Influence of Mineralogy and Plasticity on the Cyclic and Post-Cyclic Behavior of Normally Consolidated Soils

The Anchorage Landslide following the 1964 Alaska Earthquake, damage to the Moss Landing Marine Laboratory after the 1989 Loma Prieta Earthquake, and severely distressed buildings founded on clays following the 1985 Mexico Earthquake have emphasized the current deficiencies in our understanding of the cyclic and post-cyclic behavior of clays. Although researchers have begun evaluating the behavior of clays during and following cyclic loading, information presented in the literature is inconsistent. This study systematically evaluates the effect of plasticity characteristics and mineralogical composition on the cyclic behavior and the post-cyclic shear strengths of normally consolidated clays. Eighteen samples were prepared in the laboratory by mixing kaolinite with quartz (KQ) or montmorillonite with quartz (MQ) and tested in a cyclic simple shear device under a sinusoidal loading at a frequency of 0.5 Hz. The samples were then sheared monotonically to determine the post-cyclic shear strength. In addition, cyclic simple shear testing was conducted at a consulting firm for 12 natural samples. Using the test results, cyclic strength curves were developed to represent seismic performance levels of immediate occupancy (2.5% double amplitude, DA, shear strains), life safety (5%) and collapse prevention (10%). The cyclic strength curves showed that in KQ mixtures, a smaller number of additional cycles of loading were required to cause 10% DA strains after the sample had experienced 2.5% DA strain in comparison to MQ mixtures. The cyclic resistance was found to increase as plasticity index (PI) increases. The cyclic strength curves were also observed to become increasingly linear as PI increases in KQ mixtures, but the opposite trend was observed in MQ mixtures. The lower bound post-cyclic strength was found to be 45% to 100% of the static shear strength. Moreover, the post-cyclic shear strengths were significantly higher than the strength recommended by the current guidelines. Based on the results, current guidelines for seismic evaluation of clays may need to be re-visited to ensure both safe and economic design of infrastructure.