Boulanger and Idriss (2007, ASCE) recently presented an excellent summary for the evaluation of the cyclic softening in silts and clays. They correctly suggest that the first step is to evaluate the behavioral characteristics of the soil. They suggest that the transition from being more fundamentally like sands to more like clays occurs at around a plasticity index (PI) of 7. Others have suggested slightly higher values of PI to define this transition (e.g. Seed et al, 2003 and Bray and Sancio, 2006). This issue will likely become resolved in the fullness of time as more field data becomes available. I prefer a slightly higher PI criteria to define clay-like behavior (PI > 12) since sand-like soils tend to have a lower resistance to cyclic loading than clay-like soils and the simplified methodology requires a certain level of conservatism in its application for low to moderate risk projects or in the screening stages of high risk projects.

The resistance to cyclic loading for silts and clays is essentially controlled by the undrained strength ratio of the clay-like soil.  It is clear that if an earthquake applies a cyclic stress ratio (CSR = tcy/s’vc) that is close to the undrained strength ratio of the soil (su/s’vc), large deformations are likely.  The amount of deformations will likely depend on the size and duration of cyclic loading and the plasticity of the soil.  Boulanger and Idriss suggest a value of CRRM=7.5 = 0.8 su/s’vc to define the limit when deformations are likely to become large.

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