Reliquefaction behavior of sand and its mesoscopic mechanism

Instances of historical earthquakes demonstrated that sandy grounds can liquefy more than once (reliquefaction) when earthquakes occur in succession (e.g., the main shock and aftershocks). Previous laboratory experiments proved that the resistance of sand to reliquefaction might be lower after its first liquefaction despite an increase in density after the first liquefaction. To clarify the reliquefaction behavior of sand and its mesoscopic mechanism, a series of small-scale shaking table tests were performed for different shaking durations on a sand specimen to simulate multiple liquefactions. Mesoscopic images of the sand particles were taken with a stereomicroscope and an industrial camera both before and after each liquefaction. Then, a digital image processing technique was used to obtain the mesoscopic parameters of the sand particles, namely, the apparent void ratio, long-axis direction, and average coordination number. The test results demonstrated that the sand specimen could reliquefy up to three times according to various shaking durations, suggesting that the density of the sand specimen plays a significant role in the reliquefaction behavior of sand. The analysis of the mesoscopic parameters indicated that the long-axis directions of sand particles are prone to be horizontal in the initial state (before the first liquefaction), whereas after liquefaction and redeposition, the long-axis directions tend to be vertical, suggesting that the decrease in reliquefaction resistance results from the change in the mesoscopic structure of the sand.