Statistical Study of Uniform Cycles in Earthquakes

The applicability of equivalent uniform stress cycles in soil dynamics to the study of soil behavior during and after an earthquake is explored. The actual irregular time histories produced by an earthquake can be represented by uniform amplitude cyclic stresses, although there may be a considerable amount of uncertainty associated with them. The stress level of 75 percent of the maximum is suggested for such conversion, since in this case the uncertainty in the normalized soil-strength curve has a minimum effect on the value of the N_eq versus M relationship. A statistical relationship between N_eq and the earthquake magnitude is proposed here based on results available in the literature. The N_eq could be estimated adequately by considering the component of excitation containing the peak acceleration. The N_eq versus M relationship proposed is somewhat different from the relationship suggested by Seed and Idriss. This discrepancy in N_eq values may not yield significant differences in estimating the soil strength in a liquefaction study.