Abstract: Earthquake-induced liquefaction can cause significant damage to geotechnical structures, and the aftershocks accompanying the earthquakes may cause the sandy soils to liquefy again. To investigate the effect of free-field subsurface seismic history on the liquefaction resistance of saturated sandy soils at various depths, a series of shaking table tests was designed and conducted. Four shaking events with different accelerations, subdivided into seven smaller events, were input to the sandy soil in the tests. The excess pore pressure ratio, acceleration response, and soil settlement were calculated and compared for each vibration event to investigate the variation laws of soil liquefaction resistance at various depths under different seismic histories. The test results show that the magnitude of the input seismic wave acceleration is positively correlated with the acceleration response coefficient. Liquefaction of saturated sandy soils subjected to minor and moderate earthquakes is more likely to occur at shallow rather than superficial layers. In superficial soils, moderate aftershocks after strong earthquakes make the soils less sensitive to seismic intensity, pore water pressure dissipates rapidly after peak acceleration, and liquefaction time is shortened. Soils with strong seismic histories reduce the liquefaction resistance of shallow soils, while the liquefaction resistance of deep soils is enhanced after strong earthquakes, and the affected depth range depends on the earthquake intensity. Quantitative formula of the liquefaction-resistant lifting ratio of soil at different depths under different levels of cyclic earthquakes is obtained through data fitting. The test results can reflect the different effects of different seismic histories on the soils at various depths in the actual earthquake.

Key words: seismic history, sand soil liquefaction, liquefaction resistance, excess pore water pressure, shaking table test