as a source of significant seismic deformation. The study demonstrates the limitations of generalized liquefaction screening methods, and dispels the common misconception that plastic soils cannot generate high pore pressures and fail under cyclic loading. As shown here, "non- liquefiable" does not translate to "non-problematic".