PRESHAKE: Centrifuge modeling of the effect of seismic preshaking on the liquefaction resistance of sands

This effect of preshaking history on the liquefaction behavior of sandy soils is documented in this database. This database includes data from two centrifuge experiments performed on both silty and clean sand. The tests simulated the effect of several decades to several centuries of earthquake events on a 5-6 m uniform clean or silty sand horizontal deposit, including both events that liquefied the deposit and others that did not liquefy it. The two centrifuge deposits were subjected to three or four basic event types: Events A, B and C (or D). An Event A was defined as 5 sinusoidal cycles of a peak base acceleration, apb ≈ 0.035-0.045 g; an Event B was defined as 15 sinusoidal cycles of a peak base acceleration, apb ≈ 0.04-0.05 g; and an Event C (or D) was defined as 15 sinusoidal cycles of a peak base acceleration, apb ≈ 0.1-0.25 g, all in prototype units. The prototype frequency in all cases is 2 Hz. The 15-cycle duration of Events B, C and D corresponds approximately to an earthquake of moment magnitude, Mw ≈ 7.5; while the 5-cycle duration of the Events A correspond to Mw ≈ 6. An Event A represents a mild to moderate earthquake shaking; an Event B represents a mildly strong earthquake shaking; and an Event C (or D) represents a strong to very strong extensive liquefaction shaking in the field. The results of the experiments showed that the combination of mild/moderate (Events A) to mildly strong (Events B) shakings resulted in a significant increase in liquefaction resistance of the deposits over time. However, the occurrence of extensive liquefaction, caused by an Event C or D, resulted in an immediate reduction in liquefaction resistance of the deposit. This suggests a complex phenomenon, with most of the shaking events strengthening the liquefiable layer but some of them weakening it, sometimes dramatically when extensive liquefaction was involved.