On the determination of cyclic shear stress for soil liquefaction triggering in centrifuge model test

As the dynamic shear stress acting on soil column could not be measured directly during earthquakes, the simplified procedure proposed by Seed and Idriss is used to calculate the earthquake-induced cyclic shear stress based on acceleration record of strong motion station for liquefaction triggering evaluation. However, when this procedure is applied to physical modeling such as centrifuge model test, it will result in overestimation of the cyclic loading because the acceleration is monitored in liquefiable soil deposit and the acceleration time history will be distorted due to the softening process, and the whole time history is counted regardless the triggering of liquefaction or not. To address this problem, a centrifuge model test with long duration shaking was conducted to observe the difference of acceleration response between the soil deposit and the sidewall at the same elevation. And a series of stress-controlled undrained cyclic triaxial tests of the same sand used in centrifuge model test were carried out to obtain the cyclic strength at different elevations in model ground. This study shows that the cyclic loading before the time of liquefaction occurrence instead of the whole time history corresponds to the cyclic strength well, and also the calculation based on acceleration record of sidewall is closer to the cyclic strength than that in the soil. Thus a method is proposed to determine the cyclic shear stress in dynamic centrifuge model test, which takes the acceleration record before the time of liquefaction triggering at the sidewall of laminar container instead of that of the liquefiable soil as the source data to calculate the cyclic shear stress ratio.