Effects of Pre-Strain History on Cyclic Resistance of Ottawa Sand F-65

Abstract eng:
The cyclic strength of a saturated sand can be expected to evolve in response to the cumulative effects of multiple earthquake events over geologic time. Prior laboratory studies have demonstrated that cyclic pre-straining can significantly increase or decrease the cyclic strength for future events, depending on the sand’s properties and state variables (e.g., relative density, stress history), effective confining stresses, and dynamic strains (e.g., amplitudes and numbers of cycles). For example, prior studies have shown that small amplitude cyclic pre-straining tends to increase cyclic strength whereas large amplitude cyclic pre-straining can decrease cyclic strength for dense sand. The cumulative effect of repeated recurrent liquefaction and pre-straining events on the correlation between cyclic strength and void ratio have not, however, been examined. The present study examines the effects of repeated recurrent liquefaction and cyclic pre-straining on the cyclic strength of clean Ottawa F-65 sand. Cyclic direct simple shear tests are performed for a range of initial relative densities subjected to sequences of cyclic loading events. Each cyclic loading event consisted of uniform cyclic stress controlled loading to a maximum shear strain or maximum number of loading cycles, after which the specimen was allowed to reconsolidate prior to the next cyclic loading event. The evolution of cyclic loading behavior through these sequences of loading events are described. Recurrent liquefaction (i.e., ru = 100% or γmax = 3%) and cyclic pre-straining without causing liquefaction caused significant increases in cyclic strength relative to virgin specimens across a range of loose to dense conditions, with the effects being greater for denser specimens. The observed responses and their implications for liquefaction triggering correlations in practice are discussed.

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