Effect of Mineralogy on the Post-Earthquake Shear Strength of Clay-Like Materials

One of the most widely researched areas in geotechnical engineering is the liquefaction phenomena of saturated sands. Research on saturated sands has led to an understanding of the mechanism causing liquefaction and the development of methods to evaluate the liquefaction susceptibility of sands. However, disasters including the Anchorage Landslide following the 1964 Alaska Earthquake and damage to the Moss Landing Marine Laboratory buildings resulting from the 1989 Loma Prieta Earthquake have highlighted the necessity of studying the behavior of clays during cyclic loading. In particular, these disasters have all been attributed to cyclic mobility of the underlying clay layers. Cyclic mobility is the loss of shear strength due to a reduction in effective stress in a soil subjected to cyclic loading, such as that from an earthquake or explosion. The 2009 Mine Safety and Hazard Administration (MSHA) Engineering and Design Manual for Coal Refuse Disposal Facilities provides recommendations for the post-earthquake strength of clay-like materials, but the guidelines suggest values of shear strengths that are extremely low. In this study, the cyclic mobility of clay-like materials was studied in a mineralogical framework using a cyclic simple shear device. Mixtures of kaolinite with quartz and montmorillonite with quartz were prepared and subjected to sinusoidal cyclic loading of 0.5 Hz frequency at different amplitudes. Following cyclic loading, the samples were subjected to undrained monotonic loading under simple shear conditions to find the post-earthquake shear strength. From the results, cyclic strength curves were prepared for the mixtures and the reduction in shear strength due to cyclic loading was determined. It was found that montmorillonite-quartz mixtures required a higher cyclic stress ratio to obtain a given strain level when compared to kaolinite-quartz mixtures for test specimens having equivalent plasticity index values. Furthermore, the reduction in shear strength due to cyclic loading was found to be greater in kaolinite-quartz mixtures than montmorillonite-quartz mixtures. Using the study results, more reasonable recommendations can be made for the post-earthquake undrained shear strength of clay-like materials compared to the ones available in the literature.