Effects of Long-Duration Motions on Soil Liquefaction Hazards

Abstract eng:
Liquefaction has caused a great deal of damage in past earthquakes around the world. Laboratory studies have provided useful insight into the complex mechanics of liquefiable soil behavior, and field investigations of case histories have led to the development of useful empirical procedures for evaluating the potential for triggering and the consequences of liquefaction. However, conventional laboratory tests offer a simplified representation of earthquake loading and field case histories are sparse over ranges of conditions that are of interest in professional practice. Recent laboratory tests involving the type of transient loading that soils are subjected to in actual earthquakes has revealed complexities in their response to loading cycles of different amplitudes, and has confirmed the dependence of soil deformations on post-triggering loading. Under transient loading, the order in which the stress cycles are applied to the soil influences the level of pore pressure generated in the soil and also influences the strain amplitudes that develop in response to the applied loading. These results, along with the dearth of case histories from very large magnitude earthquakes in the empirical database, illustrate the need to account for ground motion duration in liquefaction hazard evaluations. This need is particularly important for long- duration motions, such as those that occur in subduction zone environments. Very long duration motions can lead to liquefaction at large source-to-site distances, can cause liquefaction of denser soils than may otherwise be anticipated, can lead to additional softening due to fabric degradation, and can be significantly influenced by pore pressure redistribution during shaking. In very long duration motions, a soil deposit in which liquefaction is triggered relatively early will be subjected to a high level of cumulative loading while in its liquefied state, which can lead to large cyclic and permanent deformations. This paper reviews liquefiable soil behavior and the implications of very long duration ground motions on that behavior, and describes a framework that allows the timing of liquefaction to be taken into consideration in the evaluation of liquefaction hazards. The framework allows separation of loading into pre- and post-triggering components, and allows different ground motion intensity measures to be used to characterize triggering and consequences. The correlation of consequences of liquefaction to post-triggering ground motion intensity is shown to improve the accuracy of, and reduce uncertainty in, predictions of the response of liquefiable soils. Such an approach has benefits for evaluation of performance, particularly in areas subject to very large magnitude earthquakes.

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